Compressed Air System Maintenance Guide -1006677

Compressed Air System Maintenance Guide

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Compressed Air System Maintenance Guide 1006677

Final Report, November 2002

EPRI Project Manager M. Pugh

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT EPRI

ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax). Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2002 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS This report was prepared by Fossil Maintenance Applications Center (FMAC) 1300 W. T. Harris Boulevard Charlotte, NC 28262 This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Compressed Air System Maintenance Guide, EPRI, Palo Alto, CA: 2002. 1006677.

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REPORT SUMMARY

The Compressed Air System Maintenance Guide provides fossil plant personnel with information on the operation and maintenance of the compressed air system. This information will assist personnel in improving performance of the compressed air system, reducing maintenance costs, and increasing air system reliability. Background As the age of compressed air system components increases, the maintenance costs required for continued system operation also increase. Unit reliability and lost energy costs are affected by performance of the compressed air system. Objectives • To provide general guidance that will enable plant maintenance and operations personnel to ensure that the compressed air system functions as required, thereby maintaining high system reliability and availability •

To identify corrective, preventive, and predictive maintenance steps that can be implemented to improve system performance and air delivery

•

To provide information that will enable plant personnel to troubleshoot compressed air system problems

•

To assist plant personnel in the identification of actions that will minimize air system leakage and result in a reduction of maintenance costs

Approach Using input from the industry, problem areas in compressed air systems at fossil power plants were identified. Technical information and data related to these problem areas were gathered and reviewed for inclusion in this report. An extensive search of industry and EPRI information was conducted to provide relevant information for plant personnel in the operation, maintenance, and performance of the compressed air system. A task group consisting of fossil utility personnel and other industry experts provided input throughout the development of this guide. Results The guide presents background information and fundamentals on compressed air system components and controls, along with technical discussions and guidance on air quality requirements (related to both monitoring and maintenance strategies). The guide is designed to provide plant personnel with information about the air system’s operational requirements and to help them to make better-informed decisions regarding maintenance strategies. As with all Fossil Maintenance Applications Center (FMAC) maintenance guides, various sections containing v

extensive troubleshooting guidance have been included to aid in the quick identification and resolution of compressed air system and component problems. Effective condition monitoring strategies, such as thermography, vibration monitoring, oil analysis, and, in particular, air quality monitoring techniques are included to aid plant personnel in setting up effective preventive maintenance programs. These strategies are designed to detect problems before they can impact plant availability and performance. EPRI Perspective Unit reliability and lost energy costs are affected by performance of the air system. Operation and maintenance costs associated with the compressed air system also increase as the age of the system increases. By addressing many of the compressed air system issues that challenge fossil power plant maintenance and engineering personnel, this guide provides a comprehensive overview of the equipment practices needed for continued reliable operation of these systems and components. Keywords Compressed air Instrument air Preventive maintenance Compressors Air dryers Air filters

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EPRI Licensed Material

ACKNOWLEDGMENTS The Compressed Air System Maintenance Guide was produced by the Fossil Maintenance Applications Center (FMAC) and the following members of the Compressed Air System Technical Advisory Group. FMAC would like to thank the following individuals for their participation in the preparation and/or review of this report. Technical Advisory Group Members: Name

Utility

Clay Boyd

Duke Energy Generation Services

Kent Coleman

EPRI Repair and Replacement Applications Center

Terry Gorton

Ingersoll-Rand Corporation

Jim Hinson

Tennessee Valley Authority

Jim McCarthy


Jim Plybon


Steve Wollmuth

Great River Energy Company

FMAC and the Technical Advisory Group were supported in their efforts to develop this guide by: Sharon Parker

Industry Consultant

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CONTENTS

1 INTRODUCTION ....................................................................................................................1-1 1.1

Background..................................................................................................................1-1

1.2

Approach .....................................................................................................................1-2

1.3

Organization of the Guide ............................................................................................1-2

1.4

Pop Outs ......................................................................................................................1-3

2 TUTORIAL..............................................................................................................................2-1 2.1

General Operation .......................................................................................................2-2

2.2

System Layout .............................................................................................................2-6

2.3

Air Quality Guidance and Standards............................................................................2-9

2.4

Components and Functions .......................................................................................2-15

2.4.1

Compressors.....................................................................................................2-16

2.4.1.1

Reciprocating Compressors........................................................................2-26

2.4.1.2

Rotary Screw Compressors ........................................................................2-30

2.4.1.2.1

Oil-Injected Rotary Screw Compressor ...............................................2-30

2.4.1.2.2

Oil-Free Rotary Screw Compressor ....................................................2-33

2.4.1.2.3

Oil-Injected Single-Screw Compressor ...............................................2-35

2.4.1.2.4

Oil-Injected Sliding-Vane Compressor ................................................2-37

2.4.1.2.5

Liquid Ring Rotary Compressors ........................................................2-38

2.4.1.3

Centrifugal Compressors ............................................................................2-41

2.4.1.4

Compressor Lubrication..............................................................................2-47

2.4.2

Motors ...............................................................................................................2-51

2.4.3

Air Dryers ..........................................................................................................2-54

2.4.3.1

Regenerative Desiccant..............................................................................2-59

2.4.3.1.1

Heat-of-Compression Dryer ................................................................2-64

2.4.3.1.2

Heat Regenerative Dryer.....................................................................2-66

2.4.3.1.3

Heatless Regenerative Dryer ..............................................................2-67

2.4.3.2

Refrigerant Dryer ........................................................................................2-68

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2.4.3.3

Deliquescent Dryer .....................................................................................2-70

2.4.3.4

Membrane Dryer .........................................................................................2-71

2.4.4

Air Filters ...........................................................................................................2-71

2.4.4.1

Filter Selection ............................................................................................2-73

2.4.4.2

Particulate Filters ........................................................................................2-76

2.4.4.3

Coalescing Filters .......................................................................................2-77

2.4.4.4

Adsorption Filters ........................................................................................2-79

2.4.4.5

Air-Line Filters.............................................................................................2-80

2.4.5

Inter- and After-Coolers.....................................................................................2-80

2.4.5.1

Inter-Cooler .................................................................................................2-82

2.4.5.2

After-Cooler.................................................................................................2-82

2.4.6

Separators and Traps .......................................................................................2-83

2.4.6.1

Moisture Separators....................................................................................2-83

2.4.6.2

Blowdown Devices/Traps............................................................................2-85

2.4.7

Receiver Tanks .................................................................................................2-90

2.4.8

Distribution Piping .............................................................................................2-92

2.4.9

Instrumentation .................................................................................................2-94

2.4.10 Safety Relief Valves ..........................................................................................2-95 2.4.11 End-Use Devices ..............................................................................................2-95 3 CONTROLS............................................................................................................................3-1 3.1

Single Compressor Control Methods ...........................................................................3-2

3.1.1

Start/Stop Control................................................................................................3-2

3.1.2

Constant Speed Control......................................................................................3-2

3.1.3

Modulating Control ..............................................................................................3-3

3.1.4

Multi-Step Controls..............................................................................................3-5

3.1.5

Variable Speed Control .......................................................................................3-6

3.2

Multiple-Compressor Control Methods ........................................................................3-7

3.2.1

Electro-Pneumatic Controls ................................................................................3-8

3.2.2

Microprocessor Controls .....................................................................................3-8

3.2.3

Custom Controls ...............................................................................................3-10

4 TROUBLESHOOTING ...........................................................................................................4-1

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4.1

General Compressed Air System ................................................................................4-3

4.2

Double-Acting Reciprocating Compressor...................................................................4-4


4.3

Rotary Compressor....................................................................................................4-12

4.4

Centrifugal Compressor .............................................................................................4-14

4.5

Regenerative Desiccant Dryer ...................................................................................4-24

4.6

Refrigerant Dryer .......................................................................................................4-25

4.7

Motors ........................................................................................................................4-28

5 PREVENTIVE MAINTENANCE..............................................................................................5-1 5.1

Background..................................................................................................................5-1

5.2

Overview of PM Information and Tables......................................................................5-3

5.2.1

PM Template.......................................................................................................5-4

5.2.2

Failure Locations, Degradation Mechanisms, and PM Strategies ......................5-5

5.2.3

PM Tasks and Their Degradation Mechanisms (including intervals) ..................5-6

5.3

General PM Tasks for Air Compressors ......................................................................5-6

5.4

Air Compressor Types .................................................................................................5-8

5.4.1

5.4.1.1

PM Tasks ......................................................................................................5-8

5.4.1.2

PM Template...............................................................................................5-12

5.4.1.3

Failure Locations, Degradation Mechanisms, and PM Strategies ..............5-14

5.4.1.4

PM Tasks and Their Degradation Mechanisms ..........................................5-20

5.4.2

Rotary Screw Compressors ..............................................................................5-24

5.4.2.1

PM Tasks ....................................................................................................5-24

5.4.2.2

PM Template...............................................................................................5-28

5.4.2.3


5.4.2.4


5.4.3

5.5

Reciprocating Compressors................................................................................5-8

Liquid Ring Rotary Compressor and Pump.......................................................5-36

5.4.3.1

PM Tasks ....................................................................................................5-36

5.4.3.2

PM Template...............................................................................................5-40

5.4.3.3


5.4.3.4


Motors ........................................................................................................................5-60

5.5.1

PM Tasks ..........................................................................................................5-60

5.5.2

PM Template.....................................................................................................5-64

5.5.3

Failure Locations, Degradation Mechanisms, and PM Strategies ....................5-66

5.5.4

PM Tasks and Their Degradation Mechanisms ................................................5-87

5.6

Air Dryers .................................................................................................................5-100

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5.7

Air Filters..................................................................................................................5-101

5.8

Inter-Coolers and After-Coolers ...............................................................................5-105

5.9

Separators and Traps ..............................................................................................5-108

5.10 Receiver Tanks ........................................................................................................5-110 5.11 Piping.......................................................................................................................5-110 5.12 Instrumentation ........................................................................................................5-111 5.13 Safety Relief Valves.................................................................................................5-116 5.14 End-Use Devices .....................................................................................................5-118 6 CONDITION-BASED MONITORING......................................................................................6-1 6.1

Predictive Maintenance ...............................................................................................6-1

6.1.1

Thermography.....................................................................................................6-1

6.1.2

Vibration Monitoring ............................................................................................6-2

6.1.3

Oil Analysis .........................................................................................................6-2

6.2

Air Quality Monitoring...................................................................................................6-3

6.2.1

Contaminants ......................................................................................................6-7

6.2.1.1

Dew Point Monitoring..................................................................................6-10

6.2.1.1.1

Monitoring Frequency .........................................................................6-10

6.2.1.1.2

Monitoring Locations ...........................................................................6-12

6.2.1.1.3

Dew Point Levels ................................................................................6-13

6.2.1.2

Particulate Monitoring .................................................................................6-14

6.2.1.2.1

Monitoring Frequency .........................................................................6-14

6.2.1.2.2

Monitoring Locations ...........................................................................6-14

6.2.1.2.3

Monitoring Methods.............................................................................6-15

6.2.1.3

Hydrocarbon Monitoring..............................................................................6-16

6.2.1.3.1

Monitoring Methods.............................................................................6-16

7 CORRECTIVE MAINTENANCE .............................................................................................7-1 7.1

Safety Issues ...............................................................................................................7-1

7.2

Component Failures ....................................................................................................7-5

7.2.1

Air Supply Components ......................................................................................7-5

7.2.2

End-Use Components.........................................................................................7-6

7.3

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Air Leakage..................................................................................................................7-7

7.3.1

Cost of Air Leakage.............................................................................................7-8

7.3.2

Leak Detection ....................................................................................................7-9


7.3.3

Leak Repairs .....................................................................................................7-13

7.3.4

Piping Replacement ..........................................................................................7-16

8 INDUSTRY INFORMATION ...................................................................................................8-1 8.1

Books and Reports ......................................................................................................8-1

8.1.1

Specialty Books...................................................................................................8-5

8.1.2

Information on the Compressor Marketplace ......................................................8-8

8.2

Brochures ..................................................................................................................8-10

8.3

Periodicals .................................................................................................................8-11

8.3.1

Other Periodicals...............................................................................................8-11

8.4

Software.....................................................................................................................8-12

8.5

Videos ........................................................................................................................8-13

8.6

Workshops and Training Courses..............................................................................8-14

9 REFERENCES AND GLOSSARY .........................................................................................9-1 9.1

References ..................................................................................................................9-1

9.2

Glossary.......................................................................................................................9-3

A PM PRACTICES ................................................................................................................... A-1 B MAINTENANCE PHILOSOPHY AND RECOMMENDATIONS ............................................ B-1 C LIFE CYCLE MANAGEMENT PREVENTIVE MAINTENANCE ACTIVITIES ...................... C-1 D OVERHAULING A ROTARY SCREW COMPRESSOR....................................................... D-1 E RENTAL COMPRESSORS................................................................................................... E-1 F CONDENSATE DISPOSAL ...................................................................................................F-1 G OUTDOOR INSTALLATIONS .............................................................................................. G-1 H COMPRESSED AIR SYSTEM SPECIFICATION ................................................................. H-1 I POP OUT SUMMARY ..............................................................................................................I-1

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LIST OF FIGURES Figure 2-1 Instrument Air System Boundary Diagram ...............................................................2-2 Figure 2-2 Typical Instrument Air Supply System ......................................................................2-3 Figure 2-3 A Typical Instrument Air System Layout for a Single-Unit Station With Service Air Backup..........................................................................................................................2-6 Figure 2-4 A Typical Instrument Air System Layout for a Multi-Unit Station With a Shared Compressor Bank ..............................................................................................................2-7 Figure 2-5 A Typical Compressed Air System Layout Supplying Service Air and Instrument Air.....................................................................................................................2-8 Figure 2-6 A Typical Instrument Air System Layout With Service Air Backup ...........................2-8 Figure 2-7 Moisture Content of Air Versus Dew Point .............................................................2-11 Figure 2-8 Particle Size Comparison .......................................................................................2-13 Figure 2-9 Types and Capacity Ranges of Compressors ........................................................2-17 Figure 2-10 Single-Acting Reciprocating Compressor.............................................................2-19 Figure 2-11 Compression Cycle of an Oil-Injected Rotary Screw Compressor .......................2-20 Figure 2-12 Centrifugal Impeller ..............................................................................................2-21 Figure 2-13 a) Crosshead Assembly for a Non-Lubricated Double-Acting Reciprocating Compressor b) Details of Oil Scraper Rings ....................................................................2-28 Figure 2-14 Cross-Section of an Oil-Injected Rotary Screw Compressor................................2-30 Figure 2-15 Helical, Screw-Type Compressor .........................................................................2-31 Figure 2-16 Rotary, Helical Screw Compressor, Typical Single-Stage Design........................2-32 Figure 2-17 Rotary, Helical Screw Compressor, Typical Axial Arranged Two-Stage Design ..............................................................................................................................2-32 Figure 2-18 Oil-Free Rotary Screw Compressor .....................................................................2-33 Figure 2-19 Water-Injected Rotary Screw Compressor ...........................................................2-34 Figure 2-20 Operation of a Rotary Single-Screw Compressor ................................................2-36 Figure 2-21 Oil-Injected Rotary Sliding-Vane Compressor ......................................................2-37 Figure 2-22 Liquid Ring Rotary Compressor ...........................................................................2-39 Figure 2-23 Cross-Section Diagram of a Double-Acting Liquid Piston Rotary Compressor ....2-40 Figure 2-24 Typical Multi-Stage Centrifugal Air Compressor...................................................2-42 Figure 2-25 Large Centrifugal Compressor Package ..............................................................2-43 Figure 2-26 Characteristic Curves for a Centrifugal Air Compressor at 90ºF (32.2ºC) ............2-44 Figure 2-27 Characteristic Curves for a Centrifugal Air Compressor at 60ºF (15.5ºC) ............2-45 Figure 2-28 Centrifugal Air Compressor—Integrally Geared Multiple Impellers ......................2-46

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Figure 2-29 Pressure Lubrication System for an Air Compressor ...........................................2-48 Figure 2-30 Kaeser Screw Compressor Package....................................................................2-52 Figure 2-31 Ingersoll-Rand Hybrid Permanent Magnet Motor Arrangement ...........................2-53 Figure 2-32 Atmospheric Versus Pressure Dew Point Conversion Chart................................2-58 Figure 2-33 Compressed Air Drying System With a Desiccant Dryer......................................2-60 Figure 2-34 Sullair Regenerative Dryers..................................................................................2-64 Figure 2-35 Heat-of-Compression-Type Dryer ........................................................................2-65 Figure 2-36 Heat Regenerative Desiccant Dryer .....................................................................2-66 Figure 2-37 Heatless Regenerative Desiccant Dryer...............................................................2-67 Figure 2-38 Refrigerant Dryer ..................................................................................................2-69 Figure 2-39 Deliquescent Dryer ...............................................................................................2-70 Figure 2-40 Membrane-Type Dryer .........................................................................................2-71 Figure 2-41 Mechanisms of Air/Gas Filtration..........................................................................2-76 Figure 2-42 Coalescing Filter...................................................................................................2-78 Figure 2-43 Adsorption Filter ...................................................................................................2-79 Figure 2-44 Air-Line Filter ........................................................................................................2-80 Figure 2-45 An Oil/Water Separator ........................................................................................2-84 Figure 2-46 Float-Type Drain Trap ..........................................................................................2-86 Figure 2-47 Solenoid-Operated Drain Valve............................................................................2-87 Figure 2-48 Electronically Controlled Drain Trap .....................................................................2-88 Figure 2-49 Pneumatically Operated Drain Trap .....................................................................2-89 Figure 3-1 Precise Pressure Regulation ....................................................................................3-1 Figure 3-2 Centrifugal Compressor Surge Diagram ..................................................................3-4 Figure 3-3 Five-Step Reciprocating Compressor Control ..........................................................3-6 Figure 5-1 Flexible Coupling for a Compressor .....................................................................5-102 Figure 7-1 Cost of Air Leakage ..................................................................................................7-9

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LIST OF TABLES Table 2-1 Particle Sizes of Various Substances ......................................................................2-12 Table 2-2 Air Quality Standards Per ISO 8573.1 .....................................................................2-14 Table 2-3 Compressor Operating Costs ..................................................................................2-25 Table 2-4 General Facts Regarding Compressors ..................................................................2-25 Table 2-5 Reciprocating Compressor Oil Types and Specifications ........................................2-50 Table 2-6 Moisture Content of Saturated Air in Gallons Per 1000 Standard Cubic Feet .........2-55 Table 2-7 Moisture Content of Saturated Air in Milliliters Per Cubic Meter ..............................2-56 Table 2-8 Pressure and Temperature Correction Factors for Compressed Air Dryers............2-57 Table 2-9 Desiccant Types ......................................................................................................2-62 Table 2-10 Filter Selection Chart .............................................................................................2-75 Table 4-1 Troubleshooting Compressed Air Systems ...............................................................4-3 Table 4-2 Double-Acting Reciprocating Compressor Troubleshooting Guidelines ....................4-4 Table 4-3 Rotary Compressor Troubleshooting Guidelines.....................................................4-12 Table 4-4 Centrifugal Compressor Troubleshooting Guidelines ..............................................4-15 Table 4-5 Regenerative Desiccant Dryer Troubleshooting Guidelines ....................................4-24 Table 4-6 Refrigerant Dryer Troubleshooting Guidelines ........................................................4-26 Table 4-7 Symptom 1: Motor Will Not Start .............................................................................4-29 Table 4-8 Symptom 2: High Vibration ......................................................................................4-31 Table 4-9 Symptom 3: High Stator Temperature—Random or Form-Wound..........................4-33 Table 4-10 Symptom 4: High Bearing Temperature Alarm—Motor With Anti-Friction Bearings ...........................................................................................................................4-35 Table 4-11 Symptom 5: Instantaneous Over-Current Trip .......................................................4-38 Table 4-12 Symptom 6: Running Above Nameplate Amps (Full Load) ...................................4-40 Table 4-13 Symptom 7: The Motor Exhibits Excessive Noise While Running Loaded ............4-42 Table 4-14 Symptom 8: The Motor Line or Phase Currents are Unbalanced ..........................4-45 Table 4-15 Symptom 9: Increased Motor Slip at a Constant Load Point .................................4-47 Table 4-16 Symptom 10: Motor Stalls Under Heavy Load.......................................................4-49 Table 5-1 Reciprocating Air Compressor PM Template ..........................................................5-13 Table 5-2 Failure Locations, Degradation Mechanisms, and PM Strategies for the Reciprocating Air Compressor .........................................................................................5-15 Table 5-3 PM Tasks and Their Degradation Mechanisms for the Reciprocating Compressor......................................................................................................................5-20

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Table 5-4 Rotary Screw Air Compressor PM Template...........................................................5-28 Table 5-5 Failure Locations, Degradation Mechanisms, and PM Strategies for the Rotary Screw Air Compressor .....................................................................................................5-30 Table 5-6 PM Tasks and Their Degradation Mechanisms for the Rotary Screw Compressor......................................................................................................................5-33 Table 5-7 Liquid Ring Rotary Air Compressor and Pump PM Template .................................5-41 Table 5-8 Failure Locations, Degradation Mechanisms, and PM Strategies for the Liquid Ring Rotary Compressor and Pump ................................................................................5-43 Table 5-9 PM Tasks and Their Degradation Mechanisms for the Liquid Ring Rotary Compressor and Pump ....................................................................................................5-50 Table 5-10 Low-Voltage Motor PM Template ..........................................................................5-65 Table 5-11 Failure Locations, Degradation Mechanisms, and PM Strategies for Motors ........5-67 Table 5-12 PM Tasks and Their Degradation Mechanisms for Motors....................................5-87 Table 5-13 Failure Locations, Degradation Mechanisms, and PM Strategies for Air Filters .............................................................................................................................5-103 Table 5-14 PM Tasks and Their Degradation Mechanisms for Air Filters..............................5-104 Table 5-15 Failure Locations, Degradation Mechanisms, and PM Strategies for Coolers.....5-106 Table 5-16 PM Tasks and Their Degradation Mechanisms for Coolers ................................5-107 Table 5-17 Failure Locations, Degradation Mechanisms, and PM Strategies for Traps........5-109 Table 5-18 PM Tasks and Their Degradation Mechanisms for Traps ...................................5-109 Table 5-19 Failure Locations, Degradation Mechanisms, and PM Strategies for Instrumentation ..............................................................................................................5-112 Table 5-20 PM Tasks and Their Degradation Mechanisms for Instrumentation ....................5-114 Table 5-21 Failure Locations, Degradation Mechanisms, and PM Strategies for Safety Valves ............................................................................................................................5-117 Table 5-22 PM Tasks and Their Degradation Mechanisms for Safety Valves.......................5-117 Table 5-23 Failure Locations, Degradation Mechanisms, and PM Strategies for End-Use Devices ..........................................................................................................................5-119 Table 5-24 PM Tasks and Their Degradation Mechanisms for End-Use Devices .................5-120 Table 6-1 Corrosive Gases and Affected Materials ...................................................................6-7 Table 7-1 Owner/Operator Safety Responsibilities....................................................................7-1 Table 7-2 Compressed Air Safety Checklist ..............................................................................7-3 Table 7-3 Estimated Leakage Costs..........................................................................................7-8 Table 7-4 Assembling a Solder Joint .......................................................................................7-14 Table 7-5 General Piping Guidelines .......................................................................................7-16 Table A-1 Preventive Maintenance Practices for Rotary Screw Compressors ......................... A-2 Table A-2 Preventive Maintenance Practices for Reciprocating Compressors ........................ A-3 Table A-3 Preventive Maintenance Practices for Centrifugal Compressors ............................. A-4 Table A-4 Preventive Maintenance Practices for Heatless Regenerative Air Dryers ............... A-5 Table A-5 Preventive Maintenance Practices for Closed Loop Heat Regenerated Air Dryers................................................................................................................................ A-6

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Table A-6 Preventive Maintenance Practices for Open Loop Heat Regenerated Air Dryers................................................................................................................................ A-7 Table A-7 Preventive Maintenance Practices for Refrigeration Air Dryers ............................... A-8 Table A-8 Kaeser Maintenance Checklist............................................................................... A-11 Table A-9 Preventive Maintenance for Centrifugal Compressors ........................................... A-14 Table A-10 Major Maintenance for Centrifugal Compressors ................................................. A-16 Table C-1 Preventive Maintenance Activities for Instrument Air System .................................. C-3 Table C-2 Maintenance Activities for Different Compressor Types .......................................... C-9

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1

INTRODUCTION

1.1

Background

The compressed air system might have different names in different power plants, including the control air system, plant air system, instrument air system (IAS), service air system, or the station air system. The service air system provides compressed air that is mainly used for the operation of pneumatic tools, such as air drills, grinders, sand blasters, paint sprayers, and tools for general cleaning. Service air can also be used as the medium in sootblowing operations and bottom ash conveyance. Because there are no national standards for service air, the quality of service air is not specified; however, it is desirable to have clean, dry air to prevent wear on tools from lubricant carryover, condensate, or other contaminants. The instrument air system provides compressed air that is used for the motive power of valves, instruments, and control functions required for plant operation and shutdown. The standard for instrument air is the American National Standards Institute/Instrumentation, Systems, and Automation Society of America (ANSI/ISA) 7.0.01-1996 Quality Standard for Instrument Air. For this guide, the compressed air system represents both the service air system and the instrument air system. The instrument air system requires high-quality compressed air that is free of contaminants. The loss of instrument air can cause the closure of air-operated valves, thus resulting in the malfunction or degradation of systems (which causes power reductions and/or unit trips). For these reasons, this guide’s content will focus on the instrument air part of the compressed air system. Although this guide is intended to cover the compressed air system for fossil generating plants, the instrument air system has been the cause of many problems in nuclear plants. In 1988, the Nuclear Regulatory Commission (NRC) issued Generic Letter 88-14, which requires nuclear power plants to evaluate their instrument air systems, including the quality of the air going to pneumatic devices. In response, EPRI published several excellent guides addressing nuclear systems. These guides were researched and used in the development of this guide for fossil plants. A partial listing of the EPRI guides that were used include: •

Instrument Air Systems: A Guide for Power Plant Maintenance Personnel. NP-7079, 1990.

•

Report of the Instrument Air Working Group. TR-103595, 1994.

•

Compressor and Instrument Air System Maintenance Guide: Revision to NP-7079. TR-108147, 1998.

1-1

EPRI Licensed Material Introduction

1.2

Approach

In the 2001 EPRI Fossil Maintenance Applications Center (FMAC) Maintenance Issues Survey, the compressed air system was identified as a system that was a chronic problem, a concern for unit reliability, and had a high impact at the station. A telephone survey to solicit problems and desired information was conducted with FMAC-member plants in February 2002. Based on survey responses, an outline was written for a maintenance guide on the compressed air system. The objectives for maintenance of the compressed air system are: •

To ensure that the system functions as required by maintaining high system reliability and availability

•

To minimize air system leakage

This guide is intended to provide information on maintenance of the compressors, motors, air dryers, air filters, inter-coolers, after-coolers, separators, drain traps, receiver tanks, air system piping, and instrumentation. It is not the intent of the guide to provide information related to the structural support of the air tubing and piping, control and power cables for the instrumentation, instrument valves, tubing, fittings, end-use devices of air-operated valves and solenoid valves, and electrical connections (breakers, relays, motor control centers).

1.3

Organization of the Guide

The Compressed Air System Maintenance Guide is organized into the following sections: 1. Introduction – includes the background, approach, and organization of the guide, and an explanation of pop-outs 2. Tutorial – includes information related to the general operation, system layout, air quality guidance and standards, and major components of instrument air system functions 3. Controls – includes control methods 4. Troubleshooting – includes troubleshooting information for the general system, doubleacting reciprocating compressor, rotary compressor, centrifugal compressor, regenerative desiccant dryer, refrigerant dryer, and motor 5. Preventive Maintenance (PM) – includes PM information for air compressors, motors, air dryers, air filters, inter- and after-coolers, separators and traps, receiver tanks, piping, instrumentation, safety relief valves, and end-use devices 6. Condition-Based Monitoring – includes predictive maintenance and air quality monitoring 7. Corrective Maintenance – includes safety issues, component failures, and air leakage 8. Industry Information – includes books and reports, brochures, periodicals, software, videos, workshops, and training classes 1-2


9. References and Glossary 10. Appendices, including: •

Appendix A – PM Practices

•

Appendix B – Maintenance Philosophy and Recommendations

•

Appendix C – Life Cycle Management Preventive Maintenance Activities

•

Appendix D – Overhauling a Rotary Screw Compressor

•

Appendix E – Rental Compressors

•

Appendix F – Condensate Disposal

•

Appendix G – Outdoor Installations

•

Appendix H – Compressed Air System Specification

•

Appendix I – Pop Out Summary

Because many sources of information were used in the compilation of this guide, it was decided that a reference system would be used in applicable sections. The use of reference numbers in brackets, [#], is used at the beginning of sections and after the captions of tables and figures to denote where the majority of information in that section/table/figure was obtained. The numbers and corresponding references are listed in Section 9 of the guide.

1.4

Pop Outs

Throughout this guide, key information is summarized in Pop Outs. Pop Outs are bold-lettered boxes that highlight information covered in the text. The primary intent of a Pop Out is to emphasize information that will allow individuals to act for the benefit of their plant. EPRI personnel who reviewed and prepared this guide selected the information included in these Pop Outs. The Pop Outs are organized into three categories: Technical, O&M Costs, and Human Performance. Each category has an identifying icon to draw attention to it when quickly reviewing the guide. The Pop Outs appear as follows: Key Technical Point Targets information that will lead to improved equipment reliability.

1-3


Key O&M Cost Point Emphasizes information that will result in overall reduced costs and/or increase in revenue through additional or restored energy production. Key Human Performance Point Denotes information that requires personnel action or consideration to prevent personal injury, equipment damage, and/or to improve the efficiency and effectiveness of the task. The Pop Out Summary section (Appendix I) of this guide contains a listing of all key points in each category. The listing restates each key point and provides a reference to its location in the body of the report. By reviewing this listing, users of the guide can determine if they have taken advantage of key information that would benefit their plants.

1-4


2

TUTORIAL

The compressed air system in fossil power plants can be considered as the fourth utility behind power, water, and gas resources. Compressed air is needed to operate air tools in the maintenance shop and in the plant, and for pneumatic controls of air-operated valves, airoperated solenoid valves, boiler sootblowing lances, sandblasting, paint spraying, and so on. Today, utilities have the option of purchasing compressed air rather than generating it with their own equipment. Companies will install, operate, and maintain a compressed air system on-site to provide required compressed air. In this way, the plant only pays for the flow rate of the compressed air supplied. The plant does not have to purchase, install, operate, or maintain the equipment. The plant is, however, typically responsible for the piping and distribution system. For the purpose of this guide, the boundaries of the compressed air system are the same as for the IAS and are listed below: •

Motor and air dryer electrical connections

•

Air intake

•

Cooling water to the compressor and inter-cooler

•

Cooling water to the after-cooler

•

Drains from the air receiver tanks, pre-filters, and air dryers

•

Piping distribution system

•

End-use devices

2-1

EPRI Licensed Material Tutorial

Figure 2-1 shows the instrument air system boundary diagram.

Figure 2-1 Instrument Air System Boundary Diagram [1]

This section covers the following areas related to the compressed air system: •

General operation

•

System layout

•

Air quality guidance and standards

•

Components and functions

2.1

General Operation

The requirements of the compressed air system are expressed in pressure and flow rate. The system pressure is expressed in units of lbs/in2 (kPa). The volume delivery of airflow or flow rate can be expressed in: •

Actual cubic feet per minute (acfm) (m3/min) – The flow rate of air measured at a reference point and based on actual conditions at that reference point.

•

Inlet cubic feet per minute (icfm) (m3/min) – The flow rate of air measured at the compressor inlet filter or inlet valve at rated conditions.

•

Standard cubic feet per minute (scfm) (m3/min) – The flow rate of air measured at a reference point and converted to a standard set of reference conditions. Typical reference conditions are 14.7 psia (101 kPa) pressure, 60ºF (15.5ºC) temperature, and 0% relative humidity.

2-2


A typical compressed air system is shown in Figure 2-2, and a discussion of the general operation of the system [2] follows the figure.

Figure 2-2 Typical Instrument Air Supply System [2]

The compressor draws atmospheric air through an intake filer. The filter removes dust and other airborne particulate prior to compression. As air enters the compressor and is compressed, the temperature of the air rises. For example, the temperature of air compressed from atmospheric pressure to 100 psig (689 kPa) could be over 450ºF (232ºC). The hot air passes through an intercooler (inter-stage cooler) on multi-stage compressors. This reduces the temperature and volume to be compressed in the succeeding stages, liquefies condensable vapors, and saves power. The compressed air then enters the after-cooler where further cooling and condensation take place. In systems with single-stage compressors, the air passes through the after-cooler from the outlet of the compressor. The inter-cooler and after-cooler can be either air- or water-cooled. Cooling water for watercooled compressors can be supplied from the filtered water or cooling water system. In some systems, air from the after-cooler enters a moisture separator for final water removal, thus protecting the receiver from moisture accumulation. The compressed air temperature at the 2-3


outlet of the after-cooler might still be above the plant ambient temperature, in which case further cooling and condensation occur in the air receiver. Plants without a moisture separator usually provide drain traps and receiver blowdown valves. Finally, the compressed air enters the receiver, which acts as a storage tank and pressure surge buffer for the distribution system. In a basic system, the pre-filter removes the particulate from the air receiver and piping. Any regenerative desiccant dryer should have a good coalescing pre-filter. Inlet pre-filtration enhances dew point depression capability and prolongs desiccant life. The pre-filter should remove all entrained liquids (both water and compressor lubricant) before they enter the dryer and foul the desiccant. Filters between 5 and 50 microns have proven effective. Micron size (porosity) of the pre-filter is not critical as long as it removes all liquids. From the pre-filter, air enters the dryer unit. Air dryers vary significantly depending on a particular plant design, but normally consist of one of the following types: •

Desiccant, heat regenerative

•

Desiccant, heatless regenerative

•

Refrigeration

Regenerative dryers usually use silica gel, activated alumina, or a combination of the two as desiccant. They typically bring the dew point down to at least -40ºF (-40ºC) at 100-psig (689kPa) pressure. Refrigerant dryers can reduce the dew point to only about +35ºF (1.7ºC) at 100psig (689-kPa) pressure. In a desiccant dryer, when the air stream passes through the desiccant, the moisture is adsorbed onto it. Effectiveness of the dryer will depend on the inlet air temperature. Saturated air at 100ºF (38ºC) has 0.3956 gallons/1000 ft3 (53 milliliters/m3) of water. This air contains almost twice as much water as saturated air at 80ºF (26.7ºC), which has 0.2046 gallons/1000 ft3 (27 milliliters/m3). Also, the adsorptive capacity of the desiccant starts to decline above 100ºF (38ºC). The dryer should be sized based on the air flow rate, so that sufficient time is allowed for moisture to be adsorbed onto the desiccant. Air might not fully dry during periods of surges. In a dual-tower system, before the moisture holding capacity of the desiccant in the operating tower is exhausted, a timer activates the air-operated control valves and shifts the air stream to the standby desiccant tower. The desiccant in the exhausted tower is then regenerated by one of the methods described below: •

In a heat regenerative dryer, heaters provide the heat to dry the desiccant and then a small percentage of the dried air purges the moisture from the dryer.

•

In a heatless regenerative dryer, dried air flows backward through the desiccant at reduced pressure. This purged air dries the moisture from the desiccant to regenerate the dryer tower.

In desiccant dryers, the operation of the inlet switching valve should be checked periodically. Improper maintenance of this valve might result in two problems; the air might not switch from one tower to the other, or the valve might stick in mid-position. Depending on the manufacturer, 2-4


the dryer might vent to atmosphere or it might block flow. Either way, the instrument air pressure to the end-use components might decrease. Refrigeration dryers operate in the same manner as a home air conditioner. The air is cooled to allow the moisture to condense. The moisture is then drained off through a trap or moisture separator. The after-filter provides the final conditioning of the air after leaving the dryer. Many systems use filter sizes of 10 microns or greater. Coalescing filters can also be used as after-filters; however, the added expense is unnecessary if the air system has moisture indicators or a highmoisture alarm. Appendix G contains information for outdoor installations of the compressed air system. Compressed air is also used by fossil plants in the sootblowing process. The purpose of the sootblowing system is to clean fireside deposits on heat transfer surfaces, such as boiler tubes, heaters, economizers, and air heaters. The cleaning is performed while the equipment is on-line and, thus, maintains proper heat transfer. The soot produced during the combustion process adheres to the exterior of the furnace tube bank and and to the internal surfaces of the boiler furnace. This soot tends to blanket these surfaces and insulate them. This blanketing impedes or lowers the heat transfer rate. The process of blowing the soot off the boiler’s surfaces is important for maintaining the proper balance of heat transfer in the boiler and for matching the steam rate with the superheat and reheat capability for any load. Sootblowing is accomplished by injecting a high-velocity media (air, steam, or water) onto the boiler tubes, heaters, economizers, and/or air heaters through a blower. Air is less damaging than steam or water to the boiler tubes and the blowers. Compressed air is less effective than steam or water in removing hard deposits, however. Off-line cleaning is possible with air-blowing sootblowers. Where air is used as the cleaning medium for sootblowing operations, a separate compressed air system is needed. The air requirements for the sootblowing system are in the range of 2,000 to 7,000 scfm (57 to 198 m3/min) of air at 325 to 400 psig (2.2 to 2.8 MPa) for each sootblower. For a complete sootblowing cycle, the air requirement can be 220,000 scfm (6230 m3/min). The motor horsepower rating for each compressor of the system is in the 4000 to 5000 hp range. The motor voltage can be 4160 or 7200 volts. Compressed air can be supplied to the sootblowing system from storage in receiver tanks or by direct supply from large, centrifugal compressors. The compressed air system is typically located below the boiler grade level in the plant. The sootblowing compressed air system typically consists of inlet guide vanes, inlet filters, multi-stage centrifugal compressors, compressor controls, motors, inter-coolers, after-coolers, traps, piping, and receiver tanks. This system does not contain dryers, moisture separators, or other filters.

2-5


2.2

System Layout

There are four typical instrument air system layouts [3] shown in Figures 2-3 through 2-6 of this section. The layouts show different series and parallel paths within the instrument air system, and differing placements of dryers, filters, and receivers.

Figure 2-3 A Typical Instrument Air System Layout for a Single-Unit Station With Service Air Backup [3]

The system layout in Figure 2-3 shows the dryers with pre-filters located before the air receivers. Pre-filters are necessary to prevent fouling of the dryer desiccant bed from lubricant carryover from the compressor. After-filters prevent desiccant fines from being carried downstream into instruments and controls. The receiver tanks located after the dryers ensure that only the maximum air capacity of the compressors can pass through the dryers, even in the event of a sudden demand for compressed air that exceeds the rated capacity of the dryer. The extra capacity air is drawn from the receivers, which have clean, dried air stored in them. Duplicate pre-filters, dryers, and receivers allow redundancy for maintenance. Some power plants use the service air compressors to provide backup service for the instrument air system in the event of a loss of pressure in the instrument’s air lines. This layout shows two possible modes of hookup for service air backup compressors. Mode 1, which connects the service air compressors before the pre-filter, is the preferred mode because it treats the air through filters and dryers before entering the system piping. This protects the system piping and 2-6


end-user equipment from moisture and other contaminants usually found in the service air. The Mode 2 option, which connects the service air compressors after the receivers, is not recommended because it injects untreated moist air directly into the system piping.

Figure 2-4 A Typical Instrument Air System Layout for a Multi-Unit Station With a Shared Compressor Bank [3]

The layout shown in Figure 2-4 is sometimes used in multi-unit power stations. A single bank of compressors is used to supply instrument air to all units at the site. This layout has definite economic advantages in terms of the initial investment and operating cost. This layout also has the air receivers located throughout the system. While this has the advantage of storage volume close to points of use that have varying demands and cycle time, the compressor controls might experience more fluctuation of pressure than with a receiver placed closer to the compressor, as in Figure 2-3. Therefore, the compressor might experience more starts and stops, thus increasing compressor wear and maintenance. The backup compressor (with dryer) in this figure is shown with a closed manual valve. This indicates that this service is designed only for use in an emergency, when an operator will open the valve to allow the backup compressors to pressurize the instrument air system. Because the backup compressor unit is provided with its own dryer, system contamination is not a consideration during its use.

2-7


Figure 2-5 A Typical Compressed Air System Layout Supplying Service Air and Instrument Air [3]

Figure 2-6 A Typical Instrument Air System Layout With Service Air Backup [3]

2-8


The layouts shown in Figures 2-5 and 2-6 generally are better suited to reciprocating-type compressors, where the air receivers provide a buffer for discharge pressure pulsations. Receivers before the dryers also provide radiant cooling and removal of condensate before the air reaches the dryers. These systems also have desiccant dryers for only the instrument air. Service air can be supplied after the receivers. While service air does not need to be of the same quality as instrument air, it is recommended that a dryer, such as a refrigerant-type dryer, be installed for service air to protect the system piping from ingress of moisture. Untreated service air should not be introduced into an instrument air system. If service air is to be used as a backup for instrument air, then appropriate drying and filtration should be provided. In summary, a comparison of Figures 2-3 through 2-6, the instrument air system boundary diagram from Figure 2-1 and the typical compressed air supply system from Figure 2-2, shows that the relative position of the air receiver and dryers can change. Numerous alternative redundant flow paths through the instrument air system can be created to achieve flexibility in operations and more reliable operating life of the equipment.

2.3

Air Quality Guidance and Standards

Dirt, water vapor, and other impurities enter the compressor with the atmospheric air. For fossil power plants, flyash, coal dust, and chemical fumes can enter the compressor inlet. During the compression process, oil liquid and oil vapor can also be introduced. If impurities are not removed, they travel downstream and cause corrosion, scaling, pressure loss, contamination, and freezing in the piping and the end-use equipment. The quality of air required for operation [3,4,5,6] is dictated by the end-use components. There are generally six levels of compressed air quality. They are: Level

End-Use

1.

Machine shop general requirements

2.

Air tools, sand blasting, and pneumatic control systems

3.

Instrument air, paint spraying, powder coating, and packing machines

4.

Food industry, chemical, and pharmaceutical industry laboratories

5.

Outdoor pipelines, pneumatic transport of hygroscopic (retains moisture) material, breweries, dairy industry

6.

Breathing air

2-9


The first three levels of air quality are used in fossil power plants. Some fossil plants use Level 6-quality air for personnel who are breathing in sand blast hoods and in asbestos abatement tents. Some plants fill air pack bottles or tanks for breathing air. The breathing air is usually supplied by a smaller, oil-free compressor with appropriate filters. Key O&M Cost Point The higher the levels of air quality, the more the costs are to produce that level. Higher quality air usually requires additional equipment. This increases the initial capital investment and operating and maintenance costs. It is important to meet but not exceed the required air quality levels with the compressed air system. Air quality is a total system issue and is affected by: •

Air quality entering the compressor

•

Type of compressor and lubricant or injected fluid, and by wear particles

•

Condensate and method of its removal

•

Type and condition of distribution piping, receivers, and valves

•

Type and condition of air dryers

•

Type and maintenance of filters

•

Plant maintenance procedures

Air samples can be analyzed to determine the air quality with regard to moisture, oil, and other foreign particles. Analysis of particles from downstream filters can also be used to determine the nature and source of the particles. Microscopic comparison can be made with particles displayed on wall charts. Fourier-transform infrared (FTIR) analysis can be used to compare with the spectra of bonding found in typical materials. Pneumatic tools should be supplied with the service air requirements (Level 2 quality air). To supply the pneumatic tools with instrument air (Level 3 quality air) is permissible, but would cost more to operate. For the purposes of a fossil plant compressed air system (and this guide), the requirements for the third level of air quality (instrument air, paint spraying, powder coating, and packing machines) are addressed. The standard established by the Instrument Society of America for air quality use in pneumatic instruments is the [American National Standards Institute/Instrumentation, Systems, and Automation Society (ANSI/ISA)] 7.0.01-1996 edition. Another standard that is used is the International Standards Organization (ISO) 8573.1.

2-10


This ANSI/ISA standard establishes four elements of instrument air quality for use in pneumatic instruments. They are: •

Pressure Dew Point – The pressure dew point as measured at the dryer outlet shall be at least 10°C (18°F) below the minimum temperature to which any part of the instrument air system is exposed. The pressure dew point shall not exceed 4°C (39°F) at line pressure. A monitored alarm is preferred; however, if a monitored alarm is unavailable, per-shift monitoring is recommended. (These criteria are met by the ISO 8573.1 Class 4 standard and, for low ambient temperatures, by Class 2.) Comments: It appears that the upper limit of 39°F (4°C) is based on the performance limitations of refrigerant dryers. Dew point is determined by the quantity of moisture in the compressed air at line pressure. Values of dew point are referenced in terms of the condensation temperature at line pressure. Typically, dew point is measured at the outlet of the dryer. Dew point criteria are used to help prevent condensation in the instrument air system. Condensation can cause blocking, air system corrosion, and failure of pneumatic devices. Therefore, it is important to meet the ANSI/ISA-7.0.01-1996 dew point requirements. Figure 2-7 shows the amount of moisture at specified dew points.

Figure 2-7 Moisture Content of Air Versus Dew Point [6]

2-11


•

Particle Size – A maximum 40-micron (0.00157-inch) particle size in the instrument air system is acceptable for the majority of pneumatic devices. Pneumatic devices that require instrument air with less than 40-micron particle sizes shall have additional filtration to meet the particulate size limit for the device. (This is also the criterion met by ISO 8573.1 Class 5.) Comments: When considering maximum particle size criteria, it is helpful to have a perspective on the particle sizes of various substances. Table 2-1 indicates the relative sizes for a spectrum of substances. The smallest particle visible to the naked eye is approximately 40 microns (0.00157 inch). Table 2-1 Particle Sizes of Various Substances [6] Particle Sizes (microns)

Substances

100

Sand, table salt

85

Industrial haze and mist

75

Human hair diameter

60

Flour

40

Smallest particle visible to the naked eye

25

White blood cells

20

Ragweed pollen

10

Talcum powder

5 to 100

Metal filings

1 to 100

Buffing compounds

1 to 30

Airborne dust, dirt, rust

3

ISA S7.3-1975 particle standard

2

Bacteria level

0.5 to 2 0.2

Tobacco smoke Smallest bacteria

Forty microns appears to be acceptable relative to most pneumatic end-user device clearances. For example, the very smallest orifice that exists in a pneumatic device is approximately 1/64 inch (397 microns) in diameter. A 40-micron diameter particle occupies only 1% of the given area of a 1/64-inch (397-micron) diameter opening. Seventy particles having 40-micron diameters could pass through a 1/64-inch (397-micron) diameter opening. See Figure 2-8 for a graphical illustration.

2-12


Figure 2-8 Particle Size Comparison [6]

2-13


•

Lubrication Content – The lubrication content should be as close to zero as possible, and under no circumstances should it exceed 1 ppm water or vapor. (This is met by the ISO 8573.1 Class 3 criteria.) Any lubricant in the compressed air system shall be evaluated for compatibility with end-use pneumatic devices. For example, the use of automatic oilers is strongly discouraged. Comments: One of the main factors in determining air quality is whether or not lubricantfree air is required. Lubricant-free air can be produced with either lubricant-free compressors or with lubricant-injected compressors that have additional separation and filtration equipment.

•

Contaminants – Instrument air shall be free of corrosive contaminants and hazardous gases, which could be drawn into the instrument air supply. The air system intake should be monitored for contaminants. If contamination exists in the compressor intake area, the intake should be moved to a different elevation or location, free from contamination. Comments: As stated earlier, fossil power plants have many airborne contaminants that can enter the air compressors. The standard suggests moving the intake area to an area that is free of contamination. Although desirable, this might not be an option in fossil plants.

The air quality classes established by the International Standard ISO 8573-1 are shown in Table 2-2. Table 2-2 Air Quality Standards Per ISO 8573.1 [3] Classa

Maximum Particle Sizeb (microns)

Maximum Particle Concentrationc mg/cu. m

oz/cu. ft.

Maximum Pressure Dew Point °C

°F

Maximum Oil Concentrationd mg/cu. me

oz/cu. ft.

1

0.1

0.1

0.1

-70

-94

0.01

0.01

2

1

1

1

-40

-40

0.1

0.1

3

5

5

5

-20

-4

1.0

1.0

4

15

8

8

+3

+37.4

5.0

5.0

5

40

10

10

+7

+44.6

25.0

25.0

6

-

-

-

+10

+50

-

-

7

-

-

-

not specified

not specified

-

-

a

Number of air classes for each contaminant is not the same due to widely varied requirements of compressed air applications.

b

Particle size based on a filtration ratio βµ=20. The Beta Ratio is the ratio of the number of particles larger than a given micron size upstream of the filter to the number of particles larger than the given size downstream of the filter. The higher the Beta Ratio, the more particles are retained by the filter and, hence, the higher the efficiency.

c

At 100 kPa (14.5 psia), 20°C (68°F), and a relative humidity of 60%.

d

Oil concentration at 100 kPa (14.5 psia), 20°C (68°F), and a relative humidity of 60%.

e

1.0 mg by weight of oil in 1.0 cu. m volume of air is approximately equal to 0.83 pm by weight.

2-14


2.4

Components and Functions

Major components considered critical for instrument air system functions [1,7] are: •

Air Compressors – Compressors take a volume of air and compress it into a smaller volume, thereby increasing the air’s pressure. The most common air compressor types are reciprocating, centrifugal, and rotary screw. Over 90% of air compressors in use today are supplied by the following manufacturers: Atlas Copco, Cooper Turbocompressor, Gardner Denver, Ingersoll Rand, Joy, Sullair, or Worthington. Lubrication of compressors can be of the external forced, internal forced or gravity drip, or bath designs.

•

Motors – The motors that drive the compressors are typically supplied as a package with the air compressors. These motors are usually alternating current induction or synchronous, continuously wound motors.

•

Air Dryers – High relative humidity is one of the effects of compressing air. This can lead to system-wide problems from corrosion and contamination. An air dryer removes water and oil vapor from compressed air, thus producing air suitable for plant use. The three types of air dryers are heated, heatless desiccant, and refrigerated air dryers. The major manufacturers are Kahn, Pall, Kemp, and Zek.

•

Air Filters – The three types of air filters are particulate, coalescent, and absorbent air filters. The major manufacturers are Deltec, Dollinger, Henderson, and Pall. An inlet filter removes particles from the air entering the compressor. Particulate matter, if not filtered, can cause compressor damage and increased maintenance costs due to contaminant build up. Much like inlet filters, air line filters remove solids and liquids from the compressed air stream. These filters can be placed throughout the system. If the filters are after the compressor, they can extract oil from compression. If the air filters are before the air tools, they can capture potentially damaging particles generated in the distribution system. Air filters can be placed at any location where particulate must be removed.

•

Inter-Coolers and After-Coolers – The process of compression leaves air both hot and wet. After-Coolers are heat exchangers that lower the temperature of the air leaving the compressor and remove the water that condenses as the air cools. The heat exchanger can be a closed or open loop design with fresh or salt water supplied as the cooling medium. The major manufacturers are Adams, Joy, and Ingersoll Rand. Inter-coolers are heat exchangers supplied with multi-stage compressors that cool the air between the compression cycles.

•

Separators and Blowdown Devices/Traps – Separators are designed to remove liquids, including oil, from the air. They can be located throughout the compressed air system, wherever there is a need to reduce the air’s moisture content. Condensate traps are used to collect and discharge these liquids to a drain. They frequently are integral components of other compressor equipment, such as after-coolers, dryers, and separators.

•

Receiver Tanks – A receiver is a holding tank that stores a large reserve of compressed air that helps maintain a smooth airflow to the end-use devices.

•

Distribution Piping – The distribution piping moves the compressed air from the receiver tanks to the end-use devices. Air moves from a main header to branch lines and subheaders, and finally to drop points connected to individual air tools or sootblowing lances for service 2-15


air systems. It also moves to instruments, solenoid-operated valves, and air-operated valves for instrument air systems. Air system piping materials can be copper, brass, stainless, galvanized, and carbon steel. •

Instrumentation – These are instruments used to monitor the air quality parameters of hydrocarbons, particulates, contaminate, and dew point.

•

Safety Relief Valves – Safety relief valves are needed to protect the system from overpressurization.

•

End-Use Devices – The air system requirements of the end-use devices should be considered in the compressed air system design.

•

Compressor Controls – A compressor controller directs the compressor’s output. These can be start/stop, constant speed control, dual control, or multiple compressor controls. Advanced controls can offer many features beyond capacity control, including machine protection and information management.

The main components of the compressed air system are covered in more detail in the following sections. The compressor controls are covered in Section 3. 2.4.1 Compressors Compressing air is the process of taking a given mass of air that occupies a given volume of space and squeezing it into a smaller space. In a given volume of space, more air mass will produce more pressure. The pressure, in essence, comes from this air attempting to return to its original volume. Compressors [2,3,7,8] come in a broad range of types and sizes with a range of different performance characteristics. Compressors are designed to produce a given amount of air flow measured in standard cubic feet per minute (scfm) (m3/min) up to a maximum air pressure measured in pounds per square inch (psi) (kPa). For the service air system, air tools are designed to operate most effectively with a specific amount of air at a specific air pressure. Air tool performance degrades at lower pressures, while air tool wear increases at higher pressures. When air demand exceeds air supply, then pressure levels decrease and tools no longer work effectively. Therefore, a compressed air system must produce and deliver enough air to supply all of the air tools in a plant at their required pressure levels. Most compressors for high-pressure applications have multiple stages, with each stage increasing the air pressure. A two-stage compressor is more efficient than a single-stage compressor. This gain in efficiency comes from the addition of an inter-cooler between the stages that cools the air, thus reducing its volume. With a lower volume of air to compress, the second stage can produce higher flow rates efficiently. There are various mechanisms used in the compression of air, resulting in a variety of compressor types and sizes. Compressor types in common use, along with their ranges of 2-16


horsepower (hp), size in cubic feet per minute (cfm) (m3/min), and pressure (psi) (kPa, MPa) capability, are listed in Figure 2-9. Only those stationary types and sizes common to the instrument air systems are discussed in the text. Normally, compressors operate in the 100–125 psig (689–125 kPa) range.

Figure 2-9 Types and Capacity Ranges of Compressors [3]

2-17


There are two different physical means used to compress air, positive displacement and dynamic displacement. Positive Displacement – The positive displacement type of compressor uses a squeezing effect to force gas molecules into a smaller space. Compression is carried out by reducing the volume that the air occupies. Positive displacement compressors have nearly constant efficiency at different operating speeds. Positive displacement types used frequently in industrial air systems include reciprocating and rotary screw compressors. These compressors can be lubricated or oil-free. Lubricated compressors leave oil vapor in the air but are more efficient than their oil-free counterparts. If the plant requires oil-free air, then more extensive filtration is required. A reciprocating compressor uses pistons within a cylinder to compress and displace the air. This is accomplished by taking successive volumes of air and elevating this air to a higher pressure. The reciprocating air compressor can be single- or double-acting. For single-acting, the pistons compress air in one direction of movement, and for double-acting, the pistons compress in both directions of movement. The double-acting compressors are more efficient than the single-acting designs, producing nearly double the air in the same range of motion. Figure 2-10 shows a single-acting reciprocating compressor.

2-18


Figure 2-10 Single-Acting Reciprocating Compressor (Courtesy of Coltec Industries, Inc., Quincy Compressor Division)

The reciprocating air compressor uses a number of automatic spring-loaded valves in each cylinder that open only when the proper differential pressure exists across the valve. Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure. Most plant reciprocating air compressors over 100 hp (75 kw) are built as multi-stage units with two or more steps of compression grouped in series. The air is normally cooled between the stages to reduce the temperature and volume entering the following stage.

2-19


Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations. They can be packaged and provide a wide range of pressure and capacity selections. Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single-stage helical or spiral lobe, oil-flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air-cooled or water-cooled oil coolers) where the oil seals the internal clearances. Figure 2-11 shows the compression cycle of a rotary screw compressor.

Figure 2-11 Compression Cycle of an Oil-Injected Rotary Screw Compressor (Courtesy of Sullair Corporation)

Because the cooling takes place inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, aircooled, or water-cooled compressor package. Because of the simple design and few wearing parts, rotary screw air compressors are easy to maintain and operate, and provide installation flexibility. Rotary air compressors can be installed on any surface that will support static weight. The two-stage, oil-flooded rotary screw air compressor uses pairs of rotors in a combined air end assembly. Compression is shared between the first and second stages flowing in series. This increases the overall compression efficiency up to 15% of the total full load consumption. The two-stage rotary air compressor combines the simplicity and flexibility of a rotary screw compressor with the energy efficiency of a two-stage double-acting reciprocating air compressor. Two-stage rotary screw air compressors are available air-cooled and water-cooled, and with full packages.

2-20


The oil-free rotary screw air compressor uses specially designed air ends to compress air without oil in the compression chamber, yielding true oil-free air. Oil-free rotary screw air compressors are available air-cooled and water-cooled and provide the same flexibility as oil-flooded rotaries when oil-free air is required. The rotary scroll compressor is used in the lower end of the horsepower range of rotary air compressors. The operating compression principle is accomplished by means of two intermeshing spirals or scrolls. One scroll is stationary and the other orbits in relation to the stationary scroll. Air enters through the suction port of the stationary scroll and fills the suction chamber. As the orbiting scroll rotates, the space occupied by the air becomes progressively reduced and moves toward the discharge port in the center of the stationary scroll. Because the application of this compressor is in the lower horsepower ranges (<10 hp), it is mentioned in this section but will not be covered in this guide. Rotary screw air compressors are available as air-cooled or water-cooled, oil-flooded or oil-free, and single-stage or two-stage. There is a wide range of availability in configuration and in pressure and capacity. Dynamic Displacement – The dynamic compressor increases the air’s velocity and then converts this kinetic energy to potential energy in the form of pressure. Unlike positive displacement compressors, dynamic-type compressors produce an air mass flow that varies with inlet and discharge pressures. The higher the discharge pressure, the lower the mass flow produced. Centrifugal is the most common dynamic air compressor used in power plant applications. Axial flow compressors are another dynamic type that produce very large air flows, but are not used in power plants. Figure 2-12 shows a centrifugal compressor impeller.

Figure 2-12 Centrifugal Impeller [7]

2-21


In a centrifugal compressor, high-speed rotating impellers impart a high velocity to the air. This kinetic energy is transferred to pressure as the air is slowed and massed in diffusers surrounding the impellers. Centrifugal compressors are capable of producing very large volumes of oil-free air over 30,000 scfm (14.2 cubic meters/sec). Generally, three or four impeller stages are used to produce 100 psi (689 kPa) or higher of air pressure. The centrifugal air compressor is an oil-free compressor by design. The oil-lubricated running gear is separated from the air by shaft seals and atmospheric vents. The centrifugal air compressor is a continuous-duty compressor with few moving parts. This design is particularly suited to high-volume applications and especially where oil-free air is required. Centrifugal air compressors are water-cooled and can be packaged. Typically, the package includes the after-cooler and all controls. There are eight key factors to consider when evaluating the cost of a compressor: initial cost, operating efficiency, maintenance, durability, pressure output, heat recovery, lubrication, and demand. Following is a description of each of these factors: Initial Cost – Reciprocating compressors are generally the most expensive compressors to purchase and install. They require more extensive foundations than rotary-type compressors because of their large physical size and the large vibrations that they create. Reciprocating compressors also produce pulsating air that requires dampening in the system, usually provided by a receiver. Centrifugal compressors have lower installation costs but are expensive because they are high-precision machines. Rotary screw compressors are typically designed as all-in-one packages. Key O&M Cost Point Rotary screw compressors are the lowest cost compressors to purchase and install. Operating Efficiency – A standard measure of compressor efficiency is the horsepower required to produce 100 scfm (28.3 m3/min). All compressors are designed to be most efficient at their full load output, when they typically will use between 19–25 hp (19.2–25.3 metric hp) per 100 scfm (28.3 m3/min) at 100 psia (689 kPa), depending on the design. At full load, reciprocating compressors are the most efficient type of compressor (although modern centrifugals can be nearly as efficient). Key O&M Cost Point Rotary screw compressors are usually less efficient than reciprocating or centrifugal compressors.

2-22


In many plants, compressors do not always operate at maximum capacity, which makes part-load performance very important when considering overall compressor efficiency. Part-load performance depends on the type of compressor, the method used to control its capacity, and the exact load profile during the compressor’s operation. Key O&M Cost Point The most efficient compressor type for part-load operation is the reciprocating compressor. Centrifugal compressors are efficient to their turn-down limit, at which point they typically must blow off excess air (modern controls can dramatically improve this problem). Rotary screws lose efficiency rapidly at part-load (with the exception of variable speed drive control), and generally are not operated below 50% capacity because of their lower efficiency beyond this point. With all compressor types, proper sizing is essential for optimal part-load performance. Compressors should be sized so that they run as close to full load as possible under expected operating conditions. Oversized compressors often operate at lower average loads, which wastes energy through inefficient operation. Maintenance – With no valves or contacting parts, centrifugal compressors have low daily maintenance requirements. Rotary screws also are characterized by minimal maintenance costs. Key Technical Point Reciprocating compressors, with many air control valves and large frictiongenerating components, usually require more frequent maintenance to sustain performance levels. Durability – If maintained properly, both reciprocating and centrifugal compressors will operate for long timeframes with minimal performance losses. Key Technical Point Rotary screw air compressors usually have shorter life expectancies than reciprocating or centrifugal compressors. Regular maintenance will extend the useful life of the rotary screw compressor but will not prevent the inevitable need for complete replacement of the air compressor component (the air end). With improper maintenance that results in large amounts of gradual wear and tear, centrifugal compressors (because they are more precise and more delicate) tend to lose efficiency more rapidly than reciprocating compressors.

2-23


Key O&M Cost Point When performance degrades to the point where replacement or overhaul of key compressor components (impellers, etc.) is necessary, centrifugal repair is very expensive. Equivalent work on a reciprocating compressor will tend to cost much less. Pressure Output – A compressor’s energy directly relates to the pressure of the air that it produces. When a compressor produces 100 psi (689 kPa) of air pressure, and demand remains constant, then every 2 psi (13.8 kPa) of pressure change will increase or decrease its energy consumption by approximately 1%. Any reduction in the compressor’s output pressure will, therefore, result in significant energy savings. There are several ways to lower output pressure including, better control, lower pressure drops in the conditioning and distribution sections of the air system, and selection of equipment that operates at lower pressures. Heat Recovery – During operation, compressors generate a large amount of heat that can be recovered for use in other areas of the plant. Some uses of this hot air are water preheating, process heating, and supplemental space heating. Heat recovery systems can be used with both air- and water-cooled compressors. A well designed system can recover over 50% of the compression heat energy. In plants that have uses for hot air, the potential for energy savings can often justify a compressor heat recovery system. Lubrication – The need for truly oil-free air also influences the type of compressors required. To maintain the ANSI/ISA standard for instrument air quality in a power plant, it is recommended that oil-free air compressors be specified. If the air supply is obtained from lubricated compressors, the air must be treated with coalescing filters and rigorously monitored to achieve the required quality. Demand – Selection of air compressors must take into account the requirements of the different points-of-use, the air capacity for each when fully loaded, and the frequency of these requirements. Demands are often intermittent but the worst case scenario must also be considered. Standby compressor capacity must be considered, taking into account the essential nature of an application and the cost of down time compared with the cost of a spare compressor. The capacity required is a major factor in determining the type of compressor chosen. Key Technical Point A general rule is that centrifugal and rotary air compressors are better suited to continuous base-load type of service. Reciprocating air compressors are better suited for swings in load. This means that a centrifugal or rotary air compressor can be sized for the minimum or average demand, while a reciprocating air compressor can then handle the swings in load from minimum or average to the peaks.

2-24


The anticipated load swings are also a determining factor in the selection of a single compressor and its type of capacity control or multiple compressors with sequential controls. Table 2-3 is a comparison of the costs to operate the different types of compressors. Table 2-3 Compressor Operating Costs [3] Type of Compressor

Energy to Operate in kW/100 cfm (kW/m3/min)

Single-acting, air-cooled reciprocating

22–24 (7.8–8.5)

Double-acting, water-cooled reciprocating

15–16 (5.3–5.6)

Oil-injected rotary screw

18 (6.4 )

Oil-injected rotary vane

21 (7.4)

Liquid ring

24 (8.5)

Oil-free rotary screw

20 (7.1)

Centrifugal

16–18 (5.6–6.4)

These calculations assume the full brake horsepower requirements of each compressor type to be 100 psig (689 kPa) discharge pressure at the compressor with a main drive motor typical efficiency of 92%. These costs are based on full-load operating conditions.

Some general facts concerning compressors are listed in Table 2-4. Table 2-4 General Facts Regarding Compressors [9] Air compressors normally deliver 4 to 5 cfm (6.8 to 8.5 m3/hr) per horsepower at 100 psig (689 kPa) discharge pressure. Depending on the size of the system, compressed air costs about 25 to 30 cents per thousand cubic feet (28.3 m3) of free air ingested by the compressor (including operating and maintenance costs). Every 2 psi (13.8 kPa) pressure drop costs 1% of compressor horsepower in efficiency. Power cost for each 1 horsepower operating constantly for one year at 10 cents per kw-hr is about $750. A 50 horsepower compressor rejects heat at approximately 126,000 btu (36.9 kw) per hour. Air receivers are sized for about 1 gallon (3.8 liters) capacity for each cfm (1.7 m3/hr) of compressor capacity. Typical compressor discharge air temperature before after-cooling: Rotary screw =175°F (79°C), single-stage reciprocating = 350°F (177°C), two-stage reciprocating = 250°F (121°C) The water vapor content at 100°F (38°C) of saturated compressed air equals about 2 gallons (9 liters) per hour for each 100 cfm (170 m3/hr) of compressor quality.

2-25


Ideally, an air compressor should be located as close as possible to the point-of-use of the compressed air. This is not always feasible and other factors must be considered. The chosen location for the compressors must take into account the type of cooling required. Air-cooled compressors eliminate the need for expensive cooling water and drainage costs. Large radiatortype coolers located outdoors in cold climates can also present problems, such as with oil temperature, oil viscosity at start-up when the compressor has been idle overnight, and freezing of condensate from the compressed air after-cooler. Immersion heaters and frost-free traps are readily available. It is recommended that outside radiators be located adjacent to a north-facing wall to minimize the heating effect of direct sunlight. The compressor room should be well ventilated. The location of the point at which air is drawn in from outside is extremely important. This should avoid the potential of exhaust fumes from other sources, that is, near welding areas, battery rooms, and so on. Inlet air to the compressor from the compressor room might be using air that has been air conditioned at a significant cost. The temperature within a compressor room might also be higher than the outside ambient temperature and will reduce the mass flow of air through the compressor. Air drawn from outside the compressor room should be from a location where contaminants from an industrial atmosphere will not be a problem. The compressor air inlet filter must be readily accessible for regular routine maintenance. When the air compressor inlet filter is mounted remotely from the compressor, the piping from the air inlet filter to the compressor inlet must be clean, non-corrosive, and can be of plastic material. As previously stated, the air compressor inlet filter is for the protection of the compressor and not for any equipment downstream. Appendix E contains information to consider when using a rental compressor. The following sections include more information about the reciprocating, rotary screw, and centrifugal compressors and their lubrication systems. 2.4.1.1

Reciprocating Compressors

A reciprocating compressor [3] is a positive displacement type of machine. A given quantity of air is trapped in a cylinder and the space that it occupies is mechanically reduced by movement of the piston. This movement causes a corresponding rise in pressure before the air is discharged. There are two basic types of reciprocating air compressors, single-acting and double-acting. Reciprocating single-acting air compressors are compressors that have one side of the piston used in the compression process. The connecting rod transmits power and motion from the crankshaft to the piston and is attached to the underside of the piston by means of a pin. This is sometimes referred to as an automotive-type piston. These air compressors are generally onestage for pressures from 70–100 psig (482–689 kPa) or two-stage for pressures from 100–250 psig (0.7–1.7 MPa). They range from a fractional hp to 25 hp.

2-26


Cylinders and cylinder heads have external fins for radiant heat dissipation and in two-stage designs, cooling between the stages is accomplished by a finned pipe inter-cooler. Belt drive is most common, and the compressor sheaves/flywheel can also serve as a cooling fan with suitably contoured spokes. Cylinders are generally lubricated by splash lubrication where a dipper on the connecting rod dips into the oil in the crankcase and splashes it toward the cylinder. A pressure lubrication system is used on some compressors of this type. Oil-free or oil-less, single-acting air compressors are also available where oil is excluded from the compressor cylinder. In this case, polytetra fluoro ethylene (PTFE) or a similar material can be used for piston rings. Single-acting air compressors are commonly supplied as packages and are mounted on an air receiver or tank. These are called tank-mounted compressors. The receiver tank provides some additional radiation cooling and requires that resulting condensate be drained regularly. Generally, these packages are installed close to the point-of-use of the compressed air. Reciprocating double-acting air compressors use both sides of the piston for air compression, doubling the capacity for a given cylinder size. A piston rod is attached to the piston at one end and to a crosshead at the other end. The crosshead ensures that the piston rod travels concentrically with the piston. These compressors can be single- or multi-stage, depending on discharge pressure and horsepower. These can range from 10 hp, which increases with pressures from 50 psig (345 kPa) to several thousand psig (2000 psig = 13.8 MPa). Details of typical construction features are shown in Figures 2-13a and 2-13b.

2-27


Figure 2-13 a) Crosshead Assembly for a Non-Lubricated Double-Acting Reciprocating Compressor (Courtesy of Ingersoll-Rand Company) b) Details of Oil Scraper Rings

2-28


Cylinder lubrication is generally by means of a forced-feed cylinder lubricator with a feed rate of several drops per minute, depending on cylinder size and piston speed as specified by the manufacturer. Oil-free versions are also available with PTFE or a similar material for piston and glide rings, and a distance piece between the crankcase and the cylinders to ensure that no part of the piston rod that enters the lubricated crankcase can enter the oil-free cylinder area. Double-acting air compressors generally have cooling water jackets around the cylinder body and in the cylinder head. This, combined with their relatively slow speed of operation and watercooled intercooling, results in excellent compression efficiency. Single- and two-cylinder compressors of this type generally require a substantial foundation due to unbalanced reciprocating forces. Some advantages and disadvantages are listed below for a typical air compressor application in a power plant. The advantages for a single-acting, air-cooled reciprocating air compressor are: •

Small size and weight

•

Can generally be located close to the point of use, avoiding the use of long piping and pressure drops

•

Does not require separate cooling systems

•

Has simple maintenance procedures

The disadvantages include: •

Oil carryover (which should be avoided in instrument air systems)

•

Relatively high cost of compression

•

Generally designed to run less than 50% of the time

•

Generally compress and store the air in a receiver at a pressure that is higher than that required at the point of use. The pressure is then reduced to the required operating pressure. This occurs without recovery of the energy used to compress to the higher pressure.

•

High noise level

The advantages of a double-acting, water-cooled reciprocating air compressor are: •

Efficient compression, particularly with multi-stage compressors

•

Three-step (0–50–100%) or five-step (0–25–50–75–100%) capacity controls, allowing efficient part-load operation

•

Relatively routine maintenance procedures

The disadvantages include: •

Relatively high initial cost compared with equivalent rotary air compressors

•

Relatively high space requirements

•

Seldom sold as a complete independent package 2-29


•

Relatively high vibrations, which require high foundation costs

•

Require sufficient flywheel mass to overcome torque and current pulsations

•

High noise level

2.4.1.2

Rotary Screw Compressors

The positive displacement rotary compressor can be an oil-injected, oil-free, oil-injected singlescrew, oil-injected sliding-vane, or a liquid ring rotary-type compressor. These types of rotary screw compressors [3,10] are described in the sections that follow. 2.4.1.2.1

Oil-Injected Rotary Screw Compressor

The oil-injected rotary screw compressor is a positive displacement type of compressor. This means that a given quantity of air or gas is trapped in a compression chamber and the space that it occupies is mechanically reduced, thus causing a corresponding rise in pressure before the air is discharged. The oil-injected rotary screw compressor consists of two intermeshing rotors in a stator housing having an inlet port at one end and a discharge port at the other. The male rotor has lobes formed helically along its length, while the female rotor has corresponding helical grooves or flutes. A typical oil-injected rotary screw compressor is shown in Figure 2-14.

Figure 2-14 Cross-Section of an Oil-Injected Rotary Screw Compressor (Courtesy of Sullair Corporation)

Air flowing in through the inlet port fills the spaces between the lobes on each rotor. Rotation then causes the air to be trapped between the lobes and the stator as the inner-lobe spaces pass beyond the inlet port. As rotation continues, a lobe on one rotor rolls into a groove on the other rotor and the point of intermeshing moves progressively along the axial length of the rotors. This 2-30


reduces the space occupied by the air and results in increased pressure. Compression continues until the inner-lobe spaces are exposed to the discharge port when the compressed air is discharged. This cycle is illustrated in Figure 2-15.

Figure 2-15 Helical, Screw-Type Compressor [11]

Oil is injected into the compression chamber during compression and serves the following three basic functions: •

It lubricates the intermeshing rotors and associated bearings.

•

It takes away most of the heat caused by compression.

•

It acts as a seal in the clearances between the meshing rotors, and between the rotors and the stator. Key Technical Point The generic term oil has been used. This is generally understood to be a hydrocarbon product, but most compressors now use cleaner and longer-life synthetic lubricants including diesters, polyglycols, polyalphaolefins, and silicon-based fluids. These newer products are suitable for a wider range of temperatures and have higher flash points. However, the compatibility of the lubricant and its volatile additives with downstream components (especially elastomeric materials) should be checked.

A mixture of compressed air and injected oil leaves the air end and is passed to a sump/separator where the oil is removed from the compressed air. Directional and velocity changes are used to separate most of the liquid. The remaining aerosols in the compressed air are then separated by means of a coalescing filter, resulting in only a few parts per million of oil carryover. Most oilinjected rotary screw compressor packages use the air pressure in the oil sump/separator, after discharge of the air end, to circulate the oil through a filter and cooler prior to re-injection to the compression chamber. Some designs might use an oil pump. 2-31


Single-stage, oil-injected screw compressor packages are available from 3–900 hp and with capacities from 8–5000 cfm (0.2–141.6 m3/min), with discharge pressures from 50–200 psig (0.3–1.4 MPa). Two-stage versions can improve the power required for a given rate of flow by up to 15% and some can achieve higher discharge pressures. Figure 2-16 shows a picture of a single-stage design and Figure 2-17 shows a picture of a two-stage design.

Figure 2-16 Rotary, Helical Screw Compressor, Typical Single-Stage Design [11]

Figure 2-17 Rotary, Helical Screw Compressor, Typical Axial Arranged Two-Stage Design [11]

Advantages of the oil-injected rotary screw compressor include: •

Compact size and complete package

•

Economic initial cost

•

Vibration-free operation that does not require a special foundation

•

Part-load capacity control systems that can match system demand

•

Routine maintenance (includes oil and filter changes)

2-32


Disadvantages include: •

Less efficient full- and part-load operation when compared to water-cooled reciprocating air compressors

•

Oil carryover into delivered air requires proper maintenance of air/oil separator and the oil itself

2.4.1.2.2

Oil-Free Rotary Screw Compressor

The oil-free rotary screw compressor is a positive displacement type of compressor. The principle of compression is similar to that of the oil-injected rotary screw compressor but without oil being introduced into the compression chamber. Two distinct types of this compressor are available—the dry type and the water-injected type. In the dry type, the intermeshing rotors are not allowed to touch and their relative positions are maintained by means of lubricated timing gears external to the compression chamber. Because there is no injected fluid to remove the heat of compression, most designs use two stages of compression with an inter-cooler between the stages and an after-cooler after the second stage. The lack of a sealing fluid also requires higher rotational speeds than for the oil-injected type. A typical air end is shown in Figure 2-18.

Figure 2-18 Oil-Free Rotary Screw Compressor (Courtesy of Atlas Copco Compressors, Inc.)

2-33


The dry type of oil-free rotary screw compressors have a range from 50–4000 hp or 200–20,000 cfm (5.6–566.3 m3/min). Single-stage units can operate up to 50 psig (345 kPa), although twostage units can generally achieve 150 psig (1 MPa). In the water-injected type, similar timing gear construction is used but water is injected into the compression chamber to act as a seal in internal clearances and to remove the heat of compression. This allows pressures in the 100–150 psig (0.7–1 MPa) range to be compressed with only one stage. The injected water, together with condensed moisture from the atmosphere, is removed from the discharged compressed air by a conventional moisture separation device. Similar to the oil-injected type, oil-free rotary screw compressors are generally packaged with all necessary accessories. The oil-free rotary screw air compressor is shown in Figure 2-19.

Figure 2-19 Water-Injected Rotary Screw Compressor (Courtesy of Gardner Denver Machinery, Inc.)

Advantages of the oil-free rotary screw air compressor include: •

Completely packaged

•

Designed to deliver oil-free air

•

No special foundations are required


Significant cost above the oil-injected type

•

Less efficient than the oil-injected type

•

Limited to load/unload-type capacity control

•

Higher maintenance costs than the oil-injected type

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2.4.1.2.3

Oil-Injected Single-Screw Compressor

The name rotary screw compressor normally refers to a compressor having two intermeshing rotors with parallel axes. The single-screw compressor, as its name implies, has only one helically grooved rotor, normally arranged with a horizontal axis. Two meshing gate (or star) rotors are arranged, one on each side of the helical rotor with their axes at right angles to it. Air enters the compression chamber through an inlet port and compression begins when a tooth of a star rotor enters and seals the groove in the helical rotor. As rotation continues, the tooth follows along the path of the groove, reducing the space occupied by the air with a corresponding rise in pressure. When the discharge port is reached, the compressed air is discharged as illustrated in Figure 2-20. The two-gate (or star) rotors allow compression to take place simultaneously on both sides of the helical rotor and to maintain axial balance of the helical rotor. Axial force is exerted on each gate rotor.

2-35


Figure 2-20 Operation of a Rotary Single-Screw Compressor (Courtesy of Grimmerschmidt Corp.)

2-36


As with the conventional rotary screw compressor, oil is injected into the compression chamber to lubricate, seal, and take away the heat of compression. A mixture of compressed air and injected oil leaves the air end of the compressor and is passed to a sump/separator where the oil is removed from the compressed air. Directional and velocity changes are used to separate most of the liquid. The remaining aerosols in the compressed air are then separated by a coalescing filter, thus resulting in only a few parts per million of oil carryover. Most oil-injected screw compressor packages pipe the oil through a filter and cooler before re-injection into the compression chamber. Some designs use an oil pump. Oil-injected single-screw compressor packages are available from 20–150 hp with capacities from 62–700 cfm (1.8–19.8 m3/min) and pressures to 150 psig (1 MPa). Advantages of the oil-injected single-screw air compressor include: •


•


•


•


•



Less efficient at full- and part-load operation, as compared with water-cooled reciprocating air compressors

2.4.1.2.4

Oil-Injected Sliding-Vane Compressor

The oil-injected rotary sliding-vane compressor is a positive displacement type of compressor and is shown in Figure 2-21.

Figure 2-21 Oil-Injected Rotary Sliding-Vane Compressor (Courtesy of Compressed Air and Gas Institute)

2-37


The basic design consists of a circular stator containing a cylindrical rotor that is smaller than the stator bore with the rotor supported eccentrically in the stator. The rotor has radial (sometimes off-set) slots in which vanes, or blades, slide. Rotation of the rotor exerts a centrifugal force on the vanes, thus causing them to slide out to contact the bore of the stator and forming cells bounded by the rotor, adjacent vanes, and stator bore. Some designs have means of restraining the vanes so that a minimal clearance is maintained between the vanes and the stator bore. The air inlet port is positioned to allow air to flow into each cell exposed to the port, filling each cell by the time it reaches its maximum volume. After passing the inlet port, the size of the cell is reduced as rotation continues and each vane is pushed back into its slot in the rotor. Compression continues until the discharge port is reached and the compressed air is discharged. Similar to the oil-flooded rotary screw compressor, oil is injected into the compression chamber to act as a lubricant and a seal, and to remove the heat of compression. Single- and two-stage versions are available with either an in-line or over-under arrangement of the stages. The sliding-vane compressor is normally sold as a completely pre-engineered package in the range from 10 to 200 hp, with capacities from 40 to 800 cfm (1.1–22.7 m3/min) and discharge pressures from 80 to 125 psig (552–862 kPa). Advantages of the oil-injected sliding-vane air compressor include: •


•


•


•


•



Less efficient full- and part-load operation as compared to water-cooled reciprocating air compressors

•

Less efficient than equivalent oil-injected rotary screw-type compressors

2.4.1.2.5

Liquid Ring Rotary Compressors

The liquid ring (or liquid piston) rotary compressor is also a positive displacement type of compressor. The mode of compression is similar to that of the sliding-vane rotary compressor but the vanes (or blades) are fixed on the rotor. The stator bore can be circular with the rotor eccentric to it or elliptical with the rotor concentric to it. The former provides one compression per revolution and the latter provides two compressions per revolution. A ring of liquid is swirled around the bore of the stator as the rotor turns. The depth of penetration of each vane, or blade, into the liquid varies as rotation occurs. The space between 2-38


the rotor hub and the liquid ring, therefore, varies. Axial inlet porting allows air to fill the space between adjacent vanes up to its maximum volume. Further rotation then causes the space to be reduced and compression to occur until the discharge port is reached when the compressed air is discharged. This is illustrated in Figure 2-22. The liquid ring also serves to remove the heat of compression. The lubricated bearings are external to the compression chamber and isolated from it; thus, oil-free compression is obtained.

Figure 2-22 Liquid Ring Rotary Compressor [3]

This type of design is most commonly used as a single-stage vacuum pump to generate up to 26 in. (66 cm) Hg. Two-stage units can achieve higher vacuum levels. Capacities range from 2– 16,000 actual cubic feet per minute (acfm) (.06–450 m3/min). As a compressor, it is limited to about 100 psig (689 kPa). Key O&M Cost Point The liquid ring rotary-type compressor is less efficient than other rotary types because of the energy required to swirl the liquid in the stator.

2-39


A double-acting liquid piston rotary compressor is shown in Figure 2-23.

Figure 2-23 Cross-Section Diagram of a Double-Acting Liquid Piston Rotary Compressor [11]

Advantages of the liquid ring type of air compressor include: •


•

Oil-free air delivery


Designed mainly for vacuum service. Very inefficient as an air compressor.

•

Much less efficient than the equivalent oil-injected rotary screw type.

•

Does not offer good part-load capacity control.

2-40


2.4.1.3

Centrifugal Compressors

A centrifugal compressor [2,3] is a dynamic compressor. A dynamic air compressor has a continuously flowing air stream that has velocity energy (or kinetic energy) imparted to it by an impeller, or impellers, that rotates at speeds that can exceed 50,000 rpm. Approximately half of the pressure energy is developed in the impeller, with the other half achieved by converting the velocity energy to pressure energy as air speed is reduced in a diffuser and volute. The most common dynamic compressor is the centrifugal air compressor with two to four stages for pressures that are in the 100–125 psig (689–862 kPa) range for the compressed air system, and in the 300–400 psig (2–2.8 MPa) range for the sootblowing application. A typical multistage dynamic air compressor skid is shown in Figure 2-24. A water-cooled inter-cooler and separator between each stage return the air temperature to approximately ambient temperature and remove condensed moisture before entering the next stage. An after-cooler and separator cool the air from the final stage and remove more moisture prior to air delivery.

2-41


Figure 2-24 Typical Multi-Stage Centrifugal Air Compressor (Courtesy of Cooper Turbocompressor, Inc.)

2-42


Figure 2-25 shows a larger compressor package for a sootblowing application.

Figure 2-25 Large Centrifugal Compressor Package (Courtesy of Cooper Turbocompressor, Inc.)

2-43


An inherent characteristic of the centrifugal air compressor is that the discharge pressure decreases as the capacity increases, as shown in Figure 2-26. The steepness of the pressure head/capacity curve is dependent upon the impeller design. The more the impeller blades lean backward from the true radial position, the steeper the curve is.

Figure 2-26 Characteristic Curves for a Centrifugal Air Compressor at 90ºF (32.2ºC) (Courtesy of Cooper Turbocompressor, Inc.)

2-44


Most standard centrifugal air compressor packages are designed for an ambient temperature of 90ºF (32.2ºC) and near sea level barometric pressure. The dynamic nature of the centrifugal compressor results in an increasing pressure head generated by each impeller as the air density increases. The compressor mass flow and actual flow capacity at a given discharge pressure increase as the ambient temperature decreases (as shown in Figure 2-27).

Figure 2-27 Characteristic Curves for a Centrifugal Air Compressor at 60ºF (15.5ºC) (Courtesy of Cooper Turbocompressor, Inc.)

Typically, a capacity control system is provided with the compressor to maintain the desired capacity and to operate within the motor’s horsepower limits. The control system regulates the air flow by means of an inlet throttle valve or inlet guide vanes. The amount of reduction in the flow rate is limited by a minimum point flow reversal phenomenon known as surge. Control systems normally unload the compressor to avoid this occurrence, which could result in excessive vibration and potential damage to the compressor.

2-45


Centrifugal air compressors range from around 500 to over 100,000 acfm (14.1–2831.7 m3/min), but the more common instrument air compressors are from 1,000 to 5,000 cfm (28.3–141.6 m3/min) flow with discharge pressures of up to 125 psig (862 kPa). The compressors can have several impellers in line on a single shaft or have separate impellers integrally geared as shown in Figure 2-28.

Figure 2-28 Centrifugal Air Compressor—Integrally Geared Multiple Impellers (Courtesy of Cooper Turbocompressor, Inc.)

Centrifugal air compressors provide oil-free air delivery because there is no oil in the compression chambers. Lubrication, for speed increasing gears and the special high-speed shaft bearings, is kept away from the compression chambers by shaft seals, which might also have air purge and vent connections.

2-46


Centrifugal air compressors are high-speed rotating machines. Shaft vibration monitoring is needed to record operational trends and protect the equipment. Automatic control of the compressors is typical and has been greatly improved by the use of microprocessors. The microprocessors monitor pressure/capacity/temperature characteristics and the main drive motor current. It is important that the manufacturer’s recommended maintenance procedures be followed and that certain maintenance procedures be carried out by factory-qualified staff. This is particularly true of attempts to remove an impeller from a shaft because special procedures and tools might be involved. Advantages of a centrifugal compressor include: •

Completely packaged for plant or instrument air up through a 500-hp rating (6000 hp for sootblowing applications)

•

Purchase cost decreases as size increases

•

Designed to deliver oil-free air

•

No special foundation required


Limited capacity control modulation, which requires unloading for reduced capacities

•

High rotational speeds, which require special bearings and sophisticated monitoring of vibrations and clearances

•

Specialized maintenance considerations

When changing from a reciprocating compressor to a centrifugal compressor, it is important to verify that the receiver size is adequate for the centrifugal compressor. 2.4.1.4

Compressor Lubrication

Not all compressors require lubrication. For the ones that are designed to operate with lubrication, this section discusses the types of lubrication systems and lubricants used [2,3]. An adequate supply of specified oils at the required temperature should always be provided to the rubbing surfaces. Oil delivered on a continual basis should be cautiously filtered to remove contaminants that are constantly entering the system. A complete change of specification grade oil should always be available in case of serious contamination due to failure or accident. Most small, single-acting, air-cooled reciprocating compressors use a splash lubrication system, where a dipper attached to the connecting rod dips into the oil in the crankcase on each revolution and throws the oil to areas requiring lubrication. Some industrial grade compressors use a pressure lubrication system with an oil pump driven by the crankshaft. Some of the oil lubricates the cylinder and is carried over with the discharged air. Relatively high discharge temperatures can result in carbon deposits in valves, discharge ports, and piping.

2-47


Double-acting water-cooled reciprocating compressors generally use an oil pump to provide lubrication for the crankshaft, connecting rod bearings, the crosshead, and its guide. A separate forced-feed cylinder lubricator is used for cylinder lubrication and the manufacturer might specify a specific lubricant. Oil-injected rotary compressors generally use the pressure differential between discharge and inlet to circulate the lubricant for the bearings, any gears, and for the compression chamber. Some designs use an oil pump. Centrifugal compressors use a pressure lubrication system similar to that shown in Figure 2-29. The main oil pump can be driven by the gearbox input shaft. A motor-driven auxiliary oil pump is supplied for pre-lubrication before start-up and for post-lubrication during a shut-down period.

Figure 2-29 Pressure Lubrication System for an Air Compressor [3]

2-48


Three types of lubrication systems that are used at power plants are: •

External forced lubrication

•

Internal forced lubrication

•

Gravity drip and bath

External Forced Lubrication – External forced lubrication systems are common in larger installations with slight variations in components. Most systems consist of the following: •

An external lube oil tank

•

An external electric pump

•

A lube oil cooler (air- or water-cooled)

•

A system pressure regulator

•

A lube oil heater

•

A bearing lube oil pressure regulator

•

A system filter or dual filtering unit

•

Oil sump (usually at the base of the unit)

Internal Forced Lubrication – Internal forced lubrication systems usually include a shaftdriven pump similar to an internal combustion engine’s oil pump. The pump draws oil through a coarse screen filter to protect the pump and forces the oil through a canister-type disposable filter, similar to the ones used in cars. The clean, filtered oil then flows under pressure through drilled holes into the bearings and returns to the sump by gravity. Some internal lubrication systems use an emergency or auxiliary pump that is connected to the pressurized lube oil line. This arrangement allows for circulation of the oil before operation of the compressor, and for cooling of the compressor after operation. The auxiliary pump is normally powered from an alternate power supply. Gravity Drip and Bath Lubrication – Gravity drip and bath systems are often found on older and slower machines or on portable and temporary compressors that have rolling element bearings. It is important to keep the lubrication pots and baths at operating levels. Bearings that run in grease must be packed and inspected periodically to ensure proper lubrication of the bearings. Even fully automatic systems require operator attention. Lubricants vary from a straight hydrocarbon mineral oil to sophisticated synthetic oils. Oil types should never be mixed or changed without the approval of the compressor manufacturer. A lubricant that meets the specifications of the compressor manufacturer should be used at all times and changed at the specified intervals.

2-49


Key Human Performance Point It is estimated that the useful life of lubricating oil is halved for every 18ºF (5ºC) above 200ºF (93ºC) of operating temperature. Compressors operating in high ambient conditions need to have the oil checked frequently, and a quarterly sampling and analysis program is recommended. For additional details on compressor lubricants, refer to the EPRI Lubrication Guide: Revision 3 (Formerly NP-4916-R2), 1003085. In the case of oil-injected cylinders for reciprocating compressors, proper lubricating oil should be used. Ordinary machine oils decompose at high compression cycle temperatures and cause the formation of carbon and sooty deposits on cylinder walls and valves. These deposits cause valves to stick and prevent them from operating properly. This might cause even higher temperatures or result in an explosion. Reciprocating compressor oil-injected cylinders of all sizes should initially be operated until the cylinder has a glazed appearance. The cylinder lubricant should have a viscosity equivalent to SAE-60 oil. This oil should be fed at an accelerated rate to help flush out wear particles and mill scale. When the cylinder bore begins to take on the glazed appearance, the lubricant feed rate can be gradually reduced until the minimum quantity recommended by the manufacturer is reached. An Ingersoll-Rand specification for cylinder oils to be used after initial operation is listed in Table 2-5. Other manufacturers have their own oil types and specifications that might or might not agree with this table. This table should only be used as a guide. Table 2-5 Reciprocating Compressor Oil Types and Specifications (Courtesy of Ingersoll-Rand Corporation) Parameter

Specification

Flash Point (open cup)

350–380ºF (177–193ºC) minimum

Viscosity @ 100ºF (38ºC)

420–780 Saybolt universal seconds maximum

Viscosity @ 210ºF (99ºC)

50–60 Saybolt universal seconds minimum

Carbon Residue

0.25–0.45 Conradson maximum

Strong Acid Number

0.00 maximum

Pour Point

10ºF (5.5ºC) above ambient

For reciprocating compressors used primarily for instrument air, a straight mineral oil or oxidation-inhibited internal combustion engine lubrication oil is acceptable. The oil must separate rapidly and produce little sludge. It must be a well-refined petroleum product containing no fats or fixed oil compounding. Straight mineral oil containing a foam depressant is preferred. It must be substantially non-corrosive to common bearing metals.

2-50


Reciprocating compressors have been around the industry for a long time and commercial grade petroleum oils change names frequently. Therefore, most compressor companies do not recommend an oil by the trade name, but provide generic specifications as in Table 2-5. All major oil companies will be able to provide oil that satisfies the recommended specifications. Because of their high speeds, dynamic air compressors require high quality petroleum-based oil. This oil will perform for years without changing, provided that a high degree of filtration is maintained. A typical oil will be oxidation- and rust-inhibited, resist foaming, and will not break down under operating temperatures and pressures. Lubrication requirements on rotary screw compressors vary greatly by manufacturer, with warranties voided if specific oils are not used. A check of the vendor package should provide the proper lubricant. Machines without the warranty considerations should begin with a SAE-30 automotive oil and inspect the machine frequently. Temperatures and temperature ranges can be checked against the oil specification on the container. A proper oil should be non-acidic and provide an oil film at all pressures. The temperatures should not cause carbon or sludge buildup. If service water is used as the cooling medium for the oil coolers and there is a leak in the tubes, then oil might get into the cooling water. Oil can then be discharged with the service water into a river, lake, pond, and so on. To minimize or eliminate this possibility, the water pressure should be increased so that a leak will result in water entering the oil. In this way, oil from a tube leak will not be discharged with service water. If water leaks into the oil, the compressor can be damaged. Checking for leaks in the oil coolers on a regular basis is important to protect the compressor from damage. 2.4.2 Motors Electric motors [7,12] are the most common method for powering an air compressor system. Modern motors have efficiencies from 80% to 97% with the larger sizes (over 50 hp) generally greater than 90% efficiency. A typical motor configuration is shown in Figure 2-30. It is assumed that the motor that drives the compressor in the compressed air system is a 480-volt motor. For fossil plants that use compressed air as the medium for boiler slag sootblowing and/or air preheater sootblowing, the size of the compressor and motor will increase. The motor size is in the 4160- and 7200-volt range.

2-51


Figure 2-30 Kaeser Screw Compressor Package

The motor must be matched in terms of part load efficiencies and sized to the expected operating conditions of the compressor. Electric motors are the most efficient under full-load operation and have varying performance at reduced loads. If the compressor will frequently run at part loads, then the part-load efficiencies of the compressor’s driver will have a large impact on the compressor’s operating costs. Most electric motors have a service factor of 1.15 and are able to handle occasional overloading. This is not a viable option if the compressor’s demand frequently overloads the motor and leads to excessive motor wear. There are two types of electric motors frequently used with air compressors: induction and synchronous. Both use alternating current to create a revolving magnetic field that turns the motor’s rotor. The motor’s magnetic fields operate at speeds that depend on the line frequency of the power supply and the design of the motor (number of poles). Induction motors tend to rotate slightly slower than the magnetic field. The difference in the two speeds is known as slip. A greater load on these motors will increase the amount of slip. The primary difference between induction and synchronous motors is that the synchronous types use a direct current component in the motor’s rotor to keep it operating at a fixed speed proportional to the line frequency and independent of motor load.

2-52


Key Technical Point Because of their complex design, synchronous motors tend to be more difficult to install and operate. In terms of energy use, synchronous motors are usually slightly more efficient (1%–3%) than similar induction motors. In addition, synchronous motors have unity or leading power factors, where induction motors have lagging power factors. The power factor describes the relationship between the current and voltage for electrical power. A larger lagging power factor requires more current for the same amount of kilowatts. Key O&M Cost Point In practical terms, induction motors with their lagging power factor will result in greater expenses for the same power usage. Although electric motors are rated at a certain horsepower, they will use power to match the load placed on them until they either stall or burn out. For this reason, motor controls with adequate overload protection are essential. Control systems for motors provide additional benefits, such as variable voltage operation, reduced voltage starting, and under-voltage protection. A combination of a good control system and a maintenance program can enhance safe and efficient motor operation. The size of the motors covered in this guide is 600 volts or less. While higher voltage motors are used for some large compressors, the majority of the motors used are in the 480-volt range. A recent introduction into the market is the hybrid permanent magnet synchronous motor for rotary air compressors by Ingersoll-Rand Company. The motor is about one-third the size of a comparable induction motor and is mounted directly onto the air end of the compressor in a cantilever fashion. This arrangement eliminates the motor bearings and shaft seals found in other compressor motors. A picture of the new design is shown in Figure 2-31.

Figure 2-31 Ingersoll-Rand Hybrid Permanent Magnet Motor Arrangement

2-53


The hybrid permanent magnet design replaces the continuous windings that surround the stator with multiple independent coil modules. This allows for physically separated windings that eliminate electrical contact between phases and result in improved reliability. The rotor design is a salient pole, flux concentration type. The magnets are not surface-mounted but are mechanically mounted within the rotor. The new design motors are controlled by variable speed inverters that match the compressor output to demand. These motors can cycle on and off as many as fifty times per hour, allowing the units to simply turn off when the air system pressure has been stabilized. 2.4.3 Air Dryers Compressed air leaving an air compressor is not of a quality suitable for the intended use. This is due to several factors. Atmospheric air, particularly in an industrial environment, contains pollutants that include moisture, particulate, and hydrocarbons. The air inlet filter on an air compressor is a particulate filter, designed to protect the compressor rather than any downstream equipment. The inlet filter might not be sufficiently fine for the equipment using the compressed air. The air compressor itself can contribute contaminants in the form of wear particles and the carryover of the compressor lubricant. A dryer is commonly used to remove moisture before it enters the distribution system [2,3,6,13]. When air is compressed, the water content is concentrated and results in very wet air. While the air naturally heats up during compression, the water remains vaporized. If the air enters the distribution system without further conditioning, it will still hold large amounts of moisture and will cause water condensation throughout the system. This condensation can: •

Corrode pipes and valves

•

Contaminate moisture-sensitive processes

•

Increase maintenance costs throughout the system

•

Cause problems in the operation of pneumatic systems, air motors, and solenoid valves

•

In winter, water in the system can freeze and cause additional problems from clogging and instrument/tool malfunctions.

Generally, it is not recognized how large a volume of water needs to be removed during the process of compressing and cooling air. The following example shows the quantity of water handled through the inter-coolers and after-coolers during the compression process. Table 2-6 shows the moisture content of saturated air in gallons per 1000 cubic feet of air at various temperatures and pressures. Table 2-7 shows the moisture content of saturated air in milliliters per cubic meter.

2-54

EPRI Licensed Material Tutorial Table 2-6 Moisture Content of Saturated Air in Gallons Per 1000 Standard Cubic Feet [3] psig

Temperature °F 35

40

50

60

70

80

90

100

110

120

0

.0392

.0479

.0702

.1016

.1450

.2046

.2857

.3956

.5443

.7460

10

.0233

.0283

.0416

.0600

.0854

.1200

.1667

.2290

.3119

.4217

20

.0165

.0201

.0295

.0426

.0605

.0849

.1176

.1612

.2186

.2939

30

.0128

.0156

.0229

.0330

.0469

.0657

.0909

.1243

.1682

.2256

40

.0105

.0128

.0187

.0269

.0383

.0536

.0741

.1012

.1367

.1830

50

.0089

.0108

.0158

.0228

.0323

.0452

.0625

.0853

.1152

.1540

60

.0077

.0093

.0137

.0197

.0280

.0391

.0540

.0737

.0995

.1329

70

.0068

.0082

.0121

.0174

.0246

.0345

.0476

.0649

.0876

.1169

80

.0060

.0074

.0108

.0155

.0220

.0308

.0425

.0580

.0782

.1043

90

.0055

.0067

.0098

.0140

.0199

.0279

.0385

.0524

.0706

.0942

100

.0050

.0061

.0089

.0128

.0182

.0254

.0351

.0478

.0644

.0858

110

.0046

.0056

.0082

.0118

.0167

.0234

.0323

.0439

.0592

.0789

120

.0043

.0052

.0076

.0109

.0155

.0216

.0298

.0407

.0548

.0729

130

.0040

.0048

.0071

.0102

.0144

.0201

.0278

.0378

.0509

.0678

140

.0037

.0045

.0066

.0095

.0135

.0188

.0260

.0354

.0476

.0634

150

.0035

.0042

.0062

.0089

.0126

.0177

.0244

.0332

.0447

.0595

160

.0033

.0040

.0058

.0084

.0119

.0167

.0230

.0313

.0421

.0561

170

.0031

.0038

.0055

.0080

.0113

.0158

.0217

.0296

.0398

.0530

180

.0029

.0036

.0052

.0075

.0107

.0149

.0206

.0281

.0378

.0503

190

.0028

.0034

.0050

.0072

.0102

.0142

.0196

.0267

.0359

.0478

200

.0027

.0032

.0048

.0068

.0097

.0136

.0187

.0254

.0342

.0455

One example is a compressor operating at atmospheric inlet pressure with an ambient temperature of 100°F and 80% relative humidity. From Table 2-6, it is noted that saturated air at atmospheric pressure and 100°F contains 0.3956 gallons of water per 1000 ft3 of air. At 80% relative humidity, the moisture content is 0.3956 x 80/100 = 0.3165 gallons. If this air is compressed to 100 psig and cooled to 80°F, the air is saturated and the water content that the air can contain is only 0.0254 gallons per 1000 ft3 of air. This is equivalent to a removal of 0.2911 (0.3165 minus 0.0254) gallons of water per 1000 ft3 of air. This translates to 17.5 gallons of water per hour for a 1000 cfm capacity air compressor. Moisture content in air more than doubles from 0°F to 20°F, and continues to double for each increase of 20°F.

2-55

EPRI Licensed Material Tutorial Table 2-7 Moisture Content of Saturated Air in Milliliters Per Cubic Meter Pressure Gauge (kPa)

Temperature °C 1.7

4.4

10

15

21

27

32

38

43

49

0

5.24

6.4

9.58

13.58

19.38

27.35

38.19

52.88

72.76

99.72

69

3.11

3.78

5.56

8.02

11.42

16.04

22.28

30.61

41.69

56.37

138

2.20

2.69

3.94

5.69

8.09

11.35

15.72

21.55

29.22

39.29

207

1.71

2.08

3.06

4.41

6.27

8.78

12.15

16.62

22.48

30.16

276

1.40

1.71

2.50

3.60

5.12

7.16

9.91

13.53

18.27

24.46

345

1.19

1.44

2.11

3.05

4.32

6.04

8.35

11.40

15.40

20.59

414

1.03

1.24

1.83

2.63

3.74

5.23

7.22

9.85

13.30

17.77

483

0.91

1.09

1.62

2.33

3.29

4.61

6.36

8.67

11.71

15.63

552

0.80

0.99

1.44

2.07

2.94

4.12

5.68

7.75

10.45

13.94

620

0.73

0.89

1.31

1.87

2.66

3.73

5.15

7.00

9.44

12.59

689

0.67

0.81

1.19

1.71

2.43

3.40

4.69

6.39

8.61

11.47

758

0.61

0.75

1.10

1.58

2.23

3.13

4.32

5.87

7.91

10.55

827

0.57

0.69

1.02

1.46

2.07

2.89

3.98

5.44

7.33

9.74

896

0.53

0.64

0.95

1.36

1.92

2.69

3.72

5.05

6.80

9.06

965

0.49

0.60

0.88

1.27

1.80

2.51

3.48

4.73

6.36

8.47

1034

0.47

0.56

0.83

1.19

1.68

2.37

3.26

4.44

5.97

7.95

1103

0.44

0.53

0.77

1.12

1.59

2.23

3.07

4.18

5.63

7.50

1172

0.41

0.51

0.73

1.07

1.51

2.11

2.90

3.96

5.32

7.08

1241

0.39

0.48

0.69

1.00

1.43

1.99

2.75

3.75

5.05

6.72

1310

0.37

0.45

0.67

0.96

1.36

1.90

2.62

3.57

4.80

6.39

1379

0.36

0.43

0.64

0.91

1.30

1.82

2.50

3.40

4.57

6.08

An example in Standard International units is a compressor operating at atmospheric inlet pressure with an ambient temperature of 38°C and 80% relative humidity. From Table 2-7, it is noted that saturated air at atmospheric pressure and 38°C contains 52.88 milliliters of water per cubic meter of air. At 80% relative humidity, the moisture content is 52.88 x 0.80 = 42.3 milliliters per cubic meter of air. If this air is compressed to 689-kPa pressure and cooled to 27°C, the air is saturated and the water content that the air can contain is only 3.4 milliliters per cubic meter of air. This is equivalent to a removal of 38.9 (42.3 minus 3.4) milliliters of water per cubic meter of air. This translates to 66 liters/hr for a 28.3 m3/min capacity air compressor. Moisture content in air more than doubles from -17.8°C to -6.7°C and continues to double for each increase of 11°C (see Table 2-7).

2-56


Different types of compressed air dryers are available and each has different characteristics and degrees of dew point suppression. Liquid moisture must be removed before the air enters the dryer. This can be accomplished with a moisture separator and/or a coalescing filter. Key Technical Point Most dryers are rated at what is commonly referred to as the three 100s, that is, 100 psig (689 kPa), 100°F (37.8°C), and 100% relative humidity (saturated air). An increase in operating pressure (within the capability of the dryer) increases the rated mass flow capacity. An increase in inlet temperature (within the capability of the dryer) decreases the rated mass flow or capacity because of the higher moisture content at a saturated condition. Typical capacity correction factors are given in Table 2-8. Table 2-8 Pressure and Temperature Correction Factors for Compressed Air Dryers [3] Inlet Pressure (psig) (kPa)

60 (414)

80 (552)

100 (689)

120 (827)

140 (965)

Percent Rated Capacity

89

95

100

104

107

Inlet Temperature °F (°C)

80 (27)

90 (32)

100 (38)

110 (49)

120 (60)

Percent Rated Capacity

157

123

100

82

64

The most important factor in determining a dryer system is the degree of drying needed. If different air uses require different degrees of drying (such as instrument quality air supplied from a general purpose plant air system) it is possible to provide extra drying only to those parts of the system that require it. Key O&M Cost Point Because more effective drying is generally more expensive (from initial cost and operating cost perspectives), it is important not to over-dry a system. Degree of dryness is specified by dew point temperature. Dew point temperature is the temperature at which vapor will begin to condense into liquid. A lower dew point corresponds to drier air. To avoid system-wide water condensation, adequate drying must provide air with a dew point lower than the lowest air temperature in the distribution system. The selection of a compressed air dryer should be based on the required pressure dew point and the estimated cost of operation. Where a pressure dew point of less than 35°F (1.7°C) is required (such as for instrument air), a refrigerant-type dryer cannot be used. The required pressure dew point for the application at each point of use eliminates certain types of dryers. Because dryer ratings are based on saturated air at inlet, the geographical location is not a concern. The dryer has a lower load in areas of lower relative humidity, but the pressure dew point is not affected. A chart is shown in Figure 2-32 to assist in finding the dew point at atmospheric pressure and at pressure. 2-57


Figure 2-32 Atmospheric Versus Pressure Dew Point Conversion Chart [3]

2-58


Typically, the pressure drop through a compressed air dryer is 3–5 psig (20.7–34.5 kPa) and should be taken into account in system requirements. There are four popular types of dryers: •

Regenerative desiccant

•

Refrigerant

•

Deliquescent

•

Membrane

Each air dryer type has different drying abilities. More effective dryers are usually more expensive and have greater operating costs. Deliquescent and membrane dryers are not common in power plant air systems. However, plants can use portable systems with these dryers, so they are included in this section. 2.4.3.1

Regenerative Desiccant

To maintain air quality that consistently meets the ANSI/ISA 7.0.01-1996 dew point criteria, a regenerative desiccant dryer (heatless or heated) is recommended. Regenerative desiccant dryers can consistently maintain the dew point at -40ºF (-40ºC) or below at line pressure with 100ºF (37.8ºC) saturated inlet air. This is well within the ANSI/ISA specifications. Desiccant dryers are the most effective type of air dryers, typically supplying air with dew points of -40ºF (-40ºC) or lower, depending on the desiccant substance used. Key Technical Point For colder climates, freezing in outside lines is a problem. One solution is to install a regenerative desiccant dryer with a lower pressure dew point such as -40ºF (-40ºC). Three types of desiccant dryers are: •

Heat of compression

•

Heated regenerative

•

Heatless regenerative

The regenerative desiccant dryers are used in applications that require compressed air at dew points as low as -150°F (-101°C). With two identical drying towers, each containing a desiccant bed, air flows through each tower alternately. While one tower is on-stream drying, the other is off-stream and being regenerated. Purged air is used to regenerate the desiccant. A typical compressed air system with a desiccant dryer is shown in Figure 2-33.

2-59


Figure 2-33 Compressed Air Drying System With a Desiccant Dryer [5]

The desiccant dryer uses a porous desiccant substance that adsorbs the moisture by collecting the moisture in its pores. This process allows large quantities of water to be retained by a relatively small quantity of desiccant. Adsorbing is the process of condensing and holding water vapor on the surface of a substance. Desiccants are regenerated (a process that removes the adsorbed moisture) with heat and low-pressure dry air. The released moisture is carried out of the drying chamber. Dryers of this type use twin towers that allow one tower to dry the airflow while the other tower is reactivated. The adjustable purge/re-pressurization times that are available on some desiccant dryers prove to be an economical way to adjust to variable air flow. 2-60


The diameter and length of desiccant beds determine drying efficiency. Bed diameter controls air velocity through the bed. If the velocity is too high, the desiccant will float or fluidize, causing desiccant degradation. Bed length determines consistency of the dew point. To reach the proper outlet dew point, the bed must be long enough to ensure sufficient contact time between the wet air and the dry desiccant. Key Technical Point Channeling occurs when an air stream finds a path through the bed and follows the path instead of distributing evenly throughout the bed. This is often a problem with desiccant dryers. Channeling can be avoided by using stainless steel diffusers in the inlet and outlet of the desiccant towers and controlling the air velocity through the desiccant bed. Desiccant dryers are either cold (heatless) regenerative or heat regenerative. In heatless regenerative dryers, 15% of dried, unheated purge air is diverted from the air outlet and is used to regenerate the desiccant. In heated regenerative desiccant dryers, purge air is heated to 300°F– 400°F (149°C–204°C) and directed through one of the desiccant towers. Depending on the heated dryer type (internal heated, externally heated, blower purge, etc.), only a small percentage of 1 to 7% of purge air is diverted from the dried air stream. Valuable purge air is saved, reducing operating costs up to 40% in applications over 500 cfm (0.2 m3/sec). Most standard regenerative desiccant-type compressed air dryers provide a pressure dew point of -40ºF (-40ºC), which is equivalent to -72ºF (-57.8ºC) at atmospheric conditions. For heatless dryers, purge air for regeneration is taken from the air already dried. The amount of purge air required can vary from 10–15% of the air flow passing through the dryer, thus reducing the air flow available to the system from the compressor by this amount. Estimated operating cost (excluding pressure drop through the dryer) is 1.8–2.7 kW/100 cfm (0.63–0.95 kW/m3/min). The most commonly used desiccants are: •

Silica gel

•

Activated alumina

•

Molecular sieve

Silica gel is available in both granular and beaded form. In beaded form, it has a high resistance to attrition and produces little or no fines. The ability of silica gel to achieve -40ºF (-40ºC) dew points at 100 to 125 psig (689 to 862 kPa) line pressure, and its low regeneration temperatures, make it an extremely cost-effective medium for instrument air drying applications. Lower dew points can be obtained at less cost using activated alumina; however, it is susceptible to a greater percentage of fines and it requires a slightly higher regeneration temperature. Molecular sieve has the capability to adsorb small molecules while excluding the large ones. Therefore, it should only be used as a polishing or trimming agent in a mixed bed operation. Use of molecular sieve as a primary agent might not remove sufficient moisture in dryers with small 2-61


desiccant capacities. Also, the regenerative temperature for molecular sieve is higher than that for silica gel or activated alumina. Table 2-9 shows the various desiccant types available and the reason to use each type. Table 2-9 Desiccant Types [2] Dryer Type Heated Regenerative

Heatless Regenerative

Deliquescent

Desiccant Type (In order of preference)

Characteristics

1. Beaded silica gel

Most resistant to attrition and lower regeneration costs

2. Beaded silica gel with water blocker

Used when in direct contact with water to resist fracturing

3. Activated alumina

Lowest initial cost

4. Molecular sieve

Used in a mixed bed as a trim or polishing desiccant only

1. Activated alumina

Most cost-efficient for heatless dryers

2. Beaded silica gel

Used to control attrition only

3. Beaded silica gel with water blocker

Used when in direct contact with water to resist fracturing

4. Molecular sieve

Used in a mixed bed as a trim or polishing desiccant only

Water soluble salts or organic compounds

Limited use

In some cases, more than one desiccant type can be used for special drying applications. In most cases, a larger particle size dessicant (1/4 in. [6 mm] or more) is used as a buffer zone at the inlet, while a smaller particle size desiccant (1/8 in. [3 mm] to 1/4 in. [6 mm]) is used for final drying. Where very low effluent dew points are required, a molecular sieve desiccant is added as the final drying agent. If the dryer manufacturer permits, the recommendations in Table 2-9 and the following guidelines can be used in desiccant selection: •

On heat regenerative dryers, silica gel beads can be used to conserve power. Silica gel beads regenerate at lower temperatures than activated alumina.

•

On heated or heatless regenerative dryers, if desiccant fines are causing particulate problems, silica gel beads might mitigate this situation. Double sieving of the dryer desiccant compartment will also aid in resolving this problem. Silica gel beads have a high fracture point.

•

On heatless regenerative dryers that are not experiencing problems with fines, use of activated alumina beads will be economical due to their ability to retain a greater amount of moisture at a lower cost than silica beads.

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•

On systems experiencing water intrusion into the dryers, use of ordinary silica gel beads is not recommended. In fact, regenerated silica gel beads are likely to shatter if they come in contact with water droplets. In such situations, specially processed water-resistant beads (water blockers) can be used as a layer of buffer desiccant near the point of air inlet. However, it is important to eliminate the source of water intrusion into the dryer as soon as possible. Major manufacturers of silica gel beads can provide assistance in selecting desiccant for mixed bed applications.

The various desiccants are listed in order by cost as follows (with the most expensive listed first and then decreasing in cost): •

Silica gel beads (water blockers)

•

Granular silica gel/silica gel beads

•

Molecular sieve

•

Activated alumina

It is highly recommended that before selecting a desiccant, the dryer vendor and a desiccant manufacturer be contacted to recommend the best possible desiccant available for the particular application. Recent developments in desiccant manufacturing have made new products available that might not have existed when the original dryers were purchased. It should also be considered that in a heated regenerative system, frequency of regeneration influences the aging of the desiccant. Therefore, if frequent regenerations occur either due to system leaks or low capacity dryer trains, desiccants might have to be replaced earlier than their recommended life. A picture of some regenerative dryers is shown in Figure 2-34.

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Figure 2-34 Sullair Regenerative Dryers

2.4.3.1.1

Heat-of-Compression Dryer

Heat-of-compression-type dryers are regenerative desiccant dryers that use the heat generated during compression to accomplish desiccant regeneration. The most common type has a rotating desiccant drum in a single-pressure vessel divided into two separate air streams. This is illustrated in Figure 2-35.

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Figure 2-35 Heat-of-Compression-Type Dryer (Courtesy of Atlas Copco Compressors, Inc.)

Most of the air discharged from the air compressor passes through the air after-cooler where the air is cooled and the moisture is condensed. The cooled air stream, saturated with moisture, then passes through the drying section of the rotating desiccant bed (a) where it is dried and exits from the dryer (b). A portion of the hot air taken directly from the air compressor at its discharge before the after-cooler, flows through the opposite side of the dryer drum (c) to regenerate the desiccant bed. The hot air, after being used for regeneration (d), passes through a regeneration cooler before being combined with the main air stream by means of an ejector nozzle before entering the dryer. This means that there is no loss of purge air.

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Drying and regeneration cycles are continuous as long as the air compressor is in operation. This type of dryer requires air from the compressor at sufficiently high temperatures to accomplish regeneration. For this reason, it is used almost exclusively with oil-free rotary screw compressors. 2.4.3.1.2

Heat Regenerative Dryer

Figure 2-36 shows a schematic of a heat regenerative desiccant dryer.

Figure 2-36 Heat Regenerative Desiccant Dryer [3]

The moist inlet air enters the dryer through the inlet switching valve (A) and is routed to either the left or right tower (B). While the air flows through the tower, the moisture is adsorbed onto the desiccant. After drying, the air leaves via the dryer outlet (C). Regeneration of the exhausted tower is accomplished by heating the desiccant. Desiccant can be heated by using embedded heaters (D) or by passing externally heated air over the desiccant bed. When the desiccant is heated, the moisture is desorbed from the desiccant. In the case of externally heated air, the flow of air carries the moisture away. In the case of internal heaters, the moisture is removed from the dryer by using approximately 5% of the dry air as reactivation air (purge air). This reactivation air is routed back through the dryer and exits through the reactivation air outlets (E) to atmosphere. This type of dryer usually operates on an 8-hour cycle. With the embedded heaters, sometimes hot spots can occur in the desiccant bed. Desiccant beads subjected to these high temperatures can crumble, creating fines. Therefore, it is important to note the maximum temperature of the bed during regeneration and the capability of the desiccant to withstand this temperature. 2-66


Normally, the desiccant is contained in two separate towers. Compressed air to be dried flows through one tower, while the desiccant in the other is being regenerated. Regeneration is accomplished by reducing the pressure in the tower and passing previously dried purge air through the desiccant bed. The purge air can also be heated to reduce the amount of purge air required. Dryers of this type normally have a built-in regeneration cycle, which can be based upon time, dew point, or a combination of the two. 2.4.3.1.3

Heatless Regenerative Dryer

A schematic of a heatless regenerative dryer is shown in Figure 2-37.

Figure 2-37 Heatless Regenerative Desiccant Dryer [3]

In heatless regenerative desiccant dryers, the moist inlet air enters the dryer through the inlet switching valve (A) and is routed to either the right or left tower (B). While the air flows through the tower, the moisture is adsorbed onto the desiccant. After drying, the air leaves the dryer outlet (C). Regeneration of the off-line tower is accomplished by taking 15% to 20% of the dry outlet air and routing it through the off-line tower and then to the atmosphere through the purge muffler (D). Regeneration and drying cycles are controlled by using timers or moisture probes (E). The moisture probes sense the moisture content of the desiccant. The purge control throttle valve is adjusted to ensure the proper flow rate of the purge air throughout the regenerating tower for optimum regeneration efficiency. The tower re-pressurization valve provides for a slow repressurization of the regenerated tower before it is placed back on line. This reduces the pressure surge in the tower and breakdown of the desiccant. 2-67


The control air isolation valve isolates the air to the control panel. These dryers usually operate on 10-minute cycles. This type of dryer might starve the distribution system in plants with marginal capacity, due to their high purge air requirements. Key Technical Point Before installing a heatless desiccant type dryer, ensure that the compressors have 15 to 20% excess capacity. 2.4.3.2

Refrigerant Dryer

Refrigeration-type air dryers cool the incoming air, which causes water vapor to condense out of the air stream. The resulting liquid is removed by a separator and trap. Many dryers of this type use heat exchangers to pre-cool incoming air with the cold discharge air. The dew point level of the outgoing air can be as low as 35ºF (1.7ºC). Key O&M Cost Point The refrigerant-type dryer is the most economical type of air dryer. Refrigeration dryers require additional energy, up to about 6% of that used by the compressor. These costs can be lowered with the use of a control system that modulates the dryer to match the compressor’s output. These controls are an added expense, but in many systems the cost will be recuperated in energy savings. Newer refrigerant types are more efficient, with energy savings that can often justify their purchase. Non-cycling and cycling are the two types of refrigerated dryers. On a non-cycling dryer, the refrigeration compressor operates continuously regardless of dryer load. A thermostatic expansion valve and hot gas bypass valve regulate the flow of refrigerant into the heat exchanger to maintain dew point and minimize freeze-up. Because the unit uses full input power at all times, a non-cycling dryer should be selected for systems with a constant airflow. In cycling dryers, the refrigerant cools an intermediate fluid, which cools and dries the air. During low-load operation, the refrigeration circuit stops its compressor and restarts it when the fluid temperature rises. The cycling-type dryer conserves energy and minimizes dryer freeze-up making cycling dryers the choice with fluctuating air flow and inlet temperatures. Over-sized cycling dryers provide additional drying capacity for future air system upgrades.

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A schematic of a refrigerant dryer is shown in Figure 2-38.

Figure 2-38 Refrigerant Dryer [3]

In refrigerant dryers, moist inlet air enters the dryer pre-cooler (A) and is cooled by the dry air leaving the dryer. The moist air then enters the evaporator (B), where it is cooled by the refrigerant. Most of the moisture is condensed and removed in the evaporator. Air leaving the evaporator is 33 to 35ºF (0.5 to 1.6ºC). Any excess moisture entrained in the air condenses in the separator (C) and is removed by using a drain trap (D). The dry air leaves the separator and returns to the pre-cooler (A), where it is heated by the moist inlet air before distribution. The air will remain dry throughout the system as long as it is not cooled below the evaporator temperature. Refrigerant dryers do not work well if the distribution system is exposed to freezing temperatures. When the instrument air temperature drops below freezing, the moisture forms sleet or ice crystals in the air line and causes a loss of air supply to end-use components. Refrigerant-type dryers can only provide dew points of 35ºF to 50ºF (1.6ºC to 10ºC), but ANSI/ISA 7.0.01-1996 does not allow dew points that exceed 35ºF (1.6ºC) at any time. For this reason, the use of refrigerant-type dryers located before the air distribution to end-use components is not recommended. 2-69


2.4.3.3

Deliquescent Dryer

A deliquescent air dryer uses a water vapor absorbing chemical to remove moisture from the air. In the process of absorbing the water, the chemical changes from a solid to a liquid that drains from the bottom of the dryer vessel. Disposal of the chemical/water mix (which is corrosive) can be an added system expense. These dryers will lower the air’s dew point by 20ºF to 36ºF (-6ºC to 2.2ºC). Assuming an inlet temperature of 100ºF (37.8ºC), the resulting dew point is between 64ºF and 80ºF (17.8ºC and 26.7ºC). A deliquescent-type dryer will be adequate if air with a relatively high dew point can be used in the plant. The deliquescent-type dryer uses no energy, but the deliquescent chemical must be replenished periodically to maintain the effectiveness of the dryer. Deliquescent chemicals have a corrosive action that can damage the distribution system, so additional filtration is required to remove any deliquescent that enters the air stream. This extra filtration adds additional pressure drop, indirectly raising the system’s energy costs. Unlike desiccant-type dryers, the drying medium in deliquescent-type dryers absorbs (rather than adsorbs) the moisture in the compressed air. This means that the desiccant medium is used up as it changes from a solid to a liquid and cannot be regenerated. The most common deliquescent chemicals for compressed air drying are salts of sodium, potassium, calcium, and those that are urea-based. Various compounds of these have been developed and sold under a variety of trade names. A dryer of this type is shown in Figure 2-39.

Figure 2-39 Deliquescent Dryer (Courtesy of Hankison International)

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Deliquescent dryers are sensitive to the saturated air inlet temperature. The lower the inlet temperature, the smaller the dew point suppression. The deliquescent dryers are limited to a maximum inlet temperature of 100ºF (37.8ºC). Because the drying medium is consumed and not regenerated, there is no requirement for purge air. Therefore, the pressure drop through the dryer (and any associated filtration) and loss of air volume during the drain cycle are the operating costs, which are estimated at 0.2 kW/100 cfm (0.07 kW/cm/m). 2.4.3.4

Membrane Dryer

Membrane technology has advanced considerably in recent years. When used as a dryer in a compressed air system, specially designed membranes allow water vapor (a gas) to pass through the membrane pores faster than the other gases (air). This reduces the amount of water vapor in the air stream at the outlet of the membrane dryer and suppresses the dew point as illustrated in Figure 2-40.

Figure 2-40 Membrane-Type Dryer (Courtesy of Hankison International)

The dew point achieved normally is 40ºF (4.4ºC), but lower dew points can be achieved at the expense of additional purge air loss. Purge air, or sweep air, displaces the moisture that passes through the permeable membranes. For comparable dew point depression with a refrigerant-type dryer, this is in the range of 15–20% of the air capacity of the dryer. Therefore, 15–20% should be added to the air compressor’s required capacity and power requirement. This amounts to 2.7–3.6 kW/100 cfm (0.95–1.3 kW/cm/m). 2.4.4 Air Filters As previously stated, compressed air leaving an air compressor is not normally of a quality suitable for the intended use. The air inlet filter for the air compressor is intended to protect the compressor but is often inadequate to protect downstream equipment. The compressor itself can add contaminants, including wear particles, carbon deposits, and lubricant. These contaminants require filtration. Filters are used throughout an air system, wherever solid, liquid, or gaseous particles need to be removed [2,3,7,13]. Usually, a compressor system has an inlet filter to reduce particle build-up in the compressor. Air leaving the compressor usually is filtered to eliminate oil, water, and other 2-71


particles. Filters can also be found before air tools, removing any solids generated in the distribution system. Filter effectiveness, which determines how well the filter cleans the air, has the largest effect on pressure drop. Every filter creates a pressure drop as air passes through it. Inlet filters with large pressure drops can reduce both the compressor’s output and its efficiency. Air-line filters can be the largest source of pressure drop in an air system. Key O&M Cost Point The air stream should not be filtered to a greater degree than the plant requires. More effective filters create more pressure drop and the filters are more expensive. Often, an oversized filter that is rated to handle a larger amount of air can reduce the amount of pressure drop by permitting a less obstructed airflow. Prudent selection of equipment in terms of effectiveness and size can reduce pressure drop, resulting in meaningful energy savings. With any degree of filtration, periodic maintenance that consistently cleans or replaces the filtering element is necessary to keep the pressure drop low. A wide variety of filters are available for compressed air systems. The most basic filtration is provided by a filtered centrifugal separator that combines separation and filtration in one housing. Using principles of centrifugal force and impaction, a well-designed filter’s first stage is 99% efficient in removing particles that are 10 microns in size and larger. The second stage is a replaceable filter sleeve that removes solids and liquids down to 3 microns in size through coalescence. Key Technical Point Filtration literature might refer to an absolute rating or a nominal rating. The absolute rating refers to the diameter, in microns, of the largest particle that will pass through the filter. This is theoretical because it requires a consistent pore size of the filter medium, which is not always practical, and particle sizes are not always spherical. The nominal rating is determined by the filter manufacturer and is an arbitrary value. Usually it is a percent retention by weight of a specified contaminant of a given size. Filters that are used in the compressed air system are: •

Intake filters

•

Air dryer pre-filters and after-filters

•

Lubricating oil filters

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2.4.4.1

Filter Selection

Filters are of four basic types: •

Particulate filters

•

Coalescing cartridges

•

Adsorption filters

•

Air-line filters

In choosing a filter, have the vendor prescribe a nominal filter that will remove all of the particulate that needs to be removed, while still meeting system flow requirements. Absolute filters are very expensive and might not adequately support flow. Dry cartridge-type filters have fibrous filter sections and are often coated with epoxy resin. If correctly sized, these filters can remove particulate down to 0.01 microns without excessive pressure drops in the system. The choices for intake filters are: •

Particulate (dry)-treated paper or felt cloth – nominal 10.0 micron

•

Oil bath – nominal 5.0 micron

•

Viscous impingement – nominal 10.0 micron

The choices for lubricating oil filters are: •

Canister, automotive type

•

Duplex strainer

•

Duplex strainer, magnetized rod

Instrument air dryers need to be fitted with pre-filters and after-filters to remove contaminants such as particulate, moisture, and hydrocarbons that are harmful to the dryers and the end-use components. The pre-filters protect the dryers and the after-filters ensure air specifications are maintained in the distribution system. Filter selection is determined by component selection and the system arrangement. In addition to plant-specific factors, the following list should be considered when selecting air dryer pre-filters and after-filters: •

The type of compressor selected and whether or not it is oil-free

•

The filtration capability of the intake filter

•

Receiver location in relation to dryers

•

The type of after-cooler, air- or water-cooler, and its efficiency

•

The presence of separators before the dryer system 2-73


•

The type of dryer installed

•

The type and condition of the desiccant

•

The end-use specification of the instrument air

•

Whether instrument air is used as breathing air or for ventilation

The choices for the dryer pre-filters are: •

Coalescing filter – 1.0 to 0.1 micron 95% + water retention

•

Particulate filter – 3.0 microns absolute

•

Particulate filter – 10.0 microns nominal

The choices for the dryer after-filters are: •

1.0- to 0.1-micron particulate filter

•

3.0-micron absolute to 0.025-micron particulate filter

•

High-temperature filter

High-temperature after-filters are primarily designed as after-filters to heated desiccant dryers. They can also be used wherever large amounts of solid particles are present in dry air. They can hold a large number of desiccant fines without plugging. Large dust particles settle in the bottom of the filter housing as a result of a reduction in velocity and a sharp shift in direction. Mediumsized particles are collected on the surface of a layer of glass fabric cloth, while fines 1 micron and smaller in size are collected on beds of in-depth fiberglass media. On refrigerant dryers, after-filters are only required where the processed air is above desired particulate specification. An absolute 3.0-micron after-filter will be sufficient to meet the ANSI/ISA specification for particulate count. After-filters for desiccant dryers should be dry filters with a filtration capability of capturing particulate to 1.0 micron nominal and 3.0 absolute. This permits the capture of desiccant fines. If tests show that many particles are in the 3.0-micron range, a finer mesh filter should be used. Desiccant fines cause service damage to end-use components. A final coalescer or adsorber can now be used to gather hydrocarbon mists. This filter is usually installed last in the order of desirability for cost reasons. A coalescent filter element is more expensive than a regular particulate filter. Placing it before the particulate filter would require more frequent coalescent filter element replacement than particulate filter change-out, thus increasing maintenance costs. All of the major manufacturers of filters can supply the filter type and size required. For a summary of the recommended filters, refer to the filter selection in Table 2-10.

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EPRI Licensed Material Tutorial Table 2-10 Filter Selection Chart [2] Component

Filter Type

Compressor Types:

Intake Filter:

Reciprocating, lubricated cylinder compressor

Oil bath, viscous impingement

High-dust area

Oil bath

Low-dust area

Viscous impingement

Reciprocating, non-lubricated cylinder

Particulate (dry) treated paper or felt cloth

Rotary screw, oil free


Rotary screw, flooded

Particulate (dry) treated paper or felt cloth, oil bath and viscous impingement

Centrifugal


Lube Oil System:

Lubricating Oil Filter:

Internal lube oil system

Canister, automotive

External lube oil system

Duplex strainer

-If unit has steel reduction gears

Duplex strainer, magnetized rod

Dryer Systems:

Dryer Pre-Filters:

Compressors without receivers or with dryers before the receivers

Coalescing filter

Compressors with Receivers

Coalescing filter and the 3.0-micron absolute and 10.0-micron nominal particulate filters

Refrigerant dryer

Coalescing filter and 3.0-micron absolute particulate filter

Desiccant dryer

Coalescing filter and 10.0-micron nominal particulate filter

Dryer Types:

Dryer After-Filters:

Refrigerant

3.0-micron particulate filter

Desiccant

1.0-micron nominal and 3.0-microns absolute

Heated desiccant

High temperature

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2.4.4.2

Particulate Filters

Particulate filter designs use different filtering mechanisms to achieve the desired degree of contaminant removal. The higher the degree of contaminant removal then the higher the pressure drop across the filter, the higher the pressure required from the air compressor, and the higher the energy costs. The maximum allowable pressure drop before the filter element is changed (usually 10 psig or 69 kPa) must also be taken into account when determining the pressure required at the air compressor discharge and downstream of the drying and filtration equipment. A differential pressure gauge is recommended for each filter. A particulate filter is recommended downstream of the air dryer, before any operational equipment or process. Figure 2-41 shows the mechanisms of filtration.

Figure 2-41 Mechanisms of Air/Gas Filtration (Courtesy of Domnick-Hunter, Inc.)

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2.4.4.3

Coalescing Filters

Coalescing filters contain masses of microfilament that entrap the moisture and oil mists, in addition to filtration of the solid particulate. Moisture can be reduced to 0.02 ppm at about 70ºF (21ºC), and to about 1.0 ppm at 100ºF (37.8ºC). The most common use of the coalescing filter is as a pre-filter, protecting dryers from moisture carryover and hydrocarbon mist. When used with a refrigerant dryer, it should remove all particulate greater than 10.0 microns and all moisture (99.9% +) at 1.0 micron. Entrapment stops aerosols and oil mists. Less than 1.0 ppm of aerosols and oil mists remain in the air stream. This affords the greatest protection to the refrigeration dryer, while limiting the fouling and corrosion caused by the liquid entrained in the air. When used with a desiccant dryer, the coalescing pre-filter should remove particulate greater than 15.0 microns and all moisture (99.9%+) at 0.3 micron. Entrapment eliminates aerosols and oil mists to less than 0.1 ppm. Coalescing oil removal filters will remove downstream oil aerosols that can contaminate end products, ruin paint jobs, and gum up air tools. In a typical 100 psig (689 kPa) air system, 72% of the oil aerosols by weight are less than 5 microns in size and 50% are below 1 micron. These size droplets will pass through a mechanical separator and cannot be completely removed by an air-line filter. A good coalescing oil removal filter has a liquid oil removal rate of over 99.999%. Air is directed through a maze of sub-micronic glass fibers where the oil aerosols are coalesced into large droplets and continuously removed. Small droplets of moisture or oil are caught by the filter medium and coalesce into larger liquid droplets. Airflow from the inside to the larger diameter outside of the element allows a lower exit velocity and is shown in Figure 2-42.

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Figure 2-42 Coalescing Filter (Courtesy of Domnick-Hunter, Inc.)

An optional anti-re-entrainment barrier prevents droplets from being re-introduced into the air stream. The coalesced liquid runs down by gravity to the bottom of the filter bowl and is drained, usually by an automatic drain. The liquid can contain both oil and water. An increase in pressure drop in a coalescing filter is normally due to particulate matter fouling the element. The coalescing filter should be preceded by a particulate filter. The rated pressure drop should be the wet pressure drop after the element has become saturated. A coalescing filter is recommended before any dryer whose drying medium might be damaged by lubricant. Materials should be compatible with the type of lubricant being used.

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2.4.4.4

Adsorption Filters

Oil vapor adsorbers are final-stage filters that adsorb oil vapor by passing the compressed air through two levels of activated carbon. The filters remove oil vapor, smell, and taste, as well as any other gaseous hydrocarbons normally adsorbed by activated carbon. They should be installed after the oil-removal filter because liquid oil aerosols will prematurely saturate the activated carbon and significantly reduce adsorptive capacity. Particulate and coalescing filters are capable of removing particles down to 0.01 microns, but not of removing oil vapors or odors. Adsorption involves the attraction and adhesion of gaseous and liquid molecules to the surface of the medium. Most filter elements contain activated carbon granules that have an extremely high surface area and filter time. This medium is for the adsorption of vapors only and is shown in Figure 2-43. An adsorption filter must be protected by an upstream coalescing-type filter to prevent liquid oil contamination and liquid condensate.

Figure 2-43 Adsorption Filter (Courtesy of Domnick-Hunter, Inc.)

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2.4.4.5

Air-Line Filters

Air-line filters are designed to remove liquid aerosols, rust, scale, dirt, and other solid particles that are 1.0 micron and larger. Air-line filters operate with two stages: a first stage of coarse media collects large particles, and a second stage of finer media separates dirt, water, and oilaerosols. Air-line filters are very versatile filters and have application in systems using both oillubricated and non-lubricated compressors. In oil-lubricated systems, a good filter can also be used as an oil-removal filter with over 70% efficiency or used in combination with other filters to remove particulates. In non-lubricated systems, they are used upstream and downstream of the desiccant dryers. A picture of an air-line filter is shown in Figure 2-44.

Figure 2-44 Air-Line Filter [7]

2.4.5 Inter- and After-Coolers On multi-stage compressors, a heat exchanger is usually provided between each stage of compression. These heat exchangers are called inter-coolers. They provide interstage cooling of the air to reduce the volume to be compressed in the succeeding stages and to remove condensed moisture and oil droplets through heat exchanger drains.

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On both single- and multiple-stage compressors, a heat exchanger called an after-cooler is installed after the final stage of compression. The after-cooler provides for cooling of the compressed air and removes condensed moisture and oil droplets from the air through a drain on the heat exchanger. Both inter-coolers and after-coolers [2,6,13] can be air-cooled on small compressors. They are generally water-cooled on larger compressors used in power plants. The effectiveness of the heat exchangers depends on the quantity and temperature of the cooling medium that comes in contact with the cooling surfaces. Therefore, it is essential that the flow of water through water-cooled heat exchangers be unrestricted. The source of the cooling water will dictate the cooler maintenance requirements. Using water from a closed cooling water system is preferable. A closed cooling water system will not typically contain mud, debris, and biofouling organisms. Using an open service water source can introduce foreign material into the tubes. When using service water as the cooling medium in the coolers, the water should be used on the tube side of the cooler. It will be necessary to clean the tube side of the coolers periodically. If the service water is used on the shell side of the coolers, it would be difficult to clean that area. Matching the pressure of the service water to the design of the coolers is very important. It might be necessary to reduce the service water pressure to use in the coolers. The temperature of service water will change with the seasons. If manual throttling is used to control the cooler temperatures, then valve position changes should occur as the seasonal water temperatures change. Cooling water flow can be monitored by installing flow or differential pressure gauges. The cooling water inlet temperature should also be maintained within the specified range. Similarly, cooling fins on an air-cooled heat exchanger should have unrestricted access to cooling air flow. An abnormal increase in the discharge air temperature or moisture content might indicate: •

Clogged heat exchanger tubes or closed circulation water valves

•

Higher-than-usual cooling water temperature

•

Inoperative drain valves or traps

•

Dirty air-cooling fins or otherwise restricted air circulation

•

High ambient air temperature for air-cooled heat exchangers

If steps are not taken to restore proper operation of the heat exchanger, heated air will pass into the air receiver where cooling and condensation will finally occur. This might ultimately result in moisture carryover and tank corrosion.

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2.4.5.1

Inter-Cooler

The function of the inter-cooler is to remove the heat of compression from the air after it leaves the first stage compressor cylinder and before it reaches the second stage of compression. This reduces the air volume to be compressed in the second stage and improves compression efficiency. At the same time, any moisture carried in the air is condensed and removed. The most common cause of inter-cooler inefficiency is the use of city, or hard scale-forming, cooling water. Any coating, either in the tubes or on the outside, will lower the heat transfer. The tubes should be inspected at frequent intervals and cleaned if necessary. The shell side of the heat exchanger should also be cleaned periodically. 2.4.5.2

After-Cooler

During the compression process, a great amount of heat is produced in the air. Air at this temperature is usually unusable in the plant and would increase general deterioration of the entire compressor system. After-coolers are heat exchangers that cool the air immediately after it is discharged from the compressor. This allows for the reduction of liquid moisture. Air-cooled after-coolers are more economical to operate than water-cooled units, and can also be piped so that waste heat can be used for greater savings. To reduce the air temperature, an after-cooler conditions the air leaving the compressor. Cooler air is more dense. Therefore, one benefit of an after-cooler is the storage of a greater amount of air in the compressed air system. This allows demand to be supplied more effectively. An aftercooler also removes some water from the air because water vapor condenses into liquid when the air is cooled. The after-cooler should be able to handle the worst potential air conditions and still deliver air that is suitable for the end user or a dryer. In an after-cooler, compressed air is cooled to within 5°F to 20°F (-15°C to -6°C) of ambient air temperature. As the air cools, up to 80% of the water vapor present condenses to a liquid and can be removed from the system. The type of coolant, after-cooler size, ease of maintenance, and control involved all play a factor in defining a cost-efficient after-cooling system. These systems are either liquid- (usually water) or air-cooled. The availability, effectiveness, and cost of the coolant determine what type is appropriate. Air coolant is more efficient with heat recovery systems that can use heat from compression for other purposes in the plant. A controller that monitors temperature and automatically regulates coolant flow can ensure that excessive coolant is not used. When water is the coolant, such a system can often pay for itself within a year. The size of the after-cooler determines the resulting air temperature. An oversized after-cooler is usually cost-effective; it will produce cooler air with less moisture and reduce the requirements of a following dryer. Additionally, a larger after-cooler will have a lower pressure drop, thus lowering the necessary discharge pressure from the compressor. 2-82


Proper maintenance will keep the after-cooler efficient. A dirty after-cooler results in both warmer air temperature and increased pressure drop. To maintain the integrity of an after-cooler, efforts should be made to monitor: •

The operability of drain traps (by monitoring the trap levels and the need to use the bypass valves)

•

Cooling water inlet and outlet temperatures (comparing these to baseline data can identify reduced heat transfer capabilities)

•

Air leakage through the traps

The after-cooler should be located in the compressor discharge line between the compressor and the receiver, and as near to the compressor as possible. The preferable location is in the compressor room where the operation can be monitored and controlled readily. In any location it must be protected from freezing temperatures and enough room must be provided for dismantling and cleaning. Full-size pipe connections between the compressor and the aftercooler must be used to prevent pulsations in the pipeline. A bypass pipe should be installed so that the after-cooler can be removed for cleaning without shutting down the system. In most types of after-coolers, the cooling water enters at the bottom and discharges at the top. Cooling water should be regulated so that the discharge air is cooled to within 15ºF (-9.4ºC) of the temperature of the inlet water. For example, if the inlet water is 50ºF (10ºC), then the outlet air temperature should not exceed 65ºF (18.3ºC). If this limit of cooling cannot be achieved with full cooling water flow, it might indicate tube fouling or an undersized heat exchanger. The colder the inlet water, the cooler the air will be leaving the after-cooler, and the more moisture will be removed from the air. In colder climates, when water-cooled compressors are shut down, the water supply lines can freeze. It might be necessary to install heat tracing on these lines. 2.4.6 Separators and Traps It is necessary to provide a method for removing condensed moisture or oil droplets collected at various points in the compressed air system. Some manufacturers use moisture separators, and others use drain traps [2,3,13]. Both should be designed to have minimum impact on air system pressure and capacity. 2.4.6.1

Moisture Separators

Because moisture removal in the after-cooler is only as effective as the after-cooler’s ability to lower the air temperature, any additional moisture is removed by the use of moisture separators. The moisture separators are usually installed in the air line between the after-cooler and the receiver tank. They can also be installed prior to the installation of components that require additional moisture protection.

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Most separators designed for instrument air use centrifugal force or redirection of flow to throw out water droplets. Others use felt disks, rotating elements, or porous stone disks to separate the moisture from the air. Some designs employ the use of a water-cooled condenser inside the separator making it similar to another after-cooler. Plants that do not have moisture separators prior to the air receiver rely more heavily on the receiver to act as a moisture separator. This can cause accelerated corrosion of the receiver walls and, if the receiver walls fall below minimum wall thickness requirements, this can become a safety hazard. An oil/water separator works in the following manner: •

Condensate enters the inlet chamber and expands

•

Liquid separates by centrifugal motion and drains into the primary tank

•

Dirt settles at the bottom of this tank

•

Condensate flows into the main tank

•

Oil separates from the water and coalesces at the surface of the main tank

•

Oil is removed and collected in an external container

•

Water passes through the pre-filter(s) for bulk contamination removal

•

Water passes through the carbon filter(s) to remove any remaining oil traces

•

Clean water is discharged from the unit

A picture of an oil/water separator is shown in Figure 2-45.

Figure 2-45 An Oil/Water Separator (Courtesy of Sullair Corporation)

Appendix F contains more considerations for the disposal of air compressor condensate. 2-84


2.4.6.2

Blowdown Devices/Traps

Various means are used to drain off separated or condensed moisture from dryers, air receivers, filters, and header drop legs. Key Human Performance Point The relatively common practice of leaving a manual drain valve open a crack should not be used because it wastes costly compressed air. Debris can flow into condensate traps and block the discharge open. This allows compressed air to escape. Strainers can be installed upstream of the traps to prevent any material from entering the traps. It is recommended that the strainers have an integral blowdown valve and upstream isolation valves to allow strainer cleaning. For all types of drain traps, bypass piping is recommended to facilitate proper maintenance. Most manufacturers use drain straps for moisture removal from inter-coolers and after-coolers, moisture separators, and receivers. The drain traps usually consist of a reservoir that allows moisture to collect to a predetermined level and a float valve that actuates, allowing the air pressure to blow the water into a drain. Some manufacturers use a timer-actuated solenoid valve, which opens for a few seconds on a preset time schedule, to blow the air line down. The automatic drain valves associated with inter-coolers must be checked regularly. If the condensate is carried into the second-stage cylinder, it will wash away cylinder lubrication on lubricated compressors and might damage the cylinder. The float is connected by linkage to a drain valve that opens when an upper-level setting is reached and closes when the drain is emptied. The float device varies from a simple ball to an inverted bucket, but the basic principle is the same. An adequately sized drain valve is essential for satisfactory operation and to prevent blockage. A float that sticks in the closed position does not allow condensate to be drained, while a float that sticks in the open position allows the costly loss of compressed air. A float-type trap is shown in Figure 2-46. The text that follows Figure 2-46 outlines the drainage process.

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Figure 2-46 Float-Type Drain Trap (Courtesy of Ultrafilter International)

Condensate enters through the trap inlet and is collected in the vessel. The float rises with the level until the control valve opens and pressurizes the surge chamber. The air pressure on the diaphragm moves the piston to open the diaphragm. Now the condensate drains through the outlet, via the passages. Drain traps are a critical, but often over-looked, component in compressed air systems. A properly operating drain trap reduces plant operating costs, lowers maintenance, and prevents air lines from flooding. It eliminates the expense of bleeding expensive compressed air through petcocks or manually draining compressed air lines and equipment. A ball valve should precede all drain traps to facilitate routine maintenance without interruption. Demand-operated drain traps automatically discharge moisture and oil-containing condensate from the system. It is important that drain traps discharge only water, and not compressed air. For high reliability, the control mechanism should be completely separated from the condensate. The best drain traps adapt to many mounting positions, operate without electricity, and do not require a maintenance-intensive strainer upstream of the condensate inlet. They can be used on air receivers, inter- and after-coolers, refrigerated dryers, separators, filters, and header piping. Timed electric traps are another option. A timer on these traps allows the setting of both the valve open time and the time between drain cycles. They are less expensive to buy than the demand-operated traps, but are not as efficient in operation due to excessive loss of compressed air.

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The electrically operated solenoid valve (see Figure 2-47) is a type of device that might be referred to as time cycle blowdown. A solenoid-operated drain valve has a timing device that can be set to open for a specified time and at specified intervals. Again, the size of the valve and any associated orifices must be adequate to prevent blockage.

Figure 2-47 Solenoid-Operated Drain Valve (Courtesy of Pneumatech, Inc.)

The potential problem with the electrically operated solenoid valve, apart from an electrical malfunction, is that the valve is set to operate without reference to the presence of condensate, or lack of it. The period when the valve is open might not be long enough for adequate drainage of the amount of accumulated condensate. On the other hand, the valve can operate even when little or no condensate is present, which results in the expensive loss of compressed air through the drain valve. Some electrically operated drain valves use a magnetic reed switch or a capacitance device to detect the level of condensate present. These operate only when drainage is needed. A drain valve of this type is shown in Figure 2-48.

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Figure 2-48 Electronically Controlled Drain Trap (Courtesy of Domnick-Hunter, Inc.)

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Another example is a pneumatically controlled drain trap. A picture is shown in Figure 2-49.

Figure 2-49 Pneumatically Operated Drain Trap (Courtesy of Drain-All, Inc.)

At the start of the cycle, the float (1), with an integral float magnet (2), is at the lowest level. The float magnet exerts a magnetic force that repels the inner magnet (3) upward and holds it seated against the valve stem (4). This prevents control air, coming in through the control filter (5), from reaching the actuating cylinder (6) (which stays in the home position with the discharge ball valve (7) in the closed position). The inner magnet and valve stem are located in the center tube and are isolated from the condensate. There is always a residual amount of condensate (8) in the bottom of the reservoir from the last discharge cycle. The drain trap stops discharging before all accumulated condensate is removed, providing a liquid seal that conserves expensive compressed air. Manual blowdowns are particularly important in older instrument air systems that have been upgraded to meet the current ANSI/ISA air standards. Systems that have been operated for a long period of time with moisture in the air might have plating of corrosion products on the pipe walls. Upgrading systems to a sub-zero dew point will dry out this plated layer and cause it to spall and contaminate the air stream. In addition to the use of drain traps and blowdown valves in the instrument air system, it is recommended that the various air lines to end-use components be manually blown down periodically to facilitate the removal of particulate contamination from the system.

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The amount of condensate will vary with geographic location and atmospheric conditions of temperature and relative humidity. Drain traps should be sized for the anticipated rate of accumulated condensate and chosen for the specific location and anticipated contamination by lubricants being used. If drains and traps are clogged, the cooler shells will fill up with condensate in a short period of time. The condensate is then carried over into the system in the form of liquid water droplets and these droplets can: •

Deposit in the receiver and cause corrosion of the tank walls and bottom.

•

Prematurely exhaust the capacities of pre-filters and desiccant dryers.

•

Overload refrigerant-type dryers.

•

Cause moisture accumulation in the system piping, thus resulting in corrosion. Some of the system piping might be installed outdoors and exposed to varying ambient temperatures. Accumulated water might freeze during winter and cause damage to piping and instruments.

•

Cause malfunction of air-operated valves (sluggish or erratic). Wash away lubricants from operating cylinders of air-operated valves or other similar equipment.

•

Cause some of the lubricants used on solenoid valve O-rings to become sticky or gummed up, thus causing the solenoid valve to become inoperable.

•

ANSI/ISA-S7.0.01-1996 states that a monitored alarm on drain traps is preferred; however, if a monitored alarm is unavailable, monitoring the traps in each shift is recommended.

The discharge of condensate into a river, lake, or pond is subject to environmental regulations. See Appendix F for some condensate disposal methods. 2.4.7 Receiver Tanks A receiver [2,6] storing a large quantity of compressed air serves several important functions in a compressor system. Air receivers provide a reserve (surge volume) of air to supply large, shortterm air demands upon the loss of an air compressor, a large system leak, or some other transient demand. With its large storage capacity, a receiver can supply sudden increases in plant air demand that would potentially cause upsetting pressure fluctuations. By acting as a buffer between the compressor and the plant, a receiver prevents frequent compressor loading and unloading that would result from changes in plant demand. Excessive cycling can cause extra wear on compressor parts (especially the capacity control valves of reciprocating compressors) and increase maintenance requirements. Because the velocity and temperature of the air is reduced as it passes through the receiver, additional water, oil, and solid particles settle out of the air stream. Pulsation in the air stream, resulting from the compression process that otherwise might travel through the system, is dampened by a receiver. In the distribution system, intermittent large-volume demands can quickly drain the air from immediate areas, causing pressure levels to fall for surrounding end-use devices. Strategically 2-90


located receivers in the distribution system can supply these sudden localized demands and maintain a consistent air flow to the affected areas. Receivers serve the following functions: •

Dampen the compressor discharge pulsations

•

Serve as reservoirs to reduce the impact of sudden pressure changes during periods of unusually heavy demand that exceeds compressor capacity

•

Prevent frequent loading and unloading of the compressor

•

Serve to precipitate some of the moisture in the air that might have failed to condense in the after-cooler and the separator

In the case of sootblowing applications, pulsations of pressure are the result of the rapid opening and closing of inlet valves at each sootblower unit at time intervals that correspond to the start and finish of blowing each unit. The elapsed time interval between the end of blowing one unit, to the start of blowing the next unit, is only a matter of seconds. However, there is sufficient time to create pulsations in the header that must be dampened by adding receiver volume. Adding receiver volume will prevent the unstable header pressure from interfering with the control of the compressor. The air receiver capacity required to stabilize the header pressure is a function of the time that is required for the compressor to load or unload. When receivers are liberally sized, the frequency of compressor loading is decreased. A receiver provides a storage capacity that can be used for a limited period of time in case of compressor failure. Key Technical Point Air receivers or storage tanks should be provided in all installations using positive displacement air compressors such as reciprocating and rotary screw air compressors. Receiver tanks for dynamic air compressors (centrifugal compressors) are used for large-flow requirements, such as boiler sootblowing applications. The use of non-American Society of Mechanical Engineers (ASME) Section VIII receivers is not recommended. Many federal, state, and local laws regulate the construction of unfired pressure vessels and these laws should be adhered to strictly. ASME receivers are furnished complete with ASME-approved safety valves, pressure gauges, hand holes or manholes, and drain valves. ASME vessels should be inspected periodically for: •

Proper pressure setting of the safety valve. It should be at least 5% less than the stamped maximum pressure on the tank.

•

Current gauge calibration.

•

Routine drainage of the receiver tank. Moisture accumulation can cause severe corrosion problems.

•

Integrity of coatings, paint, and corrosion inhibitors. Vessel should be recoated if necessary. 2-91


Recent industry water intrusion events have demonstrated the need for verifying the integrity of receivers by: •

Ensuring the operability of drain traps by monitoring their water levels

•

Ensuring that manual blowdowns are regular and effective

•

Periodically inspecting receiver internals to prevent receiver catastrophic failure and blockage of lines due to crud buildup

•

Verifying and testing relief valve setpoints and closure tightness

2.4.8 Distribution Piping The air distribution subsystem connects the major components and is an important part of the compressed air system [2,3,6]. It is made up of main trunk lines, hoses, valves, line drops to specific usage points, pressure regulators, lubricators, additional filters, traps, and supplementary air treatment equipment. It is through this subsystem that most leaks occur, energy is lost, and maintenance is required. Key Technical Point The complete drying, filtration, and distribution system should be sized and arranged so that the total pressure drop from the air compressor to the points of use is much less than 10% of the compressor discharge pressure. Compressed air distribution systems generally use piping made of carbon steel, stainless steel, or copper. Instrument air system piping materials vary a great deal between power plants. Plants generally use carbon steel, hard copper, stainless steel, galvanized steel, or a combination of these types. Each type of pipe has advantages and disadvantages for use in an air system. As expected, stainless steel piping experiences the fewest failures due to corrosion or physical damage; however, a stainless steel distribution system is expensive. Stainless and carbon steel systems have not experienced failures due to piping separations or weld failures. Incidences of through-wall corrosion have occurred in carbon steel air systems that are not adequately dried. Corrosion and leaks in carbon steel piping are common where the moisture content of air remains consistently high. When dew points are maintained below the ANSI/ISA standard, little or no corrosion will occur. It is recommended that systems be equipped with blowdown points to assist in eliminating any corrosion products that might form as a result of moisture intrusion events. When installing new or additional headers or piping, the following guidelines apply to the materials used: •

Carbon steel piping should be in accordance with ASTM-A106.

•

Stainless steel piping should be in accordance with ASTM-A213.

•

Steel tubing should be in accordance with ANSI B93.

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All piping installations should be in accordance with the standard that was originally used for the plant design (ANSI/ASME B31.1 for most U.S. plants) or any later revision that might have been adopted by the plant. Copper piping is easily damaged with physical impact or misuse. Also, the joints in a copper pipe are soldered or brazed. Soldered joints can fail due to poor joint insertion, inadequate solder penetration, or external stress. There are no national codes or specifications for soldering. The American Welding Society (AWS) considers a 70% filled joint to be adequate if the voids are small and discreetly dispersed. ASME Code provides specifications for brazing of copper piping. Brazing is preferred to soldering, particularly for copper pipes above 1 in. (2.5 cm) in diameter. This ensures that a joint has sufficient strength under pressure. Hard copper piping (Class 21B) is in common use at many facilities and is susceptible to joint failures. To strengthen the joints and lessen the possibility of failures, some plants require that copper piping joints be brazed instead of soldered; others use stainless lines for supply to user components. Some facilities that achieve the goal of clean, dry air have opted to replace their copper lines with carbon steel piping and welded joints. Key Technical Point Pressure drops between the compressor and end-use components are nonrecoverable losses. Therefore, it is imperative that pressure losses be minimized. When designing or modifying a system, the following guidelines should be considered: •

Pipe sizes should be large enough to ensure that pressure drops after the receiver tank do not exceed 10% of system pressure.

•

Long distribution lines should have liberally sized air receiver tanks located near the far ends or near points of heavy demand.

•

Where possible, a loop type of system around the plant or building is recommended. This provides a two-way distribution to the point where air demand is highest. The loop piping should be of sufficient size so that pressure drop is not excessive at any component, regardless of flow direction.

•

Each air header or main should have the outlets located as close as possible to the point of use. This is to prevent excessive pressure drops through hoses. Outlets should always tap off from the top of the pipe to prevent any condensation, oil, or other particulate from being carried over to the component.

•

All piping should be sloped toward a drop line or trap to enable condensation to be removed from the system.

•

For systems that use only oil-free compressors, it is recommended that corrosion-resistant pipe be used.

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•

On oil-lubricated compressor systems, an oil film will form on the pipe walls and help protect the pipe from the corrosive effect of warm, moist air. This protection is not available in a non-lubricated compressor system.

•

System piping should be well supported, yet allow for movement of the pipe due to operationally induced stresses. The span between hangers should be kept short enough so that the sag will not exceed the downward pitch of the pipe wall.

For the compressed air system used in sootblowing applications, the piping does not have to be sloped to drain. In addition, the piping does not have to be insulated. Usually Schedule 40 pipe is required. The piping size is determined by the maximum flow area and the allowable pressure drop from the heater/receiver tank to each blower. 2.4.9 Instrumentation Following is a description of three of the more important instruments that are used to indicate instrument air parameters [3,6]. Dew Point Monitors – In-line dew point monitors provide constant visual readouts of air system moisture content. These monitors can be equipped with alarms to alert operators to potential air system problems. The in-line monitors are usually installed downstream of the air dryers. It is recommended that portable monitors be used at end-use components to ensure that dry air is maintained throughout the system. Although air system pressures operate between 100 and 125 psi (689–862 kPa), small leaks can allow moisture to migrate into the system. Sampling at enduse components with portable monitors might indicate moisture infiltration, even though the dryer output dew point is acceptable. Air System Flow Indicators – Flow indicators installed in-line at either the compressor outlet or air dryer outlet are also recommended. By monitoring system flow rates, the system’s efficiency can be determined. A changed flow rate will also provide indication of increased or decreased system leakage. Air Header Pressure Indicators – Pressure-indicating devices that are located throughout the air system can provide information regarding the loading and isolation of the various headers. This information can prove useful in finding leaks or in determining the cause of sluggish valve or control operations. Discharge pressure and temperature indicators are essential for monitoring operating conditions, but additional indicators and safety features are also required. This includes a pressure relief valve prior to any shutoff valve or discharge check valve. Normally, on rotary air compressors, this is part of the sump/separator design. A high-temperature shutdown device is also required at the discharge from the compressor air end. Except on the smallest compressors, conventional pressure and temperature gauges and electromechanical devices have been superseded by solid-state or microprocessor controls that offer several additional features and benefits. Generally, they have an LCD or LED display with 2-94


appropriate messages and a schematic diagram on the panel so that components and status are readily identified. A communication port (RS232 or RS485) allows two-way communication with other compressor panels or computer systems. 2.4.10 Safety Relief Valves Relief valves [11] should be furnished in the air compressor suction line, in each stage before the inter-cooler or in the discharge before the cooler. Relief valves are required by ASME Section VIII. This is a minimum standard to which most pressure vessels are designed and built. Rupture discs are sometimes furnished in lieu of relief valves. The relief valve must be set to protect the component with the minimum design pressure and it must be large enough to relieve the maximum predicted capacity. For the sootblowing compressed air system, a pressure relief valve must be installed to maintain a constant pressure as the pressure is falling during sootblowing. When there is not a header/receiver tank, then an automatic valve is not required; sometimes, however, a motoroperated valve is used. 2.4.11 End-Use Devices Design basis documents state that the instrument air system is to provide a reliable supply of dry, oil-free air for the pneumatically operated instruments, controls, and valves. The discussion that follows examines the ways in which certain instruments are affected by air quality [6]: Air Pressure Regulators – The purpose of air pressure regulators is to provide a regulated air pressure to instruments, controls, and other pneumatic devices. The regulated output pressure is controlled by a spring-loaded diaphragm that opens a pilot plunger to maintain a balance between the air pressure and the spring force. Dirt particles in instrument air can accumulate at the plunger and exhaust port, and cause erratic operation. A decrease in air supply pressure can cause a failure of the valve to fully close or open. Over-pressurization can damage the diaphragm in pneumatic valve actuators. The presence of hydrocarbon in instrument air can cause deterioration of the diaphragms, which are made of elastomers. An air pressure regulator with a damaged diaphragm might improperly transmit full line pressure to its end-use devices. Air Filters – Air filters are used to remove dirt, oil, water, and other impurities from instrument air supplies. The instrument air is filtered as it passes through the filter elements. The installation of improperly sized filter elements can have adverse effects on filter performance. The installation of fine particle filter elements in coarse filtering applications can result in clogged filter elements and a substantial reduction in the volume of instrument air. Also, clogged filters can rupture and release the retained debris into the system. The installation of a coarse particle filter element in a fine particle application can admit dirt into the system.

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Positioners – The purpose of valve positioners is to ensure that the control valve plug position is directly proportional to the value of the controller output pressures. Positioners employ a forcebalance principle of operation. Usually the spring-loaded force is balanced by the control instrument pressure. To achieve the desired valve position, the instrument air is either supplied to or exhausted from the pilot assembly. An orifice that is clogged by water, oil, or dust in the air supply can prevent the valve from stroking or cause it to go to the full signal position, regardless of the actual signal. Dirt buildup in the pilot assembly can cause erratic and inconsistent operation. The presence of hydrocarbons in instrument air can deteriorate the elastomers in the positioners. I/P Transducers – The purpose of current-to-pneumatic transducers is to convert analog current control signals from field instruments into proportional pneumatic signals for positioners. Transducers are electro-pneumatic devices that balance the forces produced by the current input signal and the output pressures. An orifice that is clogged by water, oil, or dust could cause an erroneous zero output, or fulloutput pneumatic signal. Hydrocarbons in the instrument air will deteriorate the elastomers in transducers. The failure of electrical-to-pneumatic converters can cause unnecessary valve cycling and can potentially prevent valves from moving to their designated air failure positions. Faulty controllers can cause spurious alarms, sluggish responses to parameter changes, and the need for frequent recalibrations and corrections to gain settings. Recorders and indicators might not show correct process values, might drift in undesired directions, or fail altogether. Pneumatic Valve Actuators (Diaphragm Type) – The purpose of valve actuators is to modulate valve plugs and disks to control the flow through various plant valves. The modulation is in response to various input signals. A diaphragm actuator is a pneumatically powered device that uses the principle of compressed air acting on a diaphragm. The actuating force is a determined by multiplying the air pressure delivered to the diaphragm by the effective area of the diaphragm. Two problems that have affected actuators have been the presence of hydrocarbons in the air and over-pressurization. Because many of the diaphragms are made of elastomers, hydrocarbons will cause diaphragm deterioration. Over-pressurization due to setpoint drift or failure of the air regulator will stretch a diaphragm. The result can be cracks and failure of the diaphragm. Pneumatic Valve Actuators (Piston Type) – Piston actuators have a cylinder and piston that is normally sealed with an elastomer O-ring. They operate at a higher air supply pressure and are more powerful and compact than diaphragm actuators. The presence of hydrocarbons in instrument air is also detrimental to piston actuators. The elastomer O-rings will deteriorate and get stuck. The accumulated debris from dirt in the instrument air can score the cylinder and damage the O-ring. The moisture in the air can cause corrosion inside the cylinder and degrade the lubricant.

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Solenoid-Operated Valves – Solenoid-operated valves are one of the end-use components that more commonly experience failures. The nature of these failures, their root causes, and the recommendations given to prevent these failures, which are presented here, represent a compilation of industry bulletins, information from the nuclear industry, Performance and Technical Service Engineering Memoranda, industry survey responses, and vendor information. Solenoid-operated valves are used as pilot devices that receive an electric signal and direct air pressure to inlet ports of valve actuators. The actuators drive valves to their desired positions. Solenoid operators are categorized by the number of port openings and positions. The operator is composed of a coil, plunger, and spring. The spring and plunger assembly is fitted through the core of the coil. The magnetic field of the coil pulls the plunger inward when the coil is energized and the spring forces it back to its deactivated position when the coil is de-energized. The air-operated and solenoid-operated valve failures, which affect response and stroke times, fall into two categories—valve operator failures and valve body failures. The typical operator failures are air leakage at the fittings and actuator springs that are out of adjustment. Valve body failures are usually associated with air supply quality. Other reasons for failures are improper pressure, a faulty design, or improper lubrication. The most common solenoid-operated valve failures reported by nuclear power plants are caused by: •

Lubrication damage to valve bodies

•

Faulty wiring

•

Coil failures

•

Valve body leakage at the tube fitting

•

Passageway blockage and sticking, including air contaminants

•

Actuator springs out of adjustment

•

Slow response times from the fully open to fully closed valve position

•

Installation and maintenance errors

In a solenoid-operated valve, the plunger assembly is exposed to the instrument air. If the instrument air contains dirt, the particles can accumulate and score the plunger and cylinder. A significant amount of hydrocarbon in the instrument air will cause embrittlement of the elastomer O-rings. The O-rings will stick to the valve body and prevent the solenoid from operating. The combination of moisture in the instrument air and elevated temperatures will cause hydrolytic decomposition of the polyurethane seat on the plunger and cause it to become stuck to the exhaust port. Pneumatic instrumentation failures are caused by physical failures or by a contaminated instrument air supply. The common causes include corrosion due to excessive moisture in the air supply and particle sizes that exceed the vendor-specified maximum. There are also other, less common, causes, such as: •

Pneumatic actuator diaphragm failures: These are usually due to too much air pressure, which results in leaks. 2-97


•

Pneumatic actuator piston failures: These are usually due to contaminated air.

•

Problems with supply air regulators, such as: –

Wrong sizes

–

Clogging

–

Setpoint drifts

•

Pneumatic manual-loading station failure (due to fitting leaks and clogging).

•

Sluggish response of electric-to-pneumatic and pneumatic-to-pneumatic converters (due to air line contamination such as dirt and dust that plugs ports). Most converters have upstream pressure/filter regulators. These should have the petcock (at the bottom of the regulator) normally open to blow out particulates and water.

•

Inaccurate accumulator pressure gauges.

•

Problems with positioners that are mounted on control valves, such as:

•

–

Air leaks at the instrument air inlet supply or at the line to the control valve diaphragms

–

Spurious electrical signals from power supplies that result in unnecessary cycling of control valves

Problems with pneumatic instrumentation, such as panel recorders and controllers: –

Contaminants and moisture from the instrument air supply

–

Errors and damage from upstream regulator malfunctions

–

Electrical interface problems and failures

–

Setpoint drifts

–

Mechanical failures

Some end-use components that are required to operate after a loss of air have backup bottles of high-pressure air or nitrogen, which is piped through a pressure regulator to maintain operability. An example of this would be air-operated valves in the switchyard. The backup bottles are usually sized to allow either a calculated number of strokes for a valve or to guarantee an air supply for a certain period of time. When high-pressure backup bottles are used, the relief valve between the pressure regulator and the valve operator should be sized to handle the entire capacity of the bottle(s). This will ensure that lower-pressure piping is not over-pressurized, should the pressure regulator fail. Boiler Sootblowers – There are four main types of sootblowers that use air for boiler component cleaning applications. They are: •

Long retractable

•

Rotary

•

Wall blowers/deslagger

•

Sonic horns

2-98


The blowing element of the long retractable sootblower is a lance tube with two opposed venturi nozzles arranged at the front end. The rotary sootblower uses a multi-nozzled element permanently located within the tube bank to be cleaned. Wall blowers/deslaggers are short, retractable sootblowers, primarily used for cleaning boiler furnace walls. Sonic horns are airoperated and produce low-frequency, high-energy sound waves. The sound waves are transmitted by acoustic horns that resonate and dislodge ash deposits from the boiler/air preheater/Selective Catalytic Reduction (SCR) surfaces. The main concerns with using air-operated blowers is supplying the required amount of compressed air for a cycle of blowing and minimizing the water content of the air. Moisture in the air can degrade piping, receiver tanks, and blowers. For more information on the sootblowing system, please see EPRI’s Sootblowing Maintenance Guide, 1004005, 2001.

2-99


3

CONTROLS

The primary goal of a compressor control system [2,3,7,14,15] is to ensure that the compressor system produces enough air to meet plant demand. Older compressors use pneumatic and electro-mechanical systems. Modern microprocessor-based controls improve compressor performance by offering precise pressure regulation, distributed capacity control, and automatic load scheduling. For centrifugal compressors, advanced surge control can dramatically increase efficiency by reducing wasted blowdown air. Air systems can operate more efficiently and economically through the proper selection and optimization of the control system used with the compressor unit. Compressor control systems function on a demand cycle. If the demand for compressed air is steady and does not fluctuate, compressor controls simply match the capacity to the system demand. However, most air systems are more complex and demand is more variable than the one described. Figure 3-1 shows a typical system pressure fluctuation.

Figure 3-1 Precise Pressure Regulation [7]

System pressure is monitored and the control system decreases compressor output when the pressure reaches a predetermined level. Compressor output is then increased again when the pressure drops to a lower, predetermined level. The difference between these two pressure levels is called the control range. Depending on air system demand, the control range can be between 2–20 psi (14–138 kPa).

3-1

EPRI Licensed Material Controls

Key O&M Cost Point Every 2 psi (14 kPa) of control range pressure difference is equal to a 1% change in energy consumption. Processes with complicated and varying demand cycles, which can include several compressors of different sizes and configurations to maintain system pressure, require a sophisticated control system. A consideration for multiple compressor arrangements is to stagger the use of the individual compressors. If the wear on the compressors is distributed evenly, then all of the compressors will require rebuilding at the same time. By setting one or more compressors in the lead position, the time for rebuilding can be controlled.

3.1

Single Compressor Control Methods

Demand for compressed air seldom exactly matches the output from the compressor, so some form of capacity control is essential. The type of necessary capacity control is based on the type and size of the air compressor and the application. Typical single compressor capacity control systems are start/stop, constant speed, modulating, multi-step, and variable speed control. 3.1.1 Start/Stop Control Start/stop control is the simplest form of control, where a pressure switch that senses system pressure at the discharge of the compressor sends a signal to the main motor starter to stop the compressor when a preset pressure is reached. When pressure falls to another preset pressure, the pressure switch sends a signal for the compressor to be restarted. The pressure switch has an adjustable upper pressure setting and a fixed or adjustable differential between the upper and lower pressure settings. An air receiver is essential to prevent too-frequent starting and stopping. Frequent starting and stopping of the motor affects the life of the motor insulation because of the high inrush current at each start. This type of control is normally limited to compressors in the 30 hp or less range. The advantage of this type of control is that power is used only while the compressor is running. This advantage is offset, however, by having to compress to a higher receiver pressure to allow the air to be drawn from the receiver when the compressor is stopped. 3.1.2 Constant Speed Control Load/unload control is also known as constant speed control. This method allows the motor to operate continuously, but unloads the compressor when the discharge pressure is adequate. On double-acting reciprocating air compressors, when the upper pressure setting is reached, valve unloaders hold open the inlet valves so that air drawn into the cylinder is expelled through the open inlet valves without any compression or delivery taking place. When applied to only one end of the cylinder, delivered capacity drops to 50%. If applied to both ends, capacity drops to zero. 3-2


On rotary air compressors, when the upper pressure setting is reached, the pressure switch sends a signal to open a blowdown valve (which is installed in a line coming from the compressor discharge, but which is located before a discharge check valve). When the blowdown valve is opened, the compressor discharge pressure is lowered and the discharge check valve prevents back flow from the system or receiver. The lower discharge pressure reduces energy requirements. At the same time, the signal from the pressure switch is also arranged to close a valve at the inlet of the compressor. This reduces the mass flow of air entering the compressor. The power requirements are reduced, although the pressure ratio across the compressor does increase. In the case of oil-injected rotary compressors, the rate of blowdown must be limited to prevent foaming of the lubricant in the sump/separator. In oil-free rotary compressors, this is the most common type of control. It requires an adequate receiver or system volume to prevent toofrequent operation. On liquid ring compressors, when the upper pressure setting is reached, the pressure switch sends a signal to open a valve in the recirculation line from the moisture separator back to the compressor suction, which unloads the compressor. When this occurs, the discharge check valve closes and isolates the system receiver from the moisture separator. There is an orifice in the recirculation line, which maintains moisture separator pressure high enough to provide sufficient differential pressure across the seal water heat exchanger and the compressor, thus providing adequate seal water flow while in the unloaded condition for cooling. 3.1.3 Modulating Control One basic form of modulating control is called inlet valve modulation. Reducing the air flow is achieved by reducing the inlet opening through which the air enters the compressor. This, in turn, matches the compressor output to system demand. Because system pressure acts as a demand indicator, throttling is usually controlled by system pressure. A regulating valve senses system or discharge pressure over a prescribed range [usually about 10 psi (69 kPa)] and sends a proportional pressure to operate the inlet valve. Closing (or throttling) the inlet valve causes a pressure drop across the valve. This reduces the inlet pressure at the compressor and, hence, reduces the mass flow of air. Because the pressure at the compressor inlet is reduced while the discharge pressure is rising slightly, the compression ratios are increased so that energy savings are limited. Inlet valve modulation is normally limited to the range from 100% to about 40% of rated capacity. At 40% of rated capacity, the discharge pressure has reached full-load pressure plus 10 psi (69 kPa). It is assumed that demand is insufficient to require continued air discharge to the system. At this point, the compressor is unloaded as previously described in the description of constant speed control (Section 3.1.2). The modulating control method is an inefficient means of varying compressor output on reciprocating or rotary screw compressors. Better results occur when using the modulating control method on centrifugal compressors. This is particularly true with the use of inlet guide 3-3


vanes, which direct air in the same direction as the impeller inlet. The amount of capacity reduction is limited by the potential for surge and minimum throttling capacity. When the inlet valve closes in response to decreased plant demand, the centrifugal compressor will invariably approach a surge condition. To avoid surge, flow through the compressor must be maintained at a typical minimum of 60% of the maximum output. The amount that a centrifugal compressor can reduce its output, while avoiding surge, is known as turndown. If output still exceeds plant demand and the maximum turndown has been reached, then surplus air is blown down. Surge adds a degree of complexity to centrifugal control because the controls must not only modulate capacity, but must prevent the compressor from surging as well. As pressure increases and flow decreases, a centrifugal compressor approaches surge. Figure 3-2 shows this process.

Figure 3-2 Centrifugal Compressor Surge Diagram [7]

Modern controls, in comparison to conventional controls, save energy by increasing turndown and unloading the compressor. For centrifugal compressors, blowdown at minimum capacity is a significant energy waste. Modern controllers can reduce this waste by using several methods. Advanced anti-surge control provides greater turndown and allows a larger capacity modulation range before blowdown begins. Also, when the minimum capacity point has been reached, the controller can lower the compressor into an unloaded state. In an unloaded state, the compressor produces little air and, thus, blows down little air. In a distributed capacity control system, other compressors can modulate output and allow the centrifugal compressor to operate at higher capacity levels. These features combined offer great energy savings potential.

3-4


From a maintenance and protection standpoint, modern controls also offer far superior surge prevention than older systems. Advanced systems, with mathematical models of the compressor’s surge line and fast control responses, work to keep the compressor out of surge. If surge does occur, these systems detect the event and adapt the controller’s operation to avoid any recurrence. Surge can severely damage a compressor. Advanced protection is necessary to maintain a centrifugal compressor’s peak performance. 3.1.4 Multi-Step Controls Some compressors are designed to operate in two or more partially loaded conditions. The output pressure can be closely controlled without requiring the compressor to start/stop or load/unload. Reciprocating compressors are designed as two-step (start/stop or load/unload), three-step (0%, 50%, 100%) or five-step (0%, 25%, 50%, 75%, 100%) control. Figure 3-3 shows this five-step control.

3-5


Figure 3-3 Five-Step Reciprocating Compressor Control [7]

Some rotary screw compressors can vary their compression volumes by using sliding or turn valves. These are generally applied in conjunction with modulating inlet valves to provide more accurate pressure control with improved part-load efficiency. 3.1.5 Variable Speed Control Variable speed control is gaining acceptance as the cost for these drives decreases. In a rotary compressor, displacement is directly proportional to the rotational speed of the air end input shaft. However, it is important to note that, with constant discharge pressure and efficiency 3-6


remaining constant over the speed range, the input torque requirement also remains constant. This is unlike the requirement of dynamic compressors, fans, or pumps. The actual efficiency might also fall at lower speeds, requiring an increase in torque. Electric motors and controllers are currently available to satisfy these needs, but their efficiency and power factor at reduced speeds must be taken into consideration. Steam turbines and engines are also variable speed drivers but are rarely used in power plants.

3.2

Multiple-Compressor Control Methods

A multiple-compressor control system is based on the basic pressure control logic, that pressure is a function of volume. As pressure rises, it is assumed that system volume is satisfied. Conversely, as pressure decays, it is assumed that more volume is required. Multiple-compressor sequencing is desirable in larger installations so that the number of compressors in operation is sufficient to meet current demand. Compressors are started and stopped, and loaded and unloaded as required to maintain current system requirements. It is desirable to have only one compressor in the system at any given time to be in a reduced capacity mode of operation. This optimizes energy requirements. Sequencing can be arranged to equalize the running hours of each compressor, or to operate the compressors in a specified sequence (particularly where there is a mix of larger and smaller compressors). The sequence can be changed manually or automatically, as facilitated by software programming. Most individual compressor controls are run through pressure switches on microprocessors, which function as pressure controllers. Signals are sent to the pressure controller to continuously monitor the compressed air system’s air pressure. The pressure controller should have a +1% accuracy and repeatability. The job of the pressure controller is to keep the overall air system within a specified pressure range—typically 10 psig (69 kPa)—as long as the system’s air demand does not exceed the compressor capacity. This range is referred to as the control range. To maintain the control range, the pressure controller manages the functions to the inlet valve or inlet valve unloaders that regulate the amount of atmospheric air allowed to enter the compression chamber to be compressed. Simply stated, for rotary or centrifugal air compressors, the pressure controller closes the inlet device on pressure rise (unloaded) and opens the inlet device on pressure decay (loaded). Although an inverse action takes place in a reciprocating compressor, typically, the compressor pumps full capacity while loaded, and provides zero capacity at a reduced horsepower when unloaded. Depending on the installation, there are three general types of multiple compressor control systems: electro-pneumatic controls, microprocessor controls, and custom software controls.

3-7


3.2.1 Electro-Pneumatic Controls More than 70% of all existing air compressors use electro-pneumatic control systems. These compressors feature electric and mechanically controlled devices (such as pressure switches, solenoid valves, and metering pins) that adjust loading and unloading through the use of an inlet throttle and/or a bypass valve. These control systems typically rely on mechanical pressure switches to monitor the compressor’s proper discharge pressure. In addition, electro-pneumatic systems typically feature a series of mechanical trip switches that will discontinue compressor operation when pressures or temperatures reach critical levels. High quality, electro-pneumatic controllers use multiple mercury switch pressure controllers. The pressure controller allows the control pressure span to be as low as 5 psig (34 kPa) with moderate accuracy. The system still operates in a typical cascade, providing a smaller pressure range. These systems can be very capital cost-effective and the control span is moderately reduced. This provides a moderate level of energy efficiency. On the downside, electropneumatic systems do not provide much flexibility for compressor sequencing. Sequencers assign compressors different fixed pressure levels when they come on- or off-line. Key Technical Point Sequencing compressors does prevent competition between the individual compressors. It also causes plant pressure to fluctuate along the range of assigned pressure levels. For example, a four-compressor system with pressure intervals of 5 psi (34 kPa) will have a 20-psi (138-kPa) operating window. Sequencers tend to maintain an average system pressure that is higher than needed.

3.2.2 Microprocessor Controls The most advanced control system available today is an electronic or microprocessor-based control system. Instead of common pressure and metering devices, the microprocessor control system relies on electrical transducers and sensors. These devices sense air pressure and temperature values, which are then transmitted to a central microprocessor. Also, the microprocessor measures and stores compressor operating data for future maintenance needs. Microprocessor systems have been available since the late 1980s. Most compressor manufacturers offer at least one design microprocessor system and some have completely eliminated older, less efficient control systems on new units. Some manufacturers offer microprocessor control systems on compressor accessory units, such as air dryers and conversion kits that can be fitted to older compressors. Such upgrades add significant benefits and maintenance reduction, especially on reciprocating compressors.

3-8


Microprocessor systems offer significant advantages over electro-pneumatic controls. Here are some examples: Measurement Accuracy – Measurement accuracy is one of the critical functions of a control system. The risk of compressor damage and downtime increases as the measurement accuracy decreases. If a temperature alarm setting rises over time, the compressor runs the risk of operating at a level that might cause damage. Temperature and pressure switches on the electropneumatic systems can drift and change setpoints with time and wear. Microprocessor control systems, however, can provide an accurate reading. Settings will not slide or drift once they are set. This reduces the risk of compressor damage. Inaccurate measurements adversely affect the control systems’ ability to perform in extreme environments. Changes in ambient conditions over time require that pressure and temperature gauges be re-calibrated to maintain accuracy. This can be a time-consuming and costly activity that requires skilled instrument technicians. Microprocessor system transducers are less susceptible to conditions, such as vibration and heat. Re-calibration, if required, is accomplished by a simple push-button method from the microprocessor control panel. Overall downtime is, therefore, minimized. Ease of Maintenance – Many companies perform routine maintenance programs for compressors, which require that daily readings of temperatures, pressures, and functions be recorded in sequential log sheets. With most control systems, operators are required to manually record all gauge readings from the compressor. On many compressors that have electro-pneumatic systems, gauges monitor several functions on the compressor and are sometimes unlabeled. This can require operators to gather multiple readings to determine compressor functions. By incorporating all controls into one microprocessor panel, operators can obtain actual operating values from one central location at the touch of a button. For example, monitoring the air end discharge temperature of rotary screw compressors can be a critical element in reducing downtime. The microprocessor control system can alert operators to any changes in these values to allow for preventive maintenance. The addition of microprocessor controls to older centrifugal and reciprocating compressors usually eliminates a variety of complicated capacity control devices. The microprocessor panel allows operators to adjust shutdown setpoints automatically and respond to alarms. The task of mechanically resetting each protective switch is no longer necessary. The microprocessor allows resetting from the control panel. When compressor units experience shutdowns, it is sometimes difficult to pinpoint the cause because several alarms might have been activated. A microprocessor control system can monitor multiple alarms and, if shutdown occurs, recall the alarms to help identify the various problems.

3-9


Remote Access and Multiple Compressor Sequencing – Two additional benefits of a microprocessor-based system are based on its ability to program functions for remote activity, either for communications between operator and compressor unit or between two compressor units. One of the features available on some of today’s microprocessors is the ability to incorporate remote or two-way communication with the microprocessor unit. By adding a two-way communication link with the control system, operators can integrate plant-wide control systems and communicate operating functions to a remote data terminal or computer. Data links also enable operators to monitor system performance and program compressors to load, unload, and alter sequence from a remote site. Another advantage of two-way communication is the ability to retrieve maintenance observations and statistical data to develop operating trends that will enable plant operators to relegate unscheduled system outages to the routine maintenance program. Compressor control systems also allow operators to program units to automatically control and manage the functions of many compressors through one control system. Frequently called system controllers, these units can adjust to air demand fluctuations and maximize operating efficiencies, while reducing excessive energy consumption. System controllers tend to use a single pressure transducer, installed at an appropriate point in the compressed air system, to monitor system-wide pressure and to implement operating adjustments as programmed. Compared to typical sequencers that require an overlapping or cascade-style system pressure band and a fixed rotation, the controllers can operate several compressors at a common control pressure band to maximize energy conservation. In addition, system controllers can be programmed for a number of operating parameters, including target system pressure, system pressure band, compressor load and unload delay times, sequential control mode, and start/stop sequencing. 3.2.3 Custom Controls Similar to the microprocessor-based systems, custom-designed software controllers use a common, highly accurate, pressure transducer and controller. The accuracy of the pressure controller allows the control pressure span to be as low as 2 psig (14 kPa) with excellent accuracy. The system operates all compressors in a common pressure control span, allowing for the smallest pressure range and the highest energy savings. These models are expertly tailored to the user’s needs. Due to the customization, they offer superb energy savings and quite often are bundled with other operational feedback and controls. However, because these systems are built to meet a customer’s needs, their costs are very high to cover the extensive consultations and will have to be updated if the facility needs changes.

3-10


4

TROUBLESHOOTING

General problems with a compressed air system can be classified into three categories [1,3,16]: Poor air quality – Poor air quality from the presence of contaminants can result in the failure of pneumatically operated equipment. The contaminants include excessive moisture, particulates, and hydrocarbons. Where air piping is installed outdoors, colder ambient temperatures can cause the moisture in the lines to freeze. This is particularly true in northern U.S. regions. Having an adequate regenerative desiccant dryer dew point of -40ûF (-40ûC), which removes moisture from the air before it enters outside piping, is critical. Heat tracing can also be applied to outside piping to keep the moisture from freezing. Excessive moisture in the air can result in corrosion in receiver tanks and carbon steel piping. Problems with air-operated valves and solenoid valves occur with moisture in the air. Moisture problems can be resolved by re-routing outlet piping from compressors to reduce moisture collection, or by increasing the operating time of a compressor train to prevent moisture buildup. However, moisture problems might require the replacement of an air compressor and dryer. Particulates (particularly in the form of loose corrosion particles) can cause fouling of check valves and air-operated solenoid valves, and the restriction of airflow in filters and pre-filters. Particulates can also result in the common cause failure of safety-related equipment. The entry of desiccant into the instrument air system is cited as one example. Finally, the incursion of dust or rust particles into the compressor air intake can damage compressor internals, particularly on non-lubricated reciprocating compressors. The presence of excessive hydrocarbons in the air might result in the malfunction of air-operated valves, the contamination of fluids in contact with the air, and the degradation of material used in valve seals and seats. Loss of System Integrity – The potential for a loss of compressed air system integrity depends on the specific system and, in particular, on the material used in the air system piping. Stainless and carbon steel instrument air systems have not experienced failures due to piping separations or weld failures. Incidences of through-wall corrosion have occurred in carbon steel air systems that have excessive moisture. When dew points are maintained below the ANSI/ISA standards, little or no corrosion will occur. It is recommended that systems be equipped with blowdown joints to assist in eliminating any corrosion products that might form as a result of moisture intrusion.

4-1

EPRI Licensed Material Troubleshooting

For copper piping or tubing, the failure of several solder joints has resulted in major plant transients and shutdowns. Such failures are the result of poor joint insertion, inadequate solder penetration, and external joint stresses. The stabilization of questionable joints can be achieved by installing braces. To help alleviate stress due to expansion, vibration, or misalignment of fittings and end-use components, the use of tubing loops and flexible hoses is recommended. Miscellaneous Problems – In addition to air quality and system integrity problems, compressor and dryer trains can fail due to a wide variety of problems. For example, dominant failure locations and mechanisms for rotary screw air compressors are believed to be clogged or dirty air inlet filters, inlet throttle valve diaphragm problems, unloader piston diaphragm problems, and cooling water leakage in the compressor, inter-cooler, and after-cooler. In many instances, problems occur because of deficiencies in maintenance and in a flawed response of the operator to them (for example, the bypassing of a dryer should its pre-filter clog). This section is composed of charts for troubleshooting the following: •

General compressed air systems

•

Double-acting reciprocating compressors

•

Rotary compressors

•

Centrifugal compressors

•

Regenerative desiccant dryers

•

Refrigerant dryers

•

Motors

4-2


4.1

General Compressed Air System

The following general compressed air system troubleshooting guidelines (Table 4-1) were taken from Kaeser literature [9]. Table 4-1 Troubleshooting Compressed Air Systems Problem Low pressure at point of use

Low pressure at compressor discharge

Water in lines

Liquid oil in air lines

Elevated compressor temperature

Probable Cause

Remedial Action

Leaks in distribution piping

Check lines, connections, and valves for leaks

Clogged filter elements

Clean or replace filter elements

Fouled dryer heat exchanger

Clean heat exchanger

Low pressure at compressor discharge

See the following problem

Improper adjustment of air capacity control for systems with modulating load controls

Follow manufacturer’s recommendation for adjustment of control

Worn or broken valves

Check valves and repair

Improper air pressure switch setting

Follow manufacturer’s recommendations for setting air pressure switch

Failed condensate traps

Clean, repair, or replace the trap

Failed or undersized compressed air dryer

Repair or replace dryer

Faulty air/oil separation

Check air/oil separation system; change separator element

Compressed oil level too high

Follow manufacturer’s recommendation for proper oil level

Improper pressure switch setting

Readjust according to manufacturer’s recommendation

Restricted air flow

Clean cooler exterior and check inlet filter mats

Restricted water flow

Check water flow, pressure, and quality; clean heat exchanger as needed

Low oil level

Check compressor oil level; add oil as required

Restricted oil flow

Remove restriction; replace parts as required

Excessive ambient temperatures

Improper ventilation to compressor; check with manufacturer to determine maximum operating temperature

4-3


4.2

Double-Acting Reciprocating Compressor

Table 4-2 provides symptoms and probable causes for double-acting reciprocating compressor problems. This table is provided as an aid in troubleshooting commonly occurring problems with this type of compressor. The information in Table 4-2 was taken from EPRI’s Compressor and Instrument Air System Maintenance Guide: Revision to NP-7079, TR-108147, 1998 [3]. Table 4-2 Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms Low or no oil pressure

High oil pressure

4-4

Possible Cause

Solution

Defective gauge

Check the oil pressure gauge calibration.

Crankcase oil level too low

Add oil to the proper level as indicated on the sight gauge. Also see the nameplate for quantity.

Oil is too light

Check the oil specifications.

Plugged oil strainer

Clean the oil strainer as outlined in the vendor’s manual.

Leak in oil pump suction or pressure line

Check for line leakage and tighten the fittings.

Oil pressure relief valve out of adjustment

Adjust the oil pressure relief valve per the vendor’s instructions.

Dirt on oil pressure relief valve seat

Remove the relief valve. Clean the valve and seat. Use a magnet to remove foreign metal particles.

Broken oil pressure relief valve spring

Replace the relief valve spring.

Crankcase oil pump defective

Repair or replace the pump.

Defective oil pump drive

If gear-driven, check the gear alignment and security. A directdriven pump may require a new oiler pin and bushing.

Improper rotation direction of the oil pump

See the vendor’s manual for instructions

Worn crankpin or wristpin bearings

Replace the crankpin or wristpin bearings per the vendor’s manual.

Defective gauge

Check the oil gauge calibration.

Relief valve improperly adjusted

Adjust as outlined under “Oil Pressure Adjustment” in the vendor’s manual.

Oil too heavy

See the oil specification in the vendor’s manual.

Plugged oil pressure line

Remove the pump discharge piping and ensure that it is cleared.

EPRI Licensed Material Troubleshooting Table 4-2 (cont.) Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms High oil pressure (cont.)

Possible Cause

Solution

Defective or clogged oil filter

Replace or clean the element as needed.

Plugged oil return to sump

Remove the obstruction from the passage.

Erratic oil pressure

Crankpin bearing inserts worn or defective

Replace the crankpin bearing inserts per the vendor’s manual.

Does not deliver air

Controls inoperative

Check the vendor’s manual for control settings.

Restricted suction line

Service the inlet filter. Ensure that protective screens are removed and remove the obstruction from the inlet line.

Unit running “unloaded”

Verify that control switch is properly set per vendor instructions for compressor loading.

Suction and/or discharge valves missing

Replace the valves per the instructions in the vendor’s manual.

Restricted suction line

Clear the suction line.

Dirty air intake filter

Clean or replace the filter.

Loose suction or discharge valve assembly

Tighten the screw(s) holding the assembly in place.

Worn or broken valve assembly parts

Recondition valves as described in the vendor’s manual.

Suction or discharge valve improperly assembled

Re-assemble as per the vendor’s manual. Check if the suction and discharge valves have been assembled for proper flow direction.

Defective inlet valve unloaders

Service as described in the vendor’s manual.

Defective or improperly adjusted controls

Adjust or service per the instructions in the vendor’s manual.

Suction or discharge valve cages misaligned

Align the valve cap to cage as shown in the vendor’s manual. Misalignment will cause valves to leak.

Worn piston rings

Replace per the instructions in the vendor’s manual.

Leakage at piston rod packing

Tighten or replace the packing.

Low compressor speed. Veebelts slipping.

Check speed as shown on the compressor nameplate. Tighten or replace the belts.

Low capacity

4-5

EPRI Licensed Material Troubleshooting Table 4-2 (cont.) Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms Low operating pressure

Overheated cylinder

4-6

Possible Cause

Solution

Defective gauge

Check the pressure gauge calibration.

Excessive service line leakage

Check the service lines for open or leaking connections.

Suction/discharge valve loose, worn, or defective

Service the valve assemblies per the instructions in the vendor’s manual.

System demand greater than compressor capacity

Reduce demand. Detect and repair system leaks.

Inlet valve unloaders defective

Service the unloaders per the instructions in the vendor’s manual.

Improperly adjusted pressure control

Adjust per the instructions in the vendor’s manual.

Worn piston rings

Replace per the instructions in the vendor’s manual.

Low compressor speed. Loose drive belts.

Check speed on compressor nameplate. Check drive belts for tightness and/or damage. Replace if required.

No cooling water

Check the cooling water supply and controls.

Insufficient cooling water

Increase the water flow.

Scored piston or liner

Repair or replace.

Worn or broken valve assembly or parts

Repair or replace. Check the discharge valves first.

Piston rod packing too tight

Loosen or replace the packing as necessary.

Discharge pressure too high

Adjust the air pressure switch setting.

Suction/discharge valve cage not aligned

Align the valve cap to cage per the instructions in the vendor’s manual.

Cylinder cooling jackets clogged with deposits, rust, or scale

Clean the water passages of the cylinder cooling jackets.

Excessive carbon deposits on valve assemblies (for lubricated compressors)

Disassemble and clean. Check the lubrication rate.

Insufficient lubrication (for lubricated compressors)

Correct the lubrication rate.

EPRI Licensed Material Troubleshooting Table 4-2 (cont.) Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms

Possible Cause

Solution

Overheated cylinder (cont.)

Broken high-pressure relief valve or check valves

Repair or replace the valves as necessary.

Water in cylinders

Cooling water temperature too low causing condensate

Cooling water inlet temperature should be above ambient air temperature. Adjust the cooling water discharge temperature to 100–120°F (38–49°C).

Leak in head gasket

Replace the head gasket.

Cracked cylinder head

Replace the cylinder head.

Air discharge piping pitched toward compressor

Pitch the discharge piping away from the compressor.

Leak in after-cooler

Check the after-cooler tube nest and repair leaks.

Moisture separator not drained

Drain and check operation of the drain trap.

Leak in inter-cooler

Check the inter-cooler tube nest and repair leaks.

Condensate in inter-cooler

Drain and check the drain trap.

Foreign material getting into cylinder

Clean the air intake filter. Replace filter, if necessary.

Overheated cylinder

See “Overheated cylinder” in this table.

Broken valves or valve springs

Replace as needed.

Water in cylinder

See “Water in cylinders” in this table.

Excessively worn or broken piston rings

Replace as needed per the instructions in the vendor’s manual.

Insufficient lubrication (for lubricated compressor)

Correct the lubrication rate as specified in the vendor’s manual.

Improper control adjustment

Adjust the controls as needed.

Defective control components

Check the control components. Repair or replace as needed.

Clogged, pinched, or improperly connected control piping

Clean the control air tubing to the unloaders. Replace if damaged.

Clogged control air filter

Drain and clean the element.

Defective inlet valve unloaders

Check the yoke, spring, and diaphragm. Replace parts as necessary.

Scored cylinder liner and piston

Erratic or inoperative compressor control

4-7

EPRI Licensed Material Troubleshooting Table 4-2 (cont.) Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms Erratic or inoperative compressor control (cont.)

Compressor continues to pump after unloading

Broken suction or discharge valve and/or valve springs

Air receiver pressure too high

4-8

Possible Cause

Solution

Inadequate supply of control air to unloaders and control pressure switch, respectively (air supply lines from air receiver too small)

Check the installation and control panel drawing for proper piping details. Control air must be free of surging and of ample volume while unloading.

Voltage drop, incorrect voltage, or loss of power

Restore proper power supply.

Broken unloader diaphragm

Replace the inlet valve unloaders.

Broken unloader yoke

Replace the inlet valve unloaders.

Total closure valve fails to close

See “Total Closure Control” in the vendor’s manual.

Incorrect reassembly of valves

See “Valve Assemblies” in the vendor’s manual and ensure the proper spring tension with the proper number of springs.

Condensation and rust due to cooling water too cold

Cooling water inlet temperature should be above ambient temperature. Maintain 100–120°F (38–49°C) cooling water discharge temperature.

Overheated cylinder


Over-lubrication (for lubricated compressors)

Clean and remove the carbon deposits from the valves. Reduce the lubrication per the instructions in the vendor’s manual.

Insufficient lubrication (for lubricated compressors)

Increase the lubrication to the rate specified in the vendor’s manual.

Defective pressure gauge

Check the pressure gauge calibration. Recalibrate or replace gauge.

Pressure control set too high

Adjust the pressure control to a lower setting.

Restriction in pressure-sensing line

Remove any restrictions.

Defective unloader

Remove, inspect, and replace the valve unloaders per the vendor’s instructions.

Defective control or unloader

Clean, repair, or replace.

EPRI Licensed Material Troubleshooting Table 4-2 (cont.) Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms Discharge air temperature too high

Compressor stops unexpectedly

Compressor knocks

Possible Cause

Solution

Overheated cylinder


Cylinder cooling water temperature too high

Regulate the water temperature control to maintain 100–120°F (38– 49°C) water discharge temperature.

Discharge pressure too high

Lower the discharge pressure.

Insufficient cylinder lubrication (for lubricated compressors)

Correct the lubrication rate per the specification.

A protective logic initiates the drive motor trip.

Check all safety devices (oil pressure, water temperature, discharge air temperature, etc.) to determine the reason for shutdown.

Power failure

NOTE: In the event that the compressor is stopped unexpectedly or by safety devices, perform “Normal Stopping” per the instructions in the vendor’s manual to avoid start-up under load. Place the selector switch in the Off position before resetting the circuit. For units with total closure control, see “Start-Up After Abnormal Shutdown” in the vendor’s manual.

Foreign material in cylinder

Inspect the inside of the cylinder through the valve ports.

Incorrect head clearance

Adjust the piston head clearance per specifications.

Loose piston on piston rod

Tighten the piston nut.

Worn connecting rod bearings

See “Connecting Rod” in the vendor’s manual.

Worn crosshead

See “Crosshead and Guide” in the vendor’s manual.

Loose valve assembly or valve cage

Check the valve installation per the instructions in the vendor’s manual.

Loose or defective main bearings

Replace the bearing if defective. Check for proper bearing fit in the bearing bore and on the crankshaft.

Frozen piston rings

Remove and replace as necessary. Reinstall after cleaning the ring grooves to ensure free movement.

4-9

EPRI Licensed Material Troubleshooting Table 4-2 (cont.) Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms Compressor knocks (cont.)

Motor fails to start

Motor trips after start

Motor overheats

High inter-cooler pressure

4-10

Possible Cause

Solution

Bent connecting rod

Check the rod for alignment. Replace if necessary. See “Connecting Rod” in the vendor’s manual.

V-belts or sheave loose

Check the belt tension and tighten the sheave hub.

Piping vibration

Check all piping. Provide additional support, if required.

Loose counterweights in crankshaft

Tighten the counterweights.

Motor overload relay on motor starter tripped

Reset the overload relay after determining the cause for the overload.

Improper wiring

Check motor circuit wiring.

Power disconnected

Reconnect the power supply.

Low oil pressure. Oil switch not holding.

See “Low or no oil pressure” in this table.

Other safety device tripped

Check all safety devices. Determine the cause for the trip before resetting.

Voltage incorrect

Restore the correct voltage.

Excitation incorrect

Restore the correct voltage.

Dirty motor

Blow off with clean air or disassemble and clean.

Poor lubrication

Lubricate per specifications.

Motor overloaded

Correct the compressor operating pressure or speed. Ensure that compressor is running “free.”


Replace as necessary.

Broken inlet or discharge valves or valve springs in the succeeding cylinder

Repair as outlined in the vendor’s manual.

Damaged head gasket in succeeding cylinder

Replace the gasket.

Broken high-pressure check valve

Repair or replace.

Unloader yoke holding valves open in succeeding cylinder

See “Inlet Valve Unloaders” in the vendor’s manual.

EPRI Licensed Material Troubleshooting Table 4-2 (cont.) Double-Acting Reciprocating Compressor Troubleshooting Guidelines Symptoms Low inter-cooler pressure

Oil leaking past piston rod oil scraper rings

Possible Cause

Solution


Replace as necessary.

Broken inlet or discharge valves in preceding cylinder

Repair as outlined in the vendor’s manual under “Valves.”

Leak in inter-cooler

Check the inter-cooler tube bundle.

Leaking inter-cooler drain

Check the drain and trap.

Piston rod packing leaking

Adjust or replace as needed.

Unloader yoke holding valves open in preceding cylinder

Reassemble the inlet valve unloaders per instructions in the vendor’s manual.

Scraper rings worn out

Replace the worn rings.

Dirt between rings

Remove the rings and clean.

Scratched or scored rod

Clean or replace the rod.

Stretched springs

Replace the springs.

Nicks on bottom surface of stuffing box

Refinish the surface. Maintain clearance per next item.

Incorrect ring clearance in piston rod packing box.

Proper clearance is 0.002 in. (0.05 mm) per ring or total of 0.004 in. (0.1 mm) for a two-ring set and 0.006 in. (0.15 mm) for a three-ring set. (0.0015 in. min., 0.003 in. max. per ring) (0.038 mm min., 0.076 mm max.). Rings must be able to float.

4-11


4.3

Rotary Compressor

Table 4-3 provides symptoms and probable causes for rotary screw-type compressor problems. This table is provided as an aid for troubleshooting commonly occurring problems in this type of compressor. The information in Table 4-3 was taken from EPRI’s Compressor and Instrument Air System Maintenance Guide: Revision to NP-7079, TR-108147, 1998 [3]. Table 4-3 Rotary Compressor Troubleshooting Guidelines Symptoms Compressor will not start

Compressor shuts down with air demand present

Possible Cause

Solution

Main disconnect switch open

Close the switch.

Line fuse blown

Replace the fuse.

Control transformer fuse blown

Replace the fuse.

Motor starter overloads tripped

Reset. If trouble persists, check whether the motor starter contacts are functioning properly.

Low incoming line voltage

Check the voltage. If low, check the incoming power supply.

Loss of control voltage

Reset. If trouble persists, check that the line pressure does not exceed the compressor nameplate maximum operating pressure.

Excessive operating pressure

Defective pressure-sensing switch. Check the pressure at which the contact points open. Air/oil separator requires maintenance. Check the differential pressure under full-load conditions. High-pressure shut-down switch setting is too low. Correct the setting. Defective control air solenoid valve. The solenoid valve should cause the inlet valve control lever to move to the unload stop when the pressure switch contacts open. Repair or replace if defective. Defective blowdown valve. The blowdown valve should exhaust the sump pressure to 35 psig (241 kPa) or lower when the maximum operating pressure is reached. Repair if defective.

4-12

EPRI Licensed Material Troubleshooting Table 4-3 (cont.) Rotary Compressor Troubleshooting Guidelines Symptoms Compressor shuts down with air demand present (cont.)

Possible Cause Discharge temperature switch open

Solution Cooling water temperature is too high. Increase the water flow (if water-cooled). Cooling water flow is insufficient. Check the water lines and valves. Cooler is plugged. Clean the tubes. If water-cooled, install a water conditioner. Cooling air flow is restricted. Clean the cooler and check for proper ventilation (if air-cooled). Ambient temperature is too high. Provide sufficient ventilation. Low oil level. Add oil. Clogged oil filter. Change if the differential pressure is high. Cooler bypass thermal valve is not functioning properly. Replace the thermal-sensing element (aircooled). Water flow regulating valve is not functioning properly. Replace the valve. Defective discharge temperature switch. Check for a short or open circuit to the temperature probe; correct wiring.

Compressor will not build up full discharge pressure

Air demand too great

Check the service lines for leaks or open valves.

Dirty air inlet filter

Check the filter indicator and change the element if needed.

Pressure regulator out of adjustment

Adjust as directed in the compressor manual.

Defective or dirty inlet valve

Check that the inlet valve fully opens when full air demand is present. Repair or clean as needed.

4-13

EPRI Licensed Material Troubleshooting Table 4-3 (cont.) Rotary Compressor Troubleshooting Guidelines Symptoms

Possible Cause

Line pressure rises above cut-out setting on pressure switch

Leak in control system causing low-pressure signal

Check for leaks.

Defective pressure switch

Repair or replace as needed.

Defective unloader solenoid valve

Repair or replace as necessary.

Defective blowdown valve

Check that the sump pressure is exhausted to the atmosphere when the pressure switch contacts open. Repair or replace if necessary.

High-pressure shutdown switch defective or adjustment incorrect

Adjust or replace as necessary.

Clogged oil return line from air/oil separator

Clean the strainer. Clean the orifice.

Air/oil separator element damaged or not functioning properly

Change the element.

Leak in lubrication system

Check all pipes, connections, and components.

Excessive oil foaming

Drain and change the oil.

Oil level too high

Drain to the correct level.

High-pressure shutdown switch defective or out of adjustment

Readjust the pressure relief valve setting or replace.

Defective pressure relief valve

Replace the valve.

Air/oil separator plugged

Check the air/oil separator differential pressure.

Excessive compressor oil consumption (for oilinjected compressors)

Pressure relief valve opens repeatedly

4.4

Solution

Centrifugal Compressor

After a brief period, compressor operators should become accustomed to the sound of a smooth running machine. Any change in the noise or vibration level should become immediately noticeable. Often, this variation in sound indicates a potential malfunction or failure. Table 4-4 is provided as an aid for troubleshooting a centrifugal type of compressor [3] with a microprocessor control panel. The information in Table 4-4 was taken from EPRI’s Compressor and Instrument Air System Maintenance Guide: Revision to NP-7079, TR-108147, 1998 [3].

4-14

EPRI Licensed Material Troubleshooting Table 4-4 Centrifugal Compressor Troubleshooting Guidelines Symptoms

Possible Cause

Solution

No power to the control panel

Turn the panel power on. See Control Panel Troubleshooting at the end of this table.

Ready to Start LED not lit

Control system not reset after a shutdown

Press Control Reset.

Permissive Start message appears but the compressor will not start.

Drive motor switchgear not engaged

Main breakers are not racked in. Rack in and close the drive motor breakers.

Drive motor switchgear malfunction

Check and repair the drive motor switchgear.

Drive motor switchgear temperature overload

Allow the motor drive to cool down. If the motor does not start, replace or repair the overload devices.

No power to the control transformer in the switchgear

Check the control transformer power source. The control transformer may need replacement.

Blown or overloaded fuses in the switchgear

Check the fuse rating and plant voltage condition. Replace the fuse as required.

Loose or broken control wiring between current transformer, motor starter, and compressor control panel

Troubleshoot and repair the wiring.

Emergency Stop button on control panel depressed

Reset the emergency stop by pulling the button out.

Incorrect voltage

Check and compare the motor voltage rating to the switchgear rating. Refer to the nameplates.

Compressor fails to start No display on control panel

Drive motor problems Motor overheats

Incorrect voltage is applied to the motor. Check the switchgear voltage level for a possible undervoltage condition. Motor overload controls improperly set or not working

Check the motor amps when the compressor is fully loaded.

Motor bearings improperly lubricated

Refer to the motor literature.

4-15

EPRI Licensed Material Troubleshooting Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptoms

Possible Cause

Solution

Motor windings shorted out

See the manufacturer’s literature.

Seized motor

Check for free shaft movement. Be sure that the power is off.

Motor bearings not correctly lubricated


Dirty motor. The windings are fouled so that the rotor and stator are in contact.

Consult a motor service shop.

Damaged drive coupling

Inspect the coupling. It may require repair or replacement.

Loose motor hold-down bolts

Tighten the bolts.

Damaged motor bearings


Worn drive coupling

Inspect the coupling. It may require replacement.

Misaligned motor and compressor

Check the alignment and realign.

Worn or damaged pump seal

Replace the seal assembly.

Loose pipe connections to pump

Tighten the connections.

Oil lines leak

Loose connections in piping

Check the piping and threading for any damage. Tighten as required.

Oil filter leaks

Loose bolts on filter housing

Tighten the bolts.

Damaged filter o-ring or o-ring housing

Replace the o-ring or the filter as required.

Damaged o-ring on the pinion seal

Lift the cover to replace the o-ring.

Buffer air-to-pinion seals out of adjustment

Seals should not be reused. Replace from the vendor’s parts list.

Seal installed incorrectly

Lift the cover to check the arrow on the seal for shaft rotation.

Malfunctioning air ejector

Increase the ejector air flow rate.

Pressure inside the gearbox and reservoir is too high because the buffer air pressure is out of adjustment or the reservoir vent filters are plugged.

Check and clean the reservoir vent filters. Decrease the buffer air pressure.

Worn seal

Replace the seal.

Drive motor problems (cont.) Motor overheats (cont.) Unusual motor noise

Unusual motor vibration

Oil leaks Oil pump leaks

Pinion oil seals leak

Bull gear oil seal leaks

4-16


Possible Cause

Solution

Gearbox split line bolts not fully tightened

Re-torque per the vendor’s instructions.

Damaged or broken split line gasket

Lift the cover and replace the gasket.

Buffer air pressure too high

Adjust the buffer air pressure per the vendor’s instructions.

Clogged reservoir vent filters

Clean or replace the vent filters.

Ejector/filter inoperative. Oil mist exiting from the filter canister.

Plugged filter elements

Replace the element and restore the vapor lock in the drain line.

Poor aerodynamics performance

Air discharge pressure setpoint incorrectly set

Check the discharge pressure setpoint and increase within the specified maximum setting.

Inlet filter dirty or otherwise restricted (for example, ice, snow, inlet start-up screen still in place, and so on)

Clean or clear the filter as necessary.

Dirty impellers

Clean the impellers.

Air system demand beyond the compressor rating

Reduce the demand. Check for system leaks.

Maximum amp setpoint too low

Check the specified maximum amp setting and adjust as necessary.

Inlet guide vanes not operating properly

Check the lubrication of the drive screw.

Bypass valve or modulating blow-off valve not closing completely

Check the stroke of the valve actuator to ensure that the valve closes completely.

Cooling water valves partially closed

Open and adjust the valves for the cooling water flow.

High water temperature

Reduce the cooling water temperature. Cooling systems are designed for a maximum water temperature of 95°F (35°C).

Dirt or scale buildup on air or water side of inter-cooler

Check the inter-coolers and clean the tube bundles as required.

Hot air or water bypassing around inter-coolers

Remove the bundles and inspect the seals and gaskets.

Oil leaks (cont.) Gearbox leaking at split line

Low discharge air pressure

High inter-stage air temperature

4-17

EPRI Licensed Material Troubleshooting Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptoms Compressor surge

Possible Cause

Solution

Dirty inlet filter

Clean or change the filter elements.

Low cooling water flow

Open the cooling water valves fully. Check the water flow rate.

High water temperature

Reduce the cooling water temperature. Cooling systems are designed for a maximum water temperature of 95°F (35°C).

Plugged or fouled inter-coolers

Inspect and clean both the air and water side of the inter-coolers.

Air pressure setpoint too high

Adjust the discharge pressure controller.

Faulty pressure transducer

Bench-test and replace the transducer if required.

Minimum amp setpoint below recommended setting

Adjust the minimum amp setpoint to the setting recommended in the manual.

Discharge check valve not operating properly

Remove and inspect the valve seat and disc. Replace if worn.

No control panel power to the IGV motor

Restore the control panel power. Place the control panel in Manual. Cycle the IGV open/closed.

Binding or broken mechanical linkage of the IGV

Check the lubrication of the drive screw.

Malfunctioning IGV motor

Bench-test operation of the IGV. Repair or replace.

Incorrect IGV sensor

Verify the sensor rating against the motor nameplate (Full load amps plus a service factor. See the vendor’s manual).

Faulty solid-state relay or fuse

Troubleshoot. Replace as required.

No control power to the BPV solenoid valve

Test with a multimeter. Replace if defective.

Mechanical failure of the BPV

Inspect the seals and piston for wear. Replace or lubricate per the vendor’s instructions.

Faulty solid-state relay or fuse

Troubleshoot. Replace as required.

Compressor surges during normal operation (see the following Note) High inter-stage air temperatures

High discharge air pressure

Inlet guide vane (IGV) functioning improperly

Bypass valve (BPV) not functioning (if included)

4-18

EPRI Licensed Material Troubleshooting Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptoms Modulating blow-off valve (MBOV) not functioning

Possible Cause No control power to the modulating blow-off valve (MBOV)

Solution Restore the power. Place the MBOV control switch in Manual mode and cycle the blow-off valve open/closed. Verify that the MBOV opens and closes fully.

Note: Compressor surging is acceptable when setting or adjusting the controls. However, repeated surging during normal compressor operation is undesirable. Oil pressure problems Main oil pressure low

Lube oil pump not running. No power to pump motor.

The lube pump motor starter has blown or faulty fuses. Replace the fuses. The motor starter overload heaters are tripped or defective. Allow the motor to cool, then reset or replace the heaters. Check for broken, loose, or faulty wiring to the motor or starter.

Pump motor rotation incorrect

Bump the motor. Check the pump rotation per the vendor’s manual. Correct if necessary.

Pump does not turn freely

Remove the pump coupling to determine if the pump or motor is seized. Inspect for damaged bushings or pump rotors.

Pump or motor turns freely with no resistance

The oil pump coupling is damaged. Inspect and replace as necessary.

Restriction in oil suction line

Drain the tank. Remove and inspect the pump suction line and clear the restriction.

Pressure transducer inoperative or not functioning

Remove and bench-test. Replace as necessary.

Faulty wiring to control panel

Troubleshoot. Correct the wiring as necessary.

Incorrect voltage to pressure transducer

Check the voltage. Normally a +24volt supply is required. See the vendor’s manual.

Pressure regulator (relief valve) not functioning

Disassemble and inspect. Replace as necessary.

Dirty or clogged oil filter element

Remove and replace the oil filter.

Oil system check valve stuck open

Remove the check valve and inspect for binding or damage. Replace if necessary.

4-19

EPRI Licensed Material Troubleshooting Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptoms Main oil pressure low (cont.)

Main oil pressure high

Bearing oil pressure low

Possible Cause

Solution

Low oil level

Fill reservoir to proper level. Correct any leaks.

Low Main Oil Pressure message on control panel

Check the oil pump, solid-state relay, and fuse.

Incorrect pressure regulator setting

Adjust to the proper pressure per the vendor’s manual.

Pressure transducer malfunctioning



Troubleshoot. Correct the wiring as required.



Main oil pump not operating

See Probable Causes and Solutions for “Main oil pressure low” in this table.

Clogged bearing oil filter

Replace the filter element.

Faulty bearing pressure transducer


Pressure regulator not functioning

Adjust to the proper pressure per the vendor manual.

Pressure regulator diaphragm failed

Replace the regulator.

High oil temperature. Thermal mixing valve out of adjustment.

Disassemble the mixing valve and test the thermostat for the recommended temperature. Verify operation of the oil cooler. Clean the tubes if necessary.

Damaged high-pressure pump or coupling

Rotate the compressor shaft manually and observe the free operation of the pump. Disassemble the pump for inspection. Repair or replace as necessary.

None of the above, but pressure still low

Remove the gearbox cover and inspect the internal oil lines for loose fittings. Inspect the bearing oil feed lines for loose connections.

Pressure transducer malfunctioning

4-20


EPRI Licensed Material Troubleshooting Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptoms Bearing oil pressure low (cont.)

Bearing oil pressure high

Oil filter pressure drop high

Low oil temperature

Possible Cause

Solution





Incorrect pressure regulator setting

Adjust the proper pressure per the vendor’s instructions.

Regulator diaphragm failed

Replace the regulator.

Clogged filter element

Replace the filter element.

Pressure transducers malfunctioning






No flow or restricted water flow to oil cooler

Open the throttle or block valves completely. Verify the water flow through the oil cooler by measuring the water temperature in and out of the cooler. Verify that the water supply pump is operating if applicable.

Oil temperature RTD malfunctioning

Bench-test the RTD. Replace if necessary.



Faulty oil heater

Check the heater thermostat for the proper setting (80–100°F)(27–38°C). Recalibrate the thermostat or replace the heater.

Faulty thermal mixing valve

Disassemble the mixing valve and test the thermostat for the recommended temperature. Replace the thermostat if necessary.

Fouled oil cooler

Remove the cooler water headers and clean the tubes.

4-21

EPRI Licensed Material Troubleshooting Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptoms High inlet air temperature

High vibration warning on control panel. Vibration measurements are within limits.

Possible Cause

Solution

Low or no water flow to intercoolers

Open the throttle or block valves completely. Ensure an adequate water supply. Verify the in-out water differential temperature (usually 95°F or 35°C).

RTD malfunctioning

Bench-test the RTD. Replace if necessary.



High temperature differential across cooler. Measure air temperature in and out of cooler with loaded compressor to verify this symptom.

The water side of the cooler is possibly fouled. Clean the tubes.

The inter-cooler seals or gaskets are damaged. Hot air is bypassing.

Remove the tube bundles and replace as necessary.

Faulty vibration channel

Check the connections at the vibration transmitter and probe. Adjust the probe gap. Check the wiring from the panel to the vibration transmitter. Check the fuse and power supply (+ 24 V) to the control panel. Check the extension cable. Replace if damaged. Check the vibration probe. Replace if damaged. Check the vibration transmitter. Replace if defective.

High vibration

4-22

Most motor, coupling, or bull gear problems are shown by a significant increase in the firststage vibration level.

Concentrate the investigation at the specific stage that indicates a vibration problem.

Oil temperature too high

Open the cooling water valves completely. Check the cooling water inlet temperature.

Oil temperature too low

The nominal minimum temperature is 100–110°F (38–43°C). The minimum for startup is 60°F (15°C). Refer to “Low oil temperature” in this table.

EPRI Licensed Material Troubleshooting Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptoms High vibration (cont.)

Possible Cause

Solution

Bearing oil pressure too low

The optimum pressure should be 500–550 psig (3.4–3.8 MPa). Refer to “Low bearing oil pressure” in this table.

Impellers dirty or damaged

Inspect and clean the impellers.

Drive coupling faulty

Inspect the drive coupling. Reassemble and lubricate per specification. Replace if worn or damaged.

Misalignment between compressor and motor shafts

Verify that the alignment is per specification. Realign as necessary.

Excessive drive motor vibration

See the specifications for acceptable limits.

Bearings fouled or damaged

Inspect per instructions in the vendor’s manual.

Gearing damaged

Inspect per instructions in the vendor’s manual.

The next level of control panel troubleshooting involves the use of a multimeter and the control panel schematic drawing that will allow a step-by-step search for any problem. Note: Troubleshooting at this level is intended for the more electronically sophisticated operator. Some basic problems are discussed below. Other difficulties can be diagnosed using the same logic. Consult the vendor’s manual and, if in doubt, contact the vendor’s service department. Caution: Be careful when adjusting the control panel. Errors can lead to greater problems or even equipment damage. Table 4-4 (cont.) Centrifugal Compressor Troubleshooting Guidelines Symptom Control panel “dead.” (No display, display backlight, sentry, or keypad LEDs on.)

Possible Cause No power to control panel

Solution Verify that the main power supply to the panel is on and that the switch is closed. Check the main power fuse on the control panel. Check the panel control power supplies. If no voltage, check the control power supply fuses.

4-23


Possible Cause

Solution

Panel functions, but sensors do not operate

Shorted or improperly wired sensors

Troubleshoot and correct the wiring problems.

False alarm on panel

Incorrect control power supply voltage to panel

Readjust the control power supply. Check the wiring. Check the control power supply fuses.

Incorrect pressure reading

Faulty transducer

Verify the actual measurement by installing a gauge. Remove the transducer and benchtest. Replace as necessary. Check for faulty transducer wiring to the control panel.

Incorrect temperature reading

4.5

Faulty wiring to control panel from temperature sensor

Verify the actual measurement with a thermometer. Troubleshoot for a wiring fault. Repair the faulty wiring.

Regenerative Desiccant Dryer

Table 4-5 is provided as an aid for troubleshooting a regenerative desiccant-type dryer. The information was taken from EPRI’s Compressor and Instrument Air System Maintenance Guide: Revision to NP-7079, TR-108147, 1998 [3]. Prior to detailed investigation, do the following: 1. Check for loose wiring. 2. Check for damaged piping. 3. Check for parts damaged by heat or an electrical short circuit. This is usually apparent from discoloration. Table 4-5 Regenerative Desiccant Dryer Troubleshooting Guidelines Symptoms Poor dew point suppression

4-24

Possible Cause

Solution

Entrained water

Check the moisture separator.

Too little purge volume

Check the purge flow needle valve for proper setting.

Inlet pressure below design

Check the pressure and reset the purge valve.

Inlet flow rate too high

Check the flow rate and reset the purge valve.

Inlet temperature above 100°F (38°C)

Check the compressor after-cooler operation.

EPRI Licensed Material Troubleshooting Table 4-5 (cont.) Regenerative Desiccant Dryer Troubleshooting Guidelines Symptoms

Solution

Poor dew point suppression (cont.)

Desiccant soaked with compressor lubricant

Replace desiccant and check the filter elements.

High pressure drop across dryer

Excessive inlet flow

Check the flow rate.

Inlet pressure below minimum design (80 psig) (552 kPa)

Check the pressure.

No input power

Check the fuse, timer, and On/Off switch.

No pilot air

Check the pilot air lines and control air filter.

Faulty switching solenoid valve

Check the inlet solenoid valve.

Dryer fails to pressurize

Purge exhaust valve fails to close

Check the timer, solenoid, and purge exhaust valve.

Failure to purge

Purge flow control needle valve closed

Open and properly set the purge control needle valve.

Purge exhaust valve fails to open.

Check the timer solenoid and purge exhaust valve.

Clogged muffler disseminator element

Replace the disseminator.

Purge exhaust piping of excessive length for line size

Increase the pipe and fitting size.

One tower has low pressure

Defective check valve(s)

Repair or replace the defective check valve(s).

Dryer fails to repressurize purged tower to line pressure before switchover

Improperly set purge needle valve

Open the purge needle valve until re-pressurization occurs properly.

Switching failure

Excessive tower back pressure

4.6

Possible Cause

Refrigerant Dryer

The dryer consists of three basic systems: air, refrigerant, and electrical. An air leak at 100 psig (689 kPa) provides an audible signal, indicating where there is a problem. The refrigerant, R-22 has no color or odor; therefore, a small refrigerant leak is difficult to find. However, it can be detected by a bubble test, halide torch (with a flame that changes from red-orange to blue on contact with refrigerant), or an electronic detector.The electrical system consists of transformers, starter, switches, relays, and so on. The use of a volt-ohm meter or similar equipment is required for checking continuity, amperage, and voltage. Table 4-6 is provided as an aid for troubleshooting a refrigerant-type dryer. The information was taken from EPRI’s Compressor and Instrument Air System Maintenance Guide: Revision to NP-7079, TR-108147, 1998 [3]. 4-25

EPRI Licensed Material Troubleshooting Table 4-6 Refrigerant Dryer Troubleshooting Guidelines Symptoms Unit does not run

Refrigerant compressor head pressure too high

Refrigerant compressor head pressure too low

Refrigerant compressor suction pressure too low

4-26

Possible Cause

Solution

No line voltage

Follow the wiring diagram and check the voltage from the compressor terminal to the power source to find where the circuit was interrupted.

Improperly wired

Inspect the electrical components such as switches, controls, motors, and transformers.

Blown fuse or tripped circuit breaker

Supply power voltage; frequency and phase must coincide with the nameplate.

Refrigerant overcharge

Excess refrigerant. Check the unit’s nameplate for the total refrigerant charge. Refrigerant overcharge may cause the system to not perform properly or efficiently.

Condenser fouled and dirty

Dismantle and clean condenser. Clogged fins in the air-cooled condenser reduce the heat transfer efficiency.

Defective fan control switch

Replace. The fan control cuts in at 215 psig (1.5 MPa) and out at 170 psig (1.1 MPa).

Defective fan motor

Replace.

Dryer location too hot (high ambient)

Cool ambient or relocate the unit.

Compressed air leaks to refrigerant system

Evacuate. Repair the leak and recharge with refrigerant. Repair or replace the evaporator.

Low ambient temperature

Increase the ambient temperature. If too low, dryer freeze-up is possible. Add head pressure control.

Refrigerant shortage

Check for leaks in the system; repair and recharge with the correct amount of charge.

Defective fan control switch

Replace the switch. The fan should cut out at 170 psig (1.1 MPa).

Hot gas bypass valve out of adjustment or defective

Adjust or replace. Turn clockwise to increase the suction pressure. See the vendor manual for normal conditions.

EPRI Licensed Material Troubleshooting Table 4-6 (cont.) Refrigerant Dryer Troubleshooting Guidelines Symptoms Refrigerant compressor suction pressure too low (cont.)

Refrigerant compressor suction pressure too high

Water in the compressed air system

Possible Cause

Solution

Refrigerant shortage

Add enough refrigerant to maintain the desired suction pressure. Also check the amperage draw.

Excessive pressure drop on high side

Check for any restriction (plugged filter dryer). Suction pressure should be steady and vary only 1 to 3 psi (7 to 21 kPa) from high to low at this condition.

Head pressure too low due to defective fan control switch

Replace the switch.

Hot gas bypass valve out of adjustment or defective

Turn counterclockwise to lower the suction pressure to the desired reading. Compressed air dew point rises as the suction pressure increases (see Note that follows).

Refrigerant overcharge

Excess refrigerant. Check the unit’s nameplate for the total system refrigerant charge. Refrigerant overcharge may cause the system to not perform properly or efficiently.

Drain traps clogged

Disassemble and clean the traps to restore free flow of drainage. Check the drain lines. Automatic ball-float-type drain traps do not need priming but should be disassembled and cleaned after two weeks of operation.

Air bypass system open

Close air bypass valve. See the vendor’s manual for the initial startup procedure.

Improperly set or malfunctioning hot gas bypass valve, or fan switch cycling

This is indicated by suction pressure being too high. Adjust or replace.

Ambient temperature too high

Cool ambient or improve ventilation.

Note: An increase in refrigerant suction pressure causes an increase in refrigerant evaporator temperature. This, in turn, raises the dew point of the compressed air delivered into the system.

4-27


4.7

Motors

Although some motor problem symptoms were listed in the compressor troubleshooting guidelines (Tables 4-1 through 4-6), this section outlines a larger, more general troubleshooting guideline for motors. The EPRI Maintenance Working Group of the Large Electric Motor Users Group (LEMUG) has identified 10 common symptoms of problems that are encountered by electric motor users. These 10 symptoms include: 1. Failure to start or come to speed 2. High vibration 3. High bearing temperature alarm, for motors with anti-friction bearings 4. High stator temperature – random or form windings 5. Instantaneous over-current trip 6. Running above nameplate (higher amps) 7. Excessive noise while running loaded 8. The motor line or phase currents are unbalanced 9. Increased motor slip at a constant load point 10. Motor stalls under heavy load In addition to input from motor users and LEMUG members, each of these symptoms has been evaluated by a technical expert who offered insight based on actual field experience, as well as experience in motor design and manufacturing. The following symptom-troubleshooting tables (Tables 4-7 through 4-16) were copied from the EPRI report Troubleshooting of Electric Motors, 1000968, 2000 [16]. Each one of the tables in this section corresponds to one of the 10 symptoms identified by the LEMUG. The symptomtroubleshooting tables in this section have been designed by the EPRI Maintenance Working Group of the LEMUG. The tables are the supporting documents for the troubleshooting process shown in Figure 5-1 of EPRI report Troubleshooting of Electric Motors. They are intended to be living documents, to be used and customized by each plant. In instances where the table refers to Table xxx, Practice xxx, Figure xxx, and so on, users should insert their appropriate station procedure or reference to customize the table for their own use.

4-28

EPRI Licensed Material Troubleshooting Table 4-7 Symptom 1: Motor Will Not Start Process D1 1. Is the indication valid?

2. Is the value anomalous?

Method of Proof

Expected Results

A. Measure voltage and current at T1, T2, T3, in motor control center.

A. Voltage and current should be balanced within values shown in Table xxx.

B. Check motor output shaft for motion or vibration, check electrical noise.

B. Motor will produce a large electrical noise at locked rotor; shaft will try to turn and vibrate.

C. If no power is at T1, T2, T3, reset and retry start.

C. If no results, refer to control system owner.

A. A simple visual inspection.

A. Confirm that motor has started. B. Measure of output speed and line current will confirm motor is running at correct speed.

D2 Does symptom require corrective action?

Possible Causes (By order of probability)

A. Ensure that all overloads and sensors are reset.

A. Check RTD of stator and bearings, vibration sensors, or other limit switch-type devices to ensure proper position.

B. Verify OL set at proper level to allow for required in-rush current.

B. May eliminate trip

A. Check to determine if drive equipment is free to rotate and is not bound-up or jammed.

A. May need to uncouple or disengage motor-driven equipment.

B. Check load.

B. May need to release load.

C. It is critical to determine if the problem is external to motors.

C. If not, refer to owner of component or system.

Verification Method (By order of preference)

How?

Note: Once problem has been confirmed to be internal to motor and power supply is correct. DC1 The bearing has “frozen-up” or caused the rotor to bind.

A. Inspect bearing and lubricant; observe signs of overheating of shaft and seals. Look for discoloration of paint in end bell near bearing/shaft.

A. May require uncoupling and/or disassembly for accurate inspection.

4-29

EPRI Licensed Material Troubleshooting Table 4-7 (cont.) Symptom 1: Motor Will Not Start Possible Causes (By order of probability) DC2 Stator winding is damaged.


How?

A. Conduct megger, hi pot, and partial discharge tests. B. Measure stator resistance of each phase or part of winding.

A. Check Table xxx for acceptable values. B. Compare to manufacturer or benchmark data; 5% delta at correct ambient ok. C. The end turns may show signs of over-heating, also lead wire. Check exiting areas for sign of smoke/fire. D. May require disassembly to verify.

C. Inspect for external signs of smoke, fire, or over-heating.

D. Use caution not to touch motor when energized and avoid inlet/ outlet areas if trying restart. E. Compare data to previous testing; review trends. DC3 Rotor assembly is damaged.

A. Check to see if shaft will rotate freely. B. Usually will need to disassemble to verify. C. Check endplay and runout.

DC4 The stator connections are incorrect or broken.

A. Check lead-wire marking at control if more than 3 leads and in outlet box.

D3 Is source a motor issue or external issue?

If Root Cause is deemed an External Issue as determined above, notify appropriate component or system owner; otherwise, proceed to Possible Corrective Actions below.

Root Cause

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E. May reveal change from norm. A. Check the squirrel cage for signs of over-heating. B. Check for signs of contact between rotor and stator. C. Check for broken or loose bars. Check fit between core and shaft; in extreme case, shaft may be broken or bent. A. This condition should only exist if motor is new or reinstalled or connections were broken to take measurements.

Possible Corrective Action

CA1 Bearing has seized or caused rotor to bind.

A. Disassemble and replace. Check all fits, end play, and runout. Replace both bearings, purge all old grease/oil.

CA2 Stator winding damage

A. Disassembly and repair will usually require a rewinding.

A. Follow recommended Best Practice IEEE xxx.

B. Replace all stator sensors and upgrade stator insulation when applicable.

B. That is, inverter grade, add additional treatment, bracing.

EPRI Licensed Material Troubleshooting Table 4-7 (cont.) Symptom 1: Motor Will Not Start Root Cause CA3 Rotor assembly damage

CA4 Incorrect or broken stator lead connections.

Possible Corrective Action A. Overheated squirrel cage.

A. Replace if cast, re-bar if fabricated.

B. Broken or loose bars.

B. Replace and swage tightly.

C. Broken fan or blades.

C. Remove and replace or retighten.

D. Lost or broken balancing weight.

D. Replace.

E. Broken shaft.

E. Straighten or replace.

F. Bent shaft.

F. Plate, metalize, or replace according to Practice xxx.

G. Undersized journal and/or shaft.

G. Restore or replace.

A. Correct or repair.

Table 4-8 Symptom 2: High Vibration Process

Method of Proof

Expected Result

D1 1. Is the indication correct?

A. Record vibration readings as shown in Figure xxx.

A. Compare result to Table xxx for limits.


A. Check previous reading or benchmark values.

B. Note if it is tied to the driven equipment and process.

B. Monitor with respect to time.

B. Adjust and restart and repeat readiness.

C. Are the readings changing with respect to time?

C. Vary load and speed.

D. Stop and check for loose parts.

D. Isolate motor from driven equipment; confirm motor as source of vibration.

E. Is the vibration a function of the load or voltage? D2 Does the system require corrective action?

A. Compare to previous readings and Table xxx for limits

A. May reveal change

B. Compare to other parts in drive train, including mounting.

B. Isolate Problem

4-31

EPRI Licensed Material Troubleshooting Table 4-8 (cont.) Symptom 2: High Vibration Possible Causes (By order of probability) DC1 Unbalance in drive train (Motor-Driven Equipment/Process)

Verification Method (By order of preference) Conduct vibration profile of coupled drive train. Un-couple components if necessary and re-check. Check voltage for unbalance.

DC2 If cause is internal to motor, check:

Compare vibration reading to Table xxx for possible cause.

*Damaged bearing/seal.

A. Record current signature full-load, no-load, and single.

*Bent shaft or excessive runout. * Loose/open rotor bar.

B. Runout. C. Disassemble and inspect.

*Loose fit of parts. *Broken fan/balanced weight. *Thermal instability of rotor. Root Cause

D. Check all runouts and fits. E. Check fans, couplings, and rotor core for shaft fit. F. Follow procedure in Table xxx. Possible Corrective Action


If Root Cause is deemed an External Issue as determined above, notify appropriate component or system owner. Otherwise, proceed to Possible Corrective Actions that follow.

CA1 Change in balance or initially incorrect.

A. Trim balance if possible. B. Balance all rotating parts as assembly. C. Disassemble, re-balance rotor assembly. D. Straighten bent part. E. Re-fit loose parts. F. Soft foot.

CA2 Internal to Motor Damage bearing.

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A. Replace, purge old lubricants, replace seals as necessary, check shaft and end bell fits.

How?

EPRI Licensed Material Troubleshooting Table 4-8 (cont.) Symptom 2: High Vibration Root Cause


Damage to rotor cage.

A. Disassemble and tighten or replace loose bars.

Thermal instability, rotor bowing.

A. Reduce load/operating temperature and re-check. B. Refer to motor manufacturer for disposition. C. Possible heat treat and shock or cold pack. D. Resurface rotor outside diameter.

Table 4-9 Symptom 3: High Stator Temperature—Random or Form-Wound Process

Method of Proof

Expected Result


A. Take direct reading with thermocouple on terminal iron.

A. Iron temp may be lower by 10–20°C than average winding temperature.


A. Review temperature versus time profile. Compare to duplicate machines/load.

A. If trend is valid and continuous, expect eventual alarm or trip.

B. Has an alarm triggered or breaker tripped? D2 Does the symptom require corrective action?

A. Compare temperature values to Table xxx for recommended limits.

A. Measure volts, amps, and speed for additional reliance.

B. Consult operation manual.

B. Reduce load or ambient if possible and re-measure for downward trend.

C. Compare to previous readings.

C. Key action is to determine if caused by external or internal source.

D. Is the temperature steady-state or changing? How? E. Compare to previous readings.

4-33

EPRI Licensed Material Troubleshooting Table 4-9 (cont.) Symptom 3: High Stator Temperature—Random or Form-Wound Possible Causes (By order of probability) DC1 Fundamental steps

DC2 External Causes

DC3 Internal Causes A. Stator damage


How?

A. Assess if any alarm, sensor, or overload has tripped.

A. Check all phases. Is heating symmetrical?

B. Identify other symptoms—feel, touch, smell, see. Have other variables of speed or current changed?

B. Measure voltage, current, and speed.

C. What is the status? Will the motor start? Is the acceleration normal?

C. Still running; shut down?

A. Incorrect power supply.

A. Measure voltage, check variation and balance.

B. Excessive load.

B. Reduce or disconnect— recheck.

C. Too much cycling or starting.

C. Run at continuous or reduced load.

D. Variable Frequency Drive running too slow.

D. Increase speed.

A. Run no-load; measure current in each phase. B. Shut down—check for faults and grounds: check megger readings, hi pot, partial discharge, resistance. C. Verify control and motor connections

B. Rotor damage

A. Check rotor speed/motor slip. B. Measure vibration to planes on both ends.

4-34

C. Broken fan / baffles

A. Check air intakes/exhausts.

D. Mechanical, Miscellaneous

A. Inspect for rotor rub on stator, end bell, or other running clearances. Check blades, baffles. Inspect air ducts for foreign material.



A. Compare to previous benchmarks. A. May need to measure flow rate.

EPRI Licensed Material Troubleshooting Table 4-9 (cont.) Symptom 3: High Stator Temperature—Random or Form-Wound Root Cause

Possible Corrective Actions

CA1 If heating is due to internal cause: A1. Excessive current in winding

A1. Excessive load, improper voltage/hertz.

A2.Inadequate cooling

A2. High ambient, blocked ventilation, or recirculation.

A3. Excessive starting time

A3. Too many starts, too much load to start, prolonged acceleration time.

A4. Excessive current/heating in rotor

A4. Unbalanced voltage, open rotor bars, mechanical interference with running clearances, shorter laminations/ bars.

Table 4-10 Symptom 4: High Bearing Temperature Alarm—Motor With Anti-Friction Bearings Process

Method of Proof

Expected Results

A. Use backup thermocouple to verify; measure on end bell out of air stream or on iron in outlet box.

A. Will depend on source of heat, which could be ambient, driven equipment motor rotor/stator, or bearing system.

B. Refer to benchmark data or duplicate machines/load.

B. Should get similar results.

C. Conduct thermograph analysis.

C. Will quantify heat sources.


A. Track temperature with respect to time.

A. Erratic reading will usually indicate faulty instrumentation or sensor.

D2 Does symptom require action?

A. Compare to Table xxx for recommended limits.

A. Reduce load or ambient if possible and re-measure.

B. Consult operation manual.

B. If recent re-lubrication, purge grease for 30 minutes and re-measure.

C. Is the temperature steady-state?

C. Uncouple and re-test.

D. Measure vibration/noise, if exceeds values in Table xxx, shut down motor.

D. Will indicate if safe to continue to operate.


4-35

EPRI Licensed Material Troubleshooting Table 4-10 (cont.) Symptom 4: High Bearing Temperature Alarm—Motor With Anti-Friction Bearings Possible Causes (By order of probability)

Verification (By order of preference)

How?

DC1 If heat source within bearing: Improper lubricant—too much or too little? 1A. Improper lubricant— too much or too little?

A. Purge old lubricant and relubricate per manufacturer’s recommendations.

A1. Over-lubrication may be tied to new motor or recent relubrication. A2. Remove drain plug temporarily. A3. Under-lubrication will usually cause permanent damage; save lubricant for future analysis. A4. Monitor bearing temperature and vibration.

1B. Contaminated lubricant

DC2 Damaged bearing

B. Purge sample while running if possible. May require tear-down and inspection.

B. Inspect grease for foreign material, moisture, discoloration, and viscosity.

C. Compare with other motor bearing.

C. Check for consistency and discoloration.

A. Record vibration in vertical, horizontal, and axial directions.

A. Check shaft and end bell fits.

B. Remove and inspect bearing, housing, shaft, seals, and lubricant.

B1. Check for bearing damage. Is bearing preloaded? B2. Conduct analysis of lubricant.

DC3 If heat source is external to bearing: Rotor/Stator generates excessive heat.

C. Check for misalignment or other unbalanced parts.

C. Check runout measurements.

A. Verify that the motor is not overloaded.

A1. Check full-load current and motor speed. A2. Check frame and iron temperature. A3. If RTDs, check temperature.

4-36

B. Check for operation on correct and balanced voltage.

B. Measure line voltage.

C. Measure ambient and cooling air.

C. Is the ventilation restricted?

EPRI Licensed Material Troubleshooting Table 4-10 (cont.) Symptom 4: High Bearing Temperature Alarm—Motor With Anti-Friction Bearings Possible Causes (By order of probability)


How?

DC3 If heat source is external to bearing: Rotor/Stator generates excessive heat (cont.)

D. If none of the above, it may be necessary to inspect the rotor and stator by disassembly of parts. It may be necessary to uncouple from load and re-evaluate.

D. Is the heat from the rotor, stator, or driver equipment or process influencing the bearing temperature?

DC4 Excessive vibration or misalignment.

A. Measure vibration levels (6 points)

A. Check Table xxx for limits.

Check alignment and run out. Inspect mounting and base.

DC5 Incorrect bearing for application.


A. Record bearing ID number.

A. Refer to motor/bearing manufacturer.

B. Describe bearing load requirement.

B. See Figure xxx.


Root Cause


CA1 Improper amount of lubricant or quality of lubricant.

Add new lubricant and purge old until proper amount achieved. This may require a tear-down depending upon the quality of old lubricant and bearing systems or seals employed. It can usually be done without a teardown.

CA2 Bearing damage

In most cases, remove the motor and arrange for tear-down and inspection of all relevant parts and fits. Purge all old grease, measure all fits, re-balance rotor, replace bearings, re-grease, reassemble, and test no-load. Measure vibration, noise, temperature, and current. Even if only one bearing is damaged, recommend replacing both. Reinstall, run, and confirm temperature, vibration, and noise are acceptable; record data.

CA3 Heat is generated by source external to bearing but within the motor.

Reduce the load and/or speed (if variable frequency drive). Measure current, watts, and volts; correct as required. Check for inadequate air intake/exhaust and recirculation.

4-37

EPRI Licensed Material Troubleshooting Table 4-10 (cont.) Symptom 4: High Bearing Temperature Alarm—Motor With Anti-Friction Bearings Root Cause


CA4 If excessive vibration or misalignment

A. In most cases, this will be a system problem associated with the mounting, coupling, or driven equipment that must be corrected by a system alignment and rebalance.

CA5 If incorrect bearing

Refer to manufacturer of motor.

Table 4-11 Symptom 5: Instantaneous Over-Current Trip Note: This Trip May Occur Upon Start, Acceleration, or Run Condition Process D1 1. Is the indication correct?

Method of Proof A. Check all RTDs, relays, sensors, and limit switches for trip.

A. Sensors may have reset with holding coil open only

B. Confirm that trip was not “ahead” of motor control.

B. If “up stream” breaker tripped it will still be open.

C. Care must be taken if ground or line fault still exists. Hi-pot, megger, and partial discharge tests are possible tools.

C. Check for damage, smoke, or other indication of ground fault.


A. On occasion, nuisance tripping may occur for no apparent reason.

A. This condition is frequently the result of some transient voltage, load, or temperature condition.

D2 Does symptoms require corrective action?

A. Usually yes, but cause may be external to motors.

A. If ground, short, or jam exists, must clear before restart.

B. May require extensive inspection after disassembly.

B. Check Table xxx for acceptable insulation to ground values.

C. Energize stator at low voltage and inspect for hot spots, noise, arcing, smoke, and so on.

4-38

Expected Result

EPRI Licensed Material Troubleshooting Table 4-11 (cont.) Symptom 5: Instantaneous Over-Current Trip Note: This Trip May Occur Upon Start, Acceleration, or Run Condition Possible Causes (By order of probability)


DC1 Extreme loading or jam usually a transient condition.

A. Check driven equipment and process.

Too much inertia to safely accelerate with existing current trip settings.

A. Check driven equipment.

How?

B. Check overload settings. C. Refer to manufacturer for acceptable WK2 value.

Heavy overload caused by driven equipment or process.

A. Check driven equipment and process.

Improper power supply

A. Check for correct voltage and balance—low voltage can cause instantaneous trips during start, acceleration, or run conditions

DC2 If internal to motor

A. Both the rotor and stator must be inspected

DC5 Line-to-line or lineto-ground fault anywhere in system can trip motor.

A. Check from motor back to main bus. A. Look for signs of fault, burn, fire, smoke, or damage. Hi pot and megger; check partial discharge, if necessary. May require disassembly di and inspection. D. Check motor leads.



4-39

EPRI Licensed Material Troubleshooting Table 4-11 (cont.) Symptom 5: Instantaneous Over-Current Trip Note: This Trip May Occur Upon Start, Acceleration, or Run Condition Root Cause (By order of probability) CA1 Determine if cause is internal to motor. If so:

Possible Corrective Action (By order of preference) A. May need to uncouple load. B. Check distribution system.

CA2 If internal to Motor: Stator winding grounded

A. Rewind

Damaged winding leads or connection

A. Repair or replace

Stator winding shorted turn to turn, coil to coil, phase to phase

A. Rewind

The rotor is “locked up” or frozen in the stator or end bell.

A. Disassemble. Usually major repair or replacement of shafts that may be bent or broken. The squirrel cage may be severely damaged.

Table 4-12 Symptom 6: Running Above Nameplate Amps (Full Load) Note: This Must Relate to Voltage at Motor Process D1 1. Is the indication correct?

Method of Proof

Expected Result

A. Take alternate method of current readings for each phase.

A. Each phase should be balanced to the extent voltage variation allows.

B. Record voltage phase to phase.

B. Check Table xxx for permissible variation.

C. Check duplicate units at the same load.

C. Current and slip should correlate per Graph xxx on motor performance curves at rated voltage.

D. Record rpm/slip. 2. Is the value anomalous?

4-40

A. Plot amps versus time.

A. Should be a function of load.

B. Vary load and track amps/speed.

B. Current and voltage are proportional to load.

C. Check each phase.

C. Should be balanced within 5%.

D. Track voltage variation.

D. Should be within -5% and +10%

EPRI Licensed Material Troubleshooting Table 4-12 (cont.) Symptom 6: Running Above Nameplate Amps (Full Load) Note: This Must Relate to Voltage at Motor Process D2 Does the symptom require corrective action?


Method of Proof

Expected Result

A. Assess impact on all alarms and trips. Is there nuisance tripping?

A. Load reduction may bring current within limit.

B. Is the condition steady or shortterm or cycling?

B. Avoid long-term operation at service factor load and limit to 115% of full load only for short time.

C. Does the winding/bearings appear to be over-heating?

C. Check bearing RTDs and stator iron temperature with thermocouples.


DC1 Determine if load has exceeded rated fullload value.

A. Consider driven equipment performance and environmental impact of systems. Consider load graphs or benchmark data.

DC2 Determine if voltage variation is acceptable.

A. Measure at bus, control, motor terminals if possible.

How?

Check other equipment operating on same bus. DC3 A. Winding damage.

A. Measure ground insulation via megger, high pot, and partial discharge; record phase resistance and compare; check connection; disassemble and inspect.

DC4 A. Rotor core damage.

A. Check current signature while under load; run no-load and repeat; disassemble and inspect for broken, loose, or melted bars/end ring.

DC5 Mechanical parts A. Mechanical interference.

A. Misalignment.

B. Bearing/seal failure.

B. Discoloration, signs of heat, damaged lubricant.

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EPRI Licensed Material Troubleshooting Table 4-12 (cont.) Symptom 6: Running Above Nameplate Amps (Full Load) Note: This Must Relate to Voltage at Motor Possible Causes (By order of probability) DC5 Mechanical parts (cont.)

C. Will increase IR losses and impact load current.

C. Blocked ventilation and damaged cooling circuit.

D. Observe color of rotor lamination surface.


Root Cause

How?


E. Check for interference between rotating and stationary surfaces. If Root Cause is deemed an External Issue as determined above, notify appropriate component or system owner. Otherwise, proceed to Possible Corrective Actions that follow. Possible Corrective Action

CA1 Excessive load.

Reduce or modify load process.

CA2 Improper voltage supply.

Revise or consult provider.

CA3 Stator damage.

Rewind, retreat, and bake.

CA4 Rotor damage.

Replace or re-bar.

CA5 Mechanical Parts A. Bearing/Lubrication failure.

A. Repair/replace.

B. Mechanical Alignment.

B. Repair, realign.

C. Ineffective ventilation.

C. Remove blockage; replace broken part.

Table 4-13 Symptom 7: The Motor Exhibits Excessive Noise While Running Loaded Process

4-42

Method of Proof

Expected Results


Measure standard octave bands of noise per ANSI S12 (see National Electrical Manufacturer’s Association (NEMA) Motor Generator (MG)1-9, Table 9-4). If possible, measure running no-load and full-load.

Compare results of proof test, or NEMA MG-1 Table 9-4, or to similar machines.


Compare to expected results; consult motor manufacturer if still in doubt.

EPRI Licensed Material Troubleshooting Table 4-13 (cont.) Symptom 7: The Motor Exhibits Excessive Noise While Running Loaded Process D2 Does symptom require corrective action?


Method of Proof

Expected Results

Compare noise levels with OSHA and other applicable standards.

Determine if hearing protection is required.

If noise is intermittent, consider if present during start, or running, or both. Is noise escalating?

If noise appears to be associated with loose parts or interference between running and stationary parts, plan immediate shutdown.


How?

DC1 Damaged bearings

Conduct vibration analysis; measure bearing temperature.

Refer to vibration data

DC2 Broken or loose parts

A. Confirm by repeating start cycle. Most of these problems are magnified during start cycle.

Use of DC1 verification method

B. Visual inspection of rotating member and associated parts. DC3 Loss of “running” clearance in air gap or between shaft and stationary parts

A. Shut down motor as soon as possible; inspect air gap and shaft to seal, bracket, and bearing cap clearances. B. Measure geometry of all appropriate rabbit fits; check for tightness of all press or interference fits (rotor to shaft, stator to frame).

DC4 Electrical or magnetic dissymmetry, including possible turn to turn shorting

DC5 Loose conductors in stator or rotor

A. Measure all possible phase/circuit currents for balance.

A. At balanced voltage, current unbalance normally will exceed 3%.

B. Measure circuit impedance of all phases/circuits, especially noise, speed, and temperature. Vibration may also increase.

B. Should be balanced within 3%. These values can be measured with field instruments.

C. Conduct a visual inspection of winding for hot spots.

C. A surge tester will detect electrical dissymmetry, including shorting.

A. Start and stop motor; will be most noticeable during acceleration— usually a “buzzing” sound that diminishes as motor comes up to speed. B. Conduct visual inspection.

Note: DC6 and DC7 are causes that normally are present upon commissioning of the motor and do not improve with time.

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EPRI Licensed Material Troubleshooting Table 4-13 (cont.) Symptom 7: The Motor Exhibits Excessive Noise While Running Loaded Possible Causes (By order of probability) DC6 Excessive airborne noise


How?

Usually the result of too much air passing through or over the motor by design. Remove external fan or block air intakes and re-test. Consult manufacturer for solution.

DC7 Excessive electrical noise

Usually the result of electrical design chosen. Run motor and de-energize; note change in noise at instant power is removed. Consult manufacturer for solution.

Root Cause


CA1 Damaged bearings

Remove bearing; purge all lubricants; clean; remove any foreign particles. Install new bearing; add lubricant and purge excess. Check rotor for balance.

Recheck noise, vibration, and temperature

CA2 Broken or loose parts

Inspect, repair, or replace

May require rebalance

CA3 Loss of “running” clearance in air gap or between shaft and stationary parts

A. Disassemble, restore fits, straighten, resize, and balance

CA4 Electrical or magnetic dissymmetry, including possible turn to turn shorting

A. Reconnect or replace winding

CA5 Loose conductors in stator or rotor

A. Re-brace and retreat stator or rewind; swage rotor bars or re-bar

CA6 Excessive airborne noise

A. Reduce flow rate by resizing and replace external fans (within thermal limit of motor). Add baffles or noise abatement material.

CA7 Excessive electrical noise

A. Reduce the line voltage to nominal value; correct voltage unbalance; remove harmonics. B. Change structure stiffness to “move” resonant frequency. Replace rotor core with different slot design.

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A. The motor manufacturer will normally guide this effort

EPRI Licensed Material Troubleshooting Table 4-14 Symptom 8: The Motor Line or Phase Currents are Unbalanced Process D1 1. Is the indication valid?

D1 2.Is the value anomalous?

Method of Proof

Expected Results

A. Measure voltage and current at T1, T2, T3 in motor control center

A. Voltage and current should be balanced within values shown in Table xxx.

B. Check motor output shaft for motion or vibration; check electrical noise.

B. Motor will produce a large electrical noise at locked rotor; shaft will try to turn and vibrate.

C. If no power at T1, T2, T3, reset and retry start.

C. If no results, refer to control system owner.

A. A simple visual inspection

A. Confirm that motor has started. B. Measure of output speed and line current will confirm motor is running at correct speed.


If power supply is available and correct at motor terminals or motor will not accelerate to full speed.

Ensure that all overloads and sensors are reset

A. Check RTD of stator and bearings, vibration sensors, or other limits switch-type devices to ensure in proper position.

B. Verify overloads set at proper level to allow for required in-rush current.

B. May eliminate trip.

A. Check to determine if drive equipment is free to rotate and not bound-up or jammed.

A. May need to uncouple or disengage motor-driven equipment.

B. Check load.

B. May need to release load.

C. It is critical to determine if the problem is external to motor.

C. If not, refer to owner of component or system.

Note: Once problem has been confirmed to be internal to the motor and not in the cable run or power supply: Possible Causes DC1 Impedance unbalance in winding caused by shorted turns or faulty internal connection

Verification Method

How?

A. Shut down motor; discharge static voltage; measure phase resistances with a resistance bridge and record ambient temperature. If possible, repeat with winding at ambient temperatures.

A. Difference in imbalance (hot/cold) may indicate a bad internal connection.

B. Conduct a surge test comparing each phase.

B. This test may not be possible on larger motors.

C. Compare results of A and B to previous test or commissioning data.

C. A rewind maybe required.

D. Disassemble motor and inspect for faults, hot spots, and other damage.

4-45

EPRI Licensed Material Troubleshooting Table 4-14 (cont.) Symptom 8: The Motor Line or Phase Currents are Unbalanced Possible Causes

Verification Method

How?

DC2 Impedance unbalance caused by original winding geometry, configuration, or rewind

Shut down motor, discharge static voltage, open connection, and record winding resistance in each circuit of motor that can be accessed.

Refer to manufacturer for allowable impedance unbalance. A 3–5% resistance may not cause any significant damage or depreciation of performance.

Obtain vendor’s design data for stator and compare.

Some smaller concentric windings have inherent dissymmetry in coil/lamination design.

May require visual inspection to ensure that no damage has occurred. DC3 Defective or incorrect motor lead connections

Root Cause

The use of thermography is effective in identifying bad connections.


CA1 Impedance unbalance in winding caused by shorted turns or faulty internal connection

The motor must be taken out of service and the winding must be repaired or replaced.

Repairing windings in this condition is usually a short term fix unless it is an exposed connection.

CA2 Impedance unbalance caused by original winding geometry, configuration, or rewind

The motor manufacturer should be consulted for the effects of the unbalance on the motor performance and reliability.

It is possible to calculate this unbalance effect on motor performance.

It the unbalance is too great to ensure required performance and reliability, then a rewind is required.

Care must be taken with the rewind not to create a new winding with the same problem.

If incorrect, reconnect per motor manufacturer’s specification.

This information is usually on a motor nameplate or operations manual.

If defective, replace lugs, repair leads, and reconnect.

It is critical to use the correct materials for this repair.

In some cases, the motor leads will have overheated and must be replaced.

This repair may require the disassembly of the motor, the replacement of all leads, and a re-treatment of the whole stator.

CA3 Defective or incorrect motor lead connections

4-46

Inspect numbering of motor leads and compare connection to nameplate or operation manual. Using the precautions listed above, disconnect, and mark all motor leads. Inspect for hot spots, arcing.

EPRI Licensed Material Troubleshooting Table 4-15 Symptom 9: Increased Motor Slip at a Constant Load Point Process D1 1. Is the indication valid?

Method of Proof

Expected Results

Measure the motor shaft speed with a strobe light or tachometer.

Compare results to previous data, including performance data provided by motor manufacturer.

If the motor shaft is not accessible it may be necessary to measure the “output” shaft (such as gear box) and calculate the motor input speed.

Some motors have access covers that allow visual inspection of rotor assembly.


By use of 2 or more methods of determining shaft speed will confirm if value is anomalous.


A small change in slip may not be a cause for concern or corrective action, if the speed change is due to increased friction in the driven equipment.

Possible Cause (By order of possibility)


DC1 A reduction in voltage at motor terminals

Measure line voltage with respect to time.

For many applications there is a voltage swing that is time-oriented. Slip is proportional to the square of the voltage.

DC2 An increase in the load caused by the driven equipment and process

Measurement of the process outputs is a key indicator. Many processes have performance curves that are good sources of validation.

Most processes have a quantifiable output (that is, cfm, psi, ft/min.)

DC3 Excessive rotor temperature caused by lack of proper cooling

A. Measure ambient temperature and compare to history or benchmarks.

A. A 10°C change in temperature will cause a speed change.

B. Measure air intake and exhaust.

B. Measure cfm and temperature.

C. Look for blockage or reduction in heat dissipation.

C. This may be from presence of foreign material.

D. Inspect driven equipment forecirculation, air back to motor intakes

D. Compare similar equipment.

A. Broken rotor bar(s) will make noise upon starting.

A. Listen to start up.

B. Vibration and noise may increase, current may fluctuate.

B. Obtain vibration profile and measure current.

DC4 Damage to the rotor

How?

4-47

EPRI Licensed Material Troubleshooting Table 4-15 (cont.) Symptom 9: Increased Motor Slip at a Constant Load Point Possible Cause (By order of possibility) DC4 Damage to the rotor (cont.)

Root Cause CA1 A reduction in voltage at motor terminals


How?

C. Disassemble motor and conduct visual inspection of bars and end rings.

C. In some cases, the cage may have melted.

D. In a service shop a single-phase low-voltage test can be conducted.

D. Damaged cages may cause noise and current fluctuations.

Possible Corrective Action A. Possible increase in supply voltage—reduce voltage drop between motor and service.

A. Sometimes transformer taps can be changed.

B. Add voltage regulation.

B. Unfortunately, in many cases no improvement can be made.

CA2 An increase in the load caused by the driven equipment and process

Reduce the flow rate of the system— may require other system or component owners.

Many loads are speedlimited, others are volumedriven.

CA3 Excessive rotor temperature caused by lack of proper cooling

A. Reduce ambient temperature.

A. Add external blower and some shields.

B. Increase availability of cooling air. C. Minimize recirculation of hot air. D. Remove foreign material from surface of motor. E. Disassemble motor and clean all surfaces. F. Place a thermal barrier between the motor and drive equipment.

CA4 Damage to the rotor

4-48

Remove motor; disassemble and repair or replace rotor.

Copper bar rotors can be rebarred. Aluminum bar rotors may have to be replaced.

EPRI Licensed Material Troubleshooting Table 4-16 Symptom 10: Motor Stalls Under Heavy Load Process D1 1. Is the indication valid?

Method of Proof A. Start the motor, increase load until motor stalls. Note if this is a timedriven event associated with the system coming “up to” temperature.

Expected Results A. Not always possible.

B. Measure electrical inputs and torque outputs. 2. Is the value anomalous?

This must be considered with respect to time.

Environmental conditions may be a factor that must be duplicated to cause a repeat stall.

D2 Does the symptom require corrective action?

This symptom almost always requires corrective action to the motor or some other elements of the system.

If stalls are infrequent and from external sources, they can usually be tolerated.

Possible Causes (By order of possibility) DC1 Excessive load

DC2 Damage to stator winding


How?

A. Inspect system for damage, being out of adjustment, clogging, and jams.

A. This may require disassembly.

B. Inspect process output for quality.

B. By other system owners.

A. Uncouple, unload, or reduce load, and run motor.

A. Measure electrical inputs and torque outputs.

B. Visual inspection of motor.

B. May require disassembly.

C. Send to service center for complete inspection and test. DC3 Damage to rotor

DC4 A reduction in voltage or severe unbalance at motor terminals

A. Run motor at reduced load or no load.

A. May require uncoupling motor.

B. Broken rotor bar may make noise upon starting.

B. Listen to start-up, if possible.

C. Vibration and noise may increase, current may fluctuate.

C. Obtain vibration profile and measure current.

D. Disassemble motor and conduct visual inspection of bars and end rings.

D. In some cases, the cage may have melted.

E. In a service shop a single phase, low voltage test can be conducted

E. Damaged cages may cause noise and current fluctuations.

Measure line voltage of each phase while running at heavy load and, if possible, during stall.

This condition may not exist at lesser loads.

4-49

EPRI Licensed Material Troubleshooting Table 4-16 (cont.) Symptom 10: Motor Stalls Under Heavy Load Possible Causes (By order of possibility) DC5 Severe restriction of cooling air or excessive ambient temperature


How?

A. Measure intake and exhaust CFM to motor and air temperature.

A. For this problem to exist, the exhaust would have to be almost completely restricted, combined with a very heavy load.

B. Check for loss of running fits of shaft and bearings. Root Cause


CA1 Excessive load

Reduce load by changing driven equipment output or system.

This may require other system or component owners

CA2 Damage to stator winding

Will require disassembly of motor and a rewind.

Depending on cause of this damage, a new winding upgrade may be possible.

CA3 Damage to rotor core

Will require disassembly of motor and repair or replace of the bars, end ring, or laminations.

CA4 A reduction in voltage or severe unbalance at motor terminals

Refer to owner of affected component.

CA5 Severe restriction of cooling air or excessive ambient temperature

Reduce ambient temperature with auxiliary cooling or remove heat source. Note: It may be necessary to restore running fit of rotor and shaft or bearing system.

4-50

This is normally a distribution problem.

Note: This heat can cause excessive thermal growth of the rotor assembly and bearings.


5

PREVENTIVE MAINTENANCE

5.1

Background

There are two goals for maintenance of the compressed air system [6,17]: •

To maintain air quality

•

To minimize air leakage

There is a universal recognition that clean air is one of the master keys to having a reliable air system. Excessive moisture, particulates, hydrocarbons, or contaminants will fault the system and components, and degrade reliability. Dryers and filters should be of adequate size and well maintained. Key Human Performance Point Air quality is the single most important means of determining the reliability of an instrument air supply. It should be the first priority and continuously monitored. Instrument air system leakage is a never-ending concern. Leakage robs the system of capacity, lengthens compressor operating time, and wastes energy. It also increases the amount of moisture and other contaminants that must be removed from the system and it creates the need for more frequent maintenance. Leakage is a problem that is difficult to detect but that silently incapacitates an air system by giving it a full-time load of wasted air. For the plant’s instrument air system to be reliable and cost-effective: •

The plant should have a comprehensive instrument air preventive maintenance (PM) program that includes trending of critical instrument air system parameters and measures of performance. This information should be monitored by maintenance personnel.

•

Instrument maintenance procedures should provide for periodic inspection and monitoring of the moisture content and desiccant contamination of the instrument air supply lines.

•

The instrument air system should be enhanced, if necessary, to facilitate the physical inspection of all critical components. It should be possible to isolate instrument air headers to perform periodic air line blowdowns.

•

There should be regular inspections of valves and all end-use components for physical damage.

5-1

EPRI Licensed Material Preventive Maintenance

•

There should be periodic valve stroking (where practical) and the stroke time of valves should be periodically checked to uncover degradation.

•

The valve operators on randomly selected valves should be disassembled and inspected each year (20% for the first and each following year; complete cycle in five years).

•

Preventive maintenance changes should be reviewed and evaluated for their effectiveness.

•

When required, the instrument air system should be cleaned by blowing air through the piping with the air operators detached. Additional filters should be installed as required.

•

When replacing air-operated and solenoid-operated valve components, it should be ensured that the replacement part numbers are correct.

•

There should be a root cause analysis program for the instrument air system, including air supply and end-use components.

•

The plant instrument list should include the maximum allowable moisture content and particle size for each air instrument.

•

The vendor instructions and recommendations should be followed.

A comprehensive instrument air preventive maintenance program is essential to maintaining system reliability and cost-effectiveness. Preventive maintenance is defined as the conduct of pre-planned or scheduled tasks necessary to ensure the safe and reliable operation of plant equipment. PM is the total summation of the scheduled tasks and their assigned task intervals, which should occur to prevent equipment failure. To help plant personnel implement PM, EPRI has written an extensive series of PM Basis guides, with each one specific by component. This section includes information and PM tables taken from the following EPRI guides. •

Preventive Maintenance Basis: Volume 8: Low Voltage Electric Motors (600V and below), TR-106857-V8, 1997.

•

Preventive Maintenance Basis: Volume 14: Reciprocating Air Compressors, TR-106857-V14, 1997.

•

Preventive Maintenance Basis: Volume 15: Rotary Screw Air Compressors, TR-106857-V15, 1997.

•

Preventive Maintenance Basis: Volume 27: Liquid-Ring Rotary Compressor and Pump, TR-106857-V27, 1997.

One component that does not have a corresponding PM Basis guide is the centrifugal-type air compressor. For this compressor, Appendix A offers PM recommendations from other industry sources. Other components that are not covered in the PM Basis guides are air dryers, receiver tanks, and piping.

5-2


The scope of this guide does not include the complete treatment of the end-use devices of airoperated valves or air-operated solenoid valves. For reference, the PM tasks for these devices are covered in the following EPRI guides: •

Preventive Maintenance Basis: Volume 1: Air-Operated Valves, TR-106857-V1, 1997.

•

Preventive Maintenance Basis: Volume 7: Solenoid Operated Valves, TR-106857-V7, 1997.

Material from other sources can be used to augment a PM program. An EPRI-sponsored working group on Instrument Air Systems published their recommendations for PM practices in 1994. These recommendations are included in Appendix A.

5.2

Overview of PM Information and Tables

The remainder of Section 5 describes PM for the following compressed air system equipment: •

Air compressors – reciprocating compressor, rotary screw compressor, and liquid ring rotary compressor and pump

•

Motors

•

Air dryers

•

Air filters

•

Inter-coolers and after-coolers

•

Separators and traps

•

Receiver tanks

•

Piping

•

Instrumentation

•

Safety relief valves

•

End-use devices

General PM recommendations for each component are presented. Where there was information from the EPRI PM Basis guides, the general PM recommendations are followed by specific information about PM tasks, failure locations, degradation mechanisms, and PM strategies for each piece of equipment, presented in table format. Specific PM tasks provide a cost-effective way to intercept the causes and mechanisms that lead to degradation and failure. PM tasks can be used to develop a complete PM program or to improve an existing program. PM tasks noted in the following tables are intended to complement and not replace the PM recommendations given by the manufacturer. The following defines the various types of tables that are included for the applicable compressed air component.

5-3


5.2.1 PM Template The PM template summarizes the program of tasks and task intervals for the equipment type. Each plant should base their program on the manufacturer’s recommendations and their own history of equipment performance. The PM template can serve as a beginning for development of a PM program for the equipment with changes added from feedback as the program is implemented. Columns labeled 1 through 8 in the PM template list the 8 sets of conditions for the compressed air system. Each column corresponds to the combined choices of critical or noncritical equipment, high- or low-duty cycle, and severe or mild service conditions. Time intervals for the performance of each task are entered at the intersections of the task rows and columns 1 through 8. The definitions of template application conditions are: Critical: Yes – Functionally important; that is, risk-significant, required for power production, safetyrelated, or other regulatory requirement. No – Functionally not important, but economically important; that is, for any of the following reasons: high frequency of resulting corrective maintenance, more expensive to replace or repair than to do preventive maintenance, has a high potential to cause the failure of other critical or economically important equipment. Duty Cycle: High – Frequently cycled or partially loaded during the greater part of its operational time Low – Fully loaded during the greater part of its operational time Service Condition: Severe – High or excessive humidity, excessive temperatures (high/low) or temperature variations, excessive environmental conditions (that is, salt, corrosive, spray, steam, low-quality suction air), high vibration. Mild – Clean area (not necessarily air conditioned), temperatures within original equipment manufacturer specifications, normal environmental conditions. Examples of components that satisfy template conditions are as follows: •

Critical, high-duty cycle, severe service condition – instrument air compressor motors, boiler circulating pump and motor, air pre-heater and motor

•

Critical, low-duty cycle, severe service condition – fire pump and motor

5-4


•

Critical, high-duty cycle, mild service condition – service air compressor and motor –

Critical, low-duty cycle, mild service condition – condenser vacuum pump and motor

– Non-critical, high-duty cycle, severe service condition – sump pump motor –

Non-critical, low-duty cycle, severe service condition – traveling screen and motor

–

Non-critical, high-duty cycle, mild service condition – condensate polisher air compressor and motor

–

Non-critical, low-duty cycle, mild service condition – maintenance shop air compressor and motor

5.2.2 Failure Locations, Degradation Mechanisms, and PM Strategies In this table, the failure locations and degradation mechanisms that each PM task is designed to deal with are listed first. Degradation mechanisms are the means that the equipment is brought to the failure point at the specified failure location. Aspects of the environment, plant operations, maintenance, or design that cause the initiation of degradation processes, or that can affect the rapidity with which they develop, are simply referred to as influences on the degradation. The next columns of degradation progression and failure timing, give information on the occurrence of the failures. The degradation progression lists whether the degradation process is present most the time (Continuous) or whether it would not normally be present but might exist or initiate in a haphazard (Random) way. The next column lists the discovery opportunity for detecting the degradation or failure. These opportunities are listed if they represent reasonably cost-effective opportunities for intervention. The last column lists the PM Strategy, which is the choice of PM tasks that the discovery opportunity is exercised within. The failure locations, degradation mechanisms, and PM strategies are listed for the following components: •

Reciprocating air compressor (Table 5-2)

•

Rotary Screw air compressor (Table 5-5)

•

Liquid ring rotary compressor and pump (Table 5-8)

•

Motors (Table 5-11)

•

Air filters (Table 5-13)

•

Coolers (Table 5-15)

•

Traps (Table 5-17)

•

Instrumentation (Table 5-19)

•

Safety valves (Table 5-21)

•

End-Use devices (Table 5-23) 5-5


5.2.3 PM Tasks and Their Degradation Mechanisms (including intervals) The PM tasks and the degradation mechanisms are listed from the previous table. Then the corresponding PM task interval in months or years is given for each PM task. In general, an increase of 25% in the stated intervals, up to a maximum of two years is believed to not cause a significant increase in failure rates. This is in the absence of equipment history that is contrary to these intervals. The PM tasks, degradation mechanisms, and intervals are listed for the following components: •

Reciprocating air compressor (Table 5-3)

•

Rotary screw air compressor (Table 5-6)

•

Liquid ring rotary compressor and pump (Table 5-9)

•

Motors (Table 5-12)

•

Air filters (Table 5-14)

•

Coolers (Table 5-16)

•

Traps (Table 5-18)

•

Instrumentation (Table 5-20)

•

Safety valves (Table 5-22)

•

End-Use devices (Table 5-24)

5.3

General PM Tasks for Air Compressors

This section covers PM for air compressors [3]. A general listing of areas to inspect and PM tasks to perform is listed. General PM tasks include: •

Visually inspect the external surfaces of the air compressor for leaks, loose connections, belt conditions, paint condition, hot spots, and so on. This inspection can provide indicators of serious operating problems, such as high vibration, oil leaks, or inadequate cooling.

•

Conduct vibration and bearing temperature monitoring. This can simply be done using a vibration meter and a hand-held pyrometer. Because of the differences in construction, typical vibration levels are not available. The manufacturer should be asked to provide suitable vibration levels for each compressor.

•

On belt-driven machines, the drive belts should be checked for evidence of deterioration, excessive wear, and improper tightness. A proper fit is indicated by the V-belts saddling the sheave groove so that the top surface rides above the highest point of the sheave. Stresses are then evenly distributed. A low-riding belt can cause slipping and a high-riding belt loses contact.

5-6


•

Belt guards are required for safety. Be sure that the guards are replaced after any maintenance work.

•

Verify compressor operation by checking compressor load and unload times against baseline operating data. Observe load and unload pressure setpoints while checking load and unload frequencies.

•

The compressor oil pressure and reservoir level should be monitored each shift and the results recorded for reference and comparison with the next set of readings.

•

Calibrate all pressure gauges, pressure switches, temperature switches, and alarms. All indicators serving a single component or machine should be calibrated as a group to establish baseline operating data.

•

Oil filters should be changed in accordance with the manufacturers’ recommendations. A low oil pressure cut-out switch could be installed. The oil filter should be changed well before the low pressure trip point is reached.

•

Frames and fittings should be lubricated according to the manufacturer’s technical manual. If no technical manuals are available, consult the equipment vendor or lubricate in accordance with the guidelines discussed.

•

Conduct a visual examination of all internal components for wear. Bearing clearances, piston clearances, rod alignments, cylinder bore sizes, etc. should be recorded for baseline operating data.

•

A similar inspection should be conducted for helical screw compressors. With oil-injected rotary compressors, the condition of the oil is extremely important and an oil sampling procedure is recommended. Hydrocarbon lubricants need to be changed every 1,000 hours of operation. Synthetic lubricants usually have a longer life and should be changed at the recommended intervals. The oil filter should be changed when the oil is changed and sooner if the pressure differential gauge indicates the need. The air/oil separator element should be changed on the same basis. The air inlet filter element should be changed when the differential pressure indicates the need or every six months if there is no indicator. Pressure and temperature indicators and shutdown devices should be checked at least every month. Newer rotary compressors equipped with microprocessor controls generally indicate the need for specific maintenance items.

•

Centrifugal compressors generally have sophisticated monitoring devices and instruments that indicate required maintenance. Maintenance procedures called for in the operator’s manual provided by the manufacturer should be followed at all times.

Preventive maintenance information for the reciprocating, rotary screw, and liquid ring rotary compressors are given in the following sections. Additional preventive maintenance practices are listed in Appendix A.

5-7


5.4

Air Compressor Types

5.4.1 Reciprocating Compressors 5.4.1.1

PM Tasks

The PM tasks for the reciprocating compressor [18] are: •

Oil analysis – Oil sampling and analysis is particularly directed at causes of bearing wear, wear on journals, and scoring of the shaft, as well as monitoring the quality and proper type of oil. Although it is reasonable to expect many years of trouble-free service from components such as bearings, shafts, and journals, these components can also fail much earlier than their expected life due to random occurrences of lubrication failure or misalignment. The interval of six months for oil analysis is chosen to give a high probability of detecting the onset of these failure causes.

•

Total closure valve lubrication – This task is directly focused on preventing diaphragm wear and failure from lack of lubrication. The diaphragm will fail in a period of months after failure of the lubrication, with a random occurrence characteristic that does not permit any assumption of operating life before failure. The closure valves should be lubricated as necessary on a one-month schedule to ensure continued trouble-free operation.

•

External visual inspection – External visual inspection is performed to identify: –

Leaks of oil, cooling water, process air, or control air

–

Proper oil level and drip rate to the cylinders

–

Loose, damaged, or missing fasteners and other parts

–

Crushed oil or air lines

–

Loose mounting bolts

–

Worn pulleys or drive belts

–

Proper lubrication of couplings

–

Blocked or broken air filters

Leaks can occur from many different locations such as: –

Water from failed gaskets

–

Failed inter-cooler or after-cooler tubes (which will affect the color of water in condensate traps)

–

Air from packing failures, leaking gaskets, loose tubing, or fittings on pneumatic devices

– A cracked cylinder body or head (also revealed by an increase in drain trap blowdown) 5-8


–

A worn shaft seal

–

A relief valve that has failed to reseat

Worn bearings can be detected by audible noise. The diverse set of degradation processes leading to the above failures have a wide spread of failure times but some can be expected to occur much sooner than others. In particular, drift of cylinder lubricators, loose fasteners and fittings, worn pulleys and belts, wear on couplings that have poor lubrication, and blocked or broken air filters are items that can degenerate rapidly or occur randomly. The recommended external visual inspection interval is one month to accommodate these failure mechanisms. •

Operator rounds – Many of the previously listed items are visible to operators on frequent rounds. This means that items such as oil level and the lubricator drip rate (or at least that oil is still flowing) can be observed even more frequently than the one-month interval recommended for external visual inspection. One item that requires constant attention is the condition of condensate traps. The bypass should be opened to blow down the trap to avoid clogging and sticking of the drain mechanism. The amount of blowdown can indicate the existence of a cracked cylinder head or block, and the color of the water can indicate leaking cooler tubes. In addition, there are physical parameters that should be recorded and trended on a daily basis. Load time and system air pressure can indicate worn piston rings, liner, scraper rings, or packing. An inter-cooler and after-cooler ∆temperature trend can indicate progressive plugging of cooler tubes. Operator rounds should include the following: –

Inspect for air (control/compressor), oil, and water leaks, clogged or excessively dirty inlet air filters and motor air-intake screens, indications of high operating temperatures (that is, discolored paint), unusual noises and odors, visual indication of vibration, unusual color of the condensate water in the traps, and proper drainage (that is, excessive or none).

–

Ensure that oil levels and pressure are normal and the lubricator drip rate is correct.

–

Record the ∆P and ∆T across the compressor filters and coolers and compare to historical measurements. Trend for future comparisons.

–

Inspect for loose, missing, or damaged parts, wiring, and tubing.

–

Check that the vents and seal bleeds are clear and operating properly.

–

Perform a coupling inspection for wear and leaking grease, if present.

–

Perform a pulley and belt inspection for wear and indications of proper tension and alignment, if present.

–

Inspect the air filter for contamination or blockage if accessible. 5-9


•

Internal inspection – The internal inspection is particularly focused on an inspection of the condition of the inlet and outlet head valves and check valves. This also provides an opportunity to inspect the condition of the piston rings and liner by inserting a feeler gauge into the space between them. The diaphragm of the total closure valve should be inspected to ensure that it is being properly lubricated. To guard against rapid deterioration that could follow a lack of lubrication, misalignment, or excessive wear, some types of coupling should also be inspected at the internal inspection if the condition cannot be assessed at the external visual inspection. Many other items are inspected during this task, as shown in the task content below. These items are inspected because of the opportunity, not because the timing is critical. The head valves are critical items because they experience a high rate of wear and can destroy the cylinders if they fail and score the liners/rings. The valves are replaced at the annual overhaul if lubrication has been less than effective. The wear rate on piston rings and liners is affected by the amount of run time, the number of starts and stops, and whether the compressor is normally fully or partially loaded. Checking the degree of wear on these components is more important for the non-lubricated type that for the lubricated type because they experience more wear on these parts, as well as on the scraper rings and packing. An internal inspection should contain the following: –

All items from the external visual inspection

–

Inspection for worn, damaged, or loose parts not accessible during the external visual inspection

–

Inspection for indications of air or water leaks on the inlet and outlet valves

–

Inspection of inlet and outlet check valves for wear and proper operation

–

Inspection of the compressor crankcase for evidence of contamination, water, or debris

–

Inspection of the high-pressure and low-pressure cylinders for indications of wear or damage

–

Check of the piston rings for wear using a feeler gauge between the cylinder wall and the piston rings; refer to OEM specification for the proper gap

–

Inspection of the packing for wear or deterioration, if accessible

–

Inspection of the oil scraper rings for wear

–

Inspection for clogged water cooling ports if a history of problems exists

–

Inspection of total closure and relief valve diaphragms for wear and deterioration

–

Verification of belt tension, if present

–

Grease and inspection of the coupling, if present

–

Inspection of the control air filters on solenoids; replace if a history of problems exists

5-10


–

Replacement of the oil filter element

–

Cleaning of the crankcase breather cap

–

Cleaning of the cylinder cooling fins

–

Cleaning and inspection of the compressor motor

This internal inspection is performed on an as-required basis. •

Overhaul – The most important item that drives an overhaul is inspection of the degree of wear on piston rings, liners, and head valves. These will be replaced if necessary. Some components (such as, scraper rings) will be replaced regardless of condition. On nonlubricated types, the rings are made to wear out and be replaced. The valves might not be replaced if they are in extremely good condition (for example, because of a light duty cycle and mild service conditions). However, there are a large number of these valves (close to 40 of them); they are not expensive to replace and each contains a significant number of parts (for example, nine springs) that can destroy the cylinder liners and rings if they fail and enter the cylinder. Replacing the total closure and relief valve diaphragms is also a key item, as well as inspecting other parts of the valves for possible replacement. The head valves (inlet, outlet, and check) typically start experiencing failures after operating for not much longer than one year. Piston rings and liners show significant wear before one year if they are the non-lubricated type. They might not be trouble-free for much longer than one year, even if lubricated. The same is true for the scraper rings. Overhaul can be important because it provides the only opportunity to observe some types and locations of degradation. Overhaul is the only task available to: – – – – – –

Observe the condition of the shaft, crosshead, and connecting rods Check for inadequate disk clearance in the total closure and relief valves Observe failed tubesheet baffles Pressure test or perform eddy current testing on cooler tubes Check alignment Check the torque on mounting bolts

Although the number of failure mechanisms is considerable, these failure locations and mechanisms do not drive this task to be performed every year because failures are expected from them individually at times much longer than one year. For the key items of head valves and piston rings, an overhaul interval of one year is suggested. If experience is favorable this might be extended to two years or even longer. The likelihood of extending the inspection intervals beyond one year diminishes with age.

5-11


An overhaul should contain the following tasks: –

All items from the external visual and internal inspections

–

Inspection of all fasteners for damage and looseness

–

Removal and inspection of the inlet and outlet head valves and check valves for wear and proper operation

–

Inspection of the piston rings and bull rings for wear

–

Inspection of the packing and scraper rings for deterioration, wear, leakage

–

Inspection of the cylinder liner, replace if necessary

–

Performance of a cross-head inspection

–

Verification that all clearances are within OEM specifications

–

Drainage of oil and performance of a crank case inspection for damage to rods, bearings, crank

–

Inspection of piston and piston rods for wear

–

Inspection of shaft where accessible for wear and damage

–

Clearance of crank case oil pump suction strainers

–

On total closure and relief valves: replacement of packing, diaphragms, and gaskets; inspection of valves and valve springs for wear and damage; cleaning of disks and seats

– Cleaning, inspection, re-lubrication, and alignment of coupling, if present

•

–

Inspection of pulleys, belts, and alignment; replacement if worn

–

Inspection of shaft seal on motor drive for leakage and deterioration

–

Overhaul of drain traps; cleaning, and replacement of orifice if necessary

–

Inspection of inter- and after-cooler tubes and tubesheets; period performance of eddy current testing (but not at every overhaul)

–

Performance of safety relief valve inspections that are due

Functional tests – The functional test is a start/load test conducted as a post-maintenance test on the equipment to verify operability and readiness for return to service. The functional test should be performed when returning powered equipment to service. The test should be performed according to startup operational procedures.

5.4.1.2

PM Template

The PM template (described in Section 5.2.1) for the reciprocating air compressor is shown in Table 5-1.

5-12

EPRI Licensed Material Preventive Maintenance Table 5-1 Reciprocating Air Compressor PM Template [18]

Critical

Columns

1

2

3

4

Yes

X

X

X

X

No Duty Cycle

High

X

Low Service Condition Severe

X X

X

6

7

8

X

X

X

X

X X

X

Mild

5

X X

X

X

X X

X X

X

PM Tasks Oil Analysis

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

Total Closure Valve Lubrication

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

External Visual Inspection

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

Internal Inspection

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

As Req.

As Req.

As Req.

As Req.

Not Req.

Not Req.

Not Req.

Not Req.

Overhaul Functional Tests

5-13


5.4.1.3

Failure Locations, Degradation Mechanisms, and PM Strategies Key Technical Point The most common failure locations and mechanisms for reciprocating air compressors are: total closure valve diaphragm failures, suction and discharge valves on the compressor heads, inability of lubricators to provide required lubrication rates, and dirty or clogged air inlet filters.

There are many other failure locations. The leading degradation mechanisms, the main physical influences on the degradation and the time progression of the degradation for each failure location is determined. The discovery opportunities or tests that could identify the condition prior to failure are given. This information constitutes a preventive maintenance (PM) strategy and is listed in Table 5-2. Table 5-3 shows the PM tasks, the degradation mechanisms, and the recommended PM task interval.

5-14

EPRI Licensed Material Preventive Maintenance Table 5-2 Failure Locations, Degradation Mechanisms, and PM Strategies for the Reciprocating Air Compressor [18] Failure Location Lubrication

Degradation Mechanism Low oil level

Degradation Influence - Personnel error

Degradation Progression

Failure Timing

Random

Random

Discovery Opportunity Inspection

- Leakage

PM Strategy - Operator rounds - External visual inspection - Lubrication program

Incorrect oil

Personnel error

Random

Random

Oil sampling

Oil analysis

Low oil flow

- Aging of pump

- Continuous

Failure-free for many years

Inspection

- Overhaul

- Clogged/crushed lines

- Random

Random

- External visual inspection

- Clogged suction screens Lubricators

Lowpressure and highpressure cylinders

- Overhaul

Low oil flow to cylinders

Drift of lubricator settings from 8 drops per minute

Random

Random (weeks)

Inspection

Clogged supply lines and check valves

Crud buildup

Wear of inlet and outlet valves and check valves

- Run time

Random

Random

Inspection

External visual inspection

Continuous

Trouble-free for 1 year run time

Inspection on a sampling basis at ~ 6 months

Internal inspection

Random

Random but <1 year

Inspection on a sampling basis at ~ 6 months

Internal inspection

- Fatigue - Inlet air quality – contamination - Lack of lubrication

- Operator rounds - External visual inspection

- Incorrect material/assembly

5-15

EPRI Licensed Material Preventive Maintenance Table 5-2 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Reciprocating Air Compressor [18] Failure Location Lowpressure and highpressure cylinders (cont.)

Degradation Mechanism Wear of piston rings/liner

Degradation Influence


Failure Timing

Discovery Opportunity

- Run time

- Continuous

- Lack of lubrication, where appropriate

- Random

- Trouble-free for > 1 year for lubricated and < 1 year for nonlubricated

-Increase in load time and decrease in pressure

- Incorrect material/specifications

- Run time

- Continuous

- Lack of lubrication, where appropriate

- Random

- Incorrect material/specifications

Cracked cylinder body or head

- Poor cooling

- Trouble-free for > 1 year for lubricated and < 1 year for nonlubricated

Random

Random

- Wrong fasteners

-Inspection

-Increase in load time and decrease in pressure

- Silt accumulation - Low cooling water flow

5-16

- Trend data from operator rounds - Internal inspection

-Inspection

- Thermography

- Thermography

- Inspection for cracks and leaks

- External visual inspection - Operator rounds (increase in drain trap blowdown)

- Improper head tightening -Water chemistry

- Internal inspection

- Random, essentially immediate

- Manufacturing defects

Clogged water cooling parts

- Trend data from operator rounds

- Random, essentially immediate

- Contamination Wear of scraper rings and packing

PM Strategy

Continuous

Expect failurefree for 1 to 3 years

- Inspection


- Water flow and temperature trending


EPRI Licensed Material Preventive Maintenance Table 5-2 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Reciprocating Air Compressor [18] Failure Location

Degradation Mechanism

Lowpressure and highpressure cylinders (cont.)

Gasket failure

Bearings

Wear

Shaft Seal

Shaft

Degradation Influence - Improper tightening

Degradation Progression Random

Failure Timing Random


- Assembly error

PM Strategy - Internal inspection - Overhaul

- Misalignment of coupling

- Random

- Random

- Oil analysis

- Internal Inspection

- Lubrication failure

- Random

- Random

- Audible noise

- Oil analysis

- Run time

- Continuous

- Expect failurefree for 5–10 years

- Inspection

- Operator rounds - Overhaul

- Manufacturing defect

- Random

- Random

- Wear

- Run time

Continuous

Inspection

- Cracking

- Heat/friction

Expect failurefree for 5–10 years


Scoring

Lubrication failure

Random

Random

- Oil analysis

- Oil analysis

- Inspection

- Overhaul

- Oil analysis

- Oil analysis

- Inspection

- Overhaul

Journal wear

- Run time

- Continuous

- Heat/friction

- Random

- Expect failurefree for 5–10 years - Random

Cracking

Random

Random, usually after overhaul

Inspection

Overhaul


- Random

Inspection

Overhaul

- Run time

- Continuous

- Random (~ 1 year)

- Alignment - Bearing failure

Crosshead and Connecting Rods

Wear

- Expect failurefree for 5–10 years

5-17

EPRI Licensed Material Preventive Maintenance Table 5-2 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Reciprocating Air Compressor [18] Failure Location Gaskets

Degradation Mechanism Leakage

Degradation Influence - Improper tightening

Degradation Progression - Random

- Assembly error

Fasteners/ Hardware

Loose/missing

Failure Timing - Random, can be immediate

- Aging (thermal cycling)

- Continuous

- Expect failurefree for > 5 years

- Vibration

Random

Random


PM Strategy - Internal inspection - Overhaul

Inspection

- Improper tightening

- External visual inspection - Internal inspection

Total Closure Valve and Relief Unloader Valve

5-18

Spring failure

Fatigue

Continuous

Failure-free up to 5 years

Inspection

Overhaul

Diaphragm failure of the regulator

- Lack of lubrication (closure valves only)

- Random

- Random (months)

Inspection

- Closure valve lubrication

- Wear (run time)

- Continuous

- Failure-free for > 1 year

Packing failure

Aging

Continuous

Failure-free for > 5 years

Inspection

Continuous


Inspection

Overhaul

Random

Random, could be immediate

Inspection

Overhaul

Seat wear or cracking

- Fatigue

Inadequate disc clearance

Personnel error

- Contamination

- Overhaul - Operator rounds - External visual inspections

EPRI Licensed Material Preventive Maintenance Table 5-2 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Reciprocating Air Compressor [18] Failure Location Pulleys and Belts

Degradation Mechanism Wear of sheaves and belts

Degradation Influence - Misalignment


Failure Timing


PM Strategy

Random

Random, could be rapid

Inspection

External visual inspections

- Random

- Random, from weeks to months

Inspection

- Overhaul

- Improper belt tension - Improper belt - Normal wear - Contamination - Aging

Coupling

Wear

- Misalignment - Lubrication

Frame and Mounting

Cracked

or

- Failure-free for > 5 years

- Aging

- Continuous

- Vibration

Continuous

Random, on a scale of a few years

Inspection

Overhaul

Continuous

Random, on a scale of 1 year

Inspection

- External visual inspections

- Soft foot Loose mounting bolts


Vibration

- External visual inspection, depending on type

- Overhaul

5-19


5.4.1.4

PM Tasks and Their Degradation Mechanisms

Table 5-3 PM Tasks and Their Degradation Mechanisms for the Reciprocating Compressor [18]

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Operator Rounds

Interval

6 Months

1 Month

6 Months

1 Year

Daily

Location/ Degradation

Compressor Random

Low oil

Random

Incorrect oil

Random

Low oil flow— clogged/crushed lines, filter, screens

Trouble-free for many years

Pump wear

Random, weeks

Lubricator drift

X

X

Trouble-free for months

Lubricator clogged

X

X

Trouble-free for 1 year, some random in <1 year

Worn head valves— inlet, outlet, and check

X

Lubricated: troublefree for up to a few years; Non-lubricated: for less than 1 year, some random– immediate

Worn piston rings or liner

X

5-20

X

X

X X

X

X

X

X

Calibration

EPRI Licensed Material Preventive Maintenance Table 5-3 (cont.) PM Tasks and Their Degradation Mechanisms for the Reciprocating Compressor [18]

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Operator Rounds

Interval

6 Months

1 Month

6 Months

1 Year

Daily

Calibration


Compressor (cont.) Random

Cracked cylinder body or head

Trouble-free for 1–3 years

Clogged water cooling ports

Trouble-free for 5–10 years; some random

Worn bearings

Failure-free for 5–10 years

Worn or cracked shaft seal

Random

Scored shaft

Random

Cracked shaft

Trouble-free for > 20 years; some random

Worn journals

Expect failure-free for 5–10 years; some random – 1 year

Worn crosshead or connecting rods

Expect failure-free for > 5 years; some random / immediate

Leaking gaskets

Random

Loose or missing fasteners or hardware

X

X X

X

X X

X X

X

X

X

X X

X

X X

X

X

X

X

5-21


Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Operator Rounds

Interval

6 Months

1 Month

6 Months

1 Year

Daily


Total Closure Valve and Relief Unloader Failure-free for up to 5 years

Failed or weak springs

X

Random – months

Failure to lube diaphragm, closure valve only

X

Failure-free for >1 year

Worn diaphragm

X


Packing failure


Seat wear or cracking

X


Inadequate disc clearance

X

X

Pulleys and Belts Random, possibly rapid

Worn sheaves

X

Random, possibly rapid

Worn belt

X

5-22

X

Calibration


Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Operator Rounds

Interval

6 Months

1 Month

6 Months

1 Year

Daily

Calibration


Coupling Failure-free for > 5 years, some random

Normal wear

X

X

X

Random, weeks to months

Lube failure

X

X

X


Wear, misaligned

X

Random, a few years

Cracked

X

Random, about 1 year


Frame and Coupling X

X

5-23


5.4.2 Rotary Screw Compressors 5.4.2.1

PM Tasks

The PM tasks for the rotary screw compressor [19] are: •

Oil Analysis – Oil sampling and analysis is particularly directed at causes of bearing wear, and wear of the bull gear, as well as monitoring the quality and proper type of oil. Although it is reasonable to expect many years of trouble-free service from components such as bearings and gears, these components can also fail much earlier than their expected life due to random occurrences of lubrication failure or misalignment. The interval of six months for oil analysis is chosen to give a high probability of detecting the onset of these failure causes.

•

External Visual Inspection – External visual inspection is performed to look for: –

Blocked or broken air filters

–

Leaks of oil, cooling water, process air, or control air

–

Proper oil level and oil usage rate (a low rate can reveal a clogged oil filter)

–

Loose, damaged, or missing fasteners, and other parts

–

Crushed oil or air lines

–


–

Proper lubrication of couplings

Leaks can occur from many different locations, such as: –

Oil from a leaking sight glass

–

Water from failed gaskets

–

Failed inter-cooler or after-cooler tubes (which will affect the color of water in condensate traps)

–

Air from leaking gaskets

–

Loose tubing or fittings on pneumatic devices

–

A worn or cracked shaft seal (revealed by air leaking from the sump breather hole)

–

A relief valve that has failed to reseat

Worn bearings can be detected by audible noise. The diverse set of degradation processes leading to the above failures have a wide spread of failure times but some can be expected to occur much sooner than others. In particular, loose fasteners and fittings, wear on couplings that have poor lubrication, blocked or broken air filters, and low oil usage are items that can degenerate rapidly or occur randomly. 5-24


The recommended external visual inspection interval is one month to accommodate these failure mechanisms. Given the ability to observe most of the above items during operator rounds, there does not appear to be a rationale for performing the external visual inspection as frequently as at one-month intervals. It is suggested that the interval could be extended to three months or longer, depending upon operating experience. •

Operating and System Engineer Rounds – Many of the previously listed items are visible to operators or other personnel on frequent rounds. This means that items such as oil level can be observed even more frequently than the noted intervals for external visual inspection. One item that requires constant attention is the condition of condensate traps. The bypass should be opened to blow down the trap to avoid clogging and sticking of the drain mechanism. The color of the water can indicate leaking cooler tubes. In addition there are physical parameters that should be recorded and trended on a daily basis. These are: –

Oil pressure (can detect a weak relief spring)

–

Inter-cooler pressure as an indicator of moisture in the inlet air

–

Cooling water flow and temperature to indicate fouling of the low-pressure and highpressure screw element cooling ports

–

Inlet low-pressure air temperature as an indicator of a stuck unloader valve

–

Pressure drop across the inlet air filter to reveal filter condition

–

Inter-cooler, after-cooler ∆T that can indicate progressive plugging of cooler tubes

Operator and system engineer rounds should include the following tasks: –

Inspect for air, oil, and water leaks, clogged or excessively dirty inlet air filters and motor air intake screens, indications of high operating temperatures (that is, discolored paint), unusual noises and odors, visual indication of vibration, unusual color of the condensate water in the traps, and proper drainage (that is, excessive or none).

–

Ensure that oil levels and pressure are normal.

–

Verify that loading and unloading pressures are within specification.

–

Verify that the condensate is being discharged from inter-cooler and after-cooler.

–

Record the ∆P and ∆T across the compressor filters and coolers, and compare to historical measurements. Trend for future comparisons.

–

Inspect for loose, missing, or damaged parts, wiring, and tubing.

–

Check that the vents and seal bleeds are clear and operating properly.

–

Perform a coupling inspection for wear and leaking grease, if present.

–

Inspect for LED indicator light failure on compressor electronic control panel, if present.

–

Inspect air filter for contamination or blockage, if accessible. 5-25


•

Internal Inspection – The internal inspection provides an opportunity to replace intake air filters and to look closely for oil leaking from locations such as the oil seal provided by the oil receiver element. Some types of coupling should be inspected at the internal inspection if the condition cannot be assessed at the external visual inspection. An internal inspection can guard against the rapid deterioration that could follow a lack of lubrication, misalignment, or excessive wear. Other items are inspected during this task, but they are done because of the opportunity, not because the timing is critical. Many of the items are observable in the external visual inspection. The lack of a specific driver for this task appears to make it a candidate for interval extension or eventual elimination. This task is performed at 4000 operating hours according to vendor recommendations but the recommendation interval lacks a clear rationale. There could be a need to ensure that some components that might be replaced at the annual overhaul are capable of avoiding failure for the remaining period of time. These degradations would involve a worn or failed rolling diaphragm in the inlet throttle valve and a worn and sticking unloader valve. An internal inspection should contain the following tasks:

•

–

All items from the external visual inspection

–

Inspect and clean the air filter element

–

Clean and inspect the oil receiver breather pipe and refill with the proper oil

–

Check the pressure drop across the breathing extension pipe and compare it to historical values

–

Inspect the motor bearing and coupling for indications of loss of grease or oil

–

Ensure that the motor’s air intake is not clogged or dirty

–

Perform shock pulse measurement for bearing condition

Overhaul – The overhaul is focused on inspection for wear of lubricated parts due to low oil quality, the replacement of the balance piston, and inspection of the unloader valve and inlet throttle valve. Key candidates for replacement of these valves are elastomers, such as the diaphragm in the unloader valve. Some other important items are cleaning or replacing the inlet suction screen, air filter, and oil filter, and taking the opportunity to check the coupling for wear, re-lubricate it, check alignment, and clean out the condensate traps and feed lines. The need to replace the balance piston and to inspect the elastomers in the unloader valve and inlet throttle valve are the main driving influences on the timing of the overhaul. Tasks addressing the screens, filters, traps, and coupling have less influence on making the overhaul interval 8000 hours. Overhaul can be also important because it provides the only opportunity to observe some types and locations of degradation. Overhaul is the only task available to observe failed tubesheet baffles and to pressure test or perform eddy current testing on the cooler tubes, to check alignment, and to check the torque on mounting bolts. These failure locations and

5-26


mechanisms do not drive this task to be performed every 8000 operating hours because failures are expected from them individually at times much longer than that. The key items of the balance piston, the unloader valve, and the inlet throttle valve suggest a minimum overhaul interval of 8000 operating hours. If experience is favorable, this might be extended. Performing an overhaul at approximately one-year intervals does not appear to be cost-effective and might not be necessary. Although utility maintenance experience with these compressors is not extensive, the lack of rationale for the internal inspection at six months suggests that the internal inspection could move to a nine-month or one-year interval and the tasks addressing the balance piston, the unloader valve, and the inlet throttle valve could be performed at the internal inspection. This would enable an overhaul to be performed at a longer interval than 8000 operating hours. Doubling the current interval of 8000 operating hours would be a reasonable starting point for interval exploration. Not withstanding these suggestions, many utilities might still elect to have a contractor perform the overhaul at 8000 operating hours because of the overall cost-effectiveness of outsourcing this type of maintenance. See Appendix D for more considerations on overhauling a rotary screw compressor. An overhaul should contain the following tasks:

•

–

All items from the external visual and internal inspections

–

Change all oil and oil filters

–

Clean float valves on the drain receivers/coolers

–

Replace the air filter element

–

Replace the unloader piston’s rolling diaphragm

–

Dismantle and inspect unloader cylinder parts for wear and damage

–

Inspect the balance piston’s diaphragm of indications of wear and loss of flexibility

–

Inspect high-pressure discharge check valve for wear or sticking

–

Test safety valves and safety relief valves

–

Test pressure and temperature sensors

–

Check the cooling efficiency of inter-cooler and after-cooler

–

Pressure test all coolers

–

Verify motor-to-compressor alignment

–

Check condition of coupling

–

Check condition of compressor elements

Functional Tests – The functional test is a start/load test conducted as a post-maintenance test on the equipment to verify operability and readiness to return to service. The functional test should be performed when returning powered equipment to service. The test should be performed according to startup operational procedures.

5-27


5.4.2.2

PM Template

The PM template (described in Section 5.2.1) for the rotary screw compressor is shown in Table 5-4. Table 5-4 Rotary Screw Air Compressor PM Template [19]

Critical

Columns

1

2

3

4

Yes

X

X

X

X

No Duty Cycle

High

X


X X

X

6

7

8

X

X

X

X

X X

X

Mild

5

X X

X

X

X X

X X

X

PM Tasks Oil Analysis

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months


1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

1 Month

Daily

Daily

Daily

Daily

Daily

Daily

Daily

Daily

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

6 Months

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

As Req.

As Req.

As Req.

As Req.

Not Req.

Not Req.

Not Req.

Not Req.

Operator and System Engineer Rounds Internal Inspection Overhaul Functional Tests

5-28


5.4.2.3

Failure Locations, Degradation Mechanisms, and PM Strategies Key Technical Point The most common failure locations and mechanisms for the rotary screw compressors are clogged or dirty air inlet filters, inlet throttle valve diaphragm, unloader piston diaphragm, and cooling water leakage for the compressor inter-cooler and after-cooler.

There are many other failure locations. The leading degradation mechanisms, the main physical influences on the degradation, and the time progression of the degradation for each failure location is determined. The discovery opportunities or tests that could identify the condition prior to failure are given. This information constitutes a preventive maintenance (PM) strategy and is listed in Table 5-5. Table 5-6 shows the PM tasks, the degradation mechanisms, and the recommended PM task interval.

5-29

EPRI Licensed Material Preventive Maintenance Table 5-5 Failure Locations, Degradation Mechanisms, and PM Strategies for the Rotary Screw Air Compressor [19] Failure Location Lubrication

Degradation Mechanism Low oil level

Degradation Influence - Personnel error


Failure Timing

Random

Random


- Leakage

PM Strategy - Operator rounds - External visual inspection - Lubrication program

Incorrect oil

Personnel error

Random

Random

Oil sampling

Oil analysis

Low oil flow

Aging of pump

Continuous


Inspection


- Clogged/crushed lines

Random

Random

Inspection, low oil use


Weak relief spring

Continuous


Inspection—trend oil pressure

Operator rounds

Leaking sight glass

Random

Random

Inspection

- Operator rounds

- Clogged oil filter

- External visual inspection LowPressure and HighPressure Elements

Wear or damage to screw elements

Moisture in inlet air

Inlet air quality— contamination

5-30

Continuous or Random

Continuous or Random

Random, on a scale of many years

- Trend intercooler pressure

Random, on a scale of many years

Sample inspection at 8000 operating hours

Overhaul

- Sample inspection at 8000 operating hours Overhaul

EPRI Licensed Material Preventive Maintenance Table 5-5 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Rotary Screw Air Compressor [19] Failure Location LowPressure and HighPressure Elements (cont.)

Degradation Mechanism Clogged water cooling ports

Degradation Influence - Water chemistry


Failure Timing

Continuous


- Inspection

- Internal Inspection

- Water flow and temperature trending


Random

Inspection


- Silt accumulation - Low cooling water flow

Gasket failure


Random


- Assembly error Unloader

Stuck

Wear

PM Strategy

- Overhaul Continuous

Expect troublefree for at least 1 year

- Inspection

Overhaul

- Trend inlet lowpressure air temperature

Bull Gear

Wear

Lubrication failure

Random

Random

Oil sampling

Oil analysis

Bearings

Wear

- Misalignment of coupling

- Random

- Random

- Oil analysis


- Run time

- Random

- Random

- Audible noise

- Oil analysis

- Manufacturing defect

- Continuous

- Nominal bearing life

- Shock pulse measurement

- Operator rounds

- Random

- Random

- Inspection

- Shock pulse measurement


- Overhaul Shaft Seal

Gaskets

- Wear

- Run time

- Cracking

- Heat/friction

Leakage


Continuous

Expect failurefree for 5–10 years

Inspection


- Random

- Random, can be immediate

Inspection


- Continuous

- Expect failure free for > 5 years

- Assembly error - Aging (thermal cycling)

- Overhaul

5-31

EPRI Licensed Material Preventive Maintenance Table 5-5 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Rotary Screw Air Compressor [19] Failure Location


Fasteners/ Hardware

Loose/missing

Degradation Influence - Vibration


Failure Timing

Random

Random


Coupling

Spring failure

Fatigue

Continuous

Failure-free for more than 10 years

Inspection

Overhaul

Rolling diaphragm failure

- Lack of lubrication (closure valves only)

- Random

- Random (months)

Inspection

Overhaul

- Wear (run time)

- Continuous

5-32

- Failure-free for > 1 year

O-ring failure

Aging

Continuous


Inspection

Overhaul

Inadequate clearances

Personnel error

Random

Random but could be immediate

Inspection

Overhaul

Wear

- Misalignment

- Random

- Random, from weeks to months

Inspection

- Overhaul

- Lubrication

Balance Piston

- External visual inspection - Internal inspection

- Improper tightening Inlet Throttle Valve

PM Strategy

Degraded material properties

- Aging

- Continuous

Aging

Continuous

- Internal inspection or

- Failure-free for > 5 years

Trouble-free for at least 1 year

- External visual inspection, depending on type Replacement

Overhaul


5.4.2.4


Table 5-6 PM Tasks and Their Degradation Mechanisms for the Rotary Screw Compressor [19]

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration


Random

Low oil

X

Random

Incorrect oil


Pump wear


Weak relief spring

Random

Clogged/crushed lubrication lines

X

Random

Clogged oil filter

X

Random

Leaking sight glass

X

Random

Clogged oil suction screen

X

Expect trouble-free for many years, some random

Abnormal wear or damage to screw elements—moisture in inlet air

X

X X X

X X X

X

5-33

EPRI Licensed Material Preventive Maintenance Table 5-6 (cont.) PM Tasks and their Degradation Mechanisms for the Rotary Screw Compressor [19] PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Time of Failure


Expect trouble-free for many years, some random

Abnormal wear or damage to screw elements—other contamination in inlet air

Trouble-free for 1–3 years

Clogged water cooling ports

X

Trouble-free for > 1 year

Worn and stuck unloader

X

X

X

Random

Worn bull gear

X

Trouble-free for vendor stated life, expect > 5–10 years, some random

Worn bearings

X

X

X

X

Failure-free for 5-10 years

Worn or cracked shaft seal

Expect failure-free for > 5 years; some random/immediate

Leaking gaskets

X

X

Random

Loose or missing fasteners or hardware

5-34

X

X

X

X

X

Calibration

EPRI Licensed Material Preventive Maintenance Table 5-6 (cont.) PM Tasks and Their Degradation Mechanisms for the Rotary Screw Compressor [19]

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration


Inlet Throttle Valve Failure-free for > 10 years

Failed or weak springs

X

Random – months

Failure to lubricate rolling diaphragm

X

Failure-free for > 1 year

Worn rolling diaphragm

X


O-ring failure

X


Inadequate clearances

X


Normal wear

X


Lubrication failure


Wear, misaligned

X

Failed

X

Coupling X

X

X

Balance Piston Failure-free for > 1 yr.

5-35


5.4.3 Liquid Ring Rotary Compressor and Pump 5.4.3.1

PM Tasks

The PM tasks for the liquid ring rotary compressor and pump [20] are: •

Performance Monitoring – Performance monitoring addresses corrosion and erosion of the internal components of the equipment and provides assurance that the seal water system is providing sufficient flow at a low enough temperature. Corrosion and erosion affect the rotor, the cone and lobe, the pump internal casing, and the inlet and outlet flanges. Seal water flow and cooling capability can be diminished by restrictions in the seal water heat exchanger or by a stuck inlet check valve. Erosion, corrosion, and seal water flow restrictions are dominant failure mechanisms for this kind of equipment. This is especially true if the design has not ensured the compatibility of the material, or the quality of the water is poor or corrosive, such as occurs with the use of saltwater for seal water on standby pumps. In poor conditions, erosion and corrosion of pump internals could produce serious degradation within two years. Seal water flow restrictions might take from months to years to develop. The interval of one year for performance monitoring is chosen to give a high probability of detecting the onset of these failure causes. Performance monitoring should include:

•

–

Trending the ∆T across the heat exchanger

–

Performing a compressor/pump flow test

Oil Analysis – Oil sampling and analysis is particularly directed at causes of bearing wear in the pump and speed reducer, wear of the speed reducer gears, and monitoring the quality and proper type of the oil. Although it is reasonable to expect many years of trouble-free service from components such as bearings and gears, these components can also fail much earlier than their expected life due to random occurrences of lubrication failure, manufacturing defects, installation errors, or misalignment. None of the above degradations is a dominant cause of failure for this kind of equipment. Other means are available to detect the onset of degradation (for example, vibration analysis and bearing temperature trending for the larger pumps). Therefore, it is prudent to perform the oil sampling at a six-month interval for critical pumps. The interval of six months for oil analysis is chosen to give a high probability of detecting the onset of these failure causes.

5-36


Pumps with packing-type seals are likely to require more frequent oil sampling than those with mechanical seals because packing is designed to produce constant leakage of water, carrying a greater risk of contaminating the oil with water. These pumps might need to have oil sampling done at three months on critical pumps and at six months on non-critical pumps. •

Borescope Inspection – This task is focused on the condition of pump internals that might suffer erosion or corrosion. It should be possible to use a borescope to examine the rotor, cone, lobe, internal casing, and inlet and outlet flanges. No interval has been assigned for this task because it is most effective when used in conjunction with other condition monitoring techniques, such as vibration analysis or performance monitoring, which would give an initial indication of a problem. Because wear on pump internals is a dominant failure cause, the borescope is a key inspection device, especially for older pumps that have not been refurbished for 20 years. In these cases, it might be prudent to perform a borescope inspection every two years. The borescope inspection should include inspection of the pump internals and inlet and discharge piping for evidence of erosion, corrosion, wear, and damage.

•

Calibration – Calibration addresses drift in electronic and electrical instruments and control devices, and it prevents deterioration in integrated system performance by detecting degradation in pressure switches, relays, and solenoid valves. Calibration is vital for ensuring the effectiveness of performance monitoring for detecting seal water flow and temperature anomalies. Drift of electrical and electronic devices can be expected to occur randomly on a scale of a few years, and random occurrences of shorts, open circuits, and loose connections can be expected on a similar time scale. Different failure mechanisms affecting pressure switches, such as leakage from sticking, plugged orifices, burnt contacts, and failed sensors, occur randomly over several years. Individually, none of the above mechanisms appears to oblige that calibration be performed as frequently as every two years but, collectively, the significant number of devices and failure mechanisms tends to support a two-year interval, at least for critical equipment. In addition, degradation of electronic devices, electrical devices, and pressure switches is not usually specifically addressed by other preventive tasks, thus increasing the reliance placed on calibration. For these reasons, it would also be reasonable to perform calibration for noncritical pumps at two years or longer, instead of relying on operator rounds to detect failures. Items that are typically calibrated are: –

Pressure switches

–

Relays

–

Electrical and electronic devices

–

Solenoids

–

Thermocouples

5-37


•

Check Valve, Strainer, and Trap Inspection – This inspection focuses on the condition of check valves, strainers, and traps. Check valves can wear and bind, leak by, or even have internal broken parts, such as a broken spring. Traps can stick open and leak by or become clogged. Check valves can be affected by debris at random times, causing them to leak by or bind. Other check valve problems might appear over two to five years, especially for equipment in standby applications. Depending on water quality, the strainer can become clogged in just a few months, and traps can become clogged in as little as six months to one year, most likely resulting in the traps sticking open and leaking. The timing of the above mechanism supports an interval of one year for this inspection on critical equipment. Perform the following tasks when inspecting the check valve, strainer, and trap: –

Inspect and clean the check valves. Note internal seat leakage, binding, and broken or weak springs, if present.

–

Inspect and clean the strainer and traps.

–

Inspect the flow control valve orifice for evidence of wear.

•

Mechanical Seal Replacement – This task is a scheduled seal replacement to address leakage of the mechanical seal. A scheduled replacement is not very effective for random events that might occur at any time and deteriorate rapidly as a result of incorrect installation or problems in the seal water system (such as contaminated water, inadequate seal water flow, or a temperature that is too high at the seal water inlet). Apart from these randomly occurring degradations, normal aging is proportional to run time, with an expected life of more than five years for continuous service. This leads to the assignment of a five-year replacement interval for critical equipment.

•

Solenoid Valve Replacement – Solenoid valve coil failures, or elastomer or gasket failures, tend to occur only after many years of trouble-free operation. Sticking, as a result of contamination, is likely to be a random event but does not have a high chance of occurring in a period less than two to three years. Plugging of the small orifices in the unloader solenoid valve could occur in a few months for high-duty cycle equipment operating with contaminated system air. Critical solenoid valve cycling should frequently be replaced after two years, especially if it exhibits a history of rapid degradation or failure. This interval could be relaxed for low-duty cycles. Refer to EPRI report Preventive Maintenance Basis: Volume 7: Solenoid Operated Valves (TR-106857-V7) for a more comprehensive treatment of preventive maintenance for solenoid valves. The replacement interval for solenoid valves associated with liquid ring compressors and pumps is shorter than for solenoid valves in general because, in this application, they are cycled more frequently. Complete replacement of the solenoid valves is recommended.

5-38


•

Filter Replacement – Element-type air filters on a compressor might become blocked in a few months up to one year, depending on the conditions. The filter elements might break on a similar time scale if they become fully loaded. Failures of the filter housing are only expected after at least five years. These failure mechanisms support a filter replacement interval of six months for critical equipment.

•

Coupling Inspection – The coupling is expected to have a life of at least 10 years under normal conditions, but is likely to experience wear at much shorter intervals as a result of poor lubrication or misalignment. These degradation processes occur randomly on a scale of months to a few years and can lead to rapid deterioration. Vibration analysis might also detect a relatively advanced state of coupling wear. These factors suggest that the coupling inspection should be performed at a two-year interval for critical equipment. For the coupling inspection, perform the following tasks:

•

–

Inspect for signs of leaking lubricant

–

Inspect all mating surfaces for cleanliness, wear, and integrity. Record as-found and asleft conditions

–

Inspect the condition of the lubricant for evidence of wear particles and water before replacement

–

Inspect gear teeth for wear and damage

–

Inspect non-metallic parts for wear and damage

–

Inspect all bolting for evidence of damage or galling

–

Ensure that all parts are properly aligned during assembly

–

Align the coupling and record readings

Moisture Separator/Seal Water Tank Inspection – This inspection focuses on the condition of the moisture separator reservoir, especially the condition of chevrons (rust and corrosion) and the degree of sedimentation. The latter does not directly cause failure but reduces the effective tank capacity. Carryover of sediment or debris increases the probability of restricting flow through the seal water heat exchanger. The float valve is also inspected to remove rust, corrosion, or clogging from metallic particles that adhere to the (sometimes) magnetic parts of the valve. The above failure mechanisms can cause failures at random times from months to many years depending on water quality. The task is not relied on to a great extent because sedimentation is not a direct failure cause, seal water flow restrictions are monitored by other means, and operator rounds should detect problems with the float valve (usually too much overflow because it fails high) before complete failure occurs. However, the need for this task and the interval assigned to it are very dependent on water quality. If the water is very corrosive (for example, saltwater) the task could be performed every two years.

5-39


The moisture separator/seal water tank inspection should include the following:

•

–

Check for the presence of debris or the evidence of corrosion inside the moisture separator tank

–

Ensure that the moisture separator float valve/switch is operating properly and exhibits no evidence of rust or corrosion

Operator Rounds – In contrast to many of the other tasks, operator rounds is aimed at detecting a wide variety of fault conditions and generally focuses on the verification of system operation (pressure and flow within specifications), observation of leaks (mainly of water), and the detection of abnormal noises. None of the degradation mechanisms addressed by operator rounds requires observation as frequently as every shift (from the point of view of taking truly preventive action) with the possible exceptions of leakage from a packingtype seal and low oil level. Many are failure-finding items such as visible leakage from the mechanical seal, a binding check valve, or a safety relief valve that has failed to seat. Other items are truly preventive but do not require particularly frequent observation, such as noise from worn speed reducer gears, stuck-open traps, seal water heat exchanger leaks, or a binding float valve. Operator rounds should include the following activities: –

Inspect for any external leakage of oil and pumping fluid, including discharge from the relief valves

–

Ensure that the compressor/pump and speed reducers are not running hot

–

Listen for unusual noises

–

Check that the level and pressure in the moisture separator tank are within the expected ranges

–

Check that there is no evidence of air blowing out of the drain traps and a normal amount of water is draining

–

Check for obvious loose fasteners and hardware

–

Inspect for obvious damage from blockage of air filters

–

Check for proper oil levels

5.4.3.2

PM Template

The PM template (described in Section 5.2.1) for the liquid ring compressor is shown in Table 5-7.

5-40

EPRI Licensed Material Preventive Maintenance Table 5-7 Liquid Ring Rotary Air Compressor and Pump PM Template [20]

Critical

Columns

1

2

3

4

Yes

X

X

X

X

No Duty Cycle

High

X


X X

X

6

7

8

X

X

X

X

X X

X

Mild

5

X X

X

X

X X

X X

X

PM Tasks Performance Monitoring

1 Year

1 Year

1 Year

1 Year

Not Req.

Not Req.

Not Req.

Not Req.

Oil Analysis

6 Months

6 Months

6 Months

6 Months

As Req.

As Req.

As Req.

As Req.

Vibration Analysis

6 Months

6 Months

6 Months

6 Months

As Req.

As Req.

As Req.

As Req.

Borescope Inspection

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

Calibration

2 Years

2 Years

2 Years

2 Years

As Req.

As Req.

As Req.

As Req.

Check Valve, Strainer, and Trap Inspection

1 Year

1 Year

1 Year

1 Year

2 Years

2 Years

2 Years

2 Years

Mechanical Seal Replacement

5 Years

5 Years

5 Years

5 Years

As Req.

As Req.

As Req.

As Req.

Solenoid-Operated Valves

2 Years

5 Years

2 Years

5 Years

As Req.

As Req.

As Req.

As Req.

6 Months

6 Months

6 Months

6 Months

Not Req.

Not Req.

Not Req.

Not Req.

Coupling Replacement

2 Years

2 Years

2 Years

2 Years

5 Years

5 Years

5 Years

5 Years

Moisture Separator/Seal Water Tank Inspection

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

As Req.

Daily

Daily

Daily

Daily

Daily

Daily

Daily

Daily

Filter Replacement

Operator Rounds

5-41


5.4.3.3

Failure Locations, Degradation Mechanisms, and PM Strategies Key Technical Point The most common failure locations and mechanisms for the liquid ring rotary compressor and pump are solenoid pilot valves, especially those in lower-quality air systems, inadequate seal water flow, insufficient seal water tank level (which allows carryover of sedimentation), erosion/corrosion of the pump internals, oil leaks contaminating the pumped or compressed medium water, and leaks from the seal cooling water into the bearings.

There are many failure locations. The leading degradation mechanisms, the main physical influences on the degradation, and the time progression of the degradation for each failure location is determined. The discovery opportunities or tests that could identify the condition prior to failure are given. This information constitutes a preventive maintenance (PM) strategy and is listed in Table 5-8. Table 5-9 shows the PM tasks, the degradation mechanisms, and the recommended PM task interval.

5-42

EPRI Licensed Material Preventive Maintenance Table 5-8 Failure Locations, Degradation Mechanisms, and PM Strategies for the Liquid Ring Rotary Compressor and Pump [20] Failure Location Packing Shaft Seal

Degradation Mechanism Excessive leakage

Degradation Influence -Age (run time)

Degradation Progression -Continuous

Expect to be failure-free for 1 to 2 years for high-duty cycle components


PM Strategy

Inspection

Operator rounds

-Inspection

-Operator rounds

-Replace mechanical seal

-Seal replacement

-Performance monitoring


-Borescope

-Borescope

-Expect to be failure-free approximately 2 years for standby pumps with saltwater cooling

-Contaminated cooling water

-Maintenance error

Failure Timing

-Random

-Random

-Continuous

-Expect to be failure-free for > 5 years

-Incorrect packing -Damaged (scored) shaft -Inadequate cooling water flow Mechanical Shaft Seal

Visible leakage

-Age (run time)

-Random

-Random

-Contaminated cooling water

-Continuous

-Random, could be rapid

Material incompatibility (for example, hot solution of bronze in de-mineralized water)

Continuous

Expect to be failure-free for only ~2 years

-Incorrect installation -Inadequate cooling

Rotor, Cone, and Lobe

Corrosion

5-43

EPRI Licensed Material Preventive Maintenance Table 5-8 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Liquid Ring Rotary Compressor and Pump [20] Failure Location


Rotor, Cone, and Lobe (cont.)

Erosion

Pump Internal Casing, including Inlet and Outlet Flanges

Corrosion

Lubrication

Erosion

Low oil level

Degradation Influence Pumping fluid quality

Degradation Progression Continuous


PM Strategy

Expect to be failure-free for only ~2 years, especially for standby pumps in saltwater



-Borescope

-Borescope

Expect to be failure-free for only ~2 years



-Borescope

-Borescope

Expect to be failure-free for only ~2 years, especially for standby pumps in saltwater



-Borescope

-Borescope

Random

Random

Inspection

Operator rounds

Material incompatibility (for example, hot solution of bronze in de-mineralized water)

Continuous

Pumping fluid quality

Continuous

-Personnel error

Failure Timing

-Leakage

Bearings or Bushings

Incorrect or contaminated oil

Personnel error

Random

Random

Oil sampling

Oil analysis

Wear

-Coupling misalignment

- Random

- Random

-Oil sampling

-Oil analysis

-Audible noise

- Operator rounds

-Vibration

-Vibration analysis

-Lubrication failure -Age (run time) -Manufacturing defect -Installation error

5-44

- Continuous

-Expect nominal bearing life

- Random

- Random

-Bearing temperature




Bearing Housing Seals, if present

Wear

-Age (run time)

Fasteners/ Hardware

Loose, damaged, or missing

-Vibration

Coupling

Wear

-Misalignment


Failure Timing

Continuous

Expect to be failure-free for 5 to 10 years

Inspection

Operator rounds

Random

Random

-Inspection

-Operator rounds

-Vibration

-Vibration analysis

- Inspection

-Coupling inspection

-Vibration

-Vibration analysis

Random, from months to years

Inspection (makeup water flow to moisture separator tank)

-Operator rounds

Random, from months to years

-∆ T trend


-Pressure test

-Moisture separator/seal water tank inspection

-Heat

PM Strategy

-Friction -Improper installation (for example, tightening) - Random

-Lubrication

Seal Water Heat Exchanger


Leak

-Age

- Continuous

-Vibration -Corrosion

Continuous

-Fluid chemistry

-Random, from months to years -Expect to be failure-free for > 10 years

-Moisture separator/seal water tank inspection

-pH of cooling water -Debris Plugged or restricted

-Vibration -Corrosion -Fluid chemistry -Debris

Continuous

-Monitor moisture separator pressure when compressor/pump is unloaded

-Operator rounds

5-45

EPRI Licensed Material Preventive Maintenance Table 5-8 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Liquid Ring Rotary Compressor and Pump [20] Failure Location Gaskets

Degradation Mechanism Leakage

Degradation Influence -Improper torque


Failure Timing

- Random

-Random, can be immediate

- Continuous


-Improper material

- Random


-Installation error -Aging (thermal cycling)


PM Strategy

Inspection

Operator rounds

Moisture Separator Tank

Sedimentation

Water quality

Continuous

Expect to be failure-free for > 10 years

Inspection

Moisture separator/ seal water tank inspection

Float Valve

Clogged

-Debris

Continuous

Expect to be failure-free for at least 2 years

-Inspection, replace if necessary

- Operator rounds

-Water quality

Seal Water Recirculation Pump and Motor

Motor failure

Strainer

Clogged

5-46

-Age

-Tank overflow

-Moisture separator/ seal water tank inspection

Continuous


Component replacement

No task, replace component at failure

Continuous

Expect clogging in months to years

-Inspection

- Operator rounds

-Mis-operation

-Check valve, strainer and trap inspection

-Heat -Moisture Debris

EPRI Licensed Material Preventive Maintenance Table 5-8 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Liquid Ring Rotary Compressor and Pump [20] Failure Location Check Valves

Degradation Mechanism Binding

Degradation Influence -Wear


Failure Timing

- Continuous

Expect to be failure-free for 2 to 5 years, 2 years for standby compressors and pumps

-Vibration

Orifice

- Operator rounds

-Inspection

-Check valve, strainer, and trap inspection

Inspection

Check valve, strainer, and trap inspection

Expect failure in a few years

-Inspection

- Operator rounds

-Audible noise



-Inspection


- Random

-Corrosion

- Continuous

-Wear

- Continuous

Expect to be failure-free for 2 to 5 years, 2 years for standby compressors and pumps

-Debris

- Random

-Random

-Corrosion

- Continuous

Broken spring, if present

Excessive vibration

Continuous

Wear

-Aging (flow)

-Vibration

-Erosion

Continuous

PM Strategy

-Mis-operation

-Debris Leak by


-Random

-Flow monitoring


5-47





Failure Timing

Continuous

Can be as short as 1 to 2 years

-Inspection

- Operator rounds

-Excessive pressure drop while compressors or pump is unloaded

-Control valve replacement

-Expect to be failure-free for 6 to 8 years

-Vibration

-Vibration analysis

-Oil sampling

-Oil analysis

-Aging

-Inspection

-Operator rounds

-Contamination

-Audible noise

Dole Flow Control Valve, if present

Rubber embrittlement (cracked or split)

-Aging

Speed Reducer

Bearing failures

-Lubrication

-Heat

- Continuous

-Vibration

-Installation error


PM Strategy

- Random


-Bearing temperature

- Random

-Random, from months to years

-Inspection

-Coupling inspection

-Vibration

-Vibration analysis

-Vibration

-Vibration analysis

-Oil sampling

-Oil analysis

-Inspection

- Operator rounds

-Misalignment -Manufacturing defects Coupling failure

-Misalignment -Lubrication -Age

Gear wear

-Lubrication level

- Continuous


Continuous

-Random, can be very rapid

-Lubrication quality

-Expect to be failure-free for 20 years

-Aging Clogged breather

5-48

-Oil -Debris

Continuous

Expect to be failure free for 2 years

-Audible noise Clean

Coupling inspection

EPRI Licensed Material Preventive Maintenance Table 5-8 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for the Liquid Ring Rotary Compressor and Pump [20] Failure Location Speed Reducer (cont.)

Degradation Mechanism Lip seal and gasket failures

Degradation Influence -Improper torque


Failure Timing

- Random


- Continuous


- Random


-Installation error -Aging (thermal cycling)

-Improper material


PM Strategy Operator rounds

5-49


5.4.3.4


Table 5-9 PM Tasks and Their Degradation Mechanisms for the Liquid Ring Rotary Compressor and Pump [20]

Failure Location

Failure Timing

Packing Shaft Seal

Expect to be failure-free for 1 to 2 years for high-duty cycle components and <2 years for standby pumps with saltwater cooling. Some random.

5-50

Operator Rounds

1 Year to 2 Years

Seal, Filter, or SOV Replacement

As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.

Check Valve, Strainer, Trap Inspection

Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval

Performance Monitoring

PM Task

2 Years to 5 Years

As Req.

Various

Daily

Degradation Mechanism Excessive leakage

X

EPRI Licensed Material Preventive Maintenance Table 5-9 (cont.) PM Tasks and Their Degradation Mechanisms for the Liquid Ring Rotary Compressor and Pump [20]

Failure Location

Failure Timing


Mechanical Shaft Seal

Normally expect to be failure-free for > 5 years, some random, could be rapid

Visible leakage

Rotor, Cone, and Lobes

Expect to be failure-free for only ~ 2 years

Corrosion

X

Expect to be failure-free for only ~ 2 years, especially for standby pumps in saltwater

Erosion

X

Operator Rounds

1 Year to 2 Years


As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

2 Years to 5 Years

As Req.

Various

Daily

X

X

5-51


Failure Location Pump Internal Casing, including Inlet and Outlet Flanges

5-52

Failure Timing



Corrosion

X


Erosion

X

Operator Rounds

1 Year to 2 Years


As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

2 Years to 5 Years

As Req.

Various

Daily


Failure Location

Failure Timing


Operator Rounds

1 Year to 2 Years

2 Years to 5 Years

As Req.

Various

Daily


Random

Low oil level

Random

Incorrect or contaminated oil

X

Bearings or Bushings

Expect nominal bearing life, some random

Wear

X

Bearing Housing Seals


Wear

Lubrication

As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

X

X

X

X

5-53


Failure Location

Failure Timing


Operator Rounds

1 Year to 2 Years

2 Years to 5 Years

As Req.

Various

Daily


Fasteners/ Hardware

Random

Loose, damaged, or missing

X

Coupling

Expect to be failure-free for >10 years, some random

Wear

X

Random, months to years

Leak


Plugged or restricted

5-54

As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

X

X

X

X

X

X

X


Operator Rounds

1 Year to 2 Years


As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

2 Years to 5 Years

As Req.

Various

Daily

Failure Location

Failure Timing


Coupling (cont.)

Expect to be failure-free for > 5 years; some random, can be immediate

Leakage

Moisture Separator Tank

Expect to be failure-free for > 10 years

Sedimentation

X

Float Valve

Expect to be failure-free for at least 2 years

Clogged

X

X

X

5-55


Operator Rounds

1 Year to 2 Years


As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

2 Years to 5 Years

As Req.

Various

Daily

Failure Location

Failure Timing


Seal Water Recirculation Pump and Motor


Motor failure

Strainer

Expect clogging in months to years

Clogged

X

X

Check Valves

Expect to be failure free for 2 to 5 years, 2 years for standby compressors & pumps, some random

Binding

X

X

5-56


Failure Location Check Valves (cont.)

Orifice

Failure Timing

Operator Rounds

1 Year to 2 Years


As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

2 Years to 5 Years

As Req.

Various

Daily


Expect to be failure-free for 2 to 5 years, for standby compressors and pumps, some random

Leak by

X

Expect failure in a few years

Broken spring, if present

X


Wear

X

X

X

5-57


Failure Location

Failure Timing

Dole Flow Control Valve

Can be as short as 1 to 2 years

Speed Reducer

Expect to be failure-free for 6 to 8 years; some random, can be immediate

5-58

Operator Rounds

1 Year to 2 Years


As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Vibration Analysis

Not Req. – 1 Year

Calibration

Oil Analysis

Interval


PM Task

2 Years to 5 Years

As Req.

Various

Daily

Degradation Mechanism -Rubber embrittlement

X

-Cracked or split Bearing failures

X

X

X

EPRI Licensed Material Preventive Maintenance Table 5-9 (cont.) PM Tasks and their Degradation Mechanisms for the Liquid Ring Rotary Compressor and Pump [20]

Speed Reducer (cont.)

Failure Timing

Operator Rounds

X

1 Year to 2 Years


X

As Req. –2 Years


As Req. – 6 Mon.

Coupling Inspection

As Req. – 6 Mon.


Not Req. – 1 Year

Calibration

Vibration Analysis

Failure Location

Oil Analysis

Interval


PM Task

2 Years to 5 Years

As Req.

Various

Daily


Expect to be failure-free for 20 years; some random, can be very rapid

Gear wear

Expect to be failure-free for 2 years

Clogged breather

X

X

5-59


5.5

Motors

The following PM information was taken from the EPRI Preventive Maintenance Basis Volume 8: Low Voltage Electric Motors (600V and below) TR-106857-V8, 1997. For larger-size motors, the PM information can be found in the following EPRI guides: •

Preventive Maintenance Basis: Volume 9: Medium Voltage Electric Motors (Between 1kV and 5kV), TR-106857-V9, 1997.

•

Preventive Maintenance Basis: Volume 10: High Voltage Electric Motors (5kV and Greater), TR-106857-V10, 1997.

5.5.1 PM Tasks The PM tasks for the motor [21] are: •

Thermography – The main application of thermography is to provide an indication of the condition of exposed electrical connections and to complement other indications of bearing wear. Thermography plays a backup role when bearing temperature is directly measured by in situ RTDs or thermocouples. Other indications of bearing wear are oil and vibration analysis. Thermography can usually only give an indication of increased temperatures in the general region of the bearing casing, where this is accessible. Thermography can also detect clogged air passages and screens. The lubrication-related causes of bearing wear appear randomly over a period of many months up to two years. Blocked airways and high-resistance electrical connections have a similar time scale. None of the causes noted is likely to fail the motor catastrophically on short time scales, so a six-month interval seems appropriate for thermography. In the case of standby motors, thermography should be performed after the motor has been running at rated speed for four hours, in order to reach a stable operating temperature and obtain valid measurements. When thermocouples or RTDs are installed, direct bearing temperature indication is likely to be monitored frequently. Vibration monitoring provides an independent indication of bearing wear. Consequently, thermography is not a critical technology for the detection of bearing wear in this class of motors. Thermography should include: –

Inspection for unusual and unbalanced heating of the connections at the main motor and motor heater leads and their respective power cable interfaces

–

Unusual differences in exit air temperatures when compared to historical values

–

Inspection for unusual heating in motor bearings and windings that cannot be attributed to normal thermal patterns and temperatures

5-60


•

Vibration Monitoring – Vibration monitoring is very effective for addressing all causes of wear in bearings of all types. Additionally, vibration monitoring addresses all causes of failures that originate in the shaft, mechanical failures in the rotor, including loose-wound rotor windings, and in the frame enclosure and mounting. Most of the causes of bearing failures appear randomly over a period of several months and up to two years. The appearance of cracks, wear, and bowing in the shaft, and all degradation mechanisms in the rotor (although random in occurrence times), are not expected within a few years. The onset of degradation in the frame, such as deformation, cracking, and soft foot, share similar timing characteristics as the shaft and rotor, although the progression to failure could be rapid if the vibration is close to a structural resonance. The suggested interval of six months should be sufficiently frequent to make vibration monitoring an effective detection method for a wide range of failure causes. Additionally, the frequency of vibration can provide specific diagnosis or focus further investigation in many instances. However, the random nature of occurrence of many of the degradation mechanisms that can, in principle, be detected, requires this task to be performed at an interval that is no longer than six months.

•

Oil Analysis – Oil sampling and analysis is particularly directed at causes of bearing wear for all types of bearings. This includes all sources of wear for bearing seals. Other failure causes that affect oil quality are failed cooling coils and other components in the oil distribution system, and cracking and wear of the shaft. Oil temperature above the rated limit can lead to degradation. Typical anti-friction bearings will usually not exceed 45°C above ambient; two-pole motors will usually not exceed 50°C above ambient. All of the noted degradation mechanisms are random in time of onset but, in most circumstances, are expected to appear over periods of many months or years and are not expected to lead to failures on short time scales. The interval of one year for oil analysis is thought to be sufficient to detect the onset of most of the failure causes.

•

Electrical Tests (Off-Line) – This task contains three main segments: measurements of winding resistance, insulation resistance, and motor circuit evaluation. The task focuses primarily on detecting degraded insulation (whether it is associated with windings, bearings, feeder cables, or motor leads), the integrity of all electrical connections, and the detection of high-resistance shorts and grounds in electrical components such as motor heaters. Electrical insulation is subject to continuous degradation. The main causes of insulation degradation are excessive heat above the rated limit, excessive starts within a short period, winding movement and vibration, age, and contamination (which can be, for example, oil, moisture, salt). Although the initiation of these influences might be random, the degradation progresses relatively slowly and is expected to give a trouble-free period of at least several years. An

5-61


exception could be high temperatures from excessive starts within a short period, which should be controlled by operational procedures. Problems with feeder cables, motor leads, connections, lugs, and switches are likely to occur randomly on various time scales, shorter than those noted. Measurement of winding resistance can detect shorts between turns. Most of the degradation addressed by off-line electrical testing produces measurable effects before failure on a time scale of a few years. Consequently, the off-line tests could be performed every two or three years and provide effective coverage for the degradation modes discussed. It is likely that only the first three items listed below would be included in every scheduled electrical off-line test. The remaining tasks could be included every other time or as required. Electrical off-line tests can only be conducted meaningfully when all parts of the motor are within 10°F (-12°C) of ambient temperature. These tests should be trended and compared to historical data to derive their maximum benefit. The tests should include some or all of the following:

•

–

Winding resistance

–

Insulation resistance

–

Motor circuit evaluation

–

Alternating current high pot

–

Direct current step voltage

–

Surge testing generally recommended only for form-wound 480-V motors

Brush Maintenance – Brush maintenance is an inspection of the brushes and slip rings. A worn, corroded, or loose brush holder or slip ring is likely to be revealed by erratic operation and excessive sparking. To some degree, degraded insulation might also be observable. Worn brushes might lead to failures in just a few weeks in problem situations, although a much longer useful life is normally expected. Slip ring problems usually show up on much longer time scales than brush problems, in years rather than weeks. The condition of the brushes is, therefore, the key consideration that controls the task interval. Wear rates will be very dependent on the accumulated run time. Continuously running motors should have brush inspections at three-month intervals unless operating history indicates otherwise. Standby wound rotor motors are very uncommon and their brush inspection should be considered on an individual basis. Operating history is crucial to finding the appropriate intervals. The following should be included in brush inspection: –

Inspect the slip rings for unusual wear, damage, or grooving

–

Inspect slip rings for discoloration, indicating loss of electrical contact with the brushes

5-62


•

–

Inspect the brushes for wear or grooving

–

Inspect the brushes for freedom of movement and for proper spring tension

–

Inspect brushes for proper operating length, as prescribed by the manufacturer

–

Inspect the brush pigtail connection for tightness and any damage

–

Inspect the slip ring and brush housing for signs of excess carbon; clean if necessary

External Visual Inspection – The external visual inspection focuses mainly on causes of visible indications of deterioration in oil quality, visible oil and grease leakage, and low oil level (either from problems with wear of bearings or bearing seals), or from any failure in the oil distribution system. External visual inspection is also effective for detecting clogged air filters and blocked air passages or screens and blocked oil breather caps. If detectors are present, bearing temperature can be monitored frequently (for example, observed every shift during operator rounds). These frequent observations are included in the external visual inspection task, both here and in Tables 5-11 and 5-12, although plants will have a separate procedure, possibly a part of operator rounds, for how they are observed, recorded, or trended. Bearing temperature is a key indication for all causes of bearing wear, failures in the oil distribution system, and other failures that can affect the life of bearings. Certain degradation processes in electrical circuits might also be observed, such as failed space heaters. The inspection includes a general observation for loose, missing, or damaged parts, and listening for unusual noises or vibrations (for example, from mechanical interference between the rotor and stator). All of the above degradation mechanisms are random in time of onset but, in most circumstances, are expected to appear over periods of many months or years and are not expected to lead to failures on short time scales. The interval of one year for the external visual inspection is thought to be sufficient to detect the onset of most of the visible failure causes. In addition to the above discussion of bearing temperature, some other items below are observable during normal operator rounds. Such items (for example, oil level and color, unusual noises) are also assumed to be included as a formal part of operator rounds, so that operator rounds do not appear as a separate PM strategy on the template or in Tables 5-11 and 5-12 . Perform the following as part of an external visual inspection: –

Inspect for damaged, loose, missing, or vibration parts, externally visible oil leaks around bearings and bearing seals, external water leaks around bearing cooling interfaces, broken or loose grounding cables, damaged conduits and seal flex, damaged wiring and insulators, damaged junction boxes and their gaskets, blocked/clogged plugged air filters, and inlet air screens

–

Inspect bearing slinger rings for proper operation and movement 5-63


–

Verify proper oil level; oil should not be discolored

–

Inspect for plugged oil sight glass vent

–

Verify proper motor strip heater status indication

–

Listen for unusual noises

•

Function Tests – The functional test is a start/run test conducted as a post-maintenance test on the motor to verify operability, proper rotation and readiness for return to service. It is also frequently used as a post-maintenance test on the driven equipment.

•

Refurbishment – This task is focused on the inspection and replacement of bearings, the condition of rotor laminations, rotor bars, retaining rings, wound rotor windings, and stator laminations and windings. In addition, the task enables checks on the shaft, frame, enclosure, and mounting for deformation and cracks. The condition of bearings is adequately addressed by vibration monitoring and is likely the main trigger for a refurbishment. Frame and mounting degradations are also covered by vibration monitoring and by visual inspection. All of the rotor degradations are also covered by vibration monitoring. The full condition of stator laminations and windings might only be revealed during refurbishment, although a good indication of the condition of winding insulation can be obtained by electrical off-line tests. Because degradation of the winding and lamination insulation is not expected to occur for many years, perhaps for 40 years, there do not appear to be any other degradation mechanisms that require refurbishment to reveal the condition of the equipment. The degradation mechanisms are also covered by one or more of the other tasks. The refurbishment task provides protection from a large number of degradation mechanisms that can cause failures over a period of many years. Although a significant number of these mechanisms are thought to initiate randomly or to progress erratically, the expert group thought that the combination of condition monitoring tasks described would lead to full refurbishment being an on-condition task that does not require a regular schedule. Exposure to heat, age, vibration, and contamination should be a significant consideration when estimating if and when to refurbish. There might be occasions when a utility decides that only bearing replacement is necessary without performing a complete tear down and removing the rotor. This can be referred to as a partial replacement, but it is still a task that is performed only when condition data indicates the need. The decision to proceed to full refurbishment will depend on indications of the presence and degree of the other degradations mentioned.

5.5.2 PM Template The PM template is described in Section 5.2.1. The PM template for low-voltage motors is shown in Table 5-10.

5-64

EPRI Licensed Material Preventive Maintenance Table 5-10 Low-Voltage Motor PM Template [21]

Critical

Columns

1

2

3

4

Yes

X

X

X

X

No Duty Cycle

High

X


X X

X

6

7

8

X

X

X

X

X X

X

Mild

5

X X

X

X

X X

X X

X

PM Tasks Thermography (Motors > 200 hp)

6 Months

6 Months

6 Months

6 Months

Not Req.

Not Req.

Not Req.

Not Req.

Vibration Monitoring (Motors > 200 hp)

6 Months

6 Months

6 Months

6 Months

Not Req.

Not Req.

Not Req.

Not Req.

Oil Analysis (Motors > 200 hp)

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

1 Year

Electrical Tests – Off-Line (Motors < 200 hp)

3 Years

4 Years

3 Years

4 Years

Not Req.

Not Req.

Not Req.

Not Req.

Electrical Tests – Off-Line (Motors > 200 hp)

3 Years

4 Years

3 Years

4 Years

As Req.

As Req.

As Req.

As Req.

3 Months

As Req.

3 Months

As Req.

3 Months

As Req.

3 Months

As Req.

1 Year

2 Years

2 Years

2 Years

Not Req.

Not Req.

Not Req.

Not Req.

As Req.

As Req.

As Req.

As Req.

Not Req.

Not Req.

Not Req.

Not Req.

Brush Maintenance External Visual Inspection Functional Tests

5-65


5.5.3 Failure Locations, Degradation Mechanisms, and PM Strategies Key Technical Point The most common failure locations and mechanisms for low-voltage electric motors are bearings (especially from failure of a driven component, normal wear, and failures of the lube oil system), stator windings (especially from excessive consecutive starts, operation above rated temperature, and other insulation degradation), mechanical and electrical connections, and terminations (windings, motor leads). There are many failure locations. The leading degradation mechanisms, the main physical influences on degradation, and the time progression of the degradation for each failure location is determined. The discovery opportunities or tests that could identify the condition prior to failure are given. This information constitutes a preventive maintenance (PM) strategy and is listed in Table 5-11. Table 5-12 shows the PM tasks, the degradation mechanisms, and the recommended PM task interval.

5-66

EPRI Licensed Material Preventive Maintenance Table 5-11 Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Bearings, AntiFriction

Degradation Wear



Failure Timing


Degraded lubricant

PM Strategy Oil analysis

- Contaminated

Random

Random

Oil sampling

Thermography

- Temperature outside

Random, continuous after failure initiation

Random, the greater the ∆T, the shorter the life

Oil level and color

Vibration analysis

Bearing temperature


Continuous

Grease and Oil – 2 years for nominal conditions

Vibration

Refurbishment

Random

Random

Oil sampling

Oil Analysis

Oil level

Thermography

Bearing temperature

Vibration analysis

- Specifications - Age Incorrect or mixed lubricant

Vibration

External visual inspection Refurbishment

Insufficient lubricant

Random

Random

Bearing temperature

Thermography oil analysis

Oil sampling


Oil level Vibration

Vibration analysis Refurbishment

5-67

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Bearings, AntiFriction (cont.)

Degradation Wear (cont.)



Excessive grease (excessive oil can occur but is less common because oil level is generally observable; consequences are similar)

Random, once initiated the grease will fail quickly

Misalignment

Random

Failure Timing Random, at least several months to failure

Thermography Vibration

Refurbishment Random, but in a few months

Vibration

Vibration

Bearing temperature

Thermography

May not progress to failure, increased wear only—several years

Vibration

Vibration

Insulation resistance checks

Electrical off-line

Based on loading – could be very rapid

Bearing temperature

Vibration

-Wear of other components

Excessive mechanical loading

Bearing temperature

PM Strategy


-Initial assembly

Circulating electric currents


Random

Random

Vibration

Refurbishment

Refurbishment External visual inspection Refurbishment

Soft foot Maintenance-induced errors Improper fit, handling, installation

5-68

Random Random

Based on loading, could be very rapid

Vibration

Vibration

Alignment

Refurbishment

Random, but rapid

Vibration

Vibration

Bearing temperature


EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Bearings, Sleeve

Degradation Wear

Degradation Influence Degraded lubricant a. Contaminated b. Temperature outside specifications c. Age


Failure Timing


PM Strategy

a. Random

a. Random

Oil sampling

Oil analysis

b. Random, continuous after failure initiation

b. Greater the ∆T, the shorter the life

Bearing temperature

Thermography

c. Oil: 18–24 months for nominal conditions

Oil level/color

c. Continuous

Vibration

Vibration analysis External visual inspection Refurbishment

Incorrect/mixed lubricant

Random

Random

Oil sampling

Oil analysis

Bearing temperature

Thermography

Oil level/color Vibration

Vibration analysis External visual inspection Refurbishment

Insufficient lubricant

Random

Random

Bearing temperature Oil sampling

Excessive oil in vertical configuration of certain designs

Random bearing failure but oil may not have failed

Random, but rapid after onset of the failure

Thermography Vibration analysis

Oil level


Vibration

Refurbishment

Bearing temperature

Thermography

Oil sampling

Vibration analysis

Oil level


Visual inspection

Refurbishment

Vibration

5-69

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Bearings, Sleeve (cont.)


Degradation Influence Misalignment


-Initial assembly

Failure Timing Random, but in a few months

-Wear of other components


PM Strategy

Vibration

Thermography

Bearing temperature

Vibration analysis

Oil analysis

Oil analysis External visual inspection Refurbishment

Circulating electric currents

Random

Random

Vibration

Vibration

Insulation resistance checks

Electrical off-line

Oil analysis Babbitt imperfection/ cold spot

Random

Random

Random


Vibration

Oil analysis

Oil analysis

Bearing inspection


Bearing temperature Increased current

Soft foot

5-70

Random


Refurbishment

Vibration

Bearing temperature Excessive mechanical loading

Oil analysis

Refurbishment Vibration Oil analysis

Vibration


Oil analysis

Refurbishment

Vibration

Vibration

Alignment

Refurbishment

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Bearings, Sleeve (cont.)


Degradation Influence Maintenance induced errors


Failure Timing Random, but rapid

-Improper fit, handling, installation

Discovery Opportunity Vibration

Vibration

Bearing temperature


Oil analysis

Oil analysis

-Not coupled on magnetic center Failure of anti-rotation pin (horizontal motors only)

PM Strategy

Refurbishment Random

Random

Vibration

Vibration

Oil analysis

Oil analysis Refurbishment

Fatigue failure of babbitt

Slow depending upon degree of rotor imbalance

Fails after ~18 months

Oil analysis

Oil analysis

Bearing temperature


Vibration

Vibration Refurbishment

Fretting due to vibration

Slow depending upon degree of rotor imbalance

Fails after ~18 months

Oil analysis

Oil analysis

Bearing temperature


Vibration


Rough journal or runner

Random

Random

Vibration


5-71

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Bearing Seals

Degradation Wear

Degradation Influence Age


Failure Timing Non-metallic seals will last up to 15 years


PM Strategy External visual inspection Refurbishment

Bearing wear or failure

Random

Less than or equal to bearing life

Incorrect installation

Random

Months

Temperature excursions

Random

Random

Imbalance or misalignment

Random

Random

Incorrect lubricant

Random

Random

Excessive grease

Random, but rapid for excessive grease after failure onset

Random, months for excessive grease

Oil analysis

Oil analysis

Material defect

Environment

Random

Debris

Continuous

Random

Inspection


Inspection


Breather Caps and Sight Vents

Blocked

Gaskets

Age

Age

Slow

Years

Material

Personnel error

Rapid

Months

Contamination Personnel error

Installation

5-72

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Connections and Piping

Degradation Installation



Failure Timing


PM Strategy

Personnel error

Random

Random

Inspection


Personnel error

Random

Random

Inspection


-Crossed threads -Incorrect torque -Incorrect thread sealant

Machine Fits

Damaged Misaligned

Oil Slinger Rings

Fatigue

Age

Continuous

Several years

Bearing temperature inspection


Cooling Coil: Bearing Cooling Water

Corrosion

Chemistry

Continuous

Random

Chemistry sampling

Refurbishment

Erosion

Flow rate

Pressure and flow loss

Oil analysis

Eddy current testing Inspection Oil analysis Installation

Personnel error

Random

Random

Oil temperature Inspection


Joint failures

Vibration Manufacturing defects Personnel error

Random

Random

Oil level

Oil analysis

Oil analysis


Bearing temperature

Refurbishment

Visual inspection

5-73

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Cooling Coil: Bearing Cooling Water (cont.)

Degradation


Coil separation component failure

Vibration

Packing/ gasket failures

Damaged fins


Failure Timing

PM Strategy

Random

Random

Inspection

Refurbishment

Incorrect material

Random

Random

Inspection

Refurbishment

Personnel error

Random

Will last for years

Oil analysis

Oil analysis

Age

Continuous

Vibration

Random

Random

Oil analysis

Oil analysis

Inspection

Refurbishment

Bearing temperature


Manufacturing defects Personnel error

Personnel error Fouling


Debris

Random

Random, but rapid upon onset

Pressure and flow loss Inspection Chemistry

Continuous

Random

Chemistry sampling Pressure and flow loss Eddy current testing Inspection Oil analysis

5-74

External visual inspection Oil analysis

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Shaft

Degradation


Shaft integrity and cracking

Latent manufacturing defects



Cyclic loading (fatigue)


PM Strategy

Vibration

Vibration

Inspection

Refurbishment

Alignment

Oil analysis

Oil analysis Wear (journal and coupling areas)

Bowing and sagging

Age

Continuous

Contamination of lubricant

Vibration

Vibration

Random

Bearing temperature


Corrosion

Continuous

Alignment

Oil analysis

Electrolysis

Continuous

Oil analysis

Refurbishment

Shaft currents

Continuous

Inspection

Misalignment

Random

Long standby without rotation in horizontal motors

Continuous in shut down state

Continuous

Filters (Air Coolers)

Clogged or dirty

Environment

Baffles

Loose hardware

Manufacturing defect

Several years

Depends on extent of bowing— Random

Vibration

Vibration

Alignment

Refurbishment

Months

Inspection

Inspection Filter program External visual inspection

Random

Random

Inspection

Refurbishment

Random

Random

Vibration

Vibration

Bearing temperature


Inspection

Refurbishment

Vibration Personnel error

Frame, Enclosure, and Mounting

Deformation

Voids in base Improper handling Impact damage

5-75

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Frame, Enclosure, and Mounting (cont.)

Degradation Cracking

Degradation Influence Structural resonance



Improper handling


PM Strategy

Inspection

Refurbishment

Vibration

Vibration

Inspection

Vibration

Impact damage Soft foot

Installation

Random

Random

Imperfections in motor base

Vibration

Vibration

Rotor: Laminations

Clogged air passages/ screens

Environment

Insulation breakdown

Contamination

Continuous

Random

Oil leaks

Thermography Continuous

Heat

Failure-free for < 40 years

Manufacturing defects

Personnel error

Physical damage

Personnel error

Fatigue

Number of starts

Inspection

Refurbishment

Thermography

Vibration

Vibration Random

Random

Inspection

Refurbishment

Thermography Random

Random

Inspection

Refurbishment

Thermography Continuous

Magnetic cycling and transients, rotor speed Settling Vibration

5-76

External visual inspection Thermography

Vibration

Loose parts

Inspection

Random

Failure-free for < 40 years, excessive starts reduces life

Inspection

Refurbishment

Vibration

Vibration

Random, but failure is rapid after onset

Vibration

Vibration

Inspection

Refurbishment

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location

Degradation

Rotor: Laminations (cont.)

Dirty

Cast Rotor: Rotor Bars and Shorting Rings

Cracked

Degradation Influence Contamination




Environment

PM Strategy Refurbishment External visual inspection

Age

Continuous

Manufacturing/design deficiency

Random

Random

Inspection

Refurbishment

Vibration

Vibration

Single phase rotor test

Cycle fatigue Thermal fatigue Number of starts Material defect (cracks or porosity)


Random

Random

Inspection

Refurbishment

Vibration

Vibration

Single phase rotor test Deformation

Lamination stack shifts

Random

Random

Inspection

Refurbishment

Depends on other component degradation

Random (rapid after onset)

Vibration

Refurbishment

Mechanical noise

Vibration

Inspection


Excessive starts Mechanical interference between rotor and stator

Wear of other components

Alignment check Corrosion

Environment

Continuous

Random

Inspection

Refurbishment

5-77

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Other Rotor: Rotor Bars and Shorting Rings

Degradation Loose



Age

Continuous


Random



PM Strategy Refurbishment

Vibration Single-phase rotor test

Cycle fatigue Thermal fatigue Number of starts Cracked

Age

Continuous


Random

Random

Inspection

Refurbishment

Vibration Single-phase rotor test

Cycle fatigue Thermal fatigue Number of starts Brazed joint failure

Thermal fatigue

Random

Random

Manufacturing/design deficiency Fatigue – cycling

Fretting Material defect (cracks or porosity)

Deformation

Vibration Manufacturing/design deficiency

Lamination stack shifts Excessive starts

5-78

Random Random

Random

Random Random

Random

Inspection

Refurbishment

Vibration

Vibration

Single-phase rotor test

Electrical off-line

Vibration

Refurbishment

Inspection

Vibration

Inspection

Refurbishment

Vibration

Vibration

Single-phase rotor test

Electrical off-line

Inspection

Refurbishment

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Other Rotor: Rotor Bars and Shorting Rings (cont.)

Degradation Mechanical interference between rotor and stator

Corrosion

Degradation Influence Wear of other components

Environment

Degradation Progression Depends on other component degradation

Continuous

Failure Timing Random (rapid after onset)

Random


PM Strategy

Vibration

Refurbishment

Mechanical noise

Vibration

Inspection

Mechanical off-line

Alignment check


Inspection

Refurbishment Mechanical off-line

Rotor: Retaining Rings

Cracked

Rotor Cage (Lamination Support)

Loose (cage is loose from shaft)

Vibration

Continuous

Corrosion

Random

Number of starts

Random

Random

Random

Weld failure Improper fit Manufacturing defect

Breakdown of insulation

Contamination

Continuous

Heat Personnel error

Physical damage

Personnel error

Inspection

Vibration

Vibration

Refurbishment

Bearing temperature

Vibration


Inspection

Random

Inspection

Vibration Manufacturing defects

Refurbishment

Uncoupled run (test for out-of-magneticcenter)

Key failure Stator Laminations

Vibration

Random

External visual inspection Electrical off-line Refurbishment

Thermography/ core loss test Refurbishment

Thermography Random

Random

Inspection

Refurbishment

Thermography

5-79


Degradation


Stator Laminations (cont.)

Contamination

Environment (dirt build-up)

Stator Windings, Blocking, Bracing, Surge Rings – includes Wedges (only for form-wound motors)

Insulation degradation

5-80




PM Strategy External visual inspection Refurbishment

Heat above rated

Vibration

Continuous

Continuous

Depends strongly on amount of heat (years)

Insulation tests

Failure free for < 40 years

Inspection for dusting

External visual inspection Electrical off-line Refurbishment Electrical off-line

Insulation tests Age

Continuous

Random

Insulation tests

Electrical off-line

Continuous

Random

Random

Insulation tests

Electrical off-line

Excessive starts

Continuous

Random

Insulation tests

Electrical off-line

Inspection

Refurbishment

Insulation tests

Electrical off-line

Insulation tests

Electrical off-line

Inspection

Refurbishment

Insulation tests

Electrical off-line

Inspection

Refurbishment

Insulation tests

Electrical off-line

Inspection

Refurbishment

Insulation tests

Electrical off-line

Inspection

Refurbishment

Electrical and surge transients

Random


Random

Foreign material

Random

Improper installation technique

Random

Movement during start

Continuous

Random Random Random Failure-free for < 40 years


Degradation

Stator Windings, Blocking, Bracing, Surge Rings – includes Wedges (only for form-wound motors) (cont.)

Loose blocking and bracing

Degradation Influence Vibration


Failure Timing Failure-free for > 6 years

Discovery Opportunity Inspection for dusting

PM Strategy Electrical off-line Refurbishment

Insulation tests Excessive starts

Random

Random

Insulation tests

Procedural compliance Insulation tests


Random


Random

Improper installation techniques

Random


Continuous

Broken surge ring


Failure of surge ring support brackets Fretting of insulation materials

Random

Insulation tests

Electrical off-line

Inspection

Refurbishment

Insulation tests

Electrical off-line

Inspection

Refurbishment

Insulation tests

Electrical off-line

Inspection

Refurbishment


Insulation tests

Electrical off-line

Inspection

Refurbishment

Random

Random

Inspection

Refurbishment

Flexing during start and thermal expansion of stator coils

Random

Random

Inspection

Refurbishment

Movement during running

Continuous

Failure-free for years

Inspection

Refurbishment

Random Random

5-81

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Rotor: Wound Rotor Windings, Wedges, Pole Pieces, Banding

Degradation Broken ties



Coil movement

Continuous

Improper installation

Random

Improper materials

Random

Loose windings

Loose wedges

Random


Heat above rated

Failure Timing

Vibration

Continuous

PM Strategy

Random

Inspection

Refurbishment

Random (years)

Vibration

Refurbishment

Inspection

Vibration

Depends strongly on amount of heat (years)

Insulation tests



Inspection for dusting

Rotational force Continuous


Electrical off-line Refurbishment Electrical off-line

Insulation tests Age

Continuous


Insulation tests

Electrical off-line

Contamination

Random

Random

Insulation tests

Electrical off-line

Inspection

Refurbishment

Procedural compliance

Procedural compliance

Insulation tests

Electrical off-line

Excessive starts

Random


Random

Random

Insulation tests

Electrical off-line


Random

Random

Insulation tests

Electrical off-line

Inspection

Refurbishment

Insulation tests

Electrical off-line

Inspection

Refurbishment

Improper installation techniques

5-82

Random

Random

Random

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Rotor: Wound Rotor Windings, Wedges, Pole Pieces, Banding (cont.)

Degradation




PM Strategy

Insulation tests

Electrical off-line

Foreign material

Inspection

Refurbishment


Insulation tests

Electrical off-line

Inspection

Refurbishment


Vibration

Refurbishment

Inspection

Vibration

Winding resistance test

Electrical off-line

Foreign material

Random


Continuous

Loose pole pieces and banding

Age

Continuous

Temperature rating

Continuous

Degraded connections

Random


Random

Loose

Random

Contamination

Continuous


Random

Failure Timing


Random Random

Speed stability Excitation current Excitation current stability

Rotor: Wound Rotor Slip Ring

Surface condition

Polarity

Continuous

Environment

Continuous

Spring tension

Random

Random

Inspection

Brush maintenance

Brush type Sticking Brush holder positioning Arcing

5-83

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Rotor Wound: Brushes

Degradation Wear



Polarity

Continuous

Environment

Continuous

Collector ring surface

Random

Spring tension

Failure Timing Failure-free for years


PM Strategy

Inspection

Brush maintenance

Random, but failure can occur within weeks after onset

Brush type Sticking Brush holder positioning

Feeder Cables

5-84

Length

Personnel error

Random

Random, but failure can occur within weeks after onset

Inspection

Brush maintenance


Contamination, especially water

Continuous

Failure-free for years

Inspection

Electrical off-line

Age

Continuous



Random

Random

Damage

Random

Random

Temperature

Continuous

Based on degrees above operating, years

Improper testing

Random

Random

Insulation tests

EPRI Licensed Material Preventive Maintenance Table 5-11 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Motors [21] Failure Location Motor Leads

Electrical Connections

Detectors and Switches

Degradation



Failure Timing


PM Strategy

Contamination

Continuous

Years

Inspection

Electrical off-line

Age

Continuous


Insulation tests

Refurbishment


Random

Random

Damage

Random

Random

Temperature

Continuous

Based on degrees above operating, years

Improper testing

Random

Random

Vibration and starting movement

Continuous

Random


Random

Random


Random

Random

Winding resistance

Electrical off-line

Contamination

Continuous

Thermography

Refurbishment

Thermal cycling

Continuous

Ductor testing

Thermography

Vibration and movement

Continuous

Inspection

Improper material for brazed/soldered connections

Continuous


Improper installation (insulation)

Random

Broken, loose, or grounded connections

Personnel error

Random


High resistance

Random

Insulation resistance

Refurbishment

Inspection

Electrical off-line

Instrument drift

Electrical tests

Refurbishment

Heat

Calibration check

Random

Electrical off-line

Vibration

5-85


Degradation

Bearing Insulation

Broken/cracked

Space Heaters

Degradation Influence Improper handling

Random



PM Strategy Refurbishment

Electrical tests Insulation degradation

Contamination

Continuous


Personnel error

Broken, loose, or grounded connections

Personnel error

Random

Vibration

Continuous

Open element

Age

Random

Inspection

Refurbishment

Electrical tests Random

Random

Inspection

Refurbishment

Electrical tests Random

Inspection Electrical tests

External visual inspection Electrical off-line

Failure of control circuit component

5-86


Continuous

Random

Can expect ~ 10– 20 years before failure Random

Inspection Electrical tests

External visual inspection Electrical off-line


5.5.4 PM Tasks and Their Degradation Mechanisms Table 5-12 PM Tasks and Their Degradation Mechanisms for Motors [21] Anti-Friction Bearings

Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required


Random – 24 months

Wear – Degraded lubricant

X

X

X

X

X

Random

Wear – Incorrect lubricant

X

X

X

X

X

Random

Wear – Insufficient lubricant

X

X

X

X

X

Random – > several months

Wear – Excessive grease

X

X

X

X

Random – > several months

Wear – Excessive oil

X

X

X

X

Random – several months

Wear – Misalignment

X

X

X

> Several years

Wear – Electric currents

X

X

Random – rapid, based on loading

Wear – Soft foot

X

X

Random – rapid

Wear – Personnel error

X

X

X

X

5-87

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Sleeve Bearings

Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required


Random > 24 months

Wear – Degraded lubricant

X

X

X

X

X

Random

Wear – Incorrect lubricant

X

X

X

X

X

Random

Wear – Insufficient lubricant

X

X

X

X

X

May not fail – Random

Wear – Excessive oil in vertical configurations of certain designs

X

X

X

X

X

Random – a few months

Wear – Misalignment

X

X

X

X

X

Random

Wear – Electrical currents

X

X

X

Random – Rapid based on loading

Wear – Excessive mechanical loading

X

X

X

Random – Rapid based on loading

Wear – Soft foot

Random – rapid

Wear – Personnel error

5-88

X X

X

X

X

X

X

X

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Sleeve Bearings (cont.)

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

X

X

X

X

X

X

Random

Wear – Babbitt imperfection/cold spot

X

Random

Wear – Failed anti-rotational pin (horizontal motors)

>18 months

Wear – Fatigue failure of Babbitt

X

X

X

X

>18 months

Wear – Fretting (vibration)

X

X

X

X

Random

Wear – Rough journal or runner

X

X

5-89

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Oil Distribution

Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required


Random

Blocked breather caps or sight glass vents

X

Random – months to years

Failed gaskets

X

Random

Pipes/connections

X

X

Random

Machine fits

X

X

Failure free for years

Failed slinger rings

X

X

Random

Failed cooling coil

X

5-90

X

X

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Shaft

Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

X

X

X

X

X

X

Location/Degradation

Random

Integrity/cracks

Failurefree for several years

Wear

Random

Bowing/sagging

X

X

X

Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Air Filters

Time of Failure Months

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Location/Degradation Clogged

X

5-91

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Air Baffles

Time of Failure Random

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Location/Degradation Loose

X

Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Frame, Enclosure, Mounting Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required


Random

Deformation

Random – rapid near resonance

X

X

Cracking

X

X

Random

Soft foot

X

Random

Clogged air passages/screens

5-92

X

X

X

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Cast Rotor

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Time of Failure


Random – or 10–40 years

Lamination, all sources

Random – rapid

Rotor/stator mechanical interference

Random

Cracked retaining rings

X

X

X

X

X

X

X

5-93

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Other Rotor

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Time of Failure


Random – or 10–40 years

Lamination, all sources

X

X

Random

Loose laminations

X

X

Random

Failed rotor bars or shorting rings

X

X

Random – rapid

Rotor/stator mechanical interference

X

X

Random

Cracked retaining rings

X

X

5-94

X

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Wound Rotor

Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required


Random – years

Loose windings

X

Random


Random

Loose pole pieces and banding

Random

Degraded connections

Random

Worn slip ring

X

Random – weeks

Worn brushes

X

Random – weeks

Wrong length brushes

X

X

X X

X

X

X

5-95

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Stator

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Time of Failure



Insulation breakdown of laminations

X

Random

Defective/damaged laminations

X

Random

Contaminated laminations

Random – many years

Winding insulation degradation

X

Random – many years

Loose blocking and bracing

X

Random – years

Other stator winding failures

5-96

X

X

X

X

X

X

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Feeder Cables

Time of Failure Random – years

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Location/Degradation Degraded insulation

X

Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Motor Leads

Time of Failure Random – years

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Location/Degradation Degraded insulation

X

X

5-97

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Electrical Connections

Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required


Random

High resistance

X

Random

Degraded insulation

X

X

X X

Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Detectors and Switches Time of Failure Random

5-98

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

Location/Degradation Broken, loose, grounded

X

X

EPRI Licensed Material Preventive Maintenance Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Bearing Insulation

Time of Failure

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required


Random

Broken

X

Random

Degraded insulation or personnel error

X

Table 5-12 (cont.) PM Tasks and Their Degradation Mechanisms for Motors [21] Space Heaters

Time of Failure Random – many years

PM Task

Electrical Off-Line


Brush Maintenance

Thermography

Vibration

Oil Analysis

Refurbishment

Interval

3–4 years

1–2 years

3 months

6 months

6 months

1 year

As Required

X

X

Location/Degradation Failed space heaters

5-99


5.6

Air Dryers

Air dryers [3] are required to dry the instrument air supply. Proper operation of all upstream components helps to ensure that the dryers will be capable of performing to their optimum ability. Their integrity can be ensured by monitoring the following: •

The dryer tower switching valves

•

The drain valves and drain traps

•

The dew points of the dried air

•

The pressure drops through the dryers

•

The purge air flow rates (for efficient regeneration of the desiccant)

•

The desiccant levels (can indicate breakdown or carryover)

•

The desiccant conditions (for aging or contamination)

•

The dryer tower temperatures (for heat regenerated dryers)

•

The condition of the dryer moisture indicators

An added safety feature that is often installed on dryers is a purge shutoff valve. This valve automatically secures purge flow in the event of decreasing instrument air pressure or a dryer malfunction. The valve’s closure will help to minimize the system pressure drop and will prevent loss of control functions during system transients, or if a dryer switching failure occurs. The following are PM tasks for the compressed air dryers: •

Check the outlet dew point by using an in-line hygrometer, a portable hand-held hygrometer, or a moisture indicator. A silica gel moisture indicator does not produce an accurate dew point reading, but starts changing color from blue to pink or red at a dew point of approximately 0°F (-17.8°C).

•

Check the operation of the inlet switching valve on desiccant dryers. This valve, if not properly maintained, can cause the dryer towers to either not switch over or to get stuck in between towers. Depending on the manufacturer, the dryer might vent to atmosphere or might block the flow. Either way, instrument air supply pressure is lost to the downstream piping. Repair or replace the inlet switching valve as necessary to ensure that the dryer operates properly. Note: Some of the switching valves on older model dryers require frequent grease lubrication and a complete overhaul once a year. Excessive grease has been found on some occasions to be a source of air system degradation.

•

Check the operation of automatic drain valves and traps on refrigerant dryers. Repair or replace as necessary to ensure that condensate does not accumulate in the dryer.

5-100


•

Pressure drop across the air dryer should be tested periodically to ensure a free air path through the dryer. On desiccant-type dryers, a high differential pressure might indicate a breakdown of the desiccant and its subsequent compaction. On a refrigerant dryer, a high differential pressure indicates blocked or fouled tubing. A zero differential pressure might indicate a torn or incorrectly installed filter.

•

On desiccant dryers, the purge air flow rate should be checked to ensure proper flow for efficient regeneration.

•

Desiccant levels in the towers should be checked to ensure that proper amounts of desiccant are installed. Low desiccant levels might be an indication of desiccant breakdown and the need for replacement. This might also indicate desiccant carry-over. Desiccant should be replaced in accordance with the manufacturer’s recommendations.

5.7

Air Filters

Pre-filters and after-filters [3,6] are used upstream and downstream of the air dryers. Pre-filters are usually of the coalescing type to remove excessive moisture, oil, and particulates. The prefilters help to prevent desiccant bed contamination resulting in short life times. The after-filters are used to trap desiccant fines and to ensure dryer particulate outputs meet ISA standards. Some preventive maintenance activities for the air filters include: •

Conduct a visual examination of all internal and external areas of filter housing and silencer surfaces for coating condition and hydrocarbon impingement. Rusty interior surfaces require immediate correction to prevent compressor damage. External corrosion should be corrected as time permits to protect the equipment. A walkdown should be conducted semi-annually.

•

Verify inlet filter differential pressure. If a differential pressure indicator is not included by the manufacturer, check with a U-tube manometer or a differential pressure gauge set. An initial reading should be taken on each new filter element installed as baseline operating data (BOD). An increase of 4–5 in. (100–125 mm) of water over the initial reading indicates that the filter needs replacing or cleaning. A sudden drop in differential pressure might indicate a failed or torn element that needs replacement. Cleanable intake filters can be cleaned either by blowing low-pressure air in a counter-flow direction or by soaking and agitating them in warm (140–150°F/60–65°C) water containing a non-volatile cleaning solution. Air dry the washed elements for 24 hours before reuse. After two or three cleanings, filters should be replaced, regardless of their condition.

•

The flexible coupling on the inlet piping (see Figure 5-1) should be checked for tightness and material condition. A cracked surface or signs of boundary penetration indicate a need for replacement.

5-101


Figure 5-1 Flexible Coupling for a Compressor [3]

To maintain filter integrity, the following should be monitored: •

Pre-filter automatic drain traps and valves. Traps and valves that are stuck open can cause excessive air loss, and failed closed traps and valves can cause a backup of moisture and allow contamination of the desiccant.

•

Filter differential pressures. This is required to determine when filter replacement is necessary and to alert operators to potential filter failures.

The following are PM tasks for the pre-filters and after-filters: •

Check pre-filter automatic drain valves for proper operation. These valves often stick open and cause costly air losses.

•

Perform air quality tests to check for compliance with ANSI/ISA-S7.0.01-1996 requirements. Dew points can be checked using in-line hygrometers or portable measuring equipment. Particulate can be measured by using a laser-based particle counter or by blowing air through a paper filter for one minute and sending it to a lab for analysis. Hydrocarbon or oil contamination can be measured by blowing an air sample into a Tedlar bag and shipping it to a lab for analysis or by using an oil stick (which uses a dye) to indicate hydrocarbon concentration. Each of these methods has advantages and disadvantages that must be addressed for each plant’s application. The particular equipment mentioned in this paragraph is not the only equipment available to do these tests; it is mentioned here as an example of the various methods currently available.

•

Check the pressure drop across the pre-filters and after-filters to determine available filter capacity. This can be accomplished by using a differential pressure gauge or subtracting the outlet pressure from the inlet pressure. The allowable pressure drop at which the filter elements are to be replaced should be determined for each installation based on its size and vendor recommendations. Old elements should be tagged and marked for cleaning as they are removed, or they should be destroyed. New elements should be color-coded and tagged by size and type if stored in a common area.

Table 5-13 lists the failure locations, degradation mechanisms, and PM strategies for the air filters. Table 5-14 shows the PM tasks and their degradation mechanisms. 5-102

EPRI Licensed Material Preventive Maintenance Table 5-13 Failure Locations, Degradation Mechanisms, and PM Strategies for Air Filters [19] Failure Location Inlet Air Filters, Element Type

Degradation Mechanism Blocked

Degradation Influence Environment


Failure Timing

Random (element-type only)

Failure-free for months to 1 year


Continuous

PM Strategy - Operator rounds (monitor pressure drop across filter) - External visual inspection - Overhaul (annual replacement at overhaul)

Broken

- Environment

Random

Random

Inspection

- dirt - ice


- Blocked

- Overhaul (annual replacement at overhaul)

- Bypassed due to installation error

Inlet Air Filters, Oil Bath Type, Reciprocating Compressors

Failed filter housing

- Corrosion

Blocked

Environment

- Operator rounds (monitor pressure drop across filter)

Continuous

Expect failurefree period of at least 5 years

Inspection


Continuous


Inspection

Overhaul

- Application error

5-103

EPRI Licensed Material Preventive Maintenance Table 5-14 PM Tasks and Their Degradation Mechanisms for Air Filters [19] Inlet Air Filters, Element Type

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration


Expect to operate for months to 1 year

Blocked

X

X

X

Random

Broken

X

X

X


Failed housing

X

Table 5-14 (cont.) PM Tasks and Their Degradation Mechanisms for Air Filters [18] Oil Bath Type Inlet Filters for Reciprocating Compressors

PM Task

Oil Analysis


Internal Inspection

Overhaul

Operator Rounds

Interval

6 Months

1 Month

6 Months

1 Year

Daily

Time of Failure


Failure-free for 1 to 3 years

5-104

Blocked

X

Calibration


5.8

Inter-Coolers and After-Coolers

The following are PM tasks for the inter-coolers and after-coolers [3]: •

Monitor and record cooling water inlet, outlet, and approach temperatures. This provides baseline data to identify indications of reduced heat transfer in the heat exchanger due to corrosion buildup or reduced flow. It is common for the approach temperature (the difference between the water supply temperature and the air outlet temperature) to be 15°F (8°C). Where desirable, it is possible to have coolers with a much smaller approach temperature, down to 2°F (1°C), but with a significant increase in size and cost.

•

Inspect and clean the inter-cooler and after-cooler heat exchanger tubes in accordance with the manufacturers’ recommendations. The most common cause of inter-cooler and aftercooler inefficiency is a buildup of scale deposits on the heat exchanger surfaces. Where no manufacturers’ recommendations are available, the scale deposits can be removed by using a stiff brush or a strong blast of steam or air. In heavily fouled tube nests, the units should be boiled for half an hour in a cleaning solution using 4 ounces (120 ml) of trisodium phosphate per gallon (4 l) of water, followed by a thorough flushing with plain water. Scale inside the tubes can be removed by using a mild citric acid soak, followed by brushing and water flushing.

•

Closed cooling water and makeup water chemistry should be closely monitored to ensure that all rust inhibitors, biocides, and antifoaming agents are present in the proper concentrations.

Table 5-15 shows the failure locations, degradation mechanisms, and PM strategies. Table 5-16 lists the PM tasks and their degradation mechanisms.

5-105

EPRI Licensed Material Preventive Maintenance Table 5-15 Failure Locations, Degradation Mechanisms, and PM Strategies for Coolers [19] Failure Location Inter- and AfterCoolers, Tubes

Degradation Mechanism Leak

Degradation Influence - Vibration


Failure Timing

Continuous


- Corrosion - Fluid chemistry - pH of cooling water

Discovery Opportunity - Inspection – water color in traps - Eddy current – but not at every overhaul

PM Strategy - Operator rounds - External visual inspection - Overhaul

- Debris Plugged

- Vibration

Continuous

- Corrosion


- ∆T trend

- Operator rounds

- Pressure test

- Overhaul

- Fluid chemistry - Debris Loss of integrity Tubesheets and Baffles

- Corrosion

Continuous

Expect to be failure-free for months to years

Inspection

Overhaul

Random

- Random, immediate

Inspection


- Failure to maintain cathodic protection - Vibration

Gaskets, O-Rings, and Packing

Leakage

- Improper torque - Assembly error - Aging (thermal cycling) - Corrosion of O-Ring grooves

5-106

Continuous

- Expect failurefree for > 5 years

- Internal inspection - Overhaul

EPRI Licensed Material Preventive Maintenance Table 5-16 PM Tasks and Their Degradation Mechanisms for Coolers [19] Inter-Coolers and After-Coolers

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration



Leaking tubes


X

X

X

Plugged tubes

X

X

Failure-free for months to years

Failed tubesheet or baffles

X

Expect failure-free for > 5 years but come randomly, can be rapid

Leaking gaskets or O-Rings

X

X

X

5-107


5.9

Separators and Traps

The following are PM tasks for the separators and traps [3]: •

Drain traps should have bypass piping and valves to facilitate required maintenance. Inspect operation of drain traps and valves to prevent condensate accumulation in the moisture separators and subsequent carryover into the air receiver and distribution system. Open and inspect the separator internals for wear and corrosion. Remove any corrosion or debris by wiping with a wiping cloth. The wiping cloth should be moistened with the lubricant being used to help retard corrosion of the metal surfaces. If the lubrication is required for separator internals, the specified lubricant should be used.

•

Drain traps should be checked to ensure that they are functioning and that drain flow is free. If clogged, the traps should be disassembled and cleaned to restore free flow. Most drain traps do not require priming, but the manufacturer’s instructions should be checked for this requirement.

•

Check the blowdown devices (automatic drain valves and traps) for blow by and proper operation. Repair or replace as necessary to ensure that condensate and particulate do not accumulate in the air lines.

•

Periodically blow down line filters to ensure that they are not clogged with particulate or desiccant fines, which can damage the end-use component they serve.

Table 5-17 lists the failure locations, degradation mechanism, and PM strategies for traps. Table 5-18 lists the PM tasks and their degradation mechanisms for traps.

5-108

EPRI Licensed Material Preventive Maintenance Table 5-17 Failure Locations, Degradation Mechanisms, and PM Strategies for Traps [19] Failure Location Traps

Degradation Mechanism - Clogged orifices

Degradation Influence - Debris


Failure Timing

Continuous

Failure-free only for weeks

- Oil

- Plugged piping


- Rust

- Stuck open

PM Strategy - Operator rounds (daily, open the bypass and blow down) - Overhaul

- Metal particles - Filter debris

Table 5-18 PM Tasks and Their Degradation Mechanisms for Traps [19] Condensate Traps

Time of Failure Trouble-free for only a few weeks

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration

Location/ Degradation Clogged or stuck open

X

X

5-109


5.10 Receiver Tanks The following are PM tasks for the air receiver tanks [3]: •

Conduct an internal and external visual inspection of all receiver tank surfaces, including bolted and welded connections, for physical damage, pitting, erosion, and corrosion. If there are areas of severe pitting or if cracks are detected, a nondestructive test with ultrasound or radiography should be performed. Photographs should be taken so that the extent of damage can be recorded as baseline operating data (BOD).

•

Plant documents should be consulted to determine minimum wall thickness. For the first inspection after BOD and succeeding maintenance cycles, the wall thickness should be measured so that the minimum wall thickness requirement is maintained.

•

Clean the receiver inside with a hot water-soaked wiping cloth. Do not leave surface moisture. Do not wire brush, needle gun, or sandblast because these methods expose new metal and lead to accelerated corrosion of the tank walls.

ASME pressure tests and inspections should be performed in accordance with the Code requirements (ASME Section VIII: Unfired Pressure Vessels). In addition to the system safety valve, a safety valve should be installed between the hydro pump and the vessel to prevent over-pressure of the pump and vessel during the hydrostatic test. Set the receiver and system safety valves. Gauges should be calibrated prior to setting the safety valves. Note: Some QA requirements specify gauge ranges and accuracies. Typically, a pressure test gauge with an accuracy of ±0.5% and a total range of approximately two times the system pressure is used.

5.11 Piping The following are PM tasks for the air piping system [3]: •

Walkdown inspection of all headers, distribution piping, and joints should be carried out on a regularly scheduled basis. Particular care should be taken with piping located close to passageways and other equipment because physical damage can occur from forklift trucks, from movement of equipment, and from being climbed on. Where damage is observed, the piping should be checked for leaks.

•

A frequently used, reliable method for locating air leaks is to systematically apply a soap solution to the piping joints and connections. Ultrasonic leak detection devices can also be used, depending on the location of the leak and the background noise level. In some cases, tracer gases have been used to test the distribution system for leaks.

5-110


•

To ensure piping integrity, questionable piping sections and joints can be tested with ultrasonic equipment that can provide information on pipe wall thickness and joint insertion (especially for soldered/brazed joints). However, the effectiveness of the ultrasonic equipment depends on the knowledge and experience of the technician interpreting the data obtained.

•

Another method of testing questionable joints is with real-time x-ray. The x-ray picture of the joint can be evaluated for its integrity. The disadvantage of this system is that the image quality may sometimes be poor for proper evaluation. Also, due to ALARA considerations, strict administrative procedures must be in place for personnel safety during operation of the x-ray equipment.

•

All valves should be cycled periodically to ensure proper operation and freedom of movement. Solenoid valves should also be cycled and inspected for indication of physical damage that might affect proper operation (bent or pinched vent lines, etc.). In addition, solenoids should be checked for blow by or seat leakage.

•

Blowdowns should be performed on all air lines to end-use components to remove any desiccant fines or spalled particulate from the system. Air lines should be blown down through a clean white blowdown cloth such as a cloth diaper. The lines should be blown until all visible particulate is removed. During blowdown, ensure that adequate safety precautions are followed.

•

Pressure decay testing should be performed on all accumulators and associated check valves to detect leakage back into the system. If compressor loading indicates excessive system leakage, perform pressure decay tests, flow measurement tests, or soap-test the headers to identify the location of leaks.

5.12 Instrumentation Table 5-19 lists the failure locations, degradation mechanisms, and PM strategies for instrumentation. Table 5-20 shows the PM tasks and their degradation mechanisms for instrumentation.

5-111

EPRI Licensed Material Preventive Maintenance Table 5-19 Failure Locations, Degradation Mechanisms, and PM Strategies for Instrumentation [19] Failure Location Pressure Switches

Degradation Mechanism Leak by

Degradation Influence Worn orifice parts

- Stuck

- Contamination

- Plugged orifices

- Environmental conditions

Burnt contacts

- Contact alignment


Random

Failure Timing


Expect to be failure-free for several years

- Inspection

Random, on a scale of several years

- Inspection


- Inspection


- Inspection

Random

- Inspection

PM Strategy Calibration program

- Calibration check Calibration program

- Calibration check

- Moisture from gasket failure (sticking only) Random

- Failure of arc suppression

Calibration program

- Calibration check

- Pitting/contamination Failed Sensors

a. Run time/cycling

a. Diaphragms

- Vibration

a. Continuous

- Contamination

- Random

b. Capsules

b. Vibration

b. Continuous

c. Bourdon tubes

c. Vibration

c. Continuous

- Clogging from contaminated air

- Random

Loose connections

- Vibration

- Continuous

- Personnel error

- Random

Mis-calibration

Personnel error

Random

- Calibration check

Calibration program

- Calibration check Random

- Inspection - Calibration check

5-112

Calibration program

Calibration program

EPRI Licensed Material Preventive Maintenance Table 5-19 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for Instrumentation [19] Failure Location Relays




Failure Timing

- Continuous

- Expect to be failure-free for years

- Pitted or worn contacts

- Run time

- Stuck or loose contacts

- Contamination

- Random

- Vibration

- Continuous

- Drift

- Contamination

Random

- Open circuit

- Vibration

- Short circuit

- Personnel error

- Loose connections

- Environment

- Random, on a scale of years


PM Strategy

- Inspection

- Calibration program

- Calibration check

- Thermography

- Thermography

Electronic/ Electrical Devices Transmitter Temperature Control Valves Programmable Logic Controls Level/Flow Switches RTDs I/P Transducers

- Mis-calibration

Random, on a scale of years

- Inspection where accessible

Calibration program

- Calibration check

- Dirty - Temperature - Installation method and locations

5-113

EPRI Licensed Material Preventive Maintenance Table 5-20 PM Tasks and Their Degradation Mechanisms for Instrumentation [19] Pressure Switch

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration


Failure free for several years

Leaking by

X

Random, several years

Stuck or plugged

X


Burnt contacts

X


Failed sensor

X

Random

Loose connection

X

Random

Mis-calibrated

X

Table 5-20 (cont.) PM Tasks and Their Degradation Mechanisms for Instrumentation [19] Electronic/ Electrical Devices

Time of Failure Random

5-114

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration

Location/ Degradation All failures

X

EPRI Licensed Material Preventive Maintenance Table 5-20 (cont.) PM Tasks and Their Degradation Mechanisms for Instrumentation [19] Relays

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration


Failure-free for several years

Pitted or worn contacts

X

Random, a few years

Sticking or loose contacts

X

5-115


5.13 Safety Relief Valves Table 5-21 lists the failure locations, degradation mechanisms, and PM strategies for safety relief valves. Table 5-22 lists the PM tasks and their degradation mechanisms for safety relief valves.

5-116

EPRI Licensed Material Preventive Maintenance Table 5-21 Failure Locations, Degradation Mechanisms, and PM Strategies for Safety Valves [19] Failure Location Safety Relief Valves

Degradation Mechanism Weak springs

Degradation Influence - Aging


Failure Timing

Continuous

Many years of trouble-free operation

Bench-test

Plant safety valve test program

- Heat

PM Strategy


Fails to reseat

Debris

Random

Random

Inspection

Operator rounds

Stuck closed

- Corrosion

Continuous

Random

Bench-test

Plant safety valve test program

- Inactivity Table 5-22 PM Tasks and Their Degradation Mechanisms for Safety Valves [19] Safety Relief Valves

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration



Weak spring (covered by relief valve program)

Random

Failure to seat

Random

Stuck closed (covered by relief valve program)

X

5-117


5.14 End-Use Devices Table 5-23 lists the failure locations, degradation mechanisms, and PM strategies for the end-use devices of the compressed air system. Table 5-24 lists the PM tasks and their degradation mechanisms for the end-use devices.

5-118

EPRI Licensed Material Preventive Maintenance Table 5-23 Failure Locations, Degradation Mechanisms, and PM Strategies for End-Use Devices [19] Failure Location Solenoids

Degradation Mechanism Stuck

Coil failure

Pneumatic Devices

Degradation Influence Contamination


- High temperature (normally energized are most susceptible)

Continuous

Elastomer and gasket failure

- Aging (cycles)

Continuous

Leak by

- Gasket failure

- Heat

- Stuck

- Contamination

- Plugged orifices

- Environmental conditions

Failed sensors

a. Run time/cycling

a. Diaphragms

Continuous

Random

- Moisture from gasket failure (sticking only) a. Continuous

- Vibration

- Continuous

- Contamination

- Random

b. Capsules

b. Vibration

b. Continuous

c. Bourdon tubes

c. Vibration

c. Continuous

- Clogging from contaminated air

- Random

Loose tubing and fittings

- Vibration

- Continuous

- Personnel error

- Random

Failure Timing


Random, on a scale of at least 3 years

- Incorrect operation


- Inspection


- Inspection

Expect to be failure-free for several years

- Inspection


- Inspection


- Inspection

Random

- Inspection




- Calibration check

- Calibration check

- External visual inspection - Calibration program Calibration program

- Calibration check

Calibration program

- Calibration check

- Calibration check

- External visual inspection - Calibration program

5-119

EPRI Licensed Material Preventive Maintenance Table 5-23 (cont.) Failure Locations, Degradation Mechanisms, and PM Strategies for End-Use Devices [19] Failure Location



Pneumatic Devices (cont.)

Mis-calibration

Personnel error


Failure Timing

Random

Random

Discovery Opportunity - Inspection


- Calibration check

Table 5-24 PM Tasks and Their Degradation Mechanisms for End-Use Devices [19] Pneumatic Devices

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration



Leaking by – worn parts

X

X


Leaking by – failed gasket

X

X


Stuck or plugged

Random

Loose tubing or fittings


Failed sensor

X

Random

Mis-calibrated

X

5-120

X X

X

EPRI Licensed Material Preventive Maintenance Table 5-24 (cont.) PM Tasks and Their Degradation Mechanisms for End-Use Devices [19] Solenoids

Time of Failure

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration


Random, > 3 years

Stuck

X

Many years of failurefree operation

Failed coil, elastomer or gasket

X

Table 5-24 (cont.) PM Tasks and Their Degradation Mechanisms for End-Use Devices [19] Electronic/ Electrical Devices

Time of Failure Random, several years

PM Task

Oil Analysis


Internal Inspection

Overhaul

Interval

6 Months

1 Month

4000 hours

8000 hours

Operator Rounds

Calibration

Location/ Degradation All failures

X

5-121


6

CONDITION-BASED MONITORING

For the purpose of this guide, condition-based monitoring [6] includes the technologies of predictive maintenance and system monitoring parameters.

6.1

Predictive Maintenance

Predictive maintenance includes the technologies of thermography, vibration monitoring, and oil analysis. 6.1.1 Thermography Infrared thermography is directed mainly at detecting loose electrical connections, strong local eddy current heating, frictional heating, abnormal heat loss or transfer, and proper operation of heaters, coolers, fans, and motors. Thermography needs to be performed while the equipment is operating and/or is energized and the electrical panel doors are open. The following components (as applicable and accessible) should be surveyed with an infrared thermographic camera: •

Compressor motor and control panel/cabinet/cubicle

•

Compressor heads and bearings

•

Terminal blocks/strips

•

Electrical contacts, bushings, connections, splices

•

Relays, breakers, magnetic contactors

•

Dryer heaters and connections

Thermography application on motors is useful in determining defects directly related to heat as detected on the surface. Condition assessments of power cable connections, bearings, and ventilation problems are easily detected. Thermography scans on motors should be saved and trended for change. A thermographic survey should be conducted on a regular basis to detect changes from a baseline condition. It is one of the most economic and effective diagnostic tools to detect and locate incipient failure in motors, compressors, and heaters.

6-1

EPRI Licensed Material Condition-Based Monitoring

6.1.2 Vibration Monitoring Vibration analysis is a predictive maintenance task that is particularly directed at failure mechanisms such as bearing wear, coupling wear, misalignment and wear of the bull gear, as well as abnormal wear or damage to the screw elements. Vibration analysis has a wide scope of application to compressors, addressing sources of vibration in rotating parts, as well as flow noise. These sources include almost all causes of wear, especially those caused by off-design point operation, bearing wear, general alignment problems, improper internal clearances, and erosion of inter-stage sealing. It is reasonable to expect many years of trouble-free service from components such as bearings and gears; however, these components can also fail much earlier than their expected life due to random occurrences of lubrication failure or misalignment. The testing interval of three months for critical equipment vibration analysis is chosen to give a high probability of detecting the onset of these failure causes and to act as a supplement to oil analysis. Because of the high vibration levels inherent in a rotary screw compressor, it is important to obtain a good baseline to allow for trending of the data. Vibration monitoring and analysis provides a means to monitor wear in the machine bearings (compressor, motors), and to detect: •

Problems caused by the rotating parts

•

Abnormal friction

•

Parts failure (piston rings, crankshaft bearings, gears)

•

Rubbing of rotating parts and clearance problems

•

Severely cracked or damaged rotors and shafts

Vibration monitoring can use permanently installed monitors (for example, on the shaft) to continuously measure shaft deflection (peak-to-peak), or use portable equipment by taking periodic readings and comparing the data to a baseline vibration signature. Vibration analysis on motors should address vibration of rotating parts as well as bearings and seals. Commonly detected anomalies include rotor imbalance, shaft misalignment, bearing wear, and resonance. 6.1.3 Oil Analysis Oil sampling and analysis is particularly directed at causes of bearing wear and wear of the bull gear, as well as at monitoring the quality and proper type of oil. Although it is reasonable to expect many years of trouble-free service from components such as bearings and gears, these components can also fail much earlier than their expected life due to random occurrences of lubrication failure or misalignment. The testing interval of six months for oil analysis is chosen to give a high probability of detecting the onset of these failure causes. 6-2


The most important aspects of oil sampling and analysis of the compressor lube oil are to ensure that high-quality lubrication is maintained, and to detect the onset of wear in the compressor. Two annual tests are recommended: •

Laboratory test of the oil samples for anti-rust protection capability (to show depletion of rust inhibitor) (ASTM Test D 665-82)

•

Laboratory test for oxidation stability (to reveal anti-oxidant depletion and oil deterioration) (ASTM Test D 943-81 for new oil and ASTM D 2272-67 for used oil)

Ferrography is a method of analyzing wear particles deposited in the lube oil. When used to examine the size distribution, concentration, composition, and shape of techniques, the results can indicate the onset of abnormal wear and help to determine the cause and severity of machine wear. The ferrography should be able to detect wear particles in the range of 0.1 to 100 microns. At magnifications of 400X to 1000X, the particle shape and origin of particle generation can be identified. In addition, moisture, total acid, and lubricity should be determined and trended. Oil analysis is focused on processes that result in wear particles or other contaminants entering the oil or processes that cause degradation of the oil and/or bearing internal components. These include wear or failure of bearing seals, and failure or fouling of bearing cooling heat exchangers or internal bearing coolers. The sampling timeframe should only apply to those bearings with clean cavities and lubricant. Anti-friction bearings should be under a greasing program. Also, consider the use of sealed anti-friction bearings when applicable.

6.2

Air Quality Monitoring

Guidance was developed for the monitoring and testing of compressed air system integrity [2,22]. Integrity is interpreted to mean the soundness and quality of the air systems. The integrity of an instrument air system is evidenced by the efficient performance and availability of the air supply components and by their delivery of quality air per ANSI/ISA 7.0.01-1996. Key Human Performance Point Air quality monitoring is one of the keys to achieving reliable air systems. High instrument air system reliability depends on having air that is free of contaminants. Because of the importance of monitoring and the monitoring problems that occur, effort in this area is highly beneficial. Plants currently use a wide variety of techniques and practices in monitoring the quality of the compressed air that is used for component and instrument operations. A variety of monitoring devices and methods are used to monitor a variety of locations. The quality of compressed air is typically evaluated in terms of dew point, particulates, and the presence of hydrocarbons. This section identifies current air quality monitoring techniques, assesses the potential advantages and disadvantages associated with these techniques, and provides guidelines that can be used in assessing and developing individual air quality monitoring programs. 6-3


Key Technical Point There are two basic ways to monitor the corrosive properties of the air entering the system: measure the concentrations of gases known to be corrosive and measure the effects of the corrosive gases. Once a quantitative measurement of the corrosive potential of the air is obtained, the magnitude of the corrosion problem must be determined. The biggest challenge when dealing with corrosive gases is determining whether or not the gases are present in the first place. Monitoring the corrosive properties of the incoming ambient air can help determine how to avoid these problems. Pinpointing the corrosion level of the air is critical to determining the magnitude of the problem. In addition, once steps have been taken to protect the compressed air system from these corrosive gases, monitoring is needed to ensure that the system is functioning correctly. There are two methods that have been used to quantify the corrosive properties of gaseous mixtures. The first method is the direct measurement of chemical components in the air. When performing a chemical analysis of the components of the air, a sample must be taken and sent to a laboratory for analysis or, in some cases, the analysis can be performed on-site in a mobile testing facility. Companies that sell corrosive gas protection systems or independent testing services can offer an extensive array of tests. There are several drawbacks, however, of laboratory analyses. First of all, the testing will only reveal the chemicals that the laboratory personnel have been asked to identify. Secondly, a list of the chemicals in the air and their concentrations might not be the best indicator of the corrosive nature of the environment. In many cases, certain corrosive compounds can act in concert to increase or decrease the rate of corrosion of the individual compounds. In addition, the concentrations of harmful gases can also rise and fall. Therefore, occasional testing might not reveal the full extent of the problem. The second method for measuring the effects of corrosive gases is reactivity testing. This method is more cost-effective than laboratory testing, which can cost six to seven times more than reactivity testing, and could offer a more direct way of quantifying the problem. A simple pH test on a condensate sample can indicate the presence of corrosive gases in the air stream. A pH of 7 is neutral. If the pH is less than 7, the condensate is acidic and it will attack copper and steel. If the pH is greater than 7, the condensate is basic and it will attack metals such as aluminum. This test will simply indicate the presence of a corrosive compound that will dissolve in water to form an acid or a base. Compounds that do not form acids or bases when mixed with water will not change the pH of the condensate. While this test will indicate whether or not there is a problem, it will not identify the extent of the problem. If the pH test does indicate a problem, another testing method will need to be used to determine the extent of the problem. In this case, one of the best ways to quantify the effects of corrosion is to measure the reaction of the corrosive compounds in the air on a test specimen. The simplest way to do this is to place a 6-4


metal strip in the air stream, usually in the particulate filter housing, for a period of time. The strip is then sent to a laboratory for analysis. As corrosive gases react with the base metal of an air compressor component, a chemical product in the form of a film will build up on the surface of the base metal. Specially prepared copper and silver strips are usually exposed for a time period varying from 30 to 90 days. Analysis of the test strip will reveal the average film thickness buildup during the test period. There are two testing methods that will indicate film thickness. The mass gain of the metal strip can be directly correlated to the average corrosion film thickness. Precise measuring of such a small mass gain can be difficult, however. Battelle Laboratories and Bell Telephone Systems researched an electrolytic reduction method, where the exposed metal strip is placed in an electrolyte solution and an electrical current is passed through the solution from the test strip to a platinum anode. The voltage difference between the test strip and the platinum strip will change as corrosion film is reduced. The time it takes for the film to be reduced can be correlated to the average film thickness. This analysis will report the average rate of corrosion film buildup for the period of time that the strips were installed. Unfortunately, exposure to corrosive gases usually does not occur at a constant rate. As expected, as the level of technology increases to allow continuous monitoring of corrosive gases, the cost of instrumentation also increases. Once the corrosion problem is recognized, there must be some method of determining if the measured rate of corrosion is harmful. Protective measures must be taken and then it must be determined what rate of corrosion would be acceptable to the compressor or process. While these results are often compared to some standard, there are currently no standards that specify allowable levels of corrosion for the air compressor industry. As a result, compressed air system operators must define their own guidelines, or adopt a standard from another industry. A widely used corrosion standard is the Instrumentation, Systems, and Automation Society (ISA) standard ANSI/ISA-S71.04-1985: Environmental Conditions for Process Measurement and Control Systems: Airborne Contaminants. This standard is used to correlate the corrosion film thickness with electronic instrument reliability and uses four levels to describe corrosion problems: •

Level G1 describes a mild environment. Corrosion should not be a factor in determining equipment reliability. Corrosion film should build up on a copper sample at less than 300 angstroms (1 angstrom = 10-10 meters) when measured over a 30-day period.

•

Level G2 environments show measurable effects of corrosion and might have decreased reliability. Copper corrosion film buildup should be between 300 and 1000 angstroms over a 30-day period.

•

Level G3 is a harsh environment. There is a high probability that corrosive attack will occur and some effort should be made to protect the equipment. A corrosion film buildup on a copper strip of between 1000 and 2000 angstroms in a 30-day period indicates a level G3 environment. 6-5


•

Level GX is a severe environment. A corrosion film buildup of greater than 2000 angstroms over a 30-day period on a copper sample indicates a Level GX environment. Only specially designed and packaged equipment can be expected to survive. Although this standard was written for electronic instruments, it can be used to classify the corrosive nature of the air stream in a compressed air system.

Once the air has been identified as corrosive, something must be done to protect the internal mechanisms of the compressed air system. The most common method of removing chemicals from the air is gas-phase filtration or dry scrubbing the air. This compressor protection system removes gaseous contaminants from the air stream by way of adsorption, absorption, and chemisorption. An adsorbent media, such as granular activated carbon, will attract other substances and hold them on the surface of the adsorbent particle. Absorption is a similar process in which the attracted chemical penetrates into the internal structure of the filtration media. The major problem with these two types of media is that they will sometimes release the captured gases. Carbon-based media such as coconut shell carbon will release lower-molecular weight gases in favor of high-molecular weight gases. A chemisorbent media, such as potassium permanganateimpregnated alumina, will chemically react with some contaminant gases, oxidizing them, and removing the possibility that they could be released. A filtration system supplier can recommend the appropriate chemical media type. The chemical media, which must be occasionally replaced, works in conjunction with a standard particulate filter system to protect the compressor from corrosive gases. It is best to assess the corrosive nature of the environment and select a protection system prior to the purchase or rebuild of a compressor. The most common source is the atmospheric discharge of some process near the compressed air system’s operating location. As the pressure rises in a compressed air system, so does the temperature and density of the air. This density increase will cause moisture to condense out of the air. If the air is cooled after compression, more moisture will condense. When corrosive gases are present in the environment, the high temperature and moisture content inside the compressor will increase the rate of corrosive chemical reactions. The moisture content of the air depends on the relative humidity of the air taken into the compressor. High levels of relative humidity can result in more condensate forming in the compressor. While many corrosive gases will dissolve into this water, some gases, such as chlorine, will form hydrochloric acid when dissolved in water. Sulfur dioxide will combine with condensed water to form sulfurous acid. If there is an electrical potential difference between two dissimilar metals, and conductive solution of some substance dissolved in water exists between the two metals, then electrolytic or galvanic corrosion can occur. And in situations where lubrication is present, additional chemical reactions will occur to further compound the issue.

6-6


The following chart demonstrates some examples of corrosive gases and the materials they affect. While all corrosive gases listed below are harmful to air compressor components, the materials most susceptible to corrosive attack are copper, aluminum, and cast steel. Cast steel is used for many centrifugal compressor components such as the casing and some diffusers. Aluminum is used for diffusers, and copper is commonly used for inter-coolers. Admiralty brass (70–73% copper, 0.75–1.2% tin, remainder zinc) or stainless steel is sometimes used in intercoolers to reduce the possibility of corrosion. These metals are still susceptible to corrosive attack, though at a slower rate. The most susceptible components in a centrifugal air compressor are usually the coolers. Table 6-1 Corrosive Gases and Affected Materials [22] Corrosive Gas

Affected Metals

Inorganic chloride silver compounds (that is, chlorine, chlorine dioxide, hydrogen chloride)

Copper, tin, and iron alloys

Hydrogen fluoride

Copper, tin, silver, and iron alloys

Active sulfur compounds (that is, hydrogen sulfide, elemental sulfur, and organic sulfur compounds)

Copper, silver, aluminum, and iron alloys

Sulfur oxides

Reactive metals

Nitrogen oxides

Most common metals

Ammonia, amines, and ammonium ions

Copper and copper alloys

Ozone, chlorine, and chlorine dioxide

Many elastomers and plastics

6.2.1 Contaminants From the ANSI/ISA 7.0.01-1996 standard, “Instrument air should be free of corrosive contaminants and hazardous gases, which could be drawn into the instrument air supply. The air system intake should be monitored for contaminants. If contamination exists in the compressor intake area, the intake should be moved to a different elevation or location free from contamination if possible.” Key Technical Point From a review of industry-wide pneumatic system problems completed by EPRI in October 1988, results indicated that 49% of all failure events resulted from contamination in the system and only 28% were attributed to component failures. It is possible some of the component failures also occurred due to contamination in the instrument air system. This means that contaminants are the largest single contributor to instrument air system failures. Other problems that cause aggravation to maintenance are leaks and component failures.

6-7


Major contaminants [2] in the instrument air system are: •

Dirt or dust particles

•

Corrosion or rust particles

•

Moisture or water

•

Desiccant fines

•

Oil mist

It is believed that up to 90% of the problems in non-lubricated compressors are from the incursion of dust or rust particles in the intake pipe. It is suggested that intake lines be made of non-corroding materials such as galvanized steel or aluminum. For more information on piping material recommendations, see W.M. Stanton, Selection and Application of Industrial Air Compressors, proceedings of the American Power Conference, Chicago, 1965. Dirt or rust particles cause damage to a compressor cylinder by scoring. Cylinders of nonlubricating compressors are specially honed to a fine finish to reduce the friction coefficient between the TFE piston rings and the cylinder wall surface. Therefore, it is more important to prevent the incursion of foreign particles into a non-lubricated compressor than a lubricated one. A proper air intake filter is an important defense against incursion of dust or dirt into system piping. However, a clogged filter unit can starve the compressor and result in a substantial reduction in discharge pressure and capacity. Contamination levels vary greatly between installations. High levels of contamination might be inadvertently released in the vicinity of the air intake filter by unrelated construction or maintenance work. The only reliable method of determining if a filter requires cleaning or replacement is by measuring pressure drop across the filter. This can be done by a U-tube manometer or by installing a differential pressure gauge across the filter housing. Differential pressure should be measured periodically. If the pressure drop across the filter exceeds the manufacturers’ recommendation, the filter should immediately be cleaned or replaced. However, intake filters should also be replaced based on a periodic maintenance schedule, even if the differential pressure is within limits. A sudden drop in the differential pressure across the filter might indicate a torn or damaged filter element and should be investigated further for the possibility of needing replacement. Recommended sizes for intake filters are shown in the filter selection chart (Table 2-10). Rust in the system usually occurs due to corrosion of either the receiver or the system piping. The cause of corrosion of the receiver or piping is due to one or more of the following: •

Original installation of improperly cleaned equipment or piping

•

Humidity and moisture (water) incursion into the system between installation and startup phases

•

Improper operation of filter or dryer units

6-8


•

Frequent cross-connection of instrument air and service air headers

•

Leakage of cooling water into the air system via compressor cooling lines or inter- and aftercooler heat exchangers

•

Malfunction of drain traps, allowing water to accumulate and be carried through the system piping

It is impossible to predict when a corrosion product (rust particle) will dislodge itself, travel through the system piping, and lodge itself in a component, rendering it inoperable. Solenoid valves, pneumatically operated valves, pilot valves, and I/P converters are especially vulnerable to failure due to foreign particle incursion. Once the system piping is initially cleaned, incursion of foreign particles from atmosphere can be prevented by the proper use of intake filters on the compressor suction. Rust particles generated within the receiver tanks and distribution system piping up to the dryers can easily be removed from the system by using pre-filters on a refrigerant dryer. In addition, these particles will be filtered out by the desiccant on a desiccant type of dryer. Some utilities have used copper piping for their instrument air supply in an effort to prevent corrosion inside their distribution system. Copper piping is more susceptible to bend and breakage than carbon or galvanized steel, and compression fittings on copper tubing are more likely to develop leaks due to line vibration. To maintain the proper air quality required for operation, it is necessary to: •

Thoroughly review system design to ensure that the components have the capability to produce the quality and quantity of air to meet plant needs. Review should include any purge air requirement for the dryers.

•

Ensure that the system, upstream of the dryers, is either chemically cleaned or blown down to remove as much of the corrosion and contamination as possible.

•

Place the system on line with proper dew point air leaving the dryer. Perform blow-downs of the system, downstream of the dryers, to remove any loose particulate. These blowdowns need to be performed shortly after placing the dryers in service, and again two weeks to a month later. After the initial period, blowdowns should be performed on a 12- to 18-month cycle to ensure a clean system.

•

Locate any cross-connection of service air and instrument air upstream of the dryers to prevent moisture incursion into the system. This will also ensure that the air is dry before reaching the end-use components.

•

Properly maintain system traps and drains, pre-filters, and dryers.

If the drains and traps are clogged, the cooler shells will fill up with condensate in a short period of time and then be carried over into the system in the form of heavy water droplets. These droplets can: •

Deposit in the receiver and cause corrosion of the tank walls and bottom. 6-9


•

Prematurely exhaust the capacities of pre-filters and desiccant dryers.

•

Overload refrigerant-type dryers.

•

Cause moisture accumulation in the system piping, resulting in corrosion. Some of the system piping might be installed outdoors and exposed to the weather. Accumulated water might freeze during the winter and cause damage to piping and instruments.

•

Make the operation of air-operated valves sluggish or erratic. Wash away lubricants from operating cylinders of air-operated valves or other similar equipment.

•

Cause some of the lubricants used on solenoid valve o-rings to become sticky or gummed up, causing the solenoid valve to become inoperable.

Pre-filters and air dryers are not designed to remove a large volume of water. It is suggested that all automatic drains be inspected at least once per operating shift for proper operation. Also, each automatic trap installed should be provided with a manual bypass valve. In case of malfunction of the trap, water should be drained frequently via the manual valve until the trap can be repaired. Some plants with desiccant dryers have experienced a phenomenon known as desiccant carryover. Desiccant carry-over occurs when the desiccant has been pulverized by air pressure pulsations in the dryer tower. Once reduced to sizes smaller than the openings in the retention element, the desiccant can be carried by the air stream throughout the system. Such desiccant fines cause a great deal of damage to end-use components. If desiccant carry-over occurs, the desiccant and after-filter need to be changed, and the system requires blowing down until no desiccant fines are detected in the system. 6.2.1.1

Dew Point Monitoring

Information in this section covers dew point monitoring frequency and locations, particulate monitoring, hydrocarbon monitoring, and contaminants [6]. 6.2.1.1.1

Monitoring Frequency

Dew point monitoring should be performed on a regular basis to ensure that moisture does not intrude into instrument air systems. The determination of the optimum dew point monitoring frequency is a challenge faced by all power plants. Many considerations are involved in determining a frequency that helps ensure the safe, reliable operation of a plant in the most economical manner. The following information provides insights and guidelines that utilities can use in determining a suitable dew point monitoring frequency. Continuous dew point monitoring assumes that failure or degradation of moisture removal and drying equipment can occur at any time without an obvious indication. A decision for continuous monitoring also assumes that immediate detection of a problem is necessary to ensure that significant amounts of moisture do not enter the system. Continuous dew point monitoring

6-10


checks the actual operation of moisture removal equipment and does not rely on expected equipment performance. Periodic dew point monitoring depends on moisture removal and drying equipment being maintained in good working order and not prone to sudden failure or degraded operation without obvious signs of a problem. The monitoring frequency is based on the plant’s experience with the equipment and is selected to provide assurance that the equipment is continuing to operate as required. Potential problems can be detected through trending. Either continuous or periodic dew point monitoring can be the right choice for a particular system in a particular plant. To make an informed choice, many things must be considered. These considerations for continuous monitoring include: •

To monitor dew point continuously, it is necessary to install and maintain permanent dew point monitoring equipment. The use of portable test equipment for permanent monitoring is not cost-efficient. The cost of the equipment, the installation, and the maintenance are important considerations.

•

The number of monitoring locations must also be considered. A single monitoring location can be sufficient for the entire system or multiple locations might be required. The greater the number of monitoring locations, the greater the cost.

•

Backup equipment (portable dew point monitors and equipment spares) might be necessary to check the accuracy of installed equipment and to permit outages and maintenance of the permanent dew point monitors. Dew point monitors must be calibrated periodically.

•

Continuous dew point monitoring can save maintenance dollars by allowing on-line evaluation of dryer performance. With this input, some maintenance work can be scheduled based on the equipment’s actual performance rather than at specific intervals or operating hours.

•

Continuous monitoring might be shown to save money when the manpower costs that are associated with taking dew points periodically with portable monitors are included.

Considerations for periodic monitoring are: •

Periodic dew point measurements can be made with portable monitors, usually without the need for plant design changes.

•

Portable monitors can be used in a variety of locations.

•

Portable monitors do not require backup equipment as such.

•

The periodic monitoring philosophy relies on satisfactory operation of the moisture removal equipment between measurements. This requires that drying and moisture removal equipment be well maintained and inspected continuously for satisfactory operation. Some dryers are provided with humidity indicators that change color when moisture levels increase. For other dryers, it might be necessary to trend pressures and temperatures, and to observe dryer cycling behavior to determine if the dryer’s operation is satisfactory.

•

The frequency of monitoring depends on the reliability of the moisture removal and drying equipment and should be chosen based on equipment history. If no equipment history is 6-11


available, various frequencies should be chosen and trended until a satisfactory frequency is determined. •

Periodic monitoring can allow unsatisfactory dew points to go unnoticed for extended periods.

•

When performing periodic monitoring on dual-tower dryers, caution should be used so that the performance of each tower is individually evaluated.

6.2.1.1.2

Monitoring Locations

Dew point monitors should be located where they can provide assurance that no liquid has been introduced into, or exists, in the air system. The difference in monitoring locations is brought about by differences in system designs and monitoring philosophies. To discuss the considerations and reasoning that are used in selecting dew point monitoring locations, certain basic terms need to be defined. These are: •

Single-point monitoring: One dew point monitoring location is used to evaluate moisture conditions for the entire air system. This is usually associated with source monitoring.

•

Multiple-point monitoring: Multiple dew point monitoring locations are used to assess dew point conditions throughout the air system. This usually includes source monitoring and point-of-use monitoring.

•

Point-of-use monitoring: The dew point monitoring locations are as close to the end-use devices as practicable.

•

Source monitoring: The dew point monitoring is downstream of the drying equipment but upstream of any end use.

A major factor that affects the selection of dew point monitoring locations is the frequency of monitoring. If performed continuously, source monitoring can be used to evaluate the dew point condition of the entire system. When continuous monitoring shows acceptable moisture levels in the system at its source, it is logical to conclude that moisture levels throughout the system are acceptable. This is true provided there are no downstream sources of moisture infiltration into the system and that the system was free of water prior to the start of monitoring. When the dew point is checked only periodically, consideration is usually given to multiple-point monitoring. Multiple locations make it probable that dryer malfunctions will be detected even though they occur in the interval between dryer output readings. Although dew point monitoring frequency is a major factor in the selection of monitoring locations, other factors also play an important role. These include cost, accessibility, the operational history of the instrument air system, and the operating experience of the plant. For example, the use of single-point periodic monitoring can be acceptable when a strong trending, inspection, and preventive maintenance program is in place.

6-12


Key Technical Point The selection of dew point monitoring locations should be based principally on one consideration. The location or locations chosen must provide reasonable assurance that moisture and water has not and will not, be introduced into the instrument air system undetected. The following six suggestions can guide air system engineers in selecting multiple point sample locations that provide the most needed information: •

Sample system low points. Low points should be chosen because this is where moisture will collect if it enters the air system.

•

Sample pipe dead legs. Pipes with little or no flow tend to collect moisture.

•

Sample at end-use components. End-use components are at the greatest distances from the drying equipment and provide a good indication of the quality of air throughout the system.

•

Sample interface points. These are locations where the instrument air system may be susceptible to moisture in-leakage from other plant systems.

•

When possible, select sample locations that can be used without the need to modify existing plant equipment (that is, at drain valves, blowdown valves, and so on).

•

Select only the number of monitoring locations necessary to give reasonable assurance that the system does not contain excess moisture.

6.2.1.1.3

Dew Point Levels

System dew point levels depend on the operating pressures and temperatures and not on atmospheric conditions. When determining air system dew points at various locations without the benefit of installed dew point instrumentation, it is typically assumed that the system air temperature at the location is in equilibrium with the system’s surroundings. Caution is advised when making this assumption. It should be assured that the air system and surrounding temperatures really are similar. Also, when evaluating system dew points, it is important to be sure they are for the air system pressure, not for atmospheric pressure. If measurements are taken at atmospheric conditions, they must be converted to the equivalent dew point at line pressure. The evaluation of what constitutes a satisfactory dew point differs from plant to plant. What is satisfactory usually depends on the history of the plant, the monitoring frequencies, the type of dryers used, and the commitments that have been made in response to meeting the ANSI/ISA 7.0.01-1996 specification. The key criterion in specifying dew point levels is assurance that the moisture in the air system will not condense into liquid. Based on this criterion, a satisfactory dew point is any dew point that is below the lowest temperature that exists anywhere in the system. For plants that monitor continuously, it might be possible to adjust the acceptable dew points on a daily basis, based on known ambient temperatures. For plants that monitor on a periodic basis, 6-13


it is necessary to ensure that the dew point is lower than the lowest ambient temperature that was experienced during the preceding period, and lower than the lowest ambient temperature that is expected during the next period. To preclude the need for adjusting acceptable dew point levels based on changing temperatures, it is possible to set acceptable dew points based on the lowest expected ambient temperature obtained from historical meteorological data. When selecting a dew point criterion, it is important to consider the accuracy of the measurement device and potential deviations to expected temperatures. For these reasons, it is common practice to set dew point acceptance criteria below the lowest expected temperature. For example, ANSI/ISA 7.0.012-1996 sets the acceptable dew point limit at 18°F (-8°C) below the minimum ambient temperature at the site. 6.2.1.2

Particulate Monitoring

This section covers monitoring frequency and locations for particulates [6]. 6.2.1.2.1

Monitoring Frequency

Particulate monitoring is always performed on a periodic basis rather than on a continuous basis. The reasons that monitoring is periodic include the absence of monitoring equipment that is suitable for permanent installation, the cost of using portable equipment on a daily or weekly basis, and the fact that equipment used to control particulate (that is, filters) is not susceptible to catastrophic failure. The monitoring frequency that is selected by plants varies based on many factors. These include: •

Cost

•

Type of dryer (refrigerant versus desiccant)

•

Quality of compressor intake air filtration

•

Type of piping (carbon steel versus stainless or copper)

•

Filter efficiencies

•

Filter locations

•

Filter performances

•

History of particulate-related equipment failures

•

Trending of particulate monitoring results

6.2.1.2.2

Monitoring Locations

Typical sample locations for particulate monitoring include immediately downstream of the dryer after-filters, as well as at various points in the air piping throughout the plant. Source monitoring ensures that particulates are not being introduced into the system, and point-of-use monitoring ensures that particulates are not reaching end-use devices. 6-14


6.2.1.2.3

Monitoring Methods

The filter sampling method entails passing a specified quantity of instrument air through a filter that is then analyzed. The analysis of the filter can include gravimetric (the filter is weighed before and after being exposed to the sample flow), microscopic inspection of the contaminants on the filter, and counting particulates on the filter by optical particle counters at a laboratory. Plants that use optical particle counters to directly monitor a plant’s instrument or service air streams typically use portable units that are carried to the sample point. In this way, the sample results are obtained immediately. The following are some of the advantages and disadvantages of the alternate particulate monitoring methods. An advantage of the filter method is that the initial cost is low. Disadvantages of the filter method are: •

The results are not immediately available to plant personnel.

•

The sensitivities of the measurements are limited.

•

The samples can be contaminated by particles from the atmosphere.

•

The monitoring is labor-intensive.

Advantages of the optical particle counter method are: •

The monitoring results are immediately available.

•

The results represent only the air being monitored; there is little chance that the sample will be contaminated from the outside.

•

There is high measurement sensitivity.

Disadvantages of the optical particle counter method are: •

The required initial investment in equipment is high.

•

The equipment must be periodically calibrated.

Not all plants monitor the particulates in their instrument air systems, nor does this report recommend such monitoring. Some utilities have committed to meet the requirements of ANSI/ISA 7.0.01-1996 standard, which has a 40-micron criterion. See Section 2.3 for more details on this standard. Some plants are attempting to maintain a 40-micron maximum particle size throughout the instrument/control air system. This is difficult to achieve for the following reasons: •

There are corrosion products and pipe scale in copper and carbon steel air piping systems.

•

Desiccant powder does migrate through the dryer after-filters.

6-15


•

There is particulate residue in instrument air systems from earlier system construction activities.

•

It is difficult to thoroughly flush and blow down the air system piping in an operating plant.

The approach taken by some plants, either in place of particulate monitoring, or in addition to such monitoring, is to install filters at various locations downstream in the air system. There might be filter stations in branch headers, or the plant might use the more common practice of installing point-of-use filters. Some plants have both header filter stations and point-of-use filters. Filters can be sized to meet the manufacturer’s end-use requirement with the filter elements replaced at scheduled maintenance. The function of air dryer after-filters is primarily to retain desiccant powders. Such filters do not prevent pipe scale or other contaminants (which exist downstream of after-filters) from migrating through the system. 6.2.1.3

Hydrocarbon Monitoring

This section covers monitoring methods for hydrocarbons [6]. 6.2.1.3.1

Monitoring Methods

Several general types of instruments are used for monitoring hydrocarbons in instrument air systems. These are: •

Detector tubes – Detector tubes are portable monitors whose readout is by a change in color for a given amount of air.

•

Oil indicators – Oil indicators are somewhat similar permanent devices that also change colors.

•

Air-sample-to-laboratory – Air-sample-to-laboratory involves sending a gag that is filled with the air to a laboratory for analysis.

•

Change in filter weight – With the filter weight method, a filter is dried, weighed, exposed to a given quantity of air, re-dried, and re-weighed.

There are other test methods for monitoring hydrocarbons in the compressed air system. Two of these are specified in ASTM (American Society for Testing and Materials) D3416-88 and ASTM D4285-83. ASTM D3416-88 is the “Standard Test Method for Total Hydrocarbons, Methane, and Carbon Monoxide in the Atmosphere.” Air system pressure is not accounted for in this test method. However, this method does provide guidance on laboratory tests for hydrocarbons. ASTM D4285-83 is entitled “Indicating Oil in Compressed Air.” This method is used to determine the presence of oil and moisture in compressed air that is used for abrasive blast cleaning, air blast cleaning, and applying coatings. The method is non-quantitative and not intended to be highly precise.

6-16


Detector tubes, oil indicators, and laboratory testing of air samples appear to be the most appropriate methods for monitoring the hydrocarbon content of instrument air at power plants. A drawback of the filter weight method is that it does not collect non-condensables. The fine oils can slip through the filter and the resulting change in filter weight will include particulates as well as hydrocarbons. Following are some of the advantages and disadvantages of the alternate hydrocarbonmonitoring methods. Advantages of the detector tubes and oil indicators are: •

Quick results are provided

•

Tubes and indicators are precise

•

Real-time data is provided

Disadvantages of the detector tubes and oil indicators are: •

Data is open to interpretation

•

They are the least accurate

The advantages of sending the sample to a laboratory are: •

The most accurate data is provided

•

The equipment cost is low

•

No interpretation is required

Disadvantages of sending the sample to a laboratory are: •

Real-time data cannot be provided

•

Higher annual expense

An advantage of the filter weight method is: •

The interpretation of data required is minimal

Disadvantages of the filter weight method are: •

It is the least accurate

•

It does not account for non-condensables

•

It has a high cost

Detector tubes, oil indicators, and sending samples to the laboratory are the preferable methods of monitoring hydrocarbons. The test frequency is not specified by the testing standards. It would seem prudent to test for hydrocarbons at least once each year.

6-17


Sampling downstream of the after-filters and at random points is appropriate and appears to be the general practice at power plants. The ANSI/ISA 7.0.01-1996 standard on the permissible level of hydrocarbons in instrument air systems states: “The lubricant content should be as close to zero as possible and under no circumstances shall it exceed 1 ppm w/w (liquid water) or v/v (water vapor). Any lubricant in the compressed air system shall be evaluated for compatibility with end-use pneumatic devices.”

Most utilities do not have a problem holding the hydrocarbon levels in instrument air to below 1 ppm. The available test methods appear adequate with the possible exception of filter weight testing. The filter test method could allow some oil mist to escape detection and does not account for non-condensable gases.

6-18


7

CORRECTIVE MAINTENANCE

This section includes information [2,3,6] on: •

Safety issues

•

Component failures

•

Air leakage

7.1

Safety Issues

Manufacturers have become very conscious of safety considerations and this is reflected in operator manuals, parts lists, and warning decals applied to the machinery. The Occupational Safety and Hazards Act (OSHA) also legislates certain safety requirements for the work place that are the responsibility of the owner and operator rather than the manufacturer. Some of these requirements are listed in Table 7-1. Table 7-1 Owner/Operator Safety Responsibilities [3] 1. Equipment should be installed and operated in full compliance with all pertinent OSHA, federal, state, and local regulations, codes, and standards. Each operator of compressed air equipment should have been trained to follow the appropriate regulations, codes, and standards and should have a copy of the operator’s manual for each air compressor and dryer and their accessories. Each operator should read and understand the manual and operate the equipment in accordance with it. 2. A compressor should not be started unless it is safe to do so. A compressor considered to have an unsafe condition should be tagged and rendered inoperative by disconnecting and locking out all power to the compressor at its source until the unsafe condition has been rectified. 3. The safety shutdown features incorporated in the compressor or dryer package should be tested periodically for proper functioning, as specified in the operator’s manual. 4. The maximum pressure rating of any piece of pneumatic equipment should not be exceeded at any time. Pressure relief valves installed to safeguard against excessive pressure build-up should be tested at specified intervals. All internal pressure should be vented prior to opening any air line, fitting, hose, valve, drain plug, connection, or component in the system. 5. Modifications to air compressors, controls, and other pneumatic equipment should not be made without the written approval of the manufacturer.

7-1

EPRI Licensed Material Corrective Maintenance Table 7-1 (cont.) Owner/Operator Safety Responsibilities [3] 6. The area around air compressors should be kept clean. Spills of lubricant or other combustible or volatile substances such as cleaning fluid, paint, or solvent should be cleaned up immediately. 7. Electrical equipment should be properly grounded. Electrical wiring and terminals should be kept in good condition and replaced when cracked, cut, abraded, worn, or discolored. People and hand-held tools or other conductive materials should be kept at a safe distance from exposed live electrical components. 8. Some air compressors have an automatic starting feature. This might or might not be indicated at the instrument panel. It is essential to make sure that both the main driver power and the control system power (if different) are switched off prior to the start of any maintenance work. 9. Moving parts should be properly guarded in accordance with OSHA requirements. Equipment should not be operated with guards removed. Limbs and other body parts and clothing should be kept at a safe distance from moving parts, including shafts, couplings, belts, and fans. 10. Personnel should be kept out of line with, and away from, the discharge opening of valves, hoses, tools, or other compressed air discharge. Air at pressures under 30 psig (207 kPa) should be used for cleaning and then only with appropriate guarding. Protective equipment should be in accordance with OSHA Standard 29 CFR 1910.242 (b), including a flow-limiting valve. When an air hose exceeds 1/2-in. (13-mm) inside diameter, additional equipment is necessary to protect against hose failure. 11. Some air compressor enclosures are relatively large, so it is essential to be sure that all personnel are out of the enclosure before the doors or panels are closed and latched and before power to the compressor is switched on. 12. It is recommended that the manufacturer’s repair parts, or parts meeting the manufacturer’s specifications, be used for repairs. Some “look alike” parts might not meet specifications, might void the warranty, result in malfunction, or be unsafe. 13. Only the manufacturer’s recommended lubricants or approved equivalents should be used and changed at the specified intervals. The change-out interval can be extended (based upon lubricant sampling and plant predictive maintenance history) with the approval of the manufacturer. Different lubricants should never be mixed. A synthetic lubricant having an 8,000-hour specified life does not mean that the lubricant filter or air/oil separator also has an 8,000-hour life. It is estimated that the useful life of a lubricant is halved for every 18°F (10°C) above 200°F (93°C) operating temperature. For compressors operating in high ambient conditions, a quarterly sampling and analysis program is recommended. 14. Air receivers should be checked periodically for corrosion, cracks, or dents, and should be checked by a qualified inspector annually or at each refueling outage. The test should include the pressure relief valve and pressure gauge. Pressure testing should be with water and never with compressed air. The receiver must be isolated from the remainder of the system for testing and proper draining after the test is essential. For older receivers, it is recommended that the shell thickness of the bottom of the receiver be checked with an ultrasonic device. 15. Each location should have a formalized program of safety checks at specified intervals and with appropriate responsibilities assigned for carrying out the procedure, recording data, and monitoring safety operations.

7-2

EPRI Licensed Material Corrective Maintenance

Compressed Air Safety Checklist – The following questions in Table 7-2 are designed to aid power plant personnel in determining if proper safety measures have been taken in the installation and operation of their compressed air systems. If the answer to any of the following questions is No, the work conditions should be thoroughly reviewed from the work safety point of view. They are based on California/OSHA standards and are general in nature. Table 7-2 Compressed Air Safety Checklist [3] •

Are compressors equipped with automatic, temperature-activated shutoff mechanisms or with fusible plugs installed as near to the compressor as possible in the compressor discharge lines?

•

Are compressors equipped with automatic pressure relief valves, pressure gauges, and drain valves?

•

Are compressor air inlets installed and equipped so as to ensure that only clean, uncontaminated air enters the compressor?

•

Are air filters installed on the compressor inlet?

•

Are compressors operated and lubricated in accordance with the manufacturer’s recommendations?

•

Are safety devices on compressed air systems checked frequently?

•

Before any repair work is done on the pressure-containing system of a compressor, is the pressure released and the system locked out?

•

Are signs posted to warn of the automatic starting feature of the compressors?

•

Is the belt drive system totally enclosed to provide protection for the front, back, top, and sides?

•

Are employees prohibited from using compressed air above 30 psig (207 kPa) for cleaning purposes?

•

If compressed air is used for cleaning off clothing, is the pressure reduced to less than 10 psi (69 kPa)?

•

When using compressed air for cleaning, do employees wear protective chip guards and personal protective equipment for eyes, hands, and feet?

•

Are safety chains or other suitable locking devices used at couplings of all highpressure hose lines of 3/4-in. (19-mm) inside diameter or larger, and lines of smaller size where a hose or connection failure would create a hazard?

•

Before compressed air is used to empty containers of liquid, is the safe working pressure of the container checked?

•

When compressed air is used with abrasive blast cleaning equipment, is the operating valve a type that must be held open manually?

•

When compressed air is used to inflate auto tires, are a clip-on chuck and an in-line regulator that is preset to 40 psi (276 kPa) required?

•

Is it prohibited to use compressed air to clean up or move combustible dust if such action could cause the dust to be suspended in the air and cause a fire or explosion hazard?

7-3

EPRI Licensed Material Corrective Maintenance Table 7-2 (cont.) Compressed Air Safety Checklist [3] •

Is the total relieving capacity of the safety valve capable of preventing pressure in the receiver from exceeding the maximum allowable working pressure of the receiver by more than 10%?

•

Is every air receiver provided with a drain pipe and valve at the lowest point for the removal of accumulated oil and water?

•

Are compressed air receivers periodically drained of moisture and oil?

•

Does each compressed air receiver have an opening for internal inspections?

•

Are all air receivers periodically inspected externally for corrosion, dents, and so on? For older receivers, it is recommended that the shell thickness at the bottom of the vessel be tested using an ultrasonic testing device to confirm that the vessel meets minimum wall thickness requirements.

•

Are the external surfaces of air receivers kept free of oil and dust accumulation?

•

Is each compressed air receiver inspected internally at specified intervals by a qualified inspector?

•

Are all safety valves tested frequently and at regular intervals to determine whether they are in good operating condition?

•

Are the inlets of air receivers and piping systems kept free of accumulated oil and carbonaceous materials?

•

Have the following safety precautions been incorporated into the procedures for the internal inspection of air receivers? – Starting and control equipment is tagged and locked out. – Vessel is depressurized. •

Externally bolted manhole covers are first pried loose from their seats before all of the bolts or nuts are entirely removed.

– All manhole covers are fully removed to improve ventilation. •

Before workers are permitted to enter the tank, the tank atmosphere is tested for (a) oxygen, carbon dioxide concentrations, and (b) toxic, flammable, or combustible gases and vapors.

•

Respiratory equipment is required to be used (supplied-air type) if a hazardous atmosphere is present.

•

Workers entering the tank are required to be equipped with a lifeline and a safety watch is positioned at the tank opening.

•

Workers are required to wear proper footwear, eye and face protection, and work gloves to prevent injuries.

•

Portable electric lamps or tools used inside the tank are explosion-proof and grounded.

•

After cleaning, the inside is inspected for removal of loose scale, wiping rags, tools, or pieces of lint.

– New gaskets are placed on the manhole covers. •

7-4

Has a record been maintained of all discrepancies and have corrective actions been taken?


7.2

Component Failures

Component failures in the instrument air systems can be grouped into: •

Air supply component failures

•

End-use component failures

7.2.1 Air Supply Components Usually a complication in the supply side components is what causes the eventual end-use component failure. The most common failure components are: •

Filter Maintenance – The most common trouble spots among the instrument air supply side components are filters. Filters require periodic inspection, as well as cleaning and changing. The filter differential pressure limits should be marked on the filter housing and the shift routines should provide a method to ensure that limits are not reached. Even though filters in an instrument air system should be able to withstand full system pressure, the lack of air flow by a blocked filter can lead to plant air loss.

•

Filter Size – The second most common filter failure is due to the use of an improperly sized filter element. It is very easy for a 0.1-micron filter element to plug when it is housed where a 20-micron filter should be. Sometimes an old partially used filter finds its way back into the system without proper cleaning. Improper filter use is extremely expensive in both maintenance time and casualty repair costs. It is also one of the easiest to correct. A few easy rules save many maintenance dollars.

•

Compressor Failure – Compressor failure is another common casualty in a power plant. Most power plants have special alarms dedicated to this casualty because its effects can be very damaging. Some plants have one or two redundant compressors as backup and many have installed receiver banks and special accumulators to further mitigate the effects of a compressor loss. As in many other component failures, lack of proper maintenance is a big offender in these occurrences. It shows up as lack of lubrication, dirt incursion in the oil system, failed tubes in an after-cooler, crud-clogged valves, and so on. Vendor information and technical manuals provide information on how to properly maintain the air compressors. Maintenance personnel should thoroughly inspect the system with special attention to lubrication, filtration, and operating temperatures. They should also periodically test the system to ASME Code requirements.

•

Dryer Failures – Dryer failures contribute significantly to the failure of many end use components. While a dryer is off-line or not performing to full capacity, moisture is entering the system.

7-5


Key Human Performance Point One of the most common problems, especially with single-train dryers, occurs when a pre-filter clogs and the differential pressure goes past the limit. The operator manually compounds the problem by bypassing the dryer. It is better to bypass the filter or even remove the element than it is to bypass the entire dryer train. Key Human Performance Point When the dryer towers experience shifting difficulties and the desiccant in one tower is too wet, it is strongly recommended not to bypass the dryer. A desiccant that is partially working is better than none at all. When dryers shift rapidly or try to shift and cannot, the desiccant breaks down. The resulting fines are too small for many after-filters and system contamination can result. Dryers, timers, switching mechanisms, and heaters require periodic maintenance to perform properly. 7.2.2 End-Use Components Most end-use component failures are caused by contamination of the instrument air system. The most common failures in end-use components are: •

Solenoid Valve – A solenoid valve usually has O-rings that provide the sealing surface for the valve disk. If any particulate contamination becomes attached to the O-ring, it can cause the valve to not seal properly, or not work at all. Some of the lubricants used on O-rings by various utilities are water-soluble and any moisture can cause the lubricant to become sticky and feel somewhat like rubber cement. Moisture tends to gum up the valve and keep it from working properly. The rubber O-rings are also highly susceptible to damage from hydrocarbon contamination. The oil or hydrocarbon contamination breaks down the O-ring chemically and can cause it to either deteriorate or swell, thus making the solenoid valve inoperative. The vent line on the solenoid valve is specially sized to allow a certain flow rate of air to issue from the vent when the solenoid is actuated. This flow rate has a direct impact on the cycling time of the air-operated equipment. If this vent line is pinched or blocked, the air-operated valve’s stroke time might be out of specification or the valve might not work at all.

•

I/P (Current to Pneumatic) Converter – The I/P converter is very susceptible to air line contamination, particularly any dirt or rust in the I/P converter. Due to extremely small ports inside the I/P converter, any dirt or rust that enters the instrument can start plugging ports and cause sluggish response. Most I/P converters in use in the power industry have pressure/filter regulators in the air line just upstream of the I/P converter. Filter regulators usually have an intake screen to filter the

7-6


air. This intake screen is generally provided with a petcock at the bottom of the regulator. This petcock should be opened periodically to blow any particulate or foreign matter out of the regulator to keep the screen from clogging and blocking air flow through the regulator. If any water is present in the air line, it will also be blown out. •

Drain Traps – Drain trap failures are usually due to foreign matter fouling the seating surfaces so that they do not fully seat when closed. This allows the trap to constantly blow through and slowly depressurize the system. The compressor usually maintains this added load with little difficulty, but it causes more compressor run time. The result is the same as a constant leak in the system. Another common failure in the drain trap is the collection of rust and other corrosion products that can cause the trap to stick shut. This keeps the trap from draining the moisture from the line. It can lead to moisture carry-over and overload the dryer.

•

Pneumatic Valve Operators – Pneumatic valve operators are also known to fail. They usually fail due to contamination of the instrument air. The diaphragms might be made of a neoprene or rubber compound that deteriorates in the presence of hydrocarbon contamination. They might also collect dirt and rust, thus ultimately filling up, or causing the valve to stick or not fully stroke, by restricting its movement.

•

Air-Line Check Valves – Air-line check valves can be rendered inoperable by dirt and rust particles. Foreign matter on the seating surface can stop the check valve from seating completely, permitting accumulator pressure to backflow into the instrument air system. This seriously degrades the safety-related function because the accumulator will not have the design pressure and volume to operate the valve.

7.3

Air Leakage

The distribution system consists of a series of progressively smaller pipes that carry compressed air from the compressor station to the end uses. A riser carries the air up from the compressor station to one or more main header pipes. These carry air to subheaders or branch lines, supplying the drop points that bring the compressed air down to the point of use. Traps are located throughout the system to accumulate and discharge condensate in the air stream. At the drop points, combinations of filters, regulators, and lubricators condition the air for use by pneumatic devices. There are many opportunities for energy savings in a distribution system. The largest energy wastes come from artificial demand, in the form of leaks, unregulated end uses, and excessive pressure drop. Low pressure levels at end-use points often result in the purchase of extra compressor capacity to increase air pressure and flow levels. Adding compressor capacity incurs a large initial capital expense and increases compressor system operating costs. Frequently, however, the problem is in the distribution system. Pressure drop and artificial demand can combine to use nearly half of the compressor system’s air output. An increase in plant pressure, which increases the amount of air in the system, results in greater pressure drop and greater air loss. In extreme cases, adding compressor capacity and increasing pressure can have no (or even negative) effect at the end-use point. 7-7


Correcting the distribution problems can eliminate the need for extra compressor capacity, improve the air supply to end users and reduce the operating costs of the existing compressed air system. 7.3.1 Cost of Air Leakage Table 7-3 shows an estimated volume of air lost per month through various size openings and the annual cost of lost air. The total cost of wasted air has been calculated based on an installation of a two-stage reciprocating compressor, assuming an adiabatic compression and a 15% horsepower loss due to friction. This is a very conservative estimate for an efficient installation. It should be noted that the cost of lost air shown in this table is from a single opening of the size indicated and is based on electric power consumption only. Other indirect costs, such as pro-rated operation and maintenance cost, equipment depreciation, capital cost, reduced equipment life due to additional run time, etc. are not included. Table 7-3 Estimated Leakage Costs [2] Size of Leak in inches (mm)

Air Lost per Month @ 100 psi (689 kPa) in ft.3 (m3)

Energy Cost per Month (kW-hr/month)

Annual Energy Cost of Leakage @ $0.10/ kW-hr, ($)

3/8 (9.5)

6,671,890 (188,927)

14,678

17, 614.00

1/4 (6.4)

2,920,840 (82,709)

6,426

7,711.00

1/8 (3.2)

740,210 (20,960)

1,628

1,954.00

In general, the annual cost of electric power consumption for compressed air can be calculated by the following formula: C = P * H * 365 * 0.746 * R E where:

C = Annual electric power cost, $ P = Brake horsepower of the compressor drive H = Operating hours of the compressor per day R = Energy cost, $/kW-hr E = Motor efficiency (Generally between 0.85 and 0.9 for direct drive. For belt drives, efficiency will be slightly lower.)

For this formula, it is assumed that air is compressed adiabatically, although the water jacket on the compressor and the inter-cooler will bring the compression curve slightly below the true adiabatic curve. This slight difference is compensated by additional work required to force air through the valves. Therefore, an adiabatic assumption for the entire cycle is a fair assumption. In calculating the power consumption, the inlet air has been assumed to be at 60°F (15.5°C). Because annual ambient temperatures can vary from below freezing to above 100°F (37.8°C), a value of 60°F (15.5°C) has been used as an average for this calculation. It should be noted that 7-8


with higher ambient temperatures, the power requirement to compress and deliver the same amount of free air would increase. From the amounts shown in Table 7-3, it is clear that regular system walk-downs to identify and repair air leaks will result in significant cost savings. Kaeser assumes 8500 hours of operation per year and an energy cost of $0.08 per kw-hr for the cost of leakage in a 100 psig (689 kPa) system. The following is a chart of the cost of the leakage.

Figure 7-1 Cost of Air Leakage [4]

7.3.2 Leak Detection Key O&M Cost Point Leakage easily can be the largest energy problem in a compressed air system, ranging from 2 to 50% of compressor system capacity. An average plant has a leak rate of about 20% of total air production. Leaks can be reduced to 5% or lower, offering the average plant an opportunity to reduce compressor system costs by as much as 15%. The most frequently used method for locating air leaks is to systematically walk down and survey the air system using a soap solution. Ultrasonic leak detection devices are also available. Tracer gases can also be introduced to help in detecting leaks. 7-9


Identifying leaks in compressed air systems [2,6] is a task that is far more complex than it appears. Prior to starting a system walk-down to search for leaks, the latest piping and instrumentation drawings should be compiled and segmented into area piping runs. All leaks identified should be marked on these drawings as to their location and the type of leak (that is, large, small, leaks from threaded joints or compression fittings, and so on). It is also a good practice to update the drawings to as-built drawings during these walk-downs. Identifying large leaks that are close at hand is a relatively easy task because of the characteristic high resonance hissing sound. When these leaks are high in the overhead, it is far more difficult. The surest way to check these high runs is to scaffold or to use a ladder, but it can also be accomplished using a headset and a sound detector with a parabolic microphone. This method, if applied, should be conducted on the night shift when the plant noise level is at a minimum. Probably the most reliable, though expensive, method of identifying leaks is a complete soap test. A soap test will identify all leaks by exhibiting a constant run of bubbles. This is especially effective on low-pressure pinhole type leaks or leaks in threaded fasteners. Hydrostatic testing using nitrogen, or some other non-contaminating gas, is another method for ensuring an air system’s integrity. Hydrostatic testing can be performed during construction, during upgrading, or during major repair operations. Hydrostatic testing is difficult to perform on existing systems because air systems must function in all modes of plant operation. Also, there is difficulty in establishing hydrostatic testing boundaries. Key Technical Point One way of determining the integrity of air system piping is to measure the minimum wall thickness of carbon steel piping per ANSI B31.1 Power Piping Standard. This is an area of concern in un-dried portions of the compressed air system. By monitoring this portion of the system, leaks can be minimized and potential failure areas identified. Once a leak has been identified, the maintenance action is only beginning. Depending on location of the leak, different corrective action options need to be considered. If the leak is at a mechanical fastener (threaded- or compression-type fitting), just tightening it might not be sufficient. Retest again with soap after repair. If a significant amount of leaky joints (3 to 5%) are found, the piping should be inspected for hangers, snubbers, and so on. It is possible that pipe harmonics are causing some of the difficulties. In any case, strengthening the system by the use of stronger fasteners can be looked into to help solve this problem. It might be necessary to shift to steel pipe and use unions if soft copper instrument tubing is presently used. The procedures that have been developed for testing solder joints employ techniques that are similar to those used for flaw detection in other piping systems. With ultrasonic testing, the operator of the test device must interpret reflection peaks that appear on a readout screen. Solder joints are tested by comparing the amplitudes of the peaks received from the first and second reflections. By comparing the two peaks, the percentage of the joint that is bonded can be determined. The test analyst depends on individual knowledge and experience to interpret the results. Different conclusions are often reached by different analysts for the same joint. Because 7-10


of this variance in interpretations, there must be a margin for error when evaluating joint test results. This testing method frequently leads to a divergence of opinions. Maintenance personnel tend to know a solder joint is good, while quality control personnel point to their ultrasonic charts that might show the bond is inadequate. The best way to eliminate these situations is to have combined training sessions where joints are soldered, tested, and peeled apart in the presence of all interested personnel. Training in this manner allows maintenance personnel to see the void areas in the solder joint and also gives quality control personnel a chance to compare physical indications with electronic indications. Alternate methods of joint testing are also available. The most used, other than ultrasonic, is radiography testing. This method provides a picture of the joint that can be evaluated for joint make-up. Both methods of joint testing have drawbacks. With x-rays, the clarity of the readout can be a problem. As with ultrasonic testing, the test analyst is required to interpret the results. Compression and mechanical are two types of pipe fittings. Like most kinds of fittings, they can present integrity problems. Leaks or failures of fittings can cause problems ranging from the loss of controls to the introduction of moisture into the system. The problems encountered most often with fittings are improper assembly and under-tightening. Over-tightening is a problem that limits the reuse of fittings but does not usually affect fitting integrity. Fittings will not hold unless all of the pieces are installed, or unless the tubing is fully inserted into the joint. If the joint has been over-tightened, the fitting pieces might be deformed enough to prevent resealing during reassembly, or damage might have occurred to the threaded portions. When worked correctly, fittings should be able to seal even after many make-and-break operations. To help alleviate the stresses due to expansion, vibration, or misalignment of fittings and end-use components, the use of tubing, loops, and flexible hoses are recommended. When correctly installed, tubing loops will not require maintenance other than periodic inspection. Flexible hoses should be used in applications that require repeated movement or where failures of tubing loops have occurred. Flexible hoses require regular maintenance inspections at a frequency that depends on the type of flexible hose, the environment in which it is located, and the manufacturer’s recommendations. Through-wall leaks in pipes and pressure vessels are an indicator of serious problems. A pipe with a through-wall leak should not be patched except as a temporary fix. Then the pipe run should be tested by a nondestructive testing method such as ultrasonic testing or radiography testing. The section of pipe with a leak should be discarded. Adjoining pipes or pipes in similar orientation or use should be checked for similar problems. Normally, through-wall leaks are in carbon steel pipes where oxygen pitting or severe rust has thinned the pipe wall. This often occurs at low spots in the pipe where temperature changes permit the moisture to condense. Then the air flow is not enough to move the water to the drain point. 7-11


Through-wall leaks in a pressure vessel such as an accumulator, dryer, or receiver are a much greater problem. These ASME Code vessels have state requirements for testing. Notification of the leaks might be required by applicable state or local Codes. Leaks in instrument air systems are almost always indicators of far more significant system problems. Leakage can result in major operational problems adding significantly to the cost of plant operation. A large number of unattended leaks can interfere with system operation by lowering system header pressure. Most air-operated equipment and instruments in a power plant are rated for operation between 85 to 100 psig (586 to 689 kPa) air pressure. Any degradation of air pressure in the header will interfere with the proper operation of these components, resulting in unexpected responses. An industry-wide survey conducted by EPRI in 1988, recorded actual failure incidents of the following types due to a gradual degradation of instrument air pressures: •

Sluggish response of air-operated valves

•

Failure of check valves to close on gradual loss of air

In some power plants, the service air system is used as a backup to the instrument air system. When a loss of instrument air pressure occurs, the two systems are cross-connected automatically or manually and the instrument air pressure is maintained by the service air system pressure. This cross-connecting will cause gross moisture contamination of the instrument air system unless it occurs prior to dryer pre-filters. Reducing leakage requires an active leak detection and repair program. Locating leaks requires visual and acoustic (using ultrasonic leak detection equipment) inspection of the entire system. Stopping the detected leaks will often necessitate repair or replacement of faulty equipment, including pipe sections, hoses, joints, traps, and drains. Abandoned equipment that is still supplied air, and unauthorized air uses (personal cooling, etc.), are additional forms of leaks. These leaks can be prevented by stopping air flow to the leaking area. Lower-pressure air has less air mass in the same volume. Because the volume of leaking air remains constant for a given leak, a lower pressure results in less air escaping from existing leaks. Precise pressure control and reduced pressure drop are two ways to lower average system pressure.

Key Human Performance Point In order to be effective in the long term, a leak-reduction program must be constant and comprehensive. When compared to other measures, leak repair generally offers the greatest improvement in compressor system efficiency for the least expenditure. An energy savings potential of 15% for a typical plant more than justifies a leak maintenance program. 7-12


Many pneumatic tools can operate at pressures lower than the average system pressure. Pressure levels at individual tools can be controlled by regulators that are set to provide air at appropriate pressure levels. Unregulated tools, however, often use full system pressure, instead of using a regulator to convert the system air to design pressure levels. When air pressure is reduced, the amount of air used by the same tools is correspondingly reduced, lowering system air demand. In addition to using excess air, high-pressure levels can increase equipment wear, thus resulting in higher maintenance costs and reduced equipment life. Pressure drop is a reduction in air pressure that results from loss of air energy as it travels through the conditioning and distribution systems. Total pressure drop is the difference between compressor discharge pressure and pressure at the end use. A well designed system will have a pressure loss of less than 10% of the compressor’s discharge pressure. Two psi of pressure difference is roughly equivalent to a 1% change in compressor system energy use, so there are significant energy gains to be realized by lowering pressure drop. Every obstruction or restriction in the system increases pressure drop. In the conditioning system, after-coolers, separators, receivers, filters, dryers, traps, and drains all contribute. Joints, elbows, reducers, valves, and small pipes are elements of the distribution system that reduce air pressure. Minimizing pressure drop requires a complete approach to the air system. Air conditioning equipment can be selected with less air resistance. With certain elements, over-sizing the equipment will lower obstruction and justify the added initial cost. Effective maintenance will keep existing conditioning equipment from causing excessive pressure drop. The distribution system should be designed to allow as free a passage of air as possible. Larger piping, fittings, and valves with low flow resistance will not only reduce pressure drop but will allow for later system expansion as well. Corrosion is another factor that can cause increased flow resistance throughout an air system. As the system ages, pipe scale and other contaminants can collect and cause obstruction and blockages, which can significantly reduce air pressure. Proper filtration and drying can prevent much of this corrosion during the lifetime of the system. If corrosion does build up, it is a difficult problem to correct and often requires replacement of the problematic distribution sections. 7.3.3 Leak Repairs As questionable solder joints are found, repairs must be made in a manner such that the joint will not fail again and require rework, or introduce additional potentials for failure. As each joint is tested, the following actions are recommended: •

For full joint insertion and a 70% or greater bond – No action required

•

For full joint insertion and a 50% to 70% bond – Evaluation needed

•

For less than a 50% bond or less than a full joint insertion – Brace the joint and repair or replace 7-13


The AWS Soldering Manual, April 1, 1977 edition states: “A good criterion for lapped solder joints is to consider a 70% filled joint to be adequate provided the voids are small and discretely dispersed.” The stabilization of questionable joints can be achieved by installing braces. Clamps that are installed around the upstream and downstream pipes, with threaded rods installed between the clamps, can provide adequate joint support. When making repairs, the installation of new joint components is recommended if possible. This will ensure the best possible joint. For soldering, it is recommended that propane, acetylene-air, butane, or other premixed products be used. Care must be taken to prevent the mixing of solder types. There are 12 basic steps for assembling a solder joint and they are listed in Table 7-4. Table 7-4 Assembling a Solder Joint [6] Steps

Tasks

1.

Measure the correct length of tube. Too long or too short can cause stress in the joint or a poor fit-up and improper insertion.

2.

Cut the tube square. This will ensure full joint fit-up and proper insertion. A square cut also aids in joint alignment.

3.

Ream the cut end. Removing burrs aids in fit-up and also allows inspection of roundness.

4.

Clean the tube end. Cleaning removes oxidation and oils that might interfere with solder flow. Cleaning should remove surface film but no metal.

5.

Clean the fitting socket in the same manner as Step 4.

6.

Apply flux to the tube end as soon after cleaning as possible. Use care not to over-flux because flux is corrosive and can remain inside the joint.

7.

Apply flux to the fitting socket while following the cautions in Step 6.

8.

Assemble the joint by fully inserting the tube into the joint. A small scribe mark can be used to ensure full insertion. Twist the joint slightly to ensure that the flux is spread on both surfaces.

9.

Remove the excess flux with a rag.

10.

Apply heat using care not to overheat the joint. Ensure that there is full and even heating of the joint.

11.

Apply the solder. Large amounts of solder do not ensure a good joint.

12.

Allow the joint to cool. Excessively rapid cooling can crack fittings.

After the repairs are completed, a leak check at system pressure should be made to ensure that there is no leakage. Some type of joint integrity test (ultrasonic, x-ray, etc.) should also be performed. The design requirements at various plants require the use of 50/50 or 95/5 solder for joint assembly. A search for superior solders has been underway. Some of the new solders have been found to be much stronger and to have better workability through a wider range of temperatures. A wider working temperature range allows construction and maintenance personnel to fill joints completely without the need for constantly supplied heat. Although oxygen-acetylene torches are readily available, these are not the recommended heat source for solder joints. A carburizing

7-14


(low-temperature) flame should be used because, with oxy-acetylene, it is easy to overheat a joint and cause flux breakdown and poor solder flow. A decision matrix was developed by the Tennessee Valley Authority (TVA) to evaluate leaking connections and determine the appropriate fix. If a leaking connection can be taken out of service, the joint can be repaired to the original design. When the system must remain in operation, the repair must be made based on existing conditions. Mechanical joints are first tightened to see if that will stop the leak. If this fails, the joint is sealed with an epoxy putty. The epoxy provides a leak-tight seal, but is not relied on to provide axial strength to the joint. If there is concern about the stability of the joint, a commercially available fabric mesh tape, impregnated with fast-curing polyurethane resin, is applied over the epoxy putty. This combination seals the leak and ensures that the joint does not come apart until a permanent repair can be made. For soldered joints, a different approach is used. TVA has developed a crimping tool for use on copper pipe and wrought copper fittings that is adjustable from 3/8 inch (9.5 mm) to 2 1/2 inches (63.5 mm). This tool is a two-piece die that is placed around the leaking connection and tightened to produce a crimp in the fitting and pipe. The die is then removed from the joint. It has been suggested that a furmanite-type of repair might be used on some leaking joints. The main concern with this type of repair is the potential to introduce the sealing material into the air system with possible clogging of instrument lines or blockage of other air paths.

7-15


7.3.4 Piping Replacement Some piping guidelines are given in Table 7-5. Table 7-5 General Piping Guidelines [23] •

Use large enough carbon steel pipe so that the pressure loss in the line does not exceed 3 psig (20.7 kPa). The target is a maximum 10% pressure drop through the entire system from the air compressor to the farthest drop.

•

Slope the main lines approximately 1/4 inch per foot (21 mm/m) of pipe away from the air compressor. Install drop legs for condensate removal.

•

Slope the piping so that condensate travels with the flow of air and away from the compressor.

•

Use carbon steel pipe as discharge pipe material. Never use PVC piping, Schedule 40 black iron, galvanized, copper, stainless steel, or anodized aluminum.

•

Install an air receiver at intermittent high demand points such as occasional sandblasting, air motors, and so on.

•

Air receiver size should be 1 gallon of storage per 1 cfm (133.5 liters per m3/min) of air compressor output as a minimum in order to permit the compressor controls to operate correctly.

•

Always consider leakage and future expansion in order to eliminate compressed air system obsolescence. A 10% per year growth rate is common.

Nearly all of the compressed air system manufacturers recommend that customers do not use plastic piping, soldered copper fitting, and rubber hose as discharge piping for compressed air systems. Plastic piping is not recommended because some types might react with compressor fluids, soften due to heat, or shatter due to pressure or pulsation of the compressor. Soldered, copper fittings will eventually work loose due to pulsation caused by the compressed air system. Rubber hose piping is unacceptable because the hose material is easily attacked by today’s lubricants. In addition, flexible joint and/or flex lines can only be considered for such purposes if their specifications fit the operating parameters of the system.

7-16


8

INDUSTRY INFORMATION

Appendix H contains a specification listing for a compressed air system. This is provided courtesy of Duke Energy Corporation. The information listed in this section is intended to provide resources for utilities on compressed air systems. The information was obtained from the U.S. Department of Energy web site www.compressedairchallenge.org in the category of Library/Improving Compressed Air System Performance – A Sourcebook for Industry/Section III/Resources and Tools. A wide range of information is available on the application and use of compressed air systems. This section focuses on resources and tools in the following categories: •

Books and reports

•

Brochures

•

Periodicals

•

Software

•

Videos

•

Workshops and training courses

The resources and tools presented here are not intended to represent all available information pertaining to compressed air systems. Instead, this list presents the reference material and tools that would be of interest to those involved in energy-efficient compressed air systems. Neither the Department of Energy nor the Compressed Air Challenge™ imply any endorsement of the information included in this section.

8.1

Books and Reports

The books and reports listed are grouped into one of the following three categories: •

Documents on compressed air systems that focus on performance improvement

•

Specialty books on compressors (for example, compressor design)

•

Information on the compressed air market

8-1

EPRI Licensed Material Industry Information

Documents That Focus on Performance Improvement Air Compressors and the Compressed Air System Author: William Scales, P.E. Description: A comprehensive text on maintaining compressed air systems for peak performance. Available from: Scales Air Compressor Corporation 110 Voice Road Carle Place, NY 11514 Phone: (516) 248-9096 Fax: (516) 248-9639 Assessing Processes for Compressed Air Efficiency Authors: Bill Howe, P.E. and William Scales, P.E. Description: The report presents 11 questions managers should answer about their compressed air applications in order to understand whether compressed air is the right tool for the job, how compressed air is applied, how it is delivered and controlled, and how the compressed air system is managed. Available from: E SOURCE Reprints Service 1033 Walnut Street Boulder, CO 80302-5114 Phone: (303) 440-8500 Fax: (303) 440-8502 Compressed Air and Gas Handbook, Fifth Edition Author: Various Compressed Air and Gas Institute members with John P. Rollins, editor Description: A comprehensive reference work on all phases of compressed air and gas; this handbook covers reciprocating, rotary, and dynamic compressors, pneumatic tools, construction equipment pneumatic controls, materials handling equipment, and many other topics. The sixth edition was published in 1998. Available from: Prentice-Hall Publishers 200 Old Tappan Road Old Tappan, NJ 07675 Phone: (800) 223-1360 Fax: (800) 445-6991

8-2


Compressed Air Management, Energy Efficiency in Compressed Air Systems Seminar Workbook Author: T. F. Taranto Description: Used in seminars, this workbook is a resource for the industrial compressed air user. Topics include concepts of compressed air system management, compressed air system investment, cost of compressed air, system performance modeling, benchmarking system performance with data measurement, and system management strategies. Available from: Data Power, Inc. P.O. Box 182 Baldwinsville, NY 13027 Phone: (315) 635-1445 Fax: (315) 635-1445 Compressed Air Systems Author: H. P. Van Ormer Description: This handbook discusses compressed air systems including departmental and central air systems. It covers topics such as compressor types; application, selection, and installation of rotary and centrifugal air compressors, compressor capacity controls; compressor terminology; determination of air requirements; compressed air dryers; and optimization of systems. Available from: Air Power USA, Inc. P.O. Box 292 Pickerington, OH 43147 Phone: (614) 862-4112 Fax: (614) 862-4112 Compressed Air Systems Solution Series Author: Scot Foss Description: This comprehensive text discusses ways to improve the performance of compressed air systems. It is published as a two-year, bi-monthly subscription series. It covers topics such as design issues, troubleshooting, instrumentation, storage, piping, controls, demand issues, and supply issues. Available from: Bantra Publishing Phone: (704) 372-3400 Compressed Air Systems: A Guidebook on Energy and Cost Savings Author: E. M. Talbott Description: This guidebook covers topics ranging from compressed air equipment and distribution system layout to final application and system operation. Available from: Prentice-Hall Publishers Englewood Cliffs, NJ Phone: (800) 223-1360 Fax: (800) 445-6991 8-3


Compressed Air Technology Seminar Workbook: Opportunities and Solutions Author: H. P. Van Ormer Description: Used in Mr. Van Ormer’s compressed air seminars, this workbook serves as a good resource for those looking to improve the efficiency of their compressed air systems. Topics discussed include compressed air basics, supply equipment, regulation and controls, system design, receiver demand flow regulation, maintenance and reliability, power savings, leak surveys, and flow meters. Available from: Air Power USA, Inc. P.O. Box 292 Pickerington, OH 43147 Phone: (614) 862-4112 Fax: (614) 862-4112 Compressor Engineering Data Author: William Scales, P.E. Description: A handbook of reference material on compressed air systems. Available from: Scales Air Compressor Corporation 110 Voice Road Carle Place, NY 11514 Phone: (516) 248-9096 Fax: (516) 248-9639 Compressors and Expanders: Selection and Application for the Process Industry Author: Heinz P. Bloch Description: This book identifies preferred equipment types for specific uses, provides easy-tounderstand explanations and examples, examines the limitations of the machinery, and compiles data that is scattered throughout the literature. The potential audience includes engineers interested in gas separation, cryogenic processes, and compression stations, manufacturers and purchasers of compressors and turbo-expanders, and contractors involved in plant design and machinery selection. Available from: Marcel Dekker, Inc. 270 Madison Ave. New York, NY 10016 Phone: (212) 696-9000 Fax: (212) 685-4540

8-4


Compressors: Selection and Sizing, Second Edition Author: Royce N. Brown Description: This reference text provides information on compression principles, equipment, applications, selection, sizing, installation, and maintenance, allowing proper estimation of compressor capabilities and selection of designs. Updated with new American Petroleum Institute standards and current technology in areas of efficiency, 3-D geometry, electronics, and plant computer use, this guide covers reciprocating, rotary, and centrifugal compressors and compares their reliability. Available from: Gulf Publishing Company P.O. Box 2608 Houston, TX 07675 Phone: (713) 520-4444 Fax: (713) 520-4433 Pumps/Compressors/Fans: Pocket Handbook Authors: Nicholas P. Cheremisinoff, Paul N. Cheremisinoff Description: This handbook provides a concise presentation of the fundamentals—design, function, and applications—of pumps, compressors, and fans. It is organized for easy reference and illustrated with more than 80 photographs, diagrams, and other schematics. This text will help engineers and other plant operations personnel in their selection and utilization of pump, fan, and compressor equipment. Available from: Technomic Publishing Company, Inc. 851 New Holland Ave. Box 3535 Lancaster, PA 17604 Phone: (800) 233-9936 Fax: (717) 295-4538 8.1.1 Specialty Books Centrifugal Compressor Design and Performance Author: David Japikse Description: This publication is both a state-of-the-art review of the technology base of centrifugal compressors and a practical guide to designers. Available from: Concepts ETI, Inc. 4 Billings Farm Road White River Junction, VT 05001 Phone: (802) 296-2321 Fax: (802) 296-2325

8-5


Compressor Performance: Selection, Operation, and Testing of Axial and Centrifugal Compressors Author: M. Theodore Gresh Description: This book is divided into two main sections. In the theory section of the book, the author introduces aerodynamics, thermodynamics, aerodynamic components, and compressor characteristics. In the application section, the author discusses equipment selection, operation, field performance testing, troubleshooting, and flow meters. Available from: Butterworth Heinemann 225 Wildwood Ave. Woburn, MA 01801 Phone: (617) 928-2500 or (800) 366-2665 Fax: (617) 933-6333 Control of Centrifugal Compressors Author: Ralph L. Moore Description: This text provides comprehensive information on the techniques for controlling centrifugal compressors. In addition to compressor control issues, optimization of compressor operation and multiple compressor systems are topics also discussed. Available from: Instrument Society of America 67 Alexander Drive P.O. Box 12277 Research Triangle Park, NC 27709 Phone: (919) 549-8411 Fax: (919) 549-8288 Fluid Movers, Second Edition Authors: Nicholas P. Chopey and Chemical Engineering Magazine Editors Description: This text is a compilation of current articles on the movement of fluids with pumps, compressors, fans, and blowers from Chemical Engineering Magazine. Available from: McGraw-Hill P. O. Box 546 Blacklick, OH 43004-0546 Phone: (800) 722-4726 Fax: (614) 755-5654

8-6


Leak-free Pumps and Compressors, 1st Edition Author: Gerhard Vetter Description: As environmental regulations concerning leaks and emissions become more stringent, this practical reference manual targets those concerned with systems using leak-free pumps or compressors. This handbook explains the various designs and properties of leak-free pumps and helps you select the right pump or compressor to ensure leak-free systems, whatever the application. Available from: Elsevier Advanced Technology Mayfield House 256 Banbury Road Oxford OX2 7DH England Phone: 01865-512242 Fax: 01865-310981 Optimization of Unit Operations: Boilers, Chemical Reactors, Chillers, Clean Rooms, Compressors, Condensers, Heat Exchangers, HVAC Systems, Pumping Stations, Reboilers, and Vaporizers Author: Bela G. Liptak Description: This text examines the technical and practical applications of plant multivariable development control. Optimization of various systems is discussed in detail. Available from: Krawse Publications 700 E. State St. Iola, WI 54990 Phone: (888) 457-2873 Fax: (715) 445-4087 Reciprocating Compressors: Operation and Maintenance Authors: Heinz P. Bloch and John J. Hoefner Description: This book discusses the theory of operation and describes methods of proper installation, troubleshooting, overhauling, and repairing of all types of reciprocating compressors. Engineers and maintenance personnel in the process industries such as mining, food processing, pharmaceuticals, and petrochemicals will find this text useful. Available from: Gulf Publishing Company P.O. Box 2608 Houston, TX 77252-2608 Phone: (713) 520-4444 Fax: (713) 520-4433

8-7


Rotary Screw Air Compressors Author: H. P. Van Ormer Description: This guide provides a close look at the lubricant-cooled rotary compressor and its role in construction and industrial applications. It discusses the history, development, basic technology, application, selection, installation, and general maintenance of rotary screw air compressors. Available from: Air Power USA, Inc. P.O. Box 292 Pickerington, OH 43147 Phone: (614) 862-4112 Fax: (614) 862-4112 8.1.2 Information on the Compressor Marketplace Compressors - Air & Gas Wholesale Description: This annual directory features information on 4,180 wholesalers of air and gas compressors. Available from: American Business Information, Inc. 5711 South 86th Circle P.O. Box 27347 Omaha, NE 68127-0347 Phone: (402) 593-4500 Fax: (402) 331-5481 Compressors, Vacuum Pumps, and Industrial Spraying Equipment Author: Specialists in Business Information, Inc. Description: The U.S. market for air and gas compressors, vacuum pumps, and industrial spraying equipment strengthened in 1995 and 1996. Specialists in Business Information (SBI) has compiled and analyzed data on U.S. factory shipments, imports, exports, industry costs structure, and the competitive environment to uncover strategies that will allow manufacturers and marketers to penetrate growing markets in this $4-billion industry. SBI has also profiled worldwide manufacturers and reviewed their recent developments as part of an exhaustive effort to provide competitor intelligence. In addition, SBI has extracted sales and profit trends for 16 manufacturers in order to compare company performance with industry averages. Some of the major producers profiled include Dresser-Rand, Gardner Denver, Nordson, and Sunstrand. Available from: Specialists in Business Information, Inc. 3375 Park Ave. Wantagh, NY 11793

8-8


Pumps and Compressors Author: U.S. Department of Commerce, Bureau of the Census Description: This annual Current Industrial Report provides statistics on the quantity and value of manufacturer’s shipments, number of producers by product type and industry, exports, and imports. These statistics reflect market trends in the pump and compressor industry. Available from: U.S. Department of Commerce Bureau of the Census Gaithersburg, MD Phone: (301) 457-4100 Fax: (301) 457-4794 The report can be downloaded from the Census Bureau’s web site (http://www.census.gov). The U.S. Pump and Compressor Industry Author: Business Trend Analysts, Inc. Description: This market research report assesses the market for pumps and compressors, including reciprocating, rotary, and centrifugal air compressors, by gathering data and conducting analyses. The report presents data on U.S. manufacturer’s sales and analysis of enduse demand by industry for pumps and compressors. Additional information includes pump and compressor industry statistics, trade, corporate profiles, and a directory of manufacturers. Available from: Business Trend Analysts, Inc. 2171 Jericho Turnpike Commack, NY 11725-2900 Phone: (516) 462-5454 Fax: (516) 462-1842 U.S. Stationary Compressors Market Author: Frost & Sullivan Description: This report provides forecast information for the entire U.S. compressor and vacuum pump market. Market forecasts are based on revenues and growth rates, unit shipments and pricing trends, competitive analyses, and market and technology trends. The market is also analyzed by examining advancements in technology, materials, and manufacturing processes. The market comprises major segments that include positive-displacement compressors and dynamic-type compressors. Available from: Frost & Sullivan 90 West Street, Suite 1301 New York, NY 10006 Phone: (212) 964-7000 Fax: (212) 619-0831

8-9


8.2

Brochures

The following brochures are available from the Compressed Air and Gas Institute at: The Compressed Air and Gas Institute 1300 Sumner Avenue Cleveland, OH 44115-2851 Phone: (216) 241-7333 Fax: (216) 241-0105 [email protected] www.taol.com/cagi “Air Compressor Selection and Application: 1/4 HP through 25 HP” Author: CAGI Description: This publication provides a detailed summary of the types of compressors available, their intended application, and selection criteria for a variety of industries. “Compressed Air and Gas: An Introduction” Author: Lynn Adkins Guda Description: This booklet presents a brief discussion of compressed air as an important means of transmitting power. It introduces compressed air and gas theory, compressors, and uses. “Compressed Air and Gas: In Manufacturing” Author: William D. Ellis Description: This brochure describes the uses of compressed air in producing capital and consumer goods. Examples of compressed air use include lightweight pneumatic tools and controls on the assembly line and in automation. “Compressed Air and Gas: The Process Industries” Author: William D. Ellis Description: This booklet discusses the role of compressors in the process industries, where products such as chemicals, pharmaceuticals, and plastics require large volumes of compressed air and gas for production. “Compressed Air and Gas Drying” Author: CAGI Description: This brochure explains the need for air and gas drying. It includes a step-by-step dryer specifying guide, technical illustrations, and appropriate technical appendices. “Refrigerated Compressed Air Dryers—Methods for Testing and Rating” Author: CAGI Description: This brochure provides a uniform procedure to measure and rate the performance of refrigerated compressed air dryers.

8-10


“Rotary Air Compressor Selection Guide” Author: CAGI Description: This publication covers the complete range of rotary air compressors and discusses selection criteria, capacity control, compressor accessories, and examples of applications. “Safety Aspects of Compressor Lubricants” Author: CAGI Description: This publication discusses the safety aspects of petroleum-based and synthetic lubricants for air and gas compressors.

8.3

Periodicals

Compressed Air Description: Compressed Air is Ingersoll-Rand’s magazine of applied technology and industrial management and contains informative articles on compressed air and gas applications and other related technological innovations. Available from: Compressed Air 253 East Washington Avenue Washington, NJ 07882-2495 Phone: (908) 850-7817 Fax: (908) 689-3095 Impact Compressor/Turbine News & Patents Description: This newsletter (10 issues per year) describes new compressor/turbine patents and new developments in the compressor/turbine field. It includes listings of current articles, seminars, books, and industry news. Available from: Impact Publications Ketchum, ID Phone: (208) 726-2133 Fax: (208) 726-2155 8.3.1 Other Periodicals The following magazines often contain articles about improving the performance of compressed air systems, and can be a very good source of state-of-the-art information: •

AFE Facilities Engineering (Association of Facilities Engineering)

•

Maintenance Technology

•

Plant Engineering

•

Plant Services

8-11


8.4

Software

AIRMaster: Compressed Air System Assessment Software Description: AIRMaster is a software package that enables engineers, auditors, energy managers, and utility staff to assess the performance of their industrial compressed air systems. The latest version of this software, AIRMaster+, is in development and is projected to be available in late 1998. For more information and to request to be put on the AIRMaster+ mailing list, contact the Motor Challenge Information Clearinghouse. Available from: The Motor Challenge Information Clearinghouse P.O. Box 43171 Olympia, WA 98504-3171 Phone: (800) 862-2086 Fax: (206) 586-8303 www.motor.doe.gov C-MAX Engineering Software Description: C-MAX™ software’s compressor module is designed for systems analysis of centrifugal compressors, reciprocating compressors, and rotary screw compressors. Multiple compressors and “what if” case studies can be modeled for pure gases or gaseous mixtures such as dry or wet air, hydrogen, nitrogen, refinery gas mixture, fuel gas, and natural gas. The software allows users to perform “off-line” modeling of compressor performance, energy, and flow capacity calculations, and to create case studies by changing process, mechanical, or load variables. An evaluation copy is available on the Unicade web site. Available from: Unicade Inc. 13219 NE 20th Street, Suite 211 Bellevue, WA 98005-2020 Phone: (425) 747-0353 Fax: (425) 747-0316 Web: www.unicade.com e-mail: [email protected] CHEMCALC 15: Centrifugal Compressor Design and Rating Description: Based on the theories of Elliott and Ingersoll-Rand, CHEMCALC 15 will: 1) design a compressor and analyze a multi-stage compressor with up to four stages of compression; 2) analyze the performance of an existing compressor by calculating new operating conditions based on design operating conditions and curve and actual process conditions; and 3) calculate the thermodynamic properties of a gas mixture, including molecular weight, critical temperature, critical pressure, specific heat ratio, and gas constant. Available from: Gulf Publishing Company Software P.O. Box 2608 Houston, TX 77252-2608 Phone: (800) 231-6275, (713) 520-4448 Fax: (713) 520-4433 8-12


Compressed Air Survey Description: The software is a comprehensive, interactive software tool designed to provide a quantitative assessment of a compressor system’s operating costs and the potential efficiencies resulting from system improvements. The EPRI Compressed Air Handbook (CR-104546) accompanies the software. Available from: EPRIAMP 8000 Ravines Edge Court Columbus, OH 43235-9939 Phone: (614) 846-7322 Fax: (800) 832-9267

8.5

Videos

Three videos are available from CAGI: Compressed Air: Industry’s Fourth Utility Description: This video presents a broad overview of air compression, distribution, and treatment. It describes key considerations in designing and specifying a compressed air system, including compressor selection, distribution considerations, air dryers, and filters. (Running time: 13 minutes) Performance Under Pressure Description: This video discusses the role of compressed air and gas in various applications ranging from residential to industrial. (Running time: 16 minutes) Principles of Air Compression Description: In non-technical terms, this video explains the theory and principles involved in air compression. It illustrates the operation of both positive-displacement and dynamic-type compressors and introduces key terms, such as psig, scfm, relative humidity, and dew point, to the audience. (Running time: 14 minutes) All available from: The Compressed Air and Gas Institute 1300 Sumner Avenue Cleveland, OH 44115-2851 Phone: (216) 241-7333 Fax: (216) 241-0105 [email protected] www.taol.com/cagi

8-13


Safety and Use of Air Compressors Description: This video program shows how to operate an air compressor system safely and efficiently. Topics include moving the air compressor, compressor parts, lubrication, and maintenance. (Running time: 13 minutes) Available from: SafetyCare Inc. 26161 La Paz Road Suite A Mission Viejo, CA 92691 Phone: (714) 452-1555 Fax: (714) 452-1556 http://www.safetycare.com.au

8.6

Workshops and Training Courses

Workshops focusing on energy efficiency and performance improvement in compressed air systems are developed and presented by independent consultants, equipment manufacturers, distributors, and others. Many compressed air system consultants offer workshops and training courses on improving the performance of compressed air systems. In addition, some equipment manufacturers and distributors offer training to their customers. Workshops are sometimes sponsored by electric utilities, universities, and state energy offices. The Compressed Air Challenge™ is developing a training program for plant operating personnel, which was piloted in early 1999 and is to be available on a widespread basis in mid-1999. To get on the Compressed Air Challenge™ mailing list for future information, contact: Energy Center of Wisconsin 595 Science Drive Madison, WI 53711-1060 Phone: (800) 559-4776 Fax: (608) 238-8733

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9

REFERENCES AND GLOSSARY

9.1

References

1. Life Cycle Management Planning Sourcebooks - Volume 1: Instrument Air System. EPRI, Palo Alto, CA: 2001. 1006609. 2. Instrument Air Systems, A Guide for Power Plant Maintenance Personnel. EPRI, Palo Alto, CA: 1990. NP-7079. 3. Compressor and Instrument Air System Maintenance Guide: Revision to NP-7079. EPRI, Palo Alto, CA: 1998. TR-108147. 4. Compressed Air System Guide. Kaeser Compressors, Inc., August 2000. 5. Quality Standard for Instrument Air ANSI/ISA S7.0.01-1996. The Instrumentation, Systems, and Automation Society, November 1996. 6. Report of the Instrument Air Working Group. EPRI, Palo Alto, CA: 1994. TR-103595. 7. Compressed Air Handbook. EPRI, Palo Alto, CA: 1994. CR-104546. 8. Types of Air Compressors. Ingersoll-Rand Air Solutions, Internet web site http://www.air.ingersoll-rand.com. 9. Air Compressor Guide. Kaeser Compressors, Inc., May 1998. 10. Rotary Air Compressor Selection Guide, Second edition. Compressed Air & Gas Institute: 1997. 11. The Compressed Air and Gas Handbook, Fifth edition. Compressed Air & Gas Institute: 1988. 12. David Welch, “Compressing Your Plant’s Air Costs,” Plant Services (April 2001). 13. Compressed Air Treatment Guide. Kaeser Compressors, Inc. 1994. 14. Maximizing Your Compressed Air Utility Through Microprocessor Control Systems. Ingersoll-Rand Air Solutions, Internet web site http://www.air.ingersoll-rand.com.

9-1

EPRI Licensed Material References and Glossary

15. Systematic Control of Multiple Compressor Installations. Ingersoll-Rand Air Solutions, Internet web site http://www.air.ingersoll-rand.com. 16. Troubleshooting of Electric Motors. EPRI, Palo Alto, CA: 2000. 1000968. 17. PM Basis: Project Overview Report Update. EPRI, Palo Alto, CA: 1998. TR-106857-R1. 18. Preventive Maintenance Basis: Volume 14: Reciprocating Air Compressors. EPRI, Palo Alto, CA: 1997. TR-106857-V14. 19. Preventive Maintenance Basis: Volume 15: Rotary Screw Air Compressors. EPRI, Palo Alto, CA: 1997. TR-106857-V15. 20. Preventive Maintenance Basis: Volume 27: Liquid-Ring Rotary Compressor and Pump. EPRI, Palo Alto, CA: 1997. TR-106857-V27. 21. Preventive Maintenance Basis: Volume 8: Low Voltage Electric Motors (600V and below). EPRI, Palo Alto, CA: 1997. TR-106857-V8. 22. Ray Myers, “Monitoring for Corrosive Air Entering Compressed Air Systems,” Maintenance Technology: March 1998. 23. “Piping Rules of Thumb”. Ingersoll-Rand Air Solutions, Internet web site http://www.air.ingersoll-rand.com. 24. Dan Kent, “Overhauling a Used Rotary Screw Compressor,” Plant Services, July 1997. 25. “Considering Rental Compressors to Fulfill Temporary Compressed Air Needs,” IngersollRand Air Solutions, Internet web site http://www.air.ingersoll-rand.com. 26. “Disposal Issues and Options for Air Compressor Condensate,” Ingersoll-Rand Air Solutions, Internet web site http://www.air.ingersoll-rand.com. 27. “Common Sense of Compressor Air System Maintenance,” Ingersoll-Rand Air Solutions, Internet web site http://www.air.ingersoll-rand.com.

9-2


9.2

Glossary

This glossary contains material taken from the following references: [1,2,5,7,8,11]. absorb Absorb is to suck up or take up moisture (that is, a sponge taking up water). absorption Absorption is the penetration of substances into the bulk of a solid or liquid. accumulator An accumulator is a device used in a compressed air system to provide additional storage volume for the air system. ACFM Actual Cubic Feet Per Minute (m3/min). The acfm is the flow rate of air measured at a reference point and based on actual conditions at that reference point. adsorb Adsorbtion is the adhesion of an extremely thin layer of molecules to the surfaces of solid bodies or liquids with which they are in contact. adsorption Adsorption is the surface retention of solid, liquid, or gas molecules, atoms, or ions by a solid or liquid. after-cooler An after-cooler is a heat exchanger for cooling air or gas discharged from compressors. After-coolers are installed after the final stage of compression. These heat exchangers serve to cool the compressed air and remove condensed moisture and oil droplets from it. after-filter An after-filter is a filter installed in an air system that is downstream of the air dryers. air-cooled compressor An air-cooled air compressor is an air compressor that is cooled by atmospheric air circulated around the cylinders or casings. air dryer An air dryer removes water and oil vapor from compressed air, producing air suitable for plant use. Air dryers are typically desiccant dryers, with or without heat regeneration, and refrigerant dryers. air intake The air intake is the region surrounding the suction to the compressor. air intake filter An air intake filter is a dry filter capable of removing particles above 230 microns for lubricated compressors or 140 microns for non-lubricated compressors. The air intake filter is typically located in the intake duct. air-line filter Air-line filters remove solids and liquids from the compressed air stream. These filters can be placed throughout the system: after the compressor, to extract oil from compression; before air tools, to capture potentially damaging particles generated in the distribution system; and any other location where particulate must be removed. 9-3


air piping Instrument air piping typically comprises carbon steel, galvanized steel, copper, brass, or stainless steel piping. While carbon steel piping is susceptible to interior corrosion if the moisture content of the instrument air is too high, copper and brass tubing has lower strength and is subject to physical damage. Most instrument air tubing is constructed with compression fittings and is prone to leakage. ANSI American National Standards Institute brake horsepower (bop) Brake horsepower is power, expressed in horsepower, delivered to the output shaft of a motor or engine, or the power required at the compressor shaft to perform work. capacity The capacity of a compressor is the full rated volume flow of gas compressed and delivered at conditions of total temperature, total pressure, and composition prevailing at the compressor inlet. It sometimes means actual flow rate, rather than rated volume of flow. centrifugal compressor A centrifugal compressor is a dynamic air compressor that has a continuously flowing air stream with velocity or kinetic energy imparted to it by an impeller(s). Centrifugal compressors are highly reliable in base load operation but prone to surge and vibration at low flows. Their operating life will be adversely affected by entrained liquids and solids. chemisorbent media Chemisorbent media is media that chemically reacts with contaminant gases, oxidizes the gases, and prevents their release. coalescing filter A coalescing filter is a filter that collects water and oil mist, combines it into larger droplets, and allows it to drain to the bottom of the cartridge housing where they can be removed via a drain trap or blowdown valve. compression ratio Compression ratio is the ratio of the absolute discharge pressure to the absolute inlet pressure. compressor Compressors take a volume of air and compress it into a smaller volume, thereby increasing the air’s pressure. The most common air compressor types are reciprocating, centrifugal, and rotary screw. Compressors used in power plants to provide instrument air are stationary air compressors of 300 to 1000 cfm (8.5 to 28 m3/min) capacity and discharge pressure of 100–125 psig (689–862 kPa). Most utilities initially installed reciprocating air compressors; many have replaced the reciprocating compressors with rotary screw or centrifugal air compressors. compressor controller A compressor controller directs the compressor’s output. These can be microprocessor-, electro-mechanical-, or pneumatically-based. condensate trap A condensate trap is used to collect and discharge liquids from the compressed air system. The traps are usually integral components of other compressor equipment, such as after-coolers, dryers, and separators.

9-4


deliquescent dryers Deliquescent dryers reduce dew point through chemical reaction of air with desiccant tablets. desiccant Desiccant is material having a large proportion of surface pores, capable of attracting and removing water vapor from the air. desiccant dryers Desiccant dryers reduce dew point by flowing wet air through beads of desiccant. Heat-reactivated desiccant dryers use heat to regenerate the desiccant bed. The heated dryers are more economical than cold-regenerative desiccant dryers at higher air flow ratings. dew point Dew point is the temperature when moisture in the air will begin to condense if the air is cooled at constant pressure. distribution piping The distribution piping moves the compressed air from the compressor station to the end user. Air moves from a main header to branch lines and subheaders, and finally to drop points connected to individual end-use devices. drain traps Drain traps are devices that collect and discharge liquids from after-coolers, separators, receivers, dryers, filters, and drip legs. dryer pre- and after-filters Dryer pre- and after-filters remove particulates, moisture, and hydrocarbons from the compressed air. Pre-filters protect the dryer. After-filters ensure that an acceptable quality of air is maintained in the air delivered from the instrument air system. dynamic compressors Dynamic compressors are machines where air or gas is compressed by the mechanical action of rotating vanes or impellers imparting velocity and pressure to the flowing medium. filters Filters are devices for separating and removing dust, dirt, or other particulate from the air. free air Free air is air at atmospheric conditions at any specific location. This term does not mean air at standard conditions of altitude, barometric pressure, and temperature. humidity, relative Relative humidity is the ratio (expressed as a percentage) of the partial pressure of the water vapor contained in the air at a given temperature and pressure to the maximum partial pressure of water vapor that could be present at the same temperature saturated conditions. humidity, specific Specific humidity is the weight of the water vapor in air vapor mixture per pound of dry air. ICFM Inlet cubic feet per minute (icfm) (m3/min). The icfm is the flow rate of air measured at the compressor inlet filter or inlet valve, at rated conditions.

9-5


impeller An impeller is the part of the rotating element of a dynamic compressor that imparts energy to the flowing medium by means of centrifugal force. It consists of a number of blades mounted that rotate with the shaft. inlet filter An inlet filter removes particles from the air entering the compressor. Particulate matter, if not filtered, can cause compressor damage and increased maintenance costs due to contaminant build-up. inter-coolers Inter-coolers are heat exchangers for removing the heat of compression between stages of compression on a compressor. This reduces the volume of air to be compressed in the next stage. They also condense and remove moisture from the compressed air. inter-cooling Inter-cooling is the removal of heat from the air between stages of a compressor by use of inter-coolers. ISA Instrument Society of America changed to the Instrumentation, Systems, and Automation Society liquid piston compressor A liquid piston compressor is a rotary compressor with a vaned rotor that revolves in an elliptical casing. The rotor spaces are sealed by a ring of liquid rotating with it inside the casing. lubrication system The lubrication system of air compressors at power plants are normally lubricated by external or internal forced lubrication or by a gravity drip and bath system. micrometer A micrometer is a metric measure with a value of 10-6 meters or 0.000001 meter. A micrometer is also referred to as a micron. membrane dryer A membrane dryer reduces dew point by passing compressed air through a bundle of hollow membrane fibers. Water vapor and portions of the compressed air then permeate the membrane walls and vent to atmosphere. moisture separator A moisture separator eliminates water between the after-cooler and the air receiver. An automatic trap or timer-controlled blow-down valve allows the water to discharge. The absence of a moisture separator prior to a receiver (where dryers follow the receiver) might accelerate corrosion in the receiver as a result of the entry and accumulation of water. motor The motor is the prime mover for the compressor. These motors are usually alternating current induction or synchronous electric motors. pre-filter A pre-filter is a filter that precedes the compressed air dryer for the protection of the desiccant or heat transfer surfaces. pre-filter, coalescing A coalescing filter is a pre-filter that removes water and oil aerosols by combining the aerosols into larger droplets for easy removal.

9-6


pressure Pressure is force per unit area exerted by compressed air and expressed in units of lbs/in2 (kPa). pressure, discharge The discharge pressure is the absolute total pressure at the discharge flange of a compressor. pressure, intake The intake pressure is the absolute total pressure at the intake flange of a compressor. pressure regulator A pressure regulator controls the air pressure and flow at individual points of use. These are used to provide appropriate air pressure to end-use devices that do not need or cannot use the supplied plant air pressure levels. PSI Pounds per square inch (kPa). PSI is a unit for pressure. PSIA Pounds per square inch absolute (kPa). PSIA is a unit for absolute pressure. PSIG Pounds per square inch gauge (kPa). PSIG is a unit for gauge pressure. receiver A receiver is a tank used for the storage of air discharged from the air compressors. Receivers serve to reduce the impact of sudden pressure changes occasioned by high demand, dampen pulsations in compressor discharge pressure, and lower the frequency with which compressor loading and unloading is required. They also allow moisture that failed to condense earlier to precipitate the air. rotary compressor A rotary compressor is a positive displacement compressor. A given quantity of air or gas is trapped in a compression chamber and the space that it occupies is mechanically reduced. This causes a corresponding rise in pressure prior to discharge. Rotary screw compressors are characterized by low vibration and minimal maintenance. They can be lubricated or non-lubricated. rotary-lobed compressor A rotary-lobed compressor is a machine containing two mating lobed impellers. The impellers revolve within a cylinder or casing and are prevented from making contact with each other by timing gears mounted outside the cylinder. reciprocating compressors A reciprocating compressor is a positive displacement compressor. The compressing element is a piston having a reciprocating motion inside a cylinder. Reciprocating compressors can be lubricated or non-lubricated. Lubricated compressors are more tolerant of dust but will require coalescing air filters to provide oil-free air and are now generally avoided for use as a source of instrument air. Non-lubricated compressors are susceptible to scoring and damage as a result of the entry of dust particles into the cylinders. SCFM Standard cubic feet per minute (m3/min). The scfm is the flow rate of free air measured at a reference point and converted to a standard set of reference conditions. Typical reference conditions are 14.7 psia (101 kPa) pressure, 60°F (15.5°C) temperature, and 0% relative humidity. 9-7


separator A separator is a device designed to remove liquids, including oil from the air. They can be located throughout a compressor system, wherever there is a need to reduce the air’s moisture content. A trap discharges collected liquid to a drain. single-stage compressor A single-stage compressor is a machine where air is compressed from the intake pressure to the discharge pressure in a single compression stage. standard air Standard air is air at a temperature of 68°F (20°C), a pressure of 14.7 psia (101 kPa), and a relative humidity of 36%. This is in agreement with definitions adopted by the American Society of Mechanical Engineers (ASME). surge Surge is a phenomenon in centrifugal compressors where a reduced flow rate results in a flow reversal and unstable operation. two-stage compressor A two-stage compressor is a machine where air or gas is compressed from an initial pressure to an intermediate pressure in one or more cylinders or casings. water-cooled compressor A water-cooled compressor is a machine that is cooled by water circulated through jackets surrounding the cylinders or casing.

9-8


A

PM PRACTICES

This appendix lists additional preventive maintenance practices from the following sources: •

EPRI Report of the Instrument Air Working Group, TR-103595, 1994 [6]

•

EPRI Instrument Air Systems: A Guide for Power Plant Maintenance Personnel, NP-7079, 1990 [2]

•

Air Compressor Guide, Kaeser Compressors [9]

•

The Compressed Air and Gas Handbook, Fifth Edition [11]

I. PM Practices from the EPRI Report of the Instrument Air Working Group [6] A. Rotary Screw Compressor – Rotary screw compressors consist of two intermeshing rotors contained in a casing. There is a male rotor with convex lobes and a female rotor with concave flutes. The theory of operation is that the male lobes roll into the female rotor flutes reducing the volume of the air and hence increasing its pressure. As the lobes of the rotor rotate, air is continuously discharged and there fore the airflow is continuous and smooth. This type of compressor is not susceptible to the high maintenance requirements of intake and discharge valves for reciprocating compressors. The lobes of the rotors within the casing never come into physical contact with one another. Therefore, wear within the compression chamber is extremely minimal. However, preventive maintenance and monitoring of the compressors performance must be performed to ensure that the rotary screw compressor operates reliably throughout its service life. The following Table A-1 lists general recommendations for rotary screw compressor checks and preventive maintenance.

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EPRI Licensed Material PM Practices Table A-1 Preventive Maintenance Practices for Rotary Screw Compressors [6] Frequency of Maintenance Daily Checks for Trending and Maintenance Note: One weekly set of data obtained from the daily checks should be recorded and trended.

Tasks Check compressor oil level. Check compressor oil pressure. Check and trend air outlet temperature (after-cooler efficiency trend) and cooling water inlet and outlet temperatures. Check discharge pressure (load/unload pressure where practical). Check inter-cooler pressure. Confirm that condensate is discharging from the after-cooler trap. Visually inspect compressor for oil and water leaks.

Monthly Maintenance

Check motor vibration and trend to detect incipient bearing failures.

6 Month Maintenance

Remove air filter elements – inspect and clean/replace the elements as required.

Yearly Maintenance

Change gear case oil or have oil analyzed. Ensure that there is no water present in the oil. Replace gear case oil filter. Remove, inspect, and clean the inter-cooler, after-cooler, and oil cooler. Remove, clean, and verify proper operation of after-cooler and inter-cooler drain traps (if applicable). Visually inspect the condition and cleanliness of the highpressure rotor elements. Visually inspect the condition and cleanliness of the lowpressure rotor elements. Test compressor shutdown switches to verify correct setpoints. Remove and inspect the high-pressure check valve. Verify proper torque on all mounting bolts.

B. Reciprocating Compressor Air is compressed in a reciprocating compressor by a piston within a compression chamber. If the compressor consists of two stages, the first stage discharge air will be passed into an intercooler and into the second stage. The compression chamber consists of a fixed cylindrical volume, intake and discharge valves. The preventive maintenance practices outlined in Table A-2 are typical of those performed on most reciprocating compressors. Intervals vary with plant needs and trending practices. A-2

EPRI Licensed Material PM Practices Table A-2 Preventive Maintenance Practices for Reciprocating Compressors [6] Frequency of Maintenance Daily Checks for Trending and Maintenance

Tasks Check compressor oil level. Check compressor oil pressure. Check control line strainer – remove moisture if needed. If applicable, ensure traps on inter-cooler and after-cooler are functioning. Check inter-cooler pressure.

Monthly Maintenance

Check piston rod packing. Inspect oil scraper rings for leakage. Inspect filters. Lubricate as appropriate.

6 Month Maintenance

Motor inspections. Compressor oil samples.

Yearly Maintenance

Perform piston ring inspections (and other internals). Change crankcase oil and filters. Check foundation bolts.

C. Centrifugal Compressor The centrifugal compressor is a dynamic machine. The mechanical action of the compressor impeller gives velocity and pressure to the air in a radial direction. The compressors used in the power industry are often multi-stage machines having two or more impellers mounted on a single shaft or separately geared stages. Scheduled inspections, trending of parameter data, and preventive maintenance are essential to reliable centrifugal compressor performance. Table A-3 contains a list of suggested maintenance based on plant surveys and manufacturer recommendations. Because each compressor has its own service conditions and environment, a variation of this schedule might be more suitable for a given machine.

A-3

EPRI Licensed Material PM Practices Table A-3 Preventive Maintenance Practices for Centrifugal Compressors [6] Frequency of Maintenance Daily Checks for Trending and Maintenance

Tasks Check and record all interstage pressures. Check and record all interstage temperatures. Check and record air cooler water temperatures (inlet and outlet). Check inlet air filter differential pressure. Check vibration levels on each stage of the compressor. Check oil level. Check that the water traps are working properly or crack open the stage drains to ensure that moisture is being removed from stages. Check for oil and water leaks. Ensure that control air and instrumentation air line filters are free of moisture.

Quarterly Maintenance

Collect oil samples for analysis. Take vibration readings on the compressor housings and compressor drivers.

6 Month Maintenance

Lubricate the main driver couplings if necessary. Visually inspect the oil mist arrestors – clean the housings, lines, and replace the elements.

Yearly Maintenance

Perform control calibrations on panel and associated valves. Inspect and clean the oil reservoir suction screens.

Other than the above, the vendor’s recommendations generally suggest preventive maintenance on an as-needed basis. The degree of maintenance is based, to a major degree, on trending analyses. It is suggested that trending data be acquired from the daily and quarterly checks. The results of this data will assist in scheduling major inspections and corrective maintenance. D. Heatless Regenerative Air Dryer The heatless regenerative air dryer is a two-tower arrangement with one process tower in service and the other as a regeneration stage. Regeneration of the off-line tower requires from 15 to 25% of the dry outlet air. This air flows counter to process airflow in the regeneration tower and is vented to atmosphere to remove the moisture. This type of dryer usually operates on a 10-minute cycle.

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EPRI Licensed Material PM Practices

There are several manufacturers of heatless air dryers. Their maintenance requirements are similar because the dryer concept is the same. The following suggested preventive maintenance practices are shown in Table A-4 and are based on manufacturers’ recommendations and utility experience. These practices will assist the user in maintaining reliable and efficient heatless regenerative air dryers. Table A-4 Preventive Maintenance Practices for Heatless Regenerative Air Dryers [6] Frequency of Maintenance Daily Checks for Trending and Maintenance

Tasks Check dryer outlet dew point. Check dryer differential pressure. Check auto drain functioning correctly. Check airflow rate. Check proper operation of dryer switching valves to ensure dryer is operating properly. Dryer flow control switching valves are extremely important to dryer performance. The malfunction of only one valve can result in decreased dryer efficiency.

Yearly Maintenance

Inspect dryer controls. Inspect and rebuild flow control valves if required. Replace control air filter.

As Required Based on Trend Data

Replace dryer desiccant when dew point begins to degrade to utility-assigned action limit or after 5 years of service life. Calibrate dew point instrument per manufacturers recommended intervals to ensure proper data is being collected.

E. Closed Loop Heat Regenerative Air Dryer The closed loop heat regenerated air dryer is also a two-tower arrangement with one process tower in service and the other in a regenerative stage. Regeneration of the off-line tower is accomplished by a closed loop process whereby air is heated and circulated into the tower by a blower. The heated air removes the moisture from the tower and is then cooled and condensed by a heat exchanger. The condensate created by the cooling process is removed from the loop by a condensate drain trap. After the heating cycle has been completed, the heater is deactivated and the tower is cooled down to process air temperature by the blower. This type of dryer usually operates on a four- to eight-hour cycle. The cycle can be as long as 12 hours if the dryer is equipped with a dew point demand sensor, which allows a process tower to remain in service until dew point reaches a set value. Table A-5 shows a list of maintenance practice suggestions to assist in establishing an effective preventive maintenance program. These recommendations are based on input from the utilities and dryer manufacturers. A-5

EPRI Licensed Material PM Practices Table A-5 Preventive Maintenance Practices for Closed Loop Heat Regenerated Air Dryers [6] Frequency of Maintenance Daily Checks for Trending and Maintenance

Tasks Check dryer outlet dew point. Check dryer differential pressure. Check auto drain trap functioning correctly and draining condensate during the heating cycle. Check air blower oil level.

Monthly Maintenance

Clean and check condensate traps and remove desiccant fines. Monitor major sequence points of dryer operation: valve shifting/limit switch operation, heating, and cooling temperatures. Lubricate limit switches and bearing points if required.

Yearly Maintenance

Inspect blower motor. Inspect and test heater to verify proper efficiency. Confirm that dryer controls and alarms are functioning correctly.


Replace dryer desiccant when the dew point begins to degrade to a utility-designated action limit or after five years of service life. The action should occur before an unacceptable dew point value occurs. Calibrate dew point instrument per manufacturer’s recommended intervals to ensure proper data is being obtained. Inspect and rebuild the tower flow control valves at an interval recommended by the valve manufacturer or if sticking begins to occur.

F. Open Loop Heat Regenerative Air Dryer The heat regenerative air dryer is a dual-chamber system with one process tower in service and the other in a regeneration stage. Regeneration of the off-line tower requires an air purge rate of 1 to 8% of the dry outlet air. The purge air flows counter to the process air flow and is heated internal to the tower bed. The air is then vented to atmosphere to remove the moisture. This dryer requires a smaller purge rate than the heatless regenerative dryer because of the heat applied during the regeneration stage. The maintenance practice suggestions listed in Table A-6 will assist in establishing an effective preventive maintenance program. These recommendations are based on input from the utilities and dryer manufacturers.

A-6

EPRI Licensed Material PM Practices Table A-6 Preventive Maintenance Practices for Open Loop Heat Regenerated Air Dryers [6] Frequency of Maintenance Daily Checks for Trending and Maintenance

Tasks Check dryer outlet dew point. Check dryer differential pressure. Check that the outlet auto-drain is functioning correctly. Check the purge airflow rate. Check operation of dryer switching valves. Dryer flow control switching valves are extremely important to dryer performance. Malfunction of only one valve can result in decreased dryer efficiently.

Annual Maintenance

Inspect and test dryer heaters. Inspect dryer controls. Inspect and rebuild flow control valves if required. Replace control air filter. Check desiccant.


Replace dryer desiccant when the dew point begins to degrade to a utility-designated action limit or after five years of service life. The action should occur before an unacceptable dew point value occurs. Calibrate dew point instrument per manufacturer’s recommended intervals to ensure proper data is being obtained.

G. Refrigerant Air Dryer The refrigerant dryers condense moisture from compressed air by cooling the air in heat exchangers chilled by refrigerants such as Freon gas. The air exits the heat exchanger and enters a moisture separator to remove the condenser liquid. The cold air is then reheated by a heat exchanger in contact with the hot incoming air. This dryer has a limited capability to reduce moisture and is not capable of maintaining air quality that consistently meets ANSI/ISA 7.0.01-1996 dew point criteria. Refrigerant dryers are only capable of providing a dew point of +35 to +55°F (1.7 to 12.8°C). The standard does not recommend dew point values to exceed +39°F (3.9°C) at any time. Therefore, this dryer is not a recommended choice as a primary dryer for plant instrument air systems. Table A-7 is a list of the recommended maintenance practices for the refrigerant dryer.

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EPRI Licensed Material PM Practices Table A-7 Preventive Maintenance Practices for Refrigeration Air Dryers [6] Frequency of Maintenance

Tasks

Daily Checks for Trending and Maintenance

Verify that auto drains are functioning correctly.

Monthly Maintenance

Check outlet dew points.

6 Month Maintenance

Inspect and clean auto drain traps.

Check dryer differential pressures.

Check dryer control. Inspect dryer internal.

II. PM Practices from the EPRI Instrument Air Systems: A Guide for Power Plant Maintenance Personnel [2] A. Air Intake and Filter •

Conduct a visual examination of all internal and external areas of filter housing and silencer surfaces for coating condition and hydrocarbon impingement…..6 months

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Inlet filter differential pressure…monthly

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Change, or change and clean, filter elements. Inspect for material condition on cleanable elements. Return to factory to rebuild on approximately third cleaning…6 months

B. Air Compressor •

Visual inspection of air compressor leaks, loose connections, belt conditions, paint condition (hot spots), and so on……..monthly

•

Oil samples to be tested for water intrusion, particulate, contamination, and useful life ….. 3 months

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Perform equipment vibration and bearing monitoring…..3 months

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Check drive belt and drive belt guard……..weekly

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Check compressor load and unload times/pressures… monthly

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Check compressor oil pressure and level….each shift

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Calibrate compressor gauges, pressure switches, and temperature switches….annually

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Change oil filter…..per manufacturer instructions

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Lubricate frame and fittings….per manufacturer instructions

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Visual examinations of bearings, piston clearances, cylinder bores, rod alignments, etc., or internal parts inspection on non-reciprocating compressors….every 2 years

A-8


C. Inter-Cooler and After-Cooler •

Record cooling water inlet, outlet, and approach temperatures….each shift

•

Inspect and clean the inter-cooler and after-cooler heat exchanger tubes….every 2 years

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Maintain closed coolant quality by testing and make-up….monthly

D. Moisture Separator •

Inspect operation of automatic condensate drain valves and traps……..each shift

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Check separator internals for wear and corrosion….annually

E. Air Receiver •

Conduct a visual inspection of all accessible internal and external receivers. Inspect for physical damage, corrosion, erosion, bolted and welded connection conditions….every 2 years

•

Conduct ASME pressure test in accordance with Code requirements and nondestructive test with ultrasound or radiography for minimum wall thickness….annually

•

Calibrate the receiver pressure gauge and set the safety valve….every 2 years

F. Dryer Pre-Filter and After-Filter •

Check pre-filter automatic drain valves……..daily

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Check after-filter air quality per ANSI/ISA standard……..6 months

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Check pressure drop across pre-filter…….monthly

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Check pressure drop across after-filter…….monthly

G. Dryer •

Check outlet dew point. (This is also a check against installed moisture indicator.)…. 6 months

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Calibrate dew point instruments……..6 months

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Check pressure drop across dryer….monthly

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Check purge air flow rate on desiccant dryer by direct measurement or interpolation….monthly

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Check desiccant levels…..annually

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Check automatic drain on refrigerant dryer….daily

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Replace desiccant …..every 2 years

A-9


H. Blowdown Devices •

Check traps for blow by and operation…..each shift

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Blowdown line filters….3 months

I. Valves/Distribution •

Test all relief valves in accordance with ASME standards….annually

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Operate all valves by hand……..annually

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Blow down all instrument air lines to remove desiccant fines and spalled particulate…….annually

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Conduct an aggressive valve/leak program…….continually with annual check

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Perform pressure decay test on all accumulators and check valves……..annually

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If compressor loading indicates excessive system leakage, perform pressure decay test or a flow measurement test on each header……..annually

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III. PM Practices from the Kaeser Air Compressor Guide [9] Table A-8 Kaeser Maintenance Checklist [9] Component Reciprocating Piston Recommended Rebuild Interval = 5,000 to 10,000 hours

Maintenance Task Check the inlet and discharge valves for sticking due to carbon deposits. Check for excessive air through the crank case breather indicating worn piston rings. Check for low oil pressure indicating worn bearings.

Rotary Screw

Check for mechanical seal leakage.

Recommended Rebuild Interval = 50,000 to 100,000 hours

Check for inlet valve wear.

Motor

Grease motor bearings with the correct type of grease.

Check for excessive bearing “play.”

Replace bearings on a conservative schedule. Check amp draw to prevent motor overload. Ventilate compressor room and limit ambient temperature to increase motor life. Direct Drive

Verify that the direct drive system is correctly aligned. Check frame and mounting block for “settling,” which may cause misalignment and coupling damage.

Gear

Check spray bar for excessive contaminants and plugged orifices. Check for wear and “backlash.” Ensure proper lubrication.

V-Belts

Check v-belt tension. Check wear on v-belt/pulley.

Lubricants

Use proper grade from manufacturer’s instructions. Drain existing lubricant before refilling. Draw routine oil samples to determine maximum lubricant life. Use synthetic lubricant for maximum service life.

Air

Use proper micron rating as specified by manufacturer. Check pressure differential and if necessary carefully “counter flow” compressed air through the filters to clean them. Check for worn/damaged seals. Check structural integrity. Replace after filter has been cleaned 2 or 3 times

Oil

Use correct replacement parts. Change when maximum pressure differential is reached or at each oil change, whichever occurs first.

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EPRI Licensed Material PM Practices Table A-8 (cont.) Kaeser Maintenance Checklist [9] Component Refrigerated Dryers: Condenser Unit

Maintenance Task Periodically clean the fins with compressed air or a bristle brush to ensure enough refrigerant is transformed into a liquid state, for proper heat removal in the evaporator. Do not bend or damage the fins of the heat exchanger during cleaning.

Separator/Drain

Check separator and drain function regularly by monitoring discharge amount. On an average day a 100 cfm (169 m3/hr) dryer discharges about 12 gallons (45 liter) during a 24 hour operation. Even the best drains nee to be serviced or rebuilt at least once a year.

Controls

Check pressure switch setting if condenser fan is not running during full load operation.

Heat Exchangers

Ensure that ambient air does not contain traces of ammonia or acidic substances. Water and ammonia can create an acid that destroys copper heat exchangers.

Desiccant Dryers:

Check pressure drop across filters and change filter cartridge when pressure drop exceeds 8–10 psi (55–69 kPa) differential.

Filtration

Check drains for proper operation and rebuild them at least once a year. Desiccant

Sample desiccant (drain port on bottom of tower) and examine for proper size (dusting reduces size) and discoloration, indicating oil contamination. Replace desiccant every 2 to 3 years.

Valving

Check diaphragms and seals of switching and check valves for wear and replace at least once a year. A higher than normal pressure gauge reading on pressure vessels during “purge” cycle can indicate excessive leaks. Periodically check for dew point if dryer is not equipped with dew point monitor.

Control System

Both provide means to manually step through a complete operating cycle and verify proper valve sequence.

Purge Mufflers

Check purge mufflers for excessive desiccant dust, indicating desiccant breakdown. Replace when necessary. Check purge line for obstruction and excessive pressure drop.

Heater and Temperature Controls

Check operation and temperature settings of thermostat (typically between 350°F–450°F [177°C–232°C]) to avoid heater element burn out. Check safety temperature switch for proper operation.

Blower

Service blower motor per instruction manual. Grease or replace motor bearings as necessary. Clean or replace blower air inlet filter. Check clearance of fan blades or impeller (regenerative blower).

Membrane Dryer

A-12

Check requirements for pre-filtration and drain traps.

EPRI Licensed Material PM Practices Table A-8 (cont.) Kaeser Maintenance Checklist [9] Component

Maintenance Task

Air-Cooled After-Coolers

Clean heat exchanger fins with compressed air or a bristle brush. Be careful not to damage or bend fins.

Water-Cooled After-Coolers

Check for proper water pressure and inlet/outlet temperature. Check heat exchanger on water and airside for fouling. Mineral deposits on waterside negatively influence heat transfer. Clean or flush with special cleaning solvent if necessary.

Moisture Separator with Drain

Check separator filter element for pressure drop. Check drain for proper function. Rebuild mechanism if necessary. Ensure that the filter is installed in the proper flow direction. This is often overlooked and renders the filter useless. Check that airflow is between 20 and 120% of rated flow. Dramatically over- or under-sizing reduces filtration performance. Check particulate and liquid inlet loading versus rated performance. Installing an extra fine filter as a “catch all” right after an aftercooler only creates high pressure drop, plus it cannot remove the contaminant load at this point. Check pressure drop regularly or use an electronic filter monitor to indicate optimum element replacement. Check drain function periodically.

Vapor Adsorbers

Check for any liquid water, which renders the carbon ineffective. Carefully open drain valve and check for smell. Change cartridge when smell is detected or on cartridge style filters after 1,000 operating hours and on “tower” style filters after 10,000 operating hours, whichever occurs first. Install a fine filter to remove carbon dust on carbon “tower” style filters.

Condensate Drains

Check for clogged strainer installed ahead of drain. Note that a high quality drain does not require a strainer. Periodically clean off excessively large contaminants, which did not pass through the drain. Annually rebuild drains. Drains can open up to 250,000 times a year. Ensure drain is installed per instruction manual. Vent lines are often necessary to provide reliable operations. Never feed more than one drain line into drain.

Condensate Management System

Ensure that condensate is properly treated before being discharged into sewer systems. Failure to do could result in severe fines. Check instruction manual for maintenance schedule.

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IV. PM Practices from the Compressed Air and Gas Handbook [11] The following information is a preventive maintenance schedule for centrifugal compressors. It is not intended as a complete guide, but only to suggest how to devise a plan for the particular installation in question. The items selected and the schedule itself must be tailored to the installation and carried out with common sense and good judgment. Table A-9 Preventive Maintenance for Centrifugal Compressors [11] Daily Operating and Maintenance

Instructions 1. Check the lubricating oil and seal oil reservoirs for possible water accumulation by draining a small sample from the reservoir low-point drains. 2. Check the lubricating and seal system oil filters for excessive pressure differential. 3. Verify that the oil levels in seals and lubricating oil reservoirs are within a safe operating range. 4. Check the operation of all process cooler traps, separator traps and seal system oil traps by observing the liquid levels in sight glasses or blowing off trap bypass valves to drain funnels. 5. Review all supervisory and process instruments such as those indicating oil pressures and temperatures, vibration, process pressures and temperatures to verify that no unexplained deviations have occurred. 6. Listen for noise level and pitch changes around compressors, gears and drivers 7. Inspect visually for oil, gas, or water leaks and loose parts. 8. Check the differential pressures across intake air filters, inter-coolers, after-coolers and interstage separators for excessive differential that could signal plugging or other deterioration. 9. Observe the level in the seal oil drain sight flows and the bearing oil drain sight flows for abnormal level changes. Check regularly to establish that the connections have not become clogged.

Weekly Operating and Maintenance

Instructions 1. Verify the calibration and operation of all protective alarm and trip devices through actual testing. Test lockout arrangements should be provided on each device to allow safe testing with the compressor operating.

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EPRI Licensed Material PM Practices Table A-9 (cont.) Preventive Maintenance for Centrifugal Compressors [11] Monthly Maintenance

Instructions 1. Make a vibration survey of each bearing housing, including shaft readings where possible. The data at each location should include unfiltered or total wave and rotational or filtered wave at rotating frequency components. If these two values do not agree, a thorough investigation, including a frequency search, should be made to determine the cause. Any significant increase in vibration should be noted and corrected at the earliest possible moment to prevent permanent bearing or compressor damage. 2. Test the performance of all inter-coolers, after-coolers and oil coolers to evaluate their efficiency. The rate of deterioration will determine the cleaning schedule. 3. Lubricate the linkage, pins and slide bars of all control valves and valve positioners or guide vane positioners. 4. Obtain oil samples from lubricating and seal oil reservoirs for analysis by the lubricant supplier.

A-15

EPRI Licensed Material PM Practices Table A-10 Major Maintenance for Centrifugal Compressors [11] Component 1. Coupling Inspections A. Gear type

Instructions 1. Dismantle and remove all grease, taking note of the condition of the grease. If significant separation has occurred, the lubrication supplier should be consulted for further grease recommendations. The coupling grease should always be checked within the first month of initial operation of a new unit to verify that the lubricant has not separated or otherwise deteriorated. This practice should be followed until a lubricant is selected. The amount of sludge buildup in continuous oil-lubricated couplings may indicate the need for better or additional oil filtration at the coupling spray nozzles. Check the spray nozzle pattern on reassembly. 2. Clean all hubs and sleeves thoroughly and inspect gear teeth for abnormal wear and broken or cracked teeth. The hub and sleeve teeth and hub keyway should be given a thorough magnetic particle or dye check inspection for evidence of cracks. 3. Repack and replace the proper type and amount of grease using new gaskets or O-rings, where applicable. Follow the coupling supplier’s recommended bolt torque values and bolt tightening sequence.

B. Non-Lubricating Type

1. Inspect the flexible hinge member for cracked or damaged disks in the disk pack or diaphragm. 2. Inspect coupling hubs and spacers at all high stress points for cracks using the magnetic particle or dye check method.

2. Verify the Alignment of All Couplings

1. The cold offset alignment should be verified by actual thermal growth measurements at operating conditions. This procedure should be repeated whenever the process conditions are changed significantly. The cold offset may require changes due to foundation settling or shifting. 2. The ambient temperature and the location of the dial indicator, driver or driven shaft must be recorded on all alignment records for future reference. 3. A 12 inch (305 mm) diameter faceplate temporarily bolted to each coupling hub will assist greatly in improving angular alignment accuracy on small diameter, high speed couplings.

A-16

EPRI Licensed Material PM Practices Table A-10 (cont.) Major Maintenance for Centrifugal Compressors [11] Component 3. Clean and Inspect All Journal Bearings

Instructions 1. Remove and inspect each bearing for signs of babbitt damage. 2. Measure and record the bearing-to-shaft clearance following the manufacturer’s recommended procedure. Replace any bearings found to have clearances exceeding the manufacturer’s specifications. 3. Inspect the shoe-to-retainer contact point on all pivoted pad bearings for signs of fretting or excessive wear that could hamper the shoe pivot freedom. 4. Thermocouple or RTD (Resistance temperature detector) detector lead wires embedded in shoes must be installed to allow complete freedom of shoe movement. 5. Inspect bearing seal rings, if used. Garter springs or other retainer springs should be replaced if weakened by worn spots.

4. Clean and Inspect Each Thrust Bearing Assembly

Note: Record an axial setback measurement to locate the rotating element in the casing before disturbing the thrust bearing. Check the manufacturer’s drawing for reference dimensions. Always keep forward and rear thrust assemblies and related shims separated. These assemblies must not be interchanged. 1. Inspect shoes for signs of loose or damaged babbitt. 2. Inspect hardened contact buttons for self-aligning bearing shoe backs and leveling plates for flat spots or fretting. Slight wear spots should be removed by light stoning to restore the button crown or leveling plate curvatures. 3. Check the thrust collar radial and face run-out if a removable collar is used. Carefully follow the manufacturer’s instructions for reassembly of the thrust collar if it is removed. 4. Readjust the thrust bearing clearance to the proper value. Note: Forward and rear thrust shims must be adjusted together to keep the rotating element axial position in its correct relationship with the casing.

5. Inspect all oil baffles for signs of rub, plugged drain-back holes, or chipped touch points. Reinstall with specified radial and axial clearances. 6. Remove the casing cover for an internal inspection if performance level or maintenance schedule dictates this step.

1. Properly clean and inspect all impellers or blading for erosion or corrosion. All highly stressed parts should be given a magnetic particle inspection or dye checked for cracks. Casing diffusers or stator blading should be cleaned and inspected to meet the same requirements as the rotating speed. 2. Interstage seals should be replaced if damaged or eroded to the extent as to allow clearances to exceed specified values. 3. Rotating elements should be checked for balance if increase in vibration has been noted. The cause of any such lack of balance should be investigated, such as loss of a nut or accumulation of sludge.

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EPRI Licensed Material PM Practices Table A-10 (cont.) Major Maintenance for Centrifugal Compressors [11] Component 7. Lubrication and Seal Systems

Instructions 1. Drain and clean each system reservoir. Use squeegees or synthetic sponges for internal cleaning. No rags of any type should be used, since lint may get into the lubricating and seal systems. 2. Centrifuge the oil before returning it to the reservoir or replace it with new oil. Replacement oil should be carefully strained to keep foreign material that may be present in the oil drums out of the system 3. Clean or replace lubricating and seal oil supply filters. 4. Remove tube bundles from oil coolers and thoroughly clean the water and oil sides. Test the tube bundle of the cooler hydrostatically to its rated pressure before reassembling the cooler. 5. Inspect all system controls and regulating valves for foreign material, sticking pistons or valve stems and so on. 6. Inspect the lubricating and seal oil pumps for abnormal wear at pump element bearings, shaft seals and couplings.

8. Process Check Valves

1. Remove and inspect each valve for wear at hinge pins, disk guide pins, disk return springs and seals. All worn parts should be replaced.

9. Process Expansion Joints

1. Inspect the internal bellows surface for pitting and erosion that could lead to failure. If lined, inspect the liner. 2. Adjust pipe support hangers, if required, to position the expansion joint properly in its cold position.

10. Speed Increasers

1. Inspect pinion and gear bearings for damage or wear and proper clearance. Carefully check the bottom of the gear base for metallic particles. 2. Check the gear tooth contact pattern. Check gear teeth for signs of abnormal tooth wear and unequal tooth loading. 3. Gear and pinion teeth should be subjected to thorough inspection by magnetic particle process or dye check. 4. Clean all mesh spray nozzles and all internal oil supply passages. Make sure that the nozzles are properly secured on reassembly. 5. Inspect all splash pans for possible fatigue cracks or loose fasteners.

11. Instrumentation and Protective Devices 12. Main Driver

A-18

1. Check the calibration of all instruments used for monitoring operations or for obtaining maintenance data. 2. Check the calibration and functioning of all alarm and trip devices. 1. Follow the driver manufacturer’s recommended inspection and maintenance procedure.


B

MAINTENANCE PHILOSOPHY AND RECOMMENDATIONS

The following is information taken from the EPRI guide entitled Instrument Air Systems: A Guide for Power Plant Maintenance Personnel, NP-7079, 1990 [2]. Instrument air system reliability is of utmost importance for the continued reliability and operation of power plant facilities. Operation of instrument air systems has a tremendous impact on the operation of a plant. Virtually every piping system in a power plant has pneumatically operated valves or controls associated with it. Degradation or failure of an instrument air system could have an adverse effect on the operation of many other plant systems. This may even affect the plant’s availability factor. To improve instrument air system reliability and consequently the overall plant reliability, first, the system and its components should be capable of providing compressed air at a quality meeting or exceeding the ANSI/ISA 7.0.01-1996 specifications. The instrument air system should then be maintained to operate at optimum efficiency. This requires a plant to change from a reactive or corrective maintenance mode to a proactive or preventive maintenance mode. An aggressive valve testing program will locate air valves that leak. Repair or replacement of these valves should be placed on a prioritized maintenance schedule to be accomplished as required, based on operational decisions. Predictive maintenance and trending techniques should be employed that will assist in establishing preventive maintenance periodicities for such tasks as desiccant change-out, filter replacement, and system blowdowns. The maintenance recommendations for instrument air system components are general suggestions to assist in setting up a proactive or preventive maintenance program which helps to combat problems that are inherent to instrument air systems. These problems may be caused by particulate, hydrocarbon, or water incursion into the system. These problems may sometimes be attributed to lack of adequate maintenance in the past. Recommended maintenance activities and periodicities are listed below. These activities are not designed for any particular installation but are generic in nature. They will differ with types of equipment, manufacturer, and installation design. Various facilities should research their own pieces of equipment and determine their maintenance requirements, methods, and periodicities. Reliability centered maintenance programs have been found effective in determining the frequency of monitoring and maintenance activities.

B-1

EPRI Licensed Material Maintenance Philosophy and Recommendations

A. Air Intake and Filter Conduct a visual examination of all internal and external areas of filter housing and silencer surfaces for coating condition and hydrocarbon impingement. Rusty interior surfaces require immediate correction to prevent compressor damage. External corrosion should be corrected as time permits to protect the equipment. A walk down should be conducted semiannually. Verify inlet filter differential pressure with a U-tube manometer or a differential pressure gage set. An initial reading should be taken on each new filter element installed as baseline operating data. An increase of four to five inches of water over the initial reading indicates the filter needs replacing or cleaning. A sudden drop in differential pressure may indicate a failed or torn element and need for replacement. Cleanable intake filters can be cleaned either by blowing low pressure air in a counter-flow direction or by soaking and agitating in warm water (140°F to 150°F [60°C to 65°C]) containing a non-volatile cleaning solution. Air dry the washed elements for 24 hours before reuse. After two or three cleanings, the filter should be replaced, regardless of condition. The flexible coupling should be checked for tightness and material condition. A cracked surface or signs of boundary penetration indicate a need for replacement. B. Air Compressors Visually inspect the external surfaces of the air compressor for leaks, loose connections, belt conditions, paint condition, hot spots, etc. This inspection may provide indicators of serious operating problems, such as high vibration, oil leaks, or inadequate cooling. Oil samples should be taken and oil analysis should be conducted for water intrusion, particulate contamination and useful life. Other specific indicators desired by plant maintenance personnel should be included on the work order. Conduct vibration and bearing temperature monitoring. This can simply be done using a vibratach and a hand held pyrometer. On belt driven machines, the drive belts should be checked for evidence of deterioration, excessive wear, and improper tightness. A proper fit is indicated by the V belts saddling the sheave groove so that the top surface rides above the highest point of the sheave. Stresses are then evenly distributed. A low riding belt may cause slipping and a high belt loses contact. Belt guards are highly recommended for safety. Ensure that the guards are replaced after any maintenance work. Verify compressor operation by checking compressor load and unload times against the baseline data. Observe load and unload pressure setpoints while checking load and unload frequencies. The compressor oil pressure and reservoir level should be monitored each shift and the results recorded for reference and comparison with the next set of readings. B-2


Calibrate all pressure gages, pressure switches, temperature switches and alarms. All indicators serving a single component or machine should be calibrated as a group to establish baseline data. Oil filters should be changed in accordance with the manufacturer’s recommendations. A low oil pressure cutout switch may be installed. The oil filter should be changed well before a low pressure trip point is reached. Frames and fittings should be lubricated according to the manufacturer’s technical manual. If no technical manuals are available, consult the equipment vendor or lubricate in accordance with the guidelines discussed in an earlier section. Conduct a visual examination of all internal components for wear. Bearing clearances, piston clearances, rod alignments, and cylinder bore sizes, etc. should be recorded for baseline data. A similar inspection should be conducted for helical screw compressors and centrifugal compressors. The periodicity for centrifugal compressors may be extended beyond that of other compressors. Consult the manufacturer for further information regarding this inspection. C. Inter-Cooler and After-Cooler Monitor and record cooling water inlet, outlet, and approach temperatures. This will provide baseline data to identify indications of reduced heat transfer in the heat exchanger due to corrosion buildup or reduced flow. Inspect and clean the inter-cooler and after-cooler heat exchanger tubes. The most common cause of inter-cooler and after-cooler inefficiency is the buildup of scale deposits on the heat exchanger surfaces. These can be removed by using a stiff brush or a strong blast of steam or air. In heavily fouled tube nests, the units should be boiled for half an hour in a cleaning solution using four ounces of trisodium phosphate per gallon of water, followed by a thorough flushing with plain water. Scale inside the tubes can be removed by using a mild citric acid soak, followed by brushing and water flushing. Alternate methods can be suggested by the equipment vendor. Closed cooling water and makeup water chemistry must be closely monitored to ensure all rust inhibitors, biocides, and anti-foaming agents are present in the proper concentrations. D. Moisture Separator Inspect operation of drain traps and valves to prevent condensate accumulation in the moisture separators and subsequent carryover into the air receiver. Open and inspect separator internals for wear and corrosion. Remove any corrosion or debris by wiping with a wiping cloth. The wiping cloth should be moistened with a light mineral oil to help retard corrosion of the metal surfaces. If lubrication is required for separator internals, use a light mineral oil or petroleum jelly.

B-3


E. Air Receiver Conduct an internal and external visual inspection of all receiver tank surfaces, including bolted and welded connections, for physical damage, pitting, erosion, and corrosion. If there are areas of severe pitting or if cracks are detected, a non-destructive test with ultrasound or radiography should be performed. Photographs should be taken so that the extent of damage can be recorded as baseline data. Plant documents should be consulted to determine minimum wall thickness. On first inspection after baseline data and succeeding maintenance cycles, the wall thickness should be measured so minimum wall thickness requirement is maintained. Clean the receiver inside with a hot water soaked wiping cloth. Do not leave surface moisture. Do not wire brush, needle gun, or sand blast as these methods will expose new metal and lead to accelerated corrosion of the tank walls. ASME pressure tests and inspections should be performed in accordance with the Code requirements (ASME Section VIII: Unfired Pressure Vessels). In addition to the system safety valve, a safety valve should be installed between the hydro pump and the vessel to prevent overpressure of the pump and vessel during the hydrostatic test. Set the receiver and system safety valve(s). Gauges should be calibrated prior to the setting of the safety valves. F. Pre-Filter and After-Filter Check pre-filter automatic drain valves for proper operation. These valves often stick open and cause costly air losses. Perform air quality tests to check for compliance with the ANSI/ISA standard requirements. Dew points may be checked using in-line hygrometers or portable measuring equipment. Particulate may be measured by using a laser based particle counter or by blowing air through a paper filter for one minute and sending it to a lab for analysis. Hydrocarbon or oil contamination may be measured by blowing an air sample into a Tedlar bag and shipping it to a lab for analysis or by using an oil stick which uses a dye to indicate hydrocarbon concentration. Each of these methods has advantages and disadvantages that must be addressed for each plant’s application. The particular equipment mentioned in this paragraph is the only equipment available to do these tests. They are mentioned here as examples of the various methods currently available. Check the pressure drop across the pre-filters and after-filters to determine available filter capacity. This can be accomplished by using a differential pressure gage or subtracting the outlet pressure from the inlet pressure. Allowable pressure drop at which the filter elements are to be replaced should be determined for each installation based on its size and vendor recommendations. Old elements should be tagged and marked for cleaning as they are removed or they should be destroyed. New elements should be color coded by size and type if stored in a common area. B-4


G. Dryer Check outlet dew point by using an in-line hygrometer, a portable hand-held hygrometer, or a moisture indicator. A silica gel moisture indicator will not produce an accurate dew point reading, but will start changing color from blue to pink or red at a dew point of approximately 0°F (-18°C). Check the operation of the inlet switching valve on desiccant dryers. This valve, if not properly maintained, can cause the dryer towers to either not switch over or get stuck in between towers. Depending on the manufacturer the dryer may bent to atmosphere or may block flow. Either way, instrument air supply pressure is lost to the downstream piping. Repair or replace the inlet switching valve as necessary to ensure that the dryer operates properly. Some of the older model switching valves require frequent grease lubrication and complete overhaul once a year. Check the operation of automatic drain valves and traps on refrigerant dryers. Repair or replace as necessary to ensure that condensate does not accumulate in the dryer. Pressure drops across the air dryer should be tested periodically to ensure a free air path through the dryer. On desiccant type dryers, a high differential pressure may indicate a breakdown of the desiccant and its subsequent compaction. On a refrigerant dryer, a high differential pressure will indicate blocked or fouled tubing. On desiccant dryers, the purge air flow rate should be checked to ensure proper flow for efficient regeneration. Desiccant levels in the towers should be checked to ensure that proper amounts of desiccant are installed. Low desiccant levels may be an indication of desiccant breakdown and the need for replacement. In worst cases, this may also indicate desiccant carryover. Desiccant should be replaced in accordance with the manufacturer’s recommendations. If the dryer manufacturer permits, the recommendations in Table 2-9 and the following can be used as a guide in desiccant selection: •

On heat regenerative dryers, silica gel beads can be used to conserve power. Silica gel beads regenerate at lower temperatures than activated alumina.

•

On heated or heatless regenerative dryers, if desiccant fines are causing particulate problems, silica gel beads may mitigate this situation. Double sieving of the dryer desiccant compartment will also aid in resolving this problem. Silica gel beads have a high fracture point.

•

On heatless regenerative dryers which are not experiencing problems with fines, use of activated alumina beads will be economical due to their ability to retain greater amount of moisture and lower cost than silica beads.

•

On systems experiencing water intrusion into the dryers, use of ordinary silica gel beads is not recommended. In fact, regenerated silica gel beads are likely to shatter if they come in contact with water droplets. In such situations, specially processed water resistant beads (water blockers) can be used as a layer of buffer desiccant near the point of air inlet.

B-5


However, it is important to eliminate the source of water intrusion into the dryer as soon as possible. Major manufacturers of silica gel beads will provide assistance in selection of desiccant for mixed bed applications. The following desiccants are listed in order of increasing cost: •

Activated alumina

•

Granular silica gel

•

Silica gel beads

•

Silica gel beads (water blockers)

•

Molecular sieve

It is highly recommended that prior to selection of a desiccant, both the dryer vendor and a desiccant manufacturer be contacted to recommend the best possible desiccant available for the particular application. Recent developments in desiccant manufacturing have made new products available that may not have existed when the original dryers were purchased. It should also be considered that in a heated regenerative system, frequency of regeneration influences the aging of the desiccant. Therefore, if frequent regenerations occur either due to system leaks or low capacity dryer trains, desiccants may have to be replaced earlier than their recommended life. H. Blowdown Devices Check the blowdown devices (automatic drain valves and traps) for blow by and proper operation. Repair or replace as necessary to ensure that condensate and particulate do not accumulate in the air lines. Periodically blowdown line filters to ensure they are not clogged with particulate or desiccant fines that may damage the end use component they serve. I. Valves/Distribution All valves should be cycled periodically to ensure proper operation and freedom of movement. Solenoid valves should also be cycled and inspected for indication of physical damage that may affect proper operation (bent or pinched vent lines, etc.). In addition, solenoids should be checked for blow by or seat leakage. Blowdowns should be performed on all air lines to end use components to remove any desiccant fines or spalled particulate from the system. Air lines should be blown down through a clean white blowdown cloth such as a cloth diaper. The liens should be blown until visible particulate is removed. During blowdown ensure that adequate safety precautions are followed. Pressure decay testing should be performed on all accumulators and associated check valves to detect back-leakage into the system. If compressor loading indicated excessive system leakage, perform pressure decay tests or flow measurement tests or soap test the headers to identify the location of leaks. B-6


C

LIFE CYCLE MANAGEMENT PREVENTIVE MAINTENANCE ACTIVITIES

This appendix contains the preventive maintenance activities from the EPRI guide entitled Life Cycle Management Planning Sourcebooks – Volume 1: Instrument Air System, 1006609, 2001 [1]. EPRI Preventive Maintenance Basis documents are available for rotary screw air compressors, liquid ring rotary compressor and reciprocating air compressors, electric motors and instrument and control components. The documents provide a preventive maintenance template and present a strategy for preventive maintenance with which to address degradation mechanisms. The tasks identified in these templates, including the subtasks discussed in the PM task descriptions are listed among others in the following Tables C1–C2. Because these PM tasks have been established under an industry consensus, they should be considered as an optimum set of PM activities when assessing plant specific maintenance activities for Life Cycle Management (LCM) planning of components for which a PM template is available. To distinguish these from the PM recommendations from other sources, they have been typed in bold letters. The EPRI PM templates provide an optimum set of maintenance activities for a select number of important components, including the air compressor types of the instrument air system, the compressor electric motor and the instrument and control components. However, some components that contribute to a successful system operation are not addressed by the templates and some maintenance activities (such as the air quality monitoring for the instrument air system) are performed at a system level rather than a component level. Therefore a thorough survey was conducted to identify both candidate PM activities for components not addressed by the EPRI PM templates and additional recommendations (current practices). Using these, the plant engineer can select the appropriate tasks that may be effective for the particular instrument air system. The EPRI Instrument Air Working Group report [6] made recommendations for appropriate maintenance on instrument air compressors and other components within the instrument air system. These activities comprise a set of maintenance items that might be considered as a means of enhancing the reliability performance of an instrument air system if they have not been implemented already. Other reports [2] have also included recommendations as to appropriate maintenance for an instrument air system. All these activities are presented together with recommendations from the EPRI PM templates in Table C-1. Then, in Table C-2, the maintenance tasks that have been recommended for different compressor types are compared. Reviewing the tasks identified in Tables C-1 and C-2, it is noted that many tasks focus on the air quality issues that are the major source of problems in instrument air systems. It should also be stressed that maintenance activities and intervals for time directed maintenance might well and appropriately differ from the activities and times presented in Table C-1. There is little C-1

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities

consistency in recommended frequencies of maintenance tasks – a result, presumably of variations in equipment type and age and of other maintenance and diagnostic tasks performed in combination.

C-2

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities Table C-1 Preventive Maintenance Activities for Instrument Air System [1] Equipment/ Component Air Intake and Filter

Rotary Screw Compressor

Maintenance Activity

Frequencies Cited

Perform visual examination of internal and external areas of filter housing and silencer. Look for coating conditions, hydrocarbon impingement and cracking of flexile couplings, collection of rust on interior surfaces.

6 Months

Check inlet filter differential pressure

Monthly

Change or clean filter elements

6 Months

Check compressor oil level

Daily, each shift

Check compressor oil pressure

Daily, each shift

Check and trend air outlet temperature and cooling water inlet and outlet temperatures

Daily, each shift

Check discharge pressure (load/unload pressure if possible)

Daily, Monthly

Check inter-cooler pressure

Daily

Confirm condensate discharging from after-cooler trap

Daily

Visually inspect compressor for oil and water leaks

Daily, Monthly

Check motor vibration and trend to detect incipient bearing failures

Monthly, 3 Months

Remove air filter elements—inspect and clean and replace as necessary

6 Months

Change/analyze gear case oil. Ensure no water in oil.

Yearly, 3 Months, 6 Months

Replace gear case oil filter

Yearly, per OEM

Remove and inspect the inter-cooler, after-cooler and oil cooler

Yearly, 2 Years

Remove, clean, and verify proper operation of the after-cooler and inter-cooler drain traps

Yearly

Visually inspect condition and cleanliness of the high-pressure rotor elements.

6 Months, Yearly, 2 Years

Visually inspect condition and cleanliness of the low-pressure rotor elements


Calibrate compressor shutdown switches and pressure gauges

Yearly

C-3

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities Table C-1 (cont.) Preventive Maintenance Activities for Instrument Air System [1] Equipment/ Component Rotary Screw Compressor (cont.)

Reciprocating Compressor

C-4


Frequencies Cited

Remove and inspect high pressure check valve

Yearly

Verify proper torque and all mounting bolts

Yearly

Visual examination of bearings and internal parts

2 Years

Overhaul compressor (replacing wear parts, rebuilding compressor valves, etc.)

6 Months, yearly

Adjust water control valves

Yearly

Check compressor oil level

Daily, each shift

Check compressor oil pressure

Daily, each shift

Check control line strainer—remove moisture if needed

Daily

Ensure traps on inter-cooler and after-cooler function (if applicable)

Daily

Check inter-cooler pressure

Daily

Check and trend air outlet temperature and cooling water inlet and outlet temperatures.

Each shift

Check piston rod packing

Monthly, 2 Years

Visually inspect compressor for oil and water leaks

Monthly

Inspect oil scraper rings for leakage

Monthly, 6 Months, 2 Years

Inspect filters

Monthly

Lubricate as appropriate (that is, total closure valve lubrication)

Monthly, OEM

Motor inspection

6 Months

Compressor oil samples

6 Months, 3 Months

Inspect piston rings and other internals


Change crankcase oil and filters

Yearly

Check foundation bolts

Monthly, Yearly

Remove and inspect the inter-cooler, after-cooler, and oil cooler

Yearly, 2 Years

Overhaul compressor (replacing wear parts, rebuilding compressor valves, and so on)

Yearly

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities Table C-1 (cont.) Preventive Maintenance Activities for Instrument Air System [1] Equipment/ Component Reciprocating Compressor (cont.) Centrifugal Compressor


Frequencies Cited


Yearly


Yearly

Check and record all inter-stage pressures

Daily

Check and record all inter-stage temperatures

Daily

Check and record inlet and outlet air cooler water temperatures

Daily, each shift

Check inlet air filter differential pressure

Daily, Monthly

Check vibration levels on each stage of the compressor

Daily

Check oil level

Daily, each shift

Check that water traps function or crack open the stage drains to ensure moisture is being removed from stages

Daily

Check compressor load and unload times and pressures

Monthly

Check for oil and water leaks

Daily, Monthly

Ensure that control and instrument air line filters are free of moisture

Daily

Collect oil samples for analysis

Monthly, 3 Months

Take vibration readings on the compressor housings and compressor drivers

Monthly, 3 Months

Lubricate the main drive couplings if necessary

6 Months, OEM

Visually inspect oil mist arrestors and clean the housings and lines and replace the elements

6 Months

Perform control calibrations on panel and associated valves

Yearly

Inspect and clean the oil reservoir suction screens

Yearly

Remove and inspect the inter-cooler, after-cooler, and oil cooler

2 Years

Overhaul compressor (replacing wear parts, rebuilding compressor valves, and so on)

Yearly


Yearly


Yearly

C-5

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities Table C-1 (cont.) Preventive Maintenance Activities for Instrument Air System [1] Equipment/ Component Compressor Motors

Moisture Separators

Heatless Regenerative Air Dryer

Closed Loop Heat Regenerated Air Dryers

C-6


Frequencies Cited

Thermography

Yearly

Vibration Monitoring (>200 Hp Motors)

Yearly

Brush Maintenance

As required


Yearly

Refurbishment

As required

Inspect operation automatic condensate drain valves and traps. Disassemble and clean if clogged.

Each shift

Check separator internals for wear and corrosion and clean as necessary

Yearly

Check dryer outlet dew point

Daily, 6 Months

Check dryer differential pressure

Daily, Monthly

Check auto drain functioning correctly

Daily, Each shift

Check air flow rate

Daily, Weekly, Monthly

Check proper operation of dryer switching valves to ensure dryer is operating properly

Daily

Inspect dryer controls

Yearly

Inspect and rebuild control valves if required

Yearly

Replace control air filter

Yearly, 6 Months

Check desiccant levels

Monthly

Replace desiccant when dew point degrades or after 5 years

Based on trend data, 2 Years

Calibrate dew point instrument

OEM, 6 Months


Daily, 6 Months


Daily, Monthly

Check auto drain trap functioning correctly

Daily

Check air blower oil level

Daily

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities Table C-1 (cont.) Preventive Maintenance Activities for Instrument Air System [1] Equipment/ Component Closed Loop Heat Regenerated Air Dryers (cont.)

Open Loop Heat Regenerated Air Dryers


Frequencies Cited

Check and clean condensate traps and remove desiccant fines

Monthly

Monitor major sequence points of dryer operation (valve shifting/limit switch operation) and heating and cooling temperatures

Monthly

Lubricate limit switches and bearing points if required

Monthly

Inspect blower motor

Yearly

Inspect and test heater to verify proper efficiency

Yearly

Confirm that dryer control and alarms are functioning correctly

Yearly


Based on trend data, 2 Years


Monthly


OEM, 6 Months

Inspect and rebuild tower flow control valves

OEM, if sticking occurs


Daily, 6 Months


Daily, Monthly

Check outlet auto drain functioning correctly

Daily

Check purge air flow rate

Daily

Check operation of dryer switching valves

Daily

Inspect and test dryer heaters

Yearly

Inspect dryer controls

Yearly

Inspect and rebuild tower flow control valves if required

Yearly

Replace control oil filter

Yearly


Yearly


Based on trend data


OEM

C-7

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities Table C-1 (cont.) Preventive Maintenance Activities for Instrument Air System [1] Equipment/ Component Refrigerated Air Dryers

Air Receivers

Piping

C-8


Frequencies Cited

Check outlet auto drain functioning correctly

Daily


Daily


Monthly

Inspect and clean auto-drain traps

6 Months

Check dryer control

6 Months

Inspect dryer internals

6 Months

Check operability of drain traps by monitoring water level

Daily

Perform manual blow downs

6 Months

Inspect receiver internals to prevent catastrophic failure and the blockage of lines as a result of crud accumulation, loose coatings or rust

2 Years

Conduct non-destructive test with ultrasound/radiography for minimum wall thickness or ASME pressure test (if interior rusting/corrosion is a problem)

Yearly

Verify and test relief valve set points and closure tightness

2 Years

Leak tests (ultrasonic/soap solution/tracer gases). These tests should include within their scope all air piping subject to vibration from operating equipment and the instrument air headers

6 Months

Visual inspection (check for vibration, leakage, loose joints, compression fittings, excessive rusting/discoloration, hangers, and supports)

Monthly

EPRI Licensed Material Life Cycle Management Preventive Maintenance Activities Table C-2 Maintenance Activities for Different Compressor Types [1] Task Check inlet air differential pressure Check compressor oil level Check compressor oil pressure Check compressor load and unload times Check control line strainer—remove moisture if needed Ensure traps on inter-cooler and after-cooler function (if applicable) Check inter-cooler (interstage) pressure Check interstage temperature Check and trend air outlet temperature Check and trend cooling water inlet and outlet temperatures Check piston rod packing Visually inspect compressor for oil and water leaks Inspect oil scraper rings for leakage Inspect filters Lubricate as appropriate (that is, total closure valve lubrication) Motor inspection Compressor oil samples Inspect piston rings and other internals Visually inspect condition and cleanliness of high/low-pressure rotor elements Change crank case/gear case oil and filters Check foundation bolts Check vibration levels Remove and inspect the inter-cooler, after-cooler, and oil cooler Overhaul compressor (replacing wear parts, rebuilding compressor valves, and so on) Calibrate compressor shutdown switches and pressure gauges Adjust water control valves

Compressor Type Reciprocating Centrifugal No Yes Yes Yes Yes No No Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes No Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes No No No Yes Yes Yes Yes No Yes Yes No Yes Yes

Rotary Screw No Yes Yes No Yes Yes Yes No Yes Yes No Yes No Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes

Yes Yes

Yes Yes

Yes Yes

C-9


D

OVERHAULING A ROTARY SCREW COMPRESSOR

The information in this appendix was taken from the article, Overhauling a Used Rotary Screw Compressor, Plant Services, July 1997 [24]. Opportunities may arise to meet compressed air needs using existing or used rotary screw compressors. While purchasing a new compressor has its advantages, do not dismiss the existing equipment as “not worth the time to overhaul”. Since most rotary screw compressors use oil to seal the air end (the component containing the compression chambers), very little wear occurs in the compressor itself. This lack of internal wear means that frequently, the compressor can be returned to like-new condition for a fraction of the cost of a new compressor. The following information is offered to help a mechanic or engineer determine if an existing rotary screw compressor is worth the time to overhaul. •

How to test - Safety is paramount. Disconnect and lock out all electrical supplies before attempting any analysis or repairs. Vent any compressed air trapped in the equipment.

•

Do a literature search - Get the Service and Parts manual for the compressor. If you are unable to locate this manual, contact your local manufacturer’s representative for a copy. Usually, compressor manufacturers provide the manual for a reasonable fee.

•

Check the feasibility of the project - The preliminary inspection steps below provide the information to determine if a used rotary screw compressor is worth the cost and time to overhaul. Performing the inspection should take a knowledgeable mechanic approximately four hours for a V-Belt driven compressor. Gear driven or direct coupled compressors take somewhat longer.

1. Air End The best way to determine the condition of the air end is to run the compressor. Before doing so, find out if running the air end is safe or if it will cause damage. Disconnect the motor from the air end. If the air end is connected to the motor, simply remove the V-belts. If the air end is directly coupled to the motor or is gear-driven, the task is more difficult. Once the air end is disconnected, slowly rotate the air end’s input shaft by hand. If the air end rotates, continue rotating it for several full revolutions. The resistance to turning should be even—no tight spots. Try to move the air end shaft up and down and from side to side. You should detect only the slightest movement. If you find any tight spots in the rotation or considerable lateral movement, have your local manufacturer’s representative inspect and evaluate the air end for possible bearing damage.

D-1

EPRI Licensed Material Overhauling a Rotary Screw Compressor

If the air end rotates freely and lateral shaft movement is minimal, the air end should be safe to run. If the air end does not rotate, then overhauling the compressor will likely require replacing the air end. Factory rebuilt air ends offer an affordable alternative to a new air end by providing a quality replacement at a reduced price. Consult your local manufacturer’s representative for more information. Also, returning the old air end under an exchange program may further reduce the cost of the replacement. 2. Electric Motor Disconnect the air end and motor as stated in the previous step; remove any electrical wires and jumpers, and turn the motor by hand. If the motor rotates, continue rotating it several full revolutions. Listen and feel for any clunking or tight spots that indicate bearing problems. Typically, replacing the motor bearings is a relatively inexpensive repair. Check the condition of the windings using a mega-ohm meter or high-pot tester. If you are not familiar with these tests, ask your local motor repair facility for assistance. There are three electrical tests to perform on the motor: •

Measure the resistance of each winding. Open windings have considerably more resistance than the others. A short within the winding is identified by a lower reading when compared to a good winding.

•

Resistance between windings should be extremely high—infinity on some meters—because windings should not be interconnected. A lower resistance in one or two windings may indicate a short between the two windings.

•

Resistance to ground should be extremely high since the windings should not be connected to a ground.

A lower resistance in a winding may indicate a short to ground. If the motor does not rotate, the motor bearings are likely damaged. More extensive damage is likely. Have a qualified electrical motor repair facility determine if the motor is salvageable. 3. Control Panel Inspect the control circuits. Be sure to inspect starters, relay timers, terminals, wires, and other components for signs of high heat. Look for melted or heat-discolored plastic and burnt contacts. If the components look fine, use a mega-ohm meter or high pot tester to verify whether the circuit is safe to operate. If you notice damage, replace the components. 4. Inspect the Oil Much of the wear materials and dirt the compressor has been subjected to collects on the bottom of the separator tank. Drain the tank from the very bottom and analyze the contents. Local labs can identify what types of contaminants are present in the oil at a reasonable price (often below D-2


$25 at 1997 prices). You should expect to find considerable dirt. If wear metals like copper, iron, and the like are also present in some quantity, there may be other unidentified damage inside the unit. 5. Decision Time—Overhaul or Not Performing these test procedures should make it possible to estimate the cost of overhauling the compressor. Keep in mind that a compressor that requires the replacement of only one major component such as an air end or motor may still be worth overhauling. However, a unit that requires some combination of a new air end, motor, starters, and has a considerably contaminated oil circuit may require more work than it is worth. A point often overlooked is the aspect of post-rebuild efficiency. While the compressor may be sound enough mechanically to warrant a rebuild, it is prudent to investigate how the compressor holds up to a power cost analysis (consult your local manufacturer’s representative for assistance). Since over 70% of the total cost of compressed air is in energy input, you need a rebuilt compressor that is not only reliable but also efficient by current standards. Compare fullload power consumption versus compressed air output at rated pressure. Also, compare them at various levels of partial load. 6. How to Rebuild If you decide to overhaul the compressor, a knowledgeable mechanic will need 16 to 24 hours to perform the following tasks: •

Remove the air end, valves, separator tank, piping, hoses, and other mechanical components. Except for the air end, carefully disassemble and clean each component. Note: In order to maintain proper clearances, disassembling the air end is not advisable. It should be possible to clean most of the air end without disassembly.

•

Inspect each part. Replace O-rings, gaskets, V-belts, hoses, and other worn parts. Lightly lubricate moving parts as specified by the equipment manufacturer. Pay particular attention to solenoid-operated valves and valves in the compressor’s oil circuit. Typically, valves can be bench-tested for proper operation.

•

Reassemble the mechanical components. Remember to replace the filters.

•

Change the lubricant. Fill the compressor with the proper compressor lubricant as specified in the Operation & Maintenance manual.

•

Test the control circuit. Occasionally it is possible to dry run the control circuit. Simply put, this step allows you to operate the compressor’s control circuit without operating the motor and other higher voltage circuits. Dry running the controls identifies malfunctioning electric components that may have been overlooked in the previous inspection. Most of the time dry running the compressor’s control circuit can be done easily. However, if you are not familiar with how the circuit operates, it is safer to contact your local manufacturer for assistance.

D-3


7. Initial Start-Up Starting the compressor for the first time after an extensive overhaul should be done carefully. To ensure the compressor is not started without lubricant, pour some compressor lubricant directly into the air end intake and turn the air end by hand. Also, make sure the air end and motor are aligned properly. It is important to verify that the motor rotates in the correct direction before running the compressor. This can usually be done by “bumping” or “jogging” the compressor. This means starting and immediately stopping the motor to see in which direction it rotates. The safest way is to start the motor without the V-belts connected. Also, remember to check any fan motor(s). If a compressor has a separate fan motor, it is possible that one motor rotates in the correct direction and the other rotates backwards. Start the compressor by following the manufacturer’s recommended startup procedure usually found in the Service and Parts Manual. Once the compressor is up and running, keep any eye on operating pressure and temperature. If the compressor exceeds its rated pressure, be prepared to shut down the compressor. Usually a simple adjustment to the air pressure switch solves the problem. If the compressor starts to run hot, note the rate of temperature rise. If the air end temperature rises very quickly (within seconds) above 200°F (93°C), immediately shut down the compressor. This rapid rise indicates a problem with the oil circuit. It may be simple to fix, but continuing to run or re-start the compressor at this point could ruin your hard work. Instead, call your local manufacturer representative for assistance. Typically, you will find that your hard work has paid off because the compressor runs smoothly and reliably. One last task makes your overhaul complete. Flush the oil circuit. Now its time to dispose of the new compressor oil and some of the new filters you’ve installed. Don’t let this stop you from performing this last step. While you’ve done a great job cleaning the parts, there is still dirt and varnish in oil journals, piping, fittings, and other nooks and crannies. Use an appropriate compressor flush to clean the oil circuit completely before these contaminants cause unnecessary component failure. There are many flushing products on the market. One example is ThermasolvTM, a Penzoil product. When using these products, always follow the manufacturer’s instructions. It may be good to repeat the flush two, three, or even four times, changing the oil between flushes until there are no signs of varnish and dirt remaining. Once flushing is complete, replace the oil filter and oil separator element. This eliminates re-contaminating your oil circuit. 8. Summary If you have an efficient, quality compressor that is old or broken down, it is worthwhile to consider rebuilding it into a reliable and efficient part of your compressed air system. Since rotary screw compressors have few major components, the overhaul procedure is fairly straightforward, requiring only a reasonable amount of time. As a rule, do not spend more than 60% of replacement cost for the first overhaul and not more than 30% of replacement cost for the consecutive overhaul. D-4


E

RENTAL COMPRESSORS

The information in this appendix was taken from the article, “Considering Rental Compressors to Fulfill Temporary Compressed Air Needs,” Ingersoll-Rand Air Solutions, www.air.ingersoll-rand.com [25]. 1. Drive Arrangements: Electric or Diesel The first steps in evaluating the need for a rental air compressor are to determine how much compressed air you actually need and decide on the type of prime mover (engine/electric motor) you would like the rental unit to use. You can estimate the cubic feet per minute (cfm) (m3/hour) of needed air by matching the lost capacity of an out-of-service, permanently installed compressor(s) being repaired or new demand requirement. A calculation may be needed to estimate the new demand or a “trial-and-error” approach may be used in bringing rental equipment on site. The required discharge pressure must be met for the rental equipment. Next, you must determine which kind of air compressor drive arrangement will best suit your needs. There are two basic types of drive arrangements available in compressor rental equipment – diesel engines and electric motors – and both have their advantages and disadvantages. Diesel engine drive units are typically available in the range of 185 to 1600 cfm (314 to 2718 m3/hr) as lubricated compressors and 900 to 1500 cfm (1529 to 2548 m3/hr) as oil-free designs. The primary benefits of diesel drive rental units are how quickly they are deployed and the fact that no 3-phase power is required to operate them. A limited amount of 110/1/60 power is usually required to power heaters in cold weather situations. Diesel rental units do have some negative aspects, however. First, they require relatively frequent engine maintenance (typically every 200 hours of operation). Second, emissions from the engine exhaust may pose safety or environmental concerns. Generally speaking, the older the diesel rental unit, the higher the emission levels, as more stringent regulations have been placed on engine manufacturers in recent years. Other hassles presented by diesel engines include more routine maintenance, fueling logistics and potential fuel spills, and higher noise levels in operation. Conversely, electric motor drive units eliminate several of the diesel engine problems discussed above, such as emissions and fuel logistics. Depending on compressor size, the motor voltage can vary between 230/3/60 and 4160/3/60 with the most popular being 460/3/60. Rental

E-1

EPRI Licensed Material Rental Compressors

equipment is available covering the cfm range of 20 to 21,000 cfm (34 to 35,679 m3/hr), in a single unit, with multiple units being rented for large flow requirements. Also, the industry is generally more familiar with electric motors than it is with diesel engines. In many circumstances, particularly when the rental equipment is run around the clock, electric units save you money on both energy cost and rental charges. Electric units are inherently more efficient at energy conversion; most diesel rental prices rise as usage increases, which is generally not true with electric compressor rentals. And in comparison to diesel engines, electric motors require much lower routine maintenance. The primary disadvantage of using electric rental units is the time it takes to set them up on site and get them producing compressed air. Wiring for the 3-phase power required for these units, between the customer’s disconnect and the rental equipment, is usually the most time consuming part of hooking up an electric rental compressor. Therefore, when done under emergency conditions, electric units are frequently just too time consuming to be a realistic option. If electric drive is preferred and quick deployment is required, you must prepare in advance for setting up the rental equipment. If this preparatory work is done during normal plant conditions, it will be in place and ready for immediate hookup when needed. 2. Compressed Air Quality and Oil Carryover The compressed air quality that is needed is a critical element when it comes to compressor rentals. Different machines vary widely in terms of the purity of the delivered air. Most rental equipment falls into one of three “air quality” categories: •

Inherently oil-free compressors that do not add any oil to the air being compressed

•

Lubricated compressors with filtration or low carryover rental compressors, that remove most of the oil added to the air during the compression process (some low carryover rental systems are fully integrated, while others are rented in pieces for assembly on site).

•

Standard electric lubricated rotary screw compressors (usually including after-coolers) and standard construction-grade portable compressors.

The benefit of renting inherently oil-free compressors is obvious – no oil in the air. But because they are truly oil-free, they rent for higher rates than lubricated compressors. For applications that require oil-free compressed air, it is important to rent only inherently oil-free compressors, just as it is required to purchase them for your permanent installations. Lubricated compressors with downstream filtration (either diesel or electric motor driven) frequently offer a lower cost rental solution while still supplying very high quality compressed air. An industrial aftercooler can condense some oil vapor right along with the water vapor. Thus, if there are any hydrocarbon vapors in the ambient air being compressed, some of them can be condensed and removed from the air stream by high quality coalescing filter(s). Note that no coalescing filter can remove oil vapors. They pass through the filter just as water vapor does.

E-2


These low carryover systems are excellent for non-inherently oil-free applications, or for instrument air applications when used with appropriate compressed air drying equipment. However, because they are not inherently oil-free, there is a possibility of oil carryover, in the event of a primary separator element failure. Thus, an element of risk is accepted should this type of equipment be used on a true oil-free application. Standard electric rotary screw rental compressors typically have after-coolers on board, and usually operate with oil carryover rates of 2-5 ppm downstream of the aftercooler. For many industrial compressed air systems, commonly referred to as plant air, this level of oil carryover is acceptable. Finally, standard construction grade diesel drive portable air compressors usually do not include an aftercooler, and they operate with oil carryover rates of 5 to 20 ppm, or higher. This may be acceptable in some plant air systems but this level of carryover can frequently cause contamination problems, sometimes with expensive consequences. A recent development in this area is the introduction of “instrument quality” diesel rental compressors. These machines are intended specifically for industrial, low oil carryover rental situations, and they include onboard after-coolers and coalescing filtration systems. They produce very high quality compressed air, but still offer the advantages of diesel rental units, and at a lower rental cost. One manufacturer even provides a means of disposing of all condensate by injecting it directly into the engine exhaust system, where it instantly flashes into steam. Dirt and particulate in compressed air systems are another important element in this overall discussion. Generally speaking, the big quality filtration needed for oil carryover reduction provides more than adequate particulate filtration. In general, the higher the quality of compressed air delivered, the higher the rental cost. 3. Compressed Air Moisture Content Another major element is ensuring a successful compressed air rental program lies in addressing the moisture content of the compressed air. Most manufacturing plants today already use compressed air after-coolers and air dryers to remove liquid water and water vapor from their compressed air systems. This same care and concern must be extended tot he rental air system, to ensure consistent high quality compressed air being delivered to critical manufacturing systems. With most permanently installed compressors in use today, after-coolers are an integral part of the design. Older models usually have external after-coolers installed. Both air-cooled and water cooled after-coolers are common. The two most common air dryer designs are the refrigerated air dryer and the twin tower desiccant air dryer. As the name suggests the refrigerated dryers use a refrigeration system to physically reduce the temperature of the air, condensing water vapor, which is then separated and removed from the compressed air system. There are several control variations used, such as E-3


direct expansion, thermal mass, etc. Refrigerated dryers usually achieve a pressure dewpoint of 35 to 39ºF (1.7 to 4ºC). Twin tower desiccant dryers use an entirely different principle of operation. Desiccant materials, usually activated alumina or silica gel, adsorb water vapor directly into the compressed air. Once the desiccant bed has adsorbed sufficient water vapor, the two towers switch operation. While the second tower is actively drying the compressed air stream, the first tower is being “regenerated.” Regenerated means that the vapor is dried out of the desiccant material. The towers then switch and the process is repeated over and over. Desiccant dryers can reduce moisture to a much lower level than refrigerated dryers, typically producing a pressure dewpoint of -40 to -100ºF (-40 to -73ºC). How does one handle compressed air moisture in a rental situation? In one of two ways – the rental compressors can be connected to the plant air system upstream of the permanently installed cooling/drying equipment or after-coolers and either refrigerated or desiccant air dryers can be rented along with the compressors, to supply dry air directly to the system. Rental suppliers that specialize in industrial compressed air rental equipment will usually have a selection of both refrigerated and desiccant rental air dryers. 4. Monitoring and Operating the Rental System The rental equipment must be operated and monitored. In particular, if compressed air demand is fluctuating or if the rental system is on site as a “back up,” the rental equipment controls need to be able to react quickly to changes in demand and system pressure. As a general statement, the more advanced and automatic the control system is, the better. The latest generation rental equipment, both diesel or electric drive, offer automatic control features like automatic starting and stopping. These systems produce air on demand, without the need for attendant personnel. They also shut off automatically when not needed, saving you energy. Most rental equipment is well instrumented and some of the newer units are even microprocessor controlled. Some microprocessor controlled units can even be connected to your central energy management system, or a strategically located personal computer, to allow for remote operation and status monitoring. 5. Preparation for Rental Equipment The key elements in preparing for a successful rental experience are: •

Space for the equipment – If possible, a dedicated space is preferable for your rental equipment. Otherwise, the space must be able to be cleared out very quickly when needed. This space should be chosen with required utility access in mind, such as the compressed air connection that connects to your system at the correct point, i.e., upstream of permanently installed compressed air treatment equipment, if possible, electrical power, adequate ventilation, and so on.

•

Ventilation – There should be adequate ventilation for the rental equipment to cool down. Diesel driven machinery needs very good ventilation (preferably outdoors, in an open area)

E-4


to avoid problems with exhaust emissions and carbon monoxide. The ambient air must not contain more than trace amounts of sulfur, chlorine, or other potentially corrosive elements, or excessive dirt. •

Compressed Air Connections – Ensure that the air connection is properly sized from the airflow being put through it. Try to avoid long hose runs between the compressor and the connection, as the pressure drop through the hose is greater than it is through pipe, and the extra rental hose is an added expense.

•

Fuel Supply (Diesel Equipment) – A workable setup for refueling the diesel compressors must be established. Bear in mind that a 1500 cfm rental air compressor consumes approximately 20 gallons (76 liters) of fuel per hour. Pre-arrange for fuel deliveries with your fuel supplier. If the machinery is run 24 hours per day, most machines will need to be refueled every 8 hours. It may be preferable to use a large diesel fuel tank, or even arrange to park a tank truck filled with fuel on site for easier handling. For longer term rentals, most compressors can be set up to draw fuel directly from an external tank, bypassing the on-board tanks. Since fuel spills are possible when using this type of equipment, great care must be taken during refueling to avoid them. It also is important to remember that all relevant environmental regulations must be followed:

•

Electrical connections (for electric motor driven equipment) – It is the utility’s responsibility to properly size electrical disconnects or circuit breakers for the equipment being rented. They must be fused correctly to protect the motors on the rental equipment. External power for heaters used in cold weather (usually 110/1/60) must be available as well. If the rental equipment is not suitable for outdoor, cold weather operation, it should be installed indoors.

•

Cooling water – The very large rental compressors (2500 cfm [4247 m3/hr] and above) are frequently water-cooled. When renting this type of equipment, you will need to provide for the supply and return of this cooling water requirement, along with an adequate cooling tower. As an alternative plan, large self-contained cooling towers can be rented.

E-5


F

CONDENSATE DISPOSAL The information in this section was taken from the article, “Disposal Issues and Options for Air Compressor Condensate,” Ingersoll-Rand Air Solution, www.air.ingersoll-rand.com [26]. The proper disposal of wastewater generated at plant facilities is subject to environmental regulations by the local, state, and federal authorities. The regulations governing contaminant disposal vary substantially. Each plant has to determine the specific requirements that apply to their location. 1. Methods of Condensate Separation The most commonly used methods and use of biodegradable compressor fluids is discussed in this section. There are four commercially accepted methods for condensate separation. These processes include: •

Gravity – This method uses the differences in specific gravity between water and the various coolants and lubricants to purify the condensate. For example, in an oil and water mixture, the oil floating to the top will be collected and disposed. The primary drawback with this approach is the tendency of various liquids to form solutions that remain in the condensate. No matter how long an oil and water emulsion stands, some of the oil, an appreciable amount by federal standards, will remain in the water, regardless of what type of oil is used.

•

Filtration – This method uses various types of filter media to collect and separate contaminants from condensate prior to disposal. Currently, the only filter type that complies with limits established by federal regulations is activated carbon, which can remove contaminants down to 10 parts per million. The primary drawback with filtration systems is the required constant monitoring of the filter element. Without careful monitoring, the filter element reaches the end of its operating life and contaminant carryover may climb above acceptable limits. Often, filtration systems featuring carbon elements are used in conjunction with another separation process, such as gravity or membranes. When used in tandem, filtration elements typically last longer than when only a single element screens the condensate.

•

Semi-Permeable Membranes – Similar to filtration systems, this method can use different membranes to screen contaminant out of condensate prior to disposal. Today, membranes can be produced to remove a variety of most known contaminants. The primary drawback with this approach is the rate at which membranes will foul with contaminants. Some membrane designs require back flushing as frequently as every eight hours of operation. Over time, this maintenance procedure can cause premature membrane failure due to the stresses generated by frequent flow reversals and high differential pressures. F-1

EPRI Licensed Material Condensate Disposal

In addition, membranes are designed to screen specific lubricants or coolants. For example, a membrane designed for one base stock coolant may not work well with a different base stock coolant. Depending on the individual situation, several different membranes may be required if coolants and lubricants vary in each compressor. In addition, membrane separation efficiency is affected by the concentration of the contaminant in the condensate. In other words if condensate contains unusually high levels of contaminants, the discharge downstream of the membrane will contain relatively higher levels as well. •

Evaporation – During the evaporation process, the condensate is directed into a vessel and heated until the liquid water begins to vaporize. After the water has evaporated, the residue is collected and disposed of. Since nothing is sent to the drain, there is no conflict with any water pollution regulations. The main drawback with this approach is the energy costs required to conduct the evaporation process. Separating a gallon of condensate in this manner requires 2.3 kW of energy. At $0.10/kW that amounts to $0.23/gallon ($0.06/liter) or $12.65 for a 55 gallon (208 liter) drum.

Each of the disposal methods has advantages. Two common disadvantages are high start-up costs and significant operating costs. 2. Biodegradable Fluids A cost-effective and environmentally acceptable alternative to condensate separation systems may be the use of biodegradable compressor fluids. When approved by waste treatment authorities, these products require no separation from condensate. While there is no universally accepted definition of biodegradability, compressor fluids should pass the EPA’s test method 796.3100. A commonly used standard, this analysis determines compound biodegradability by evaluating two factors: The amount of compound degraded after 28 days (70% is the passing mark for this test component) and the percent conversion of carbon dioxide generated as a byproduct of the compound’s degradation (60% is the passing mark for this component). Even though a coolant or lubricant has proven to be biodegradable, it should not be discarded into the ground or bodies of water. Rather, disposal of these fluids should follow one of two paths: •

Recycle the fluids as long as they have not been contaminated. To assist plant engineers in this area, the National Oil Recyclers Association has published a list of companies that collect and reuse recyclable lubricants.

•

Send the biodegradable fluids in the compressor condensate directly to local waste treatment facilities. When presented with results of the EPA test method 796.3100, most waste treatment authorities are amenable to waiving any pretreatment requirements and accept the biodegradable fluid in the condensate.

F-2


G

OUTDOOR INSTALLATIONS

The information in this appendix was taken from the article, “Common Sense of Compressor Air System Maintenance,” Ingersoll-Rand Air Solutions, www.air.ingersoll-rand.com [27]. A compressor may be installed outside because of plant space limitations. When this occurs there are certain items that should be incorporated into the installation to help ensure trouble-free operation, including: •

The compressor should be purchased with the Outdoor Modification Option to provide NEMA 4 electric’s and a cabinet exhausting on the end of the unit rather than the top to prevent re-circulation of cooling air.

•

The compressor should be installed on a concrete pad designed to drain water away. If the concrete pad is sloped, the compressor must be leveled. In order to properly pull cooling air through the aftercooler, the base/skid must be sealed to the concrete pad.

•

The roof of the shelter should extend a minimum of four feet around all sides of the compressor to prevent direct rain and snow from falling on the unit. Air-cooled machines must be arranged in a way that prevents air re-circulation (i.e., hot exhaust back to the package inlet).

•

If the installation includes more than one compressor, the hot air exhaust should not be directed toward the fresh air intake of the second unit or an air dryer.

•

Arrange the machine with controller/starter enclosure facing away from the sun as radiant heat can affect starter performance. In addition, direct sunlight and UV rays will degrade the membrane touch panel.

•

Power disconnect switch should be within line of sight and in close proximity to the unit operating panel.

•

Incoming power connections must use suitable connectors for outdoor weather tight service.

•

A minimum of three feet clearance must be allowed on all four sides of the unit for service access. If possible, access by a forklift and or an overhead beam hoist should be kept in mind (for eventual service to air end or motor).

•

Some type of protection such as a fence or security system should be provided to prevent unauthorized access.

G-1


H

COMPRESSED AIR SYSTEM SPECIFICATION

The following blank specification sheet for compressors and air dryers was provided by Duke Energy Corporation. It provides a general listing of items for the design of a compressed air system. It is intended as a starting point for the design and can be modified as needed.

H-1

EPRI Licensed Material Compressed Air System Specification

Compressed Air System Datasheet Plant:___________________ Date: ___________ By: ___________________________ 1. Compressor Vendor: ___________________________________________________ 2. Quantity: Unit 1: ________ Unit 2: ________ Unit 3: ________ Total:__________ 3. Header Pressure Required: _______________________psig 4. Flow Capacity Required: _______________scfm 5. Inlet Conditions: Pressure ______________psia Temperature ___________ûF 6. Compressor Discharge Pressure: ______________psig 7. Compressor Type: Single Stage Reciprocating _______ Two-Stage Reciprocating ___________ Centrifugal ___________ Rotary Screw _____________ Other _______________ 8. Duty: Continuous ______________Intermittent _________________ 9. Type of Controls __________________________________________ 10. Compressor Lubrication: Oil Free Type: Yes__________

No _________

Type of Lubrication System___________________ Lubricant Used ______________ 11. Motor(s): Induction ______________ Synchronous ________________ Voltage _________________ Hp________________ 12. Air Dryers: Refrigerant ____________Desiccant ____________Other ___________ Heat of Compression _____________ Heat Regenerative _______________ Cold Regenerative _________________ No. of Dryers _______________ Inlet Conditions ________________psig Temperature _________________ûF Capacity ________________scfm

H-2

EPRI Licensed Material Compressed Air System Specification

13. Filters: Particulate _____________ Coalescing ______________ Adsorption _____________ Air-Line __________________ Intake Filter-Silencer Required: ___________ Type of Inlet Filter __________________________ 14. Inter-Coolers Type _____________________ Air-Cooled __________________ Water-Cooled ______________________ Inlet Water Temperature ___________ûF Discharge Water Temperature ______________ûF Water Source ______________ 15. Aftercoolers: Capacity _____________scfm Air-Cooled ________ Water-Cooled ________ Water Source:____________ Water Inlet Temperature _______ûF Water Outlet Temperature ______ûF Design Pressure (Water Side) ________psig Design Pressure (Air Side) _______psia 16. Separators/Traps: Moisture Separator _______________ Traps_______________ Type of Traps ____________________ No. of Traps _____________________ 17. Receiver Tank(s): Quantity _____________ Size _____________ft3 Design Pressure ___________psig ASME Section VIII _____________ 18. Distribution Piping: Sizes _________________ Material: __________________ 19. Instrumentation: Dew Point Monitors: __________ Air Flow Indicators _________ Header Pressure Indicators ______________________ 20. Safety Relief Valves: Pressure Relief Valve ______________psig Rupture Disc ___________psig 21. Special Features:_______________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ____________________________________________________________

H-3


I

POP OUT SUMMARY

The following list provides the location of the key Pop-Out information in this report. Key Technical Point Targets information that will lead to improved equipment reliability. Section

Page

Key Technical Point

2.4.1

2-23

Reciprocating compressors, with many air control valves and large frictiongenerating components, usually require more frequent maintenance to sustain performance levels.

2.4.1

2-23

Rotary screw air compressors usually have shorter life expectancies than reciprocating or centrifugal compressors. Regular maintenance will extend the useful life of the rotary screw compressor but will not prevent the inevitable need for complete replacement of the air compressor component (the air end).

2.4.1

2-24

A general rule is that centrifugal and rotary air compressors are better suited to continuous base-load type of service. Reciprocating air compressors are better suited for swings in load. This means that a centrifugal or rotary air compressor can be sized for the minimum or average demand, while a reciprocating air compressor can then handle the swings in load from minimum or average to the peaks.

2.4.1.2.1

2-31

The generic term oil has been used. This is generally understood to be a hydrocarbon product, but most compressors now use cleaner and longerlife synthetic lubricants including diesters, polyglycols, polyalphaolefins, and silicon-based fluids. These newer products are suitable for a wider range of temperatures and have higher flash points. However, the compatibility of the lubricant and its volatile additives with downstream components (especially elastomeric materials) should be checked.

2.4.2

2-53

Because of their complex design, synchronous motors tend to be more difficult to install and operate.

2.4.3

2-57

Most dryers are rated at what is commonly referred to as the three 100s, that is, 100 psig (689 kPa), 100°F (37.8°C), and 100% relative humidity (saturated air). An increase in operating pressure (within the capability of the dryer) increases the rated mass flow capacity. An increase in inlet temperature (within the capability of the dryer) decreases the rated mass flow or capacity because of the higher moisture content at a saturated condition.

2.4.3.1

2-59

For colder climates, freezing in outside lines is a problem. One solution is to install a regenerative desiccant dryer with a lower pressure dew point such as -40ºF (-40ºC).

I-1

EPRI Licensed Material Pop Out Summary

Key Technical Point Targets information that will lead to improved equipment reliability. Section

Page

Key Technical Point

2.4.3.1

2-61

Channeling occurs when an air stream finds a path through the bed and follows the path instead of distributing evenly throughout the bed. This is often a problem with desiccant dryers. Channeling can be avoided by using stainless steel diffusers in the inlet and outlet of the desiccant towers and controlling the air velocity through the desiccant bed.

2.4.3.1.3

2-68

Before installing a heatless desiccant type dryer, ensure that the compressors have 15 to 20% excess capacity.

2.4.4

2-72

Filtration literature might refer to an absolute rating or a nominal rating. The absolute rating refers to the diameter, in microns, of the largest particle that will pass through the filter. This is theoretical because it requires a consistent pore size of the filter medium, which is not always practical, and particle sizes are not always spherical. The nominal rating is determined by the filter manufacturer and is an arbitrary value. Usually it is a percent retention by weight of a specified contaminant of a given size.

2.4.7

2-91

Air receivers or storage tanks should be provided in all installations using positive displacement air compressors such as reciprocating and rotary screw air compressors. Receiver tanks for dynamic air compressors (centrifugal compressors) are used for large-flow requirements, such as boiler sootblowing applications.

2.4.8

2-92

The complete drying, filtration, and distribution system should be sized and arranged so that the total pressure drop from the air compressor to the points of use is much less than 10% of the compressor discharge pressure.

2.4.8

2-93

Pressure drops between the compressor and end-use components are non-recoverable losses. Therefore, it is imperative that pressure losses be minimized.

3.2.1

3-8

Sequencing compressors does prevent competition between the individual compressors. It also causes plant pressure to fluctuate along the range of assigned pressure levels. For example, a four-compressor system with pressure intervals of 5 psi (34 kPa) will have a 20-psi (138-kPa) operating window. Sequencers tend to maintain an average system pressure that is higher than needed.

5.4.1.3

5-14

The most common failure locations and mechanisms for reciprocating air compressors are: total closure valve diaphragm failures, suction and discharge valves on the compressor heads, inability of lubricators to provide required lubrication rates, and dirty or clogged air inlet filters.

5.4.2.3

5-29

The most common failure locations and mechanisms for the rotary screw compressors are clogged or dirty air inlet filters, inlet throttle valve diaphragm, unloader piston diaphragm, and cooling water leakage for the compressor inter-cooler and after-cooler.

5.4.3.3

5-42

The most common failure locations and mechanisms for the liquid ring rotary compressor and pump are solenoid pilot valves, especially those in lower-quality air systems, inadequate seal water flow, insufficient seal water tank level (which allows carryover of sedimentation), erosion/corrosion of the pump internals, oil leaks contaminating the pumped or compressed medium water, and leaks from the seal cooling water into the bearings.

I-2


Key Technical Point Targets information that will lead to improved equipment reliability. Section

Page

Key Technical Point

5.5.3

5-66

The most common failure locations and mechanisms for low-voltage electric motors are bearings (especially from failure of a driven component, normal wear, and failures of the lube oil system), stator windings (especially from excessive consecutive starts, operation above rated temperature, and other insulation degradation), mechanical and electrical connections, and terminations (windings, motor leads).

6.2

6-4

There are two basic ways to monitor the corrosive properties of the air entering the system: measure the concentrations of gases known to be corrosive and measure the effects of the corrosive gases.

6.2.1

6-7

From a review of industry-wide pneumatic system problems completed by EPRI in October 1988, results indicated that 49% of all failure events resulted from contamination in the system and only 28% were attributed to component failures. It is possible some of the component failures also occurred due to contamination in the instrument air system. This means that contaminants are the largest single contributor to instrument air system failures. Other problems that cause aggravation to maintenance are leaks and component failures.

6.2.1.1.2

6-13

The selection of dew point monitoring locations should be based principally on one consideration. The location or locations chosen must provide reasonable assurance that moisture and water has not and will not, be introduced into the instrument air system undetected.

7.3.2

7-10

One way of determining the integrity of air system piping is to measure the minimum wall thickness of carbon steel piping per ANSI B31.1 Power Piping Standard. This is an area of concern in un-dried portions of the compressed air system. By monitoring this portion of the system, leaks can be minimized and potential failure areas identified.

I-3


Key O&M Cost Point Emphasizes information that will result in overall reduced costs and/or increase in revenue through additional or restored energy production. Section

Page

Key O & M Cost Point

2.3

2-10

The higher the levels of air quality, the more the costs are to produce that level. Higher quality air usually requires additional equipment. This increases the initial capital investment and operating and maintenance costs. It is important to meet but not exceed the required air quality levels with the compressed air system.

2.4.1

2-22

Rotary screw compressors are the lowest cost compressors to purchase and install.

2.4.1

2-22

Rotary screw compressors are usually less efficient than reciprocating or centrifugal compressors.

2.4.1

2-23

The most efficient compressor type for part-load operation is the reciprocating compressor.

2.4.1

2-24

When performance degrades to the point where replacement or overhaul of key compressor components (impellers, etc.) is necessary, centrifugal repair is very expensive. Equivalent work on a reciprocating compressor will tend to cost much less.

2.4.1.2.5

2-39

The liquid ring rotary-type compressor is less efficient than other rotary types because of the energy required to swirl the liquid in the stator.

2.4.2

2-53

In practical terms, induction motors with their lagging power factor will result in greater expenses for the same power usage.

2.4.3

2-57

Because more effective drying is generally more expensive (from initial cost and operating cost perspectives), it is important not to over-dry a system.

2.4.3.2

2-68

The refrigerant-type dryer is the most economical type of air dryer.

2.4.4

2-72

The air stream should not be filtered to a greater degree than the plant requires. More effective filters create more pressure drop and the filters are more expensive.

3.0

3-2

Every 2 psi (14 kPa) of control range pressure difference is equal to a 1% change in energy consumption.

7.3.2

7-9

Leakage easily can be the largest energy problem in a compressed air system, ranging from 2 to 50% of compressor system capacity. An average plant has a leak rate of about 20% of total air production. Leaks can be reduced to 5% or lower, offering the average plant an opportunity to reduce compressor system costs by as much as 15%.

I-4


Key Human Performance Point Denotes information that requires personnel action or consideration to prevent personal injury, equipment damage, and/or to improve the efficiency and effectiveness of the task. Section

Page

Key Human Performance Point

2.4.1.4

2-50

It is estimated that the useful life of lubricating oil is halved for every 18ºF (5ºC) above 200ºF (93ºC) of operating temperature. Compressors operating in high ambient conditions need to have the oil checked frequently, and a quarterly sampling and analysis program is recommended. For additional details on compressor lubricants, refer to the EPRI Lubrication Guide: Revision 3 (Formerly NP-4916-R2), 1003085.

2.4.6.2

2-85

The relatively common practice of leaving a manual drain valve open a crack should not be used because it wastes costly compressed air.

5.1

5-1

Air quality is the single most important means of determining the reliability of an instrument air supply. It should be the first priority and continuously monitored.

6.2

6-3

Air quality monitoring is one of the keys to achieving reliable air systems. High instrument air system reliability depends on having air that is free of contaminants. Because of the importance of monitoring and the monitoring problems that occur, effort in this area is highly beneficial.

7.2.1

7-6

One of the most common problems, especially with single-train dryers, occurs when a pre-filter clogs and the differential pressure goes past the limit. The operator manually compounds the problem by bypassing the dryer. It is better to bypass the filter or even remove the element than it is to bypass the entire dryer train.

7.2.1

7-6

When the dryer towers experience shifting difficulties and the desiccant in one tower is too wet, it is strongly recommended not to bypass the dryer. A desiccant that is partially working is better than none at all.

7.3.2

7-12

In order to be effective in the long term, a leak-reduction program must be constant and comprehensive. When compared to other measures, leak repair generally offers the greatest improvement in compressor system efficiency for the least expenditure. An energy savings potential of 15% for a typical plant more than justifies a leak maintenance program.

I-5

Target: Steam Turbines, Generators, and Balance-of-Plant Fossil Maintenance Applications Center

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Compressed Air System Maintenance Guide -1006677

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