Pulverizer Maintenance Guide Volume 3: Ball/Tube Mills
Technical Report
Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export
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Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.
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Pulverizer Maintenance Guide Volume 3: Ball/Tube Mills 1010443
Final Report, March 2006
EPRI Project Manager A. Grunsky
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ELECTRIC POW ER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1395 ▪ PO Box 10412, Palo Alto, California 94303-0813 ▪ USA 800.313.3774 ▪ 650.855.2121 ▪
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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT W AS 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 W ARRANTY 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 Electric Power Research Institute (EPRI)
NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail
[email protected]. Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.
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Copyright © 2006 Electric Power Research Institute, Inc. All rights reserved.
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CITATIONS This report was prepared by Electric Power Research Institute (EPRI) Fossil Maintenance Applications Center (FMAC) 1300 W.T. Harris Boulevard Charlotte, NC 28262 Maintenance Management and Technology (MM&T) 1300 W.T. Harris Boulevard Charlotte, NC 28262 Principal Investigator S. Parker This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Pulverizer Maintenance Guide, Volume 3: Ball/Tube Mills. EPRI, Palo Alto, CA: 2006. 1010443.
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REPORT SUMMARY
The Pulverizer Maintenance Guide, Volume 3: Ball/Tube Mills provides fossil plant personnel with current maintenance information on ball/tube mills from different manufacturers. This guide will assist a plant in improving the maintenance of their pulverizer mills. Background This is the third guide produced for pulverizer mills; it is preceded by: •
Pulverizer Maintenance Guide, Volume 1: Raymond Bowl Mills (EPRI report 1005061)
•
Pulverizer Maintenance Guide, Volume 2: B&W Roll Wheel™ Pulverizers (EPRI report 1009508)
Two Electric Power Research Institute (EPRI) groups, Fossil Maintenance Applications Center (FMAC) and Maintenance Management and Technology (MM&T), sponsored this guide. Objectives • To identify preventive and predictive maintenance practices for ball/tube mills •
To assist plant maintenance personnel in the identification and resolution of ball/tube mill equipment problems
Approach A Technical Advisory Group (TAG) was formed that consisted of pulverizer equipment owners from EPRI FMAC and MM&T members. There are five manufacturers’ mills covered in this report. The manufacturers are Allis-Chalmers, Foster Wheeler, Kennedy Van Saun, Riley Power Inc., and Stein Industrie. Input was solicited for the current maintenance issues for the ball/tube mills. An extensive search of industry and EPRI information was conducted to provide relevant information for this guide. Results This guide includes general information on the ball/tube mill function in the coal-handling process, the operation and safety of the ball/tube mill, and the performance testing of the mills. The technical description, failure modes, troubleshooting, and predictive and preventive maintenance sections are the main sources of information in the guide. A preventive maintenance (PM) basis was also developed for the guide. This guide covers the coal pulverizer system from the feeder to the pulverizer outlet.
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EPRI Perspective The maintenance of the ball/tube mills affects the availability and reliability of the operating unit. The efficiency of the mill in providing the desired coal and air mixture to the furnace has increased cost consequences with the addition of NOx controls. Repairs and modifications to the mill will ensure that the mills operate reliably. Keywords Pulverizer mill Ball mill Tube mill Maintenance Reliability Troubleshooting
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ABSTRACT The pulverizer mill is a critical component in a coal-fired power plant. As the age of the mill increases, the maintenance costs required for continued operation also increase. With the addition of NOx controls, the efficiency of the unit is affected to a greater degree by the air quantity and fineness of the coal-air mixture going to the furnace. Monitoring critical dimensions and parameters on the mill makes sure that the mill is functioning correctly. Performing routine preventive inspections and anticipating component replacements ensures that the maintenance activities are planned activities and not the cause of forced outages. This guide covers several ball/tube mill manufacturers. It is intended to improve the maintenance practices and reliability of the equipment.
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ACKNOWLEDGMENTS This report was produced by the Electric Power Research Institute (EPRI) Fossil Maintenance Applications Center (FMAC), Maintenance Management and Technology (MM&T), and the following members of the Technical Advisory Group (TAG). EPRI would like to thank these individuals for their participation in the preparation and review of the report: TAG Members: Name
EPRI Member Utility/Location
Robert Baca J. Corie Biggs Jesse Billings Mark Breetzke Marlize Dreyer Chris Du Toit Wolf Hahn Scott Hall Andre Van Heerden Ken Isaacson Mahomed Jhetam
Salt River Project/Coronado Hoosier Energy/Merom Hoosier Energy/Merom Eskom/Kendal Eskom/Arnot Eskom/Tutuka Eskom/Tutuka Salt River Project/Coronado Eskom/Lethabo Salt River Project/Coronado Eskom/Majuba
Ken Johnson Robert Jones Ken McDonald Mce Matanda
Salt River Project/Coronado Wisconsin Energies/Corporate Office Hoosier Energy/Merom Eskom/Kendal
Hennie Pretorius Russell Tarr Christo Van Wyk
Eskom/Matimba Eskom/Corporate Office Eskom/Matimba
Manufacturers: Dan Smith
Riley Power Inc.
EPRI and the TAG 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.............................................................................................................. 1-3
1.4
Key Points................................................................................................................. 1-4
2 GLOSSARY......................................................................................................................... 2-1 3 SYSTEM APPLICATION ..................................................................................................... 3-1 3.1
Coal Handling System............................................................................................... 3-1
3.2
Coal Characteristics .................................................................................................. 3-5
3.3
Coal Pulverizer Mills.................................................................................................. 3-7
3.4
Environmental Regulations ....................................................................................... 3-9
4 TECHNICAL DESCRIPTION – GENERAL .......................................................................... 4-1 5 TECHNICAL DESCRIPTION – ALLIS-CHALMERS ............................................................ 5-1 5.1
Inlet/Outlet Boxes...................................................................................................... 5-4
5.2
Rotating Shell, Liners, and Balls................................................................................ 5-7
5.3
Trunnion Bearings..................................................................................................... 5-9
5.4
Classifier ..................................................................................................................5-15
5.5
Drive Motor ..............................................................................................................5-17
5.6
Gearbox Unit ............................................................................................................5-23
5.7
Girth Gear and Pinion Shaft .....................................................................................5-29
5.8
Power-Sonic Mill Conditioning System .....................................................................5-33
5.9
Technical Specification List ......................................................................................5-35
6 TECHNICAL DESCRIPTION – FOSTER WHEELER .......................................................... 6-1 6.1
Conveyor Assembly .................................................................................................. 6-4
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6.1.1 6.2
Conveyor Support Assembly ............................................................................ 6-6
Drum Assembly......................................................................................................... 6-8
6.2.1
Double-Wave Liners ........................................................................................6-10
6.2.2
Double-Size, Double-Wave Access Doors.......................................................6-12
6.2.3
Flight Bars .......................................................................................................6-12
6.2.4
Grinding Balls ..................................................................................................6-12
6.3
Conveyor Shaft Bearing and Seal ............................................................................6-13
6.4
Trunnion Main Bearing and Dust Seal ......................................................................6-14
6.5
Gearing ....................................................................................................................6-16
6.5.1
Pinion Bearings ...............................................................................................6-17
6.6
Trunnion Tube..........................................................................................................6-18
6.7
Classifier ..................................................................................................................6-20
6.7.1
Classifier Reject Damper .................................................................................6-20
6.7.2
Adjustable Blade Classifier ..............................................................................6-21
6.7.3
M-Type Classifier.............................................................................................6-23
6.7.4
Dynamic Classifier ...........................................................................................6-24
6.8
Exhausters ...............................................................................................................6-27
6.9
Lubrication Systems .................................................................................................6-28
6.9.1
Cardwell Lubrication System ...........................................................................6-28
6.9.2
Farval Lubrication System ...............................................................................6-30
7 TECHNICAL DESCRIPTION – KENNEDY VAN SAUN....................................................... 7-1 8 TECHNICAL DESCRIPTION – RILEY POWER INC. .......................................................... 8-1 8.1
General Description .................................................................................................. 8-1
8.2
System Components ................................................................................................. 8-4
8.2.1
Feeder .............................................................................................................. 8-5
8.2.2
Crusher-Dryer................................................................................................... 8-6
8.2.3
Rotating Drum or Barrel.................................................................................... 8-7
8.2.4
Grinding Ball Makeup ....................................................................................... 8-8
8.2.5
Classifier........................................................................................................... 8-9
8.2.6
Shutoff Valves .................................................................................................8-10
8.2.7
Speed Reducer Gearbox .................................................................................8-10
8.2.8
Clutch ..............................................................................................................8-12
8.3
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Systems ...................................................................................................................8-15
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8.3.1
Primary Air System ..........................................................................................8-15
8.3.2
Seal Air System ...............................................................................................8-16
8.4
Modifications ............................................................................................................8-17
8.4.1
Trickle Valve Addition ......................................................................................8-17
8.4.2
Trunnion Air Seal Redesign .............................................................................8-18
8.4.3
Mill Conditioning System Upgrade ...................................................................8-19
8.4.4
Hydrodynamic Slide Shoe Bearing Conversion................................................8-19
8.4.5
Crusher-Dryer Crusher Block...........................................................................8-21
9 TECHNICAL DESCRIPTION – STEIN INDUSTRIE............................................................. 9-1 9.1
General Description .................................................................................................. 9-1
9.2
Systems .................................................................................................................... 9-5
9.2.1
Coal Inlet System ............................................................................................. 9-5
9.2.2
Primary Air System ........................................................................................... 9-6
9.2.3
Seal Air System ................................................................................................ 9-8
9.2.4
Lubrication Oil Systems .................................................................................... 9-9
9.2.5
Drive System ...................................................................................................9-13
9.2.6
Blow-Down System .........................................................................................9-14
9.2.7
Ball Loading System ........................................................................................9-14
10 OPERATION AND SAFETY – ALLIS-CHALMERS ..........................................................10-1 10.1
Operations...........................................................................................................10-1
10.2
Fire Protection .....................................................................................................10-2
11 OPERATION AND SAFETY – FOSTER WHEELER ........................................................11-1 11.1
General Operation ...............................................................................................11-1
11.2
Operation Indications...........................................................................................11-3
11.3
Startup Procedures..............................................................................................11-4
11.4
Fire Detection System .........................................................................................11-6
11.5
Fire Protection .....................................................................................................11-7
12 OPERATION AND SAFETY – KENNEDY VAN SAUN.....................................................12-1 12.1
Load Changes .....................................................................................................12-1
12.2
Fuel Oil Support...................................................................................................12-2
12.3
Blocked Fuel Pipe................................................................................................12-3
12.4
Wet Coal .............................................................................................................12-4
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12.5
Mill Stripping ........................................................................................................12-5
12.6
Cold Startup ........................................................................................................12-6
13 OPERATION AND SAFETY – RILEY POWER INC. ........................................................13-1 13.1
General Operation ...............................................................................................13-1
13.2
Control System ....................................................................................................13-2
13.3
Handling Ball Charge...........................................................................................13-8
13.4
Primary Air Damper Operation.............................................................................13-8
13.5
Seal Air System .................................................................................................13-11
13.6
Feeder Calibration .............................................................................................13-12
13.7
Instrumentation Settings ....................................................................................13-13
13.8
Fire Detection ....................................................................................................13-15
13.9
Fire Protection ...................................................................................................13-17
14 OPERATION AND SAFETY – STEIN INDUSTRIE ...........................................................14-1 14.1
Layup ..................................................................................................................14-1
14.2
Fire Protection .....................................................................................................14-2
15 PERFORMANCE ..............................................................................................................15-1 15.1
Fineness..............................................................................................................15-1
15.2
Grindability ..........................................................................................................15-2
15.3
Moisture ..............................................................................................................15-2
15.4
Capacity ..............................................................................................................15-3
16 FAILURE MODES ............................................................................................................16-1 16.1
Abrasion ..............................................................................................................16-1
16.2
Erosion ................................................................................................................16-3
16.3
Failed Components .............................................................................................16-4
17 TROUBLESHOOTING......................................................................................................17-1 17.1
Allis-Chalmers .....................................................................................................17-1
17.2
Foster Wheeler ....................................................................................................17-5
17.3
Kennedy Van Saun..............................................................................................17-7
17.4
Riley Power Inc....................................................................................................17-9
17.5
Stein Industrie ...................................................................................................17-10
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18 PREDICTIVE MAINTENANCE .........................................................................................18-1 18.1
Vibration Analysis ................................................................................................18-3
18.2
Oil Analysis..........................................................................................................18-3
18.3
Condition-Based Maintenance – Kennedy Van Saun ........................................18-12
18.4
Condition Based Maintenance – Stein Industrie ................................................18-15
19 PREVENTIVE MAINTENANCE ........................................................................................19-1 19.1
Allis-Chalmers .....................................................................................................19-1
19.1.1
Inspection Criteria ........................................................................................19-2
19.1.2
External Mill Inspection.................................................................................19-3
19.1.3
Internal Mill Inspection..................................................................................19-6
19.1.4
Classifier Inspection .....................................................................................19-7
19.1.5
Drive Train Inspection ..................................................................................19-9
19.1.6
Shell and Trunnion Liner ............................................................................19-10
19.1.7
Trunnion Bearing Insert Replacement ........................................................19-11
19.1.8
Girth Gear Replacement.............................................................................19-13
19.1.9
Gearbox Rebuild ........................................................................................19-18
19.1.10
Miscellaneous Equipment...........................................................................19-22
19.1.11
Equipment Lubrication List .........................................................................19-22
19.2
Kennedy Van Saun............................................................................................19-24
19.2.1
Pinion, Girth Gear, and Lubrication System................................................19-25
19.2.2
Reversing the Worm Gear ..........................................................................19-25
19.3
Riley Power Inc..................................................................................................19-27
19.3.1
Feeder........................................................................................................19-27
19.3.2
Crusher-Dryer ............................................................................................19-28
19.3.3
Inlet/Outlet Box and Air Seals.....................................................................19-31
19.3.4
Mill Liners ...................................................................................................19-31
19.3.5
Ball Charge ................................................................................................19-32
19.3.6
Speed Reducer Gearbox............................................................................19-33
19.3.7
Drive Train..................................................................................................19-38
19.3.8
Driveshaft ...................................................................................................19-44
19.3.9
Lubrication Heat Exchanger .......................................................................19-46
19.3.10
Classifier ....................................................................................................19-47
19.3.11
Primary Air Fan and Ductwork....................................................................19-48
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19.3.12
Coal Shutoff Valves ....................................................................................19-49
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19.3.13
Lubrication Schedule ................................................................................... 19-49
19.3.14
Spare Parts .................................................................................................. 19-50
19.3.15
Example Preventive Maintenance Inspections ............................................ 19-51
19.4
Stein Industrie ..................................................................................................... 19-51
19.4.1
Preventive Maintenance Tasks.................................................................... 19-53
19.4.2
Inspection Tasks .......................................................................................... 19-58
19.4.3
Pinion and Girth Gear Replacement ............................................................ 19-61
20 PREVENTIVE MAINTENANCE BASIS.............................................................................. 20-1 20.1
Background ........................................................................................................... 20-1
20.2
Failure Locations, Degradation Mechanisms, and PM Strategies ........................ 20-2
20.3
PM Tasks and Their Degradation Mechanisms .................................................... 20-9
20.4
PM Template ....................................................................................................... 20-14
21 REFERENCES ................................................................................................................... 21-1 A KEY POINTS......................................................................................................................... A-1 B TRANSLATED TABLE OF CONTENTS .............................................................................. B-1 日本語 (Japanese) ............................................................................................................... B-2 Español (Spanish) .............................................................................................................. B-19
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LIST OF FIGURES Figure 3-1 A Typical Coal Handling System Diagram from Unloading to the Plant .................. 3-2 Figure 3-2 A Typical Coal Handling System Diagram from Plant to Unit Bunkers.................... 3-3 Figure 3-3 Foster Wheeler Ball Mill ......................................................................................... 3-8 Figure 3-4 Fuel-Bound Nitrogen Evolution to NOx ..................................................................3-10 Figure 5-1 Allis-Chalmers Ball/Tube Mill Outline ..................................................................... 5-2 Figure 5-2 Inlet/Outlet Box ...................................................................................................... 5-4 Figure 5-3 Mill Sealing Arrangement ....................................................................................... 5-6 Figure 5-4 Ball Charge Hopper ............................................................................................... 5-8 Figure 5-5 Trunnion Bearing Low-Pressure Lubrication System.............................................5-10 Figure 5-6 Manual High-Pressure Jacking Pump ...................................................................5-13 Figure 5-7 Static Classifier .....................................................................................................5-15 Figure 5-8 Mill Drive Motor .....................................................................................................5-17 Figure 5-9 Barring Gear Unit ..................................................................................................5-19 Figure 5-10 Barring Coupling .................................................................................................5-21 Figure 5-11 Barring Brake ......................................................................................................5-22 Figure 5-12 Gearbox Unit.......................................................................................................5-23 Figure 5-13 Gearbox Output Coupling ...................................................................................5-25 Figure 5-14 Gearbox Lubrication System ...............................................................................5-26 Figure 5-15 Gear Teeth Lubrication .......................................................................................5-27 Figure 5-16 Mill Outline ..........................................................................................................5-30 Figure 5-17 Girth Gear Lubrication System ............................................................................5-31 Figure 5-18 Power-Sonic Mill Conditioning System ................................................................5-33 Figure 6-1 Foster Wheeler Ball Mill Diagram ........................................................................... 6-1 Figure 6-2 Air/Coal Flow Diagram ........................................................................................... 6-2 Figure 6-3 Flight Ribbon Conveyor Assembly ......................................................................... 6-5 Figure 6-4 Flight Ribbon Spring Support Assembly ................................................................. 6-6 Figure 6-5 Conveyor Support Assembly.................................................................................. 6-6 Figure 6-6 Conveyor Removal with the Eight-Spoke Design ................................................... 6-8 Figure 6-7 Complete Drum Assembly ..................................................................................... 6-9 Figure 6-8 End Casting ..........................................................................................................6-10 Figure 6-9 Double-Wave Liners .............................................................................................6-11
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Figure 6-10 Double-Size, Double-Wave Access Door ............................................................6-12
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Figure 6-11 Ball Wear Rates ..................................................................................................6-13 Figure 6-12 Conveyor Bearing Modification ...........................................................................6-13 Figure 6-13 Original Bearing Assembly ..................................................................................6-14 Figure 6-14 New Design Bearing ...........................................................................................6-15 Figure 6-15 Trunnion Seal .....................................................................................................6-16 Figure 6-16 Pinion and Bull Gear Assembly ...........................................................................6-17 Figure 6-17 Roller Bearings ...................................................................................................6-18 Figure 6-18 Classifier Trunnion Tube .....................................................................................6-19 Figure 6-19 Original Scroll-Type Classifier .............................................................................6-20 Figure 6-20 Classifier Reject Dampers...................................................................................6-21 Figure 6-21 Adjustable Classifier ...........................................................................................6-22 Figure 6-22 Classifier Comparison .........................................................................................6-22 Figure 6-23 Adjustable Classifier Fineness Improvement.......................................................6-23 Figure 6-24 M-Type Classifier ................................................................................................6-24 Figure 6-25 Dynamic Classifier ..............................................................................................6-25 Figure 6-26 Exhauster Diagram .............................................................................................6-27 Figure 6-27 Exhauster Spider ................................................................................................6-28 Figure 6-28 Cardwell Lubrication System...............................................................................6-29 Figure 6-29 Farval Lubrication System...................................................................................6-30 Figure 7-1 Kennedy Van Saun Mill System ............................................................................. 7-2 Figure 7-2 Kennedy Van Saun Mill Components..................................................................... 7-3 Figure 8-1 Riley Power Chain-Driven Ball/Tube Mill System ................................................... 8-2 Figure 8-2 Riley Power Gear-Driven Ball/Tube Mill System..................................................... 8-3 Figure 8-3 Drum-Type Feeder................................................................................................. 8-5 Figure 8-4 Crusher-Dryer ........................................................................................................ 8-6 Figure 8-5 Mill with Pinion/Ring Gear Drive Set....................................................................... 8-8 Figure 8-6 Static Classifier ...................................................................................................... 8-9 Figure 8-7 Speed Reducer Gearbox ......................................................................................8-11 Figure 8-8 Clutch Assembly ...................................................................................................8-13 Figure 8-9 Rotorseal ..............................................................................................................8-15 Figure 8-10 Clutch Air Control System ...................................................................................8-15 Figure 8-11 New Trickle Valve Design ...................................................................................8-18 Figure 8-12 Trunnion Air Seal Designs ..................................................................................8-19 Figure 8-13 Hydrodynamic Slide Shoe Bearing......................................................................8-20 Figure 8-14 Thrust Bearing ....................................................................................................8-21 Figure 9-1 Stein Industrie Tube Mill......................................................................................... 9-2 Figure 9-2 Detailed View of the Stein Industrie Tube Mill ........................................................ 9-3 Figure 9-3 Mill Airflow.............................................................................................................. 9-7
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Figure 9-4 High- and Low-Pressure Lubricating Oil System ...................................................9-10
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Figure 10-1 Mobile Gas Bottle Unit ........................................................................................10-3 Figure 10-2 Permanently Installed Carbon Dioxide Fire Protection System............................10-4 Figure 11-1 Fire Detection Sensor Head ................................................................................11-7 Figure 12-1 Wet Coal Operation ............................................................................................12-4 Figure 13-1 Riley Power Operation Diagram ..........................................................................13-4 Figure 13-2 Motor Power Versus Product Charge ..................................................................13-5 Figure 13-3 Mill Parameters Using Product Charge Control ...................................................13-5 Figure 13-4 Effects of Reduced Ball Charge ..........................................................................13-6 Figure 13-5 Water Spray System Nozzle Locations .............................................................13-18 Figure 16-1 Pulverizer Component Failure Frequency ...........................................................16-1 Figure 18-1 Early Warning for Pulverizer Failure ....................................................................18-2 Figure 19-1 Link-Belt Single-Gear Reducer..........................................................................19-34 Figure 19-2 Link-Belt Double-Gear Reducer ........................................................................19-35 Figure 19-3 Link-Belt Triple-Gear Reducer...........................................................................19-36
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LIST OF TABLES Table 1-1 Conversion Factors ................................................................................................. 1-2 Table 5-1 Alarm Conditions for the Power-Sonic Mill Conditioning System ............................5-34 Table 5-2 Data for Allis-Chalmers Mills ..................................................................................5-35 Table 9-1 Normal Lubrication Oil Pressure Values .................................................................9-12 Table 9-2 Mill Lubrication Systems.........................................................................................9-13 Table 10-1 Ball Replacement Tasks.......................................................................................10-2 Table 11-1 Shutting Down One Side of a Double-End Mill .....................................................11-2 Table 11-2 Normal Operation Checks ....................................................................................11-4 Table 11-3 Initial Mill Preparation for Startup .........................................................................11-4 Table 11-4 Charging the Mill for Startup.................................................................................11-5 Table 11-5 Placing the Mill in Service ....................................................................................11-6 Table 11-6 Procedures for Fire Extinguishing in the Foster Wheeler Mill................................11-8 Table 11-7 Emergency Shutdown Procedures for the Foster Wheeler Mill .............................11-9 Table 11-8 Recommended Procedures for Shutting Down the Foster Wheeler Mill Filled with Coal ........................................................................................................................11-9 Table 12-1 Tasks for Stripping the Mill ...................................................................................12-5 Table 12-2 Cold Startup Tasks...............................................................................................12-6 Table 13-1 Coal Feeder Calibration Tasks ...........................................................................13-12 Table 13-2 Sample Riley Mill Instrumentation Parameters ...................................................13-13 Table 13-3 Sample Equipment Parameters..........................................................................13-14 Table 13-4 Fire Detection Temperature Sensor Locations ...................................................13-16 Table 15-1 Standard Screen Dimensions...............................................................................15-1 Table 16-1 Abrasive Wear Coefficients ..................................................................................16-3 Table 16-2 Allis-Chalmers Failure Components .....................................................................16-4 Table 17-1 Troubleshooting for Allis-Chalmers Mills ..............................................................17-1 Table 17-2 Troubleshooting Chart for Foster Wheeler Mills....................................................17-5 Table 17-3 Troubleshooting Chart for Kennedy Van Saun Mills .............................................17-7 Table 17-4 Troubleshooting Chart for the Chain-Driven or Gear-Driven Riley Power Ball Mills ................................................................................................................................17-9 Table 17-5 Troubleshooting for Stein Industrie Mills.............................................................17-10 Table 18-1 Particle Count Range Numbers ............................................................................18-5
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Table 18-2 Elements in Oil Additive Package.........................................................................18-9
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Table 18-3 Kennedy Van Saun Mill Components for Lubrication Program ...........................18-13 Table 18-4 Condition-Based Monitoring Values at Majuba Power Station ............................18-16 Table 18-5 Monitored Conditions at Majuba Power Station ..................................................18-19 Table 18-6 Condition Monitoring Instrumentation for Majuba Power Station ........................18-20 Table 18-7 Scheduled Condition Monitoring at Majuba Power Station .................................18-24 Table 19-1 External Mill Inspection Tasks ..............................................................................19-4 Table 19-2 Internal Inspection Tasks .....................................................................................19-6 Table 19-3 Classifier Inspection Tasks...................................................................................19-8 Table 19-4 Mill Drive Train Inspection Tasks..........................................................................19-9 Table 19-5 Shell and Trunnion Liner Replacement Tasks ....................................................19-10 Table 19-6 Trunnion Bearing Insert Replacement Tasks......................................................19-12 Table 19-7 Girth Gear Replacement Tasks ..........................................................................19-14 Table 19-8 Gearbox Rebuild Tasks......................................................................................19-19 Table 19-9 Equipment Lubrication List .................................................................................19-23 Table 19-10 Inspection Tasks for Pinion, Girth Gear, and Lubrication Systems ...................19-25 Table 19-11 Tasks for Reversing the Gearbox Worm Gear..................................................19-26 Table 19-12 Wear Liner Replacement..................................................................................19-32 Table 19-13 Speed Reducer Gearbox Oil Capacities ...........................................................19-33 Table 19-14 Triple-Speed Reducer Gearbox Disassembly Tasks ........................................19-37 Table 19-15 Clutch Inspection Tasks ...................................................................................19-40 Table 19-16 Chain Repair Tasks..........................................................................................19-41 Table 19-17 Driven Sprocket Removal and Reinstallation Tasks .........................................19-42 Table 19-18 Driveshaft Replacement Tasks.........................................................................19-44 Table 19-19 Heat Exchanger Inspection Tasks ....................................................................19-46 Table 19-20 Lube Oil Heat Exchanger Replacement Tasks .................................................19-47 Table 19-21 Classifier Preventive Maintenance Tasks .........................................................19-48 Table 19-22 Lubrication Schedule........................................................................................19-49 Table 19-23 Riley Power Recommended Spare Parts .........................................................19-50 Table 19-24 Feeder Inspection Tasks for the 7000 Operating Hour Interval.........................19-53 Table 19-25 Preventive Maintenance Tasks ........................................................................19-54 Table 19-26 Inspection Tasks for the 5500 Operating Hour Interval .....................................19-59 Table 19-27 Inspection Tasks for the 18-Month Interim Inspection.......................................19-60 Table 19-28 Inspection Tasks for the Nine-Year General Overhaul......................................19-61 Table 19-29 Replacement Tasks for the Pinion and Girth Gear ...........................................19-61 Table 20-1 Failure Locations, Degradation Mechanisms, and PM Strategies for Ball/Tube Mills ................................................................................................................20-4 Table 20-2 PM Tasks and Their Degradation Mechanisms for Ball/Tube Mills .....................20-10 Table 20-3 PM Template for the Ball/Tube Mills ...................................................................20-15
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1 INTRODUCTION
The purpose of this guide is to provide information for ball/tube mills. The information is intended to provide a comprehensive maintenance guide for plant maintenance personnel. This guide should assist plant maintenance personnel in the identification and resolution of pulverizer equipment problems.
1.1
Background
Maintenance costs on pulverizers are a significant part of a plant’s maintenance budget. The loss of pulverizer availability results in operational impacts from load reduction, NOx control difficulties, and an increased heat rate. Pulverizer failures increase demands on maintenance crews to perform reactive maintenance, which directly affects scheduled work and crew utilization and effectiveness. Most pulverizers in service today are at least 25 years old, and many are being maintained with smaller maintenance crews that have a limited amount of experience. Equipment age, technician knowledge, operational practices, and dated monitoring techniques have impacted pulverizer performance in various stations. These issues and others prompted the development of a series of comprehensive maintenance guides for pulverizers. With input from utility members, it was decided to produce three maintenance guides. The selection of the mill types was determined from survey information submitted from member utilities. The three maintenance guides are: • Pulverizer Maintenance Guide, Volume 1: Raymond Bowl Mills (EPRI report 1005061), August 2004 • Pulverizer Maintenance Guide, Volume 2: B&W Roll Wheel™ Pulverizers (EPRI report 1009508), December 2004 •
Pulverizer Maintenance Guide, Volume 3: Ball/Tube Mills (EPRI report 1010443), March 2006
Two Electric Power Research Institute (EPRI) groups, the Fossil Maintenance Applications Center (FMAC) and Maintenance Management and Technology (MM&T), sponsored this guide.
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Introduction
1.2
Approach
The proposed guide content and schedule was sent to the EPRI-member coal-fired plants in January 2005. Input for the guide and participation in a Technical Advisory Group (TAG) was then solicited. The TAG for the guide consisted of 16 utility members and one manufacturer representative. The TAG members provided input for the guide, reviewed the first draft, and provided comments. An extensive search of existing EPRI guides and industry literature was conducted during the development of this guide. Because many sources of information were used in the compilation of this guide, it was decided to use a reference system for the appropriate sections. The use of reference numbers in brackets is used at the beginning of sections and after the titles on tables and figures to denote where the majority of information in that section was obtained. The numbers and corresponding references are listed in the Section 21 of this guide. The following conversion factors in Table 1-1 should be used to convert from English to Standard International units of measurement. Table 1-1 Conversion Factors Parameter Area
Flow
Energy
Length
English Units to Standard International Units
Standard International Units to English Units
1 in2 = 6.45 cm 2
1 cm 2 = 0.155 in2
1 ft2 = 929 cm 2
1 cm 2 = 0.001 ft2
1 ft3/min = 28.317 liter/min
1 liter/min = 0.0353 ft3/min
1 ft3/sec = 28.317 liter/sec
1 liter/sec = 0.0353 ftp.3/sec
1 Btu = 2.9307 x 10-4 kwh
1 kwh = 3412 Btu
1 Btu = 1055 Joules
1 Joule = 9.48 x 10-4 Btu
1 inch = 0.0254 m
1 m = 39.37 inch
1 inch = 2.54 cm
1 cm = 0.3937 inch
1 inch = 25.4 mm
1 mm = 0.03937 inch
1 inch = 25,400 µm (micron)
1 µm = 39.37 x 10 inch
1 ft = 0.3048 m
1 m = 3.28 ft
1 ft = 30.48 cm
1 cm = 0.0328 ft
1 ft = 304.8 mm
1 mm = 0.00328 ft
1 ft = 304,800 µm (micron)
1 µm = 3.28 x 10 ft
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Introduction Table 1-1 (cont.) Conversion Factors Parameter Pressure
English Units to Standard International Units
Standard International Units to English Units
1 psi = 6.89 kPa
1 kPa = 0.145 psi
1 psi = 0.006895 mPa
1 mPa = 145 psi
psig = psi gauge psia = psi absolute Temperature
ºF = 1.8 (ºC) + 32
ºC = (ºF-32)/1.8
Torque
1 ft-lb = 1.356 N-m
1 N-m = 0.738 ft-lb
Weight
1 oz. = 28.35 g
1 g = 0.0353 oz
1 lb. = 0.454 kg
1 kg = 2.2 lb
1 U.S. ton = 2000 lbs. = 0.907 metric tons
1 metric ton = 1000 kg = 1.1 U.S. tons
1 in/sec = 2.54 cm/sec
1 cm/sec = 0.394 in/sec
1 ft/sec = 0.3048 m/sec
1 m/sec = 3.28 ft/sec
1 gal = 3.78 liter
1 liter = 0.264 gal
1 gal = 3785 milliliters
1 milliliter = 2.642 x 10-4 gal
Velocity
Volume
1.3
Organization
This guide is organized into the following sections: 1. Introduction – Background, approach, organization, and key points 2. Glossary 3. System Application – Coal-handling system, coal characteristics, coal pulverizer mills, and environmental regulations 4. Technical Description – General 5. Technical Description – Allis-Chalmers 6. Technical Description – Foster Wheeler 7. Technical Description – Kennedy Van Saun 8. Technical Description – Riley Power Inc. 9. Technical Description – Stein Industrie 10. Operation and Safety – Allis-Chalmers
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Introduction
11. Operation and Safety – Foster Wheeler
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Introduction
12. Operation and Safety – Kennedy Van Saun 13. Operation and Safety – Riley Power Inc. 14. Operation and Safety – Stein Industrie 15. Performance – Fineness, coal grindability, moisture, and capacity 16. Failure Modes – Abrasion, erosion, and failed components 17. Troubleshooting – Allis-Chalmers, Foster Wheeler, Kennedy Van Saun, Riley Power Inc., and Stein Industrie 18. Predictive Maintenance – Vibration analysis, oil analysis, and condition-based maintenance 19. Preventive Maintenance – Allis-Chalmers, Kennedy Van Saun, Riley Power Inc., Stein Industrie, inspection criteria, and inspection tasks 20. Preventive Maintenance Basis Component Module – Background, failure locations, preventive maintenance (PM) tasks, and PM templates 21. References Appendix A – Key Points Summary The words mill and pulverizer are used interchangeably in this guide.
1.4
Key Points
Throughout this guide, key information is summarized in key points. Key points are bold lettered boxes that highlight information covered in the text. The primary intent of a key point 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 key points. The key points are organized into three categories: Human Performance, O&M Costs, and Technical. Each category has an identifying icon to draw attention to it when quickly reviewing the guide. The key points are shown in the following way: Human Performance Key Point Denotes information that requires personnel action or consideration in order to prevent personal injury, equipment damage, and/or improve the efficiency and effectiveness of the task. O&M Cost Key Point Emphasizes information that will result in overall reduced costs and/or an increase in revenue through additional or restored energy production.
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Introduction
Technical Key Point Targets information that will lead to improved equipment reliability. The Key Points Summary section (Appendix A) 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 this guide can determine if they have taken advantage of key information that the writers of this guide believe would benefit their plants.
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2 GLOSSARY
AGMA – American Gear Manufacturer Association. balls – Balls are composed of hardened carbon steel, high-chrome steel, or forged chromemolybdenum and are 3/4–2 1/2 inches in diameter. The balls are also called cylpebs. ball mill – A ball mill is a low-speed mill (10–20 rpm) that uses the grinding action of steel balls in a rotating horizontal cylinder or drum. If the diameter of the drum is greater than the length of the drum, then the mill is called a ball mill. barr – Barr means to rotate or turn the mill. base capacity – Base capacity is the amount of coal the mill will process using a coal with a grindability index of 50 and a final product fineness of 70% passing through a 200-mesh screen. bituminous coal – Bituminous coals are the largest group of coals available. The name of bituminous is derived from the fact that when heated, the coal is reduced to a cohesive, binding, sticky mass. The volatile matter is complex and high in heating value. These coals burn easily in pulverized form. Bituminous coals can be further classified as high-volatile, medium-volatile and low-volatile coals. Bituminous coals are typically composed of 65% carbon, 32% volatiles, and 3% water. capacity – Capacity is the measured output of the pulverizer in pounds of coal per hour. Cardwell lubrication system – For the Foster Wheeler ball mills, the Cardwell lubrication system supplies lubricating oil to the trunnion tube journal bearings. classifier (dynamic) – The dynamic classifier is a rotating vane assembly for the separation of coal particles. classifier (scroll) – The scroll classifier directs the coal and airflow upward, over, and down a curved plate surface. The heavier particles drop down and do not exit the classifier. coal mill – A coal mill is a machine that reduces the size of coal particles, dries the coal, classifies the coal, and transports the coal to the boiler piping. A coal mill is also called a coal pulverizer.
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Glossary
coal pulverizer – A coal pulverizer is a machine that reduces the size of coal particles, dries the coal, classifies the coal, and transports the coal to the boiler piping. A coal pulverizer is also called a coal mill. conveyor – The conveyor assembly admits the primary air and coal flow into the drum. The conveyor assembly resides in the trunnion tube and is connected to the drum. It consists of an inlet air screen, an air tube, a flight ribbon with chains, and spokes. cylpebs – Cylpebs are another name for the balls in a ball/tube mill. crusher-dryer – The crusher-dryer is equipment used to reduce the coal particles by hammer action and to provide drying before the coal material enters the rotating drum. double-ended mill – A double-ended mill is a ball mill that admits coal into the drum at both ends of the drum. drum – The drum is a rotating steel cylinder where the balls crush the coal. exhauster – For the Foster Wheeler ball mills, the exhauster is a six-blade paddle wheel located at the discharge of the ball mill. Farval lubrication system – For the Foster Wheeler ball mills, the Farval lubrication system supplies oil to the pinion and bull gear that turn the drum. feeder – A feeder supplies coal at a metered rate to the pulverizer. Feeders can be gravimetric or volumetric in design. fineness – Fineness is the measured particulate size distribution of pulverized product as determined by standard screens. A standard fineness is 70% passing through a 200-mesh screen. fires – Fires consist of the active and ongoing combustion of coal and/or debris in the pulverizer. flight ribbon – In the Foster Wheeler ball mills, the flight ribbon is a spiral metal ribbon with chains attached to the outside of the air tube. As the drum rotates, the flight ribbon also rotates and coal is fed into the drum. gravimetric feeder – The gravimetric feeder weighs material on a length of belt between two fixed rollers located in the feeder body. The gravimetric feeder compensates for variations in bulk density due to moisture, coal size, and other factors. The gravimetric feeder provides a more precise weight flow rate of coal to the pulverizers than the volumetric feeder. grindability – Grindability is a measure of the ease that a coal can be pulverized when compared with other coals. The higher grindability index indicates a coal that is easier to grind.
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Glossary
Hardgrove grindability – The Hardgrove grindability index is based on the use of the Hardgrove grindability machine. Grindability is determined by the amount of new material that will pass through a 200-mesh screen. ignition support – Ignition support is the addition of supplemental oil or gas for startup and lowload stabilization of the fire in the boiler. inerting substance – An inerting substance is deficient in active properties. The substance lacks the usual or anticipated chemical or biological action. For fire fighting, the inerting substance can be carbon dioxide, water, or steam. inertant – An inertant substance is noncombustible, nonreactive, and incapable of supporting burning with the contents of the system being protected. lignite – Lignite coal is brown in color, has a laminar structure, and remnants of woody fibers may be apparent. They are high in volatile matter and moisture content, but are low in heating value. Lignite coal typically contains 38% carbon, 19% volatiles, and 43% water. loss on ignition (LOI) – Loss on ignition is the weight percentage of combustibles in fly ash based on American Society for Testing and Materials (ASTM) test standard D-3174-04, “Standard Test Method for Ash in the Analysis Sample of Coal and Coke from Coal.” LOI is a measure of all components (including carbon) that are volatized by the heat applied during the ASTM test procedure. lubrication system – The lubrication system for the gearbox is external to the pulverizer and performs the pumping, filtering, heating, and cooling of the oil for the gearbox. moisture – Moisture is the amount of water retained by the coal and is expressed as a percentage of a coal sample’s weight. Moisture reduces the mill capacity since it takes time for the hot air to dry the coal for grinding. Mw – Mw is the abbreviation for the unit of power called megawatts or 106 watts. NOx – NOx is an abbreviation for all combinations of nitrogen and oxygen. Typically, NOx as a combustion product in a power plant is 90% NO and 10% NO2. PF – PF is the abbreviation for pulverized fuel. PRB – PRB is the abbreviation for Powder River Basin coal. primary air – Primary air is the air required for the drying and transport of the pulverized coal through the pulverizer and into the boiler. pyrite – Pyrite can mean any material that is rejected from the mill. Pyrites are actually a compound of iron and sulfur, FeS2, found in coal.
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Glossary
seal air system – The seal air system supplies sealing air to the trunnion bearings in order to prevent contaminants from entering the bearing area. single-ended mill – A single-ended mill is a ball mill that admits coal into the drum from one end only. spokes – The spokes center and support the conveyor assembly inside the trunnion tubes. sub-bituminous coal – Sub-bituminous coals are brownish black or black, and are typically composed of 45% carbon, 25% volatiles, and 30% water. PRB coal is a sub-bituminous coal. tramp iron – Tramp iron is any metal that enters the pulverizer with the coal, such as nuts, bolts, scrap steel, tools, and so on. trickle valve – In the Riley Power ball mills, a trickle valve is used to keep the flow of rejected coal particles and air flowing from the classifier to the ball mill. trunnion tube – A trunnion tube is an integral part of the drum end castings. The drum is supported through the bearings that support the trunnion tubes. tube mill – A tube mill is a low-speed mill (10–20 rpm) that uses the grinding action of steel balls in a rotating horizontal cylinder or drum. If the diameter of the drum is less than the length of the drum, then the mill is called a tube mill. vertical spindle mill – Vertical spindle mills are medium-speed pulverizer mills with a vertical shaft that turns the grinding table. Vertical spindle mill designs include bowl mills, ring roll mills, and ring and ball mills. volumetric feeder – Volumetric feeders deliver coal at a uniform controlled rate based on volume. Some examples of volumetric feeders are drag, table, pocket, apron, and belt.
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3 SYSTEM APPLICATION
In a coal-fired power plant, the fuel handling system consists of the following functions: •
Delivering the coal
•
Unloading the coal
•
Weighing the coal
•
Initial crushing of the coal
•
Conveying the coal to an active pile and/or into the plant
•
Unloading the coal into bunkers or silos for each unit
•
Metering feeders and the control of the coal into the coal mills
•
Moving the pulverized coal and primary air for entry into the boiler
The fuel handling system can be divided into two systems: the coal handling system and the coal pulverizing system. The coal handling system, general coal characteristics, coal pulverizer mills, and environmental regulations are included in this section.
3.1
Coal Handling System
In a coal-fired power plant, the coal handling system provides the following functions: •
Unloads the coal from railroad cars, dump trucks, barges, ships, and so on
•
Weighs the coal being received into the plant
•
Transports (typically by conveyor belts) the coal from the unloading site to the crushing equipment, to an active coal pile or inside the plant, to bunkers or silos, and then to the coal feeders
• Crushes the coal so it can be moved by a conveyor system into the plant. The equipment used to crush the coal may be located before or after the coal goes to the active coal pile and before the coal is moved into the plant. •
Separates tramp iron from the incoming coal
• Stores coal in bunkers or silos to provide an adequate supply of coal to the plant if a malfunction of the coal handling equipment should occur. The bunkers are sized to store a 12–24 hour or more supply of coal.
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System Application
Figures 3-1 and 3-2 show typical one-line diagrams of the coal handling system.
Figure 3-1 A Typical Coal Handling System Diagram from Unloading to the Plant (Courtesy of SCANA McMeekin Station Units 1 and 2)
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System Application
Figure 3-2 A Typical Coal Handling System Diagram from Plant to Unit Bunkers (Courtesy of SCANA McMeekin Station Units 1 and 2)
For stations with railroad delivery of coal, the railroad cars are capable of holding 70–110 tons of coal. It is necessary to weigh the coal in each railroad car. This can be done using electronic scales on the track to weigh the car full, weigh the car empty, and then subtract to find the weight of the unloaded coal. Also, the coal can be weighed on a scale below the unloading area grating or on belt scales along the conveyor.
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System Application
A locomotive or mechanical puller is used to position the cars directly over the unloading hoppers. The cars can be unloaded from the bottom doors with car shakers to loosen the coal
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System Application
from the cars. The cars can also be turned upside down in a rotary dumper. From the unloading hoppers, the coal is transported to the crushers, where the coal is broken into smaller, finer particles. Coal sampling equipment is positioned near the conveyor belt to remove samples of uncrushed coal for testing. Typically, crushers are motor-driven equipment that use rolling rings or ring hammers to reduce the chunks of coal to less than 1-in. pieces. The crushed coal is then placed on a series of conveyor belts. They are propelled by a drum that is belt driven from a speed reducer gearbox and a motor. The belt rests on idlers that are evenly spaced under the belt. These belts transport the coal to the active storage pile. Coal is temporarily stored on an active storage pile before being transported into the plant. Coal from the active storage pile gravitates into the active storage reclaim hopper. A vibrator feeder or variable-speed rotary feeder is located at the discharge of the hopper. Variable-speed rotary feeders provide improved control of the feed rate. The coal falls onto the conveyor and is transported into the plant. For economic or environmental reasons, many plants burn more than one type of coal. Coals with various sulfur contents or other characteristics may be blended to achieve the most effective mixture. Each type of coal is stored in separate piles and reclaimed at specific rates in order to achieve the desired blend. Reclaim conveyors under each pile release the coal onto the main conveyor that transports the coal into the plant. In the plant, the coal travels beneath a magnetic separator. This device pulls out any metal material that can be attracted by a magnet, such as iron or steel. The transfer conveyor then unloads the coal onto a conveyor with a movable tripper device. The tripper device is positioned over each silo or bunker. Some plants use a cascade system of conveyors instead of a moveable tripper. The coal then flows to a coal silo (circular shape with conical outlet) or a coal bunker (rectangular shape with a pyramidal outlet). The outlet from the silo or bunker is usually equipped with a fully enclosed slide gate. The slide gate can be manually operated or motor operated. There is usually one silo or bunker for each feeder and one feeder for each pulverizer mill. The coal moves through the silo or bunker, through the feeder, and then enters the pulverizer. Because of the strict regulations concerning fugitive dust emissions and the explosive nature of coal dust, dust control is required on the coal handling system. The dust control systems may inject a water/chemical mixture at different points along the coal path. The dust control system may also use water to cover the surface of the coal on the belt. Other types of dust control include transfer chutes that are designed to direct the coal onto the belt at the same speed and angle of the belt in order to minimize dust creation. Additional control is obtained by using dry fogging systems or dust collectors. Some very dusty coals may require the addition of air-supported conveyors in place of the more conventional idler-supported conveyors. The air-supported conveyors use a cushion of lowpressure air to support the belts and are totally enclosed on the load side to reduce any dust creation.
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System Application
3.2
Coal Characteristics
Coal [1] is classified by the rank or degree of transformation of the original plant material to carbon. The ranks of coal containing the most carbon to the coal containing the least amount of carbon are: •
Anthracite
•
Bituminous
•
Sub-bituminous
•
Lignite
A continuous gradation occurs between these ranks. Anthracite, bituminous, and sub-bituminous coals are known as black coals. Lignite is known as brown coal. In addition to carbon, coals contain hydrogen, oxygen, nitrogen, and sulphur. Low-rank coals contain small amounts of carbon and large amounts of hydrogen and oxygen. High-rank coals contain large amounts of carbon (higher heating value) and small amounts of hydrogen and oxygen. High-rank coals require the finest grinding and are usually ground in ball mills. Anthracite is the highest ranked coal and is composed of low-volatile matter (less than 10%) and high carbon content (~90%). Anthracite has a semi-metallic luster and is capable of burning without smoke. It is used primarily for heating homes and in gas production. The main producers of anthracite include South Africa, China, Vietnam, Germany, and the United Kingdom. Bituminous coals are the largest group of coals available. The word bituminous is derived from the fact that when heated, the coal is reduced to a cohesive, binding, sticky mass. The volatile matter is complex and high in heating value. These coals burn easily in pulverized form. Bituminous coals can be further classified as high-volatile, medium-volatile, and low-volatile coals. The major producers of bituminous coals are China, the United States, India, South Africa, Australia, Russia, Poland, Ukraine, Kazakhstan, Indonesia, Germany, and the United Kingdom. Sub-bituminous coals are dull and dark brown to black in color. The quality of the coal ranges from soft and crumbly to bright, jet black, with a hard, strong texture. Sub-bituminous coals are used for power generation and industrial processes. These coals have typical moisture levels between 10–30% and a carbon content between 71–77%. Lignite coals are dark brown to black in color with low organic maturity. They are high in volatile matter and moisture content (>45%), with low carbon/energy content as compared to high-rank coals such as anthracite. Because of the high moisture content and relatively low calorific value, lignite is usually consumed close to where it is mined. There are seven coal producing regions in the United States: •
Eastern – Pennsylvania, Rhode Island, Virginia, North Carolina, Ohio, Kentucky, West Virginia, Tennessee, and Alabama. This region has the largest deposit of high-grade
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System Application
bituminous and semi-bituminous coals.
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System Application
•
Interior – Mississippi Valley, Illinois, Texas, and Michigan. Bituminous coals with a lower value and a higher sulfur content than the eastern region are found in the interior regions of the United States.
•
Gulf – Alabama, Mississippi, Louisiana, Arkansas, and Texas. The lowest value coals are found in this region. The coals are lignites with moisture content as high as 55% and heating values below 4000 Btu/lb.
•
Northern Great Plains – North Dakota, South Dakota, Wyoming, and Montana. The Dakotas have lignite deposits. Wyoming and Montana have bituminous and sub-bituminous coals.
•
Rocky Mountain – Montana, Wyoming, Utah, Colorado, and New Mexico. Coals in this region range from lignite to sub-bituminous and high-grade bituminous to anthracite.
•
Pacific Coast – Washington, Oregon, and California. The coals in this region range from sub-bituminous to bituminous to anthracite.
•
Alaska – The coal reserves in Alaska are estimated to be 15% bituminous and 85% subbituminous and lignite.
Powder River Basin (PRB) coal is a sub-bituminous coal. The PRB is a 12,000–14,000-ft-deep depression filled with sediments eroded from land uplifted during the formation of the Rocky Mountains. The PRB is located in Montana and Wyoming between the Bighorn Mountains and the Black Hills. PRB coal has an average heating value around 8500 Btu/lb. The most attractive quality of PRB coal is its low sulfur content. With an average of about 0.3% sulfur, most of the coal meets the environmental compliance requirements for utility boilers without scrubbers. Coal from the eastern region of the United States is a high-sulfur bituminous coal. Typical coal costs for PRB coal per ton are about 23% less than eastern bituminous coals. However, it takes about 113 lbs of PRB coal to equal the same energy content of 80 lbs of eastern coal. This means that it takes 30% more PRB coal to equal the energy content of eastern coal. Coal prices were stable for many years in the United States. Recent years have seen market forces changing and the cost of coal increasing. High petroleum and natural gas prices, increased coal exports, the depletion of accessible coal veins in the eastern region of the country, fuel switching, and higher transportation costs are some market forces that are influencing the price of coal. The Clean Air Act Amendments (CAAA) of 1990 have required utilities to reduce sulfur emissions. Methods of compliance include flue gas desulfurization, fuel switching, fuel blending, and emission allowance trading. For the fuel blending, some utilities are blending the PRB coals with the eastern coals to meet air quality requirements. A common area of significant concern with fuel blending is the pulverizer grinding capacity with PRB coal or coal blends. PRB coals typically have a reduced heating value and higher moisture content compared to eastern coals. Because of the higher moisture content in PRB coal, a higher level of mill coal drying is required.
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System Application
Mill grinding capacity requirements depend on the PRB blend ratio, the maximum boiler load required, and the amount of reserve mill capacity desired. For example, a plant may relax its normal requirement of attaining full load with five of six mills in service, as long as full load can be attained using PRB coals with six mills. However, if maintaining full load capacity with five mills in service is required, then mill capacity upgrades may be necessary. Inadequate mill drying capacity will result in lower than normal mill outlet temperatures. This can occur because of higher coal mass flow rates, higher coal moisture content, and capacity limitations of the hot, primary air supply system. Lower acceptable mill outlet temperature requirements for PRB coals may offset the hot, primary air drying requirements to some extent. However, in general practice, an increase in primary airflow has been associated with the use of PRB coals. If the primary airflow requirements are sufficiently high, the velocities in the coal piping can increase significantly and erosion problems can occur. Specific pulverizer-related issues that should be evaluated when burning PRB coals include: • Mill grinding capacity and fineness requirements • Coal drying capacity requirements (primary air/fuel ratio) • Primary air (PA) fan capacity, fan discharge pressure, and air temperature • Feeder discharge pluggage and cleaning practices for PRB coal • Mill fire protection, carbon dioxide or steam inerting, water wash systems, water fogging nozzle installation at the air inlet, and coal dust dampening/removal for explosion prevention to work in conjunction with carbon dioxide inerting system • Mill fire detection system (carbon monoxide detection) • Coal pipe line velocities and potential long-term erosion • Mill outlet temperatures (possible reduction from ~150ºF to approximately 130–135ºF for PRB coals to offset some of the increased primary air requirements) For additional information on coals in the United States, the following EPRI guides can be referenced: • Effects of Coal Quality on Power Plant Performance and Costs, Volumes 1–4 (EPRI report CS-4283) • Coal Quality Information Book, Volumes 1–2 (EPRI report GS-7194)
3.3
Coal Pulverizer Mills
The purpose of a pulverizer mill is to: •
Reduce the coal to small particles by grinding for better combustion
•
Dry the coal
•
Classify the particle size of the coal leaving the mill
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System Application
•
Transport the coal from the classifier to the boiler burners
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System Application
There are several styles of pulverizer mills. They are: •
Ball/tube mills – Ball/tube mills are low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. If the diameter of the cylinder is greater than the length of the cylinder, then the mill is called a ball mill. If the length of the cylinder is greater than the diameter of the cylinder, then the mill is called a tube mill. For the mills covered in this guide, the terms ball and tube are used interchangeably.
•
Vertical spindle mills – Vertical spindle mills are medium-speed pulverizer mills with a vertical shaft that turns the grinding table. Vertical spindle mill designs include bowl mills, ring roll mills, and ring and ball mills.
•
Impact mills – Impact mills are high-speed impact machines that use beater wheels to crush the coal.
The mills covered in this report are ball/tube mill designs. For the purpose of this guide, the ball/tube will be called ball mills or the manufacturer designation. Some of the manufacturers of the ball mills are Allis-Chalmers, Foster Wheeler, Kennedy Van Saun, Riley Power Inc., and Stein Industrie. Figure 3-3 is a diagram of the coal pulverizer system for the Foster Wheeler ball mill.
Figure 3-3 Foster Wheeler Ball Mill [2]
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Coal is fed from a bunker or silo to a coal feeder. A feeder supplies and meters the coal going to the pulverizer mill. One or more feeders are provided for each pulverizer. The feeders can be volumetric or gravimetric designed and are typically driven by induction motors. Coal flows from the feeder through a spiral ribbon conveyor mounted on the outside of the drum trunnion tube. Coal can be fed through one (single-ended) or both (double-ended) ends of the drum. Hot air is supplied from the boiler secondary air system through a forced draft or primary air fan and flows into the drum through the inside diameter of the trunnion tube. The drum is a large cylinder with integral trunnions bolted to each end of the drum. One journal bearing supports each trunnion. The drum is rotated through the pinion and drive gears coupled to a mill motor. The journal bearings, the pinion, and the drive gears are lubricated by a pumped supply of oil. The alignment of the journal bearings, the pinion, and the drive gears is critical for reliable operation. The drum contains steel balls and rotates slowly (10–20 rpm). The rotation causes the balls to cascade over each other and crush the coal between the balls. The drum has steel liners that are made of a harder material than that of the steel balls. Hot air flows out of the drum and dries the incoming coal. After the coal is ground, the coal flows into the classifier that allows smaller size particles to pass through and larger size particles to return to the drum. The pulverized coal and air mixture flows from the mill outlet to the boiler burners. The earlier designed mills are operated in negative pressure with an exhauster fan located at the discharge of the classifier. The coal and air particles flow through the exhauster fan and into the boiler piping. The newer designed ball mills are operated in positive pressure supplied by a primary air fan located upstream of the mill.
3.4
Environmental Regulations
The CAAA of 1990 established lower NOx emission rates for utility boilers [3]. Because NOx formation is largely dependent on how the fuel is combusted, the efforts to reduce NOx emissions have focused on modifying the combustion process. NOx includes NO, NO2, and N2O formations during combustion. Three primary sources for the formation of NOx are: • Thermal NO – Thermal NO is the oxidation of molecular nitrogen (N2) to form NO. The triple-bonded N2 requires significant energy for oxidative attack and occurs only at high temperatures. Thermal NO accounts for approximately 20–30% of the final NOx emissions. •
Prompt NO – Prompt NO describes the hydrocarbon radical attack of N2 to form-fixed nitrogen compounds (NHx, XCN) that can subsequently react to form NO. Prompt NO
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accounts for approximately 5–10% of the final NOx emissions.
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•
Fuel NO – Fuel NO is the oxidation of fuel-bound nitrogen in the coal to NOx compounds. Typically, fuel-bound nitrogen will evolve as an amine or cyano compound and will be oxidized to NO or reduced to N2. Fuel NO accounts for approximately 60–70% of the final NOx emissions.
Figure 3-4 shows how the fuel-bound nitrogen evolves to form either NOx pollutants or nitrogen gas. Char is the combustible residue remaining after the destructive distillation of coal.
Figure 3-4 Fuel-Bound Nitrogen Evolution to NOx [4]
The amount of NOx formed when coal burns is a function of the nitrogen content of the coal, the flame temperature, the amount and distribution of air during combustion, and the flame structure. Three technologies used for reducing the NOx formed are low-NOx burners, the addition of overfire air, and selective catalytic reductions (SCRs). The addition of SCRs involves adding a catalyst bed in the boiler flue gas that converts the NOx leaving the boiler. Low-NOx burners control the mixture of fuel and air to create larger and more branched flames, reduce peak flame temperatures, and lower the amount of NOx formed. The improved flame structure also improves burner efficiency by reducing the amount of oxygen available in the hottest part of the flame. In principle, there are three activities in a conventional low-NOx burner: combustion, reduction, and burnout. In the first stage, the combustion occurs in a fuel-rich, oxygen-deficient zone where the NOx is formed. In the reduction stage, hydrocarbons are formed and react with the already
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System Application
formed NOx. In the burnout stage, internal air staging completes the combustion. Additional NO x
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is formed in the burnout stage; however, the additional NOx can be minimized by an air-lean environment. Low-NOx burners can be combined with overfire air technologies that create two stages for combustion. This requires a primary and a secondary source of combustion air. The secondary air nozzles are located above the burners. This system results in a more complete burnout of the fuel and the formation of N2 rather than NOx. The operation of low-NOx burners tends to increase the unburned carbon in the ash. Unburned carbon can occur in both the bottom ash and fly ash. Loss on ignition (LOI) is a weight percentage of all the components (including carbon) in fly ash. With NOx control, the LOI for tangentially fired furnaces increases an average of 2% and for a wall-fired furnace the LOI increases 3–5%. O&M Cost Key Point The increases in LOI from NOx combustion control the increase in the heat rate. The average industry loss is 12 Btu/kWh per 1% change in unburned carbon. This increase in LOI creates a need for greater fineness. Some units have increased fineness from 70% passing through a 200-mesh screen to 75–80% passing through a 200-mesh screen and 99–99.5% passing through a 50-mesh screen. The increase in fineness settings requires more work from the pulverizer. In other words, the increase of LOI in the boiler increases the heat rate for the unit. In order to offset the heat rate increase, the mill is required to perform more work. Performing more work for the given amount and type of coal can increase the maintenance costs for the mill.
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4
TECHNICAL DESCRIPTION – GENERAL
The ball/tube mill [3] is designed for hard, highly abrasive coal materials and for some very soft coals. Ball mills are used widely in countries where high-ash, abrasive coals are available, such as China, India, and South Africa. Typical availabilities of the ball mills in the 95% range are common. In the technical description, Sections 5–9, there are five manufacturers of ball/tube mills that are covered. The manufacturers are: •
Allis-Chalmers
•
Foster Wheeler
•
Kennedy Van Saun
•
Riley Power Inc.
•
Stein Industrie
In general, the main components of the ball/tube mill are: •
Feeder – The feeder provides the raw coal in a measured amount.
• Crusher/Dryer – In one style mill, a hammer-type crusher and hot air dryer are used to reduce the coal size and dry the coal before it enters the drum. • Balls – Steel balls ranging in size from 1.2–2.4 in. (30–60 mm) are located in the drum. As the mill is rotated, the balls crush the coal in the drum. • Drum – The rotating tube or drum ranges in size up to 33 ft (10 m) long and 20 ft (6 m) in diameter. • Liners – Liners are steel plates fastened to the inside of the drum to form a replaceable, protective layer for the drum. • Classifier – The classifier regulates the fineness of the pulverized coal leaving the mill and entering the piping to the boiler. • Mill motor – The mill motor is the electrical motor that connects to the drive system that turns the drum. • Drive system – The drive system connects to the motor and consists of a reduction gearbox and clutch that drives the pinion and girth gear in order to rotate the drum.
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Technical Description – General
Technical Key Point If the diameter of the drum is greater than the length of the drum, then the mill is called a ball mill. If the length of the drum is greater than the diameter of the drum, then the mill is called a tube mill. The terms ball and tube are used interchangeably in this guide. Ball mills have a large footprint in a power plant. Because of the large rotating drum and the weight of the steel balls, there is a large dynamic loading that requires extensive mill foundations. The noise level for the ball mill is relatively high. An acoustic cover is sometimes required over the mill in order to reduce the noise to an acceptable level. The power requirement for turning the drum is high, with most of the power used for lifting the ball weight. The power requirement is essentially constant as the mill loading is decreased. The power requirement for a ball mill is about twice the requirement for that of a vertical spindle mill. For this reason, the ball mill is regarded as the least efficient type of mill with respect to grinding. Ball mills contain large quantities of coal that can be released to the boiler piping if a sudden increase of coal is needed. This stored quantity is not affected by a temporary coal feed blockage. However, the large quantity of coal available is a disadvantage if the coal contains a fire. For this reason, the airflow is limited for this type of mill. The limited airflow makes this mill unsuitable for high-moisture coals. The time needed to start a ball mill and place the mill in service is in the 30-minute range. For comparison, the vertical spindle mill require about 5–7 minutes to place a mill in service. For a ball mill operation, the grinding action is ball-against-ball with a cushioning layer of coal between the balls. The tumbling, grinding action of the balls is generated by the rotation of the drum. As the mill speed increases, the grinding action increases until a critical rotational speed is reached. At the critical rotational speed, the balls are held against the drum by centrifugal force and the grinding stops. Mills are usually operated in the range of 65–80% of the critical rotational speed. The grinding process of the coal in the drum begins with the larger diameter balls impacting and crushing the coal. The smaller balls do the fine grinding by rubbing the coal between the balls and the liner. Initially, the balls were made of forged, high-carbon steel, heat-treated to a Brinell hardness of 550–560. Many of the balls now consist of a high-chrome material. The rate of wear on the balls depends on the abrasiveness of the coal. As the balls wear, new balls are added to maintain grinding efficiency.
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Technical Description – General
The drum liners require replacement after an interval of 20,000–30,000 operating hours. Replacing the drum liners requires an extended outage. To keep the drum liner from wearing, the liner material should be harder than the ball material. Using a softer ball material causes a higher ball wear rate. There is no provision for removing tramp iron or other foreign material after it has entered the drum. Ball mills cannot be operated for lengthy times without coal in the mill. The rotation of the cylinder creates heat in the ball charge, and the small amount of coal in the cylinder can ignite. When the mills are shut down with coal still in them, provisions are made for the mill to be turned slowly or intermittently to cool the coal. Sections 5–9 contain a technical description of the following manufacturers’ ball mills: • Allis-Chalmers • Foster Wheeler • Kennedy Van Saun • Riley Power Inc. • Stein Industrie
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5
TECHNICAL DESCRIPTION – ALLIS-CHALMERS
The information in this section was provided by the Eskom Lethabo Power Station in South Africa. The station has six units with six mills per unit for a total of 36 mills at the station. The unit layout has three mills located on each side of the boiler. Figure 5-1 shows the outline of the Allis-Chalmers ball/tube mill.
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Technical Description – Allis-Chalmers
Figure 5-1 Allis-Chalmers Ball/Tube Mill Outline (Courtesy of Lethabo Power Station)
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Each tube mill is supported by trunnion bearings. The mill is rotated by the mill drive motor via the reduction gear unit, pinion shaft, and girth gear. A barring drive is provided that can be coupled to the mill drive motor when required. The coal is supplied to the ball/tube mill from two stock volumetric coal feeders that discharge coal through raw coal chutes into the mill inlet ductwork. The ball/tube mill is hot-air swept and arranged for double-ended operation. At each end of the ball/tube mill is an inlet box that allows the coal and heated primary air to pass into the mill shell through the trunnion throats. The double-ended operation causes the air stream within the mill to circulate as if a dividing wall were present inside the rotating shell. The grinding of the coal is achieved by the rotational movement of the mill shell that causes a charge of hardened steel balls to rise up and cascade down on the coal and pulverize it. The charge of steel balls is fed into the mill through one of the inlet boxes and conveyed to the classifiers above the mill ends by the heated primary air. The mill inlet and outlet boxes are contained in one housing that also contains the mill sealing arrangement. The pulverized coal fineness is achieved by the static classifiers that reject the oversize particles. Under design load conditions with an established ball charge and coal level, the average throughput of five mills in operation is 19.62 kg/second per mill. Of this throughput, 99% of the pulverized coal will pass through a 300-micron screen and a minimum of 70% will pass through a 75-micron screen. The pulverized coal is contained within the mill by a seal air system consisting of one 100% capacity seal-air fan and ductwork supplying pressure at 17.7 kPa. The seal air system maintains a seal at the mill head extension. Seal air is also piped to the coal feeders in order to prevent hot air from escaping into the bunker supplying coal to the feeder. A separate seal air system is provided for the girth gear in order to prevent any entrance of pulverized coal into the gearing. The level of coal in the mill is controlled by a Power-Sonic mill conditioning system. The system uses measurements of the mill motor power drawn and the noise produced by the grinding process in order to generate a trimming signal for the coal feeder speed controllers. The noise increment is sensed by a transducer located on the mill acoustic housing. A dip tube system is provided as a standby to the mill conditioning system. The dip tube system uses a differential pressure measurement from the probes fitted on the inlet/outlet boxes to control the mill coal level. Changeover from the mill conditioning system to the dip tube system is a manual operation. Equipment for the Allis-Chalmers mills is discussed in this section. Also, a technical specification list for the equipment is included. The equipment consists of: • Inlet/outlet boxes • Rotating shell, liners, and balls • Trunnion bearings
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Technical Description – Allis-Chalmers
• Classifier
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•
Drive motor
•
Gearbox unit
•
Power-Sonic mill conditioning system
5.1
Inlet/Outlet Boxes
Figure 5-2 shows the inlet/outlet box.
Figure 5-2 Inlet/Outlet Box
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Technical Description – Allis-Chalmers (Courtesy of Lethabo Power Station)
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Technical Description – Allis-Chalmers
Integral inlet/outlet boxes are located at each end of the mill and provide the following functions: • An inlet for the coal and primary air into the mill • An outlet for the pulverized coal to the classifiers • An inlet for ball addition • A housing for the bypass damper for the primary air bypass • A sealing interface for the pulverized coal between the rotating and fixed structure • A mounting point for the dip tube system • An access point for entry into the mill The inlet/outlet boxes are a steel fabrication with selected surfaces fitted with replaceable liners. A division plate extends from the center line of the fabrication and carries the dip tubes. The division plate pulverized coal side and the outlet box casing are tiled using 6-mm and 12-mm high alumina content ceramic tiles, respectively. A hinged access door is located on the underside of the inlet section. A yoke clamping arrangement is provided to ensure a good seal. The 200-mm ball charge pipe from the ball charge hopper is fed into the upper section of the inlet side of the box. A coarse rejects return pipe from the classifier is also connected to the inlet side of the box. This return pipe is equipped with an integral trickle valve that allows the return of oversized coal particles to the mill inlet while preventing the backflow of primary air. A bypass damper is provided between the inlet and outlet side of each inlet/outlet box. The damper blade is mounted 7° off the vertical line toward the outlet side of the box on a shaft supported by bearings at either end. The bearings are external to the inlet/outlet box. An operating range of 49.5° travel toward the outlet side of the box is provided from the closed position. The drive for each bypass damper is provided by an electrical actuator. The bypass dampers for each pair of inlet/outlet boxes per mill are controlled in parallel as a function of the control system. Technical Key Point The bypass dampers accomplish three functions: • Assist in maintaining the minimum pulverized coal and air velocity in the piping • Assist in increasing the flow of drying, hot air during periods when the coal is wet • Allow the mill to operate at lower loads A sealing arrangement is fitted to the mill side of the inlet/outlet boxes. Sealing is achieved by a pad seal on the inlet/outlet box bearing against a mill head extension. The pad seal maintains its contact with the mill head extension by steel spring fingers. The cavity formed behind the pad seal is supplied with seal air in order to maintain a pressure greater than the interior pressure of
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Technical Description – Allis-Chalmers
the mill.
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Technical Description – Allis-Chalmers
The sealing arrangement is provided with three spare pad seals that are located on extended securing bolts and protected by a pad seal shroud. The spare location permits the fitting of a new pad seal without removing the inlet/outlet box from the end of the mill. Figure 5-3 shows the sealing arrangement.
Figure 5-3 Mill Sealing Arrangement (Courtesy of Lethabo Power Station)
The dip tube system provides a standby level measuring system for the mill conditioning system. The dip tube system consists of two lengths of 15-mm piping mounted horizontally on the division plate with one pipe above the other. The mill end of the two pipes is open ended. A removable end extension is provided for the lower-pipe probe to facilitate the removal of the inlet/outlet box for mill maintenance. The removable end is designed for easy replacement and has a thick wall for extra wear life. The other end of the two pipes is connected through isolating valves to a pressure differential transmitter.
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Technical Description – Allis-Chalmers
The pipes are located on the inlet side of the division plate, and a half-tube section cover is welded over them for protection. A continuous supply of purge air from the station instrument air supply is passed through the pipes to prevent blockage.
5.2
Rotating Shell, Liners, and Balls
The mill shell is a fabricated steel cylinder supported by trunnion bearings at each end in order to allow rotation of the mill in a horizontal plane. The mill is driven by a girth gear that is bolted to the driving end of the mill shell. The shell liners perform two functions. The first function is to protect the mill shell from wear. The second function is to raise the grinding medium to a high angle in order to assist in the grinding action. In order to achieve the second function, the liners are contoured to prevent slippage of the grinding medium. There are two types of liners, a single-hole liner and double-hole liner. Single-hole liners are provided for the location at the end of each alternate row to offset the gap pattern. The liners are secured through the mill shell using a full nut and locknut. The sealing between the full nut face and the shell is achieved using a silicon washer seated in a retainer cup. Liners are manufactured from high-chrome iron material or Roq-Last. The trunnion ends of the mill shell are equipped with liners that are similarly secured to the liners in the shell. A filler ring is inserted between the trunnion end liner and the shell liner. Sealing between the trunnion throat liner and trunnion end liner is achieved using three asbestos/graphite packing rings. Trunnion throat and trunnion end liners are designed to protect the trunnion ends and should be replaced when worn. Technical Key Point As the liner contour wears, the grinding medium begins to slip. This slippage leads to a lowering of efficiency in the grinding action because the grinding medium cannot be raised to a high angle. As slippage continues, the wear will accelerate. Replacement of liners is recommended when the minimum thickness is reached. The rate of wear of the liners can be predicted based on the grade of coal being ground and the experience gained from inspections. Whenever the mill is opened for inspection, the liners should be examined for wear and breakage. It is important that broken liners be replaced immediately. The impact of the ball charge and the coal being ground tends to loosen the liner bolts. Loose bolts allow the coal to work between the liner and the shell, making bolt tightening impossible. Looseness also causes impacting and breakage of bolts. A hole (normally sealed with a blanking plate) is provided in the shell. The hole is provided in order to remove the ball charge and coal from the mill if there is an emergency shutdown
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Technical Description – Allis-Chalmers
followed by a long outage. Figure 5-4 shows the ball charge hopper.
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Technical Description – Allis-Chalmers
Figure 5-4 Ball Charge Hopper (Courtesy of Lethabo Power Station)
One ball charge hopper per mill is provided for charging the mill with the required quantity of 50-mm steel balls (cylpebs). Each hopper consists of an upper and lower section. The upper
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Technical Description – Allis-Chalmers
section is capable of storing 1 metric ton of steel balls. A top loading cover is fitted to the upper
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Technical Description – Allis-Chalmers
section and secured by three swing bolts. A worm gear, operated by an extended spindle, is used to open the lower charging door. An access door, located in the lower section, facilitates inspection of the lower charging door seal. It is important that the lower charging door seal be maintained in good condition so that there is no leakage of pulverized coal. Because of the grinding action of the balls, ball replacement is required frequently. Ball charging can be carried out during milling operation. The replenishment ball charge is provided in 850-kg barrel loads. Special trolleys are provided for transporting the barrels. When released, the ball charge drops rapidly through the 200-mm bore piping, through the inlet/outlet box, and into the mill.
5.3
Trunnion Bearings
The mill trunnion bearings are journal bearings that support the shell on both sides of the trunnion. The mill trunnion bearings are fitted with babbitted rocker inserts. Cooling is achieved by passing 0.46 liters/second of cooling water at 310 kPa pressure through the coils in the liner inserts. The bearings are designed for an oil-wedge principle of operation. The two mill trunnion bearings are provided with an independent, self-contained lubrication system. These systems are identical and ensure that the mill trunnions are floated on a film of oil and are adequately lubricated during mill operations. In oil-wedge bearings, the journal is stationary and comes in contact with a section of the bearing surface. A wedge clearance exists on either side of the contact area. Before rotation of the drum, high-pressure oil is used to raise the rotor journal clear of the bearing surface. The high-pressure oil is introduced through an oil opening in the bottom center of the bearing insert. As the journal starts to rotate, oil is drawn into the clearance to form an oil wedge that is forced under the journal. As the rotation continues, the journal becomes completely supported on a film of oil. The high-pressure lubrication is then stopped. To provide an oil wedge in the mill trunnion bearings, an oil bucket arrangement is used with a combined high- and low-pressure lubrication system. The oil bucket arrangement consists of four buckets spaced at 90° intervals around the journal. As the mill journal rotates, the oil buckets scoop up oil from the sump in the trunnion housing and discharge it onto the journal surface. This action helps to maintain the wedge of lubricant between the journal and the bearing surface. The oil buckets do not provide adequate lubrication alone for the trunnion bearings. This is provided by the low-pressure lubrication system. Figure 5-5 shows the low-pressure lubrication system.
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Technical Description – Allis-Chalmers
Figure 5-5 Trunnion Bearing Low-Pressure Lubrication System (Courtesy of Lethabo Power Station)
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Technical Description – Allis-Chalmers
Each low-pressure oil system consists of an electric heater, pump, filters, and a water-cooled oil cooler. The low-pressure oil system provides a continuous flow of filtered and temperaturecontrolled oil through a distribution manifold onto the trunnion bearings. Under normal operation, the low-pressure oil system also provides the oil supply for the high-pressure jacking oil pump. The oil temperature is maintained at 30°C by an in-line electric heater and a water-cooled oil cooler. If the oil temperature falls below 30°C, the mill start control is inhibited. The lowpressure pump delivers oil at 680 kPa to one of two filter paths through a three-way valve. Differential pressure across the filters is monitored, and a warning signal is initiated if an excessive differential exists. Check valves on the outlet side of the filters are fitted in order to prevent a backflow in the filter that is not being used. From the filter, the oil passes through a water-cooled oil cooler. In the event of a reduction in the flow rate or a pressure build-up in the oil cooler, the flow switch initiates a signal to the mill control that automatically shuts down the mill. From the oil cooler, the low-pressure oil divides into two paths. One path passes oil to the supply side of the high-pressure pump, and the other path passes oil to the bearing housing. At the bearing housing, the oil passes through a distribution manifold inside the top of the bearing cap, over the bearing journal, and into the bearing sump. The low-pressure pump is a twin-cam rotor and is driven by a flange-mounted electric motor. The pump delivers oil at 0.38 liters/second to the trunnion bearing lubrication system. The pump consists of a rotor with two cams phased at 90° and supported at each end by roller bearings. In addition, a rolling element bearing is fitted at the non-drive end to locate the rotor within the pump body. A rocker arm, center plate, and a set of check valves are fitted to provide a pressurizing non-return pumping operation. Sealing is achieved at the non-drive end by an Oring and an end cap. Sealing is achieved at the drive end by an oil seal. The rotor extends from the drive end of the pump body and is keyed to the drive coupling. Each filter unit is flange mounted and designed to permit a continuous flow of filtered (75-µm) oil. The filter element is paper, and sealing is achieved using O-rings. In the event of filter blockage, a visual color-coded indicator operates automatically, and a full-flow bypass opens at 17.3 kPa differential pressure. The oil is cooled by a water-cooled, four-pass oil cooler. The cooler is mounted on a common bedplate with the high- and low-pressure oil system components. The tube bundle consists of a number of copper-nickel, 6.3-mm diameter tubes fixed at 57-mm intervals by segmented brass baffle plates. The tube bundle is housed in a brass shell. This arrangement provides 4.98 m2 of outside cooling tube surface area. The water connections are made through an end bonnet bolted to the end hub. Sealing is achieved using an asbestos-neoprene bonded gasket. At the opposite end of the cooler, sacrificial
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Technical Description – Allis-Chalmers
anodes are fitted to protect the cooling tubes.
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Technical Description – Allis-Chalmers
A high-pressure oil system consists of a high-pressure jacking pump and a manual, standby highpressure jacking pump. The high-pressure oil system provides the initial trunnion lift through a supply oil hole in the bottom bearing insert. The lift of the trunnion is critical during startup, inching (slow shell rotation), and shutdown. The high-pressure jacking pump takes suction from the low-pressure oil system. A low-pressure bypass line is provided in order to maintain the oil supply in the event of a failure in the lowpressure oil system. A manual, high-pressure jacking pump is connected into the high-pressure oil discharge line. The manual pump is provided for use when the mill is being inched or there is a failure in the highpressure oil system. The manual pump is isolated from the oil supply line by a non-return check valve. The high-pressure jacking oil pump is a gear-type pump driven by a flange-mounted electric motor. The advantages of using a gear-type pump is low noise operation, large suction, and pressure zones resulting in small filling and displacement fluid velocities. The manual, high-pressure jacking pump (shown in Figure 5-6) is mounted on the side of the mill trunnion bearing.
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Technical Description – Allis-Chalmers
Figure 5-6 Manual High-Pressure Jacking Pump (Courtesy of Lethabo Power Station)
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Technical Description – Allis-Chalmers
The pump is provided as an emergency facility in the event of a failure in the high-pressure oil system. The pump is a reciprocating piston type operated by oscillating the handle through an arc of approximately 40°. This oscillating movement drives a gear segment that is meshed with a rack on the pump piston, causing the piston to reciprocate. The pump piston is double-acting in the bore. The discharge per stroke of the pump is 7.5 ml. When in operation, the pump draws oil from the integral 2.8 liter reservoir. The reservoir is fitted with a fine mesh filter screen. It is important that an external fresh supply of clean oil is available when using the pump in order to maintain the level in the oil reservoir. Two-piece piston ring seals are fitted to prevent oil leakage from the trunnions. The piston rings are grease-lubricated and connected by piping to a charging point on the side of the trunnion housing. The trunnion housing is used as a mounting point for an emergency, manual, highpressure pump that is provided for each trunnion. The trunnion bearing temperatures are monitored.
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5.4
Classifier
Figure 5-7 shows the static classifier.
Figure 5-7 Static Classifier (Courtesy of Lethabo Power Station)
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Technical Description – Allis-Chalmers
Each mill is provided with two static classifiers. The classifiers are hung by spring supports and located above the respective inlet/outlet boxes. Each classifier consists of a fabricated outer cone, an inner cone, 16 directional vanes, a segmented discharge chamber, an inverted control cone, and a coarse rejects return pipe leading back to an inlet/outlet box. A flanged top cover is provided to facilitate access to the inside of the classifier for maintenance. The directional vanes are adjustable, and the optimum setting is determined during commissioning by coal fineness tests. Once the directional vanes are set, they should be regarded as fixed vanes. The inverted control cone that controls the flow pattern within and the rate of reject flow from the inner cone is preset and not adjustable. A mixture of primary air and pulverized coal enters the bottom of the classifier between the inner and outer cones. At the top of the inlet section of the classifier, the mixture enters the inner cone through the directional vanes. The oversized particles in the inner chamber are flung outward by centrifugal force. By nature of their weight, the coal particles fall down the slope of the inner cone into the coarse rejects return pipe. At the same time, the mixture of air and fine particles passes upward into the discharge piping. When sufficient coal accumulation has built up in the rejects return pipe, the trickle valve opens. This opening permits the rejects to discharge against the existing differential pressure into the mill through the inlet/outlet box. The rejects are then reground. The differential pressure closes the trickle valve.
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Technical Description – Allis-Chalmers
5.5
Drive Motor
Figure 5-8 shows the mill drive motor.
Figure 5-8 Mill Drive Motor (Courtesy of Lethabo Power Station)
The mill drive motor is a horizontal foot-mounted, induction cage motor fitted with a topmounted air-to-air heat exchanger. The rotor is double-ended to provide an output drive to the mill gear and an input connect point for the mill barring gear. The stator frame is a fabricated steel box construction. The stator core is formed by highpermeability steel laminations pre-built on a mandrel and then welded to steel clamping members to form a core pack. The stator winding high-tension coils are formed from rectangular copper strips that are pre-insulated with polyester enamel and a layer of film-backed mica tape. The main insulation is provided by mica glass taping applied as a number of half-lapped layers. The coils are fitted into open slots in the stator core packs that are then closed with resin-bonded fabric slot wedges. The end windings are securely blocked and braced. The rotor core is built from laminations that are keyed to a steel shaft and clamped between two end plates to form a rigid mass. Impellers, secured to the rotor shaft on the inner side of the bearings, circulate air within the motor frame through the windings. At the non-drive end, the rotor shaft carries an outer impeller that draws in ambient air and directs it through the heat exchanger tubes. The rotor shaft is supported by two external foot-mounted, split-sleeve bearings located at both
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ends of the shaft. The bearing shells are spherically seated in the pedestal housing and bearing
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Technical Description – Allis-Chalmers
cover. The housing is made of cast iron and pinned in order to assist in heat dissipation. The bearing shells are made of steel and lined with white metal. Floating labyrinth seals are fitted, and an external water flinger is provided. Each bearing is self-lubricated by a system that uses a central loose oil ring to pick up oil from the reservoir in the pedestal housing and deposit the oil onto the shaft journal as it rotates. A window on the side of each pedestal housing permits visual checking of the oil level in the reservoir. A provision is made on each of the pedestal housings for fitting thermocouples in order to monitor the temperature of the bearings. The bearings are insulated from the motor frame by insulation packers and sandwiched under the pedestal feet. Insulated jacking blocks are provided at the pedestal feet to assist in the accurate alignment of the pedestals. Anti-condensation heaters are fitted on each side of the stator core. The 714-watt heaters are supplied with 240-volt voltage through a separate terminal box on one exterior side of the motor casing. The motor and bearings are mounted on a sub-base plate that is secured to the floor by eight foundation bolts. Each of the mounting points has a 30-mm thick square pad fitted to the underside that is set into the grouting during installation. The mill drive coupling consists of two steel hubs that are keyed to the gear unit input shaft and the motor shaft. The mill drive motor coupling is a smaller version of the gear unit output coupling. The hubs are connected by a segmented grid spring in order to form a resilient coupling. The grid spring segments fit into grooves machined in the peripheries of the hubs. The assembly is packed with grease and enclosed in two half covers that are bolted together. Figure 5-9 shows the barring gear unit.
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Technical Description – Allis-Chalmers
Figure 5-9 Barring Gear Unit (Courtesy of Lethabo Power Station)
The barring gear unit is provided to rotate the tube mill and gear unit for maintenance and inspection purposes. The barring drive is connected through a special coupling to the mill motor shaft that is double-ended. The barring gear consists of a double-reduction gear unit, a motor drive, and an electromagnetic brake on the barring drive motor coupling. All units are mounted on a common bedplate.
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Technical Description – Allis-Chalmers
The double-reduction worm gear unit provides the barring drive to the mill drive motor. The first reduction case is made of cast iron. The unit consists of an over-driven worm shaft and worm wheel supported by rolling element bearings. The first reduction wheel is keyed onto the second reduction worm. The first reduction unit is fitted with a fan and cowl for cooling purposes. The assembly of the first reduction unit is bolted onto the input side of the second reduction unit. The second reduction case is also made of cast iron. The unit consists of an under-driven worm shaft with a worm wheel mounted on a special output shaft for the barring coupling. The shafts are supported by rolling element bearings. The first and second reduction casings have self-contained lubrication systems, each having separate filling, draining, ventilation, and level indication facilities. Lubrication of the gears and bearings is achieved by oil bath and splash. Oil seals are provided for shaft extensions. An oil level plate provides the indication of a correct oil level for the first reduction casing. A dipstick located in a tube on the outside of the casing gives the indication of the oil level in the second reduction casing. Figure 5-10 shows the barring coupling.
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527
Technical Description – Allis-Chalmers
Figure 5-10 Barring Coupling (Courtesy of Lethabo Power Station)
The barring coupling drive uses two half-bevel gears. The driven-half gear is located on the shaft extension of the mill drive motor. The driving half, along with the operating mechanism, is
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Technical Description – Allis-Chalmers
located on the output section of the double-reduction worm gear unit.
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529
Technical Description – Allis-Chalmers
The two coupling halves are engaged by rotating the engagement nut, causing it to move toward the driven-half coupling. The movement of the engagement nut toward and away from the driven-half coupling is affected by a square thread drive. The thread drive is a male, fixed-square thread that is cut on an extended stub of the special oil catcher located on the worm gear unit. An interlock system is used to prevent the barring gear motor and the mill drive motor from being operated at the same time. The interlock system is initiated by two limit switches that function respectively as a main motor interlock and a barring motor interlock. When the coupling is engaged, the main motor interlock switch is set so that the main motor is turned off. At the same time, the locking catch is moved from the locked position, permitting the barring motor to be operated. If the engagement nut is screwed fully back and the locking catch is set in the locking position, the barring motor is turned off, and the main motor can be operated. Figure 5-11 shows the barring brake.
Figure 5-11 Barring Brake (Courtesy of Lethabo Power Station)
The barring brake is provided to positively brake and hold the barring gear drive during maintenance events. When applied, the brake has a remaining torque of 163 N-m on the cone ring input coupling of the barring gear unit. The brake is spring-applied, electrically released, and incorporates a hand-release mechanism. The brake shoes are fiber lined and self-aligning. A welded-steel hinged cover protects the complete magnet assembly against dust and damage.
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Technical Description – Allis-Chalmers
5.6
Gearbox Unit
Figure 5-12 shows the gearbox unit.
Figure 5-12 Gearbox Unit
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Technical Description – Allis-Chalmers (Courtesy of Lethabo Power Station)
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Technical Description – Allis-Chalmers
The mill gearbox unit is a single-reduction, double-helical unit that transmits the drive from the mill drive motor to the mill pinion shaft. The output shaft from the gearbox unit is connected to the mill pinion shaft by a grid spring coupling. The gearbox unit is mounted on a separate base plate and is completely encased by an acoustic hood. The gearbox unit case consists of two flanged half casings bolted together at midheight, with the alignment of the two half casings being achieved by four suitably spaced dowels. On the top of the top half casing is a removable inspection cover and four eyebolts that are used for lifting the top half casing only. Four lugs are provided on the bottom half casing that are used for lifting the complete gearbox unit. Other components fitted to the gearbox case are a sump drain plug and a labyrinth seal cover. The seal cover is packed with grease in order to protect the gearbox case from the entry of pulverized coal. The double-helical gears are mounted together to form a single-reduction output drive. The input and output shafts are supported by double-row, tapered roller bearings. On the output shaft the double-helical wheel is both pressed and keyed to the output shaft. Figure 5-13 shows the gearbox output coupling.
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Technical Description – Allis-Chalmers
Figure 5-13 Gearbox Output Coupling (Courtesy of Lethabo Power Station)
The gearbox unit output coupling consists of two steel hubs that are keyed to the gearbox unit output shaft and pinion shaft, respectively. The hubs are connected together by segmented grid springs to form a resilient coupling. The grid springs fit into grooves machined in the peripheries of the hubs. The assembly is packed with grease and enclosed in two half covers that are bolted together. Figure 5-14 shows the gearbox lubrication system.
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Technical Description – Allis-Chalmers
Figure 5-14 Gearbox Lubrication System (Courtesy of Lethabo Power Station)
The mill gearbox lubrication system consists of a two pump arrangement, a filter, an oil cooler, and instrumentation components. The system is mounted on top of the lubricating oil supply tank. The pumps draw oil from the oil tank and discharges the oil at 140-kPa pressure to the mill gearbox. The oil then flows to the internal oil distribution fittings. The two pump arrangement is designed to have one pump on-line and one pump in standby mode. At startup, the discharge oil pressure of the duty pump is monitored. If the discharge oil pressure fails to reach 140 kPa after 10 seconds, the standby pump is automatically started.
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535
Technical Description – Allis-Chalmers
Lubricating oil is drawn from the oil tank through a suction line and is circulated at 140-kPa pressure through a filter and an oil cooler. From the oil cooler, the oil passes to the gearbox through piping. Oil enters the gearbox casing through a flanged connection and is distributed to the bearings of both shafts and gear teeth. Figure 5-15 shows the gear teeth lubrication.
Figure 5-15 Gear Teeth Lubrication
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Technical Description – Allis-Chalmers (Courtesy of Lethabo Power Station)
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Technical Description – Allis-Chalmers
The supply to the gear teeth is passed through spray nozzles. Restrictors are fitted within the gearbox distribution lines to ensure that each lubrication point receives the correct quantity of oil at 140-kPa pressure. Four spray nozzles provide an oil flow of 1.12 liters per second to the gear teeth. The input shaft bearings are provided with 0.25 liters per second of oil flow and the output shaft bearings are provided with 0.07 liters per second of oil flow. Two pressure gauges are used, one before the filter and one after the cooler. The first gauge indicates the pressure of the oil leaving the pump and also indicates the cleanliness of the filter or cooler. The second gauge indicates the oil pressure entering the gearbox unit. Three spring-controlled relief valves are used in the system. One relief valve acts as a safety valve if the filter becomes blocked. Another relief valve serves as a cooler bypass and normally operates during the early stages of operation when the oil is cold and the resistance to flow through the cooler is high. The relief valve acts as a safety valve if a total blockage of the cooler occurs. The third relief valve acts as a pressure regulator and is set to maintain the specified working pressure of the gear unit at the normal working oil temperature. A pressure switch is located in the oil cooler discharge pipe. This switch is connected in the starting circuit of the mill drive motor. The starting circuit ensures that the drive motor will not start until the oil pressure has risen above 80 kPa and will trip the motor when the oil pressure falls below 50 kPa. A dial-type thermometer, fitted in the delivery pipe at the exit from the cooler, indicates the temperature of the oil being supplied to the spray nozzles and bearings in the gearbox unit. A flow indicator with an alarm indicating a low-flow condition is also connected in this line. A manually operated drain valve, fitted directly downstream of the oil pumps, facilitates rapid system and tank draining into suitable containers. The remainder can be drawn off by the drain plug provided. The oil pumps are gear pumps driven by electric motors through flexible, gear-type couplings. Each pump consists of two intermeshing rotors that operate in rolling element bearings. The top rotor shaft extends from the pump body and is keyed to the flexible drive coupling. One oil seal is located in a housing attached to the drive-end cover and provides sealing for the drive shaft. Orings, located in the covers at each end of the pump, provide sealing for the pump body. Oil entering the inlet port in the pump body is drawn around the space between the body and the rotor teeth and is discharged through the outlet port on the opposite side of the body. The oil filter is a biplex filter designed to permit a continuous flow of filtered oil when one of the filter baskets is removed for cleaning. The filter body consists of twin chambers located on each side of the main valve where the two filter baskets are located. The main valve directs the flow of oil through the filter and is operated by a handle that is pinned to the valve spindle at the top of the filter body. With the handle in the central position (in line with the inlet and outlet flanges), the flow of oil passes through both filter baskets. When the handle is positioned over
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Technical Description – Allis-Chalmers
one of the two basket chambers, the filter basket under the handle is the only one in use. The other basket may be removed for maintenance. A cover on top of each chamber is secured by two wing nuts and provides access for the removal of the filter baskets. A drain plug is provided at the bottom of each chamber. The oil enters the filter chamber at its head and flows down through one of the filter baskets, depending on the position of the handle and out through the sides of the basket. Particles of foreign matter in the oil are retained within the basket forming a layer of sediment at the base of the basket. The oil cooler is an oil/water heat exchanger consisting of a tube stack with two headers at each end. One header is for the shell pass fluids, and the other header is for the tube pass fluids. Both headers are contained in a steel tube. The tube stack consists of stainless steel tubes sealed at both ends under compression using elastomer seals that allow for expansion. A similar arrangement seals each tube and shell pass header to the end cap and shell, respectively. The oil cooler is provided with a special set of pressure plates and tensioning bolts in order to ensure that the shell pass of the cooler remains sealed when the tube pass is opened for inspection or maintenance. Oil enters the cooler at one end through the shell header and is directed over the tubes by segmented baffles separated by distance pieces. The distance pieces ensure uniform flow and the elimination of hot spots. Cooling water enters the unit through the tube header at the oil outlet end and, after passing through the tubes, is discharged from the tube header at the oil inlet end. This opposed flow ensures maximum cooling at the oil outlet. The oil pressure switch is a bellows-operated switch electrically connected to close on rising pressure and open on falling pressure. Adjustment of the operating point can be made by a screw driver adjustment located at the top of the switch. An adjusting screw for altering the differential is accessible by removing the cover of the switch.
5.7
Girth Gear and Pinion Shaft
The girth gear and pinion shaft transmit the torque to rotate the mill shell. The pinion shaft gear wheel and girth gear ring are enclosed by a girth guard. The pinion shaft is supported by two bearings. A large inspection door is provided in the girth guard adjacent to the pinion shaft gear wheel. The girth guard seal is achieved on the rotating interface using a Teflon seal strip fixed in a radial groove and bearing against the girth gear wheel side flat surface. The seal strip is maintained in contact with the gear side by a light radial spring that is adjustable. The sealing arrangement is needed to prevent the ingress of pulverized coal into the girth gear guard enclosure. The girth gear seal air system also keeps the pulverized coal contamination out of the enclosure. Figure 516 shows the location of the girth gear and girth guard.
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Technical Description – Allis-Chalmers
Figure 5-16 Mill Outline (Courtesy of Lethabo Power Station)
The two pinion shaft bearings are spherical roller bearings. The bearing adjacent to the mill gear unit coupling is a stationary bearing. The other bearing is free to float in the housing. Both bearings are packed with grease for lubrication purposes. Each mill is provided with a girth gear seal air system that supplies an air pressure of 0.5 kPa to the girth gear enclosure. The air pressure is required to assist the girth gear seal and to prevent contamination of the girth gear lubricant. The air supply is provided by a motor-driven, radial-bladed fan located adjacent to the ball mill on the ground level. The fan is fitted with a twin inlet facility, each inlet having an isolating ball valve and air filter. The air supply is piped to the girth gear guard through 100-mm diameter piping with a section of rubber hose fitted at the mill end of the piping. A schematic of the girth gear lubrication system is shown in Figure 5-17.
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Technical Description – Allis-Chalmers
Figure 5-17 Girth Gear Lubrication System (Courtesy of Lethabo Power Station)
The girth gear is lubricated by a four-nozzle spray lance arrangement that is supplied with lubricant from a pneumatic pumping system. The spray lance is attached to the girth side of the girth gear teeth. The spraying is carried out in intervals, the frequency being controlled by a timer control circuit. The quantity of lubricant is metered by a piston displacement within a distributor block.
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541
Technical Description – Allis-Chalmers
The pneumatic pumping system consists of the following: •
Pneumatic pump
•
Air service unit containing a pressure switch, filter, regulator, lubricators, and solenoid valve
•
Four-nozzle spray lance
•
Control panel with timer
•
Lubricant distributor with cycle counter
•
Air lubricator at pump motor
The air supply is connected to a pressure switch on the side of the air service unit. The air supply passes through a filter, a regulator, and a lubricator to a solenoid valve. The solenoid valve is controlled by the timer. When the solenoid valve is open, air flows to the spray lance and the air pump. The lubricant supply is provided from a standard commercial container where the air pump is mounted. The air pump maintains a supply of lubricant from the container to the distributor block. The flow of air through the spray lances clears the nozzles and atomizes the lubricant at the nozzles when it is supplied from the distributor. After lubricating the gear teeth, residue lubricant collects in a drip tray under the gearing. The girth gear lubrication system has a heating system that improves the pumping ability of the lubricant during times of low-ambient temperatures. The heating system contains trace heating elements and cartridge heater elements controlled from independent temperature controllers. The trace heating tape is wrapped around the grease pump outlet piping and controlled by a Danfossmanufactured controller. The cartridge heater is fitted into an aluminum manifold and mounted between the distribution block and the mill casing. The cartridge heater is controlled by using a Brainchild-manufactured temperature controller. One of the distributor sections contains a piston with an external indicator pin that trips a contact for each displacement of the distributor piston. The contact circuit causes a pulse to be recorded by the cycle counter fitted to the distributor. When the preset number of pulses has been reached, the control circuit closes the air solenoid valve and stops the pump action. The pump remains stopped until a preset time delay expires. The system then repeats the operation. The system is provided with alarm functions as follows: • Loss of supply voltage or an open circuit in the air pressure switch circuit causes an interlock relay to disable the system. • Loss of air pressure or excessive pumping time initiates a visible and audible alarm, stops the cycle timer, and disables the system. • Excessive pump time that may be caused by lubricant leakage or pumping from an empty lubricant container shuts off the air solenoid valve.
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Technical Description – Allis-Chalmers
The girth gear lubricating pump is an air-driven pump mounted on top of a standard 180-kg lubricant drum. The pump requires an air pressure of 500 kPa to operate. The pump consists of a top-mounted controlling air motor and a long pump tube assembly designed to reach the bottom of the supply drum. Operating air enters the motor valve body and is directed by a ported piston into the cylinder under the operating piston. The operating piston is attached by connecting rods to a lower piston at the bottom of the pump tube. As the lower piston rises, the upper ball is held on its seat, and the lower ball is lifted off its seat due to the ingress of the lubricant. When the piston rod reaches the top of its travel, the ported piston then redirects the operating air to the top of the operating piston, causing it to move downward. As the operating piston moves downward, the lower ball is forced onto its seat, and because of the pressure buildup, the upper ball is lifted. This allows the pressurized lubricant to pass out of the discharge port and into the outlet supply line to the distributor block.
5.8
Power-Sonic Mill Conditioning System
Figure 5-18 shows a line diagram of the Power-Sonic mill conditioning system
Figure 5-18 Power-Sonic Mill Conditioning System
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543
Technical Description – Allis-Chalmers (Courtesy of Lethabo Power Station)
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Technical Description – Allis-Chalmers
Under normal operating conditions, the coal level in each mill is controlled by a mill conditioning system. The system is provided to maintain the coal level in the mill for the various operating conditions including moisture content, size of coal, and so on. The system primarily uses a measurement of power drawn by the mill motor along with a measurement of the noise generated by the grinding process in order to produce a composite error signal. The error signal is then passed to the coal feeder speed controllers. The system consists of a power transducer, a sonic transducer, a control cabinet, and a control panel. One control cabinet houses the individual control circuitry for three ball mills. The power transducer monitors the power used by the mill drive motor when driving the rotation of the mill. The sonic transducer monitors the noise generated by the grinding action of the balls in the mill. The mill shell and girth gear are enclosed by an acoustic housing. The sonic transducer for the mill conditioning system is mounted on the side of the acoustic housing, 1700 mm above ground level and 600 mm from the access door edge towards the center of the mill. The power control circuit provides the major control path for the mill conditioning system. A signal representing mill motor power is generated by the power transducer. The signal is processed and passed to the power meter, the low-power alarm, and the power set point. With this set point, a power error signal is generated that is then passed through a summation network where both the sonic inputs and the mill load demand signals are compared. From the signal summator, a composite error signal is derived and passed on to the coal feeder speed controllers. A signal proportional to the grinding noise is generated by the sonic transducer. The signal is amplified, filtered, and passed to a sonic meter, a sonic alarm, a sonic-swamp set point, and a sonic-trim set point. When the sonic intensity exceeds the sonic-swamp set point, the signal triggers a comparator to affect the power error signal. This action forces the composite error signal to indicate to the controller that the product charge is out of range or on the low side. The low limit is used to provide a fixed reference for the power control circuit when the swamping circuit is not triggered. Table 5-1 shows the alarm conditions for the Power-Sonic mill conditioning system Table 5-1 Alarm Conditions for the Power-Sonic Mill Conditioning System (Courtesy of Lethabo Power Station) Mill Condition
Normal operation within control range Mill starting to overload – high product charge Mill is reducing the product charge and indications are moving toward the stripping range or loss of a feeder in the control range
Alarm Output Low-Power
High-Sonic
Normal
Normal
Alarm
Normal
Normal
Alarm
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Technical Description – Allis-Chalmers Mill in stripping range and almost void of product charge
Alarm
Alarm
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Technical Description – Allis-Chalmers
From Table 5-1, a high-sonic alarm condition indicates a low-product charge. If the ball charge weight has not been maintained, then the following symptoms appear: • At rated high-load output, the mill becomes unstable because of the inability to grind the required throughput because of insufficient ball charge. • At lower outputs permitted by reduced ball charges, the system initiates a low-power alarm because the power is lower than the alarm set point. • A reduced ball charge drops the stripping point power down below the control setting and the high-sonic alarm is initiated. This is because the controller attempts to increase power to the set value by reducing the coal feed to the mill.
5.9
Technical Specification List
Data for the Allis-Chalmers mill are listed in Table 5-2. Table 5-2 Data for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Equipment/System Mill shell
Parameter
Technical Data
Diameter
4267-mm
Length
5790-mm
Maximum ball charge
Weight
97,000-kg
Initial ball charge
Size
20% – 60-mm balls 80% – 40-mm balls
Make-up ball charge
Size
60-mm balls
Trunnion bearings
Type
Babbitted rocker type insert with individual lubrication system
Trunnion bearing cooling
Type
Water-cooled coils in inserts
Water flow rate
0.46 liters/second
Pressure
810 kPa
Type
Single-helical pinion shaft and girth gear
Ratio
10,071:1
Size
614.5-mm outside diameter
Number of teeth
28 teeth
Size
5820.3-mm outside diameter
Number of teeth
282 teeth
Speed
15.7 rpm
Drive gear
Pinion shaft
Girth gear
Rotating mill
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547
Technical Description – Allis-Chalmers
Table 5-2 (continued) Data for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Equipment/System Girth gear seal air fan, single inlet
Girth gear seal air fan motor
Girth gear lubrication system
Mill trunnion bearing lubrication system
Gearbox unit
Acoustic hood
Gearbox lubrication system
Parameter
Technical Data
Size
228-mm diameter
Speed
2800 rpm
Pressure
0.5 kPa
Voltage
380 volt
Type
3-phase
Frequency
50 Hz
Frame
KDY 71
Type
Four-nozzle lance with spray valves
Pressure
400 kPa
Voltage
110 volts AC
Operating pressure – high pressure
20.7 mPa
Relief valve setting
31 mPa
Low pressure
680 kPa
Oil temperature
30°C
Oil flow rate
0.38 liters/second
Minimum oil flow rate
0.125 liters/second
Oil reservoir capacity
2.8 liters
High oil temperatures
60°C auto start high-pressure pump, 82°C auto trip mill
Weight of gearbox without oil
13,000 kg
Input speed
985 rpm
Output speed
158,871 rpm
Size
2400 mm by 1810 mm by 1180 mm
Lining material thickness
30 mm
Tank capacity
675 liters
Oil tank temperature
54°C
Oil spray temperature
48°C
Operating pressure
140 kPa
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Technical Description – Allis-Chalmers Table 5-2 (continued) Data for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Equipment/System Pump
Parameter
Technical Data
Type
Gear
Flowrate
1.76 liters/second
Power rating
4 kW
Voltage
380 volt
Type
Three-phase
Frequency
50 Hz
Frame
DX112MD
Speed
1,425 rpm
Filter
Size
119 microns
Cooler
Water flow rate
0.9 liters/second
Pressure drop
157 kPa
Settings
Closes on rising pressure of 80 kPa
Pump motor
Pressure switches
Opens on falling pressure of 50 kPa Relief valves
Settings
350–360 kPa, 300 kPa, 154 kPa
Flow meter
Size
2 in.
Mill drive motor
Enclosure
CACA
Cooling
Air
Shaft length
3.700 mm double-ended
Supply
3300 volt, Three-phase, 50 Hz
Output
1550 kW
Speed
993 rpm
Insulation
Class F
Weight
9.75 metric tons
Bearings
Lubrication
Loose ring
Winding
Temperature alarms
120°C alarm, 130°C trip
Bearing bushing
Temperature alarms
85°C alarm, 90°C trip
Bearing sump
Temperature alarms
75°C alarm, 80°C trip
Motor/gear coupling
Manufacturer
Falk
Type
T10
Size
170T
Weight
443 kg
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549
Technical Description – Allis-Chalmers Coupling gap
6 mm
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5-50
Technical Description – Allis-Chalmers Table 5-2 (continued) Data for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Equipment/System Barring gear unit
Parameter Gear ratio
Technical Data 1st gear – 4.875:1 2nd gear – 30.5:1 Total reduction – 148,687:1
Input coupling
Output coupling
Barring gear drive motor
Barring gear brake
Input speed
1460 rpm
Output speed
9819 rpm
Barring speed
0.157 rpm
Bearings
Ball race
Weight
1900 kg
Type
Heavy-duty cone ring
Size
03
Coupling gap
3 mm
Type
Special CUD14 barring coupling
Coupling gap
6-mm
Frame size
D180L
Enclosure
TEFC
Supply
380 volt, Three-phase, 50 Hz
Speed
1460 rpm
Size
203-mm diameter
Supply
380 volt, Three-phase, 50 Hz
Retaining torque
163 N-m
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551
6
TECHNICAL DESCRIPTION – FOSTER WHEELER
The Foster Wheeler ball/tube mill system [5] consists of a single- or double-feed system that delivers coal to a large rotating drum partially filled with hardened steel balls. The ball/tube mills can process up to 70 tons/hour of coal. The following is a description of a negative pressure Foster Wheeler ball mill. When charged with approximately 50 tons of forged steel balls of 7/8-in., 1-1/4-in., and 2-in. diameter, the Foster Wheeler ball mills are capable of grinding 83,000 lbs of coal per hour with a pulverized coal fineness of 70% through a 200-mesh screen and 98.5% through a 50-mesh screen. This is based on 10% maximum raw coal moisture and a coal grindability of not less than 55 on the Hardgrove scale. Figure 6-1 shows a diagram of the Foster Wheeler ball mill. The pulverized fuel system can be a single- or double-feed system. The system described here is comprised of two raw coal feeders and motors, one ball mill pulverizer and motor, two classifiers, two exhausters and motors, two distributors, interconnecting raw coal and fuel piping, and all related dampers and controls.
Figure 6-1
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6-1
Foster Wheeler Ball Mill Diagram [2]
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6-2
Technical Description – Foster Wheeler
During operation, two table-type feeders provide coal to each end of the mill. Each feeder has a single discharge and supplies coal into one end of the mill. The rotating table of the feeder is driven by a 1-horsepower, two-speed, squirrel-cage induction motor. The flow of coal from the feeder table is regulated by the position of an adjustable shear plate. An access door with a glass observation window permits inspection of the feeder table and shear plate. The access door facilitates removal of any foreign material that may interfere with feeder operation. Raw coal flows from the feeders in a pipe through the classifier. The raw coal is mixed with coarse rejects drawn out from the classifiers. The coal is then carried into the mill by the spiral ribbon conveyors that rotate with the mill. Figure 6-2 shows the air/coal flow diagram for the Foster Wheeler ball mill.
Figure 6-2 Air/Coal Flow Diagram [6]
After the 1960s, ball mills were designed for pressure operation and supplied with a primary air fan; the exhauster fan was removed. The primary air fan supplies air to the mill inlet. Preheated air is taken from the secondary air duct downstream of the air heaters and enters the mill from both ends through a centrally located air tube. This preheated air is called primary air and is used to carry pulverized coal through the pulverizer to the classifiers, exhausters, and fuel piping to the boiler. The desired air temperature is achieved by blending the hot primary air with cold tempering air through an adjustable damper arrangement upstream of the pulverizer. A 700-horsepower, 327-rpm, 4160-volt, squirrel-cage induction motor drives each mill through two flexible couplings to a herringbone pinion and gear. The mill is rotated at a speed of
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Technical Description – Foster Wheeler
approximately 18 rpm. The mill motor is protected by a low-voltage auxiliary relay, current overload relays, and ground relays. As the drum rotates and coal is fed into the mill, the balls and coal are carried up on the periphery of the drum. The balls and coal then cascade toward the mill axis and the coal is ground. The pulverized coal and air leave the mill through the annular space between the air inlet tube and the mill trunnion tube. Technical Key Point There is no tramp iron removal system. The mill is designed to grind the raw coal and foreign matter together. Considerable amounts of pyrites will accelerate the wear rates of the grinding media. The flow of primary air transports the ground coal to the classifiers. There may be one or two classifiers that receive the coal and air mixture exiting the drum. With two classifiers, each classifier receives the coal and air mixture from one end of the drum. Each classifier separates coarse particles from the mixture of primary air and fuel leaving the mill, allowing only the fine particles to continue on to the exhauster. The heavy particles fall and are separated by centrifugal force as the fuel mixture passes over the baffle plate. The coarse particles are mixed with the raw coal being fed into the classifier and are returned to the mill for further grinding by the spiral ribbon conveyor. The differential pressure across the classifier is the primary indication of mill output. Before the 1960s, Foster Wheeler-manufactured ball mills were negative pressure mills equipped with exhauster fans. The exhauster fans are located at the discharge of the mill and provide suction pressure for the mill. The primary air and fuel exit the classifier and are drawn through the exhauster inlet elbow and into the exhauster. A coal distributor is located on the exhauster outlet to distribute fuel uniformly to the piping that leads to the boiler. The exhauster is essentially a six-bladed paddle wheel that receives the fuel mixture at the center of the wheel and discharges the fuel mixture at the periphery of the blades. The exhauster pulls air through the mill to pick up the coal and discharges the coal/air mixture to the boiler burners. The exhauster is driven by a 200-horsepower, 1165-rpm, 4160-volt, squirrel-cage induction motor. The maximum fuel flow capacity of one exhauster is approximately 85,000 lbs per hour. Mill suction should be maintained at approximately 2 in. of water negative pressure. This is necessary to ensure proper functioning of the feeder controller. If the mill suction is low, then the differential signal will be low. This will cause the feeder to operate continuously and fill the pulverizer with coal. If the mill suction is too high, the air leakage will be too great, and the coal level in the mill will be reduced.
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Technical Description – Foster Wheeler
From the discharge of the exhauster, a distributor box distributes the coal/air mixture to the fuel piping that leads to the boiler burners. The shape of the distributor box assists the distributor dampers in dividing the coal equally among the burners. Normally, when the distributor dampers are set, it is not necessary to make any changes. The burner shutoff valves are located at the discharge side of the distributor box. The burner shutoff valves enable the operator to take any burner or burners out of service without affecting the operation of the other burners. The output of the pulverized fuel system depends on the airflow through the mill. The airflow is regulated by the position of the exhauster output control damper located in the exhauster inlet elbow. This section covers the technical description for the Foster Wheeler ball mill on the following components/systems: • Conveyor assembly • Drum assembly • Trunnion main bearings • Gearing • Trunnion tube • Classifier • Exhauster • Lubrication systems • Seal air system
6.1
Conveyor Assembly
Figure 6-3 shows the flight ribbon conveyor assembly. The conveyor assembly consists of an inlet air screen, an air tube, a flight ribbon with chains, and spokes. The conveyor assembly resides inside the trunnion tube and is connected to the drum.
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Technical Description – Foster Wheeler
Figure 6-3 Flight Ribbon Conveyor Assembly [2]
The primary air flows through an inlet screen, through the air tube, and into the drum. The coal is fed from the piping through the classifier and onto the flight ribbon. The flight ribbon is a spiral metal ribbon attached to the outside of the air tube. There are chains attached between the spiral ribbons. As the drum rotates, the flight ribbon rotates, and coal is fed into the drum. Attached to the drum end of the conveyor assembly, a ring with spokes keeps the air tube centered in the trunnion tube and the drum. A number of changes have occurred in the conveyor assembly when compared to the original design provided on older units. The following changes have occurred from the original design: •
The flight ribbons have additional supports that reduce the possibility of breakage.
•
The flight ribbon supports on the drum end are not fixed. The new standard support is a type of spring support that provides some flexibility when objects such as tramp iron, pyrites, or other objects become wedged in the flight ribbons.
•
The screen at the mill end of the air tube is now located at the inlet elbow end of the air tube. The new location for the screen reduces damage to the screen due to balls and other debris getting tangled in the screen.
•
A reject ribbon is welded inside the air tube. This reject ribbon kicks the balls and other debris back into the mill before it reaches the screen at the other end of the air tube.
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Technical Description – Foster Wheeler
6.1.1 Conveyor Support Assembly The flight ribbon spring support assembly is shown in Figure 6-4.
Figure 6-4 Flight Ribbon Spring Support Assembly [2]
The redesigned conveyor support systems have had two major upgrades in order to provide increased reliability. Figure 6-5 shows the conveyor support assembly.
Figure 6-5
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Technical Description – Foster Wheeler Conveyor Support Assembly [2]
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Technical Description – Foster Wheeler
The first upgrade was to add four spokes to the assembly for a total of eight spokes. The use of eight spokes reduces the loading on the individual spokes and decreases the load transferred to the remaining spokes if one spoke breaks. The second upgrade includes spokes that are not threaded. This reduces the chance of stress concentrations and cyclic-fatigue failures. The spoke material has also been upgraded. Finally, the hole in the reject liner that the spoke fits in has a sleeve insert that keeps the spoke tight in the reject liner hole. For earlier designs, a significant amount of time was spent in aligning the air tube after the spokes were cut out or removed. It is critical for the air tube to be precisely centered in the mill, or the spokes will fail after some period of time. The original design has the air tube centered and the ring and spokes welded into position. With the redesign, a fixed ring and its spokes remain in position, and the air tube is bolted into the ring. When the conveyor needs repair or replacement, the air tube is easily removed from the ring, but the ring and spokes can remain in position. When the air tube is reinstalled, it is bolted back into the ring, and alignment is not required. Figure 6-6 shows the conveyor removal with the eight-spoke ring design.
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Technical Description – Foster Wheeler
Figure 6-6 Conveyor Removal with the Eight-Spoke Design [2]
6.2
Drum Assembly
The pulverizer drum assembly consists of two cast-iron end castings bolted to a rolled-steel plate cylinder. The ends are cast with integral trunnions, and the entire assembly is supported at each
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Technical Description – Foster Wheeler
end by large, self-aligning, water-cooled, babbitted bearings. Flight bars are bolted to the end
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Technical Description – Foster Wheeler
castings. Flight bars are designed to direct debris and small grinding balls back into the drum. Figure 6-7 shows the complete drum assembly.
Figure 6-7 Complete Drum Assembly [2]
O&M Cost Key Point It is less expensive and more time efficient to purchase the complete drum assembly instead of having the end castings and drum shell plate assembled on site. The drum assembly shell plate is pre-drilled for the installation of liners, and the end castings are finished machined. The end castings enclose the drum shell and provide the rotating bearing surfaces when the drum assembly is seated in the trunnion bearing. Figure 6-8 shows an end casting.
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Technical Description – Foster Wheeler
Figure 6-8 End Casting [2]
The castings are susceptible to long-term failure from cyclic fatigue and notch effect if the routine maintenance procedures, such as flight bar replacement and ball charge reclassification, are not regularly completed. 6.2.1 Double-Wave Liners The drum liners are designed to lift and tumble the ball charge as the drum is rotated. Foster Wheeler has standardized double-wave liners for the grinding drum on all of the new mills. Figure 6-9 shows the double-wave liners.
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Technical Description – Foster Wheeler
Figure 6-9 Double-Wave Liners [2]
The primary benefits of the double-wave liners are: •
Reduced maintenance – This design eliminates the need to align separate wedge bars and liners.
•
Improved efficiency – An improvement in mill performance and coal fineness characteristics is typically noted because wedge bar systems can rarely be maintained to the design specification.
•
Liner longevity – The double-wave liners last longer than the old wedge bar and liner system.
•
Ease of installation – Compared to the wedge bar and liner system, the labor required to install the double-wave liners is 30% less because there are fewer parts and no required liner adjustments are necessary (each liner automatically bolts into position).
•
Increased durability – In the older system when the wedge bars were worn, there was a degradation of mill performance because of the loss of the original lift profile. The original wedge bars had a hardness of 300 BRN (Brinell hardness number). The wedge bars would wear more rapidly than the liners and needed to be replaced more often. The double-wave liners have a hardness of 600 BRN. Additionally, as the double-wave liner wears, the contour remains the same for the life of the liner without loss of mill performance.
• Reduced inventory – A typical wedge bar and liner design can require as many as 14 different patterns. The typical double-wave design uses only six different patterns (assuming the double-size, double-wave access door replaces the original access door). The improved double-wave liners can be installed when the wedge bars are scheduled for replacement or during the next liner replacement outage.
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Technical Description – Foster Wheeler
6.2.2 Double-Size, Double-Wave Access Doors In response to requests for larger access doors for the ball mills, Foster Wheeler is producing double-size, double-wave access doors. The original door can be retained with the larger door providing additional access, or the larger door can be installed over the original door, resulting in fewer liner patterns. Figure 6-10 shows a double-size, double-wave access door.
Figure 6-10 Double-Size, Double-Wave Access Door [2]
6.2.3 Flight Bars Between the trunnion tube and drum end casting is a gap where debris and small grinding balls can get lodged. Over time, this debris can wear grooves at the most critical area of the end casting. The resulting wear and stress concentration eventually causes the end casting to fail. Bolted to the end casting are flight bars that are designed to push the small grinding balls and debris back into the drum. If the flight bars are worn out or missing, they should be replaced. The flight bar material has been upgraded in order to improve wear and abrasion properties. 6.2.4 Grinding Balls Foster Wheeler has standardized high-chrome grinding balls for the grinding media. The increased cost of the steel-forged balls is offset by longer wear life properties. The material of the high-chrome balls matches the high-chrome, double-wave liner material. Foster Wheeler recommends the reclassification of balls every three years or at some reasonable cycle. The largest ball size is usually 2.5 in. in diameter, while the smaller ball size ranges from 0.75–1 in. in diameter. Figure 6-11 shows the wear rates on the forged-steel, carbon-molybdenum, and high-chrome balls for different grindability coals.
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Technical Description – Foster Wheeler
Figure 6-11 Ball Wear Rates [2]
6.3
Conveyor Shaft Bearing and Seal
The bearing and stuffing seal in the inlet elbows is used to support the outboard end of the ribbon conveyor assembly. The latest design uses an air-cooled roller bearing in an internally braced inlet elbow. Figure 6-12 shows the conveyor bearing modification.
Figure 6-12 Conveyor Bearing Modification [2]
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Technical Description – Foster Wheeler
This upgrade has three advantages over the old design and the advantages are: •
The bearing is air-cooled to extend the operating life.
•
A labyrinth seal is included in the bearing support enclosure. The cooling air provides a purging action that keeps coal particles away from the bearing.
•
The internally braced inlet elbow provides adequate stiffness to the bearing support and eliminates the need for the old pipe support.
Because of the close machining tolerances required for this modification, it is difficult to accurately locate the new bearing and seal in the field, as well as install the internal support in the elbow area. For these reasons, new elbows are normally purchased with the modification. If there is a need to replace the inlet elbows because of fire damage or mill puffs, then this modification would be an improvement from the current design. If the existing elbows are in good condition, then the elbows can be retrofitted during a future outage.
6.4
Trunnion Main Bearing and Dust Seal
The trunnion main bearing assembly supports the drum end casting and consists of the main bearing, thermocouples, and spring-loaded dust seals. Figure 6-13 shows the original bearing assembly.
Figure 6-13 Original Bearing Assembly [2]
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Technical Description – Foster Wheeler
The trunnion bearing has been modified to be used in the driven or the non-driven end of the drum casting. The new design bearing is shown in Figure 6-14.
Figure 6-14 New Design Bearing [2]
The thermocouples protrude into the babbitted bearing surface at the 4 o’clock position. These thermocouples provide the operators with a high-temperature alarm in the event of possible bearing distress. The spring-loaded dust seals are self-adjusting so that the outboard gap between the trunnion and the oil retainer is sealed. This prevents coal dust and other contaminants from entering the lube oil. The ball mills with exhausters installed before the 1960s may leak coal dust during mill excursions. In this design, the exhausters provide a negative pressure operation in the drum. The loss of drum level or control problems can cause the mill to go from negative to positive pressure operation, and coal dust may leak out of the drum. Figure 6-15 shows the trunnion seal.
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Technical Description – Foster Wheeler
Figure 6-15 Trunnion Seal [2]
Ball mills supplied after the 1960s use a positive pressure system that uses a primary air fan to pressurize the mill. To prevent coal dust from blowing out between the end casting and trunnions, a trunnion dust seal arrangement was added. Seal air, at a pressure that is slightly higher than the mill pressure, is provided to a trunnion seal chamber. During operation, the seal air is introduced to prevent coal from escaping through the clearance between the rotating drum and the trunnion. Spring-loaded mechanical seals, consisting of a steel ring attached to the pulverizer trunnion and a flexible gasket attached to the classifier, keep the seal air contained. This trunnion seal area is vented to the lower-pressure classifier in order to remove any coal fines that migrate there during operation. A plow, rotating with the trunnion, stirs up the coal to ensure that it is removed through the scavenging vent system. The modification for the dust seals is available for the negative pressure mills.
6.5
Gearing
The Foster Wheeler ball mill drive system consists of gearboxes, couplings, support rings, inching drives, pinions, and bull gear assemblies. Bull gears are available as either cast or fabricated alloy steel materials. Pinions are available as carburized and ground, forged-alloy steel material. Figure 6-16 shows the pinion and bull gear assembly.
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Technical Description – Foster Wheeler
Figure 6-16 Pinion and Bull Gear Assembly [2]
6.5.1 Pinion Bearings The original babbitted pinion bearings required a large amount of maintenance. It is possible to convert the babbitted pinion bearings to roller bearings. Figure 6-17 shows the new roller bearings.
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Technical Description – Foster Wheeler
Figure 6-17 Roller Bearings [2]
The roller bearings have several advantages: •
The lubrication system used for the old babbitted pinion bearings is not required. This reduces maintenance costs, power consumption, and the chance for bearing failures.
• The roller bearings can be mounted in split pillow blocks. This facilitates the flipping of the pinion gear to operate on the unused side of the pinion teeth. The pillow blocks do not require removal from the foundation sole plates, and this eliminates the need to realign the pinion gear.
6.6
Trunnion Tube
The classifier trunnion tube is the part of the classifier housing that extends through the trunnion and into the drum. As originally designed, a thorough inspection and replacement of the flight bars is difficult. An access door in the trunnion tubes is now provided for inspections and flight bar replacement. Figure 6-18 shows the new classifier trunnion tube.
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6-21
Technical Description – Foster Wheeler
Figure 6-18 Classifier Trunnion Tube [2]
The lower portion of the inside of the trunnion tube is protected from wear by replaceable steel liner plates. The liner plates can be supplied in different grades of steel for corrosion and erosion protection.
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Technical Description – Foster Wheeler
6.7
Classifier
The original scroll-type classifier is shown in Figure 6-19.
Figure 6-19 Original Scroll-Type Classifier [6]
There are several options or upgrades from the original design scroll-type classifier. The options/upgrades are: • Classifier reject damper • Adjustable blade • M-type classifier • Dynamic classifier 6.7.1 Classifier Reject Damper The classifier reject damper [2] is an on-line adjustable device used to optimize the return flow path of large coal particles from the classifier to the mill’s raw feed inlet conveyor. Proper adjustment minimizes and/or eliminates bypassing of the large particle sizes (greater than or equal to 50 mesh). Figure 6-20 shows the classifier reject dampers.
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Technical Description – Foster Wheeler
Figure 6-20 Classifier Reject Dampers [2]
If the ball mill performance has been optimized and coal fineness still requires minor adjustment in order to assist with the reduction of NOx and unburned carbon, then these dampers can provide the desired results. A 3–5% increase in fineness is the expected range of improvement for these reject dampers. 6.7.2 Adjustable Blade Classifier The modification for an adjustable blade classifier consists of substituting a static classifier with adjustable inlet vanes. This classifier is similar to a cyclone-type classifier with the inlet area around the circumference and fitted with adjustable vanes. The primary air and coal/air mixture is centrifuged by the dual scroll before exiting the mill. Figure 6-21 shows the adjustable classifier.
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Technical Description – Foster Wheeler
Figure 6-21 Adjustable Classifier [6]
Figure 6-22 shows the original classifier and the static classifier with adjustable inlet vanes.
Figure 6-22 Classifier Comparison [6]
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Technical Description – Foster Wheeler
Figure 6-23 shows the fineness improvement with the adjustable classifier.
Figure 6-23 Adjustable Classifier Fineness Improvement [6]
6.7.3 M-Type Classifier The Foster Wheeler M-type classifier is similar to the static classifier used on vertical shaft mills. It offers a higher coal fineness output for the same coal flow as the original ball mill classifier. Additionally, the coal fineness can be controlled by the use of movable vanes that can be adjusted while the mill is operating. Depending on the coal piping configuration, the M-type classifier may also provide a better coal flow balance among the burners. Figure 6-24 shows an M-type classifier.
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Technical Description – Foster Wheeler
Figure 6-24 M-Type Classifier [2]
6.7.4 Dynamic Classifier The dynamic classifier is designed to improve the coal classification from the static, cyclonetype classifier. Using the dynamic classifier improves the pulverizer capacity, coal fineness distribution, and fineness control. The M-type classifier can be directly converted into a dynamic classifier and results in minimal changes to the existing hardware and components. The dynamic classifier consists of: •
An outer housing with coal distribution control vanes
•
A vertical rotor assembly
•
A ring of fixed, flow-directing vanes
•
A reject hopper
•
A modular shaft and bearing assembly
•
A drive system, complete with a drive belt, sprockets, an ac electric motor, and a
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Technical Description – Foster Wheeler
frequency inverter drive
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Technical Description – Foster Wheeler
Figure 6-25 shows a dynamic classifier for the ball mills.
Figure 6-25 Dynamic Classifier [2]
On horizontal ball mills, the classifier can be either close-coupled to the mill discharge or mounted from the above elevation floor and connected to the mill discharge through a short transition pipe. The dynamic classifier rotor design is similar to a radial flow fan. The rotor consists of a central conical hub that is encircled by a ring of vertically-mounted, evenly spaced blades. The blades are held in place by several stabilizing rings and a lower-support ring. The support ring is directly attached to the conical hub. Each stabilizing ring is attached to the hub by spokes. The rotor and stationary vane ring are manufactured from abrasion-resistant steel plate. After fabrication, the rotor is dynamically balanced. The rotor assembly is bolted to the lower end of the main drive shaft. The drive shaft is hollow to provide a central passage for the coal feed pipe. The shaft is constructed from forged steel and is supported by a pair of anti-friction roller bearings. The bearing housings are designed with multiple fittings for periodic grease lubrication. Labyrinth seals retain grease in the bearing housings, and pressurized seal air is introduced into the bearing housing cavities in order to protect the bearing from coal dust contamination. Thermocouples or resistance temperature detectors (RTDs) are supplied for the temperature
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Technical Description – Foster Wheeler
monitoring of each bearing.
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Technical Description – Foster Wheeler
The bearing and shaft assembly is a modular unit. The rotor is driven by an ac, inverter-duty motor. The motor is connected to the rotor by a set of sprockets and a drive belt with teeth. The motor is bolted to a base on the top of the classifier housing. The drive sprocket is connected to the motor shaft using a bushing. The driven sprocket is bolted to the top of the rotor shaft and supports a labyrinth seal. The labyrinth seal seals the annular space between the coal feed pipe and the rotor shaft. Surrounding the rotor is a ring of fixed, static vanes. The vanes direct the air and coal particles into the classification zone between the static vanes and the rotor. The lower end of the static vane ring is attached to a reject cone that directs rejected material back to the coal conveyor. The cylindrical classifier housing is constructed from carbon steel plate and is fitted with an ellipsoidal-shaped roof. The roof has a flanged opening for the installation of the rotor, shaft, and bearing assembly. All major sub-assemblies are flanged and piloted for ease of assembly and maintenance. The drive bearings and shaft assembly can be removed without removing the rotor when servicing of the bearings is required. The operation of the dynamic classifier starts as the pulverized coal/air mixture enters the classifier and flows through the annulus between the outer periphery of the classifier static vane ring and the outer housing. The mixture passes through the ring of fixed vanes and is directed into the rotor. Here, the coal particles are segregated by size as the rotor imparts an outward force proportional to the rotor speed. The higher the rotor speed, the finer the outgoing coal particle fineness. Heavier or larger coal particles that cannot accelerate through the rotor lose their speed and are returned to the coal conveyor by the reject cone and the hopper assembly. When the weight of the rejects in each hopper is sufficient, the hopper swing flap opens, and the rejected material is directed to the coal conveyor entering the mill drum. The speed of the rotor is controlled by a variable-speed drive motor. The motor can be set to vary automatically as a function of coal flow or airflow. The typical speed range of the rotor is between 75 and 150 rpm. As the rotor speed increases, a larger quantity of coal particles is rejected to the conveyor for further size reduction. The overall higher separation efficiency of the dynamic classifier means that a lower percentage of large particles are discharged by the mill, and a lower percentage of small particles are retained in the mill. The small coal particles exit the mill by passing through the discharge divider and into each coal pipe by coal distribution vanes. The vanes are made of abrasion-resistant steel. The vanes are independent from one another, manually adjusted, and locked into position.
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Technical Description – Foster Wheeler
6.8
Exhausters
An exhauster diagram is shown in Figure 6-26.
Figure 6-26 Exhauster Diagram [5]
The exhauster assembly contains a housing fan (spider) and a motor to drive the fan. Exhauster fans and wear liners in the housing can be replaced if necessary. Figure 6-27 shows the exhauster spider.
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Technical Description – Foster Wheeler
Figure 6-27 Exhauster Spider [2]
6.9
Lubrication Systems
There are two lubrication systems for the Foster Wheeler ball mill. The first system is called the Cardwell lubrication system and supplies oil and starting lifting hydraulic pressure to the trunnion bearings. The second system is the Farval lubrication system and supplies oil to the pinion and bull gear that turns the drum. 6.9.1 Cardwell Lubrication System The Cardwell lubrication system for the trunnion bearing consists of a main oil reservoir with an 80-gallon capacity, an auxiliary oil reservoir with a 25-gallon capacity, a low-pressure pump, a high-pressure pump, strainers, filters, relief valves, and interconnecting tubing. The low-pressure pump feeds both trunnion bearings and the high-pressure pump and supplies oil for lubrication of the bearings. The high-pressure pump is a hydraulic lift pump used to lift the trunnions off the bearings during start up. Figure 6-28 shows the Cardwell lubrication system.
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Technical Description – Foster Wheeler
Figure 6-28 Cardwell Lubrication System [5]
The low-pressure pump, the high-pressure pump, and the mills are interlocked in a starting sequence.
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Technical Description – Foster Wheeler
An integral control system monitors the output of the oil pump and switches to the standby pump if necessary. Isolation valves allow for the replacement of pumps and filters without shutting down the system. Integration of the lubrication system and plant controls ensures that: •
The trunnions are lifted before the mill is started.
•
The lubricating mode is changed when the mill starts.
•
The mill will trip if the lubrication system fails.
6.9.2 Farval Lubrication System The Farval lubrication system provides lubrication to the gear system that turns the mill. It is controlled by a system timer that is interlocked to the mill motor switchgear breaker. Figure 6-29 shows the Farval lubrication system.
Figure 6-29 Farval Lubrication System [5]
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Technical Description – Foster Wheeler
The time clock starts each cycle of lubrication by energizing a solenoid-operated, air-control valve on the air-control panel. A pre-set air pressure of 40 psi is applied to the air-operated pumping unit that delivers lubricant from the supply drum to the spray valve panel through the hydraulic reversing valve. As the lubricant enters one of the two supply lines leading to the dual-line measuring valves, the measuring pistons force a predetermined quantity of lubricant through the valve discharge lines to the spray control valves and nozzles. Here, the air stream atomizes the lubricant and blows it onto the gear surface. The air supply pressure to the spray valve is set at 80 psi. After all measuring valves have discharged the predetermined quantity of lubricant, the pressure flows through one of the two return lines to actuate the pistons at the hydraulic reversing valve and shuts down the system. The movement of flow-directing pistons in the reversing valve automatically relieves the pressure in the line that was last pressurized and sets up the proper porting to direct lubricant to the opposite supply line when the time clock starts the next cycle. The lubricator may be hand-tripped by loosening the thumb nut on the spraying cycle control dial until it is loose on the shaft and then turning the star wheel clockwise.
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7
TECHNICAL DESCRIPTION – KENNEDY VAN SAUN
The Kennedy Van Saun mills for the Kendal Power Station of Eskom in South Africa consist of 30 air-swept ball mills. The mills are 15 ft, 5 in. in diameter and 22 ft, 1 in. in length from flange to flange. The mill speed is 14.88 rpm. The mill motor supplies power for each mill and the power requirement is 1.85 MW. Figure 7-1 shows the Kennedy Van Saun mill system.
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7-1
Technical Description – Kennedy Van Saun
Figure 7-1 Kennedy Van Saun Mill System (Courtesy of Kendal Power Station)
The Kennedy Van Saun mill components are shown in Figure 7-2.
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7-2
Technical Description – Kennedy Van Saun
Figure 7-2 Kennedy Van Saun Mill Components (Courtesy of Kendal Power Station)
The following are components of the mills: •
Feeder discharge boxes made of welded steel fabrication with ceramic lining on the discharge side and chrome-iron lining on the feed plate.
•
Trunnion sleeve made of chrome-moly steel with helical flights.
•
Ball charge hopper made of fabricated steel.
•
Gear reducer consisting of a single-stage reduction helical gear.
•
Falk Steel flex couplings for low speed and high speed.
•
Gear reducer and bearing-forced lubrication system – The lubrication system is located on top of the steel oil tank. Oil is circulated under pressure by a gear pump and driven by a 4-kw motor at 1450 rpm. Oil is drawn from the oil tank by the pump and is fed to a dual-basket oil filter. The oil is then passed to a tube oil cooler. The oil is conveyed through piping to the mill drive reducer gear case and drive pinion bearings. The reducer gear case has an internal oil distributor. The lubrication system includes three pressure relief valves for bypassing the filters and the cooler and to acts as a pressure regulator. Pressure switches, thermometers, flow indicators,
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7-3
Technical Description – Kennedy Van Saun
pressure gauges, and thermostats are used for system control and monitoring. Sensor switches activate alarms in the event of malfunction or failure. • Ring gear and lubrication system – The lubrication system is an air-operated, automatic gear spray system. Supplied at a constant pressure, air is filtered, regulated, and lubricated. The air then passes through a solenoid valve that is energized from an electrical switchboard timer. The air operates a 40:1 ratio pump that delivers grease from either a 50- or 180-kg standard grease container to a progressive single-line distributor. The grease then goes to the spray nozzles where the grease is air atomized and sprayed onto the pressure side of the gear teeth. The system continues to operate until a pre-selected number of operating cycles is completed. An alarm system provides visual and audible alarms on low air pressure, high pressure in the nozzle line, and excessive pumping time. Under alarm conditions, the electrical switchboard will automatically shut down the lubrication system. If the condition is not corrected, the ball mill will automatically be shut down. •
Barring (or rotating) gear – The barring gear is driven by the mill motor and produces a barring speed of 0.15 rpm. The barring gear system consists of a worm reducer driven by a 18.5-kW, 1460-rpm motor. A 203-mm diameter magnetic brake and brake drum is fitted between the motor and reducer. A gear-type coupling is fitted between the worm reducer low-speed shaft and one end of a double-ended electric motor shaft.
•
Ball charge – An initial ball charge for each mill consists of 124,989-kg of high-carbon, heattreated, forged steel balls with a nominal Brinell hardness of 600, composed of 60-mm, 50mm, 40-mm, and 30-mm balls.
•
Twin cone classifiers – The classifiers are made of all welded steel plate construction. The classifiers consist of a manually operated vane set, an inlet and outlet connection, two inspection ports, an inner and outer cone, a set of deflector vanes, a conical-shaped top with four 203-mm. inspection ports, ceramic tiles on the high-wear surfaces, and a reject line. The reject line has a cut-off gate and lining in the sloped area going to the mill feed box.
•
Silencer housing – The housing consists of a steel angle frame, 3-mm cover sheets, insulation to meet 85 db(a) specification, an access door, and an inspection hole with cover.
•
Piping ducts – The piping ducts consist of ducts from the feeder outlet valves to the mill feed connections, from the mill discharge connections to the classifier inlet connections, and from the classifier reject connections to the feed box reject return connections.
•
Ball charge hopper – The ball charge hopper is supplied with an isolation valve and chute from the hopper to the mill feed/discharge box.
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Mill bypass damper assemblies.
•
Seal air fans – The seal air fans deliver 5.29-m3/second airflow at 38°C at an elevation of 1615 m. The design discharge pressure is 12.93 kPa, and the suction pressure is -0.25 kPa. The air fans include an inlet silencer with a filter, a 120-Mw motor, acoustic dampening to achieve 85 db at a 3-m sound power level, gear-type coupling, and ductwork from the seal air fan to the various mill components.
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8
TECHNICAL DESCRIPTION – RILEY POWER INC.
Riley Power Inc. is part of the Service Business Unit of Babcock Power Inc. Riley Power Inc. was previously called the Riley-Stoker Corporation. This section covers a general description, components, systems, and modifications for the Riley Power ball/tube mill.
8.1
General Description
The Riley Power ball/tube mill [7–9] is a cylindrical, low-speed grinding mill designed for highcapacity grinding of coals ranging from anthracites to low-rank sub-bituminous and lignites. Grinding capacities range from 20,000–300,000 lbs of coal per hour. The Riley Power mill systems that are currently offered are gear-driven, double-feed, and double-discharge systems. There are also chain-driven units in service. The ball/tube mill is characterized by the size of the rotating drum, specifically the inside diameter and the length. The mill consists of a steel barrel lined with cast alloy liners, partially filled with hardened steel balls. Coal, mixed with heated primary air, enters each end of the mill from a crusher-dryer or directly from a feeder. With a crusher-dryer, the size of the steel balls is smaller than a mill without a crusher-dryer. Figure 8-1 shows a typical Riley Power chain-driven ball/tube mill system.
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Technical Description – Riley Power Inc.
Figure 8-1 Riley Power Chain-Driven Ball/Tube Mill System [7]
Figure 8-2 shows the outline of the ball/tube mill system with trunnion bearings and gear drive.
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Technical Description – Riley Power Inc.
Figure 8-2 Riley Power Gear-Driven Ball/Tube Mill System [8]
Coal flows from the feeders to the crusher-dryers, which are installed at the inlets to the mill, the crusher-dryer performs the primary crushing process. Hot primary air is introduced in the crusher-dryer and evaporates most of the surface moisture in the coal. A reduction in the coal size in the crusher-dryers results in lowering the power requirements of the mill. Primary air conveys the coal from the crusher-dryer to the mill drum. Pre-crushed and partially dried coal and preheated air (primary air reduced in temperature after going through the crusherdryer) enters the rotating drum through the mill inlet box located at each end of the drum. The mill drum is connected to the mill inlet/outlet boxes by pressurized air seals. As the mill rotates, the balls and coal are lifted by the corrugated shape of the liners. The balls and coal then cascade down the drum and pulverize the coal by impact and attrition. The pulverized coal and primary air leave the drum through the outlet box, and the coal goes to the external centrifugal classifiers. Fine coal particles exit the classifier through the shutoff valves into the coal piping for transport to the boiler burners. Classifier rejects are returned to the mill for further grinding through the coarse particle return pipe. The classifiers reduce overgrinding and control the coal fineness.
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Technical Description – Riley Power Inc.
The rotating drum is driven by the mill motor, speed reducer gearbox, air clutch reducer, and chain drive assembly or pinion/ring gear set. The rotating drum is supported by two spherical roller bearings or by two oil film trunnion bearings. Temperature detectors are installed on all bearings associated with the mill system. These temperature detector readings, the load cell readings from the thrust roller assembly, the clutch air pressure, and the oil pressures are all monitored by the mill alarm system for safe operation of the mill. In order to grind and deliver the coal efficiently, the amount of coal in the mill must be accurately controlled, which is accomplished by the Power-Sonic mill conditioning system. This system uses a measurement of the noise made by the grinding process, sensed by a microphone located on the mill silencer housing, and a measurement of the mill motor power to generate a signal proportional to raw coal demand. This signal and a feed-forward signal from the combustion controls determine the feeder speed. The chain-driven ball mill is supplied with 24 temperature detectors. Nineteen temperature detectors are located on the mill drive bearings, four are on the speed reducer, and one is in the chain oil bath. Some characteristics of this type of mill are: •
The grinding balls can be replaced while the mill is in service.
•
Mill liners require replacement up to a 10-year interval, depending on fuel abrasiveness.
•
Each mill end has a feeder and crusher-dryer set that may be taken out of service without taking the mill out of service.
•
Coal fineness of 99% or higher through 50-mesh screen and 80% or higher through 200mesh screen can be achieved.
The mill is completely enclosed in a sound insulation housing in order to reduce the noise of mill operation. Access doors are provided on both sides of the housing for maintenance access.
8.2
System Components
The Riley Power ball/tube mill system consists of the following components: •
Feeder
•
Crusher-dryer
•
Rotating drum/barrel
•
Grinding balls
•
Classifier
•
Shutoff valve
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Technical Description – Riley Power Inc.
• Speed reducer gearbox • Clutch • Chain drive 8.2.1 Feeder The feeder supplied for the Riley ball mills can be a gravimetric or volumetric feeder. A Riley Power drum-type volumetric feeder is shown in Figure 8-3.
Figure 8-3 Drum-Type Feeder [9]
The feeder consists of the following components: •
Rotating drum
•
Adjustable leveling apron
•
Apron liner
•
Wiper blade
•
Shearing pin device
•
Seal air connections
•
Feeder housing
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Technical Description – Riley Power Inc.
•
Speed reducer
•
Isolation gate valve with manually operated handwheel
The feeder has a housing that contains a rotating feed drum that delivers a measured amount of coal to the crusher-dryer and is designed with eight longitudinal arc-shaped pockets. This drum is fabricated from an alloy metal that is non-magnetic and corrosion resistant. An adjustable leveling apron covers the discharge side of the drum to level the coal in the pockets and to ensure uniform delivery with each drum revolution. A spring attached to the apron shaft allows the leveling apron to swing out and pass larger objects or foreign materials. An adjustable apron liner at the rear of the drum acts as a seal to prevent fine coal particles from leaking down behind the drum. Below and behind the drum is a revolving wiper that is synchronized with the drum to clear the drum pockets of remaining coal. Through gearing, the wiper is timed to clean each drum pocket with each revolution of the drum. A shearing pin device is provided to prevent damage to the feeder and drive mechanism when foreign objects jam or obstruct the drum. A variable speed motor is used to drive the feeder. Speed changes may be accomplished manually or automatically by the control mechanism. The feeder is pressurized from the primary air system. When a feeder is pressurized, sealing air is provided to prevent infiltration of coal dust into the shaft bearings. 8.2.2 Crusher-Dryer The crusher-dryer is designed to feed partially dried, granulated coal and primary air to the mill. The crusher-dryer is a pressurized, constant-speed, swing hammer-type coal crusher that operates with hot primary air for drying the coal. There are typically two crusher-dryers supplied for each mill. A crusher-dryer is shown in Figure 8-4.
Figure 8-4 Crusher-Dryer [9]
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Technical Description – Riley Power Inc.
The crusher-dryer is mounted on a base plate and is driven by a constant-speed induction motor through a self-aligning coupling. The shaft is supported by double-row roller bearings. Forcedfeed lubrication is provided to maintain a low bearing temperature. Raw coal and preheated air enter the hammer circle where the coal is crushed by impact between the hammers and breaker plate. The crusher section is equipped with six rotors that have evenly spaced hammers mounted on each rotor. Each hammer is cast and secured to the rotor disc with a pin. An adjustable crusher block assembly is located below the breaker plate to allow in-service adjustments to maintain crushing efficiency. The lubrication system consists of two oil pumps, a cartridge filter, a gravity oiler, an oil bath air filter, a check valve, a needle valve, a pressure gauge, and an overflow sight glass. The crusher-dryer is equipped with an air seal connection mounted on each side of the crusherdryer where the shaft enters the housing. The purpose of the air seal arrangement is to prevent coal/air leakage from the crusher-dryer. Seal air requirements to each connection are approximately 30 scfm per seal or 60 scfm per crusher-dryer at a pressure of 10 in. water gauge above the static pressure existing at the crusher-dryer inlet. A 2-in. pipe connection is provided at each seal for the seal air piping. The normal inlet air temperature for the crusher-dryer is between 450°F and 700°F with a maximum air temperature of 900°F. The rotational speed of the crusher-dryer is 900 or 1200 rpm depending on the size. 8.2.3 Rotating Drum or Barrel The rotating drum or barrel contains the coal and balls that grind the coal. The drum is a cylindrical steel barrel with welded steel heads and integral steel roller tires or large trunnion bearings on each end of the mill barrel. At the ends of the barrel are welded boxes through which the coal/air mixture passes into the mill barrel and out of the mill barrel. The mill barrel is connected to these inlet/outlet boxes by a set of lubricated air seals at each end of the mill barrel. Seal airflow behind the seal prevents any coal/air mixture leakage from the seal into the plant. For the chain-driven ball/tube mills, the roller tires rotate on trunnion support rollers and support the mill barrel. The support roller assemblies consist of a pedestal base, a support assembly, bearing housing, and roller. There are four assemblies on each mill. The thrust roller assembly is used to hold the mill drum or barrel in place axially. The drive end of the drum is positioned between the thrust rollers to restrict the axial movement of the mill during operation. The clearance between the sides of the mill barrel and the two thrust rollers should be 0.010 in. ± 0.005 in. A load cell (strain gauge) is connected to the roller assembly and wired to the mill alarm system. The load cell monitors any excessive load put on the thrust roller assembly that could move the barrel. Detection of excessive barrel movement by the load cell activates an alarm. Limit
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Technical Description – Riley Power Inc.
switches are positioned on the thrust roller assembly. They are connected to the alarm and trip the circuitry whenever a severe radial movement of the barrel is detected. A multi-strand roller chain is used to rotate the barrel. The assembly consists of a sprocket assembly on the outside of the barrel, a drive sprocket on the drive shaft, and an idler sprocket for chain adjustment. The drive shaft and idler shaft are supported by pillow block bearing assemblies. The roller chain is lubricated by an oil bath system. For the gear-driven ball/tube mills, a single helical gearing is used for the drive arrangement. A high-speed induction motor and speed reducer supply the power. A direct-drive, low-speed synchronous motor and coupling may be furnished. Trunnion bearings are the ball-and-socket type, have easily removable babbitted bearing sockets, and are water cooled. A high-pressure, motor-driven oil pump is provided for each main bearing to float the mill trunnions on the bearing surfaces before the start-up cycle. Oil is continually circulated to the bearings when the mill is operating. The ring gear assembly is equipped with an automatic spray lubrication system. The mill barrel is shown in Figure 8-5.
Figure 8-5 Mill with Pinion/Ring Gear Drive Set [9]
8.2.4 Grinding Ball Makeup Riley Power Inc. recommends specific ball sizes and ratios of each ball size for ball/tube mills with and without crusher-dryers.
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Technical Description – Riley Power Inc.
The ball-charging hopper enables makeup ball charges to be added to the mill during operation and without mill service interruption. Balls may be added to an operating mill by closing the hopper outlet valve on the ball charging hopper and filling the hopper with balls. The cover should then be latched securely in place and the stop valve opened to allow the balls to enter the mill. This procedure is then repeated until the required addition to the ball charge is complete. 8.2.5 Classifier The classifiers for the Riley Power ball/tube mills can be static or dynamic. 8.2.5.1
Static Classifier
The static centrifugal classifier ensures that the coal is accurately sized, uniformly mixed, and evenly distributed to all boiler burners in service. Oversized coal particles are returned to the mill. Thermocouples are installed to monitor classifier exit temperatures. The static centrifugal classifier consists of an outer cone, an inner cone equipped with adjustable vanes, a segmented discharge cylinder, and a coarse return pipe with an integral trickle valve. The trickle valve is a self-closing door mechanism that allows the return of oversized coal particles to the mill inlet while preventing the backflow of primary air. A mixture of primary air and pulverized coal enters the bottom of the classifier between the inner and outer cones where a rotational spin is imparted to the mixture by fixed vanes. At the top of the inlet section of the classifier, the mixture swings around and enters the inner cone through the adjustable control vanes. Classifier discharge fineness can be altered by adjustment of these control vanes. The forced rotation of the mixture into the discharge chamber provides substantial uniformity in particle fineness and density leaving the classifier. In the inner chamber, the oversized particles fall downward into the coarse return pipe leading back to the mill. When the accumulation in the coarse return pipe becomes heavy enough, the counterbalanced door opens to permit the contents to be discharged against the existing pressure differential and into the mill inlet for regrinding. The efficient classification of coal size is accomplished by the swirling effects imparted on the coal/air mixture within the centrifugal classifier. The static classifier is shown in Figure 8-6.
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Technical Description – Riley Power Inc.
Figure 8-6 Static Classifier [7]
8.2.5.2
Dynamic Classifier
To achieve better coal fineness and increase mill capacity, a dynamic classifier is an option. The Riley Power dynamic classifier for ball/tube mills consists of a set of stationary vanes and a set of rotating vanes (cage) located within the stationary set. The rotating cage is driven by a variable-speed drive motor. Adjusting the speed of the rotating cage can change the intensity of the centrifugal force field to achieve fineness control and high efficiency of classification. For the mill equipped with a dynamic classifier, zero residue on a 50-mesh screen can be obtained at a fineness of 80% passing through a 200-mesh screen. 8.2.6 Shutoff Valves Isolating the mill from the burner is done by the coal shutoff valves that are designed to be installed in each coal supply line to the burners of a pulverized coal-fired furnace. These valves can be equipped with remote operating devices, either pneumatic cylinders or electric motors, that receive signals to open or close the valves from the burner management system. The coal shutoff valve is installed in the vertical run of coal piping just above the classifier outlet so when closed, the coal/airflow is completely stopped. 8.2.7 Speed Reducer Gearbox The speed reducer gearbox between the mill and the mill motor is shown in Figure 8-7.
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Technical Description – Riley Power Inc.
Figure 8-7 Speed Reducer Gearbox [7]
The Food Machinery Corporation Link-Belt parallel-shaft speed reducer uses helical gears that experience a lateral thrust on each shaft. The parallel-shaft reducer can be a single-, double-, or triple-gear reducer. Adjustments made with shims enable the use of ball and roller bearings at the bearing retainers. The intermediate- and high-speed shaft bearings are adjusted correctly when they have 0.005–0.015-in. of lateral play. The speed reducer is equipped with an oil bath lubrication system. The oil bath system for the
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Technical Description – Riley Power Inc.
speed reducer and the chain case has individual pumps driven by the same motor. The clutch is
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Technical Description – Riley Power Inc.
air-actuated from the plant air supply system. The air is introduced through the rotorseal into the unused end of the reducer output shaft. 8.2.8 Clutch The neoprene rubber and cord actuating tube is contained within a steel rim that is drilled for mounting to the driving component. As air pressure is applied to the air-actuating tube, the tube inflates, forcing the friction shoe assemblies uniformly against the drum that is attached to the driven component. The friction shoe assemblies consist of friction blocks attached to aluminum backing plates and are guided by torque bars that are secured to side plates. As actuating air is exhausted, release springs and centrifugal force ensure positive engagement. The torque flow is from the driving shaft, through the element mounting component, through the rim/side plate structure, through the torque bars to the backing plates and friction material, where the torque is transmitted, and through the friction couple to the components mounted on the driven shaft (clutch drum and drum mounting component). The air-actuating tube automatically compensates for friction shoe wear, eliminating the need for adjustment. Centrifugal force and release springs ensure total disengagement of the friction shoes from the drum at the moment the air is expelled. Power is transmitted from the friction shoes through the torque bars to the side plates of the assembly. The maximum recommended air pressure is 150 psi. The clutch assembly is shown in Figure 8-8.
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Technical Description – Riley Power Inc.
Figure 8-8 Clutch Assembly [7]
The clutch carrier is mounted on the reducer shaft that has a drilled air passage for supplying air to the clutch. The clutch drum hub is mounted on the ball mill drive sprocket shaft. Bore diameters are machined to provide a standard interference fit when installed on the shafts. The interference is a maximum of 0.0005 in. for each inch of shaft diameter.
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Technical Description – Riley Power Inc.
Plant-supplied compressed air is passed through an air amplifier system to the air receiver and distributed under flow control conditions to the clutch arrangement. The air amplifier system consists of the following items: •
Air amplifier
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Solenoid air valve
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Flow control valve
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Flow switch
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Relief valve
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Pressure gauge
•
Filter
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Regulator with gauge
•
Check valve
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Air cock
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Gate valve
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Oil pump and filter with cooler set
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Oil pump motor
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Two hydraulic pumps
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Sight glasses
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Heat exchanger for the reducer
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Heat exchanger for the chain reservoir
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Strainer
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Valve
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Pressure gauge
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Needle valve
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Gate valve
•
Pressure switch
The rotorseal is a positive seal for introducing air under pressure into a rotating shaft. The rotorseal operates continuously or intermittently at high speeds in either direction. It is flange mounted to the machinery shaft or assembly. The rotating seal of the rotorseal is established by a lapped surface on the rotating carbon seal that is held against the lapped surface of the stationary shaft by spring pressure. The high-quality lapped finish of the contacting surfaces and the applied contact pressure ensure a good seal from the stationary member to the rotating member. Air can pass through the bore of the stationary shaft and through the bore of the rotating seal into
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Technical Description – Riley Power Inc.
the clutch assembly. Figure 8-9 shows the rotorseal.
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Technical Description – Riley Power Inc.
Figure 8-9 Rotorseal [7]
An air control system for the clutch is shown in Figure 8-10.
Figure 8-10 Clutch Air Control System [7]
8.3
Systems
The following systems are used in the Riley Power ball/tube mill: •
Primary air system
•
Seal air system
8.3.1 Primary Air System The primary air system consists of ductwork and dampers from primary air fans and air preheaters. The function of a primary air system is to convey the coal at a desired temperature to the boiler burners. This is accomplished by the position of various dampers for the control of the temperature, flow, and air velocity. In a cold primary air system, air from the atmosphere is moved by the primary air fan to a tempering air header, through the air preheater, and then to the hot air duct. In a hot primary air system, the primary air fan receives tempering air from the cold air duct and hot primary air from the secondary air ducts. The temperature of the coal/air mixture at the classifier outlet is maintained at the desired value by the movement of the tempering air dampers and the hot air dampers. Both dampers are butterfly-type dampers and are used for their tight
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Technical Description – Riley Power Inc.
closing feature.
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Technical Description – Riley Power Inc.
When the supplied coal is very wet and the action of the tempering air damper and hot air damper is not sufficient to maintain the required classifier exit temperature, the bypass damper is used. The bypass damper allows a portion of the primary air to go around the mill and to enter back into the classifier discharge. The flow of coal from the mill to the boiler burners is controlled by modulation of the rating damper. 8.3.2 Seal Air System The coal dust is contained within the mill system by automatic modulation of the seal air dampers. The seal air fans take suction from the tempering air duct, through an inlet filter box, and then deliver the suction to the seal air header. Each mill system has its own seal air differential pressure controller. This controller maintains a constant differential between the pressure in the seal air header and the pressure at the mill inlet. If seal air pressure is lost, the mill will trip. The sealing air system prevents coal dust leakage into and from the components of the ball/tube mill system by providing a positive flow of sealing air. Seal air is provided to the feeder, the feeder discharge isolation gate, and the crusher-dryer in order to prevent coal dust from damaging bearings and other rotating parts outside the coal stream. It also maintains a seal at the mill trunnion air seals (connection of mill barrel to inlet/outlet boxes), which prevents coal dust leakage into the plant’s environment. Seal air is forced from the discharge side through a transformation section, into the connecting pipe section, and to the discharge header. Air from the discharge header flows to the main distribution branches and then to each individual piece of equipment. Each branch section of the air distribution system is designed on the basis of the specific seal air requirements of the equipment. A solenoid-operated butterfly valve is located before each mill sub-system and modulates in response to the mill differential pressure. The seal air fan is typically provided by Buffalo Forge Company. The fan has a flanged inlet and outlet, independent pedestals with sole plates, variable-inlet vanes, an inlet screen, a housing split for wheel removal, sleeve bearings, outlet dampers, a housing drain and access door, bearing thermocouples, bearing seals, and a coupling guard. The seal air fans are generally located adjacent to the ball/tube mill system for accessibility. These single-width, single-inlet fans have a cleanout door that includes both a housing drawer and a coupling guard. The seal air fan is a booster fan that draws air from the tempering air duct, through the filter box, and supplies the required seal air to the various pieces of equipment. One fan is necessary to meet operating requirements. The second fan serves as a 100% backup unit.
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Technical Description – Riley Power Inc.
8.4
Modifications
Riley Power Inc. has introduced several modifications [8] to the ball/tube mill. The modifications covered in this section are the trickle valve addition, trunnion air seal re-design, the mill conditioning system upgrade, the hydrodynamic slide shoe bearing conversion, and the crusher-dryer crusher block. 8.4.1 Trickle Valve Addition The addition of a trickle valve is used in the classifying process. In order for the classifying process to operate successfully, a smooth flow of coal particle rejects should occur from the classifier back to the drum. Also, the primary air should not flow from the mill to the classifier through the reject line. A trickle valve is similar to a check valve and is installed to keep the flow of rejected coal particles and air flowing in one direction, from the classifier to the drum. The valve opens as the rejected coal pressure increases above the primary air pressure from the mill. In general, the valve moves continuously from the open and closed position during normal operation. The actual rejects flow rate is dependent upon coal properties and primary air pressure changes during operation. A new design trickle valve with an adjustable counterweight controls the rejects level above the valve for various operating conditions. The new trickle valve design consists of the valve housing with flanged connections to the rejects pipe, a valve plate, a valve shaft with an external indicator, shaft support bearings, and an adjustable counterweight. The new trickle valve design is shown in Figure 8-11.
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Technical Description – Riley Power Inc.
Figure 8-11 New Trickle Valve Design [8]
8.4.2 Trunnion Air Seal Redesign Pressurized air seals are needed between the rotating mill and the mill inlet/outlet boxes. These seals prevent leakage of coal dust or air from the mill. The old seal design was a lip-seal design, and the new seal design uses a self-tightening pad seal arrangement. The new design seal improves the seal distribution with an enlarged air chamber, a longer service life, and a higher tolerance for surface irregularities in the mill head extension. Figure 8-12 shows the old and new trunnion air seal designs.
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Technical Description – Riley Power Inc.
Figure 8-12 Trunnion Air Seal Designs [8]
8.4.3 Mill Conditioning System Upgrade A Riley Power Power-Sonic mill conditioning system was developed in the 1970s to control the total coal inventory in the mill by using the measurements of two mill variables. One variable is obtained by comparing the coal inventory to the mill power (kW input) required for the mill motor to rotate the mill barrel. The second variable is the sonic level (sound) of the steel balls colliding with each other and with the mill liners. This sonic level is also related to the amount of coal inventory in the mill. Riley Power recently upgraded the Power-Sonic mill conditioning system design by adding an Allen-Bradley programmable logic controller-based system with a control setup display. The display allows the operator to adjust parameters and setpoints for various applications. The new design is compatible with a digital control system and capable of peak display and recording. 8.4.4 Hydrodynamic Slide Shoe Bearing Conversion Merom Generating Station of Hoosier Energy installed a hydrodynamic slide shoe bearing for the mill drum. The bearing assembly consists of the bearing housing, bearing segments, and an oil supply. Figure 8-13 shows the hydrodynamic slide shoe bearing.
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Technical Description – Riley Power Inc.
Figure 8-13 Hydrodynamic Slide Shoe Bearing (Courtesy of Hoosier Energy – Merom Generating Station)
The two identically designed segments are mounted on the bearing seat in the bearing housing. The bearing shoe is supported on the bearing base through the axial bearing that permits gyratory movement. The thrust bearing is hydrodynamically lubricated. The lubricating oil is distributed through annular grooves in the segments. Figure 8-14 shows the thrust bearing.
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Technical Description – Riley Power Inc.
Figure 8-14 Thrust Bearing (Courtesy of Hoosier Energy – Merom Generating Station)
The lubrication system for the bearings consists of a high-pressure pump and motor, a lowpressure pump and motor, a thrust pump, backup pumps and motors, an oil cooler, a filter, and associated piping. The high-pressure pump provides the lift for the main shaft at a maximum pressure of 3000 psi. The low-pressure pumps provide bearing lubrication. Each pump provides their lubrication at a pressure of 225 psi. The control system will not allow the mill to start until the pumps have been operational at least 3–5 minutes. After the mill has started, the high-pressure pump will operate for another minute, and then it will shut down. The low-pressure pump and thrust pump operate continuously with the mill operating. If insufficient flow is detected, then the backup pumps will begin operation. 8.4.5 Crusher-Dryer Crusher Block Riley Power Inc. has introduced modifications to the crusher-dryer with a limited float crusher block. The purpose of the crusher block assembly is to maximize the effect of the swing hammers in reducing the size of coal passing through the crusher-dryer. As the swing hammers
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Technical Description – Riley Power Inc.
and crusher block wear during their service life, the adjustable crusher block can be repositioned
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Technical Description – Riley Power Inc.
to maintain the proper gap between the swing hammer tips and block to achieve maximum size reduction. The original design resulted in excessive float between the adjustment screw and the crusher block. The excessive float prevented the block from being accurately withdrawn to maintain the proper gap after impact with the rotating swing hammers. The end result was less than optimum coal size reduction. The latest design features a modified adjustment mechanism that substantially reduces crusher block float. This permits the crusher block to be precisely positioned with respect to the rotating swing hammers. The crusher components provide uniform wear across the width of the crusher block. Riley Power Inc. conducts regular wear tests to determine the most wear-resistant materials.
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9
TECHNICAL DESCRIPTION – STEIN INDUSTRIE
This section covers the ball/tube mills found in the Eskom Matimba Power Station, Majuba Power Station, and the Tutuka Power Station. The tube milling plant is composed of the following: •
Tube mill bunkers and hoppers
•
Coal feeder system
•
Tube mill pulverizing system
•
Tube mill primary air system
•
Tube mill sealing air supply
The following topics are included in this section: •
General description
•
Systems
9.1
General Description
The coal pulverizing system is comprised of five identical pressurized tube mill assemblies per boiler unit. Four tube mills are normally used when the boiler is operated at the maximum continuous rating. The fifth tube mill is kept in reserve and used when one of the four running tube mills is taken out of service. Each tube mill supplies pulverized coal to six burners in the boiler. Each group of six burners is arranged in a row on the same level, three in the front wall and three in the rear wall. Mill 1 supplies pulverized coal to the bottom row of burners. Each successive row above the bottom row is supplied by mills 2–5, with mill 5 supplying the top row. Figure 9-1 shows the arrangement of the Stein Industrie tube mill. A more detailed view of the Stein Industrie tube mill is shown in Figure 9-2.
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Technical Description – Stein Industrie
Figure 9-1 Stein Industrie Tube Mill (Courtesy of Majuba Power Station)
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Technical Description – Stein Industrie
Figure 9-2 Detailed View of the Stein Industrie Tube Mill (Courtesy of Tutuka Power Station)
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Technical Description – Stein Industrie
Overland conveyors supply graded raw coal to the tube mill bunkers, which stores raw coal to be used by the tube mill. Each bunker supplies only one tube mill. The conical, steel bunker directs the coal through two hoppers attached to the bottom of each bunker. Coal is fed by gravity into two raw-coal feeders that work in conjunction with the bunker and hoppers. Each raw-coal feeder supplies one end of one tube mill. The amount of coal fed to the tube mill depends on the unit load. As the unit load increases, the speed of the raw-coal feeders increases, and more coal is supplied to the tube mill. Each feeder feeds coal through the coal and primary air mixing box, where it is mixed with heated primary air. The heated primary air raises the temperature of the coal and dries it, making the coal easier to pulverize. If the pulverized coal is warm, it will be closer to the coal combustion temperature. Hot primary air is taken from the hot air leg of the combustion air system, after the air heater. The primary air passes through a hot air control damper and into the hot and cold primary air mixing box. Cold tempering primary air is tapped from the cold air leg of the combustion air system and passes through the tempering air control damper into the hot and cold primary air mixing box. The tempering air control damper provides cold air to mix with the hot air to produce the mill outlet temperature. The tempered air then passes through a quick-closing isolating damper to the primary air inlet piping, bypass piping, and purge air piping. The primary air inlet piping is the main inlet into the tube mill. The primary air conveys the pulverized coal from the mill to the boiler burners. The bypass piping feeds the primary air into the raw coal and primary air mixing boxes to dry and raise the temperature of the coal prior to pulverizing. The purge air piping feeds the primary air to the pulverized coal ductwork after the classifier to maintain a pulverized coal flow of not less than 20 m/second. The velocity of 20 m/second prevents the pulverized coal from settling in the ductwork. The raw coal falls down the raw-coal chute into the mill coal inlet, where the screw conveyor feeds it into the tube mill. Inside the tube mill, the raw coal mixes with the forged chromemolybdenum steel balls (cylpebs). Monobloc liners, attached to the inside of the mill body, lift the coal and cylpebs when the tube mill is turned. The falling action of the coal and cylpebs pulverizes the coal. Primary air, fed into both ends of the tube mill, lifts and conveys the pulverized coal out of the tube mill, through the classifiers, and to the boiler burners. The tube mill is driven through a girth gear fitted onto its circumference in order to provide the maximum torque. An electric motor and reduction gearbox provide the main drive to the drive pinion gear. Slow movement, normally barring or inching, of the tube mill is achieved through an auxiliary drive system. Because of the mass of the tube mill (~120 tons), a high degree of friction occurs at its turning points. To overcome this, oil is pressure-fed to the journal bearings, also known as trunnion bearings. An oil film is created in which the tube mill journals (trunnions) can turn.
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Technical Description – Stein Industrie
Cylpebs are added, when required, to maintain the pulverizing efficiency of the tube mill. A ball feed mechanism, located adjacent to the raw-coal feeders, introduces new cylpebs to enter the drive end of the tube mill with the raw coal. Pulverized coal passes from the tube mill to the classifier, which classifies the size of the coal particles. Heavier coal particles drop out of the pulverized coal stream and return to the tube mill through the rejects line for finer pulverization. The finer pulverized coal particles continue through the coal ducts to the boiler burners. A carbon dioxide fire protection system is incorporated within the tube mill. Carbon dioxide can be fed into the tube mill to extinguish any fire.
9.2
Systems
This section covers the following systems for the Stein Industrie tube mill: •
Coal inlet system
•
Primary air system
•
Seal air system
•
Lubrication systems
•
Drive system
•
Blowdown system
•
Ball loading system
9.2.1 Coal Inlet System The coal inlet system from the feeder to the mill is composed of a continuous chain fitted with flights and moved in an enclosed steel trough. The steel trough has a rectangular cross section. The chain/scraper conveyor slides along the casing bottom and conveys the coal. The conveyor is guided around the end station shaft by a non-toothed idler. The reverse run of the chain is supported by a central guide rail above the coal being conveyed. The coal is extracted from the bunker by the upper returning chain. The coal then drops to the lower bottom of the adapter trough. The coal is transported by the lower chain toward the outlet at the terminal end. The chain is pulled by a rotating toothed wheel. The toothed wheel is driven by a 380-V motor through a variable-speed gearbox in order to control the coal level in the mill on the coupling connecting the toothed wheel and the variable-speed gearbox. A shearing pin is fitted to protect the feeder. If any foreign matter such as steel pieces or rocks enter the feeder chain and wedge between the flights, the foreign matter will break the shearing pin and not the feeder chain or flights. The variable-speed gearbox speed for the feeder can be adjusted manually.
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Technical Description – Stein Industrie
9.2.2 Primary Air System Primary air is taken from the hot and cold air (tempering air) ducts from the primary air fans. Each duct is equipped with control dampers that are controlled by the mill outlet temperature control system. The mill outlet temperature is set from the unit control desk. After the hot and cold air are mixed, the air goes through the quick-close damper. The ductwork is then divided into four lines. Two lines go to the drive-end side of the mill and the other two lines go to the non-drive-end side of the mill. One line on each side supplies the mill directly with primary air through a control damper. This damper controls the amount of airflow entering the mill for transporting the pulverized fuel to the burners. A flow transmitter is installed on these lines to give a flow reading in the unit control room. The other two lines are for bypassing the mill. These lines, located on each side of the mill, have a bypass control damper on each line. This damper controls the amount of air through the coal pipes and mixing box. A flow transmitter is installed to measure the flow. Each bypass line is again divided into two lines. One line is for direct connection to the coal pipes through an isolating damper. The purpose of this bypass air line on the coal pipes is to maintain the velocity in the coal pipes as constant as possible during load changes. It is also used for the purging of the coal lines. The other line is connected to the mixing box through an isolating damper. The purpose of this line is to dry and increase the coal temperature before it enters the mill. Figure 9-3 displays the airflow through the mill.
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9-6
Technical Description – Stein Industrie
Figure 9-3 Mill Airflow (Courtesy of Tutuka Power Station)
9-7
Technical Description – Stein Industrie
9.2.3 Seal Air System As the pulverized coal is conveyed in an air stream, it tends to leak from the tube mill through the air gaps between the rotating and stationary parts of the mill. Seal air, at a greater pressure than the primary air, is used to prevent the coal-laden primary air from escaping and damaging sensitive equipment and machined surfaces. Cold air is taken from the tempering air duct and is boosted by a sealing air fan. Therefore, seal air is at a slightly higher pressure than primary air. Seal air is used to contain the pulverized coal at the following points: •
Ball feed mechanism delivery tube
•
Both hot air box stuffing boxes
•
Both conveyor body seal air boxes
•
Trunnion
•
Girth gear
Seal air is also fed to the hinges of the coal level detector on both raw-coal feeders. The seal air prevents coal dust from getting into the detector and causing it to jam or stick. The girth gear surrounding the tube mill is supplied by its own seal air system. The seal air system prevents coal dust and pulverized coal from getting to the machined surfaces of the girth gear, the pinion gear, and the lubricating system. The seal air system is composed of four different lines from the trunnion seal air system and the girth gear seal air system. In the trunnion seal air system, the seal air is supplied by the seal air fan that receives air from the tempering air duct through a hand-isolating damper. Two lines, one for each side of the mill, ensure a seal between the rotating parts of the mill and the stationary parts. A control damper controls the seal air pressure. Limit switches are installed to indicate whether the damper is open or closed. The seal air pressure will increase or decrease as the pressure inside the mill increases or decreases. The seal air pressure must be higher than the primary air pressure inside the mill. Two lines supply seal air upstream of the seal air control damper. One line goes to the coal feeder to keep the coal detection flaps clean. The other line goes to the ball loading for sealing purposes. On the girth gear seal air system, a separate seal air fan is installed at ground level. This fan receives air through an air filter from the atmosphere. It delivers the air to the girth gear enclosure to prevent any dirt from entering the gear system. This occurs by pressurizing the
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9-8
Technical Description – Stein Industrie
enclosure above atmospheric pressure.
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9-9
Technical Description – Stein Industrie
9.2.4 Lubrication Oil Systems There are several lubrication systems for this mill. The systems are: • Turbo lubrication system – The turbo lubrication system is divided into three compartments: the oil return, preparation, and working oil. • Low-pressure sprinkle system – The purpose of the low-pressure sprinkle system is to sprinkle oil on top of the mill bearings and to supply the working oil compartment of the oil tank with clean oil. The sprinkle oil acts mainly as a cooling medium for the bearings. The two mill bearings support the mill trunnion and the bearing ball and socket support. If any of the bearing temperatures reach 80°C, the mill will trip. A warning alarm in the unit control room will indicate when the bearing temperatures reach 60°C. The low-pressure oil pump receives oil from the preparation compartment through a suction filter that is located inside the tank. At the pump discharge, a safety valve is installed that operates at a pressure of 1 MPa. The oil from the pump discharge returns to the oil return compartment. From the pump, the oil flows through an oil/water cooler. A control valve on the water inlet controls the oil cooler oil outlet temperature in conjunction with the three heaters inside the oil tank. If the differential pressure increases, the two filters on the low-pressure system can be changed over manually by a three-way valve. After the filters, the lines split into two sections, and some of the oil flows through an orifice at a rate of 40 liters/minute to the working oil compartment of the oil tank. This flow ensures suction to the high-pressure pump. The rest of the oil flows to the drive-end and non-driveend bearings of the mill at a rate of more than 21 liters/minute and is sprayed on top of the bearings. An alarm will sound in the unit control room if the flow drops to 21 liters/minute. The maximum oil temperature at the low-pressure pump outlet is 55°C. The minimum oil temperature for low-pressure pump operation is 20°C. The high alarm oil temperature is set at 56°C, and the oil heater will stop at 45°C. The low-pressure pump will not start if the oil temperature is less than 19°C. Figure 9-4 shows the low-pressure lubricating oil system.
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910
Technical Description – Stein Industrie
Figure 9-4 High- and Low-Pressure Lubricating Oil System (Courtesy of Tutuka Power Station)
•
High-pressure oil system – The high-pressure oil system receives oil from the working oil compartment of the oil tank. This system consists of three motors and eight pumps. Four of the pumps supply jacking oil, two pumps supply thrust oil to the mill, and two pumps supply oil for the ball and socket lubrication. The jacking oil causes the bearings to float and prevents metal-to-metal contact. Two jacking oil pumps and one thrust oil pump are driven by one electrical motor for the drive-end side of the mill. The same arrangement is installed for the non-drive-end side of the mill with the exception of the thrust oil going only to the drive-end side. Two ball and socket pumps (positive displacement) are driven by one electrical motor. The jacking oil is supplied to the drive-end and non-drive-end bearings by four lines. Two lines go to the drive-end and two lines go to the non-drive-end side bearings. Each of these lines is equipped with a pressure gauge, a non-return valve, and a safety valve that operates at 8 mPa. The thrust bearing oil line provides oil to the thrust bearing. The mill girth gear teeth are fitted at an angle, and when the gear is turned by the motor, a thrust movement occurs toward the drive end. To counteract this movement, thrust oil is supplied in order to prevent damage to the mill bearings and seals.
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9-11
Technical Description – Stein Industrie
The oil to the mill thrust bearing is supplied by two lines, each equipped with a pressure gauge, a non-return valve, and a safety valve. The safety valve operates at 8.9 MPa. This oil is supplied to the drive-end side only. The ball and socket oil is supplied to the drive-end and non-drive-end bearings by one motor and two pumps. Each line is equipped with a pressure gauge, a non-return valve, and a safety valve. The safety valve operates at 8 MPa. A sump is installed at the bottom of the drive-end and non-drive-end bearings. From this sump, the oil flows back to the oil return compartment of the oil tank by gravity. The oil from the safety valves on the oil lines also drains to the oil return compartment of the oil tank. •
Gearbox oil system – The main speed gearbox oil system uses oil pumped from the gearbox through a filter back to the gearbox top. The oil then passes over the gears and the lubricating oil cooler. The lines are composed of two filters, one in service and one on standby. Each filter is equipped with a differential pressure gauge over the filter with a maximum difference of 50 kPa. A pressure gauge, temperature gauge, and flow meter are installed in the line and located after the pump. At the pump, a safety valve is installed that will return the oil back to the gearbox. Inside the gearbox, a cooler that is fitted with a control valve on the water outlet line controls the temperature of the oil. At the gearbox, high- and low-level switches are installed to warn against high or low oil level.
• Girth gear lubrication system – In the girth gear lubrication system, grease is pumped from a grease drum by an air-operated pump and is used as a lubricating medium on the girth gear. Air from the service air range is supplied at a constant pressure through an air filter, a regulator with a pressure gauge, and an air lubricator to a normal two-three-way solenoid valve. The solenoid valve is energized by the interval timer, allowing clean oil and air at the correct pressure to pass to the air-operated grease pump. A separate air line goes to the spray lance (nozzle) for the atomizing of the grease. The air-operated grease pump discharges the lubricant through a pressure gauge to the singleline distributor. From the distributor, the grease is discharged to the five spray nozzles at the girth gear. The pump continues to operate until a pre-selected number of operating cycles have been completed. The number of cycles is determined by a timer in the control box. The timer gets a signal from a counting switch next to the block. Because there is air flowing through the nozzles, the lubricant is atomized and delivered onto the girth gear teeth. If this system fails to operate, an alarm will sound after 10 minutes. The mill will trip after 30 minutes of the system failure. • Main motor lubrication system jacking system – The main motor bearings use ring lubrication. Because the oil rings do not work when the mill is on barring, there is no lubrication on the bearing. To prevent damage to the bearings, a Hytec lubrication system is used to ensure sufficient lubrication of the bearings and remove the hot oil from the sumps for cooling.
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912
Technical Description – Stein Industrie
Oil is drawn from the tank through a suction filter by two pumps. A bypass is fitted to the filter in order to allow the oil to bypass the filter when it becomes dirty. A vacuum gauge fitted before the pump allows monitoring of the suction pressure. This is an indication of the filter condition. Pump A discharges the oil through a cooler back to the tank. No cooling water is connected to the cooler because of low ambient temperature. Pump B discharges the oil to the forced feed and jacking system. The forced feed lubrication system is used under normal working conditions. Oil is discharged by the pump through a discharge filter to the directional control valve. The filter is supplied with a spring-loaded non-return valve. The valve acts as a bypass in the event of a blocked filter. In the forced feed mode of operation, the directional control valve is deenergized. Oil flows through the directional control valve out of port A and to the mill motor bearings. A pressure relief valve is located in the bearing supply line to control the oil pressure at 2 MPa. Oil from the bearings returns to the tank by gravity. The jacking system is used when the mill motor is started or when the mill is on barring. In this mode of operation, the directional control valve is energized. Oil flows through the directional control valve out of port B and to the mill motor jacks. The jacks lift the shaft clear of the bearings. When the directional control valve is deenergized, the forced feed system is in operation. Oil expelled from the jacks flows back to the tank through port T of the directional control valve. A pressure relief valve is located in the oil supply line to the jacks. The relief valve opens at a pressure of 4 MPa. Table 9-1 shows the normal operating oil pressure values. Table 9-1 Normal Lubrication Oil Pressure Values (Courtesy of Tutuka Power Station) Lubrication System
Parameter
Value
Low-pressure
System pressure
5–6 Bar or 500–600 kPa
High-pressure
Jacking oil pressure
80 Bar or 8 MPa
Ball and socket oil pressure
30 Bar or 3 MPa
Thrust bearing oil pressure
15 Bar or 1.5 MPa
Suction pressure
0 kPa
Discharge pressure with mill on-line
2 MPa
Discharge pressure with mill off-line
4 MPa
Main motor bearings
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9-13
Technical Description – Stein Industrie Recirculating oil pressure through cooler
300 kPa
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914
Technical Description – Stein Industrie
Table 9-2 shows the mill lubrication systems for the Tutuka Power Station. Table 9-2 Mill Lubrication Systems (Courtesy of Tutuka Power Station) Lubrication System
Manufacturer
Lubricant Type
Lubricant Volume
Filtration
Bearing Type
Main gearbox
David Brown Model 360X535 CRS
British Petroleum (BP) Energol GRXP 150
445 liters
Vokes (E238L/62982) 17 micron nominal at 61 liters/minute
Rolling element
Inching gearbox
Citroën Messian
BP Energol GRXP 150
145 liters
None
Rolling element
Main lube oil tank
Stein Industrie Turbolub
BP Energol HL 460
1315 liters
EPE 40 micron nominal at 17 liters/minute
All white metal
Mill main motor lube oil tank
Hytec
BP Energol THB 32
45 liters
Hydac 10 micron nominal at 60 liters/minute
All white metal
9.2.5 Drive System The mill drive system consists of a 380-V motor for the barring or inching rotation and a main drive motor (3.3 kV). The main mill motor is provided with two tail shafts. At the drive-end side of the mill, the tailend shaft is connected to the main speed gearbox. On the other tail-end shaft, the barring motor is connected by a gearbox. The barring motor is used to drive the mill and the main motor at a very slow speed when the mill is off-line. Technical Key Point Barring is performed when the mill needs to be cooled down. Inching of the mill is performed during maintenance periods and allows the precise positioning of the mill drum inspection door. The gearbox contains a free wheel system that uncouples the low-speed barring gear when the mill is on-line. A safety device will trip the mill when the free wheel system is not operating correctly. The barring motor is provided with an electromagnetic brake. The purpose of the brake is to lock the mill when barring is stopped at a certain position for maintenance. The shoes of the brake can be released locally by a padlock lever. The lever position is monitored by limit switches.
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9-15
Technical Description – Stein Industrie
9.2.6 Blow-Down System To prevent the lines and probes from blockage and from a malfunction of the automatic controls, a blow-down line is installed. Station air is used to clear any blockage of the instruments. The system works automatically. When the blow-down system is in operation, the drive-end and then the non-drive-end side probes will be purged. After 1 hour, the program will start the purge program again. It is important to know that the mill controls on the side where purging is performed will be frozen during the purge period on the set point that existed before the purging. 9.2.7 Ball Loading System Additional grinding media must be introduced into the mill. The wear of the balls depends on the quality of the coal. Less abrasive coal allows the balls to remain in service for a longer time. The mill must be loaded with balls whenever the power output decreases. A square ball container is lifted onto a platform located above the ball loading system. The loading system consists of a hopper, a quick-opening gate, a lock chamber, and a slow-opening gate. When the balls are loaded, the quick-opening gate is opened. The balls are then dropped into the lock chamber, and the quick-opening gate is closed. The slow-opening gate is then opened so that the balls drop gently into the mill through the fuel inlet line. The two gates are fitted to prevent air and coal from blowing into the plant during ball loading. The feed trunnion includes a screw feeder that drives the balls toward the inside of the mill drum. The line of the ball loading system consists of pins specifically located in order to reduce the speed of the falling balls. The mill must be on-line or at least on barring before the ball loading can be performed.
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916
10
OPERATION AND SAFETY – ALLIS-CHALMERS
This section (courtesy of Lethabo Power Station) covers the operations and fire protection for the Allis-Chalmers ball/tube mills.
10.1 Operations Before starting the mill, the low-pressure trunnion bearing lubrication system is operated to ensure that the oil temperature has reached 21°C. The high-pressure jacking pump is then started to raise the oil pressure in order to lift the trunnion clear of the bearing. After the mill has started, the high-pressure jacking pump stops after 5 minutes, leaving the low-pressure system to provide lubrication. When the mill shuts down, the high-pressure pump starts an automatic cycle operation of 5 minutes on and 30 minutes off until the cycle is manually stopped. The cycling operation is required until the mill shell has cooled to ambient temperature and all mill contractions have ceased. If the high-pressure jacking pump fails during a mill shutdown, the manual jacking pump is used. A suitable operating cycle for the manual jacking pump is a minimum of 60 seconds pumping every 30 minutes for the period when the high-pressure pump jacking would normally be operating.
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10-1
Operation and Safety – Allis-Chalmers
Table 10-1 lists the tasks for ball replacement. Table 10-1 Ball Replacement Tasks (Courtesy of Lethabo Power Station) Tasks 1.
Barricade the lifting area.
2.
Lower the cross beam with the mill ball hoist.
3.
Use the cross beam chains to attach the ball drum and ensure tightness.
4.
Load the mill ball drum onto the mill ball trolley and unhook the cross beam chains.
5.
Pull the trolley to the selected mill ball hopper.
6.
Verify the hopper is the correct unit and mill.
7.
Open the mill ball hopper top door.
8.
Discharge the balls into the hopper.
9.
Close the top hopper door.
10.
Use the bottom hopper valve spindle to discharge the balls into mill.
11.
Close the bottom hopper valve when completed.
12.
Remove the trolley and cross beam.
13.
Clear the lifting area.
10.2 Fire Protection If a fire in the mill or classifier occurs, it will be extinguished with carbon dioxide. Centrally located at the fire fighting station are two mobile trailers. Figure 10-1 shows a mobile gas bottle unit.
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10-2
Operation and Safety – Allis-Chalmers
Figure 10-1 Mobile Gas Bottle Unit (Courtesy of Lethabo Power Station)
Each trailer is provided with carbon dioxide gas bottles. One trailer is located at the non-drive end and the other trailer is located at the drive-end side of the mill. The gas trolleys will be connected by quick fit couplings to the existing purge pipework on the mills. The existing shutoff valves at the trolley and the shutoff valves on the purge pipework on the mill are to be opened. During purging, the carbon dioxide flows out of the bottles through the flexible hose, into the mill pipework, and into the mill. After purging both trailers, the mill shutoff valves should be closed, and the carbon dioxide bottles should be refilled. A permanently installed carbon dioxide fire protection system is shown in Figure 10-2.
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10-3
Operation and Safety – Allis-Chalmers
Figure 10-2 Permanently Installed Carbon Dioxide Fire Protection System (Courtesy of Lethabo Power Station)
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10-4
11
OPERATION AND SAFETY – FOSTER WHEELER
The operational and safety aspects for the Foster Wheeler mills [5] include the following topics: •
General operation
•
Operation indications
•
Startup procedures
•
Fire detection system
•
Fire protection
11.1 General Operation The double-ended ball/tube mill [3] can be operated as two mills with each end of the mill serving a different set of boiler burners. In this way, six single-ended mills can be replaced with three larger, double-ended mills. The advantage of using fewer mills is compared to the increased complexity of operating the larger mills. There are problems in balancing the flow of air and coal from the double-ended mills. The dual supply and discharge of the coal and air requires a complicated control system to overcome any imbalance problems. At low load, the instabilities of the coal/airflow can be avoided by shutting down one side of the mill. The following conditions are possible with a double-ended mill: •
Double-end startup
•
Single-end startup
•
Single-end operation
•
Double-end operation
•
Adding double-end operation while operating in single-end operation
•
Shutting down double-end operation and operating in single-end operation
•
Shutdown of double-end operation
•
Shutdown of single-end operation
Because of the many different operating modes, the airflow for each mode must be changed. The control system dictates damper adjustments for each flow requirement.
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11-1
Operation and Safety – Foster Wheeler
The mill operating system [3] can control the: •
Coal flow to the mill
•
Air inlet temperature
•
Airflow through the mill
•
Classifier settings Technical Key Point The ideal proportion of the weight of the balls to the weight of the coal is between six and seven. This means that for every 330 lbs of coal in the mill, there should be 2205 lbs of balls to effectively pulverize the coal. If the proportion of coal in the mixture is too low, the balls will strike each other and grind against the cylinder. If the proportion of coal in the mixture is too high, the coal will tend to flow into the feed scrolls.
The output of coal from the mill is controlled by the airflow through the mill. The airflow required to transport the coal mixture to the boiler is greater than the airflow required in the cylinder. Therefore, some of the airflow bypasses the mill. At low load, most of the air bypasses the mill. A mill control system varies the rate of coal flow in order to maintain a constant level in the mill. During startup, the mill sounds very loud as the balls are contacting each other. During full load operation, the coal level is monitored using differential pressure sensors. Table 11-1 lists the operating tasks for the shutting down for one side of a mill. Table 11-1 Shutting Down One Side of a Double-End Mill [3] Tasks 1. Decrease the coal feed on both ends to below 60%. 2. Stop the drive-end coal feeder. 3. Close the drive-end raw coal gate valve. 4. Set the classifier temperature control to single-end operation. 5. Close the drive-end primary air damper and bypass damper. 6. Close the damper to the drive-end pre-drying chamber. 7. Open the cold purge air damper to the pulverized coal lines. 8. Purge the pulverized coal lines for 10 minutes.
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11-2
Operation and Safety – Foster Wheeler 9. Close the purge air damper.
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11-3
Operation and Safety – Foster Wheeler
As the drive-end primary air and bypass air dampers are closed, the flow settings of the corresponding dampers for the other side of the mill are automatically increased to keep the total airflow through the mill constant. After shutdown of the drive-end coal feeder, the mill level is controlled by varying the input of the remaining coal feeder. The control system ensures that temperatures and transport velocities remain within critical limits. The operation [2] of the classifier is controlled through a programmable, variable-frequency drive (VFD) controller. The controller accepts an external 4-20 milliamp (mA) signal for motor speed control purposes and provides a 4-20 mA motor speed feedback signal. A remote alarm signal is generated whenever the controller trips and/or is operating outside of the normal operating parameters. The controller also typically incorporates an override feature to allow for remote, manual speed control of the classifier. Programming of the controller is lock-out protected and typically has a self-diagnostic capability while displaying fault causes.
11.2 Operation Indications For the negative pressure Foster Wheeler ball mill, the temperature of the coal/air mixture leaving the mill should be 150–160ºF. Temperatures higher than 160ºF could result in a fire in the mill. Opening the tempering air damper allows cooler air to flow into the mill and reduces the exiting coal/air temperature. The coal/air temperature leaving the mill should not be below 140ºF except during startup. Coal/air temperatures below 140ºF may result in an unstable ignition of the coal and could require the addition of fuel oil in the boiler. Low coal/air temperatures also mean that the coal is not being properly dried and may result in plugged coal piping. Excessive moisture content in the coal will result in the need for a higher classifier differential for a given steam flow. A higher classifier differential can be obtained by adjusting the exhauster loading signal bias. Excessive moisture may also cause the fuel level control lines to become plugged more frequently. Normal mill and feeder operation, including mill level gauges, mill amperage, and coal/air mix temperatures, provide a good indication of the coal level in the mill. Variation of the normal relationship between the high- and low-level lines indicates a problem. The problem is usually plugged mill level control lines which would need to be blown out. Foster Wheeler provides a purge system to prevent coal from entering the mill level sensing lines. This purge system is needed on pressurized mills and may be applicable to mills with exhausters. Positive pressure operation of the mill during mill swings can cause the sensing lines to become plugged. The basic idea for the purge system is to supply air at a slight pressure to each of the sensing lines in order to prevent the coal from entering the lines. Because the mill level controls are based on differential pressure, the added air pressure is nullified when measuring the differential pressure. When properly calibrated, the mill controls read the correct differential pressure.
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11-4
Operation and Safety – Foster Wheeler
Differences of 0.5–1 in. between the high- and low-level lines are normal assuming the low level line is not below 1 in. Mill motor amperage will normally be between 114 and 116 amps. The shear arm on the feeder table is adjusted to keep the feeders on slow speed approximately 90% of the time. The shear plate arm position is not usually changed. When a mill is operated at more than one-half load using one feeder, instead of two, the shear plate arm should be adjusted to a larger opening. Table 11-2 lists the normal operation checks for the negative pressure Foster Wheeler ball mill. Table 11-2 Normal Operation Checks [5] Operational Tasks 1. Check the feeders periodically to see that coal is flowing. When a feeder is off, check to ensure that coal is not flowing through this feeder, as this would plug the mill with coal. 2. Visually inspect the mill for any coal that is spilling because of loose liner bolts. 3. Visually inspect all lubricated parts. 4. Check to see that the air supply pressure to the Farval pump is approximately 40 psi. The atomizing air supply should be a minimum of 80 psi. 5. Change the strainer scraper five or six turns each shift to clean the strainer on the Cardwell lubrication system. 6. Check to see that oil is flowing through the sight glass to the trunnion bearings.
11.3 Startup Procedures Three tables are listed for the startup procedures for a negative pressure Foster Wheeler ball mill. Table 11-3 lists the initial preparation tasks, Table 11-4 lists the tasks for charging the mill, and Table 11-5 lists the tasks for placing the mill in service. Table 11-3 Initial Mill Preparation for Startup [5] Operational Tasks 1. In cold weather, energize the strip heaters in the Cardwell tank a few hours before the mill is to be used. 2. Check the lubrication of the mill gear and pinion, all motor bearings, all exhauster bearings, and the air tube bearings. Check the oil level in Cardwell reservoir tank. 3. Turn on the cooling water to the mill trunnion bearings. 4. Close the purge damper, the output damper, and the mill hot air shutoff damper. 5. Open the inspection plate at the outside end of the air inlet tube.
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11-5
Operation and Safety – Foster Wheeler
Table 11-3 (continued) Initial Mill Preparation for Startup [5] Operational Tasks 6. Purge the mill level control lines. Check the oil level in the Farval barrel. 7. Check the Farval nozzles for spraying. 8. Open the burner-switching valve above the exhauster to be used for charging the mill and place a 3-in. plug in the coal pipe. 9. Place the burner cleanout plugs in the burner that is used for charging the mill. Set the feeder shear plates to 50% open. 10. Check all controls for proper settings. 11. Check that the following dampers are closed: mill suction, tempering air damper, capacity dampers, and exhauster purge dampers.
Table 11-4 Charging the Mill for Startup [5] Operational Tasks 1. Open the auxiliary air damper cover plates to 35–40% open. 2. Start the exhauster that is selected for startup. 3. Open the purge damper. 4. Open the output damper approximately 20%. 5. Adjust the mill suction to approximately 2 in. of negative water pressure. If the mill suction is too high, open the auxiliary air damper enough to reduce the suction to the desired level. If the suction is too low, start closing the auxiliary air damper. If this fails, start closing the output damper. 6. Start the low-pressure mill lube oil pump, wait 5 seconds, then start the high-pressure lube oil pump. 7. Start the mill 20 seconds after the high-pressure pump is started. 8. Turn off the high-pressure pump after the mill has reached operating speed. 9. Open the bunker coal gate. 10. Start the feeders. 11. Feed coal to the mill by operating the feeder intermittently 1 minute on and 2 minutes off. 12. NOTE: Be careful not to feed too much raw coal to the mill. The mill will not grind the coal if it is fed too fast. When the first coal level indication appears, (0.2–0.3 in.), discontinue operating the feeder until the level stabilizes. 13. If necessary, feed more coal until the low level line shows 1–1.2 in. When the level has reached this point, as indicated by low level line gauge, the mill may be placed in service
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Operation and Safety – Foster Wheeler Table 11-5 Placing the Mill in Service [5] Tasks 1. Close the auxiliary air damper of the exhauster to be placed in service. 2. Open the mill suction control damper about 10%. 3. Check to see that all plug cocks in the coal pipe are closed. 4. Open the burner shutoff valve. 5. Open the hot air shutoff damper wide and hold open until the damper has latched. 6. Readjust the mill suction damper to give 2 in. negative water pressure in the mill. Place the mill suction damper control on automatic as soon as two burners are in service on that mill, or as soon as possible thereafter. 7. The output damper must first be closed manually at the output damper control station. 8. The purge damper is then opened by a control switch. 9. The output damper is then reopened manually approximately 20% at the output damper control station. 10. The hot air mill suction shutoff damper is then opened by a reset pushbutton. 11. NOTE: The mill suction shutoff damper is tripped closed automatically when both purge dampers are closed, regardless of how the purge damper closes. 12. The purge damper can be closed with the operation of the respective control switches and operation is below minimum exhauster differential. 13. NOTE: The operation of dampers is identical for all mills and exhausters. Damper open means open to fuel flow. 14. Place the feeders on automatic as soon as possible. At least two burners should be in service on any one mill. 15. Adjust the mill output damper for the desired output and place this damper control on automatic. 16. Open the auxiliary air damper, as required, to give enough exhauster discharge pressure to keep the burners from plugging with coal. The minimum exhauster discharge pressure should be 5 in. 17. Close the inspection plate at the outside end of the mill air inlet tube.
11.4 Fire Detection System The fire detection system designed by Foster Wheeler is normally set up to detect a spark or flame in the classifier area. The system is based on the detection of light emissions in the visible to near infrared spectrum. A standard system contains sensor head assemblies, a junction box at each mill, an interface enclosure with a power supply, a computer, a monitor, and a printer. A
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fire detection sensor head assembly is shown in Figure 11-1.
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Figure 11-1 Fire Detection Sensor Head [2]
Holes are cut and mounting brackets are welded to the mill housing at the appropriate locations. A mounting flange, including dual-quartz sight glasses, is bolted to the mounting bracket sealing the mill. The sensor head is quick-coupled to the mounting flange. Electronic circuitry responds to minute currents generated when light is present in the visible to near infrared spectrum. The resulting signal is reviewed by a microprocessor system. As each head is polled for data, the corresponding microprocessor communicates with the control system. Only the corresponding head assembly that matches the address sent by the control system will respond with data. Every 15 minutes, the control system commands all the assemblies to enter the self-test mode by turning on light-emitting diodes within the sensor head. The control system sequentially polls each head for the self-test data and notifies the user of any sensor problems.
11.5 Fire Protection The National Fire Protection Association (NFPA) now requires that ball mills be provided with a means of inerting inside the mill during a trip. Although older mills may not be equipped with this safeguard, it is recommended that ball mills be retrofitted with an inerting system, especially if highly volatile fuels are being used.
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Human Performance Key Point The NFPA identifies two areas of protection for ball mills. The two areas are preventing an explosion (inerting) and extinguishing a fire. The inerting process introduces a vapor medium (steam, nitrogen, or carbon dioxide) that lowers the oxygen concentration to prevent an explosion. This will also reduce the intensity of or extinguish any existing fire. The use of the inerting system is intended for out-of-service equipment and would not be effective for an operating mill, even at the minimum airflow through the mill. Therefore, the inerting concept should only be used when the pulverizer is out of service. Extinguishing a fire is primarily accomplished with water. In addition to extinguishing the fire, it is necessary to remove the ignition source so that a dust or gaseous explosion does not occur. Foster Wheeler offers a steam inerting system that is skid-mounted with one skid per ball mill. The associated steam piping to the skid and the ball mill are typically provided by the plant. The steam supply for the inerting system is in the 50–100 psig pressure range and between 0 and100°F of superheat temperature. Table 11-6 lists the procedures required when a fire is detected in the negative pressure ball mill. Table 11-6 Procedures for Fire Extinguishing in the Foster Wheeler Mill [5] 1. Stop the exhauster(s). This should automatically stop the mill and feeders. 2. Allow sufficient time for the pulverized coal in suspension to settle, approximately 5–10 minutes. Flood the mill by allowing a low-velocity stream of water to flow into the feeder until the water runs out the bottom of the trunnion. The mill should then be turned over for approximately 5 minutes to thoroughly mix the water and coal. The remainder of the system should then be carefully inspected and any existing fires should be extinguished. A carbon dioxide (CO2) extinguisher can be readily adapted to the mill. 3. Fires in the pulverized coal system cause coke formations on the inside of the fuel piping, distributors, and burners. If the coke deposits are not removed, they may cause future fires. The thorough cleaning of these parts following a fire is necessary to prevent future fires. 4. Blow out the mill level lines immediately after a mill has been flooded and after all fires have been extinguished. 5. Water and coal may wash back into these lines and deposit a layer of silt that would be difficult to remove. Blow out the level lines frequently until the coal in the mill has dried. 6. To dry the mill after it has been flooded, the equipment and dampers should be set up for normal operation with the exception that no coal is fed to the mill. The unit should be operated until the hot air flowing through the mill and the heat generated by the cascading ball charge has evaporated all the moisture in the mill. 7. When the mill is dry, the presence of coal at the burners will appear.
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Table 11-7 lists the emergency shutdown procedures for the negative pressure Foster Wheeler ball mill. Table 11-7 Emergency Shutdown Procedures for the Foster Wheeler Mill [5] 1. Close the exhauster output damper. In case of extreme emergency, omit this step. 2. Trip the exhausters. The tripping of both exhausters should automatically trip the mill, both feeders, and the hot air shutoff damper. 3. If after tripping the exhausters, the mill, feeders, and hot air shutoff damper do not immediately trip, manually trip the mill, feeders, and hot air shutoff damper. 4. Trip the mill low-pressure lube oil pump. 5. Close the burner switching valves. 6. Remove the coal conduit plugs, burner front plugs, and blow out if necessary.
Circumstances may necessitate shutting down a mill filled with coal. Table 11-8 lists the recommended procedures that should be used when shutting down the mill filled with coal. Table 11-8 Recommended Procedures for Shutting Down the Foster Wheeler Mill Filled with Coal [5] 1. Lower the fuel level in the mill to a point where the ball charge is no longer covered with coal. Mill fuel level gauges will indicate almost 0 in. while maintaining a mill suction pressure of 2 in. This should be done in all cases except emergency shutdown. 2. The mill should be started up and turned over several revolutions every hour in order to prevent the coal in the mill from igniting due to spontaneous combustion. 3. Check the mill to ensure that no dormant fires have started before turning the mill over. Fires may be detected by the odor of burning coal at the feeder inspection door and the inspection hole on the air tube, or by visual inspection through the inspection hole on the air tube.
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12
OPERATION AND SAFETY – KENNEDY VAN SAUN
The Eskom Kendal Power Plant has Kennedy Van Saun ball mills. The following topics are covered in this section: •
Load changes
•
Fuel oil support
•
Blocked fuel pipe
•
Wet coal
•
Mill stripping
•
Cold startup
12.1 Load Changes During normal operation, the load is met with four mills in service, but it is possible to operate with three and a half mills in service. The one-half mill is a ball mill with only single-end operation. When increasing the load from 586–686 MW with three and a half mills in service, the singleend operation should be changed to double-end operation at 586 MW. If three mills are in service, then the fourth mill should be started at 586 MW. For the load range of 486–586 MW, the normal operation in this load range can be achieved with three, three and a half, or four mills. When reducing load to the 386–486 MW load range with four mills in service, the top mill should be changed to single-end operation at 486 MW. With three and a half mills in service, one of the double-ended mills should be shut down at 386 MW. With three mills in service, the top mill should be changed to single-end operation no later than 386 MW. The single-end operation mills should be changed to a double-end operation at 386 MW when increasing the load in the 386–486 MW load range with two and a half mills in service. When operating in the load range of 300–386 MW, normal operation in this load range is with two and a half mills in service.
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12.2 Fuel Oil Support Should any mill fuel flow fall to 34% in double-end operation or 20% in single-end operation, the affected mill master must be placed in manual mode, and oil must be added for the affected burner. Should the oil addition not be available, the mill demand should be increased to maintain the fuel flow above 30% in double-end operation or 15% in single-end operation. The oil addition can be safely removed when the fuel flow on that mill is above 38% in double-end operation or 23% in single-end operation and the mill is stable. Human Performance Key Point Fuel flow of the mill feeding the lower burner elevation should not drop below 45%. Fuel oil should be added on the bottom mill in service if the fuel flow drops below 45% and is not stable. The ignition of coal on the lower burner elevation mill in service could become unstable at lowfuel flow if the bottom mill is not supported by an adjacent mill. When shutting down, stripping the mill of fuel, or working on the mill controls on any lower burner elevation mill, fuel oil support must be inserted on that elevation and on the next burner elevation mill upward. The two mills that feed the bottom boiler burner levels may not be started or stopped without fuel oil support. In order to save fuel oil when starting these mills, the mills should be prewarmed. Pre-warming the mill is accomplished by opening the hot mill primary air isolation gate and both mill outlet gates at the back panel. The hot air control damper should then be opened to a maximum of 20 kg/second of airflow through the mill. The mill outlet temperature should be monitored, and when the outlet temperature exceeds 80°C, the dampers can all be closed, and the mill started normally. If any instability occurs while warming the mill during this process, the primary air on the mill should be shut down as soon as possible. Pre-warming should not be done if the unit load is less than 486 MW. When starting the mill in an emergency situation, the mill does not have to be pre-warmed. Any mill can be changed from double-end operation to single-end operation or from single-end operation to double-end operation without fuel oil support, provided that all of the following conditions are satisfied: •
The unit load must be above 386 MW.
•
There must be at least three mills in service in either single- or double-end operation.
•
There must be at least two mills in service below the corresponding boiler burner level. The two mills do not have to be the mills supplying the burner level directly below the mill being
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changed, but can be the mills for a lower burner elevation.
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If the above criteria are not met, then it is necessary to add fuel oil before the mill is changed in operation. During the change from single-end operation or double-end operation, it is important to monitor the drive-end mill outlet gate run time. If after 70 seconds, the appropriate limit switch has not been made, an attempt can be made to change back to the previous selection. If the limit has not been made within 80 seconds, the mill will trip. When changing a mill from double-end operation to single-end operation, the mill bias will automatically be set to minus 40%. When starting a mill in single-end operation, the mill bias must be manually set to minus 40% except during startup conditions.
12.3 Blocked Fuel Pipe Human Performance Key Point If a blocked coal pipe is discovered, every effort should be made to correct this defect as soon as possible. Continued mill operation with a blocked coal pipe is not allowed for more than 14 days. As soon as the blocked fuel pipe is discovered, operating personnel will complete an “out of normal” notification and tag the associated mill at the mill motor. The pipe should be isolated and cleaned as soon as possible. If the blocked fuel pipe is on the drive-end side, the mill should be changed to single-end operation as soon as possible, and the pipe should be blanked off for cleaning. If required, the mill can then be returned to double-end operation and operated with seven of the eight fuel pipes in service. If the blocked fuel pipe is on the non-drive-end side, the mill will have to be shut down as soon as possible, and the pipe will need to be blanked off for cleaning. While the pipe cleaning occurs, the mill can be returned to service and safely operated with the one pipe blanked off. If a second blocked fuel pipe occurs on the same mill and the blocked pipes are on the same end of the mill, then: •
If the blocked pipes are on the drive-end side, the mill can be changed to single-end operation to allow for the blanking and cleaning of the blocked fuel pipes. The mill may be returned to double-end operation only when one of the pipes has been cleaned and the blank removed.
• If the blocked pipes are on the non-drive-end side, the mill must be shut down immediately to allow for the isolation of the affected pipes. The mill may be returned to service when one of the pipes has been cleaned and the blank removed. •
If one pipe on both ends of the mill is blocked, then the mill must either be shut down immediately or changed to single-end operation to allow for the immediate cleaning of one of the pipes to continue. If selected for single-end operation, the mill may be operated with one
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blocked fuel pipe for a maximum of 14 days.
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The pulverized fuel trapped in the blocked fuel pipe does occasionally begin burning. This causes the pipe surface temperatures to increase dramatically to over 200°C. Because the fuel sediment is compact, the combustion proceeds slowly with only the exposed surface of the sediment burning. The ash layer that remains and the combustion gases reduce the availability of oxygen for continued combustion. When blocked fuel pipes are found to be burning, the pipes should be left to burn out and cool. Additional blanks should be put in place on both ends of the burning pipe to limit the ingress of more oxygen. Without additional oxygen, the combustion process will not continue, and the heat will dissipate. Once the combustion has ceased, the remaining ash and pulverized fuel can be cleaned out from the pipe.
12.4 Wet Coal Operation with wet coal can be identified by comparing the mill demand versus the inlet temperature according to the following graph in Figure 12-1.
Figure 12-1 Wet Coal Operation (Courtesy of Kendal Power Station)
If a mill is operating with wet coal, the following actions should be taken: •
The affected mill bias must be set to minus 40%.
•
The mill power must be monitored. Should the mill power drop significantly by more than 80 kW, the feeders must be stopped immediately and the mill stripped empty.
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• When the hot air control damper opens fully, the mill differential pressure is in alarm values, and the mill fails to maintain primary airflow, it is an indication of partial blocking of the feed box with wet coal. The affected mill feeders must be stopped immediately. If the primary airflow does not return to normal values but continues to decrease, then the mill must be shut down immediately. • Oil support on mills feeding the lower burner elevations will require fuel support. • The maximum number of mills should be put in service depending on the permissible load contract.
12.5 Mill Stripping Mill stripping is emptying the mill of coal. If the level instrumentation indicates that there is coal in the mill when the mill is empty, then the instruments should be recalibrated. Table 12-1 lists the tasks for stripping the mill. Table 12-1 Tasks for Stripping the Mill [Kendal-Kennedy Van Saun Mills] Tasks Note: The mill should be in service for at least three hours before the stripping process begins. 1. Initiate oil support on burner where required. 2. Shut down the feeders. 3. Place the mill master in manual and at 53%. 4. When the mill level has reached a steady state (~10 minutes), monitor the mill power until it is constant at 1360 kW. The mill is assumed to be empty. 5. Place the mill master at the 40% setting. For the mill supplying the lower burner elevation, the mill master should be set at 50%. 6. Check the primary airflow. The airflow should be 42.5 kg/second. 7. Set the bypass dampers to 100% open. 8. The mill level should be checked for 0% indication. 9. Turn off both seal air fans to trip the mill. 10. After the mill trips, return both seal air fans to normal operation (one fan operating and one fan in standby). 11. Open the drive-end and non-drive-end mill outlet gates. 12. The primary airflow should be at 42.5 kg/second and the bypass dampers set to 100% open. 13. The level instrumentation can now be set to 100% indication.
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Table 12-1 (continued) Tasks for Stripping the Mill [Kendal-Kennedy Van Saun Mills] Tasks 14. Return the mill to the normal shutdown position. All pressure switches should be reading zero. 15. Restart the mill but do not load the mill. Recheck the zero position of the level instrumentation. 16. After the instrumentation is calibrated, then the mill can be shut down for an internal inspection.
12.6 Cold Startup After a major outage, it is necessary for the mills to go through a cold startup. The tasks for a cold startup are shown in Table 12-2. Table 12-2 Cold Startup Tasks (Courtesy of Kendal Power Station) System Trunnion high-pressure lube oil system
Tasks Fill the lube oil tank. Set the high-pressure lube oil system pressure relief valve to lift at 15 MPa. Place the lube oil tank heaters in service and verify that the temperature is within the indicated range. Check the lube oil tank level alarm. Check the lube oil temperature alarm. Check the lube oil pressure alarm.
Trunnion low-pressure (LP) lube oil system
Verify the LP lube oil flows to be 20 liters/min. Check the LP lube oil flow alarm. Check the LP lube oil pressure alarm.
Pinion lube oil system
Fill the pinion lube oil tank. Check the pinion lube oil flow low alarm. Check the oil pressure low alarm to be <100 kPa.
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Table 12-2 (continued) Cold Startup Tasks (Courtesy of Kendal Power Station) System Mill motor lube oil system
Tasks Fill the mill motor lube oil tank. Verify the oil pressure relief valve setting to be 10 MPa. Verify the oil pressure setting to be 3.5 mPa. Check the oil pressure alarms to be <3.0 mPa and <1.5 mPa. Perform the mill lift test and record the results. Clean all lube oil filters after the pressure/flow adjustments.
Mill feeders
Change the feeder switchgear setting to remote. Ensure the feeders internal lighting is available. Perform the feeder belt training. Check the clean out conveyors chain tension. Check the feeders. No coal on belt switch. Check the clean out conveyors shearing pin broken alarm. Ensure the ball loading hoppers door is lock-fitted.
Mill dampers
Check the hot air isolating damper stroke. Check the cold air isolating damper stroke. Check the hot air control damper stroke. Check the cold air control damper stroke. Check the drive-end bypass damper stroke. Check the non-drive-end bypass damper stroke. Check the drive-end mill outlet damper stroke. Check the non-drive-end mill outlet damper stroke. Check the drive-end feeder outlet damper stroke. Check the non-drive-end feeder outlet damper stroke. Check the drive-end reject gate stroke. Check the non-drive-end reject gate stroke.
Seal air fans
Balance and test both of the seal air fans. Check the seal air pressure to be >10 kPa and the differential pressure from the drive-end and non-drive-end to be 1 kPa. Check the seal air pressure alarm. Check the seal air system for leaks.
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Table 12-2 (continued) Cold Startup Tasks (Courtesy of Kendal Power Station) System Mill barring and protection devices
Tasks Place the mill on barring and check the girth gear spray patterns. Check the bearing alarms for 60°C. Check the mill motor bearing alarms for 80°C. Simulate the trunnion lube oil pressure to be low and check the mill trip. Check the mill seal air trip. Check the mill motor bearing temperature trip. Check the mill bearing temperature trip. Check the girth gear spray trip. Check the mill outlet gate open limit lost trip.
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13
OPERATION AND SAFETY – RILEY POWER INC.
The following topics are covered in this section: •
General operation
•
Control system
•
Handling ball charge
•
Primary air damper operation
•
Seal air system
•
Feeder calibration
•
Instrumentation settings
•
Fire detection
•
Fire protection
13.1 General Operation During normal operation, the ball/tube mill temperature monitoring is accomplished through the mill alarm system. The system monitors the mill bearing temperatures and the oil temperature to the reducer and chain case, alarming and tripping when these temperatures increase beyond their predetermined set points. The most commonly monitored pressures are: • The seal air differential that protects the shafts and bearings of rotating parts from coal dust and prevents coal dust leakage from the mills, crusher-dryers, and feeders •
The mill differential that gives the operator an indication of fuel flow
• The classifier to furnace differential, a measure of coal line velocity, the maintenance of which protects against coal line layout •
Primary air system pressures that give an indication of the system’s overall condition
• The pressure exerted on the thrust rollers is transmitted by the load cell to the mill alarm system. This pressure should be monitored, as it warns the operators that the mill barrel is not in alignment. Excessive pressure will set off the alarm, and corrective action must be taken immediately.
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For flow indication, the operator makes continual reference to the following flow indications: •
Coal flow measurement is made in two ways. The mill differential is an indication of the amount of coal/air leaving the mill and, therefore, is used as the total fuel signal that is being sent to the combustion control system. This differential is also useful when trying to maintain equal fuel flow from all mills in service. The coal flow determined from the feeder counters is an essential tool used to determine when to add balls to the mill and when to check hammers in the crusher-dryers. These values are helpful when calculating the amount of coal fed to the system over time.
•
The oil flow to the chain case and reducer oil baths is monitored through the mill alarm system. Periodic inspections are necessary to ensure that this system is functioning properly and that an adequate supply of oil is maintained.
The coal charge is an inventory of coal in a mill. The proper inventory is measured by a combination of sound (dB) and power (kW) values that are fed into the Power-Sonic mill conditioning system. Trending the classifier exit temperature provides the early warning of a fire present in the mill system and also whether there is proper drying of the coal in order to ensure ignition at the boiler burners. Detailed startup procedures are available from the Riley Power Service Guide for: •
Ball/tube mill startup procedure with steam inerting
•
Startup of ball/tube mill containing a coal inventory using steam inerting
•
Operation of double-ended ball/tube mill with one feeder out of service
•
Operation with burners out of service
•
Normal ball/tube mill shutdown procedure with steam inerting
•
Steam inerting procedure following a ball/tube mill trip
13.2 Control System The Power-Sonic mill conditioning system includes a kilowatt transducer, a sonic transducer, and a kilowatt set point control station with sound decibels, kilowatt meters, and interconnecting multi-conductor cables. Through this equipment, the system maintains control of the total coal inventory in the mill by using measurements of two mill variables. One variable is obtained by comparing the coal inventory to the mill power required for the mill motor to rotate the mill barrel. The second variable is the sound or sonic level of the steel balls colliding with each other and with the mill liners. This variable is also related to the amount of coal inventory in the mill. From these variables, an error signal is generated by which the system directs the feeder speed
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controller calling for either an increase or a decrease in the feed rate.
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The power required to rotate the mill barrel is related to the total balls plus the coal inventory weight. With the ball inventory traction being held reasonably constant, the power input becomes a function of the coal inventory. As the coal is fed into an empty rotating mill that is holding just the balls, the power input to the mill motor increases with the coal inventory. The incoming coal eventually fills the void between the colliding balls resulting in a decreased sound level. When this condition is reached, the combined inventory bulk density and power input are at a maximum peak power. Further addition of coal decreases the bulk density by forcing the balls apart, swelling the inventory, and displacing its center of gravity toward the mill axis. This reduces the power input despite increasing the total weight. Maximum grinding efficiency is obtained near the peak power requirement. The region where power increases with increasing coal inventory is called the stripping range of the kW curve. The region where power decreases with additional coal inventory is called the operating or control range of the kW curve. It is necessary to operate with a mill that has a coal reserve in it so the mill system can respond to sudden increased demand on output without waiting for additional new coal to be fed into the mill for pulverization. The ball/tube mill is operated in the control range of the operating range of the kW curve. Thus, a kW set point is selected in this range, and the mill feeders automatically reposition on a change in mill load to satisfy this set point. The second control factor is the sonic function (sound level in dB versus coal inventory). Sonic intensity increases as the coal inventory decreases. This is caused by the steel balls making direct contact with each other when the coal is moving toward the stripping range of the sonic curve. This portion of the sonic curve indicates that there is not enough coal in the mill to fill the voids between the steel balls. The control strategy of the Riley Power ball mill is shown in Figure 13-1.
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Figure 13-1 Riley Power Operation Diagram (Riley Power Service Guide)
The rate of the airflow is controlled in response to a mill output or fuel demand signal. The coal feeder rate is controlled by the feeder speed controller in response to the error signal generated by the mill conditioning system. This error signal is generated in response to three signals representing the mill motor power, the sonic (sound level) intensity of the mill, and the mill output of the fuel demand signal. When the mill is operating at the rated capacity, the coal feeder rate is primarily controlled by a signal representing the power consumed by the mill motor. When the mill is operating above rated capacity, the coal feeder rate is controlled by signals from the mill motor power and the sonic intensity of the mill. When the mill is operating below rated capacity, the coal feeder rate is controlled by the mill motor power and by the mill output demand signal. The control strategy is based on maintaining the weight of the coal charge in the mill rather than the coal level at the ends of the mill. The power needed to rotate the mill drum is related to the total charge weight of the coal and balls. Therefore, if the ball weight is held constant, the power required is a function of the coal charge weight. Restoration of ball charge weight from wear at 10-day intervals should maintain the coal charge to mill motor power relationship within
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practical limits. This is achieved by adding steel balls without taking the mill out of service.
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The maximum grinding efficiency is obtained at the peak power requirement. However, it is desirable to maintain a reserve capacity in the mill to supply sudden required output increases without waiting for additional coal to be added and pulverized. For this reason, the control system is set to maintain a product charge in excess of that existing at peak power. Figure 13-2 shows the desired control point.
Figure 13-2 Motor Power Versus Product Charge [7]
The control system uses motor power as the primary input. The drum sound level (Riley Power Inc. calls this sonics) and feed forward input is used as input to the power control. The power controller operates on a full-range output of less than 5% change in motor power. Figure 13-3 shows the response characteristics of the mill parameters using the product charge control.
Figure 13-3
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Operation and Safety – Riley Power Inc. Mill Parameters Using Product Charge Control [7]
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From these parameters it can be seen that as the demand on the mill is increased, the total product charge increases, the motor power decreases, the sound level decreases, then remains constant and begins to increase with higher loads, and the product level remains steady and increases with higher loads. The alarm outputs are: •
Normal operation – no alarms
•
High product charge – low power alarm only
•
Low product charge – high sonics alarm only
•
Low-low product charge – low power alarm and high sonics alarm
The high sonics alarm condition indicates low product charge. If the ball charge weight falls below the control range value, three events can occur: •
At rated, high-load output, the mill will become unstable due to the inability of the mill to grind the required throughput because of inadequate ball charge.
•
At lower outputs, the system will alarm at low power, because the power is now lower than the alarm set point.
•
The reduced ball charge will lower the power required below the control setting. The high sonic alarm will be activated as the controller attempts to increase power to the set value by reducing the feed rate to the mill.
Figure 13-4 shows the effects of reduced ball charge.
Figure 13-4
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Operation and Safety – Riley Power Inc. Effects of Reduced Ball Charge [7]
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During normal mill operation, the Power-Sonic mill conditioning system maintains the coal product charge weight as determined by the motor power set point and the feed forward signals. Control may be switched to the Power-Sonic mill conditioning system after initial startup as soon as the motor power reaches its peak. If a loss of one of the mill feeders occurs, the sonic signal will indicate a stripped (empty) condition, and the other feeder will go to full-open operation. If the second feeder does not operate in a short time, the load on the mill should be reduced. The control system can provide stable mill operation when the coal is wet. With a moderately wet coal, the reduced charge provides greater air volume for drying. With a very wet coal, the coal can build up in the ducts between the mill and the feeder and block the flow into the mill. However, the lower level of product in the mill tends to prevent this situation. The effects of moisture content in the coal charge to the mill power is as follows: • Over 12% moisture, the mill power is 10–15% below normal operating input and steady. • For 8–12% moisture, the power increases and becomes very erratic as the drying process proceeds. • For 2–8% moisture, swinging decreases and power drops to an empty mill value. The effect of excessive moisture on the mill operation is evident by rapidly swinging current indications on the mill motor. If a mill upset occurs because of an interruption of coal feed from the source to the feeders, the controls should be changed to a manual operation. The manual operation should maintain a constant mill motor current reading until the normal coal supply resumes. Then the control system can be returned to an automatic mode. A change in the voltage to the mill motor will not affect the operation of the Power-Sonic mill conditioning system. Unless the coal is of very poor quality or very wet, the mill motor set point should not require changing. When removing a mill from service, the control system is placed in manual, and the mill is stripped of coal by the operator. Delivering the coal in measured amounts is accomplished by the drum-type coal feeders, which receive a signal from the Power-Sonic mill conditioning system and by the modulation of the rating dampers. The feeders consist of a housing containing a rotating drum that delivers a given volume of coal and provides a seal against the backflow of primary air into the bunkers. A variable-speed transmission drives the feeder, changing its speed when the signal from the conditioning system indicates the need.
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13.3 Handling Ball Charge The ball charge hopper permits makeup ball charges to be added to the mill during operation and without mill service interruption. Balls may be added to an operating mill by closing the hopper outlet valve on the ball-charge hopper and filling the hopper itself with balls. The cover should then be latched securely in place and the stop valve opened to allow the balls to enter the mill. This procedure is then repeated until the required addition to the ball charge is complete. The charging frequency for each mill should be approximately one or two 2000-lb drums every 10 days. The outside dimensions of a 55-gallon drum are approximately 23.75-in. outside diameter × 35-in. long. The common method of storing these drums is on end, although some are stored on their side. The storage area is usually located on a different level from that of the mill ball-charge hopper. The drum is transported to the charging hopper by truck. Once the drum has been oriented over the charging hopper, cut a small flap on the drum to control the flow of balls to the charging hopper. The ball charge is added through a 2-ft, 10.5-in. × 2-ft, 10.5-in. opening in the concrete floor covered by 0.25-in. checkered cover plate of 1-in. thick grating. The ball charge hopper flange is recessed below floor level and is centered in the opening. The hopper door cover is 2-ft, 4-in. in diameter and is bolted to the hopper flange with eight eye bolts that extend 1.5 in. above the flange. The transporting truck must be able to straddle the opening, or the drum must be maneuvered so that one edge is over the charging hopper opening so that the flap can be cut in the drum to control the flow of balls to the charging hopper. When the drum is being emptied, its lowest point should be a minimum of 3 in. and a maximum of 24 in. above floor level. The truck used to move the drums must have load bearing wheels of the largest type possible and not less than 1.5-in. wide. The truck must be able to move over grating usually spaced with 1-in. wide openings. The heavy 2000-lb load dictates that the wheels should be of steel or hard synthetic composition that is oil resistant. The wheels should have a non-marking tread. It is suggested that any equipment that tilts the drum by chain or cable, which would allow such a load to swing, must also be able to drop this load on a base of safe transportation to the charging hopper. The use of four-wheel vehicles as opposed to three-wheel vehicles is suggested for the same reason. The trucking vehicle must be small enough to be put on an elevator and light and maneuverable enough to be handled by one or two people. A means of lifting the truck and drum by crane would be required if elevators were not available.
13.4 Primary Air Damper Operation Delivering the coal and maintaining its temperature requires the interaction of the many dampers and controls in the primary air system that performs three functions. The functions are temperature control, flow control, and velocity control. For temperature control in a cold primary air system, air from the atmosphere is taken by the
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primary air fan and discharged to a tempering air header, through the air preheater to the hot air
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duct. In a hot primary air system, the primary air fan draws tempering air from the cold air duct and hot primary air from the secondary air duct discharging the hot air to an individual mill system. The temperature of the coal/air mixture at the classifier outlet is maintained at the desired value by the manipulation of the tempering air dampers and the hot air dampers. This temperature is necessary to ensure proper handling and ease of combustion while minimizing the effects of wet coal on crusher-dryers, ductwork, and ill inlets. The tempering air dampers and hot air dampers have a fixed relationship to one another. This relationship helps maintain a stable primary air pressure through the load range, calming the system’s response. Both dampers are butterfly-type dampers, selected for their tight closing feature. When coal is supplied that is so wet that the action of the tempering air damper and hot air damper is not sufficient to maintain the required classifier exit temperature, a wet coal override comes into action with the bypass damper. For flow control, the flow of coal from the mill to the burners is controlled by modulation of the rating damper. This louver-type damper acts on demand from the boiler master through the fuel master, controlling the flow of primary air to the mill system. The amount of coal flowing to the burners is proportional to the amount of air flowing through the mill. The mill differential is the value used to measure the flow through the mill. At periods of low load or wet coal, some of the primary air delivered to the system through the rating damper is bypassed around the mill. For velocity control, the coal-laden air stream in the coal pipes to the furnace must be maintained at a certain velocity or the coal particles will drop out of suspension. The coal can collect in the coal pipe, form a barrier to flow, and create a fire hazard in the pipe or at the burner. Another problem is the unbalanced heat input to the furnace. The bypass damper performs two functions. It helps to maintain the coal/air velocity above the minimum values at low loads, and it helps to increase the flow of drying air during periods when wet coal is being supplied. The classifier to furnace differential pressure is a measure of coal/air velocity. The bypass damper’s controller reacts to this differential when it approaches a minimum value. When this value drops during operation, primary airflow will lower due to the movement of the rating damper, and the bypass damper will begin to open. This causes some of the air supplied to bypass the mill to join the coal/air mixture leaving the mill, lowering the mill differential. The rating damper, in response, will open slightly to regain the required mill differential. Variable relationship between the bypass damper and the rating damper will then exist, at low loads. The bypass damper enables the mill to operate at low loads. The bypass damper also assists the temperature control system when wet coal exists. The bypass damper will begin to open if the coal is so wet that, with the hot air damper wide open and
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Operation and Safety – Riley Power Inc.
tempering air damper completely closed, the classifier exit temperature falls below 150°F. This
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causes the mill differential to decrease and the coal flow from the mill to drop. Sensing the drop, the rating damper will open more, delivering more primary air to the system than is required for coal transport. This added air contains additional heat that is used to dry the wet coal. The sequence of operation for the primary air fan dampers is as follows: •
The primary air fan inlet dampers are closed when the primary air fan is not operating. Although the seal air fan draws its supply from the ducts served by these fans, there is enough leakage by the damper to allow sufficient air quantities to cool and clean the flame scanners.
•
The inlet damper remains closed while the primary air fan is started, and is opened only after the isolating dampers have been opened and the fan is up to speed. Thereafter, the inlet damper is modulated to satisfy the hot air header pressure set point and is placed on automatic control.
•
The system isolation dampers that are in the cold air and hot air ducts from each fan prevent backflow into an idle system while the other system is in service. The system isolation dampers are opened after starting a primary air fan.
•
Before starting a ball/tube mill, the tempering air damper should be fully opened and the hot air damper closed. The hot and tempering air dampers are set to maintain the classifier exit temperature at its normal set point. Technical Key Point The mill system’s rating and bypass dampers control the fuel flow responding to the demand signal from the combustion controls. The mill bypass dampers operate in a fixed relationship to the mill rating damper over the entire demand signal range of the combustion control system.
These dampers are characterized so the coal pipe velocity will decrease moderately with a decreasing mill load to a predetermined minimum value for proper coal transport. At mill startup, open the bypass dampers and follow the procedures outlined in the Riley Power Service Guide. Increase the mill load by opening the rating damper while simultaneously closing the bypass damper. The rating damper is in the primary air duct downstream of where the hot and tempering air blend. When all the burners serviced by the mill are in service, place the rating and bypass damper in automatic. The bypass damper is integral with the mill inlet/outlet box. The rating damper responds to fuel demand. The bypass damper responds to the rating damper position to maintain a required air/fuel ratio over the loading range of the mill. A manual bypassing or proportional adjustment of both the rating and bypass dampers will increase the level of primary airflow and will be used during conditions of extremely wet coal
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operation.
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13.5 Seal Air System The seal air system is placed in operation before starting the first forced-draft fan. Seal air pressure is required as a permissive to begin the purge sequence of the boiler. This pressure must be maintained during the operation of the ball/tube mill system. Loss of the seal air pressure will result in a master fuel trip. In order to isolate the operating fan from the backup fan, the power-operated butterfly valve located in the discharge side of the backup fan is placed in the closed position. The inlet side manual damper to the backup fan is placed in the open position. A system shutdown of the operating fan would result in the automatic opening of the backup fan discharge damper and the automatic starting of the fan. The inlet damper to the idle fan should always remain in the open position, except during isolation of the fan for maintenance. The sealing air to the mill trunnion arrangement confines the coal dust within the mill system. Replace the trunnion seal if the required pressure drop cannot be maintained. Sealing air is also provided to the feeder discharge gate to restrict a backflow of hot air from the primary air system into the feeder and bunker when a feeder or feeders are out of service. Except for the mill, each piece of equipment has its own manual control damper valve located in the seal line. The mill damper modulates to control and to maintain a constant differential between the pressure in the seal air header and the pressure at the mill inlet. Maintaining the selected set point is accomplished by the use of a single-element control loop using a differential controller interlocked with each mill. Alarm settings are established to monitor the seal air pressure in the seal air system. The differential controller must be released to the combustion controls before starting the primary air fan. The seal air system then remains on automatic control during the operation of the boiler and the ball/tube mill system.
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Operation and Safety – Riley Power Inc.
13.6 Feeder Calibration Table 13-1 shows the tasks for coal feeder calibration. Table 13-1 Coal Feeder Calibration Tasks [7] Calibration Tasks 1. Remove the feeder from service. Allow the feeder to cool. 2. Open the access doors for inspection. Remove the shear pin from the drive socket or from the shear pin alarm coupling to permit rotation of the feeder pinion shaft, feed drum, and pocket wiper, using the hand crank. Note: Eight crank revolutions equals one drum revolution of the feeder. 3. Rotate the feed drum by hand-cranking at least one complete revolution to determine the condition and operation of the drum pocket wiper, leveling apron, functional revolution counter, and so on. Set the leveling apron-to-drum clearance at 1/16 in. maximum, and correct any mechanical deficiencies prior to calibration. 4. After placing and securing sample chutes prepared for calibration through the crusher access door, open raw coal spout gates above feeder. Rotate the feed drum by hand cranking through at least two revolutions to establish uniform coal flow from bunker to feeder, and to determine and verify sample acquisition procedure. 5. Calibrate the feeder in terms of pounds of coal per revolution. This is done by rotating the feeder drum by hand exactly eight complete revolutions, catching coal on and removing same from the catch chutes during the feeder rotation. The coal should be immediately transferred to a 55gallon steel drum for weighing on an appropriate scale. Weigh the full 55-gallon drum and determine the coal weight per cubic foot (55 gal. = 7.353 cu. ft) 6. Immediately after removing the coal from the chute for weighing, rotate the feeder one revolution to obtain additional coal from which a sample for size and moisture content should be obtained and sent to the lab for analysis. The laboratory is to determine the approximate analysis, grindability, higher heating value in Btu/lb, and relay information to the persons doing the calibration to those keeping records in the plant. 7. In the event that the coal cannot be weighed within a reasonable time after depositing in the steel drums, the drums should be capped or covered in order to prevent the loss of moisture and to prevent a subsequent error in determining the results. 8. For other coals, calibration can be done simply by determining coal weight per cu. ft and calculating the ratio of weight per cu. ft to determine the new pounds of coal per drum revolution. Samples should be sent to the laboratory for results. Recalibration can be done for each different coal source or for the same source of coals to have a plot for average values of lb coal/drum revolution and records kept. 9.
The following date is provided to help in the test: Feeder drum size is 17.75-in. diameter and 30in. long. The approximate lb/rev. is 105. This varies with raw coal size and moisture content. The number of steel drums of 55-gallon capacity required for eight feeder revolutions is four.
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13.7 Instrumentation Settings Table 13-2 lists the various parameters for the mill instrumentation. Table 13-2 Sample Riley Mill Instrumentation Parameters [7] Equipment
Alarm Setting
Trip Setting
No coal
N/A
Chain oil temperature – high
185°F
200°F
Mill bearings temperature – high
200°F
220°F
Gear reducer bearings temperature – high
225°F
240°F
Gear reducer oil temperature – high
Varies
200°F
Mill inlet temperature – high
Varies
N/A
Mill outlet temperature – high
110°F
N/A
Mill end pressure differential – high
14-in. W.G.
N/A
Mill seal air pressure differential – high and low
22-in. W.G. 12-in. W.G.
N/A
90 psig
N/A
Mill displacement load cell weight
10,000 lbs
1/4-in. movement (16,000 lbs)
Classifier exit temperature – high
Varies
N/A
Classifier to burner differential pressure – low
Field determined
N/A
Mill motor power – low
Field determined
N/A
Mill sonics – high
Field determined
N/A
Feeder
Clutch air pressure – low
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Typical operating parameters are shown in Table 13-3. Table 13-3 Sample Equipment Parameters [7] Equipment Parameter Mill drum speed Mill capacity Crusher-dryer inlet temperature – normal Crusher-dryer inlet temperature – maximum Crusher-dryer seal airflow Crusher-dryer rotating speed Crusher-dryer bearings operating temperature range – normal Crusher-dryer bearings operating temperature – maximum Crusher-dryer bearings – normal vibration limit
Value 16.3 rpm 151,140 lbs/hr 450–700°F 900°F Approximately 450 #/hr 900 rpm 150–170°F 190°F 3-4 mills displacement
Feeder capacity, bituminous coal
133,800 lbs/hr
Feeder capacity, sub-bituminous coal
120,400 lbs/hr
Feeder shaft speed – maximum
21–25 rpm
Feeder shaft speed – minimum
0.73 rpm
Feeder motor shaft output speed – maximum
1700 rpm
Feeder reducer and wiper speed – minimum
5.8 rpm
Feeder reducer and wiper speed – maximum
170 rpm
Feeder available speed variation ratio Feeder drum size, diameter Feeder seal airflow
29:3:1 17.75 in. outside diameter 45 scfm
Chain water recirculation inlet temperature
85°F
Chain water recirculation temperature rise
32°F
Chain water recirculation inlet pressure
40 psi
Chain water recirculation pressure rise
0.08 psi
Chain water recirculation flow rate
3 gpm at 85°F
Chain oil recirculation inlet temperature
190°F
Chain oil recirculation temperature drop
15°F
Chain oil recirculation flow rate
15 gpm at 85°F
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Operation and Safety – Riley Power Inc. Chain oil recirculation outlet pressure
<15 psi
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Operation and Safety – Riley Power Inc. Table 13-3 (continued) Sample Equipment Parameters [7] Equipment Parameter Chain oil recirculation pressure drop
Value 3.75 psi
Reducer water recirculation inlet temperature
85°F
Reducer water recirculation temperature rise
9°F
Reducer water recirculation inlet pressure
40 psi
Reducer water recirculation flow rate
6 psi
Reducer water recirculation pressure drop
3.8 psi
Reducer oil recirculation inlet temperature
180°F
Reducer oil recirculation temperature rise
20°F
Reducer oil recirculation flow rate
3.6 gpm at 85°F
Reducer oil recirculation outlet pressure
<50 psi
Reducer oil recirculation pressure drop
8 psi
13.8 Fire Detection Inspections of the system should be performed on a regular basis for evidence of fires. Peeling paint, hot spots, and/or glowing or discolored surfaces can all be signs of a fire. Abnormal temperature differences and/or excursions can indicate a fire is present in the mill system. The recommended method of ensuring a startup without incident is to completely strip the mill of coal upon shutdown. If the system has been tripped with a coal charge in it, it must be physically inspected for the evidence of fire before startup. The following inspection sequence is recommended: • Examine the area above the coal level in the bunker supplying the mill for smoke, odor, or other indications of fire. Carbon monoxide or methane gas detection systems can be used to detect combustion. • Inspect the external surface of the downspouts between the bunker and feeders and the feeder housings for hot areas that may indicate an internal fire. • Inspect the top of the feeder housings and the top of the classifier housings. •
Inspect the surface areas of the crusher-dryer housings, the mill inlet/outlet boxes, the mill bypass dampers, the classifier inlet ducts, the coarse rejects pipe, the trickle valve housing, and the coal lines from the classifiers to the burners.
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Table 13-4 lists the temperature sensor locations for fire detection. Table 13-4 Fire Detection Temperature Sensor Locations [7] Location
Number of Sensors Required
Temperature Range (°F)
Single-Ended Mill
Double-Ended Mill
Hot Primary Air Fan Discharge
1
1
0–700
Hot Primary Air Header
1
1
0–700
Cold Primary Air Fan Discharge
1 per fan
1 per fan
0–700
Cold Primary Air Duct After Mill Rating Damper
1
1 (2 when damper is < 6 ft from duct split)
0–700
Mill Inlet
1
2
0–500 with crusher-dryer, 0–700 without crusher-dryer
Mill Discharge
1
2
0–300
Classifier Exit
1
2
0–300
An active fire anywhere in the mill system will usually cause abnormal temperature differences and/or excursions of the mill inlet, outlet, or classifier exit temperatures that cannot be accounted for by occurrences such as a sudden change in coal moisture, faulty control of primary air temperature or flow, or a sudden stoppage or decrease of raw coal feed into the mill. For mills with one classifier, the continuously recorded exit temperature can identify fires by indicating rates of temperature rise and abnormally high temperatures. For double-ended mills with two classifiers, the two continuously recorded exit temperatures, in addition to indicating rates of temperature rise and abnormally high temperatures can isolate fires by identifying the differences between classifier exit temperatures. It is rare, but possible, for fires to exist simultaneously in both ends of the mill. When a fire is detected anywhere in the mill system, the system should be taken out of service by simultaneously activating the water deluge system and the ball/tube mill system emergency trip. After a fire is extinguished, conduct a thorough inspection and repair any fire-damaged equipment. Remove any agglomerated coal, coke, deposits, or other debris remaining after the fire is extinguished. Close all inspection ports and all access doors except those at the mill ends. Completely vent the mill with the air fans. Thoroughly remove any debris from the inlet/outlet boxes manually. Proceed to remove slurry by
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pumping it through trunnions. Remove all material down to the top of the ball charge. An
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intermittent spray application at reduced pressure adjacent to the pump hose inlet in slurry may be required to assist in pumping if dry pulverized coal pockets are encountered. Any odor of burning coal detected inside the mill after the removal of coal down to the top of the ball charge may indicate hot, unburned coal remaining in the mill. Continue spraying and pumping slurry until all evidence of the fire disappears. It is recommended that further removal of slurry be accomplished by inserting a pipe into the bottom of the ball charge and pumping until suction is completely lost. Check the trunnion seal air local header loops for flooding and drain if needed.
13.9 Fire Protection If ignition occurs anywhere in a ball/tube mill system, a continued flow of hot air and fuel past the ignition source results in a rapid downstream spread of fire that is both damaging and dangerous. For this reason, fire control and extinguishing procedures must be initiated immediately upon detection of fire at any location in the system. The basic method of extinguishing fires in a ball/tube mill system is the application of water spray directly to the fire to quench it, plus an additional spray application to the mill interior to prevent accidental or spontaneous ignition of the pulverized coal in and above the ball charge. A fire fighting system should be installed for each ball mill system. Water spray nozzles should be installed in the 10 system locations as shown in Figure 13-5.
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Figure 13-5 Water Spray System Nozzle Locations [7]
The system should be permanently piped, have a remote manual activation switch in the control room, and should be activated on both ends of the system. Care should be taken to ensure that water leaks into the system cannot occur. Unintentional activation should not occur. The valve that directly supplies water to the deluge system should be automatically regulated in order to avoid overflooding the mill system.
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Deposits of unground or pulverized coal anywhere in the system between the feeder discharge and the boiler furnace are potentially dangerous and should be minimized. Raw or pulverized coal accumulations exposed to air above normal operating temperatures can spontaneously ignite. The probability of ignition is lowest with anthracite or low-volatile bituminous coals, but increases with both volatile and sulfur content coals. Midwestern U.S. bituminous, western subbituminous, and lignite coals, in that order, are increasingly prone to oxidation and self-ignition under the previously described conditions. System equipment maintenance involving welding or torch cutting may cause fires, especially following an emergency shutdown that has prevented the usual system purge. Emergency hot shutdown of a mill system containing a coal charge can also cause a fire. This could occur within two hours or within even a shorter period for the more readily ignitable coals. The practice of feeding hot coal into an operating mill system from a smoldering or active raw coal bunker fire is dangerous and should not be done. Fires can also be caused by operating at higher than normal temperatures.
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14
OPERATION AND SAFETY – STEIN INDUSTRIE
This section covers the mill layup and fire protection of the Stein Industries ball/tube mills.
14.1 Layup If the mill will be out of service or on standby for a period of 60 days or longer, then certain preservation tasks should be performed on the mill. The following are the equipment and tasks to be performed: •
Mill bunkers – The mill bunkers will not be able to hold the same coal for a period of more than six weeks. A full spontaneous combustion test should be performed.
• Mill barring or rotation – The mills should be rotated every 14 days for a period of at least 1 hour. A mill checksheet is completed and any problems with the rotation are corrected. •
Main bearing lubrication system (turbo lube system) – The lubrication system should be operating at all times except when maintenance is being performed on the system.
•
Gearbox lubrication system – The gearbox lubrication system should be operating when the rotation or barring occurs, but not operating when the mill is off-line.
•
Girth gear greasing system – The girth gear greasing system should be operated when the mill is being rotated, but not operated when the mill is off-line.
•
Girth gear seal air fan – The girth gear seal air fan should be operated when the mill is being rotated, but not operated when the mill is off-line.
•
Trunnion seal air system – One seal air fan should be operated when the mill is being rotated while the other fan should be tested for operation. The seal air regulating damper should be placed on manual and reduced to 10% flow in order to prevent the fan motors from experiencing high amperage.
•
Air system – The control air and service supply lines should remain open with the purge air systems on automatic operating mode.
•
Dampers – All mill dampers should be operated for one full cycle every 60 days. The primary air isolation damper can not be operated if there are other mills in operation.
•
Feeders – Feeders should be operated in the test mode every 60 days for one full revolution of the belt.
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14.2 Fire Protection After a mill is taken out of service, the mill is put on barring until the internal ball load and fuel has cooled down to prevent internal combustion. When the mill barring is stopped, the mill outlet temperatures should be checked for a temperature rise. The mill should be checked for any hotspots or discolored areas. It must also be checked for the smell of sulfur that would indicate combustion. If a major temperature rise exists, it will be necessary to inject carbon dioxide. Before the carbon dioxide trailer is started, the following should occur: •
Ensure the trunnion seal air fan is shut down.
•
Ensure the girth gear seal air fan is shut down.
•
Shut down the compressed air purge of differential pressure level control.
•
Ensure the mill is off barring.
The carbon dioxide system is activated by the following actions: •
Position the carbon dioxide trailer about two meters from the control panel to ensure the hoses are uncoiled completely.
•
Connect the two quick-acting couplings from the trailer hoses to the control panel.
•
On the female coupling, pull back the spring-loaded outer sleeve and push the male coupling firmly into the mating piece. Release the sleeve and pull hard on the hose to ensure the hose is firmly coupled.
•
If you do not need to inject carbon dioxide into the mill body, move the mill body valve bar to the closed position to allow injection to the classifiers only.
•
Open the carbon dioxide cylinders isolating valve levers to allow injection into the classifiers.
• Empty the top three cylinders first. The top three cylinders are for cooling down the mill and to reduce the oxygen level. • If the fire still exists, discharge the remaining four cylinders at a continuous flowrate over a period of 40 minutes. • After the cylinders have been discharged, close the cylinder valves and wait five minutes for any residual gas to clear the hosepipe. • Disconnect the coupling and remove the trailer and cylinders from the mill area. • Recharge the cylinders.
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15 PERFORMANCE
Acceptance testing for mills is performed according to the American Society of Mechanical Engineers (ASME), Performance Test Code (PTC) 4.2, Coal Pulverizers, 2003. Other testing may be performed as part of an overall boiler test or to determine if maintenance or adjustments are needed. The following are parameters that affect mill performance: •
Fineness
•
Grindability
•
Moisture
•
Capacity
15.1 Fineness Fineness is an indication of the quality of the pulverizer action. Fineness is defined as the measured particulate size distribution of pulverized product as determined by standard screens. Specifically, fineness is a measurement of the percentage of a coal sample that passes through a set of test screens usually designated at 50, 100, and 200 mesh. Table 15-1 shows the standard screen dimensions. Table 15-1 Standard Screen Dimensions Mesh
In.
Microns
20
0.0331
840
30
0.0234
595
40
0.0165
420
50
0.0117
297
60
0.0098
250
100
0.0059
149
140
0.0041
105
200
0.0029
74
325
0.0017
44
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15 PERFORMANCE 400
0.0015
37
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Performance
A standard fineness is 70% passing through a 200-mesh screen. The coal retained on the 50mesh screen should be in the 1–2% range. There are two standards that are used in sampling and fineness testing. The standards are the ASME PTC 3.2-1054- Solid Fuels and the American Society of Testing Materials (ASTM) D 197 (1980) - Sampling and Fineness Test of Pulverized Coal. Tutuka Power Station samples and tests the coal for fineness every 3500 operating hours.
15.2 Grindability Grindability indicates the ease that coal can be ground. A higher grindability number indicates coals that are easier to pulverize. Grindability is not just a matter of hardness. Coal of the same hardness can have a range of different grindability indexes because of other constituents such as moisture. Also, materials that are fibrous, sticky, or plastic can be difficult to grind. One test procedure is the Hardgrove Machine Method to determine the Hardgrove Grindability Index. This procedure is described in ASTM D 409 – Method of Test for Grindability of Coal by Hardgrove Machine Method. The grindability is determined by the amount of new material that will pass through a 200-mesh screen. A 50-gram air-dried coal sample, sized to less than 16 mesh and greater than 30 mesh, is placed in the Hardgrove machine with eight 1-in. steel balls. A weighted race is placed on the balls, and the machine is turned for 60 revolutions. The resultant coal size is then compared to an index and a value assigned from the index. Another test procedure is the bond ball mill work index (BMWI) that was developed for use in the minerals industry to determine the grindability of ores. The bond BMWI work index models continuous grinding in a horizontal ball mill. It was devised as a feature of the bond method for sizing ball mills. A standard cylindrical test mill is loaded with balls of a designated diameter and weight. The coal is air-dried and screened over a No. 6-mesh screen. The sample is then placed into the mill and rotated 100 times at a speed of 70 rpm. The product is then screened over a 200-mesh screen. Fresh sample coal is added to the oversized coal, and the mixture is then retested. Using the screening process, the number of mill revolutions required to give a product containing 28.6% passing through 200-mesh sieve and 71.4% (250% of 28.6) oversize is estimated. The process is continued until a constant value is obtained for the mass of undersize coal produced per mill revolution. This value is called the grindability (G). Correlations between the Hardgrove Grindability Index and the bond BMWI have been developed.
15.3 Moisture Moisture content in the coal can influence grindability for sub-bituminous and lignite coals. The surface moisture plus inherent moisture influence the amount of drying needed and the air inlet temperature. Coal moisture is highly variable and depends more on coal type than on the amount
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Performance
of water introduced after mining. Moisture may be present in the geological deposit or may be surface moisture that is introduced during handling, transport, processing or storage. Moisture found in the coal ranges from levels of 2% for Appalachian bituminous to 40% for lignites. Brown coals can range up to 70%. Moisture in coal is determined according to the procedures in ASTM D3302-02a – Standard Test Method for Total Moisture in Coal.
15.4 Capacity The following is an example of a performance improvement project with the mills. The Arnot Power Station of Eskom was initially designed to produce 350 MW. The ball mills are Stein Industrie mills that are serviced by Alstom. Three mills producing 54 tons/hour of pulverized coal were required to produce the 350 MW. To perform maintenance on one mill, it was necessary to reduce the load on the unit to 280 MW. It was decided to derate the unit to a nominal 320 MW. The station investigated the feasibility of operating two mills and achieving the full load of 320 MW. It was decided to increase the ball volume from 17–21%, which is equivalent to 80 tons of balls total. This was done over a six to eight month period. As a result, the motor mill current increased from 260 to 310 amps. The current trip is set at 340 amps, which is 110% of the load. A safe minimum air to fuel ratio is 1:5, and the minimum primary air velocity is 18 m/second. It was necessary to modify the classifiers to achieve the desired fineness. The following modifications were performed: •
A reject box with a loose sealing flap door was added to stop the coarse pulverized coal from bypassing the classifier vanes.
•
Every second classifier vane was removed to eliminate the blocking of vanes by foreign objects.
•
The Vortex finder was reintroduced to improve fineness.
•
The feed chute was covered to stop the raw coal from entering the classifier before it was milled.
The conclusions of the test were that two mills could produce 330 MW with an acceptable fineness. It was necessary to operate an additional primary air fan in order to allow more boiler control. The ability to operate the unit at full load with only two mills allowed a spare mill for maintenance work and produced the power with considerable savings to the plant. EPRI is currently looking at pulverizer performance issues to include the impact of blending coals, mill control, and particle size distribution measurement.
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16 FAILURE MODES
Information from earlier studies of pulverizer components is noted below. The failure of the pulverizer components can be shown as the relative frequency of component failures [10]. Figure 16-1 shows the frequency of pulverizer component failures.
Figure 16-1 Pulverizer Component Failure Frequency [10]
A discussion of failure mechanisms is given and an excerpt is provided from EPRI report CS5935 [11]. Failure modes analysis is defined by failure mechanisms and recommended action to reduce or eliminate the failure mechanisms. Failure mechanisms for the pulverizer mills include erosion and three types of abrasion.
16.1 Abrasion The three types of abrasion are: •
Gouging abrasion – Gouging abrasion is a heavy plastic deformation of a surface by hard mineral fragments under heavy pressure or an impact causing deep surface grooving or gouging and the removal of relatively large wear debris particles. Examples of gouging
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16-1
abrasion are seen in jaw crushers and hammer mills.
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16-2
Failure Modes
•
High-stress grinding abrasion – High-stress grinding abrasion is a three-body process caused by mineral fragments under sufficient contact stress to cause the scratches in the contacted surface. Examples of high-stress grinding abrasion are found in pulverizers and ball mills.
•
Low-stress scratching abrasion – Low-stress scratching abrasion is the wear by cutting or plowing of mineral fragments under contact stress below their crushing strength. Examples of low-stress scratching abrasion are found in coal chutes, sand pump impellers, and screens.
In coal pulverization, 5–20% of the material being crushed is abrasive mineral. A large part of the power in coal pulverization is used to crush coal. Coal is not abrasive by itself. The minerals in coal that are the most abrasive are quartz and pyrite. These minerals in the coal cause a less severe, high-stress grinding abrasion than the minerals alone. Because of the cushioning effect of coal powder, the size and shape of the mineral particles found in coal influence the severity of the abrasion process. Abrasion involves the sliding of particles under normal load over a surface. The abrasion rate is influenced by the particle hardness and the normal load of the abrasive medium. The removal of material during the abrasive wear process can occur by cutting or plowing. The cutting process is more efficient and results in severe wear. The probability of cutting by abrasive particles increases with the sharpness and the angularity of the particles. Therefore, quartz particles crushed in a mineral processor are more aggressive than rounded sand particles sliding over a metal surface. The angle of attack by each individual abrasive particle determines whether cutting will occur. This cutting occurs when the angle between the leading facet and the plane of sliding reaches a critical value. The critical angle for cutting is influenced by metal alloy properties. For example, the critical angle for the cutting abrasion of nickel is 60–70º. One of the parameters that influence abrasion resistance is the quantity of carbides in the metal part. Some materials that contain massive carbides are Ni-Hard, high-chromium cast iron, and Stellite. The Ni-Hard and high-chromium white cast iron materials are considered to be the most resistant to mineral abrasion in high-stress grinding abrasion conditions. Alloying elements are important in the design of abrasion resistant alloys. Carbon content is the most effective parameter in abrasion control. As the carbon content increases, abrasion resistance increases. Increasing silicon content will significantly improve fracture toughness in a cast material. Molybdenum, in quantities up to 1%, will improve abrasion resistance with no discernable effect on toughness. Usually an increase in abrasion resistance is accompanied by a decrease in toughness. High-chromium cast iron materials are used for improved abrasion resistance. These alloys have a variety of compositions to choose from. The alloy could be selected on the basis of optimizing required toughness, hardenability, corrosion resistance, and abrasion resistance. In comparing wear coefficients for several materials having wear data available, the values shown in Table 16-1 have been obtained.
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16-3
Failure Modes Table 16-1 Abrasive Wear Coefficients [10] Material
Wear Coefficient -4
High-Chrome Cast Iron (18 Cr, 2 Mo)
1.2 × 10
Ni-Hard (3C, 4Ni, 2Cr)
1.5 × 10
Stellite No. 6 hardfacing
2.4 × 10-4
Star J (Stellite) hardfacing
5.4 × 10
1090 Steel (Rockwell C hardness 55)
8.0 × 10-3
-4
-4
16.2 Erosion Erosion by mineral particles picked up in the air stream carrying pulverized coal through the mill, classifier, and transport pipe is a recognized problem. The erosion process is more selective than the abrasive wear and tends to remove metal in localized areas. Erosion can produce holes in steel liners and deep depressions in large section cast parts. A localized attack is typical of erosive damage because of the sensitivity of the material removal rate to the angle of impingement and the impingement velocity. The following parameters affect erosion rates: •
Velocity – Erosion rates increase exponentially with velocity. For ductile materials, the exponent is about 2.3; for brittle materials, the exponent ranges between 1.4 and 5.
•
Impingement angle – Maximum erosion rates occur at 30º for ductile materials and at 90º for brittle materials.
•
Particle size – Erosion rates increase with particle size up to a critical size. Particle sizes larger than the critical size do not increase the erosion rate. For very small particles, all materials act like ductile materials. EPRI studies have determined that pyrite particles above the 200-micron size cause pronounced damage [11].
•
Particle hardness – Hard particles relative to the surface being eroded are more aggressive. EPRI studies have determined that mill wear is primarily dependent on the quartz content and, to a lesser extent, the pyrite content of the coal. In general, the damaging effect of quartz is about two to three times that of pyrite [11].
•
Material structure – Single-phase materials improve erosion resistance with increasing hardness. Multiphase materials are insensitive to hardening.
The approach to erosion control requires using wear-resistant material mechanical properties for a predicted impingement angle. There has been success in the industry using ceramic materials. Ceramics are ideal for erosion corrosion conditions because of their inertness in the corrosive environment and their ability to handle low impingement angle erosion.
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Failure Modes
16.3 Failed Components Table 16-2 lists the components, failure, cause of failure, and the corrective action for the AllisChalmers mills. Table 16-2 Allis-Chalmers Failure Components (Courtesy of Lethabo Power Station) Components
Damage
Cause Of Damage
Corrective Action
Acoustic hood
Bent/damaged
Aging/incorrect assembly
Repair
Feed tube
Erosion/deformed
Misalign/ageing
Replace/align
Bypass damper
Worn/bent
Wear
Repair
Bypass damper shaft
Worn/bent
Wear
Replace/repair
Bypass damper pillar blocks
Worn
Wear
Refurbish
Reject flap valve
Hole/deformed
Wear
Replace
Division plate
Erosion/deformed
Wear
Repair/replace
Slope liners inlet box
Worn/cracked
Wear/impact
Replace
Slope liners outlet box
Worn/cracked
Wear/impact
Replace
Slope liner bolts
Loose/missing
Vibration
Replace/tighten
Square to round seal
Leaking/damaged
Wear
Repair
Classifier outer cone
Hole/erosion
Wear
Repair
Classifier inner cone
Hole/erosion
Wear
Repair
Classifier inverted cone
Worn/misaligned
Wear/incorrect assembly
Repair/align
Classifier vanes
Worn
Wear
Repair/replace
Reject wear sleeve
Hole/erosion
Wear
Replace
Reject pipe bend
Worn/hole
Wear
Replace
Ball loading pipe bend
Damaged/hole
Impact/wear
Repair/replace
Ball loading hopper
Leaking/deformed
Impact/jammed
Repair/clear
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17-11
17 TROUBLESHOOTING
This section consists of troubleshooting tables for the ball/tube mills provided by AllisChalmers, Foster Wheeler, Kennedy Van Saun, Riley Power Inc., and Stein Industrie.
17.1 Allis-Chalmers Table 17-1 lists the symptoms, possible causes, and actions for troubleshooting the AllisChalmers mills. Table 17-1 Troubleshooting for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Symptoms
Causes
Actions
Gearbox Lubrication System High oil temperature
Low oil pressure
Vibration
Spray nozzles in gearbox unit blocked
Remove spray nozzles and piping, flush, and clear.
Filter unit dirty
Clean filters.
Cooler not functioning properly.
Check cooling water flow. The minimum flowrate is 0.88 liters/second.
Blocked cooler
Clean cooler.
Pump over-heating
Check pump and motor speed (1425 rpm). Inspect and repair pump.
Pressure relief valves set too low
Reset relief valves.
Leakage in system
Check system for leaks and repair.
High oil temperature
Verify temperature indication and operation of cooler.
Oil pump worn
Repair pump
Pump not operating at rated speed
Check pump and motor speed (1425 rpm). Inspect and repair pump.
Air in suction line
Check gear unit oil level and add oil as needed.
Pump-motor misalignment
Check coupling alignment.
Worn or loose pump bearings
Rebuild pump.
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16-4
Piping exerting forces on pump
Re-align piping.
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17-11
Troubleshooting
Table 17-1 (continued) Troubleshooting for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Symptoms
Causes
Actions
Trunnion Bearing Lubrication System High oil temperature
Low jacking oil pressure
Vibration
Filter unit dirty – associated with high low pressure pump pressure
Change filter unit.
Cooler not functioning properly
Check cooling water flow when oil temperature is above 30°C. Clean cooler.
Low-pressure pump overheating
Check low-pressure pump and motor speed (1410 rpm). Inspect pump and rebuild if needed.
Low oil flow
Check flow rate to be 0.38 liters/second.
Regulating pressure relief valve set too low
Reset relief valve to 31 mPa.
Leakage in system
Check system for leaks and repair.
High oil temperature
Verify temperature indication and operation of cooler.
Jacking oil pump worn
Repair pump.
Pump not operating at rated speed
Check pump and motor speed.
Air in suction line
Check bearing sump oil level and add oil as needed.
Pump-motor misalignment
Check coupling alignment.
Worn or loose pump bearings
Rebuild pump.
Piping exerting forces on pump
Realign piping.
Power-Sonic Mill Conditioning System Power or sound meters inoperative
Power set point or read/set switch inoperative
Defective meter
Replace meter.
Loose terminal
Tighten all terminal connections.
Defective cable
Repair or replace cable.
No system output
Repair main electronics.
Defective potentiometer or switch
Install new potentiometer or switch.
Loose or defective cable
Tighten or repair cable.
Main electronic system defective
Repair main electronics.
Feeder speed controller defective
Repair or replace speed controller.
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17-2
Troubleshooting Table 17-1 (continued) Troubleshooting for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Symptoms
Causes
Actions
Power-Sonic Mill Conditioning System (continued) Mill tends to overfill
Mill tends to empty
High sound alarm inoperative
Low power alarm inoperative
Mill does not change coal level with output demand
Incorrect power set point
Adjust power set point.
Incorrect sound set point
Adjust sound set point.
Incorrect feed forward set point
Adjust feed forward set point
Incorrect output polarity
Change switch.
Incorrect feed forward input polarity
Change switch.
Defective electronic system
Repair or replace electronics.
Defective feeder speed controller
Repair or replace speed controller.
Low ball charge
Replenish ball charge.
Incorrect power set point
Adjust power set point.
Incorrect feed forward set point
Adjust feed forward set point.
Incorrect output polarity
Change switch.
Defective electronic system
Repair or replace electronics.
Defective feeder speed controller
Repair or replace speed controller.
Defective relay
Repair or replace relay.
Defective electronic system
Repair or replace electronics.
Defective or incorrect wiring to terminal block
Repair or replace wiring.
Incorrect high sound set point
Adjust high sound set point setting.
Defective relay
Repair or replace relay.
Defective electronic system
Repair or replace electronics.
Defective or incorrect wiring to terminal block
Repair or replace wiring.
Incorrect low power set point
Adjust power set point.
Incorrect feed forward set point
Adjust feed forward set point.
Incorrect feed forward gain setting
Adjust feed forward gain setting.
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17-3
Troubleshooting
Table 17-1 (continued) Troubleshooting for Allis-Chalmers Mills (Courtesy of Lethabo Power Station) Symptoms
Causes
Actions
Power-Sonic Mill Conditioning System (continued) No ± 15-volt on PCB
No input power
Connect power and turn on as appropriate.
Blown fuse
Replace fuse.
PCB not located correctly in connector.
Insert PCB correctly.
Defective transformer
Replace transformer.
Defective regulator
Replace regulator.
Defective transistor
Replace transistor.
Defective input switch
Replace switch.
Defective IC in system
Replace IC.
Switch S4 open
Close switch S4.
Inputs holding output high or low
Correct other inputs.
Feed forward input has no effect on output.
Look for power input in previous fault
Look at previous fault.
Set switch between positions.
Set switch to correct position.
Sound input has no effect on output.
Look for power input in previous fault
Look at previous fault.
Defective filter
Replace filter.
Filter not tuned to correct frequency
Tune filter.
Set switch between positions.
Set switch to correct position.
Defective IC
Replace IC.
Defective output transistors
Replace output transistors.
No input to transducer
Check connection to transducer.
No input power to amplifier terminals or input out of range
Connect correct input to terminals.
Defective transducer
Replace transducer.
Microphone input clogged
Clear microphone area.
No input power
Connect ± 15-volt dc from main electronics.
Defective IC
Replace IC.
Defective microphone
Replace microphone.
Power input has no effect on output
Input has no effect on output
No output from power transducer
No output from sound transducer
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17-4
Troubleshooting
17.2 Foster Wheeler Table 17-2 shows a troubleshooting chart for the operation of a negative pressure Foster Wheeler ball mill. Table 17-2 Troubleshooting Chart for Foster Wheeler Mills [5] Problem Loss of pulverized coal
Possible Causes Mill level control lines may be plugged or leaking. Coal feeder may be plugged. Feeder motor may be stopped, burned out, or have blown fuses. Feeder rotating table is binding. Coal feed pipe to the mill is plugged. Mill is plugged or overloaded with raw coal. Mill is receiving coal faster than it can be ground. Classifier settings are too fine. Coal ribbon conveyor is not working.
Pulverized coal is spilling out of the mill
The mill level control lines may be plugged or leaking. The feeder is operating continuously on manual. The automatic feeder controller is malfunctioning. Dry, fine coal is spilling off the feeder rotating table into the mill. The liner bolts on the liner plates are loose. The mill is receiving coal faster than it can be ground. The mill suction pressure is lost or the mill is being pressurized. The fuel level block is excessively worn.
Exhauster is tripped
The exhauster has been accidentally tripped. There is a loss of one or more forced draft or induced draft fans There is a loss of voltage for the exhauster motor. The exhauster overload trip has been activated.
Mill is tripped
The mill is accidentally tripped. There is a loss of exhausters, forced draft or induced draft fans. There is a loss of voltage for the mill motor. The mill motor overload trip is activated.
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17-5
Troubleshooting
Table 17-2 (continued) Troubleshooting Chart for Foster Wheeler Mills [5] Problem Feeder is tripped
Possible Causes The feeder is accidentally tripped. There is a loss of exhausters, forced draft or induced draft fans. There is a feeder motor overload trip. There are problems with the thermal heaters in the circuit. The feeder motor fuses are activated. The feeder rotating table is binding.
The dampers are tripped
There is a loss of exhausters, forced draft or induced draft fans. The dampers are accidentally closed. There is a loss of exhauster differential. The air operated pistons or air solenoid valves are not operating properly.
Bearing (mills, exhausters, or feeders) temperatures are too high
Bearing vibration is too high. There is a loss of lubricant in the bearings. The lubricant supply lines are plugged. The lubrication system is not operating. The lubricant type is incorrect. There is a failure of the bearing wear parts.
Mill gear and pinion damage
There is misalignment between the gear and pinion. The Farval lubricator is not operating. There is coal or foreign material on the gear surface. There is excessive loading on the gear teeth.
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17-6
Troubleshooting
17.3 Kennedy Van Saun Table 17-3 lists the problems, possible causes, and actions for the Kennedy Van Saun ball/tube mills. Table 17-3 Troubleshooting Chart for Kennedy Van Saun Mills (Courtesy of Kendal Power Station) Problem Low primary airflow
High mill motor stator winding temperature
Oil support required
Possible Cause
Actions
Low airflow <37.5 Kg/second
Check the cold air and hot air damper control. If the primary airflow does not increase, change the damper control to manual and open the damper to clear the alarm/trip. Note: There is one minute to trip the mill.
Faulty primary airflow transmitter
Establish which transmitter is faulty. Select the cold air and hot air dampers to manual to restore airflow. Contact the instrumentation and controls group.
Closed auxiliary cooling return valve
Open the auxiliary cooling valves to >30 L/minute flowrate. Contact maintenance group. Change to spare mill.
Overloading mill motor
Check motor current to be <203 amps. Reduce the mill load.
Bearing temperature tracks the winding temperature
Check the rate of change of the bearing temperature and the stator winding temperature. Contact maintenance. If the rate of change indicates a trip condition, shutdown the mill.
Too high or too low motor voltage/current readings
Determine proper values for mill motor voltage/current. Increase voltage and decrease current on the mill motor.
Failed instrument
Verify thermocouple reading. Contact the instrumentation and controls group.
Tripped both feeders
Establish oil support. Start feeders in the automatic mode.
Mill motor just started
Verify oil support. Ensure alarm clears three minutes after the mill starts.
Low fuel flow <38%
Establish oil support. Stop the loading program. Increase the output setpoint by 10 Mw. Change the loading program to automatic. Ensure fuel flow increases.
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17-7
Troubleshooting
Table 17-3 (continued) Troubleshooting Chart for Kennedy Van Saun Mills (Courtesy of Kendal Power Station) Problem High mill bearing temperature
Low seal air pressure
Abnormal mill coal level
Possible Cause
Actions
Low oil level
Verify oil level. Monitor bearing temperatures. If needed, add oil and inspect for leaks.
Faulty thermocouple
Determine which thermocouple is faulty. Replace thermocouple. Monitor bearing vibration.
High load on motor
Verify motor amps are <203 amps. Lower mill level to normal.
Failed bearing
Check rate of change of bearing temperature on plant instrumentation. If rate is high, shut down mill. Contact maintenance personnel.
Incorrect oil ring operation
Check oil ring operation. Inform maintenance personnel. Shut down mill.
Lack of oil
Shut down mill. Inform maintenance personnel.
Low cooling water to oil cooler
Check auxiliary cooling flow to be ≥30 L/min. If not, shut down mill. Contact maintenance personnel.
Broken/leaking impulse line to transmitter
Try to contain leak. Start standby mill.
Ruptured seal air ductwork
Shut down mill. Start standby mill. Inform appropriate mill.
Faulty instrumentation
Try to repair instrument. Start standby mill.
Tripped seal air fan motor
Ensure standby seal air fan motor starts automatically or manually. Investigate reason for seal air fan motor trip. Contact appropriate personnel.
Blocked suction filters
Investigate and correct blocked filters. Contact appropriate personnel.
Low mill level at -1
Investigate feeder operation. Monitor mill level.
Low mill level at -2
Mill motor will trip at 35% in shutdown program. Monitor mill level.
High mill level at +1
Check feeder speeds. Remove control from automatic to manual. Reduce speed and place back in automatic. Monitor mill level.
Faulty card
Change feeder control to manual. Add oil support to burners. Monitor mill level.
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17-8
Troubleshooting
17.4 Riley Power Inc. Table 17-4 lists the problems and possible causes for the chain-driven Riley Power ball/tube mills. Table 17-4 Troubleshooting Chart for the Chain-Driven or Gear-Driven Riley Power Ball Mills [7] Problem
Possible Causes
Speed Reducer Gearbox Overheating
Overloading – Load exceeds capacity of reducer. Improper lubrication – Insufficient oil level, oil level too high, bent or missing oil wipers, clogged air passages, wrong grade of oil, forced lubrication, and oil cooling system on high speed drives may be clogged
Noise and vibration
Loose foundation bolts Bearings failed from wear and overloading Excessively worn gears from overloading Insufficient oil Loose parts from excessive shock loads Excessively high speeds
Excessive end play in low speed shaft or excessive radial motion in high speed shaft
Worn bearings from exposure to an abrasive substance
Excessive backlash or slack
Worn gear or loose parts. Backlash is greater in double- and triplereduction units.
Oil leakage
Leakage between housing and cover, too much oil, clogged breather
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17-9
Troubleshooting
17.5 Stein Industrie Table 17-5 lists the alarms, possible causes, and operator actions for the Stein Industrie ball/tube mills. Table 17-5 Troubleshooting for Stein Industrie Mills (Courtesy of Tutuka Power Station) Alarm
Possible Causes
High motor and bearing temperatures
Mill motor drive-end and non-drive-end bearing temperature high on video
Operators Action
Oil leaks
Check for oil leaks.
Incorrect oil level
Check oil level. Add oil if needed.
Contaminated oil
Sample and test oil.
High inlet oil temperature
Check cooling water flow. Check filter differential pressure. Change filter if required.
Motor/gear misalignment
Check alignment.
Worn bearings
Inspect bearings.
Motor overheated
Check motor winding temperature.
Thermocouple faulty
Check for loose connections and check thermocouple. Alarm value >60°C Trip value >80°C
Mill motor winding temp high on video
Low/High Primary Air Inlet Flow
Motor overloaded
Reduce mill load.
Worn bearings
Inspect bearings.
Gearbox oil level low
Check oil level and add oil if needed.
Low-supply voltage
Check equipment.
Faulty measuring equipment
Alarm value >120°C
Faulty dampers
Check dampers at normal working position.
Mill blocked
Verify mill inlet or outlet differential pressure and coal level normal.
Faulty pressure control of PA fans
Verify PA fans on automatic and PA fan outlet pressure is normal.
Damaged expansion joints
Verify pipework is in good condition
Faulty measuring equipment
Check equipment.
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17-10
Troubleshooting Table 17-5 (continued) Troubleshooting for Stein Industrie Mills (Courtesy of Tutuka Power Station) Alarm Low primary air bypass flow during mill startup
High bearing temperature
Possible Causes
Operators Action
Faulty dampers
Verify dampers are open in plant.
Damaged pipework
Verify pipework is in good condition.
Faulty measuring equipment
Check equipment.
Faulty startup program
Check startup program.
Bearing temperature on the drive-end and non-drive-end high on video Damaged bearings
Inspect bearings.
Hot air leaks onto bearings
Inspect for hot air leaks.
High-oil temperature
Check cooling water flow.
Blocked filter
Check filter differential pressure and change filters if needed.
Oil leaks
Check for oil leaks.
Faulty lube oil pressure control valves
Check control valves.
Faulty measuring equipment
Check equipment Alarm value >60°C Trip value >80°C
Low oil flow spray
Blocked filters
Check filter differential pressure and change-over filter if needed.
Damaged pipework
Check for oil leaks.
Lube oil pump faulty
Check pump coupling.
Faulty measuring equipment
Check equipment.
Contaminated oil
Sample and analyze oil.
Low lube oil temperature
Check in on cooling water flow to increase oil temperature. Check temperature control on cooling water system. Trip value <21 liters/second after 60 seconds.
Orifice plate wear
Check size.
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1711
Troubleshooting
Table 17-5 (continued) Troubleshooting for Stein Industrie Mills (Courtesy of Tutuka Power Station) Alarm Faulty lube oil level
Faulty mill gear wheel lubrication
Possible Causes
Operators Action
Mill oil tank level low/ high. Mill reduction oil tank level low/high on video. Oil leaks
Check for any oil leaks.
Tank temperature low/high
Verify tank temperature to be normal >35°C and <65°C.
Contaminated oil
Sample and test oil.
Operating error
Purify oil if necessary. Check oil in sight glass. Add or drain oil if needed.
Faulty measuring equipment
Check equipment.
No grease
Verify grease drum level is correct.
No air supply
Verify air supply.
No electrical supply
Verify electrical supply is energized.
Faulty timer
Check timer.
Pump faulty
Inspect pump.
Loss of girth gear seal air fan
Verify fan is on-line. Alarm after 10 minutes. Trip after 30 minutes.
Faulty seal air system
Seal air fan tripped
Verify fan is on-line.
Faulty dampers
Verify dampers in plant are open. Verify control damper actuator is intact.
Air leaks
Check for air leaks in piping.
Faulty measuring equipment
Check equipment. Trip mill after 15 seconds.
Faulty feeder
Shearing pin broken
Verify shearing pin is intact.
Feeder tripped
Check switch gear.
Bunker empty
Verify coal level in bunker to be correct.
Coal flow blocked/hang-up
Verify coal in feeder and clear hang-up if needed.
Faulty measuring equipment
Check equipment.
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17-12
18 PREDICTIVE MAINTENANCE
Predictive maintenance philosophy assumes that the pulverizer equipment will deteriorate and a partial or complete loss of function will occur. Predictive maintenance monitors the condition or performance of plant equipment through various technologies. The data are obtained, analyzed, trended, and used to predict equipment failures. When equipment failure timing is known, then actions can occur to prevent or delay failure. This allows equipment reliability to remain high. Predictive maintenance is accomplished by integrating all available data in order to predict the impending failure of equipment. This process depends on the ability to recognize undesirable operating conditions as measured by diagnostic equipment. This process allows equipment to continue operating in an undesirable condition while it is being monitored until maintenance can be scheduled. Early warning can be broken into catastrophic failure or minor failure and deterioration. These can be further broken into maintenance or operations effects and to actual costs. Figure 18-1 shows the effects of early warning for pulverizer equipment failure.
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18-1
Predictive Maintenance
Figure 18-1 Early Warning for Pulverizer Failure [12]
Some advantages of detecting equipment problems early include: •
Reduced catastrophic failure rate – This rate is reduced by diagnosing equipment conditions and taking action before the equipment fails.
•
Reduced forced outage rate – By detecting equipment problems early, the inspection and repairs can be performed during scheduled outage time and not during a forced outage.
•
Increased inspection/overhaul interval – The inspection and overhaul interval can be increased by knowing the equipment condition and not basing the interval on elapsed time alone.
•
Reduced maintenance outage length – The time to perform inspection and repairs is reduced when adequate planning for the outage can occur. This can include having the correct parts and tools on site, the labor force planned, the isolation tags requested, and so on.
One of the main predictive maintenance tasks is the measurement and estimation of wear. This applies to drum liner wear and ball wear. Other areas of wear are on the inlet and outlet ducts and pipes. The main technologies used in predictive maintenance are vibration analysis, oil analysis, and
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18-10
Predictive Maintenance
thermography. This section covers the vibration analysis and oil analysis.
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18-1
Predictive Maintenance
Thermography may be used on the mill motors to detect overheating, loose connections, and so on. For more information on thermography for use on motors, see Electric Motor Predictive Maintenance Program (EPRI report TR-108773-V2) [13]. Thermography may also be used on the gear teeth to detect misalignment or areas of high temperature that can eventually lead to lubrication breakdown.
18.1 Vibration Analysis Vibration readings should be taken on the trunnion tube bearings and the pinion and bull gears. The recommended frequency of taking vibration readings on the gearing is a three to six month interval depending on the operation of the mill. Some general guidelines on interpreting vibration data are given in this section. If the dominant frequency is one times the operating speed (1 × rpm), then the vibration could be caused by an unbalance, a misalignment, or a bent shaft. If the dominant frequency is two times the operating speed (2 × rpm), then the vibration can be caused by looseness or misalignment. Multiple values of the operating speed indicate vibration caused by bearing or gear problems. Identifying the specific details of the bearings and gears, such as the number of rollers in the bearings, the number of teeth on the gears, and the individual gear speed, allows the measured high frequencies to be used as a diagnostic tool for the bearings or gears. For example, a bearing with 18 rollers on a shaft rotating at 1800 rpm would have a characteristic frequency of 540 hertz (1800 rpm/60 seconds per minute × 18 rollers). The mill motor vibration should be checked on a quarterly basis. The horizontal and vertical vibration readings should be taken on the inboard and outboard bearings. Axial vibration readings should be taken on the outboard bearing. The vibration amplitude and phase readings should be trended. High axial vibration readings could be the result of a coupling problem or misalignment between the motor and gearbox.
18.2 Oil Analysis Technical Key Point Lubricant testing is recommended for several reasons. These include: •
To study the condition (wear) of the machine being lubricated. If there is a problem with the lubricant, there is a strong possibility that the machine will need maintenance.
•
To determine if the lubricant is meeting the specifications
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18-3
Predictive Maintenance
There are numerous lubricant tests that can be performed on an oil sample. The task is to perform the minimum tests that produce the condition of the oil and the condition of the machine. The first and most crucial step in lubricant testing is to get a representative sample. Recommendations for taking samples are to: •
Take the sample when the system is stabilized, not before or just after makeup lubricant has been added.
•
Take the sample ahead of filters so that contaminants are still in the lubricant.
•
Put the oil sample in a suitable, clean, well-labeled container.
•
Take the sample using a consistent method. Take the sample from the same location and under the same operating conditions.
•
Ensure that contaminants from the component do not enter the sample.
It is recommended that the bearing lubricating oil sample be taken and analyzed every 6 months for the Riley ball/tube mills. A general discussion of lubricant testing is given in this section. The following are laboratory tests [14] performed on oil samples. •
Particle count (International Standards Organization (ISO) 4405, 4406) – Particles have long been recognized as the main cause of failure in hydraulics and rotational machinery. Particles are also a leading indicator of a machine’s condition. Since all contaminants in the oil are counted as particles, the particle count includes wear particles, soot, dirt, and other contaminates. This test provides information on lubricant cleanliness. As oil cleanliness becomes more important, particle counters have taken on an increasingly important role in maintenance strategies. Most particle counters use light or infrared energy to illuminate individual particles and are referred to as optical particle counters. The ISO Solid Contaminant Code (ISO 4406:99) is probably the most widely used method for representing particle counts (number of particles/milliliter) in lubricating oils and hydraulic fluids. The current standard employs a three-range number system. The first range number corresponds to particles larger than 4 microns, the second range number for particles larger than 6 microns, and the third for particles larger than 14 microns. As the range numbers increment up by one digit, the associated particle concentration roughly doubles. A typical ISO Code for a turbine oil would be ISO 17/15/12. Particle counts can be obtained manually using a microscope or by an automatic instrument called a particle counter. There are many different types of automatic particle counters used by oil analysis laboratories. There are also a number of different portable and online particle counters on the market. The performance of these instruments can vary considerably depending on the design and operating principle.
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Table 18-1 shows the particle count range numbers and the corresponding number of particles. Table 18-1 Particle Count Range Numbers [15] Number of Particles per Milliliter Sample Greater Than
Up To and Including
Range Number (R)
80,000
160,000
24
40,000
80,000
23
20,000
40,000
22
10,000
20,000
21
5,000
10,000
20
2,500
5,000
19
1,300
2,500
18
640
1,300
17
320
640
16
160
320
15
80
160
14
40
80
13
20
40
12
10
20
11
5
10
10
2.5
5
9
1.3
2.5
8
0.64
1.3
7
0.32
0.64
6
0.16
0.32
5
0.08
0.16
4
0.04
0.08
3
0.02
0.04
2
0.01
0.02
1
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•
Fourier transform infrared analysis (FTIR) – The FTIR monitors the chemical composition of the oil in certain key wavelengths. The infrared absorption spectrum of a lubricant furnishes a means of fingerprinting organic compounds and functional groups. Test results are trended, and quantitative and qualitative determinations can be made. Infrared analysis is often used for identifying additives and their concentrations, reaction products, and contamination by organic materials in used lubricants. Oxidation (carboxylic acids and esters), nitrate esters, water, soot, and glycol can be quantified.
•
Spectrometric analysis/Emission spectroscopy/Rotrode filter spectroscopy (RFS) – Elemental analysis is performed in accordance with atomic emission spectroscopy (AES). A specific volume of lubricant is energized using an electrical arc. The light frequencies and intensities are measured and reported in parts per million of various elements. Elemental analysis is useful for identifying contamination, confirming additive content, and indicating system wear. The following elements are analyzed: Fe, Cr, Al, Pb, Sn, Cu, Ag, Ni, Na, V, Cd, Ti, Mo, Ca, Ba, P, Zn, B, K, Mg, and Si.
•
Additive package condition – Additives present in a lubricant improve and strengthen the performance characteristics of the lubricant. Chemically active additives are able to interact with metals and form a protective film with the metallic components present in the machinery. The designer of the additive package must ensure that the additives will not produce unacceptable side effects. If an additive is present in excessive levels or interacts in an unsatisfactory manner with other additives that are present, it can be detrimental to the equipment. Over a period of time additive packages can deplete, leaving machinery unprotected, and vulnerable to failure. The additives in a lubricant can also be referred to as the performance package. Some of the more commonly used additives include: –
Anti-foam agents – Almost every lubricant foams to some extent due to the agitation and aeration that occurs during operation. Air entrainment due to the agitation encourages foam formation. The presence of some detergent and dispersant additives tends to promote foam formation. Foaming increases oxidation and reduces the flow of oil to the bearings. In addition, foaming may cause abnormal loss of oil through orifices. Anti-foam agents are used to reduce the foaming tendencies of the lubricant. Foam inhibitors may be added to a lubricant in service if a foaming problem is detected. The lubricant and equipment manufactures should be consulted before adding foam inhibitors. The foaming characteristics of lubrication oils are tested per the ASTM D892 standard. The test makes a determination of the foaming characteristics of lubricating oils at a specific temperature. The test results monitor the foaming tendency and stability of the foam.
–
Anti-wear and EP additives – Both anti-wear and extreme-pressure (EP) additives form a protective layer on metal parts by decomposition and absorption. Anti-wear additives function in moderate environments of temperature and pressure while EP additives are
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effective in the more extreme environments.
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Molybdenum disulfide and graphite additives are a special form of anti-wear additives known as anti-seize agents. They form a protective layer on the metal parts by deposition of the graphite or molybdenum disulfide. Anti-seize agents work independent of temperature and pressure. Typical applications include engine oils, transmission fluids, power steering fluids, and tractor hydraulic fluids. EP additives are common in gear oils, metalworking fluids, and some hydraulic fluids. – Dispersants – The purpose of this additive is to suspend or disperse harmful products within the lubricant. Thus, the additive neutralizes the effect of these products. Harmful products include contaminates such as dirt, water, fuel, and process material, and lube degradation products such as sludge, varnish, and oxidation products. Typical applications include diesel and gasoline engine oils, transmission fluids, power steering fluids, and in some cases gear oils. – Detergents – Detergents, like dispersants, are blended into lubricants to remove and neutralize harmful products. In addition, detergents form a protective layer on the metal surfaces to prevent deposition of sludge and varnish. In engines, this can reduce the amount of acidic materials produced. A detergent’s protective ability is measured by the total base number or the reserve alkalinity. The metallic basis for detergents includes barium, calcium, magnesium and sodium. Typical applications for detergent additives are primarily diesel and gasoline engines. – Friction modifiers – Friction modifiers are lubricant additives blended with the base stock to enhance the oil’s natural ability to modify or reduce friction. Friction modifiers reduce wear, scoring, and noise. Typical applications include gasoline engine oils, automatic transmission oils, power steering fluids, metalworking fluids, and tractor hydraulic fluids. – Anti-oxidants – Anti-oxidants, also known as oxidation inhibitors, interfere with the oxidation process by chemically converting oxidation products to benign products. In addition, some oxidation inhibitors interact with the free catalytic metals (primarily copper and iron) to remove them from the oxidation process. Almost all modern lubricants contain anti-oxidation additives in varying degrees. Lubricants for extreme operating conditions such as diesel and gasoline engines, for hightemperature situations, and for applications that involve high lubricant agitation require higher levels of anti-oxidants than other lubricants. – Pour point depressants – The pour point is the lowest temperature that a lubricant will flow. In order to obtain flow of oil at low temperature (fluidity), pour depressants are added to the lubricating oil to lower the pour point. These additives tend to inhibit the formation of wax at the low temperatures. In many formulations, especially those containing viscosity improvers, supplemental pour depressants are not needed since other
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additives also have pour point depressant properties.
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Typical applications include diesel and gasoline engine oils, transmission fluids, tractor fluids, hydraulic fluids, and circulation fluids. – Rust and corrosion inhibitors – Rust and corrosion are the result of the attack on the metal surfaces by oxygen and acidic products, and are accelerated by the presence of water and impurities. Rust and corrosion inhibitors work by neutralizing acids and forming protective films. These inhibitors must work in the lubricant and on surfaces above the liquid level. The rust preventing characteristics are tested per the ASTM D665 standard. The test evaluates the ability of inhibited mineral oils to aid in preventing the rusting of ferrous parts should water become mixed with the lubricant. Typical applications include engine oils, gear oils, metalworking fluids, and greases. – Viscosity index improvers – Mineral lubricants tend to lose their lubricating ability at high temperatures due to viscosity reduction. Viscosity improvers are added to a lubricant to retain satisfactory lubricating capabilities at the higher temperatures. At low temperatures the viscosity characteristics of the base stock prevail while at high temperatures the viscosity improver maintains the viscosity at satisfactory levels. In addition to these additives, there are numerous other ones such as dyes to mark lubricant types, seal-swell agents to counteract the adverse effect of other additives on seals, and biocides to retard or prevent bacterial growth. Additive packages are proprietary information, and lubricant manufacturers do not offer detailed information on the additives present in their products. There are, however, several laboratory tests available to determine additive depletion or loss in a lubricant. It is important to monitor your additive package through laboratory tests. When your additive package depletes, your lubricant’s performance decreases, and your equipment is left unprotected.
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Common elements found in lube oil additives are shown in Table 18-2. Table 18-2 Elements in Oil Additive Package [16]
•
Common Elements
Additive Function
Barium
Detergent or dispersant
Boron
Extreme-pressure
Calcium
Detergent or dispersant
Copper
Anti-wear additive
Lead
Anti-wear additive
Magnesium
Detergent or dispersant
Molybdenum
Friction modifier
Phosphorus
Corrosion inhibitor, anti-wear
Silicon
Anti-foaming
Sodium
Detergent or dispersant
Zinc
Anti-wear or anti-oxidant
Viscosity testing (ASTM D445) – Viscosity is one of the most important characteristics of an oil because it ensures that the proper film strength is present to minimize metal-to-metal contact and machine wear. Viscosity is a factor in the formation of lubricating films under both thick and thin film conditions. It affects heat generation in bearings, cylinders, and gears. It governs the sealing effect of the oil and the rate of consumption or loss. It determines the ease with which machines may be started in cold conditions. For any piece of equipment, the first essential for satisfactory results is to use oil of proper viscosity to meet the operating conditions. If the viscosity is too low, the oil may not have the necessary film strength required to maintain a proper oil film. An inadequate oil film results in excessive wear. A decrease in viscosity may indicate contamination with a solvent or fuel or with lower grade viscosity oil. If the viscosity is too high, additional fluid friction is generated. This increases the operating temperature of the bearings and increases the rate of oxidation. A change in viscosity over time can indicate oxidation, shearing, the presence of contamination, and additive depletion. However, in most cases, an out-of-specification viscosity value indicates the use of an incorrect oil or the addition of an incorrect oil during refilling of the reservoirs. Viscosity testing is performed to characterize a fluid’s flow/resistance to flow at a given temperature. Almost all industrial lubricating oils are specified by the International Standards
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Predictive Maintenance
Organization (ISO) viscosity grade system. The system specifies standard viscosities at 40°C
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from 2 Centistokes (cSt) to 460 cSt. The most common viscosity grades for bearing applications are 32, 46, 68,100,150, and 220 cSt. To meet the specifications of the ISO viscosity grade system, oils must be within ± 10% of the viscosity grade from the lube oil suppliers. From the ASTM D445/446 standard, kinematic viscosity is measured by adding a small portion of sample oil to a calibrated capillary tube viscometer in a temperature-controlled bath. The time it takes the fluid to flow between two fixed points in the viscometer is measured and then compared to the standard. From this, viscosity is calculated and reported in centistokes. •
Total Acid Number (TAN) (ASTM D664 and D974) – Acidity indicates the extent of oxidation of a lubricant and its ability to neutralize acids from exterior sources such as combustion gases. The acidity of lubricants is measured by the amount of potassium hydroxide required for neutralization (mg KOH/g), and the resultant number is called the TAN. The additives in most new oils contribute a certain TAN or acidity. Therefore, it is critical to determine and monitor changes from the new oil reference. An increase in TAN may indicate lube oxidation or contamination with an acidic product. A severely degraded lubricant indicated by a high TAN may be very corrosive.
•
Total Base Number (TBN) (ASTM D4739, D664, D974, and D2896) – The TBN is determined by titration of a known substance, such as HCl, in order to determine an unknown quantity. Weighed samples are titrated using an automatic titration system. TBN of a used lubricant is a measurement of its ability to neutralize the acid using basic buffers.
•
Crackle test/Karl Fischer water test (ASTM D-4928 and D1744) – Water in a lubricant not only promotes corrosion and oxidation, but also may form an emulsion having the appearance of a soft sludge. In many bearing applications, even a small amount of water can be detrimental, especially in journal bearing applications where the oil film thickness is critical. Some of the major causes of water in the oil include seal leaks, heat exchanger leaks, and condensation. The sources of these leaks must be identified if the reoccurrence of this problem sis to be prevented. The purpose of the crackle test is to monitor the lubricant for water contamination. Since the presence of water can cause accelerated oxidation, corrosion, and excessive wear, it is essential that the oils are monitored for water. In the crackle test, a drop of oil from an eyedropper is placed on a hot plate, heated to 100° C, and monitored for the characteristic crackle that occurs as water explodes into steam. This test is a simple, go-no go test that indicates either a positive or negative for the presence of water. If the drop of oil crackles, it indicates that at least 0.1% water or greater is present. The lab will report this as a “positive” test. Typically, a Karl Fisher test is then performed to quantify the amount of water. The Karl Fisher test is a quantitative measure of moisture in oil, reported in parts per million, or as a percentage. Per the ASTM D1744 standard, a fixed amount of water reactive reagent is added to a mixture of sample and solvent to achieve a pre-selected electric response. The instrument calculates the amount of water present based on the amount of reagent required.
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•
Smells – EPRI has been working with Cyrano Sciences to develop a library of composite smells of lube oils, both for new oil fingerprinting and additive concentration and also for used oil degradation without ever having to sample the equipment. The technology consists of individual thin-film carbon-black polymer composite detectors configured into an array. The collective output of the array is used to identify an unknown vapor using standard data analysis techniques. The sensor array, along with data analysis algorithms, forms the main components of the electronic nose. The output from the device is an array of resistance values as measured between each of the two electrical leads for each of the detectors in the array. When the detector is exposed to vapors, the polymer matrix acts like a sponge and swells while absorbing the vapors. Moreover, for well-defined applications, the polymers used in the detector array can be chosen to maximize chemical differences between target compounds to increase the discrimination power of a smaller array. This underscores the power of Cyrano Sciences’ polymer composite sensor technology because it is not reliant on any particular polymer type or limited to a particular set of polymers. Additionally, the simplicity of reading resistance values and the low cost of materials of the detectors makes this an ideal technology for a low-cost, hand-held electronic nose. By establishing a library of the composite smells of oxidized oils in different degrees of oxidation, a library can be established that will allow for a quick check of a sample using the electronic nose to determine what state of degradation the oil is in. Because of the desire to concentrate the vapors in an available headspace, the vented areas of storage drums and operating equipment reservoirs become the ideal location to perform in situ analysis of the condition of the lubricants. Without ever sampling the oil from equipment or drums, an evaluation of the vapors present in the headspace can provide important information about the condition of the lubricating oil present.
• Flash Point (ASTM D92) – Flash point indicates the presence of highly volatile and flammable materials in a relatively nonvolatile or nonflammable material. An example is an abnormally low flash point on a test specimen of engine oil that can indicate fuel contamination. The lubricant sample temperature is raised at a constant rate as the flash point is approached. At specified intervals, a small test flame is passed across the cup containing the sample. The lowest temperature that the application of the test flame causes the vapors above the surface of the liquid to ignite is determined as the flash point. • Oxidation Stability Test (ASTM D2272) – This was formerly called the rotating bomb oxidation test. The test is used to assess the remaining oxidation test life of in-service lubricants. The test lubricant, water, and a copper catalyst coil contained in a covered glass container are placed in a pressure vessel equipped with a pressure gage. The vessel is charged with oxygen to a pressure of 620 kPa, placed in a constant-temperature oil bath set at 150°C, and rotated axially at 100 rpm at an angle of 30° from the horizontal. The number of minutes required to reach a specific drop in gage pressure is the oxidation stability of the test sample. • Demulsibility (ASTM D1401-96) – The test provides a guide for determining the water separation characteristics of oils subject to water contamination and turbulence. A 40-ml sample and 40-ml of distilled water are stirred for 5 minutes at 54°C in a graduated cylinder.
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The time required for the separation of the emulsion thus formed or volumes of water, oil, and emulsion remaining after 30 minutes is recorded. •
Pour Point (ASTM D97) – The pour point is the determination of the lowest temperature that a petroleum product may be used if fluidity is necessary to the application. After preliminary heating, the petroleum sample is cooled at a specified rate and examined at intervals of 3ºC for flow characteristics. The lowest temperature that movement of the specimen is observed is recorded as the pour point.
•
Foam Test (ASTM D892-95) Sequence I, II, III – The test determines the foaming characteristics of lubricating oils at specified temperatures. The foam test is a means of empirically rating the foaming tendency and the stability of the foam. A defined volume of air is forced through a set volume of sample lubricant at a specified temperature. The resulting volume of foam is measured.
•
Cone Penetration of Lubricating Grease (ASTM D 217) – This test measures the consistency of grease. Harder grease will have a low National Lubricating Grease Institute (NLGI) rating number such as 00 or 1. Most industrial greases penetrate in the 265–295 ranges and have a NLGI rating of 2. A measured amount of grease sample is placed under a cone apparatus. The cone is attached to a gauge that measures from 85–475 gradations on the apparatus. The cone is dropped into the grease sample from a specified height and for a specified time. The measured amount that the cone penetrates into the grease is the cone penetration.
•
Dropping Point of Lubricating Greases (ASTM D566) – This test is a determination of the maximum operating temperature of grease. A grease sample is heated in the dropping point apparatus. The point at which the grease starts separating and the oil drops out of the apparatus is the dropping point. The temperature is measured in degrees Celsius.
• Percent Sediment in Lubricating Oils – This test is an excellent determination of sediments suspended in lubricating oil. Excessive amounts of sediments can impede oil capability and can clog filters.
18.3 Condition-Based Maintenance – Kennedy Van Saun (Courtesy of Kendal Power Station) The following pertains to the condition-based maintenance program on the Kennedy Van Saun ball/tube mills at the Kendal Power Station. Vibration readings are taken monthly on all mill components. Historical data are captured for monitoring the critical components like the girth gear, the pinion gear, and the main gearbox. Oil samples from the motor oil tank and trunnion bearing oil tank are tested for viscosity, water content, total acid number, spectrometric analysis, and wear particle concentration. Table 18-3 lists the components that are included in the lubrication program.
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Table 18-3 Kennedy Van Saun Mill Components for Lubrication Program Equipment Trunnion bearings
Component for Lubrication Lube oil tank motor bearings Lube oil tank
Mill main gearbox
Lube oil tank Labyrinth seals Lube oil tank motor bearings
High-speed coupling Low-speed coupling Mill pinion bearings Mill drive motor bearings
Lube tank Lube oil tank motor bearings
Barring gear unit
Motor Perigrip brake (pivots) Gearbox Coupling (Hand wheel)
Girth gear
Spray Lubricator on control air system
Seal air fans
Motor bearings High speed coupling Fan bearings
Mill isolation valve (non-drive-end and drive-end)
Bushings for air cylinder Pin on fulcrum Glands on valve Lubricator on control air
Classifier reject isolation valve (nondrive-end and drive-end)
Bushings for air cylinder Pin on fulcrum Glands on valve Lubricator on control air
Reject flap gate
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Predictive Maintenance Table 18-3 (continued) Kennedy Van Saun Mill Components for Lubrication Program Equipment Control damper
Component for Lubrication Actuator Links/fulcrums
Drive-end reject shutoff gates control air Ball charge hopper control air isolation valve
Glands (2) Pin Lubricator on control air
Hot primary air control damper
Gearbox worm Actuator
Cold air isolator
Gear motor Gearbox Metro-flex isolator
Stock Volumetric Feeders: Belt drive conveyor
Motor bearings Eddy current coupling Gearbox worm
Cleanout conveyor
Motor bearings Gearbox worm Belt head pulley (DE [2], NDE [1]) Chain head pulley (DE [2], NDE [1]) Head support rollers (2) Tension pulley Booster pulley Impact rollers (13) Belt take-up pulley Belt take-up Chain take-up
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Table 18-3 (continued) Kennedy Van Saun Mill Components for Lubrication Program Equipment Feeder inlet gate – Bunker outlet
Component for Lubrication Gearbox handwheel and chain Shaft end cap Drive-end bearing housing Valve body (1 roller per side) Position indicator
Feeder outlet gate – Air/coal valve
Valve Valve actuator Shaft end cap Drive-end bearing housing Limitorque main gear case Zerk fitting in housing cover Geared limit switch Motor bearings
18.4 Condition Based Maintenance – Stein Industrie (Courtesy of Matimba and Majuba Power Stations) Matimba Power Station has placed many of the ball/tube mill components on a condition-based maintenance program. For example, the trunnion seal air fan motors are monitored and repaired based on their condition. Vibration readings are taken once a month for the mill drive train (main motor and gearbox), the girth gear, and the pinion. Vibration readings are taken every two months for the hot seal air fan and the cold seal air fan. The following equipment is monitored for temperature readings monthly: •
Girth gear and pinion tooth flank
•
Pinion bearing
• Lubrication system motors for the Turbolub, Hytec, and main gearbox and girth grease systems •
Hot and cold seal air fan motors
•
Feeder belt drive gearbox and motors
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Predictive Maintenance
Main motor winding temperatures (every two months).
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Oil samples are taken every six months for the main gearbox, motors and annually for the auxiliary gearbox. The following equipment is rotated or replaced based on the condition of the equipment: •
Main gearbox
•
Screw conveyor – 40,000 operating hours life
•
Girth gear – 90,000 operating hours life
•
Mill liners – rotations at 35,000 operating hours and replacements at 65,000 operating hours
•
Melroth gasket
The following discussion pertains to the condition-based monitoring program for the Majuba Power Station. A history of all wearing or eroding parts should be established to monitor the rate of wear or erosion. After the history has been established, any drastic change must be investigated. The system is running in an acceptable state if no faults are found during condition monitoring checks by the operational or maintenance personnel and all temperatures and pressure are correct. A potential failure mode is an operating condition that may lead to failure if allowed to continue. The potential failure modes for equipment are listed in all relevant tables in this section. Table 18-4 shows the system parameters that are monitored. Table 18-4 Condition-Based Monitoring Values at Majuba Power Station System Raw-coal feeder
Parameter Chain speed
Value 0.34 rpm, 0.741 m/minute minimum 3.47 rpm, 7,562 m/minute maximum
Drive motor speed
1,445 rpm
Reduction gearbox – input
1,445 rpm
Reduction gearbox –output
17.89 rpm
Reduction gearbox – reduction ratio Inverter speed reduction ratio
80.76:1 10:1
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Predictive Maintenance Table 18-4 (continued) Condition-Based Monitoring Values at Majuba Power Station System Main bearing lubrication system
Parameter Low-pressure system – cooling oil flow
40°C
Low-pressure system – oil differential pressure
100 kPa
High-pressure system – pump supply pressure
150 kPa
High-pressure system – pump outlet pressure
7 MPa
High-pressure system – thrust pump outlet pressure
4 MPa 1,750 mm (1,800 liters)
Mill speed
16.2 rpm
Mill motor speed
991 rpm
Main reduction gearbox – input
991 rpm
Main reduction gearbox – output
Main reduction gearbox lubrication system
7 liters/minute
Low-pressure system – cooling oil temperature
Oil tank level Drive train
Value
123.8 rpm
Main reduction gearbox – reduction ratio
8:1
Number of teeth on drive pinion gear
30
Number of teeth on girth gear
234
Oil quantity Oil temperature
270 liters 45°C
Oil pressure
300 kPa
Oil flow rate
80 liters/minute
Oil differential pressure
100 kPa
Girth gear lubrication system
Pump delivery
40:1
Grease temperature
Ambient temperature
Main motor lubrication system
Flooding oil pressure
1.5 MPa
Jacking oil pressure
20 MPa
Oil flow rate per bearing
4.3 liters/minute
Oil differential pressure
100 kPa
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Table 18-4 (continued) Condition-Based Monitoring Values at Majuba Power Station System Mill air system
Parameter
Value
Primary air collector – air temperature
245°C
Primary air collector – air pressure
10 kPa
Direct hot air box – air temperature
245°C
Direct hot air box – air pressure
10 kPa
Direct hot air box – airflow
28 kg/second
Bypass piping – air temperature
245°C
Bypass piping – air pressure
10 kPa
Bypass piping – airflow
28 kg/second, 32 kg/second maximum
Purge air piping – air temperature
245°C
Purge air piping – air pressure
2.63 kPa
Purge air piping – airflow
Seal air subsystem
8.2 kg/second
Air inlet/outlet differential pressure
3 kPa
Fan suction pressure
9 kPa
Fan discharge pressure
14 kPa 8,000 m 3
Fan airflow
1,083 kg/m 3
Fan air density Seal air pressure at air boxes
11.2 kPa
Seal air to mill air differential pressure
1.5 kPa
First fan differential pressure
1.15 kPa
Second fan differential pressure
1.2 kPa
Condition monitoring covers the following: •
Continuous remote operational monitoring using alarm sensors
•
Scheduled condition monitoring
The conditions listed in Table 18-4 are monitored by the operating staff in the control room. These indications are checked locally by operational staff on the local alarm panel or directly on the local sensors at the equipment. All the alarm-based checks are listed in Table 18-5.
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Predictive Maintenance Table 18-5 Monitored Conditions at Majuba Power Station Operating Performance Parameter
Acceptance Criteria
Potential Failure Modes
Trunnion bearing drive-end/non-drive-end temperature
<60 °C
>60 °C
Main motor bearing temperature
<85 °C
>85 °C
>1.0 kg/s
<1.0 kg/s
>75 °C
<75 °C
<90 °C
>90 °C
>2.5 kPa
<2.5 kPa
<4.5 kPa
>4.5 kPa
>5 kPa
<5 kPa
>4.5 kPa
<4.5 kPa
>100%
<100%
<160 kPa
>160 kPa
Low
>75 °C
<75 °C
High
<100 °C
>100 °C
Mill primary air/pulverized fuel duct purge airflow Drive-end/non-drive-end outlet primary air temperature – Low High Burner pulverized fuel duct supply pressure Low High Seal air/primary air differential pressure Mill non-drive-end/drive-end inlet primary air pressure Mill quick-closing damper position Duplex filter differential pressure Burner pulverized fuel duct supply temperature
A list of condition monitoring instruments supplied by Stein Industrie and a description of their particular function and location are given in Table 18-6. The instrumentation covers the raw-coal feeder, the mill air system, the seal air system, and the mill main motor.
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Table 18-6 Condition Monitoring Instrumentation for Majuba Power Station Instrumentation Description
Working Point at the Maximum Continuous Rating
Raw-Coal Feeder Speed transmitter monitoring speed of raw-coal feeder Current transmitter monitoring current drawn by feeder motor
3.3 m/minute (96.53%) Load dependent
Limit switch monitoring coal level
200 mm
Inductive pick-up monitoring feeder rotation stall monitor
3.3 mm
Limit switch monitoring single gate slide damper position
100%
Limit monitor monitoring speed - control frequency inverter Current transducer monitoring main drive motor kW indication
3.3 m/minute 1.508–1.660 kW
Limit switch (barring) monitoring electrohydraulic brake position
Disengaged
Limit switch (inching) monitoring electrohydraulic brake position
Engaged
Centrifugal switch monitoring auxiliary gearbox speed
50 r/minute
Thermocouple monitoring trunnion drive-end/non-drive-end bearing
55°C
Thermocouple monitoring main drive motor bearing
70°C
Thermocouple monitoring main drive motor winding
100°C
Thermocouple (fire detection) monitoring tube mill classifier temperature
75°C
Flow switch monitoring trunnion bearing NDE oil flow - connected to flow transducer (SM1 supplied)
20–25 liters/minute
Flow switch monitoring trunnion bearing DE oil flow - connected to flow transducer (SM1 supplied)
20–25 liters/minute
Flow switch monitoring main reduction gearbox oil flow Low -level switch monitoring trunnion bearing oil reservoir level
90 liters/minute 1.450 liters
Low-level switch monitoring main reduction gearbox oil level
245 liters
Local sight glass monitoring trunnion bearing oil reservoir level
750 mm
Differential pressure switch monitoring trunnion bearing lowpressure differential pressure
100 kPa
Pressure switch monitoring trunnion bearing high-pressure pump inlet pressure
100 kPa
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Pressure switch monitoring trunnion bearing non-drive-end lubrication pressure
Predictive Maintenance 2–4 MPa
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Instrumentation Description
Working Point at the Maximum Continuous Rating
Raw-Coal Feeder Pressure switch monitoring trunnion bearing drive-end lubrication pressure
2–4 MPa
Differential pressure switch monitoring reduction gearbox filter differential pressure
100 kPa
Pressure switch monitoring trunnion bearing thrust face lubrication pressure
1–4 MPa
Local pressure gauge displaying trunnion bearing cooling low pressure oil pressure
250 kPa
Local pressure gauge displaying trunnion bearing inlet pressure
150 kPa
Local pressure gauge displaying trunnion bearing non-drive-end lubrication pressure
3 MPa
Local pressure gauge displaying trunnion bearing drive-end lubrication pressure
3 MPa
Local pressure gauge displaying tube mill reduction gearbox lubrication pressure
100 kPa
Local pressure gauge displaying trunnion bearing thrust face lubrication pressure
1.5 MPa
Temperature sensor controlling trunnion bearing lubrication
40°C
Temperature switch controlling trunnion bearing oil reservoir heater
40°C
Temperature switch controlling trunnion bearing oil tank temperature interlock
40°C
Temperature switch controlling trunnion bearing oil tank temperature
40°C
Temperature switch monitoring trunnion bearing high pressure
40°C
Temperature switch monitoring reduction gearbox oil temperature
60°C
Local temperature gauge displaying trunnion bearing oil reservoir temperature
40°C
Local temperature gauge displaying reduction gearbox inlet temperature
60°C
Level switch monitoring mill main motor lube oil tank level
80 liters
Sight glass displaying mill main motor lube oil tank level
80 liters
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1826
Predictive Maintenance Table 18-6 (continued) Table 18-6 (continued) Condition Monitoring Instrumentation at Majuba Power Station Condition Monitoring Instrumentation at Majuba Power Station Instrumentation Description
Working Point at the Maximum Continuous Rating
Pressure switch monitoring mill main motor jacking oil pressure
10 MPa
Pressure switch monitoring mill main motor jacking oil pressure
10 MPa
Pressure switch monitoring mill main motor jacking oil pressure
10 MPa
Pressure switch monitoring mill main motor flooding oil pressure
600 kPa
Pressure switch monitoring mill main motor flooding oil pressure
600 kPa
Pressure switch monitoring mill main motor flooding oil pressure
600 kPa
Differential pressure switch monitoring mill main motor filter differential pressure
20 kPa
Local pressure gauge displaying mill main motor lube oil pressure
20 MPa
Local pressure gauge displaying mill main motor lube oil filter differential pressure
20 kPa
Local pressure gauge displaying mill main motor flooding oil pressure
600 kPa
Temperature switch monitoring mill main motor lube oil temperature -
50°C
Temperature switch monitoring mill main motor lube oil temperature
50°C
Temperature switch monitoring mill main motor lube oil temperature
50°C
Local temperature gauge displaying mill main motor lube oil temperature
50°C
Mill Air System Primary air collector pressure measurement - connected to differential pressure transducer
10 kPa
Resistance thermal device, monitoring primary air collector temperature measurement
245°C
Venturi measuring purge air piping flow
2.93 kg/second
Venturi measuring bypass piping non-drive-end flow
8.8 kg/second
Venturi measuring direct piping drive-end flow
19 kg/second
Venturi measuring direct piping non-drive-end flow
19 kg/second
Venturi measuring direct piping drive-end flow
19 kg/second
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18-27
Predictive Maintenance
Instrumentation Description
Working Point at the Maximum Continuous Rating
Direct piping non-drive-end pressure measurement
8 kPa
Direct piping drive-end pressure measurement
8 kPa
Resistance thermal device measuring direct piping non-drive-end temperature
245°C
Resistance thermal device measuring direct piping drive-end temperature
245°C
Tapping point for drive-end hot air box differential pressure
10 kPa
Tapping point for non-drive-end hot air box differential pressure
10 kPa
Seal Air System Tapping point for seal air differential pressure
15.4 kPa
Connection to pressure transmitter showing differential pressure between drive-end hot air box primary air and main seal air piping
10 kPa
Connection to pressure switch showing differential pressure between non-drive-end hot air box primary air and main seal air piping
10 kPa
Mill Main Motor Type-K thermocouple monitoring drive-end bearing temperature
±75°C
Type-K thermocouple monitoring non-drive-end bearing temperature
±75°C
Resistance temperature device monitoring U-phase winding temperature
±140°C
Resistance temperature device monitoring W-phase winding temperature
±140°C
Resistance temperature device monitoring V-phase winding temperature
±140 °C
Resistance temperature device monitoring U-phase winding temperature - connected to SM1 instrumentation
±140 °C
Resistance temperature device monitoring W-phase winding temperature
140°C
Resistance temperature device monitoring V-phase winding temperature
140°C
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1828
Predictive Maintenance Table 18-6 (continued) Condition Monitoring Instrumentation at Majuba Power Station
All scheduled (time-related) condition-based maintenance tasks are listed in Table 18-7.
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18-29
Predictive Maintenance Table 18-7 Scheduled Condition Monitoring at Majuba Power Station Equipment
Condition Monitored
Acceptance Criteria
Potential Failure Modes
Scheduled Monitoring Interval
<10°C
>10°C
No vibration
Vibration
<10°C
>10°C
No Vibration
Vibration
<10°C
>10°C
No Vibration
Vibration
<10°C
>10°C
No Vibration
Vibration
Check for vibration.
<2.8 mm/s rms
Check temperature increase.
<10°C
>2.8 mm/s rms
Check gland for leaks.
No leaks
Leaks
Check for damaged cables.
No damage
Damage
Secure
Loose
No leaks
Leaks
Check for vibration.
No vibration
Vibration
Check temperature increase.
<10°C
>10°C
Seal Air
Check airflow.
Slight increase
Marked increase
one month
Raw-coal feeder bearings
Check chain tension adjuster gap.
>30 mm
<30 mm
200 hours
<1 mm
>1 mm
Conveyor bearings
Check temperature increase.
one month
Check for vibration. Drive pinion bearing drive-end/non-drive-end
Check temperature increase.
one month
Check for vibration. Main reduction gearbox bearing
Check temperature increase.
one month
Check for vibration. Auxiliary reduction gearbox bearing
Check temperature increase.
one month
Check for vibration. Mill main motor bearings
Mill and seal air system dampers
Check for loose mounting.
one month
<10°C one month
Check for air duct leaks. Seal air fan bearings
one month
Check clamping plate distance between either side.
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1830
Predictive Maintenance Table 18-7 (continued) Scheduled Condition Monitoring at Majuba Power Station Equipment
Main Seal Air Fan
Condition Monitored
Acceptance Criteria
Potential Failure Modes
Check for leaks:
No leaks
Leaks
Outlet seal, inlet seal, and shaft seal.
No cracks or corrosion
Cracks corrosion
Check fan for cracks or corrosion.
No cracks, secure
Not secure
Check bearing plummer block bases for cracks and security.
No vibration
Vibration >4 mm/ second
<10°C
>10°C
No vibration
Vibration
Scheduled Monitoring Interval three months
Check fan alignment. Raw-coal feeder bearings
Check temperature increase.
800 hours
Check for vibration. Electrohydraulic Brake
Check brake shoe adjustment.
>1–3 mm
<1–3 mm
100 operations
Mill main motor
Check desiccator crystals for motor dampness.
Blue
Pink
one year
Secure
Loose
Check fixing bolts, couplings, and guards.
Tight
Loose
No leaks
Leaks
Check electrical connections. Quick-close damper
Check gland for leaks.
one year
At Tutuka Power Station, the manufacturer recommended oil change intervals for the mill main lubrication system, the main reduction gearbox, the inching gearbox, and the mill main motor lubrication system of 4000 operating hours. This means that each mill would require a full lubricant service twice a year. The station decided to use condition-based monitoring of the oil to determine the interval of lubricating oil changes. The following practices were adopted: •
Lubricant sampling and analysis was conducted between 1500 and 1750 operating hours.
• Modifications were made to the existing shaft sealing arrangement to eliminate points of contaminant ingress.
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18-31
Predictive Maintenance
•
Additional lubricant sampling and analysis was conducted based on vibration analysis and other plant monitoring parameters.
•
Blockage of lube oil filters was investigated quickly.
•
Lubricants with moisture contamination were replaced.
Because of these lubrication system practices, the current average interval for oil changes is 39,450 operating hours. The change from the manufacturer-recommended interval of 4,000 operating hours to 39,450 operating hours has realized cost savings to the plant. There have been no forced outages from lubricant failures, but the plant has been unavailable because of severe lubricant contamination. Tutuka Power Station has a well-equipped, on-site oil laboratory that is used primarily to test lubricants for basic parameters such as viscosity, moisture content, and acid number on a routine basis. The on-site laboratory is used for first-line investigations and has a very fast turnaround time for testing. An independent, off-site oil analysis laboratory was contracted to provide the in-depth oil testing and analysis. The off-site laboratory was selected based on the offerings of testing available, the repeatability of test results, the reliability of analysis, and rapid turnaround times. The following lubricant tests are performed by an off-site testing laboratory: • Viscosity at 40°C in centistokes (cSt) • Total acid number in mg KOH/g • Water content in parts per million (ppm) • Solid Contamination Code from ISO (International Standards Organization) 4406 – Hydraulic Fluid Power – Fluids –Methods for Coding the Level of Contamination by Solid Particles • Elemental analysis by atomic emission spectroscopy • Additive package condition • Fourier transform infrared analysis • Wear debris analysis
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1832
19 PREVENTIVE MAINTENANCE
This section includes the PM information for the ball/tube mills manufactured by AllisChalmers, Kennedy Van Saun, Riley Power Inc., and Stein Industrie. Human Performance Key Point There are three areas of work that are critical to performing maintenance on the mills. The areas are temporary lighting, lifting and rigging practices, and temporary scaffolding. It should be noted that temporary lighting should be installed when working in and around the mill. Generally, the environment is dark, and the black color of the coal makes the environment even darker. When lifting components, it is very important to know the weight of the components so that the appropriate lifting devices can be used. In some of the tables in this section, the weight of the components is given. Using the correct device to lift the components prevents personnel injury. In a number of areas around the mill, it is necessary to access manholes, dampers, or piping. Temporary scaffolding should be inspected prior to use.
19.1 Allis-Chalmers Information in this section was provided by Eskom’s Lethabo Power Station. There are several tables that list the inspection tasks for the mill components. The following tables cover: •
External inspection
•
Internal inspection
•
Classifier inspection
•
Drive train inspection
•
Shell and trunnion liner replacement
•
Trunnion bearing insert replacement
•
Girth gear replacement
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19-1
•
Gearbox rebuild
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19-2
Preventive Maintenance
•
Miscellaneous equipment
•
Equipment lubrication list
Before the tables are presented, the general layout of the Lethabo Power Station is given. Entry to the interior of the ball mill is through an access door in one of the two inlet/outlet boxes. Externally, access to the milling plant equipment can be achieved using low scaffolding. The primary air piping and ducting above the ball/tube mill is supported by constant load supports enabling disconnect from the inlet/outlet boxes without additional support being provided. The inlet/outlet boxes require support platforms when withdrawing them from the mill trunnions. This support is also required when relocating the inlet/outlet boxes to align the mill sealing arrangement. A special jacking cradle is provided to facilitate limited upward movement of each ball/tube mill for maintenance purposes such as bearing changes. A portable hydraulic system consisting of a power pack with four lifting cylinders is used under the lifting cradle to raise the mill evenly. The system is designed to extend and retract the lifting cylinders at the same time, but individual cylinder control is not provided. The system is protected by an overload trip switch in the electrical circuit and by pressure relief valves in the hydraulic circuit. Each cylinder is fitted with a safety valve that locks the cylinder in the event of a power or hydraulic hose failure during operation. 19.1.1 Inspection Criteria The following inspections should be performed every 3000 operating hours: •
Seal air fan system – impeller, bearings, breather, coupling, springs, foundation bolts, trunnion air seal, and trunnion vent valve
• Ball charge hopper • Trunnion lube oil system – sight glass, filters, strainer, flexible hoses, oil cooler, sample points, and oil pump • Gearbox lube oil system – oil pumps, filters, oil cooler, flexible pipe, bypass regulating valve, and drain valve • Girth gear lube system – lubricators, air supply line water trap, air regulator, flexible hoses, spray bar distribution block and pump, nozzles distribution block and overhaul pump, flexible pipe, air filter, and seal air fan •
Dampers – secondary air, primary air, isolation, hot air, tempering air, bypass, rating, and core air
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19-3
Preventive Maintenance
• Feeders – screens, tension pulley bearings and bushings, main conveyor drive pulley, main drive coupling, conveyor main drive gearbox, gearbox oil level, inspection windows, main conveyor belt and joint, conveyor tensioning unit, suspended support roller, support idlers and bearings, impact idlers and bearings, rubber scraper and bushings, feeder casing, door seals, inspection cover seals, and feeder profile bar height • Cleanout conveyor system – drive sprocket, heat and tail sprocket, chain and links, first and second reduction gearbox, drive adaptor coupling, and tensioning unit • Drive train alignment – mill motor to gearbox, gearbox to pinion gear, barring gear to motor, drive motor magnetic center, pinion bearing clearances, trunnion bearing insert wear, reduction gearbox internal condition • Mill motor heat exchanger – remove, clean, and replace • Classifiers – vanes and inverted cone • Motors – mill drive, gearbox lube oil pump, mill girth gear seal air fan, mill barring gear, trunnion pump, feeder conveyor drive, secondary air damper, bypass damper actuator , primary air isolating damper, seal air fan damper actuator, primary air rating damper, hot air damper, tempering air damper, seal air fan, and core air fan In addition, the following inspections should occur annually: • Pinion gear – Remove, clean, and reinstall. • Girth gear – Verify axial and radial run-outs, root clearance, backlash, gear contact, and alignment. 19.1.2 External Mill Inspection Table 19-1 lists the inspection tasks for an external mill inspection.
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19-2
Preventive Maintenance
Table 19-1 External Mill Inspection Tasks (Courtesy of Lethabo Power Station) Equipment
Tasks
Ball charge hopper
Check around hopper flanges and access doors for air leaks. Check the status of the ball charge and confirm the storage of replenishment balls. Ensure operability of the operating extending spindle for the lower charging door. Check the 200-mm ball-charge piping for damage.
Classifier
Check around the flanges and top cover for leakage of coal. Check the spring hanger supports for damage. Check the outer cone for deterioration. Check the coarse rejects pipe for leakage and deterioration.
Primary air ductwork
Check the expansion joints and flanges for leakage and deterioration. Check the fixed and spring hanger supports for damage. Check the ductwork for deterioration.
Inlet/Outlet boxes
Check the mill sealing arrangement for leakage. Check the inlet/outlet boxes and ductwork for deterioration. Note: Discolored areas of paint may indicate internal hot spots or fires. It is recommended any discolored areas be repainted. Check around the flanges and access doors for leakage. Check the service supply connections to the inlet/outlet boxes for leakage.
Rotating mill
Check the acoustic housing for damage and deterioration. Check the acoustic housing for external noise level. Check the trunnion bearings for overheating. Check that the power and sound transducers are in normal operating limits. Check the trunnion piston ring sealing system and add grease as needed. Check that all drive coupling guards are secure.
Trunnion bearing lubrication system
Check the oil levels in each trunnion sump and add oil as needed. Check the lubricating systems for leakage. Check the low-pressure (680 kPa) readings on each lubrication system. Check the cooling water supply flow rates (0.46 liters/second). Check the oil temperatures on each system (30–60°C).
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19-4
Preventive Maintenance Table 19-1 (continued) External Mill Inspection Tasks (Courtesy of Lethabo Power Station) Equipment
Tasks
Trunnion bearing lubrication system (continued)
Check the oil filter color indicators with coding as follows: Green - Full filtration, bypass closed. Green/Red - Change element, full filtration but bypass valve is starting to open. Red - Element change overdue, partial filtration, bypass valve is opening. Check low press pumps for unusual noise and heat.
Gearbox unit and lubrication system
Check the gearbox unit for unusual noise and vibration. Check the gearbox unit acoustic housing for damage and deterioration. Check the acoustic housing for external noise level. Check the oil level and add oil as needed. Check the oil temperature (48–54°C) and pressure (140 kPa) Check the system for leakage. Check the input and output coupling guards for security. Check the cooling water supply flow rates (0.9 liters/second).
Girth gear lubrication system
Check the lubricant level in the supply drum. Check the air pump, panel, and piping for leakage. Check that the air supply pressure is above the minimum required (400 kPa). Cleanout the residue from the drip tray under the gearing. Confirm that all spray lances are providing lubricant over the girth gear.
Girth gear seal air system
Check the fan bearings for unusual heat and noise. Check that both fan inlet filters have no obstructions. Check the fan discharge pressure (0.5 kPa). Check the flexible air supply piping for leakage. Check around the girth gear guard for air leaks.
Motor and barring gear
Check that the motor cooling air inlet and outlet have no obstruction. Check the motor air temperatures (<110°C). Check the oil level in the motor bearings. Check that the bearing oil temperatures are in the normal range (<80°C). Check the oil level in the barring gear unit. Check that the barring coupling engagement handwheel is not obstructed and is correctly in the engaged or disengaged position
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19-5
Preventive Maintenance
19.1.3 Internal Mill Inspection Table 19-2 lists the internal inspection tasks for the Allis-Chalmers mill. Table 19-2 Internal Inspection Tasks (Courtesy of Lethabo Power Station) Equipment
Tasks
Disassembly
Wear appropriate safety equipment at all times. Observe safety precautions. Use gas monitor at all times. Grease drums to be removed to prevent contamination. Clean acoustic hood, chutes, and pinions. Install access ladders at all mill doors.
Miscellaneous during disassembly
Repair or replace trunnion division plates. Renew reject flaps and wear sleeves where identified. Repair mill bypass dampers and shafts. Set open and close stops on actuator. Replace feedtubes where needed. Measure grinding media levels.
Shell liners
Measure the thickness of the shell liners using an ultrasonic thickness tester. The measurements should be taken in the trough of the liner contour and on the crest. Measure the thickness of the end inner and outer liners. Examine the trunnion throat liners for wear. Record the measurements to establish a rate of wear. The high-chrome iron liners are 25-mm thick initially and should be replaced when worn to 18-mm thickness. The Roq-last liners can be used until completely worn through. When these liner plates are between 2- to 3-mm thick, then replacement should be considered.
Shell liner bolts
Check the tightness of all liner bolts. Tighten the liner bolts to 750 N-m. When using new liners, the liner bolts must be retightened within the first eight hours of full load operation. A second retightening must be performed within the first week of full load operation.
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19-6
Preventive Maintenance
Table 19-2 (continued) Internal Inspection Tasks (Courtesy of Lethabo Power Station) Equipment
Tasks
Trunnion bearing insert
Remove the small access cover on the top of the trunnion bearing cap at each end of the mill. Measure the distance between the trunnion journal and the outer face of the access cover hole using a steel rule. Compare this measurement with previous measurements. W hen the measurement has increased by 2 mm, the trunnion seal should be checked. When the measurement has increased by 6 mm, the bearing insert should be replaced.
General
Repair inlet and outlet boxes for ceramic tile installation, if needed. Remove and replace bottom coal gates. Repair reject pipe bends. Inspect and replace tiles in the mill. Inspect and repair tiles for the reject wear sleeves. Retile the mill outlet boxes if needed.
Outside the Drum
Check all coupling bolts for tightness. Check all hold down bolts for security. Check all piping brackets for security. Check all concrete foundation platforms and surrounding areas for cracking and deterioration.
Assembly
Remove all tools and debris from mill. Replace grease drums. Replace girth gear grease system. Close mill doors. Remove access ladders from mill doors. Clean outer mill area from tools and debris.
19.1.4 Classifier Inspection Table 19-3 lists the tasks for inspecting the classifier.
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19-7
Preventive Maintenance
Table 19-3 Classifier Inspection Tasks (Courtesy of Lethabo Power Station) Tasks 1. Open inspection plug on classifier roof. 2. Remove side inspection covers. 3. Inspect inner cone for blockage. 4. Inspect inverted cone. 5. Inspect vanes for blockage. 6. Record findings. 7. Close inspection plugs and covers. 8. Open classifier by removing the pipe bend on top of the classifier. Move bend sideways to allow access and anchor in position. 9. Inspect vanes and shafts for wear. 10. Inspect all lined surfaces for wear. Measure the thickness of the liners using an ultrasonic thickness tester. Record the thickness readings to establish a rate of wear. 11. Inspect classifier roof for wear. 12. Inspect unlined surfaces for wear and deterioration. Small areas can be strengthened by welding plate patches externally. 13. Inspect inverted cone for position and wear. 14. Inspect thermocouple pockets for wear. 15. Mark repairs and record findings. 16. Perform ultrasonic thickness test on inner cone. Record findings. 17. Repair classifier inner cones using the window repair method. Open the inspection covers on the outer cone before welding. Repairs to be welded from inside of cone and ground flat on the outside. 18. Inspect tiles on classifier inner cones. 19. Inner cones to have tiles replaced from the outside of the inner cone. Open inspection covers on outer cone to ensure draft for welding. Rough grind area to be repaired before applying tiles to ensure bonding. 20. Replace inspection covers. 21. Perform final inspection of repairs. 22. Inspect sealing surfaces and clean. 23. Inspect gasket and lightly grease. 24. Remove scaffold and tools from inner cone. 25. Ensure no tools or debris are dropped in inner cone.
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19-8
Preventive Maintenance 26. Close pipe bend.
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19-9
Preventive Maintenance
19.1.5 Drive Train Inspection Table 19-4 lists the tasks for inspecting the mill drive train. Table 19-4 Mill Drive Train Inspection Tasks (Courtesy of Lethabo Power Station) Tasks 1. Clean and inspect mill pinion gear for destructive pit marks. 2. Remove, clean, and inspect grid couplings. 3. Open, clean, and inspect pinion bearings. 4. Re-pack pinion bearings with grease and close bearings. 5. Replace all oil filters. 6. Replace all breathers. 7. Open gearbox cover, disconnect coupling, and inspect. 8. Check and correct the drive train alignment. 9. Clean and inspect bearing engagement nut. 10. Inspect gearbox oil level. 11. Inspect gear brake drum linings. 12. Inspect gear motor coupling. 13. Drain drive motor bearing oil and inspect bearings. 14. Clean motor bearing oil cavity and level sight glass. Replace sight glass when stained or damaged. 15. Lubricate and replace grid couplings.
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19-10
Preventive Maintenance
19.1.6 Shell and Trunnion Liner Table 19-5 shows the tasks for replacing the shell and trunnion end liners. Table 19-5 Shell and Trunnion Liner Replacement Tasks (Courtesy of Lethabo Power Station) Tasks 1. Remove the ball charge from the mill using a vacuum device or remove manually. 2. Remove the mill acoustic housing. 3. Remove the inlet/outlet boxes. 4. Position and support a runway beam through the center of the mill shell end and through the ends to give clearance for lowering the liners onto a trolley. 5. Install portable air extraction and supply equipment inside the mill. 6. Install portable lighting inside the mill. 7. Install a communication system between the inside and outside of the mill. 8. Arrange for the barring gear to be available. Note: When turning the mill, the trunnion bearing jacking oil system or manual jacking pumps must be used. The mill gearbox lubrication system should be in operation. Note: The liner bolts, nuts, and seal washers are not to be reused but replaced with new items. The seal washer retainers can be reused. As the old bolts are removed, they should be put in bags for disposal. 9. Starting at a point well below the level of the portable runway beam, remove the shell liner bolts. 10. As each liner is released, use rigging attachments and raise the liner up and out of the shell along the portable runway beam. 11. Remove all liners that are accessible using the lifting arrangement. 12. Remove the securing bolts from the trunnion end liners and filler ring that can be reached and move the bolts out of the mill. 13. When all the liners that can be reached are removed from the shell, raise and remove all tools and equipment from inside the bottom of the shell. 14. Using the barring gear, the manual jacking pumps, and the gearbox lubrication system, rotate the mill to the next working position. 15. Repeat the procedure to remove the shell liners, trunnion liners, and securing bolts. 16. Clean the inside of the shell and inspect for corrosion and erosion. Note: New liners should be interchangeable within the type of liner. The weights of the new liners are: Trunnion end liner - inner = 174 kg, trunnion end liner outer = 184 kg, filler ring = 70 kg, 2-hole shell liner = 114 kg, 1-hole shell liner = 57 kg. 17. Fit new filler ring segments, new filler ring bolts, nuts, seal washers, and original seal washer retainers to the lower working area of the trunnion ends.
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1911
Preventive Maintenance
Table 19-5 (continued) Shell and Trunnion Liner Replacement Tasks (Courtesy of Lethabo Power Station) Tasks Note: The liner bolts should not be finally tightened until all the gaps between the liners have been equalized. 18. Fit new trunnion end outer liners using new attachment pieces. Check each liner against the trunnion inner face for rocking. If more than 2.5 mm of rocking is present, the liner should be adjusted to fit. 19. Fit new trunnion end inner liners using new attachment pieces. Check each liner against the trunnion inner face for rocking. If more than 2.5 mm of rocking is present, the liner should be adjusted to fit. 20. Fit new shell liners and attachment pieces. The shell liners consist of the single-hole and double-hole types that allow the offset gaps. Note: Shell liner rock should be checked. Only the lip around the liner edges should be in contact with the shell. 21. Rotate the mill with the barring gear until all the liners are installed. 22. Check and equalize the gaps between the liners. 23. Tighten the liner bolts to 750 N-m using a torque wrench. 24. Remove all tools from the inside of the mill shell. 25. Remove the portable runway beam, air extraction equipment, portable lighting, and communication system. 26. Replace the inlet/outlet boxes. 27. Replace the acoustic housing. 28. Return the mill to service.
19.1.7 Trunnion Bearing Insert Replacement Table 19-6 lists the tasks for replacing a trunnion bearing insert.
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19-12
Preventive Maintenance Table 19-6 Trunnion Bearing Insert Replacement Tasks (Courtesy of Lethabo Power Station) Tasks 1. Attach a flexible hose to the drain valve on the trunnion bearing base sump. 2. Open the drain valve and drain the oil into an appropriate container. 3. Shut the drain valve and remove the flexible hose. 4. Disconnect the oil supply pipe from the bearing cp. 5. Plug the oil supply piping to prevent ingress from dirt. 6. Disconnect the grease supply pipes to the upper halves of the two sealing rings. 7. Remove the nuts, washers, and bolts securing the bearing cap. Then remove the tapered dowel pins. 8. Fit eye bolts in the two rapped holes in the bearing cap. 9. Attach lifting slings to the eye bolts and remove the bearing cap using the hoists attached to the mill runway beams. 10. Remove and discard the gasket. 11. Disconnect the cooling water supply and return hoses from the bearing insert. 12. Disconnect the high-pressure jacking oil supply hose from the bearing insert. Plug the system side of the pipe to prevent the ingress of dirt. Release the pipe clamps. 13. Disconnect the resistance temperature detector from the bearing insert. 14. Remove the retainer plates, deflector plate, and felt from the bearing insert. 15. Fit the eye bolts in the ends of the bearing insert. Attach slings to the eyebolts and lift the bearing insert using hoists attached to the mill runway beams. Note: It is important that cleanliness standards be maintained during assembly to prevent the ingress of foreign material into the trunnion bearing lubrication system. 16. Fit eye bolts in the tapped holes in the sides and ends of the new bearing insert. 17. Lightly oil the trunnion surface. 18. Attach slings to the eyebolts on the side of the bearing insert. 19. Lift the bearing insert and lower the bearing onto the trunnion surface. 20. Rotate the insert around the trunnion surface until the insert is in the correct position in the bearing housing. Remove all eyebolts. 21. Using the hydraulic jacks, lower the mill carefully into the bearings. 22. Check the oil wedge clearance by inserting a 0.5-mm thick feeler gauge. The sum of the depths of penetration of the feeler gauge on both sides of the bearing should be an average of 200 mm.
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1913
Preventive Maintenance 23. Fit the retainer plates, deflector plate, felt, and clamps. Adjust the felt so that it is in contact with the trunnion.
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19-14
Preventive Maintenance Table 19-6 (continued) Trunnion Bearing Insert Replacement Tasks (Courtesy of Lethabo Power Station) Tasks 24. The retainer plate on the deflector plate side of the bearing should be adjusted with shims if necessary to ensure that it contacts the bearing housing. The retainer plate on the other side of the bearing should be adjusted with shims to give a clearance between 1.5–7.8 mm. After the retainer plate is adjusted, tighten the retainer plate securing bolts to 2,650 N-m. 25. Re-connect the resistance temperature detector to the bearing insert. 26. Remove the plug from the system side of the jacking oil supply hose and re-connect the pipe to the bearing insert. Fit the pipe clips and tighten to 22 N-m. 27. Install a new gasket on the bearing housing. Lift the bearing cap and lower the cap into position on the bearing housing. 28. Fit the taper dowel pins and secure the bearing cap with the bolts, washers, and nuts. Tighten the bolts to 185 N-m. 29. Reconnect the grease pipe to the upper halves of the sealing rings and fill the lubricating system with grease. The initial grease charge is 3.5-kg followed by 0.1-kg six times hourly for 20 hours. After 20 hours, add 0.5-kg every second week or when leakage is detected. Note: The gap between the sealing rings and bearing cap should be 0.1–2.2 mm maximum. 30. Remove the plug from the system side of the lubricating oil pipe and reconnect the pipe to the bearing cap. 31. Record the bearing clearances. 32. Carry out an inspection of the drive pinion and gear to check alignment. 33. Refit the input/output box. 34. Open one of the hand-hold covers and replenish the oil level in the bearing housing. Use a funnel or a hand pump to transfer oil from the container to the bearing. 35. Close the hand-hold cover.
19.1.8 Girth Gear Replacement Table 19-7 lists the tasks to replace the girth gear.
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1915
Preventive Maintenance
Table 19-7 Girth Gear Replacement Tasks (Courtesy of Lethabo Power Station) Tasks Disassembly 1. Remove sections of the acoustic housing from the drive-end of the mill for access to the girth gear. 2. Remove the four top half sections of the girth gear guard. 3. Remove the ball charge from the mill using a vacuum device or remove ball charge manually. 4. Rotate the mill using the barring gear until the split line of the girth gear halves is horizontal. Note: When turning the mill, the trunnion bearing jacking oil system or manual jacking pumps must be used. The mill gearbox lubrication system should be in operation. 5. Remove the mill drive pinion assembly. 6. Using a system of pulleys, slings, and winches, secure the mill to prevent rotation. 7. Remove the split pins, locknuts, and nuts from the girth gear split joint alignment bolts. 8. Remove the parallel stud bolts and drive out the fitted taper stud bolts. 9. Drive out the split taper bushings from the taper stud bolt holes. Ensure that match markings on the stud bolts, taper bushings, and corresponding holes are clearly visible. 10. Attach slings to the girth gear top half and support the weight (8000 kg) of the gear with the aid of hoists attached to the runway beam. 11. Remove the locknuts, nuts, and bolts securing the girth gear half to the mill head. 12. Lift the girth gear half clear of the mill. 13. Rotate the mill 180° so that the remaining girth gear half is on the top position of the mill. 14. Secure the mill to stop any rotation. 15. Attach slings to the girth gear top half and support the weight (8000 kg) of the gear with the aid of hoists attached to the runway beam. 16. Remove the locknuts, nuts, and bolts securing the girth gear half to the mill head. 17. Lift the girth gear half clear of the mill. New Girth Gear Installation 18. Inspect the new girth gear halves and remove any burrs, grease, paint, and protective coatings from the machined surfaces. 19. Ensure that all centering bolts for adjusting radial alignment have been fitted before mounting the gear halves. 20. Inspect the mill head flange and remove all burrs, grease, and paint from the gear mating surface.
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19-16
Preventive Maintenance 21. Check that all mill shell flange bolts are correctly tightened.
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1917
Preventive Maintenance
Table 19-7 (continued) Girth Gear Replacement Tasks (Courtesy of Lethabo Power Station) Tasks 22. Check all connecting bolts for damage and compatibility with nuts. 23. Check tapered bolts and bushings for damage and compatibility with nuts. 24. Attach slings to the girth gear half and lift the gear half into position. Install the gear half with every third bolt connected to the mill head flange. Ensure that the true face marking on the girth gear is facing toward the mill drive. 25. Rotate the mill so that the gear half is on the bottom half of the mill. 26. Check the clearance bolt assembly by inserting a clearance bolt from the top of the split into one of the clearance holes located nearest the outside diameter of the gear half. If the bolt passes completely through the hole, proceed with mounting the other gear half. Note: If the bolt does not pass completely through the hole because of interference at the inside rim diameter of the gear, all clearance bolts must be inserted into this half before assembly. 27. Attach slings to the remaining girth gear half and raise the gear half into position. Ensure that the true face marking is on the correct side. 28. Draw the girth gear half against the mounting flange with three bolts. Do not fully tighten bolts until after alignment of the gear halves. 29. Insert the stud bolts and draw together the ring gear halves. Do not fully tighten the bolts until the correct match marks of the stud bolts and holes are verified. 30. Coat the internal and external surfaces of the split tapered bushing with Coppercoat and insert the tapered bushing into the hole. Place the flange of the tapered bushing at the top of the flange seated against the counterbore. Ensure the correct match marking of the tapered bushing and holes. 31. Insert the tapered alignment bolt into the tapered bushing with the small end down. Ensure the correct match marking of the tapered bolts and holes. 32. Place a washer and nut on the small end thread and tighten with a torque spanner wrench to 3,343 N-m. 33. Place a washer and nut on the large end thread of the tapered bolt and tighten with a torque spanner to 2522 N-m. Fit the large end locknut and tighten. 34. Check the torque at the small end of the tapered bolt, fit the locknuts, and tighten. 35. Repeat steps 30–-34 of these tasks for the remainder of the alignment bolts. Tighten the parallel alignment bolts to 5865 N-m. 36. Check the joint true face alignment with a straight edge and feeler gauges. The true face alignment should be within 0.076 mm. 37. Tighten all stud bolts and adjust to give equal lengths on either side of the joint.
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19-18
Preventive Maintenance Table 19-7 (continued) Girth Gear Replacement Tasks (Courtesy of Lethabo Power Station) Tasks 38. Heat shrink the clearance bolts into place. Place nuts on the bottom of the clearance bolts and adjust the bolts so that equal amounts of bolt length extend out both sides. After adjusted, remove the nut from the bottom, making sure that the nut can be easily removed. Slip the bolt out of the hole so that the unthreaded portion is completely exposed. Do not damage the threads. Heat the unthreaded portion uniformly to a temperature of 234°C above ambient temperature. Check the temperature every three or four minutes with a surface pyrometer or Tempilstik. Make sure the bolts are heated evenly and that the torch is not held too close to the bolts. Do not heat above the specified temperature. After heating, replace the bolt quickly and put the nut on and tighten by sledging. Lock nuts should then be put on and tightened. Note: Do not fit split pins until the final alignment is complete. Note: The gear is now ready to be aligned for axial wobble and radial runout. Note: Joint tighteners should prevent an entry of 0.0381-mm feeler gauge. 39. Reinstall the mill drive pinion assembly. Note: Pinion shaft seals should be set to fit as snug as possible. Girth Gear Alignment Note: Suitable gear alignment data sheets should be developed based on the original installation records. 40. Insert all flange bolts and finger-tighten the bolts. 41. Adjust the gear centering screws to position the girth gear on the shell flange to obtain a uniform gap between the gear flange inside diameter and the shell flange outside diameter. 42. Tighten every third flange bolt snug. 43. Mark the girth gear in 12 equal positions starting at the split line of the gear and number 1 to 12. Note: Box axial wobble and radial runout test should be made at the same time. Four dial indicators are required. The gauge used for radial runout should be fitted with a large mushroom button. 44. For radial runout, place the indicator mushroom squarely against the top land of the gear teeth and set the indicator to zero. 45. Rotate the gear so that each of the numbered stations is brought under the indicator. Note: If the indicator does not return within 0.05 mm of zero after one revolution, the readings must be repeated as the dial may have moved or the gear is not properly located. 46. Record the readings of the indicator at each of the 12 locations and determine the amount and location of the largest positive runout. 47. Rotate the mill so that the point of largest positive runout is in the 12 o’clock position. 48. Back off the centering screws on the top half of the gear to the amount of correction required.
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1919
Preventive Maintenance 49. Back off the flange bolts and allow the gear to settle back on the centering screws under its own weight.
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19-20
Preventive Maintenance Table 19-7 (continued) Girth Gear Replacement Tasks (Courtesy of Lethabo Power Station) Tasks 50. Tighten all flange bolts to secure the gear in position. 51. Tighten all centering screws into contact with the gear. 52. Continue taking runout readings and make corrections until the maximum runout is 0.608-mm is obtained. Record the final runouts. 53. For axial wobble, place two indicators against the trunnion journal face and one against the gear face. 54. Rotate the mill and take a reading at 12 positions. Note: The axial wobble is the algebraic difference between the readings. If the axial wobble is greater than 0.608 mm, then the mill manufacturer should be consulted. 55. Tighten all the gear securing bolts to the correct torque of 5,865 N-m. Check the full circumference with a feeler gauge ensuring full contact between the gear face and shell flange face. 56. Back off the centering screws and secure with Locktite. 57. Check the gear teeth contact marking and backlash by rotating the gear to a position that the point of maximum runout is in contact with the pinion. 58. Ensure that the gear can not rotate. 59. Tighten the pinion against the gear in the direction of normal rotation. 60. Check the contact and backlash clearance on both sides of the pinion and gear with feeler gauges. The backlash is 2.79-mm maximum and 2.54-mm minimum measured at 20°C ambient temperature. Note: Contact and clearance should be made at four points that are 90° apart. If the gear and pinion axis are parallel, the contact and backlash will be the same on either end of the tooth. If not, shimming of the pinion may be necessary and should not affect the drive alignment. Assembly 61. Replace the girth gear guard by joining the segments of the guard from the markings. 62. Seal all guard flange joints with RTV sealant. 63. Screw all the air seal adjusting screws down, then back off five full turns for the initial setting. 64. Replace the acoustic housing. 65. Replace the ball charge. 66. Check the gear teeth contact marking with the full ball charge in the mill and record the readings. If the contact is not correct, then the gear may require shimming. Note: If any shimming of the pinion was performed, the alignment of the pinion to the gearbox and the gearbox to the motor coupling must be checked and realigned.
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1921
Preventive Maintenance
Table 19-7 (continued) Girth Gear Replacement Tasks (Courtesy of Lethabo Power Station) Tasks Mill Startup 67. During startup of the mill, the following must be monitored for the first 48 hours of operation: •
Final alignment of the girth gear to pinion by using a strobe light
•
Vertical and horizontal vibration readings on the drive train
•
Temperatures across the pinion and girth gear measured at three places. If a temperature difference of >10°C between any one of the measured values occurs, the mill should be shut down and the alignment checked.
•
Girth gear guard air seal checked for clearance at the normal operating conditions
19.1.9 Gearbox Rebuild Table 19-8 lists the tasks for rebuilding the mill gearbox unit.
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19-22
Preventive Maintenance Table 19-8 Gearbox Rebuild Tasks (Courtesy of Lethabo Power Station) Tasks Note: The component weights are: Acoustic hood = 494 kg, complete main gear unit = 13,000 kg, main gear unit casing = 4400 kg, output coupling = 1420 kg, input coupling = 443 kg, input shaft assembly without the coupling = 415 kg, output shaft assembly without the coupling = 5080 kg. Disassembly 1. Remove the coupling halves from the input shaft and the output shaft. 2. Remove the dowels, nuts, and bolts from the casing. 3. Remove the studs, nuts, and locknuts from the bearing housings. 4. Remove the setscrews and screws from the top half of the input shaft end cover and oil catcher. 5. Attach the lifting devices to the four lifting lugs on the top half casing and with a straight lift, raise the top half casing carefully and remove. The approximate weight of the top half casing is 2000 kg. Disassemble Pinion Shaft 6. Remove the setscrews from the bottom half of the input shaft end cover and oil catcher. Remove the end cover. 7. Disconnect the flanged joint and remove the oil spray nozzle piping. 8. Protect the shaft and attach lifting devices around the shaft on both sides of the double helical pinion. Raise the assembly carefully. The weight of the pinion shaft is 415 kg. 9. Remove the labyrinth cover from the oil catcher. Remove the oil seals and slide off the oil catcher. 10. Remove the keeper plate from the non-drive-end of the shaft and the locknut from the drive-end. Disassemble Output Shaft Assembly 11. Remove the screws from the bottom half of the output shaft end cover and oil catcher. Remove the end cover from the assembly. 12. Protect the shaft and attach the lifting gear around the shaft on either side of the wheel. Raise the assembly carefully. 13. Remove the labyrinth cover from the oil catcher. Remove the oil seals and slide off the oil catcher. 14. Remove the keeper plate from the non-drive-end of the shaft. Remove the lock plate, setscrew, and washer from the locknut. Remove the locknut. 15. Remove both bearings from the shaft. 16. Remove the distance piece.
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1923
Preventive Maintenance 17. If required, remove the complete wheel center and wheel rim from the output shaft using a hydraulic press.
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19-24
Preventive Maintenance
Table 19-8 (continued) Gearbox Rebuild Tasks (Courtesy of Lethabo Power Station) Tasks Clean and Inspect 18. Clean all removed components for inspection. Clean the gear case. 19. Examine the gears for cracked or broken teeth. If there is damage on the gears, then both gears should be replaced. 20. Examine the bearings for damaged rollers or races. Replace bearings if needed. 21. Examine the internal lubrication piping, restrictors, and nozzles for damage or blockage. 22. Examine the shafts for cracking, damage, and wear. If wear is found on the shaft bearing surfaces from inner race creep, the seating scan be machined slightly undersize, plated with chromium, and reground to the correct size. Assemble Output Shaft 23. If it was necessary to remove the complete wheel center and wheel rim from the shaft, then perform the following tasks: •
Grease the mating surfaces of the wheel and the shaft.
•
Fit the key to the shaft.
•
Ensure the helix is correct and press the complete wheel center and wheel rim onto the shaft until it makes solid contact with the shaft should using a 60-metric ton hydraulic press.
24. Fit the distance piece ensuring the piece makes contact with the final wheel. 25. Prepare the bearings by heating in an oil bath not to exceed 121°C. 26. Fit the bearings to the shaft ensuring a solid fit against the shaft end distance piece and shaft shoulder. 27. Fit the keeper plate onto the short end of the shaft and secure with setscrews and spring washers. Lock the setscrews with wire. 28. Fit the bearing locknut and secure with the lock plate, setscrew, and washer. 29. Attach the lifting devices to the shaft on both sides of the wheel and lift the assembly. The weight of the shaft assembly is 5080 kg. 30. Lower the assembly into the bottom half gear casing. Set the bearings carefully into the bearing housings to avoid damage. 31. Position the output shaft to maintain 925 mm from the shaft end to the center line of the gear unit. 32. Fit the end cover and oil catcher. Ensure the oil slots coincide with those in the gear casing. 33. Check that the bearing axial end float is between 0.81–1.13 mm. 34. Fit the new oil seals and spacer. Pack the cavity with grease. Fit the labyrinth cover.
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1925
Preventive Maintenance
Table 19-8 (continued) Gearbox Rebuild Tasks (Courtesy of Lethabo Power Station) Tasks Assemble Input Shaft 35. Prepare the bearings by heating them in an oil bath not to exceed a temperature of 121°C. Ensure that the oil is clean. 36. Fit the bearings to the shaft ensuring they are in close contact with against the shaft shoulder. 37. Fit the keeper plate to the short end of the shaft with setscrews and spring washers. Lock the setscrews with wire. 38. Fit the bearing locknut and lock washer. 39. Protect the shaft and attach the lifting devices around the shaft on both sides of the pinion. The shaft assembly weight is 415 kg. Lift the assembly. 40. Lower the assembly into the bottom half gear casing. Set the bearings carefully into the bearing housings to avoid damage. 41. Position the input shaft to maintain 825 mm from the shaft end to the center line of the gear unit. 42. Apply blue dye to the pinion teeth and roll the gears under light load to ensure the input shaft is centered. Check that the teeth make contact over 80% of the meshing line. 43. Fit the end cover and oil catcher. Ensure the oil slots coincide with those in the gear casing. 44. Check that the bearing axial end float is between 0.498–0.675 mm. 45. Fit the oil seals and spacer. Pack the cavity with grease. Fit the labyrinth cover. Assemble the Top Half Casing 46. Ensure that the top and bottom half casing mating surfaces are clean. 47. Apply a thin layer of jointing compound to the top and bottom half casing mating surfaces. Fit new brown paper packing. (0.15-mm thick). 48. Fit the oil spray nozzle piping and re-connect at the flanged joint. Secure bolts with steel wire. 49. Attach the lifting devices to the four lifting lugs on the top half casing. The casing weight is approximately 2000 kg. Position the top half casing over the bottom half casing and lower the top half casing carefully into position. 50. Insert the dowels to center the top half casing. 51. Fit the studs, nuts, and locknuts to the bearing housings. 52. Fit the nuts and bolts to the casing joint. 53. Fit the screws to the top half of the output shaft end cover. 54. Fit the screws to the top half of the input shaft end cover. 55. Fit the coupling halves to the input shaft and output shaft.
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19-26
Preventive Maintenance
19.1.10
Miscellaneous Equipment
The sealing air fan system uses a single fan during operation, leaving the second as a backup source. This arrangement provides an opportunity to alternate the usage of the fans and allows for a scheduled maintenance program. The inlet filter boxes must be cleaned periodically, with filters being replaced or cleaned as operation conditions dictate. All dampers must be lubricated and operated to order to ensure that the shafts are free to rotate in the bearing assemblies. Linkages and controls must be free in the entire range of movement. The dampers in the air system are integral with the boiler control system. All damper valves must be checked for leaks and operated periodically for ease in shutting and to determine if replacement parts are needed. All supports, internal and external, should be inspected periodically to ensure their integrity. Any evidence of corrosion or erosion should be recorded and repaired. 19.1.11
Equipment Lubrication List
Table 19-9 lists the equipment that requires lubrication.
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1927
Preventive Maintenance
Table 19-9 Equipment Lubrication List (Courtesy of Lethabo Power Station) Equipment
Component
Girth gear seal air fan
Intermediate bearings
Mill drive motor
Bearings
Trunnion
Bearings Low-pressure pump motor High-pressure pump motor Piston ring seal
Seal air fan
Bearing box Drive motor bearing Coupling
Feeder
Belt take-up pulley Belt take-up screws Cleanout conveyor take screw Inlet span rollers Tension roller shaft Tension roller pivot Support roller (suspended) Belt drive shaft Cleanout conveyor shaft Belt paddle switch shaft Belt drive motor counter shaft Cleanout conveyor motor booster roller Idle rollers
Reducer gearbox
Belt drive-MVF 150 P Cleanout conveyor
Inlet valves
Operating shaft Gate rollers Actuator adaptors Position indicators
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19-28
Preventive Maintenance Table 19-9 (continued) Equipment Lubrication List (Courtesy of Lethabo Power Station) Equipment Outlet valves
Component Operating shaft Gate rollers Actuator adaptors Position indicators
Primary and FD fans
Lube oil Pump – Motor bearing Jacking oil pump – Motor bearing Lubricating oil tank Motor bearing Vane control actuators (Siemens Linkage) Gearbox
PF isolating dampers (Babcock)
Isolating dampers gearbox
Bypass dampers Siemens actuator
Linkage ball joints
Automatic mill ball loading machine
Conveyor grease points
Milling plant gear unit
David Brown gear unit
Air line mist spray
Input Coupling Output coupling Mill gear unit labyrinth Barring gear unit
First and second reduction cases Output gearing
Girth gear and pinion shaft
Girth gear spray grease pump Girth gear spray air service unit Pinion shaft bearing
19.2 Kennedy Van Saun This section (Courtesy of Kendal Power Station) includes tables for the inspection of the pinion,
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1929
Preventive Maintenance
girth gear, and lubrication systems and for reversing the worm gear.
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19-30
Preventive Maintenance
19.2.1 Pinion, Girth Gear, and Lubrication System Table 19-10 lists the tasks for inspecting the pinion, girth gear, and the lubrication systems. Table 19-10 Inspection Tasks for Pinion, Girth Gear, and Lubrication Systems (Courtesy of Kendal Power Station) Tasks 1. Ask operations about oil leaks and overheating conditions during operation. Check the condition of the pinion gear seals and lantern rings. 2. Inspect the clearances between the stuffing box and pinion shaft (19 mm). 3. Check the stuffing box alignment measured at 4 places. 4. Check the mounting bolts torque for the pinion mounting bolts (2700 N-m) and the pinion bearing pedestal bolts (4200 N-m) 5. Inspect the pinion bearing. Remove the old grease and replace with new grease. 6. Check the backlash using lead wire readings. The contact and backlash average should be >1.964 mm and <7.856 mm. 7. Check the contact pattern using Prussian blue. Parallel and center patterns covering 80% tooth load surface should be recorded. 8. Check the girth gear lubrication system for train 1 and 2. 9. Clean the drip trays and surrounding area. 10. Service the girth gear spray pumps to deliver 15 MPa. 11. The girth gear spray pumps air supply lubricators should be filled with oil. Girth Gear Distribution Blocks and Nozzles 12. Unblock the strainers, distribution blocks, nozzles, and piping. 13. Ensure there is a 2-mm gap between the moving plunger and the proximity switch. 14. Simulate the lube spray distribution and ensure the spray is uniform across the tooth flank. 15. Verify the girth gear sealing arrangement is intact with no air leaks.
19.2.2 Reversing the Worm Gear Table 19-11 lists the tasks for reversing or flipping the gearbox worm gear.
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1931
Preventive Maintenance
Table 19-11 Tasks for Reversing the Gearbox Worm Gear (Courtesy of Kendal Power Station) Tasks 1. Remove gearbox coupling guards, housings, and coupling springs on both high- and low-speed sides. 2. Remove mounting bolts and pipework. 3. Secure flange holes. 4. Lift out gearbox from mill using approved crane and rigging hardware. 5. Transport the gearbox from the mill area into a workshop area. 6. Remove both couplings. 7. Remove all bolts and nuts and lift the top casing half. 8. Remove the two M20 dowels from the casing joint. 9. Remove the three top half bolts on the input line end cover and oil catcher. 10. Remove the oil spray lance at the flanged joint. 11. Lift out the pinion shaft by using suitable slings on both sides of the bearings. 12. Remove the labyrinth cover from the oil catcher and slide off the oil seals. 13. Remove the keeper plate at the non-drive-end side from the shaft and the locknut on the driveend side. 14. Pull off both bearings, inspect the bearings, and replace if necessary. 15. Using suitable slings on both sides of the bearings, lift the output shaft assembly straight up. 16. Clean the casing halves and ensure the casing is dry. 17. Inspect the pinion and output shaft gears for high wear, pitting, damage and so on. 18. Record findings by using a camera. 19. Turn the gears 180° and reinstall the complete assembly into the bottom casing. 20. Adjust input bearing bench endplay to 0.26–0.42 mm. 21. Adjust input bearing mounting endplay to 0.05–0.21 mm. 22. Adjust output bearing bench endplay to 0.50–0.75 mm. 23. Adjust output bearing mounted endplay to 0.13–0.39 mm. 24. Reassembly of gearbox is the reverse of the disassembly procedure. 25. Install gasket onto center joint and refit the top half of the gearbox. 26. Fit nuts and bolts on center joint and tighten to correct torque specifications. 27. Refit mill reduction gearbox into mill drive train. 28. Align gearbox.
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19-32
Preventive Maintenance
19.3 Riley Power Inc. The PM information in this section [7] covers the PM inspection criteria and tasks associated with the following components/issues: •
Feeder
•
Crusher-dryer
•
Inlet/outlet boxes and air seals
•
Mill liners
•
Ball charge
•
Speed reducer gearbox
•
Drive train
•
Driveshaft
•
Classifier
•
Lubrication heat exchanger
•
Primary air fan and ductwork
•
Coal shutoff valves
•
Lubrication schedule
•
Spare parts
•
Example PM inspections
19.3.1 Feeder The PM tasks for the feeder include the following: •
Grease bearings monthly. This includes the drum shaft bearing, the wiper shaft bearing, the release apron shaft bearing, the shear pin sprocket assembly, and the conveyor pulley shaft.
•
Apply a light oil (SAE #40) weekly to the roller chain drive.
•
Flush and refill the transmission gear reducer every six months with AGMA #8 compound.
•
Check wiper blades for wear once every six months.
•
Check the release apron gap (1/16 in.), the adjustable apron gap (1/32 in. ±1/64 in.), and the wiper blade gap (between 1/8–1/4 in.) every six months.
• Inspect all wearing parts once a year for excessive wear. • Inspect counter linkage monthly to prevent loosening of parts.
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1933
Preventive Maintenance
There should be a 1/16-in. clearance between the tip of the leveling apron and the drum. This clearance can be obtained by adjusting the external support under the leveling apron arm. A minimum clearance is necessary for the adjustable apron plate. Loosen the holding screws, move the apron liner forward until it touches the drum rib, retract it slightly for a tolerance of 1/32 in. ±1/64 in., and tighten the holding screws. Recheck by rotating the drum one full revolution. The wiper blades should have 1/4-in. maximum and 1/8-in. minimum clearance at the inlet and outlet points for moving through the drum pockets to ensure maximum cleaning. After adjusting the wiper blades, revolve the drum by hand to make sure the wiper blades pass through all the pockets without interference. A maximum 10% pocket loss from teeth tip to pocket bottom is the allowable limit. 19.3.2 Crusher-Dryer On a monthly schedule, the hammer-to-crusher block clearance should be reset to a clearance of 1/8 in. This procedure is done with the crusher-dryer in service and no doors on the crusher-dryer open. To adjust the clearance requires the following actions: • Loosening the locknuts on the crusher block adjusting screws • Turn each crusher block adjusting screw1/4 turn clockwise until a ticking noise is heard • Back off each crusher block adjusting screw one full turn counterclockwise and retighten the locknuts, thereby securing the crusher block adjusting screws Change the crusher-dryer lube oil annually. The lube oil has a viscosity of 225 Saybolt Seconds Universal (SSU) at 100°F and a Society of Automotive Engineers (SAE) rating of 20. It is recommended to flush the entire lube oil system before changing the oil. This is accomplished by adding a solvent with no greater than 5% by volume of the total quantity of used oil in the reservoir. This can be done while the crusher-dryer is in a normal operating mode. Allow the crusher-dryer to operate normally with the solvent-oil mixture for a 24-hour period but not more than 36 hours. The lube oil system is then drained and filled with new oil. Clean the oil filter element periodically. The filter element can be cleaned with the crusher-dryer in service. Using the handle at the top of the filter housing, make one complete turn of the filter element. In addition, remove the plug at the bottom of the filter sump housing to drain the filter annually. The grids, swing hammers, and crusher block of the crusher-dryer can be removed and replaced easily through the front access door that is located above the adjustable crusher block. Removal of this door exposes the back of the breaker plate and frame assembly. Remove the breaker plate support rods, and with the help of an overhead lifting device, withdraw the breaker plate from the crusher. Remove the breaker frame assembly holding bolts. The breaker frame assembly can then be removed. Then, remove the adjustable crusher block and inspect the solid grids and hammers.
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19-34
Preventive Maintenance
Swing hammers can be worn to within 1.5 in. of the shank opening as measured from the side with the least wear of the hammer before replacing. Grids can be worn through one-half of the thickness. The swing hammers can be easily removed after the cotter pin, that holds the swing hammer pins in position, has been removed. Each swing hammer pin may be slid out through the hand hole provided in the crusher-dryer housing. The swing hammers are matched and wired in balanced pairs, and care must be taken to install these hammers diametrically opposite each other to maintain the balance of the crusher-dryer. The adjustable crusher block should be set for 1/4– 1/2-in. clearance from the hammers before starting the crusher-dryer. Refasten the breaker plate, the frame assembly and the front access door. Refasten any other access doors that were removed. To remove the bearings from the shaft, support the weight of the crusher-dryer shaft on a sling or blocking, remove the bearing housing end cover along with the shaft coupling and any other obstructions. Remove the bearing lock nut and lock-washer. Then, using the bearing removal nut furnished with the crusher-dryer tools, loosen and extract the tapered adapter sleeve, and remove the bearing from the housing. To install a new bearing, visually inspect the bearing seat to ensure that the machined finish is not scratched or marred. Make a similar visual inspection of the housing base. The housing base and the mounting surface must be flat. Place the bearing housing on the sole plate, check for any unevenness or distortion, and correct any irregularities. Wipe the inside of the bearing housing and the shaft with a clean, lint-free, lightly oiled rag, remove the bearing from the wrapping, and place it in the housing. Do not remove the oil-soluble protective coating from the bearing. Place the housing and the bearing on the shaft with tapered sleeve lightly pushed into position. Make sure that the inside flinger, the collar, and the end cover are already in place. Pay close attention to cleanliness in handling these bearings, especially during installation. Clean, lint-free rags and hand protection should be used when cleaning the bearing housing and shaft. Be careful to keep dirt and corrosive chemicals from coming in contact with the polished surfaces. Do not remove new bearings from their carton or wrapper until they are required to be mounted on the crusher-dryer shaft. Before replacing the bearing, all the parts and tools necessary for the complete assembly and disassembly of the shaft should be available. With the bearing partially assembled on the shaft, and the weight of the crusher-dryer rotor and shaft supported by a sling or block so that the bearing has no load, and after properly centering the rollers in the outer race, the unmounted initial clearance between the rollers and the outer race can be determined by the use of feeler gauges. The allowable range for unmounted initial clearance is 0.0031–0.0043 in. The outer race should be lifted, and the clearance measured. Keep the bearing assembly covered. When tightening the bearings, clean the threads on the shaft and in the locknut. Dirt on the
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1935
Preventive Maintenance
threads may cause damage by galling. Apply a very light coating of Molykote to the shaft
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19-36
Preventive Maintenance
threads, to the faces of the tapered sleeve, and to the lock nut that meets during assembly. Position the tapered sleeve in the bearing. Run the locknut up to a hand-tight position. Tighten the nut further with a light spanner wrench or by tapping with a hammer and drift. Maintain a steady force on the locknut with the spanner wrench. Hold a heavy punch parallel to the shaft with the end against the locknut face. Hit the punch with an 8-lb hammer. The impact on the nut drives the sleeve inward if the nut is kept firmly against the sleeve. This is achieved by maintaining a steady pressure on the spanner wrench. Continue tightening in the above manner until the desired reduction in bearing unmounted internal clearance is obtained. During field disassembly or assembly, the crusher weight should be removed from the bearing by jacking underneath the shaft next to the bearing so that the lower rollers are free to turn. Always tighten the locknut. Never release tension to obtain the correct internal clearance. Should it become necessary to release tension because of over-tightening, use the sleeve removal nut, and begin the tightening process again. Care must be used during the tightening operation as the races and adaptor are hard and brittle, and it is important that no chips or dirt enter the bearing or housing. If the bearing assembly is too loose, the adaptor may creep or turn in the shaft. Both the shaft and bearing would be damaged beyond repair before corrective action could be taken. Consequently, it is extremely important that the crusher-dryer shaft bearing adaptor sleeve and inner race be even and free from dirt or grit. The locknut should be rigid in position and retained by the lock washer. By driving the tapered sleeve by the locknut and reducing the internal clearance between the rollers and the outer race, a definite tightness is set up between the inner bearing race and the shaft sleeve. It is important that the allowable minimum reduction is obtained. Assemble the remaining bearing parts, such as covers and so on. Avoid unnecessary hammering or rough handling when assembling bearing parts. Bearing housing covers and flingers must be assembled with proper clearance. There should be approximately a 1/16-in. longitudinal clearance between the flingers and cover in order to allow for shaft expansion. The flingers must not rub on the housing cover, and there should be equal clearance around the entire circumference of the flinger. This equal clearance should be maintained when the housing is bolted to the pedestal, and care should be taken when bolting the housing to avoid housing distortion, as the bearing outer race must float in the housing. Tight fit or distortion of the bearing housing that restricts the movement of the outer race causes the bearing to overheat and become damaged. The parts that need to be replaced the most frequently in the crusher-dryer section are solid grinds, swing hammers, and crusher block and breaker plates. The design of the machine enables these parts to be easily replaced without removing the housing.
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1937
Preventive Maintenance
19.3.3 Inlet/Outlet Box and Air Seals The mill inlet/outlet boxes are normally not a high-maintenance item. However, a few routing items should be checked periodically. These items include: • The bypass damper, which should move freely, and the shaft bearing should be lubricated at regular intervals. • The foundation bolts, which should be checked and tightened as required. • The inlet/outlet boxes, which should be checked for distortion from overheating caused by fires in the mill system. This could cause binding of the bypass damper or misalignment with the mill head extension causing damage to the air seal. The inlet/outlet air seals should be inspected and greased monthly according to the lubrication schedule. There are two grease fittings on the perimeter of the seal at each end of the mill. These seals should be replaced when excessive leakage occurs and the preset mill/seal air differential cannot be maintained. With the mill rotating, stand at the end of the mill, and listen for rubbing noises in the area of the mill inlet/outlet box seal sleeve. If rubbing noises are heard, the air seals should be inspected and repaired. Visually inspect the air seal for leaking coal dust. If coal dust is observed leaking from the area of the air seals, verify that the seal air pressure is approximately 15 psig. Open the ring header air piping drain valve to blow out any water. The following should be performed when inspecting the air seals: • Remove the existing seal and visually inspect the seal lip before cleaning. • If the seal lip appears undamaged and pliable, carefully clean the seal using rags and mineral spirits (do not damage seal lip). If the cleaned seal has a satisfactory sealing lip, it may be reused. • If the seal lip is damaged or hardened, it must be replaced. • Inspect the sealing surface of the mill head extension for signs of wear and grooving. Replace the mill head extension if wear and grooving is significant, as this will prematurely wear out the new seal and/or a poor seal will result. • Lubricate the new seal immediately while rotating the mill. • If the new air seals are still leaking, it may be necessary to replace the sealing gasket or seal sleeve. 19.3.4 Mill Liners The mill liner bolts must be kept tight to prevent coal dust leakage from the mill barrel and to keep the liners secured to the mill barrel. These bolts should be checked annually and tightened
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19-38
Preventive Maintenance
as required. Check the Riley Power Service Guide for specific torque values for these liner bolts.
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1939
Preventive Maintenance
Table 19-12 lists the tasks for replacing the wear liners. Table 19-12 Wear Liner Replacement (Courtesy of Hoosier Energy - Merom Generating Station) Wear Liner Replacement Tasks 1. Remove the ball charge using a vacuum truck. 2. Disconnect and remove the drive chain. 3. Block the mill to prevent the mill from rotating. 4. Unbolt and remove a row of liners at the bottom of the mill. Take each liner out of the mill with slings. Note: To prevent damage to the mill bearings and drive train, the mill must be rotated in the normal direction of rotation. 5. Unblock the mill and rotate it 180° using the rigging, then reblock the mill. Note: The wear liners should be removed and installed one row at a time, alternating across quadrants or 90°of the mill. 6. Inspect the mill board liner for wear and repair if needed. 7. Install one row of new liners into position in the bottom of the mill. 8. Lubricate the threads of the liner bolts and nuts using Molykote. 9. Install the bolts, rubber washers, retainers, and nuts. Do not install the lock nuts at this time. Tighten the nuts to 170 ft-lbs for the shell liners or to 400 ft-lbs for the head or throat liners. 10. Unblock the mill and rotate it 180°. Then re-block the mill. Install the new liners one row at a time alternating between 90° of mill. 11. When all new liners are installed, rotate the mill 10–12 revolutions. 12. Tighten the liner nuts. 13. Install and connect the drive chain. 14. Load the ball charge and return the mill to service for 30–60 minutes. 15. Retighten the liner nuts and install the lock nuts. Note: The liner nuts should be retightened after one month of operation.
19.3.5 Ball Charge At the annual mill inspection, a visual check of the ball charge size distribution should be made. For mills without the crusher-dryers, the ball charge should show a uniformly graded size distribution with approximately 25% of the ball charge in sizes between 1.5-in. and 2.5-in. in diameter and the remaining balls between 0.75-in. and 1.5-in. There are non-spherical geometric
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19-40
Preventive Maintenance
wear patterns, in the smaller sizes.
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1941
Preventive Maintenance
19.3.6 Speed Reducer Gearbox The approximate oil capacities for the speed reducer gearbox are shown in Table 19-13. Table 19-13 Speed Reducer Gearbox Oil Capacities [7] SingleReduction Type HS
Oil Capacity (gallons)
DoubleReduction Type HD, HDS
Oil Capacity (gallons)
TripleReduction Type HT, HTS
Oil Capacity (gallons)
400
0.50
1000, 1028
2.5
1000, 1028
2.5
500
0.75
1300, 1350
4
1300, 1350
3.5
600
1
1500, 1566
6
1500, 1566
5.5
700, 800
2
1650, 1700, 1750
7.5
1650, 1700, 1750
6.5
900,1000
3.5
2000, 2050, 2100
13
2000, 2050, 2100
11.5
1100,1200
5.5
2250, 2300, 2350
20
2250, 2300, 2350
18
1300,1400
9.5
2450, 2500
29
2450, 2500
24
1500,1600
14
2900, 2950, 3000
41
2900, 2950, 3000
34
1800
18
3400, 3450, 3500
46
3400, 3450, 3500
38
1900, 2000
26
4100, 4150, 4200
76
4100, 4150, 4200
64
2400, 2500
45
4900, 4950, 5000
150
4900, 4950, 5000
130
3200, 3300
65
6200, 6250, 6300
240
6200, 6250, 6300
200
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19-42
Preventive Maintenance
Figure 19-1 shows the single-gear reducer.
Figure 19-1 Link-Belt Single-Gear Reducer [7]
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1943
Preventive Maintenance
Figure 19-2 shows the double-gear reducer.
Figure 19-2 Link-Belt Double-Gear Reducer [7]
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19-44
Preventive Maintenance
Figure 19-3 shows the triple-gear reducer.
Figure 19-3 Link-Belt Triple-Gear Reducer [7]
Table 19-14 lists the disassembly tasks for the triple-speed reducer gearbox.
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1945
Preventive Maintenance Table 19-14 Triple-Speed Reducer Gearbox Disassembly Tasks [7] Disassembly Tasks 1. Note: Never remove the housing without disconnecting the motor and ensuring that the motor cannot start. The gears are retained in the bottom housing by their weight only. Drain the oil completely from the housing or to a point below the bore of the lowest bearings. 2. Remove the cap screws that fasten the bearing retainers to the housing cover. Loosen the fastening retainers to the housing base. 3. Remove the oil dipstick. 4. Remove the taper pins in the housing cover flange that positions the cover on the base. 5. Remove the cap screws that fasten the housing cover to the base. 6. Break the bond between the cover and base by tightening jack screws at both ends of the housing cover flanges. 7. Attach eyebolts and a sling to remove the cover. Removing the flexible coupling: 8. Unfasten and separate the coupling halves. 9. With the housing cover removed, vertically lift the shaft assembly out of the housing. Be careful not to damage the parts. 10. The couplings halves can be repaired or replaced without disturbing the foundation or alignment of the speed reducer or the mill motor. Removing the high speed shaft and supporting bearings: 11. Remove the key from the high-speed shaft projection. The key can be unseated by making an indentation in the end of the key with a center punch and tapping the punch with a hammer. 12. Remove the cap screws that fasten the high-speed bearing retainers to the housing base. 13. To prevent the sharp keyseat edges from causing damage to the seals, slide the high-speed bearing retainer with the oil off the shaft. 14. Lift the high-speed shaft with the bearings from the housing as one piece. Be sure to use softeners when lifting to prevent damage to the parts. Remove the bearings from the high-speed shaft with a bearing ring puller. 15. Lift the low-speed pinion shaft with the intermediate gear from the housing as one piece. C-clamps can be used to fasten the sides of the intermediate gear in lifting. 16. Remove the retaining rings from the intermediate pinion shaft. Remove the retaining rings from the housing bore. 17. Remove the intermediate integral pinion shaft by pushing on the end of the shaft so it slides through the gear and out the opposite bearing bore. The bearing located on the side from which the shaft was pushed will separate, leaving the outer race in the housing bore. The inner and outer races of the bearing on the opposite side will remain on the end of the shaft as it is pushed through the bearing bore. 18. Clean housing joints and use an acceptable sealing compound on all the metal-to-metal parts when reassembling.
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19-46
Preventive Maintenance Note: For assembly, the intermediate- and high-speed shaft bearing are adjusted correctly when they give 0.005–0.015 in. lateral play. The end float should be adjusted at normal room temperature.
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1947
Preventive Maintenance
19.3.7 Drive Train There are several components in the drive train that require periodic maintenance. Some of the components are the rotorseal, the clutch, the speed reducer, the oil conditioning system, the drive chain and sprocket, the idler shaft bearings and sprocket, the mill tires and trunnion rollers, the thrust rollers and bearings, and the trunnion shaft bearings. •
Rotorseal – Four or five drops of light machine oils should be applied every three months through the oil hole sealed with the pipe plug. Leaks in the air lines or in the rotorseal should be repaired as needed.
•
Clutch – Areas to inspect on the clutch include: –
The condition of the friction surface of the drum. If the drum surface is badly grooved or worn, the surface may be remachined. The minimum drum diameter is given in the Riley Power Service Guide.
–
The condition of the friction shoe assemblies. If friction linings are glazed, this condition can be corrected by sanding the friction lining to remove the glaze. Linings that have been worn to minimum allowable thickness should be replaced.
–
The condition of the rubber tube. If there are air bubbles or signs of ply separation on the tube, the tube must be replaced.
–
Oil or grease on the friction surface. If oil or grease accidentally gets on a clutch actuating element, the oil or grease should be wiped off with a cloth dampened with gasoline or naphtha, and then wiped dry. If friction shoes have been contaminated, they should be removed and cleaned. For light contamination, Fuller’s earth media may be used to remove grease or oil from the friction-lining surface. If the lining is saturated with oil or grease, a solvent should be used to degrease the lining. A cloth dampened with the solvent may be used to wipe the grease off the lining, or the friction shoe assembly may be dipped into the solvent. Severely saturated linings may have to be replaced.
–
Friction shoes that do not retract: o Inspect the quick-release valve. Contamination of the quick-release valve may not permit complete air exhausting of the tube and will not allow the shoes to retract properly. o Inspect the release springs. If the release springs are broken, the shoes will not retract properly and should be replaced. o If the element is exposed to severe contamination, a deposit may form that can prevent the friction shoes from retracting. The friction shoes should be removed, and the deposits should be cleaned off. o Inspect the rubber for hardening of the actuating tube. If hardening occurs, then the rubber resists proper friction shoe retraction. The tube should be replaced.
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19-48
Preventive Maintenance
Due to environmental conditions in the ball tube mill operations, the clutches can be subjected to contamination from dust. This mill dust is usually very abrasive and tends to shorten the life of the clutch components. Sealing air may be provided to the clutch housing to eliminate dust contamination of the clutch components. Technical Key Point Excessive accumulation of dust deposits within clutch elements can eventually restrict shoe retraction during clutch disengagement, resulting in dragging and overheating. Protective shielding or total enclosure is recommended to keep the dust out, and if it is not provided, the clutches must be inspected frequently (every three months) to ensure normal shoe release. The clutch should be de-energized briefly each month in order to maintain the flexibility of the rubber tube. To effectively clean the clutch elements, it is necessary to completely dismantle them. The removal of the friction shoes is also necessary to ensure a thorough cleaning. With a vacuum cleaner or an air hose, remove the dust deposits from all of the components. In most instances, it is not necessary to remove the clutch drums for this operation. Regularly scheduled inspection of the friction lining is important in order to determine when replacement is necessary. Do not exceed the limits of liner wear because the rivets or screw heads will score the drum and release spring breakage can occur. The clutch air system should be inspected at regular intervals for possible leakage and contamination of the air filter, which should be repaired and replaced as required. The air receivers should also be drained periodically in order to remove any condensate accumulation. Instructions for replacing the clutch friction shoes, the lining, and the actuating tube are given in the Riley Power Service Guide. Table 19-15 lists the tasks for inspecting the clutch.
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1949
Preventive Maintenance
Table 19-15 Clutch Inspection Tasks (Courtesy of Hoosier Energy – Merom Generating Station) Tasks 1. Unbolt the clutch enclosure. Rig and remove the clutch enclosure. 2.
Visually inspect the clutch friction surfaces (shoe linings and drum) for oil or grease. If a small amount of oil or grease is found on the shoe friction lings, the linings should be cleaned using solvent and rags. If the linings are saturated with oil or grease, the linings should be replaced.
3. Visually inspect the friction shoe assemblies for damage and excessive wear. Glaze on the shoe friction linings can be removed by sanding. If the thickness of the lining is 3/8 in. or less, the lining should be replaced. 4. Visually inspect the clutch drum for damage and excessive wear. Measure the drum diameter in several locations to determine the amount of wear. The maximum allowable drum wear diametrically is 1/4 in. 5. Visually inspect the clutch rubber tube for air bubbles or other signs of ply separation. 6. Remove all tools from the clutch area. Rig, install, and bolt the clutch enclosure.
•
Speed reducer – The speed reducer is lubricated by an oil bath, and the oil is supplied by the oil conditioning system. This oil bath should be full at all times.
•
Oil conditioning system – The oil conditioning system supplies oil to the chain bath and speed reducer, and should be serviced annually. The heat exchangers should be disassembled and cleaned, and the suction oil strainers should be cleaned during the annual outage. At six month intervals, the oil filter elements should also be replaced.
•
Drive chain and sprocket – Monthly visual examinations of the chain drive should be made to ensure that adequate lubrication, chain alignment, and adjustments are maintained. An oil level sight gauge is mounted outside the silence housing to allow monitoring of the oil level in the chain case. The chain case oil level should be kept full at all times. The chain tension should ensure that the sag of the lower strand is approximately 4 in. from the horizontal. Adjustments, if necessary, are made by lowering the idler sprocket and relocking the jack screws in place. Make sure the chain is lined up on the sprockets and rides correctly to prevent unusual and premature wear of this equipment. The chain must be kept lubricated during outages to prevent rust from forming and damaging the chain rollers. This can be accomplished by the periodic rotation of the mill during the outage period and drawing the chain through its oil bath. Chain case seals should be replaced if oil leakage develops. Oil leakage creates a cleanliness problem that can be hazardous to personnel inspecting and servicing the equipment.
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19-50
Preventive Maintenance
Table 19-16 lists the tasks for repairing a break in the drive chain. Table 19-16 Chain Repair Tasks (Courtesy of Hoosier Energy – Merom Generating Station) Chain Repair Tasks 1. Remove the back access door of the chain case. Locate the break in the chain. 2. Isolate the mill. 3. Drain oil from the chain case. 4. Remove the chain tensioning covers (Plexiglas and steel on sides). Remove the top tensioning device covers on both sides. 5.
Install the jacking device above both tensioning idler bearings. Check the “out of service” chain position before jacking out the idler bearings. Measure/count the threads on the idler tensioning pad for the “as found” condition on both sides. Record the findings.
6. Use the jacking device to remove the idler bearings. 7. Loosen the jamb and tensioning nuts to relieve pressure from the chain. Cut the chain case as needed for access to the broken chain segments. 8. Install small steel chokers on the chain on the bottom side of the break. Attach a come-along to the steel on the back of the mill. Pull the slack out of the bottom of the chain to allow replacement of the breaker segments. 9. Have personnel clean the chain case components while the chain repair is being performed. Remove the sealant and gaskets from the cover. Grind components out of the chain case. 10. Remove the pins from the links to be replaced. Replace one link at a time. Drive the new link in as the old link is removed. The new link is installed in the opposite direction of the broken link. Repeat until all the broken links are replaced. 11. Allow slack in the chain. Remove the rigging from the chain. The slack in the chain should be 4in. sag ±1/2 in. 12. Weld repair the chain case. 13. Verify the level of the idler shaft. Verify the jamb nuts are tight. 14. Complete cleaning the openings, remove the jacking devices, and reseal covers and doors. 15. Reinstall the door on the backside of the mill. 16. Install the chain break glass rod. 17. Remove the mill isolation tags.
Table 19-17 lists the tasks for removing and reinstalling the driven sprocket.
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1951
Preventive Maintenance
Table 19-17 Driven Sprocket Removal and Reinstallation Tasks (Courtesy of Hoosier Energy - Merom Generating Station) Tasks for Sprocket Removal and Re-Installation Safety requirements: Full protection must be worn when working on the upper levels of the mill. Burn permits are needed for any cutting or welding. When rotating the mill barrel, the barrel must be locked down by rigging to prevent rotation. When the barrel is rotated manually, all personnel and equipment must be clear of the barrel. The welding ground procedure must be used for any welding done on the ball mill. Removal of Sprocket Segments: Note: Three segments can be removed for each roll of the mill. The lubrication and lift oil pumps must be in service when rotating the mill manually. Manual rotating is accomplished by using two 1-1/2-ton comealongs. W hen removing and replacing sprocket segments, it is easier to perform this task from the top of the mill. 1. Use a cutting torch to remove the bolt keepers from the segment bolts and the washers from the dowel keepers 2. Fabricate a slide hammer on a 5/8-in. solid stock rod and tack weld to the dowel pin. 3. Remove the dowel pin by hammering and cut the pin from the slide hammer rod. 4. Grind off the slide hammer rod and move to the next pin. 5. Mark the sprocket segments by stamping so the segments can be replaced in the same order as the segments are removed. 6. Insert a 3/4-in. eye bolt in the sprocket segment and connect with a 1-in. wide by 2- or 3-ft long nylon choker to the mobile crane hook. 7. Remove the bolts from the segment. Remove the sprocket segment. If the sprocket segment will not come loose, then a straight torch can be used to cut the nut off under the sprocket segment. 8. Move all sprocket segments to a laydown area. Clean the segments and inspect all surfaces for burrs and damage. If necessary, grind the mounting surfaces of the segments. Oil Seal Replacement: 9. Cut off the segment bolt nuts and grind the surface. A cutting torch can be used on the nuts. 10. Cut out the oil seal plate. Leave a lip for welding on the new oil seal. 11. Overlay the new oil seal to the lip of the old seal. Tack weld and check the fit. 12. Seal weld the new oil seal into place. The material is carbon steel. Use the proper welding procedure. Installing the Segment Nuts: 13. Position the nuts by inserting the centering hole apparatus into the bolt hole. Make sure the apparatus has shoulder sleeves on the bolt. 14. Screw on the nut and tack weld the nut. Use the proper welding procedure.
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19-52
Preventive Maintenance 15. Remove the apparatus and weld the nut solid around the nut. Continue to the next nut. There are 90 nuts total.
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1953
Preventive Maintenance
Table 19-17 (continued) Driven Sprocket Removal and Reinstallation Tasks (Courtesy of Hoosier Energy - Merom Generating Station) Tasks for Sprocket Removal and Re-Installation Install the Sprocket Segments: 16. Inspect each segment for burrs. Note: It is recommended that the segments be installed from the top of the mill barrel. Three segments can be installed before the barrel is rotated. Each time the barrel is rotated, the barrel should be locked into place with come-alongs to secure the barrel. 17. Using the threaded hole in the center of the segment, attach an eye bolt shackle to a 1-in. wide by 2- or 3-ft long nylon choker to the crane hook. Lift the segment into place on the top of the mill. 18. Fabricate two 1-1/2-in. rod coupling nuts for each side of the segment. Install a 30- or 36-in. long all thread rod into the rod couplings. 19. Using a heavy piece of channel (3/8-in. thick, 4-in. wide, 36-in. long) as a strong back, jack the ends of the segment into place with 10-ton hydraulic jacks. 20. Align the dowel pins and install the dowel on each end first, leaving 1/8 in. of dowel sticking out, then install the bolts (six per segment). 21. Hold the opposite end of the segment down while jacking the other end into place. Use a 3/4-ton come-along and a 3-ft steel choker around the segment through the hand holes under the ribs, and pull down. 22. After the segment is in place, the dowels and bolts are in place, tighten the bolts snug with a 1-1/2-in. combination wrench. Move to the next segment until all segments are installed. Tighten the segments to 1495 ft-lbs. 23. Using the weld grounding procedure, weld the washer onto the dowel pins and the mill rib. Weld all bolt keepers and dowel washers. 24. Remove all rigging. Reinstall the chain. Set the chain tensioner. 25. Close up the oil casing covers. 26. Reinstall the lagging.
•
Idler Shaft Bearings and Sprocket – The idler shaft bearings require monthly lubrication. Refer to the lubrication recommendations in the Riley Power Service Guide. When raising or lowering the idler shaft sprocket, its horizontal alignment with the chain is very important. Premature and uneven wear can result if the idler sprocket is not properly aligned.
• Mill Tires and Trunnion Rollers – The mill tires and trunnion rollers should be inspected periodically to check for uneven tire/roller wear. If uneven wear exists, the mill must be realigned according to the Riley Power Service Guide. • Thrust Rollers and Bearings – The thrust rollers are set with the proper clearances during
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19-54
Preventive Maintenance
the installation of the mill. The mill end thrust is monitored through the load cell attached to
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1955
Preventive Maintenance
the thrust roller assembly and alarms if the end thrust increases beyond the alarm point (usually 10,000 lbs). An end thrust alarm is indicative of mill barrel misalignment and should be corrected immediately as outlined in the Riley Power Service Guide. The thrust rollers require periodic adjustment back to the 0.010–0.005 in. clearance as they wear into the tire. Inspections and adjustments should be monthly during the wear-in period. Excessive wear, as with frequent alarms, is caused by mill misalignment and should be corrected immediately. The thrust roller bearings should be cleaned and repacked during the annual outage. 19.3.8 Driveshaft The driveshaft bearings are pillow block bearings that should be cleaned and repacked during the annual outage. The operating records for the bearing should be checked with the highest operating temperature bearing for inspection. The bearing metal thermocouples and vibration detectors should be removed, and the grease supply tubing should be removed from the bearing cap. The bearing cap can then be unbolted, the rigging hardware can then be attached, and the bearing cap can be removed. The bearing can now be inspected and replaced if needed. Refer to the lubrication schedule in the Riley Power Service Guide. Table 19-18 lists the tasks for replacing the driveshaft. Table 19-18 Driveshaft Replacement Tasks (Courtesy of Hoosier Energy – Merom Generating Station) Driveshaft Replacement Tasks Note: The mill must be rotated in the normal direction of rotation. Verify the chain is lined up and located correctly in the sprocket teeth. Caution: The mill motor must be tagged out. Mill Shaft, Drive Chain, and Sprocket Removal: 1. Remove grease lines from the mill housing and bearings. 2. Remove thermocouples and instrumentation. 3. Remove water lines to chain spray cooler. 4. Remove doors. Remove the necessary sections of mill housing to access the upper chain case. Use 15/16-in. socket for the divider strips. Use a torch for removal of the frame above the case. Mark the bolts and place in a bag. 5. Drain oil from the idler tensioner housing. 6. Remove the upper chain case housing. Mark the bolts and place in a bag. 7. Remove the covers on the tensioner housing. Use a 9/16-in. socket and adjustable wrench to remove the flexible hose. Mark the bolts and place in a bag.
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19-56
Preventive Maintenance
Table 19-18 (continued) Driveshaft Replacement Tasks (Courtesy of Hoosier Energy – Merom Generating Station) Driveshaft Replacement Tasks 8. Raise the idler sprocket to the maximum adjustable height to relax the chain tension. Remove the top cover on the tensioner. Install a special pallet on top for the hydraulic jack. Measure the thread length exposed on the lower tension rod. Loosen nuts on both tension rods at the bottom by installing horseshoe plates at the bottom and pressing the tension off with the hydraulic jack at the top. 9. Disconnect the chain at the drive sprocket. 10. Using rigging equipment, position the chain to gain access to the idler and drive shafts. 11. Remove the housing for the clutch assembly. Unhook the air supply pipe. 12. Remove the clutch assembly. Remove the left connecting bolts, holding the hub to the driveshaft. Remove the right bolts holding the outer hub to the gearbox hub. Remove enough bolts on the left side at the rear of the outer hub to allow removal past the driveshaft. Mark the bolts and place in a bag. Remove the clutch assembly using rigging apparatus and mobile crane. 13. Before removing the driveshaft, take the following readings: Tight wire to sprockets, face-to-face dimension on the hubs at four points, and the run out on the hubs at four points. 14. Remove the driveshaft bearing caps and mark the location. 15. Attach rigging for the driveshaft removal. 16. Remove the driveshaft. 17. Clean all areas in preparation for reinstallation. Installation and Alignment: 18. Apply rigging and lift the new driveshaft into position above the bearing housings. Slowly lower the new driveshaft into the bearing housings. Remove the rigging. 19. Install the bearing caps and tighten the bolts. 20. Align the driveshaft in the bearings. Pre-load the center bearing to 0.006 in. total. Use powdered graphite to ease the preload. The axial movement is 0.017 in. for each 0.001 in. of preload. Install a tight wire through the driven sprockets and align within 0.060 in. Perform four face-toface readings on the clutch. The readings should be within 0.00025 in. Take four rim runout readings. The runout should be 0.0005 in. Remove the driveshaft. Install the bearing caps and tighten the bolts. 21. Remove the driveshaft. Install a tight wire in the center bearing with the east and west bearings with caps on and bolts tightened. Reinstall the driveshaft. Pre-load the center bearing to 0.0040– 0.0055-in. range total using powder graphite to ease the preload. Replace the bearing caps and tighten the bolts. 22. Install the clutch assembly. 23. Reconnect the chain.
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1957
Preventive Maintenance 24. Reset the idler shaft tension chain. Use a level on the idler sprocket.
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19-58
Preventive Maintenance
Table 19-18 (continued) Driveshaft Replacement Tasks (Courtesy of Hoosier Energy – Merom Generating Station) Driveshaft Replacement Tasks 25. Lock the tension rods and remove the jacks and special plates. 26. Replace all tension covers. Install the chain case cover. Install the mill housing and clutch housing. 27. Install the water lines to the chain spray cooler. 28. Install the thermocouples and instrumentation. 29. Fill the case with oil (Mobil DTE Extra Heavy) 30. Clear isolation tagging.
19.3.9 Lubrication Heat Exchanger Table 19-19 lists the tasks for inspecting the lubrication heat exchanger. Table 19-19 Heat Exchanger Inspection Tasks (Courtesy of Hoosier Energy – Merom Generating Station) Inspection Tasks 1. External inspection of the heat exchanger shell includes examination for dents in the shell, extruded or damaged gaskets, corrosion, damaged flanges, bonnets, hub, and tube sheets, stress marks in the shell, signs of a previous repair, and shell bulge. 2. Internal inspection should include draining the shell completely and inspecting the heat exchanger to determine the extent of cleaning needed. Detection of severe fouling or corrosion of the tubes and/or bonnets may require further disassembly. 3. If further disassembly of the heat exchanger is needed, visually inspect the tube ends for thinning, anodes for wear (if applicable), damage to the nozzle threads on the tube and shell side, and erosion of the tubes under the shell side nozzle. Note: Signs of any damage may be an indication of high velocity, particle entrainment in the fluid, or a corrosive environment. Depending on the severity of the damage, the heat exchanger may require replacement. 4. Clean the inside of the tubes with a rotary brush driven by an air or electric rotating tool. The use of nylon brushes is recommended because metal brushes or bristles will break the protective film, scratch the metal surfaces, and accelerate corrosion. After brushing, the tubes should be flushed with clean water to remove loosened dirt and scale. 5. Before assembly of the heat exchanger, the shell side should be pressure tested to verify the integrity of the tubes and all joints.
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1959
Preventive Maintenance 6. Clean the bonnets as required and reassemble to the heart exchanger. New gaskets are required for assembly. Use of oiled gaskets is acceptable. Perform a pressure test on the tube side at the specified test pressure with air under water.
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19-60
Preventive Maintenance
Table 19-20 lists the tasks for replacing the lube oil heat exchanger. Table 19-20 Lube Oil Heat Exchanger Replacement Tasks (Courtesy of Hoosier Energy – Merom Generating Station) Heat Exchanger Replacement Tasks Note: The following tasks are for replacing the lube oil heat exchanger with the mill on-line. The cold start bypass valve, heat exchanger lube oil inlet and outlet ball valves, and the cooling water inlet and outlet isolation ball valves will need to opened and closed during the online heat exchanger replacement. The replacement should occur when the ambient temperature is low since no cooling of the oil will take place during the time (~2 hours) of the replacement. Caution: The minimum oil pressure on the low-pressure system is 10 psi and the thrust shoe pressure system is 4 psi. When the oil pressure drops below these pressure settings, the mill will trip. The bearing pad alarm temperature is 160°F and trip temperature is 165°F. These pressure and temperature indications should be monitored during the heat exchanger replacement. 1. Remove the 2-in. diameter inlet and outlet oil lines. 2. Remove the inlet and outlet water lines. 3. Remove the fittings on the inlet and outlet on the top of the cooler. 4. Remove the four bolts from the base. 5. Remove the cooler and install a new cooler. 6. Set up two electric fans on the outside of the mill housing to blow air across the bearing surfaces on each end of the drum. 7. Disconnect the oil and water lines from the heat exchanger. Remove the heat exchanger. 8. Install the new heat exchanger. Connect the oil and water lines. Leave the fitting loose at the oil inlet isolation ball valve so the air can be purged out of the heat exchanger and oil lines. 9. Purge the air from the heat exchanger. Tighten the fitting at the oil inlet. 10. Remove the air fans.
19.3.10
Classifier
Classifier vanes require the most maintenance attention in the classifier. These vanes impart the swirling effect to the coal/air mixture that results in an erosive effect on the vanes. The erosion tends to be more prevalent at the top edge as a result of the design flow pattern. The trickle valve is not an integral part of the classifier assembly; however, the trickle valve can cause problems in the operation of the mill. A deformed, missing, or sticking trickle valve may prevent coarse coal from returning to the mill or allow air and coarse coal to bypass the vane arrangement. Table 19-21 lists the PM tasks for the classifier.
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1961
Preventive Maintenance Table 19-21 Classifier Preventive Maintenance Tasks [7] Preventive Maintenance Tasks 1. Remove the classifier from service and isolate from the system. 2. Remove the bolted cover on the top of the classifier. 3. Inspect the vanes for erosion and excessive movement. The vanes should be replaced if the erosion or other physical effects are severe. 4. Inspect the internal services for abnormal wear or metal failure. 5. Investigate any area of coal buildup. 6. Inspect the deluge system and clean the nozzles from any coal pluggage. 7. Inspect the classifier exit thermocouple. Replace and/or calibrate as needed. 8. Inspect the trickle valve and associated coarse return pipe for deformation, sticking, and so on. 9. Document all conditions in the classifier.
19.3.11
Primary Air Fan and Ductwork
The cold primary air fans draw ambient air and forces it through the air preheater into the system. The inlet silencers and screens must be cleaned periodically or fan capacity will be reduced. The shaft, damper, and drive motor bearings must be assured of receiving the proper lubrication. The drive coupling should be inspected for general condition and wear. All dampers should be lubricated and worked to ensure the shafts are free to rotate in the bearings. Linkages and controls must be free in the entire range of movement. The dampers in the air system are integral in the control of the boiler, and their maintenance schedule must be adhered to rigidly. Inspect the mating seating surfaces of these dampers to ensure good contact and make sure the damper blades are neither warped nor improperly positioned on their shafts. Inspect all supports, internal and external, to ensure their integrity, and record and deal with any evidence of corrosion or erosion as required. Also check the position of any spring hangers, noting and correcting any problems. Inspect all expansion joints for evidence of cracks or tears that can result from cyclic stress or uneven expansion. Repair these leaks early, either by welding or by a partial or full replacement of the expansion joint. An accumulation of debris (fly ash, coal, and so on) in the bottom convolutions of these joints is a major cause of failure. Remove these accumulations at every opportunity.
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19-62
Preventive Maintenance
19.3.12
Coal Shutoff Valves
Exercise the shutoff valve once a month for several cycles to ensure proper functioning of the unit. During the annual outages, disassemble the valve, wire-brush the parts to remove the oil lubricant and coal deposits, and apply Molykote spray. Reassemble and grease the valve, but do not use any grease on the valve sealing surface. 19.3.13
Lubrication Schedule
Table 19-22 shows the lubrication schedule for the Riley Power ball mills. Table 19-22 Lubrication Schedule [7] Component
Number of Bearings
Bearing Type
Lube Quantity
Lubrication Frequency
Lubrication Specification
Trunnion support
4
Sleeve
Trunnion support roller bearings (main bearings
16
Pillow block
1 oz.
Monthly
Grade II grease
Drive shaft bearings
3
Pillow block
1 oz.
Monthly
Grade II grease
Idler shaft bearings
2
Rolling element
As needed
Monthly
Grade II grease
Thrust roller
2
Rolling element
0.3 oz.
Monthly
Grade O grease
Change at six months. Add as needed.
EP 75
Drive chain case
Grade II grease
205 gallons
Oil conditioning system filters Rotorseal Air seals (inlet/outlet boxes)
Change at six months or as required 4–5 drops
Add monthly
Light machine oil
Add monthly as required
Type M-77 Molykote
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1963
Preventive Maintenance
19.3.14
Spare Parts
Table 19-23 lists the recommended spare parts for 1–5 ball mills. Merom Generating Station stocks a complete clutch and gearbox assembly. The complete assembly allows for a faster response time when changing these components because of a component failure. Table 19-23 Riley Power Recommended Spare Parts [7] Spare Parts
Quantity
Reducer high-speed pinion gear
1
Reducer high-speed bearings
2
Reducer high-speed thrust bearings or Spare reducer complete with coupling half and clutch spider
2 or 1
Reducer high-speed coupling
1
Clutch element assembly
1
Clutch drum
1
Clutch rotorseal with coupling and hose
1
Clutch solenoid valve
1
Clutch flow control valve
1
Clutch air regulator
1
Clutch air amplifier
1
Trunnion roller with bearings and seals
1
Thrust roller assemblies
2
Drive sprocket segments
3, 2–14 teeth, 1–15 teeth
Drive shaft bearings with seals
3
Drive shaft chain case seals
2
Idler shaft chain case seals
2
Felt chain case seals
2
Chain – 62.5-ft length
1
30 gpm oil pump
1
5 gpm oil pump
1
Oil filter elements
12
2 hp motor for oil pump
1
Motor coupling for 5 gpm oil pump
1
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19-64
Motor coupling for 30 gpm oil pump
Preventive Maintenance 1
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1965
Preventive Maintenance
19.3.15
Example Preventive Maintenance Inspections
The Merom Generating Station has the following PM inspections: • Inspect the motor coupling every two years. • Clean and inspect the heat exchangers annually. • Replace the air drive amplifier annually. • Inspect the gearbox annually. • Inspect the chain case, gear reducer, and lube oil skip pump annually. • Inspect the air seal, clutch, bearings, and heat exchangers annually. • Replace the heat exchanger every two years. • Lubricate the driveshaft, idler shaft, and gear reducer seal monthly. The drive chain oil reservoir holds approximately 215 gallons of EP oil. The oil filter should be replaced every 750 hours of operation. The oil should be changed every 2500 hours of operation. The oil level on the speed reducer gearbox should be checked weekly and oil added if needed. The grease seals should be purged monthly and grease added to all of the couplings. Take an oil/grease sample for testing every two months. Change the oil every 6 months or 2500 operating hours. Change the grease at the seals at the same time as the oil change.
19.4 Stein Industrie This section contains the following items: •
Feeder inspection tasks for the 7000-hour interval
•
Preventive maintenance tasks
•
Inspection tasks for a 5500-hour interval
•
Inspection tasks for an 18-month interval
•
Inspection tasks for a nine-year general overhaul
•
Replacement tasks for the pinion and girth gear
As an example, the following is the maintenance strategy for the Stein Industrie mills at the Matimba Power Plant: •
One mill at a time is planned for off-line maintenance. The mill will not be scheduled off-line unless the remaining mills are providing reliable operation.
•
Mills on the same unit cannot be scheduled for planned maintenance within the same week.
•
The screw conveyor replacement outages are scheduled during a unit outage.
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19-66
•
Preventive Maintenance
Liner rotations and liner replacements are performed during unit outages.
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1967
Preventive Maintenance
The following are components and their maintenance strategies: •
Mill drum liner wear – The cylindrical, conical, and oblique liners have to be rotated and replaced. The drum liners are used for 35,000 operating hours and then rotated. After rotation, the liners are used for an additional 30,000 operating hours. After the resultant 65,000 operating hours, the liners are replaced.
•
Screw conveyors – A new design screw conveyor was installed starting in 1999. The modification was made in the inlet area from 110 × 12 mm to 120 × 12 mm, and a third liner was added to the hot air tube. As a result, the life of the screw conveyor was extended from 22,500 to 30,000 operating hours. The wear rate is monitored during the 5500-hour operating hour inspection intervals, while the screw conveyors are typically replaced at six-year intervals. Rubbing of the flights on the liners will increase the wear rate on the liners and shorten the life of the liners.
•
Classifier inlet piping – Visual inspections and thickness measurements are made during each unit outage. Repairs are also performed at the same time.
•
Feeders – For Matimba Power Station, feeder inspections are performed at 5500 operating hour intervals and any repairs are completed at that time. A general overhaul is scheduled every nine years. The scope of the general overhaul is to inspect all bearings, the gearbox, the clutch, and the motor. The feeder level bar height should be inspected during the general overhaul. For the Tutuka Power Station, the feeder is inspected at an interval of 7000 operating hours. Tables 19 through 24 list the feeder inspection tasks.
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19-68
Preventive Maintenance Table 19-24 Feeder Inspection Tasks for the 7000 Operating Hour Interval (Courtesy of Tutuka Power Station) Tasks 1. Open inspection door on feeder and make sure it is safe to work on feeder. 2. A chain block must be used to pull the chain. The crane should not be used. 3. The return idlers should be checked for correct nuts and tightness. 4. The feeder bar heights should be checked. Clean the feeder bar and base plate. Measure the distance between the bottom of the feeder bar and the base plate of the coal feeder on the left hand side of the feeder bar. This distance should be perpendicular to the bottom of the feeder bar and parallel to the feeder bar. Measure the same distance in the middle and on the right hand side of the feeder bar. Record the measurements. 5. Inspect the general condition of the feeder bar assembly for wear, holes in body, and so on. 6. Check the general condition of the pedestal bearings. 7. Check the feeder speed. Check the motor frequency versus feeder speed. Check for 5 Hz at 80 rpm and 24 Hz at 360 rpm for the short and long feeders. 8. All bent flights should be straightened. 9. The chain should be tensioned to clear the guide shoe by ± 3 mm. 10. Inspect and replace the feeder shaft seals if needed. 11. Clean the classifiers.
•
Couplings – The mill motor-to-gearbox coupling was originally a gear coupling that required regreasing. The 5500 operating hour interval was too long because the lubricant experienced soap separation and coupling failure. The couplings are being replaced with spring couplings.
• Electrical motors – The motors include the main bearing oil pumps, the main motor bearing oil pump, the main gearbox lubrication oil pump, the girth gear grease pump, the seal air fan, and the feeder. These motors are inspected and cleaned every 18 months with a general overhaul at nine years. • Girth gear and pinion gear – Inspection of the girth and pinion gears to detect cracks, broken teeth, pitting, wear, and so on should be performed every 18 months and during the general overhaul. Any problems found during inspections should be monitored until the next repair outage. Repairs should be made for tooth flank relief, pitting removal, sharp edges, and crack removal. The expected life of the girth gear is 90,000 operating hours, and then the gear is turned or flipped. A new pinion gear is used with the turned girth gear. 19.4.1 Preventive Maintenance Tasks Table 19-25 lists the PM activities for the Stein Industrie ball mills at the Majuba Power Station.
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1969
Preventive Maintenance Table 19-25 Preventive Maintenance Tasks (Courtesy of Majuba Power Station) Tasks
Frequency (Operating Hours)
Pulverized fuel sampling
Annual
Airflow checks
Every 3 years
Mill load line checks
Annual
Internal inspections: Ball charge measurement
5000
Liner wear measurement
10,000
Screw conveyor wear measurement
10,000
Wind box liner wear measurement
10,000
Trunnion tube liner wear measurement
10,000
Reject chute clack box inspection
5000
Classifier inspection: Chinese hat position
5000
Vane position
5000
Wear
10,000
Feeder inspections: Liner wear
10,000
Flights condition
5000
Feeder calibration
5000
Level bar height
5000
Power transducers calibration check
Annual
Electric ear calibration
Annual
Ball wear rate calculations
Monthly
Ball loading
Daily
Mill operating hours
Monthly
Operating conditions – temperatures, power, damper positions
Daily
Oil analysis
5000
Vibration monitoring performed by: Maintenance
Monthly
Performance
Every three months
Girth gear condition monitoring
Every two weeks
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19-70
Preventive Maintenance Table 19-25 (continued) Preventive Maintenance Tasks (Courtesy of Majuba Power Station) Tasks
Frequency (Operating Hours)
Raw-coal feeder: Clamping Plate – Measure distance between either side
200
Scraper chain – Check chain tension
200
Head and tail section bearings – Change grease for first time
200
Head and tail section bearings – Greasing
200
Head and tail section bearings – Check vibration and temperature
800
Head and tail section bearings – Change bearing grease routinely
4000
Single-gate slide damper – Change grease on spindle for first time
800
Single-gate slide damper – Change grease on drive gear for first time
8000
Single-gate slide damper – Grease the spindle and drive gears
4000
Double-gate slide damper – Change spindle grease routinely
800
Double-gate slide damper – Change drive gear grease routinely
8000
Double-gate slide damper – Grease both gearboxes and bearing pedestals
Annual
Liners – Inspect liner
6000 to 7000
Reduction gearbox and drive gears – Change oil for first time
10,000
Reduction gearbox and drive gears – Change oil routinely
25,000
Drive arrangement – Check gear backlash
15,000
Mill body liners: Mill Liners – Change liners
28,000 to 40,000
Conveyor body liners – Change liners
28,000 to 40,000
Mill conveyor bearing: Bearings – Change grease for first time
500
Bearings – Change grease routinely
8000
Bearings – Grease routinely
2000
Bearings – Check vibration and temperature
Monthly
Girth gear and drive pinion: Drive pinion bearings – Check vibration and temperature
Monthly
Drive pinion bearings – Lubricate bearings
2000
Drive pinion bearings – Change bearing grease for first time
800
Drive pinion bearings – Change bearing grease routinely
8000
Drive pinion and girth gear – Clean
8000
Drive pinion and girth gear – Check backlash
40,000
Drive pinion and girth gear – Check contact marks
40,000
Grease drum – Replenish
As required
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1971
Preventive Maintenance
Table 19-25 (continued) Preventive Maintenance Tasks (Courtesy of Majuba Power Station) Tasks
Frequency (Operating Hours)
Main reduction gearbox: Bearings – Check vibration and temperature
Monthly
Gearbox oil – Change oil first time
500
Gearbox oil – Change oil routinely
4000 to 8000
Gearbox oil – Oil sampling and testing
8000
Gearbox oil – Check oil level
500
Air breather – Change
6 months
Oil cooler – Clean
Annual
Labyrinths – Grease
800
Gears and bearings – Inspect and repair
150,000
Main reduction gearbox high-speed coupling: High-speed coupling – Inspect coupling pins
5000 to 10,000
Main reduction gearbox low-speed coupling: Low-speed coupling – Perform routine greasing
2000
Low-speed coupling – Change grease
2 years
Low-speed coupling – Inspect coupling grid members
2 years
Auxiliary Reduction Gearbox: Gearbox oil – Change oil for first time
500
Gearbox oil – Check oil level
500
Labyrinths – Grease routinely
800
Gearbox oil – Sample and test oil
4000
Gearbox oil – Change oil
4000 to 8000
Breather – Change
6 months
Bearings – Check vibration and temperature
Monthly
Gears and bearings – Inspect
25,000
Auxiliary reduction gearbox low-speed coupling: Low-speed coupling – Inspect coupling pins
6 months
Electrohydraulic brake: Brake pads – Inspect
100 operations
Brake pads – Change
As required
Fluid drive coupling: Coupling bolts – Inspect
6 months
Coupling oil – Change oil
8000 or Annual
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19-72
Preventive Maintenance Table 19-25 (continued) Preventive Maintenance Tasks (Courtesy of Majuba Power Station) Tasks
Frequency (Operating Hours)
Electromagnetic brake: Friction disk – Check wear
Monthly
Mill main motor: Lubricating oil system – Change oil
10,000
Lubricating oil system – Sample and test oil
10,000
Motor bearings – Check vibration and temperature
Monthly
Desiccator – Check for dampness
Annual
Fixing bolts – Check tightness
Annual
Electrical connections – Check tightness
Annual
Main bearing oil lubrication system: Oil – Change oil
12,000
Oil – Sample and test oil
12,000
Duplex oil filter – Clean
As required
Oil cooler – Clean
During outage
Fire protection system: Carbon dioxide bottles – Check pressure
Weekly
Carbon dioxide bottles – Change
As required
Bursting disks – Change
As required
Ball feed mechanism: Slide gate – Grease spindle
500
Mill air system: Air system dampers – Inspect
Monthly
Gland – Check for leaks
Monthly
Cables – Check for damage
Monthly
Mountings – Check for security
Monthly
Ducting – Check for leaks
Monthly
Quick-close damper – ball and socket – Change grease
3 months
Quick-close damper – hydraulic actuator – Sample and test oil
Annual
Quick-close damper – hydraulic actuator – Change oil
2 years
Quick-close damper – hydraulic actuator – Check oil seals
2 years
Classifier – Inspect liners
During outage
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1973
Preventive Maintenance
Table 19-25 (continued) Preventive Maintenance Tasks (Courtesy of Majuba Power Station) Tasks
Frequency (Operating Hours)
Main seal air sub-system: Fan bearings – Check vibration and temperature
Monthly
Fan bearings – Inspect
3 months
Fan bearings – Grease routinely
2000
Fan bearings – Change grease
10,000
Sub-system flow test points – Check flow
Monthly
Fan coupling guard – Inspect
3 months
Fan ducting – Inspect
3 months
Fan shaft seal – Inspect
3 months
Fan casing – Inspect
6 months
Fan shaft – Inspect
6 months
Fan coupling alignment – Inspect
6 months
Fan coupling tire – Inspect
6 months
Fan plummer blocks – Inspect
6 months
19.4.2 Inspection Tasks At the Matimba Power Station, there are several types of inspections and intervals for mill inspections. The first type is the 5500 operating hour inspection based on equipment experience and failure investigations. The interim repair occurs every 18 months. There is also a partial overhaul at four and a half years. The last inspection is the general overhaul that is scheduled for a nine-year interval.
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19-74
Preventive Maintenance
Table 19-26 shows the inspection tasks for the 5500 operating hour interval. Table 19-26 Inspection Tasks for the 5500 Operating Hour Interval (Courtesy of Matimba Power Station) System or Area for Inspection General
Inspection Task Check the damper positions. Inspect the splitter boxes.
Classifier
Inspect the classifier. Verify the classifier vane angle settings.
Feeder
Inspect the volumetric feeders. Verify the feeder speed versus actual belt speed.
Girth gear
Check girth gear pattern with the stroboscope. Check the oil spray pattern for the girth gear. Clean the mill driven train. Disassemble the high and low speed couplings. Inspect the girth gear grease system.
Lubrication systems
Inspect the main gearbox lubrication oil system. Inspect the main motor lubrication system – Hytec. Inspect the main bearing lubrication system – Turbolub.
Mill internals
Inspect the mill liner profiles. Verify the ball filling degree versus the power readings. Inspect the screw conveyor bearing and stuffing box. Inspect the mill screw conveyor on both ends of the mill. Inspect the pipe thermocouple protection rod.
Seal air system
Inspect the cold and hot seal air fan couplings and bearings.
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1975
Preventive Maintenance
Table 19-27 lists the tasks for the interim inspection and repairs every 18 months. Table 19-27 Inspection Tasks for the 18-Month Interim Inspection (Courtesy of Matimba Power Station) System or Area for Inspection General
Inspection Task Perform a pre-shutdown inspection. Perform a mill barring function test. Inspect the mill ball loading gates inspection and repair. Perform a general mill cleaning. Perform a general mill inspection. Open and close all mill doors. Install and inspect mill mix box door.
Classifier
Inspect, clean, and repair the classifier. Refurbish raw coal chute. Inspect epoxy wear protection on classifier pipe. Inspect the mixing box. Inspect and repair the wear on the top cover.
Dampers
Inspect and service the cold air regulation damper. Inspect and service the hot air regulation damper. Inspect and service the isolating damper. Test the primary air and bypass dampers.
Feeder
Inspect the bunker slide gates and liners.
Gearbox
Perform main gearbox internal inspection and oil system service.
Girth gear
Inspect the girth gear and pinion condition.
Mill internals
Perform a mill internal inspection. Inspect the mill screw conveyor on both ends of the mill.
Seal air system
Inspect the seal air ducts.
Table 19-28 lists the tasks for the general overhaul of the Stein Industrie ball mills. These tasks are in addition to the 18-months and 5500 operating hour inspection tasks. The general overhaul is scheduled for a nine-year interval.
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19-76
Preventive Maintenance Table 19-28 Inspection Tasks for the Nine-Year General Overhaul (Courtesy of Matimba Power Station) System or Area for Inspection
Inspection Task
Classifier
Adjust vanes and perform thickness measurements.
Dampers
Inspect the primary air isolation damper gearbox. Inspect and service the primary regulating damper. Inspect and service the bypass regulation damper. Inspect and repair the purge air isolating damper.
Feeder
Perform volumetric feeder major service.
Girth gear
Perform girth gear and pinion inspection and repairs.
Lubrication systems
Drain and clean the main bearing lubrication system – Turbolub
Mill internals
Perform mill internal inspection.
19.4.3 Pinion and Girth Gear Replacement Table 19-29 lists the tasks for the replacement of the pinion and girth gear on the Alstom Stein Industrie mills at the Arnot Power Station. Table 19-29 Replacement Tasks for the Pinion and Girth Gear (Courtesy of the Arnot Power Station) Tasks 1. Remove the soundhood panel top section and pinion section. 2. Remove the girth gear seal air fan pipe and girth gear guard top section and pinion section. 3. Clean all flange bolts on the girth gear and loose all the bolts. 4. Clean all the girth gear mounting bolts on station 1/10 and 5/6. Loosen and remove the mounting bolts 5. Position the first section of the girth gear. Install lifting and rigging equipment. Remove all flange bolts. Lift the girth gear. 6. Rotate and clean the first section of the girth gear mounting face and the flange face station 1 to 5 and 6 to 10. 7. Erect the first girth gear section onto the mill and install the flange bolts. 8. Rotate the mill 180° and remove the second section of the girth gear.
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1977
Preventive Maintenance
Table 19-29 (continued) Replacement Tasks for the Pinion and Girth Gear (Courtesy of the Arnot Power Station) Tasks 9. Rotate and clean the second section of the girth gear mounting face and the flange face station 1 to 5 and 6 to 10. 10. Erect the girth gear second section onto the mill and install the flange bolts. 11. Tighten the girth gear mounting bolts on station 1/10 and 5/6. 12. Perform runout checks on the girth gear alignment. Tighten all flange bolts. 13. Remove the pinion from the base plate and clean the base plate. 14. Install the new pinion and check the backlash and contact with the girth gear. 15. Install the pinion guard, girth gear guard, and the seal air pipe. 16. Install the soundhood panel at the pinion side and girth gear top section. 17. Align the low speed side of the reduction gearbox to the pinion coupling. 18. Align the main motor to the reduction gearbox high-speed side. 19. Align the auxiliary reduction gearbox to the main motor coupling. 20. Align the auxiliary motor to the auxiliary reduction gearbox coupling. 21. Grease and close all couplings. Install safety guards. 22. Clean area around the mill and drive train.
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19-78
20 PREVENTIVE MAINTENANCE BASIS
It is the intent of this section to establish a PM Basis document for the ball/tube mills.
20.1 Background Many power plants are in the process of reducing PM costs and improving equipment performance by matching PM tasks with the functional importance of the equipment. For this to succeed, utilities require information on the most appropriate tasks and task intervals for the important equipment types, while accounting for the influences of functional importance, duty cycle and service conditions. An early approach to optimizing the PM activities was the use of reliability centered maintenance (RCM). RCM was developed in the 1960s by the commercial airline industry to apply reliability concepts to maintenance and the design of maintenance programs. The RCM approach to preventing equipment failure is to perform maintenance tasks that are specifically aimed at preventing component failure mechanisms from occurring. Many nuclear power plants used the RCM process to improve their PM programs. In 1991, the Nuclear Regulatory Commission issued 10 CFR 50.65, Requirements for Monitoring the Effectiveness of Maintenance at Nuclear Power Plants, also called the Maintenance Rule. In brief, the Maintenance Rule required the nuclear power plants to develop a reliability and availability monitoring program for the systems, structures, and components considered to be within the scope of the rule. The monitoring part of the rule included determining the effectiveness of the maintenance performed on the components. In addition, the Maintenance Rule required the utility to evaluate industry operating experience and to use that experience when modifying the maintenance program. When maintenance practices have been changed, the most common action is to modify the PM tasks for the components. Initially, PM tasks were assigned based on vendor recommendations and plant experience. In modifying or optimizing the PM tasks, one vital piece of information was missing - the time to failure for the components. Because the time to failure was not known, it was difficult to justify the PM task intervals. Also missing was the understanding of the factors that influence the progression of the degradation mechanisms for the component.
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20-1
Preventive Maintenance Basis
As a result of the need to comply with the Maintenance Rule and to optimize the PM tasks for more effective maintenance, the PM Basis project was proposed by EPRI. The PM Basis objective was to: •
Provide a summary of industry experience that the PM tasks and task intervals were based on
•
Establish the relationship between the degradation mechanism, the progression of the mechanisms to failure, and the opportunities available to discover the failure mechanisms before component failure occurred
During the 1996-1998 timeframe, 39 PM Basis documents were developed for major components in the nuclear power plants. The components included various style valves, switchgear, motor control centers, motors, pumps, compressors, HVAC (heating, ventilation and air conditioning) components, inverters, batteries, relays, heat exchangers, turbines, transformers, and instrumentation and control (I&C) components. The PM Basis documents can be found in Preventive Maintenance Basis Volume 1–38 (EPRI report TR-106857) [17]. Currently, there are over 73 component types in an electronic PM database. The database can be accessed by logging onto www.epri.com, searching for Preventive Maintenance Database Version 5.1.1 (EPRI report 1010919) [18]. The product can be downloaded from www.epri.com; however, it is best to order the CD from the EPRI Orders and Conferences Center at 1-800-3133774 (press 2). Although the fossil power plants do not have the same regulatory requirements as the nuclear power plants, the establishment of the PM Basis for critical components provides valuable information for the optimization of the maintenance program. The next step in the development of this PM Basis document is an additional review with plant personnel. The PM tasks are further analyzed for criticality, condition monitoring options, and other parameters before being added to the EPRI PM database. The information used in the development of the PM Basis was gathered from the manufacturer, industry literature, and input from utility maintenance personnel. The following is a description of the tables generated by the PM Basis document.
20.2 Failure Locations, Degradation Mechanisms, and PM Strategies The information for the mills can be contained in several tables. Table 20-1 lists the failure locations of the pulverizer. The first table (Table 20-1) contains the: •
Failure locations – A list of the most common components
•
Degradation mechanisms – The cause of the component failing at the specified failure location
• Degradation influence – Aspects of the environment, plant operations, maintenance, or design that cause the initiation of degradation processes or can affect how rapid the
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20-2
Preventive Maintenance Basis
degradation progresses
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Preventive Maintenance Basis
• Degradation progression – Whether the degradation progress is present most of the time (continuous) or whether it would not normally be present but might exist or initiate in a haphazard (random) way • Failure timing – The relevant time period that the component would be free from failure • Discovery opportunity – Reasonable, cost-effective opportunities for detecting the failure mechanism • PM strategy – The choice of PM tasks that can discover the failure mechanism
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Preventive Maintenance Basis
Table 20-1 Failure Locations, Degradation Mechanisms, and PM Strategies for Ball/Tube Mills Failure Location
Degradation Mechanism
Degradation Influence
Degradation Progression
Failure Timing
Discovery Opportunity
PM Strategy
Grinding balls
Cracking
Coal level too low
Random
Random
Operator checks
Operator checks
Chain drive
Broken sprocket
Lack of lubrication
Random
Random
Visual inspection, Operator checks
Visual inspection, Operator checks
Classifier – Scroll type, if present
Wear
Normal use
Continuous
Based on amount of coal processed
Visual inspection, Fineness Test
Visual inspection, Performance test
Foreign Material
Random
Random
Visual inspection
Visual inspection
Blade wear
Normal use
Continuous
Based on amount of coal processed
Visual inspection, Fineness Test
Visual inspection, Performance test
Cone wear
Normal use
Continuous
Based on amount of coal processed
Visual inspection, Fineness test
Visual inspection, Performance test
Cone impact damage
Foreign Material
Random
Random
Visual inspection
Visual inspection
Stuck open or closed
Foreign Material
Random
Random
Visual inspection, Operator checks
Visual inspection, Operator checks
Will not close fully
Wear
Continuous
Continuous
Visual inspection
Visual inspection
Slipping
Wear
Continuous
Continuous
Visual inspection
Visual inspection
Insufficient air
Random
Random
Operator checks
Operator checks
Broken swing hammers
Wear
Continuous
Continuous
Visual inspection
Visual inspection
Failed bearings
Contamination
Random
Random
Vibration analysis, Visual inspection
Vibration analysis, Visual inspection
Classifier – Adjustable or dynamic, if present
Classifier trickle box, if present
Clutch
Crusher-dryer, if present
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Preventive Maintenance Basis Table 20-1 (continued) Failure Locations, Degradation Mechanisms, and PM Strategies for Ball/Tube Mills Failure Location Airflow dampers
Drum/trunnion bearings
Drum liners
Exhauster, if present
Degradation Mechanism Stuck
Degradation Influence
Degradation Progression
Failure Timing
Discovery Opportunity
PM Strategy
Wear
Continuous
Continuous
Operator checks, Visual inspection
Operator checks, Visual inspection
Controls failure
Random
Random
Calibration, Operator checks
Calibration, Operator checks
Improper lubrication type
Random
Random
Oil analysis, Operator checks
Oil analysis, Operator checks
Insufficient lubricant
Random
Random
Operator checks
Operator checks
Oil cooling system clogged
Random
Random
Operator checks
Operator checks
High vibration
Wear
Continuous
Continuous
Vibration analysis, Oil analysis
Vibration analysis, Oil analysis
Thinning
Wear
Continuous
Continuous
Visual inspection
Visual inspection
Broken wedge bolts
Wear
Continuous
Continuous
Visual inspection
Visual inspection
Foreign material
Random
Random
Visual inspection
Visual inspection
Bearing wear
Age
Continuous
Months to years
Vibration analysis, Visual inspection
Vibration analysis, Visual inspection
Lack of lubrication
Random
Random
Check oil level, Vibration analysis
Operations check, Vibration analysis
Lubrication contamination
Random
Random
Oil analysis, Seal Air System check
Oil analysis, Operations check
Vibration
Continuous
Random
Vibration analysis, Oil analysis
Vibration analysis, Oil analysis
High temperature
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20-5
Preventive Maintenance Basis Bearing housing wear
Fan vibration
Continuous
Continuous
Vibration analysis, Visual inspection
Vibration analysis, Visual inspection
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Preventive Maintenance Basis Table 20-1 (continued) Failure Locations, Degradation Mechanisms, and PM Strategies for Ball/Tube Mills Failure Location
Degradation Mechanism
Exhauster, if present continued
Fan blade wear
Degradation Influence
Degradation Progression
Failure Timing
Discovery Opportunity
PM Strategy
Normal use
Continuous
Months to years
Visual inspection, Vibration analysis, Fan Tests
Visual inspection, Vibration analysis, Performance test
Foreign Material
Random
Random
Fineness Test, Visual inspection, Vibration analysis
Performance test, Visual inspection, Vibration analysis
Fan housing wear
Normal use
Continuous
Random
Visual inspection
Visual inspection
Foreign material
Random
Random
Visual inspection
Visual inspection
Loose or broken fan shaft
Coal erosion, fatigue
Random
Random
Visual inspection, Vibration analysis
Visual inspection, Vibration analysis
Leaking seals
Age
Continuous
Continuous
Operations check, Visual inspection
Operations check, Visual inspection
Feeder Bearings
Failed bearings
Contamination
Random
Random
Oil analysis
Oil analysis
Feeder - Belt table
Incorrect coal flow
Wear
Continuous
Continuous
Calibration, Visual inspection
Calibration, Visual inspection
Feeder
Obstruction
Foreign material
Random
Random
Calibration, Visual inspection
Calibration, Visual inspection
Wear
Normal use
Continuous
Years
Calibration, Visual inspection
Calibration, Visual inspection
Belt wear, if applicable
Age
Continuous
Years
Calibration, Visual inspection
Calibration, Visual inspection
Inaccuracy
Controls failure
Random
Random
Calibration
Calibration
Feeder Gearbox drive
Feeder controls
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Preventive Maintenance Basis
Table 20-1 (continued) Failure Locations, Degradation Mechanisms, and PM Strategies for Ball/Tube Mills Failure Location Girth gear and pinion
Inlet/outlet box
Mill motor
Speed reducer gearbox
Degradation Mechanism
Degradation Influence
Degradation Progression
Failure Timing
Discovery Opportunity
PM Strategy
Misalignment
Wear
Continuous
Years
Vibration analysis, Visual inspection
Vibration analysis, Visual inspection
Gear damage
Lack of lubrication
Random
Random
Vibration analysis, Operator checks
Vibration analysis, Operator checks
Contamination
Random
Random
Vibration analysis, Oil analysis
Vibration analysis, Oil analysis
Ribbon wear
Age
Continuous
Continuous
Visual inspection
Visual inspection
Broken ribbon
Foreign material
Random
Random
Visual inspection
Visual inspection
High temperature
Insufficient bearing lubrication
Random
Random
Operator checks
Operator checks
Lubrication contamination
Random
Random
Oil analysis, Visual inspection
Oil analysis, Visual inspection
Oil cooler not functioning
Random
Random
Operator checks
Operator checks
Faulty thermocouple
Random
Random
Operator checks, Calibration
Operator checks, Calibration
Low voltage
Random
Random
Operator checks
Operator checks
High load
Random
Random
Operator checks
Operator checks
Improper lubrication type
Random
Random
Oil analysis, Operator checks
Oil analysis, Operator checks
Insufficient lubricant
Random
Random
Operator checks
Operator checks
Overheating
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20-8
Preventive Maintenance Basis Oil cooling system clogged
Random
Random
Operator checks
Operator checks
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Preventive Maintenance Basis
Table 20-1 (continued) Failure Locations, Degradation Mechanisms, and PM Strategies for Ball/Tube Mills Failure Location Speed reducer gearbox continued
Degradation Mechanism Failed bearings
High vibration
Pitted and galled gears
Degradation Influence
Degradation Progression
Failure Timing
Discovery Opportunity
PM Strategy
Wear
Continuous
Continuous
Vibration analysis, Visual inspection
Vibration analysis, Visual inspection
Insufficient lubricant
Random
Random
Operator checks
Operator checks
Lubrication contamination
Random
Random
Oil analysis, Visual inspection
Oil analysis, Visual inspection
Loose foundation bolts
Random
Random
Operator checks, Vibration analysis
Operator checks, Vibration analysis
Worn gears
Continuous
Continuous
Visual inspection
Visual inspection
Loose parts
Random
Random
Vibration analysis, Visual inspection
Vibration analysis, Visual inspection
Incorrect lubrication
Random
Random
Oil analysis
Oil analysis
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Preventive Maintenance Basis
20.3 PM Tasks and Their Degradation Mechanisms Table 20-2 contains the PM tasks and intervals. The PM tasks and the degradation mechanisms are listed from the previous table. The corresponding PM task interval is then given for each applicable PM task.
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Preventive Maintenance Basis
Table 20-2 PM Tasks and Their Degradation Mechanisms for Ball/Tube Mills Component
Location/Degradation
PM Task and Interval Calibration
Oil Analysis
Operator Checks
Performance Testing
Vibration Analysis
Visual Inspection
1–2 years
6 months – 1 year
Daily
Monthly
3–6 months
6 months – 1 year
Grinding balls
Cracking
X
Chain drive
Broken sprocket
Classifier - Scroll type, if present
Wear/normal use
Classifier - Adjustable or dynamic, if present
Blade wear
X
X
Cone wear
X
X
X
X X
X
Wear/foreign material
X
Cone impact damage
X
Classifier trickle box, if present
Stuck open or closed
X
Will not close fully
X
Clutch
Slipping/wear
X
Slipping/insufficient air Crusher-dryer, if present
Broken swing hammers
Airflow dampers
Stuck/wear
Drum/trunnion bearings
X X
Failed bearings
Stuck/controls failure High temperature/improper lubrication
X
X X X
X X
X X
X
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Preventive Maintenance Basis High temperature/insufficient lubricant
X
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Preventive Maintenance Basis
Table 20-2 (continued) PM Tasks and Their Degradation Mechanisms for Ball/Tube Mills Component
Drum/trunnion bearings - continued
Location/ Degradation
Exhauster, if present
Calibration
Oil Analysis
Operator Checks
Performance Testing
Vibration Analysis
Visual Inspection
1–2 years
6 months – 1 year
Daily
Monthly
3–6 months
6 months – 1 year
High temperature/oil cooling system clogged High vibration
Drum liners
PM Task and Interval
X X
X
Thinning
X
Broken wedge bolts/wear
X
Broken wedge bolts/foreign material
X
Bearing wear
X
Bearing wear/lack of lubrication
X
Bearing wear/ lubrication contamination
X
Bearing wear/vibration
X
X
X
X X
Bearing housing wear
X
X
Fan blade wear/normal
X
X
X
Fan blade wear/foreign material
X
X
X
Fan housing wear/normal use
X
Fan housing wear/foreign material
X
Loose or broken fan shaft
X
X
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Preventive Maintenance Basis Table 20-2 (continued) PM Tasks and Their Degradation Mechanisms for Ball/Tube Mills Component
Location/ Degradation
PM Task and Interval Calibration
Oil Analysis
Operator Checks
Performance Testing
Vibration Analysis
Visual Inspection
1–2 years
6 months – 1 year
Daily
Monthly
3–6 months
6 months – 1 year
Feeder Gearbox drive
Leaking seals
X
Feeder - Bearings
Failed bearings
Feeder - Belt, if applicable
Incorrect coal flow
X
X
Obstruction
X
X
Wear
X
X
Belt wear, if applicable
X
X
Feeder controls
Inaccuracy
X
Girth gear and pinion
Misalignment
X
X
Gear damage Gear damage Inlet/outlet box
Mill motor
X
X X
X
X X
Ribbon wear
X
Broken ribbon
X
High temperature/insufficient bearing lubricant High temperature/lubrication contamination
X
X
X
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Preventive Maintenance Basis
Table 20-2 (continued) PM Tasks and Their Degradation Mechanisms for Ball/Tube Mills Component
Mill motor (continued)
Location/ Degradation Calibratio n
Oil Analysis
Operator Checks
Performance Testing
Vibration Analysis
Visual Inspection
1–2 years
6 months – 1 year
Daily
Monthly
3–6 months
6 months – 1 year
X
X
High temperature/oil cooler not functioning High temperature/faulty thermocouple
Speed reducer gearbox
PM Task and Interval
X X
X
High temperature/low voltage
X
High temperature/high load
X
Overheating/improper lubrication type
X
X
Overheating/insufficient lubricant
X
Overheating/oil cooling system clogged
X
Failed bearings/wear Failed bearings/insufficient lubricant Failed bearings/lubrication contamination
X X
High vibration/loose foundation bolts
X X
X
High vibration/worn gears
X
High vibration/loose parts Pitted and galled gears
X
X
X
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Preventive Maintenance Basis
20.4 PM Template The following describes the information contained in the PM template shown in Table 20-3. The PM template summarizes the program of tasks and task intervals for the equipment type. Each plant should base their program on the manufacturer 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 1–8 in the PM template list the 8 sets of conditions for the ball/tube mills. Each column corresponds to the combined choices of critical or non-critical 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 1–8. The definitions of template application conditions are: •
Critical Yes – Functionally important, that is, risk significant, required for power production, safety related, 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 operation 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 OEM specifications, normal environmental conditions.
PM tasks – The PM tasks provide a cost-effective way to intercept the causes and mechanisms that lead to degradation and failure. The PM tasks can be used to develop a complete PM program or to improve an existing program. These tasks are intended to complement and not to
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Pre
replace the PM recommendations given by the manufacturer. A brief description of the PM tasks follows the PM Template.
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Table 20-3 PM Template for the Ball/Tube Mills Conditions
1
2
3
4
Critical: Yes
X
X
X
X
No Duty Cycle: High
X
Low
X X
Service Condition: Severe
X
6
7
8
X
X
X
X
X X
X
Mild
5
X X
X
PM Tasks
X X
X
X
X
X
Frequency Interval
Calibration
1–2 years
1–2 years
1–2 years
1–2 years
2 years
2 years
2 years
2 years
Oil analysis
6 months
6 months
6 months
6 months
1 year
1 year
1 year
1 year
Daily
Daily
Daily
Daily
Daily
Daily
Daily
Daily
Monthly
Monthly
Monthly
Monthly
2 months
2 months
2 months
2 months
Vibration analysis
3 months
3 months
3 months
3 months
6 months
6 months
6 months
6 months
Visual inspection
3000 hrs
3000 hrs
3000 hrs
3000 hrs
1 year
1 year
1 year
1 year
Operator checks Performance testing
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The following is a brief description of PM tasks used in the PM Template. •
Calibration – Calibration of pulverizer equipment includes the setting and verification of instruments and components. Instruments include thermocouples, RTDs (Resistance Temperature Detectors), pitot tubes, pressure gauges, and so on. The calibration of components includes setting the clearances/tolerances for the mill inlet/outlet boxes, classifier blades, trickle valves, feeders, and so on. The frequency for calibration can be time based or initiated based on equipment condition. For example, if mill fineness testing indicates a drop in performance, then adjusting the classifier blades should be performed. New on-line monitoring techniques may trigger an instrument that needs calibration.
•
Oil analysis – Oil analysis is very valuable predictive maintenance technology in detecting problems in equipment before failure occurs. For the ball/tube mills, oil samples should be taken and analyzed for the speed reducer gearbox, mill motor bearings, trunnion/drum bearings, chain drive oil reservoir, and so on. Samples should be analyzed for water and particle contamination and oil properties. The results of the oil analysis can alert personnel that bearings are failing and plans can be made to monitor the operation of the equipment, take more frequent samples, or shut the equipment down.
•
Operations check – Operations check includes an external visual inspection of the mills, listening for noises, smelling for smoke, checking temperatures, checking pressures, and so on.
• Performance testing – Performance testing includes fineness testing, airflow tests, fan tests, and so on. Fineness testing is especially important with the mills to determine the efficiency or effectiveness of the mill. •
Vibration analysis – Vibration analysis on rotating equipment is very valuable in detecting bearing problems before the bearings fail. Vibration analysis is recommended for the speed reducer gearbox, trunnion/drum bearings, mill motor bearings, and so on.
• Visual inspection – A visual inspection is an internal inspection of mill components to determine their condition. Components to be inspected include classifier blades, classifier cone, inlet/outlet box ribbon conveyor, trickle valve, shut-off valves, drum liner and wedge bolts, speed reducer gearbox, girth and pinion gears, feeders, crusher-dryers, and so on. Inspections include taking physical measurements, ultrasonic thickness checks, nondestructive testing, general condition assessment, wear areas, foreign material/debris, and so on.
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21 REFERENCES 1. Guidelines for Evaluating the Impact of Powder River Basin (PRB) Coal Blends on Power Plant Performance and Emissions. EPRI, Palo Alto, CA: 1996. TR-106340. 2. Foster Wheeler Ball Mill Upgrades and Features, July 2003, http://parts.fwc.com/ballmills. 3. David H. Scott, “Coal Pulverizers – Performance and Safety,” International Energy Agency Coal Research, London, England. June 1995. 4. Pulverizer and Fuel Delivery Guidelines. EPRI, Palo Alto, CA: 2004. 1009490. 5. Allegheny Power, Armstrong Station Unit #1 Pulverized Coal System (description for training purposes). 6. J. Garcia-Mallol, K. McCarthy, J. Fernandez, P. Ventin, P. Yague, and J. Martinez, “HardCoal Burnup Increase with Adjustable Static Classifier for Ball Mill,” Power-Gen International Conference, Orlando, FL, November 14–16, 2000. 7. “Riley Power Service Guide Steam Generating Equipment for Hoosier Energy Rural Electric Cooperative, Inc., Merom Station Units 1 and 2,” Merom, IN, 1981. 8. Q. Lin and C. Peterson, “Coal Pulverizer Design Upgrades to Meet the Demands of Low NOx Burners,” Electric Power 2004, Baltimore, MD, March 30–April 1, 2004. 9. Internet web site http://www.babcockpower.com 10. Component Failure and Repair Data for Coal-Fired Power Units. EPRI, Palo Alto, CA: 1981. AP-2071. 11. Evaluation of Coal Pulverizer Materials. EPRI, Palo Alto, CA: 1988. CS-5935. 12. On-Line Predictive Condition Monitoring System for Coal Pulverizers. EPRI, Palo Alto, CA: 2003. 1004902. 13. Electric Motor Predictive Maintenance Program. EPRI, Palo Alto, CA: 1999. TR-108773-V2. 14. Predictive Maintenance Guide Primer Revision. EPRI, Palo Alto, CA: 2003. 1007350. 15. Lubrication Guide: Revision 3. EPRI, Palo Alto, CA: 2001. 1003085. 16. Lana Robin, “Improving the Life and Capabilities of Lubricants,” Maintenance Technology, May 1999. http://www.pdma.com. 17. Preventive Maintenance Basis, Volumes 1–38. EPRI, Palo Alto, CA: 1998. TR-106857. 18. Preventive Maintenance Basis Database, Version 5.1.1. EPRI, Palo Alto, CA: 2004. 1010919.
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Key Points
Human Performance Key Point Denotes information that requires personnel action or consideration in order to prevent personal injury, equipment damage, and/or improve the efficiency and effectiveness of the task. Section
Page
Key Point
11.5
11-8
The NFPA identifies two areas of protection for ball mills. The two areas are preventing an explosion (inerting) and extinguishing a fire.
12.2
12-2
Fuel flow of the mill feeding the lower burner elevation should not drop below 45%. Fuel oil should be added on the bottom mill in service if the fuel flow drops below 45% and is not stable.
12.3
12-3
If a blocked coal pipe is discovered, every effort should be made to correct this defect as soon as possible. Continued mill operation with a blocked coal pipe is not allowed for more than 14 days.
19.0
19-1
There are three areas of work that are critical to performing maintenance on the mills. The areas are temporary lighting, lifting and rigging practices, and temporary scaffolding.
O&M Cost Key Point Emphasizes information that will result in overall reduced costs and/or an increase in revenue through additional or restored energy production. Section
Page
Key Point
3.4
3-11
The increases in LOI from NOx combustion control the increase the heat rate. The average industry loss is 12 Btu/kWh per 1% change in unburned carbon. This increase in LOI creates a need for greater fineness. Some units have increased fineness from 70% passing through a 200-mesh screen to 75–80% passing through a 200-mesh screen and 99–99.5% passing through a 50-mesh screen. The increase in fineness settings requires more work from the pulverizer.
6.2
6-9
It is less expensive and more time efficient to purchase the complete drum assembly instead of having the end castings and drum shell plate assembled on site.
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A KEY POINTS
Technical Key Point
Targets information that will lead to improved equipment reliability. Section
Page
Key Point
4.0
4-2
If the diameter of the drum is greater than the length of the drum, then the mill is called a ball mill. If the length of the drum is greater than the diameter of the drum, then the mill is called a tube mill. The terms ball and tube are used interchangeably in this guide.
5.1
5-5
The bypass dampers accomplish three functions: • Assist in maintaining the minimum pulverized coal and air velocity in the piping • Assist in increasing the flow of drying, hot air during periods when the coal is wet • Allow the mill to operate at lower loads
5.2
5-7
As the liner contour wears, the grinding medium begins to slip. This slippage leads to a lowering of efficiency in the grinding action because the grinding medium cannot be raised to a high angle. As slippage continues, the wear will accelerate. Replacement of liners is recommended once the minimum thickness is reached. The rate of wear of the liners can be predicted based on the grade of coal being ground and the experience gained from inspections.
6.0
6-3
There is no tramp iron removal system. The mill is designed to grind the raw coal and foreign matter together. Considerable amounts of pyrites will accelerate the wear rates of the grinding media.
9.2.5
9-12
Barring is performed when the mill needs to be cooled down. Inching of the mill is performed during maintenance periods and allows the precise positioning of the mill drum inspection door.
11.1
11-2
The ideal proportion of the weight of the balls to the weight of the coal is between six and seven. This means that for every 330 lbs of coal in the mill, there should be 2205 lbs of balls in order to effectively pulverize the coal. If the proportion of coal in the mixture is too low, the balls will strike each other and grind against the cylinder. If the proportion of coal in the mixture is too high, the coal will tend to flow into the feed scrolls.
13.4
13-10
The mill system’s rating and bypass dampers control the fuel flow responding to the demand signal from the combustion controls. The mill bypass dampers operate in a fixed relationship to the mill rating damper over the entire demand signal range of the combustion control system.
18.2
18-3
Lubricant testing is recommended for several reasons. These include: •
To study the condition (wear) of the machine being lubricated. If there is a problem with the lubricant, there is a strong possibility that the machine will need maintenance.
•
To determine if the lubricant is meeting the specifications
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Key Points
19.3
19-37
Excessive accumulation of dust deposits within clutch elements can eventually restrict shoe retraction during clutch disengagement, resulting in dragging and overheating. Protective shielding or total enclosure is recommended to keep the dust out, and if it is not provided, the clutches must be inspected frequently (every three months) to ensure normal shoe release.
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B TRANSLATED TABLE OF CONTENTS
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, (IV) THAT ANY TRANSLATION FROM THE ENGLISH-LANGUAGE ORIGINAL OF THIS DOCUMENT IS WITHOUT ERROR; 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. THE TRANSLATION OF THIS DOCUMENT FROM THE ENGLISH-LANGUAGE ORIGINAL HAS BEEN PREPARED WITH LIMITED BUDGETARY RESOURCES BY OR ON BEHALF OF EPRI. IT IS PROVIDED FOR REFERENCE PURPOSES ONLY AND EPRI DISCLAIMS ALL RESPONSIBILITY FOR ITS ACCURACY. THE ENGLISH-LANGUAGE ORIGINAL SHOULD BE CONSULTED TO CROSS-CHECK TERMS AND STATEMENTS IN THE TRANSLATION. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Electric Power Research Institute (EPRI)
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Translated Table of Contents
レポートの概要
目的 •
ボール/チューブミルのための予防および予知保全方法を明らかにすること
•
ボール/チューブミル装置問題を明らかにして解決して、プラント保全の人員を助け ること
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目次
1はじめに.................................................................................................................................. 1-1 1.1背景 ................................................................................................................................. 1-1 1.2アプローチ ...................................................................................................................... 1-2 1.3構成 ................................................................................................................................. 1-3 1.4キーポイント................................................................................................................... 1-4 2用語 ......................................................................................................................................... 2-1 3システム.................................................................................................................................. 3-1 3.1石炭の取扱いシステム .................................................................................................... 3-1 3.2石炭の特性 ...................................................................................................................... 3-5 3.3石炭の微粉化ミル............................................................................................................ 3-7 3.4環境の規則 ...................................................................................................................... 3-9 4技術的な記述–一般 ................................................................................................................. 4-1 5技術的な記述– ALLIS-CHALMERS ...................................................................................... 5-1 5.1入口/出口ボックス........................................................................................................... 5-4 5.2回転シェル、ライナーおよびボール ............................................................................... 5-7 5.3トラニオンベアリング .................................................................................................... 5-9 5.4分類機 ........................................................................................................................... 5-15
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5.5駆動モーター ................................................................................................................ 5-17
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5.6変速機ユニット ............................................................................................................. 5-23 5.7ガースギヤおよびピニオンシャフト ............................................................................. 5-29 5.8パワーソニックミル調節システム ................................................................................ 5-33 5.9技術仕様のリスト.......................................................................................................... 5-35 6技術的な記述–FOSTER-WHEELER....................................................................................... 6-1 6.1コンベヤーアセンブリ .................................................................................................... 6-4 6.1.1コンベヤーサポートアセンブリ .............................................................................. 6-6 6.2ドラムアセンブリ............................................................................................................ 6-8 6.2.1ダブルウェイブライナー ....................................................................................... 6-10 6.2.2ダブルサイズ、ダブルウェイブアクセスドア....................................................... 6-12 6.2.3フライトバー ......................................................................................................... 6-12 6.2.4粉砕ボール............................................................................................................. 6-12 6.3コンベヤーの軸受およびシール .................................................................................... 6-13 6.4トラニオンの主要なベアリングおよびダストシール..................................................... 6-14 6.5伝動装置 ........................................................................................................................ 6-16 6.5.1ピニオンベアリング .............................................................................................. 6-17 6.6トラニオンの管 ............................................................................................................. 6-18 6.7分類機 ........................................................................................................................... 6-20 6.7.1分類機のリジェクトダンパー ............................................................................... 6-20 6.7.2調節可能な刃の分類機........................................................................................... 6-21 6.7.3 Mタイプ分類機 ..................................................................................................... 6-23 6.7.4ダイナミック分類機 .............................................................................................. 6-24 6.8排気装置 ........................................................................................................................ 6-27
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6.9潤滑油システム ............................................................................................................. 6-28 6.9.1 Cardwellの潤滑油システム ................................................................................... 6-28 6.9.2 Farvalの潤滑油システム ....................................................................................... 6-30 7技術的な記述–KENNEDY VAN SAUN .................................................................................. 7-1 8技術的な記述–RILEY POWER株式会社............................... .................... .............................. 8-1 8.1概説 ................................................................................................................................. 8-1 8.2システムコンポーネント ................................................................................................. 8-4 8.2.1送り装置 .................................................................................................................. 8-5 8.2.2粉砕機ドライヤー ................................................................................................... 8-6 8.2.3回転ドラムまたはバレル ......................................................................................... 8-7 8.2.4粉砕ボール............................................................................................................... 8-8 8.2.5分類機 ...................................................................................................................... 8-9 8.2.6遮断弁 .................................................................................................................... 8-10 8.2.7減速変速機............................................................................................................. 8-10 8.2.8
クラッチ ............................................................................................................ 8-12
8.3システム ........................................................................................................................ 8-15 8.3.1一次空気システム .................................................................................................. 8-15 8.3.2シール空気システム .............................................................................................. 8-16 8.4改造 ............................................................................................................................... 8-17 8.4.1逆流抑制（トリクル）弁の付加 ............................................................................ 8-17 8.4.2トラニオンの空気シールの設計変更 ..................................................................... 8-18 8.4.3ミル調節システムアップグレード......................................................................... 8-19 8.4.4流体力学のスライドシューベアリング変更 .......................................................... 8-19
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9技術的な記述–STEIN INDUSTRIE ........................................................................................ 9-1 9.1概説 ................................................................................................................................. 9-1 9.2システム .......................................................................................................................... 9-4 9.2.1石炭入口システム .................................................................................................... 9-5 9.2.2一次空気システム .................................................................................................... 9-5 9.2.3シール空気システム ................................................................................................ 9-6 9.2.4潤滑油システム ....................................................................................................... 9-7 9.2.5 駆動システム ....................................................................................................... 9-12 9.2.6排出システム ......................................................................................................... 9-13 9.2.7ボールのローディングシステム ............................................................................ 9-13 10運転および安全– ALLIS-CHALMERS ............................................................................... 10-1 10.1運転 ............................................................................................................................. 10-1 10.2防火 ............................................................................................................................. 10-2 11運転および安全–FOSTER-WHEELER ............................................................................... 11-1 11.1運転の概要 .................................................................................................................. 11-1 11.2運転指示値 .................................................................................................................. 11-3 11.3起動プロシージャ........................................................................................................ 11-4 11.4火検出システム ........................................................................................................... 11-6 11.5防火 ............................................................................................................................. 11-7 12運転および安全–KENNEDY VAN SAUN .......................................................................... 12-1 12.1ロード変更 .................................................................................................................. 12-1 12.2燃料サポート............................................................................................................... 12-2 12.3ブロックされた燃料管 ................................................................................................ 12-3
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12.4湿った石炭 .................................................................................................................. 12-4 12.5ミルストリッピング .................................................................................................... 12-5 12.6冷態起動 ...................................................................................................................... 12-6 13運転および安全–RILY POWER株式会社............................................................................. 13-1 13.1運転の概要 .................................................................................................................. 13-1 13.2制御システム............................................................................................................... 13-2 13.3ボール供給取扱い........................................................................................................ 13-7 13.4一次空気のダンパー運転............................................................................................. 13-8 13.5シール空気システム .................................................................................................. 13-11 13.6送り装置のキャリブレーション ................................................................................ 13-12 13.7計器の設定 ................................................................................................................ 13-13 13.8火災探知 .................................................................................................................... 13-15 13.9防火 ........................................................................................................................... 13-17 14運転および安全–STEIN INDUSTRIE ................................................................................. 14-1 14.1 長期停止時の対策 ...................................................................................................... 14-1 14.2防火 ............................................................................................................................. 14-2 15パフォーマンス ................................................................................................................... 15-1 15.1微粉の細かさ............................................................................................................... 15-1 15.2 粉砕性 ........................................................................................................................ 15-2 15.3湿気 ............................................................................................................................. 15-2 15.4容量 ............................................................................................................................. 15-3 16故障モード .......................................................................................................................... 16-1 16.1摩耗 ............................................................................................................................. 16-1
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16.2腐食 ............................................................................................................................. 16-3
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16.3機器の故障 .................................................................................................................. 16-4 17トラブルシューティング .................................................................................................... 17-1 17.1 Allis-Chalmers ........................................................................................................... 17-1 17.2 FOSTER WHEELER .................................................................................................. 17-5 17.3KENNEDY VAN SAUN ............................................................................................... 17-7 17.4RILEY POWER株式会社 .............................................................................................. 17-9 17.5 STEIN Industrie ....................................................................................................... 17-10 18予知保全.............................................................................................................................. 18-1 18.1振動分析 ...................................................................................................................... 18-3 18.2油分析 ......................................................................................................................... 18-3 18.3状態監視保全–KENNEDY VAN SAUN ..................................................................... 18-12 18.4状態監視保全–STEIN Industrie ................................................................................ 18-15 19予防保全.............................................................................................................................. 19-1 19.1 Allis-Chalmers ........................................................................................................... 19-1 19.1.1点検基準 .............................................................................................................. 19-2 19.1.2ミルの外部点検 ................................................................................................... 19-3 19.1.3ミルの内部点検 ................................................................................................... 19-6 19.1.4分類機の点検 ....................................................................................................... 19-7 19.1.5駆動機構のトレインの点検 ................................................................................. 19-9 19.1.6シェルおよびトラニオンライナー..................................................................... 19-10 19.1.7トラニオンベアリングインサート取替 ............................................................. 19-11 19.1.8ガースギヤ取替 ................................................................................................. 19-13 19.1.9変速機の改造 ..................................................................................................... 19-18 19.1.10様々な装置....................................................................................................... 19-22
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19.1.11装置の潤滑油のリスト..................................................................................... 19-22 19.2KENNEDY VAN SAUN ............................................................................................. 19-24 19.2.1ピニオン、ガースギヤおよび潤滑油システム................................................... 19-25 19.2.2ワームギヤの逆転 .............................................................................................. 19-25 19.3RILEY POWER株式会社 ....................................................................................... 19-27 19.3.1送り装置 ............................................................................................................ 19-27 19.3.2粉砕機ドライヤー ............................................................................................. 19-28 19.3.3入口/出口ボックスおよび空気シール ................................................................ 19-31 19.3.4ミルライナー ..................................................................................................... 19-31 19.3.5ボール供給......................................................................................................... 19-32 19.3.6減速変速機......................................................................................................... 19-33 19.3.7トレイン駆動 ...................................................................................................... 19-38 19.3.8ドライブシャフト .............................................................................................. 19-44 19.3.9潤滑油の熱交換器 .............................................................................................. 19-46 19.3.10分類機 .............................................................................................................. 19-47 19.3.11一次空気ファンおよびダクト .......................................................................... 19-48 19.3.12石炭の遮断弁 ................................................................................................... 19-49 19.3.13潤滑スケジュール ............................................................................................ 19-49 19.3.14予備品 .............................................................................................................. 19-50 19.3.15予防保全点検 ................................................................................................... 19-51 19.4STEIN Industrie ........................................................................................................ 19-51 19.4.1予防保全タスク .................................................................................................. 19-53 19.4.2点検タスク.......................................................................................................... 19-58 19.4.3ピニオンおよびガースギヤ取替 ......................................................................... 19-61
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20予防保全の基礎 ................................................................................................................... 20-1 20.1背景 ............................................................................................................................. 20-1 20.2故障の位置、劣化メカニズムおよびPM戦略 .............................................................. 20-2 20.3 PMタスクおよび劣化メカニズム ............................................................................... 20-9 20.4 PMテンプレート ...................................................................................................... 20-14 21参照 ..................................................................................................................................... 21-1 Aキーポイント ......................................................................................................................... A-1
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図のリスト 図3-1荷降ろしからプラントまでの典型的な石炭取扱い系統図 ............................................... 3-2 図3-2プラントからユニットバンカまでの典型的な石炭取扱いの系統図 ................................. 3-3 図3-3 FOSTER WHELLERチューブミル.................................................................................. 3-8 図3-4燃料に含まれる窒素の窒素化合物への変化 ................................................................... 3-10 図5-1 Allis-Chalmersのボール/チューブミルの概要 ................................................................. 5-2 図5-2入口/出口ボックス ........................................................................................................... 5-4 図5-3ミルのシーリング配置 ..................................................................................................... 5-6 図5-4 ボール供給のホッパー ................................................................................................... 5-8 図5-5トラニオンベアリングの低圧潤滑システム ................................................................... 5-10 図5-6手動高圧ジャッキングポンプ ........................................................................................ 5-13 図5-7 静的な分類機................................................................................................................. 5-15 図5-8ミル駆動機構モーター .................................................................................................. 5-17 図5-9バーリングギヤユニット................................................................................................ 5-19 図5-10バーリングカップリング.............................................................................................. 5-21 図5-11バーリングブレーキ ..................................................................................................... 5-22 図5-12変速機ユニット ............................................................................................................ 5-23 図5-13変速機出力カップリング.............................................................................................. 5-25 図5-14変速機潤滑油システム ................................................................................................. 5-26 図5-15ギヤ歯の潤滑油 ............................................................................................................ 5-27 図5-16ミルの概要 ................................................................................................................... 5-30 図5-17ガースギヤ潤滑油システム .......................................................................................... 5-31 図5-18パワーソニックミルの調節システム............................................................................ 5-33
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図6-1FOSTER WHEELチューブミルの図 ................................................................................ 6-1 図6-2空気/石炭の流れ図 ........................................................................................................... 6-2 図6-3フライトリボンのコンベヤーアセンブリ ........................................................................ 6-5 図6-4フライトリボンのばねサポートアセンブリ ..................................................................... 6-6 図6-5コンベヤーサポートアセンブリ ....................................................................................... 6-6 図6-6 8スポーク設計のコンベヤーの取り外し ......................................................................... 6-8 図6-7完全なドラムアセンブリ.................................................................................................. 6-9 図6-8端の鋳造部品.................................................................................................................. 6-10 図6-9ダブルウェイブライナー................................................................................................ 6-11 図6-10ダブルサイズ、ダブルウェイブアクセスドア ............................................................. 6-12 図6-11ボールの摩耗率 ............................................................................................................ 6-13 図6-12コンベヤーベアリング改造 .......................................................................................... 6-13 図6-13オリジナルベアリングアセンブリ ............................................................................... 6-14 図6-14新設計のベアリング ..................................................................................................... 6-15 図6-15トラニオンのシール ..................................................................................................... 6-16 図6-16ピニオンおよびブル・ギヤアセンブリ ........................................................................ 6-17 図6-17ローラ軸受 ................................................................................................................... 6-18 図6-18分類機のトラニオンの管.............................................................................................. 6-19 図6-19オリジナルのスクロールタイプ分類機 ........................................................................ 6-20 図6-20分類機のリジェクトダンパー ..................................................................................... 6-21 図6-21調節可能な分類機 ........................................................................................................ 6-22 図6-22分類機の比較................................................................................................................ 6-22 図6-23調節可能な分類機の適合性（フィットネス)の改善 ..................................................... 6-23 図6-24Mタイプ分類機............................................................................................................. 6-24 図6-25ダイナミック分類機 ..................................................................................................... 6-25 図6-26排気装置の図表 ............................................................................................................ 6-27 図6-27排気装置のスパイダー ................................................................................................. 6-28
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図6-28 Cardwellの潤滑油システム ......................................................................................... 6-29
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図6-29 Farvalの潤滑油システム ............................................................................................. 6-30 図7-1KENNEDY VAN SAUN Millシステム................................................................................ 7-2 図7-2KENNEDY VAN SAUN Millの構成部................................................................................ 7-3 図8-1RILEY POWERのチェーン駆動のボール/チューブミルシステム.................................... 8-2 図8-2RILEY POWERのボール/チューブミルシステム ............................................................. 8-3 図8-3ドラム型送り装置 ............................................................................................................ 8-5 図8-4 粉砕機ドライヤー .......................................................................................................... 8-6 図8-5ピニオン/リングギヤ駆動機構セット付きのミル.............................................................
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図8-6分類機 .............................................................................................................................. 8-9 図8-7減速変速機 ..................................................................................................................... 8-11 図8-8クラッチアセンブリ ....................................................................................................... 8-13 図8-9 ロータシール................................................................................................................. 8-15 図8-10クラッチ空気制御システム .......................................................................................... 8-15 図8-11新逆流抑制（トリクル)弁デザイン .............................................................................. 8-18 図8-12トラニオンの空気シール設計 ...................................................................................... 8-19 図8-13流体力学のスライドシューベアリング ........................................................................ 8-20 図8-14スラスト・ベアリング ................................................................................................. 8-21 図9-1STEIN Industrieのチューブミル....................................................................................... 9-2 図9-2 STEIN Industrieのチューブミルの詳細鳥瞰図 ................................................................ 9-3 図9-3ミルの空気流れ ................................................................................................................ 9-6 図9-4高圧、低圧の潤滑油システム .......................................................................................... 9-9 図10-1移動式ガス容器ユニット.............................................................................................. 10-3 図10-2恒設二酸化炭素の防火システム ................................................................................... 10-4 図11-1火災探知センサーヘッド.............................................................................................. 11-7 図12-1湿り石炭運転................................................................................................................ 12-4 図13-1RILEY POWER運転の図表 .......................................................................................... 13-4 図13-2モーター出力対生産性 ................................................................................................. 13-5
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図13-3プロダクト制御によるミルパラメータ ........................................................................ 13-5
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図13-4ボール供給減少の効果付き .......................................................................................... 13-6 図13-5水スプレーシステムノズルの位置 ............................................................................. 13-18 図16-1 微粉化装置の機器故障頻度 ......................................................................................... 16-1 図18-1微粉化装置の故障に対する早期警報............................................................................ 18-2 図19-1リンクベルト単一ギヤ減速機 .................................................................................... 19-34 図19-2リンクベルトダブルギヤ減速機 ................................................................................. 19-35 図19-3リンクベルト三重ギヤ減速機 .................................................................................... 19-36
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テーブルのリスト テーブル1-1換算率.................................................................................................................... 1-2 テーブル5-1パワーソニックミルの調節システムのアラーム条件..........................................
5-34
テーブル5-2 Allis-Chalmersのミルのデータ ........................................................................... 5-35 テーブル9-1正常な潤滑油の油圧値 ........................................................................................ 9-11 テーブル9-2ミルの潤滑油システム ........................................................................................ 9-12 テーブル10-1ボールの取替タスク .......................................................................................... 10-2 テーブル11-1ダブルエンドのミルの一方を停止 .................................................................... 11-2 テーブル11-2通常運転時の点検.............................................................................................. 11-4 テーブル11-3起動のための最初のミルの準備 ........................................................................ 11-4 テーブル11-4起動のためのミルチャージング ........................................................................ 11-5 テーブル11-5ミルの運転開始 ................................................................................................. 11-6 テーブル11-6 FOSTER WHEELERミルでの消火プロシージャ ............................................. 11-8 テーブル11-7 FOSTER WHEELERミルの緊急停止プロシージャ ......................................... 11-9 テーブル11-8石炭の入ったFOSTER WHEELERミルを停止する為の推薦手順...................... 11-9 テーブル12-1ミルストリップのタスク ................................................................................... 12-5 テーブル12-2冷態起動タスク ................................................................................................. 12-6 テーブル13-1給炭機のキャリブレーションタスク ............................................................... 13-12 テーブル13-2ミルの計器パラメータ .................................................................................... 13-13 テーブル13-3典型的な装置パラメータ ................................................................................. 13-14 テーブル13-4火災探知の温度センサの位置.......................................................................... 13-16 テーブル15-1標準スクリーン寸法 .......................................................................................... 15-1 テーブル16-1研摩の摩耗係数 ................................................................................................. 16-3
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テーブル16-2 Allis-Chalmers故障部品 .................................................................................... 16-4 テーブル17-1 Allis-Chalmersミルのトラブルシューテ-ィング............................................... 17-1 テーブル17-2 FOSTER WHEELERミルのトラブルシューテ-ィングの図 ............................. 17-5 テーブル17-3 KENNEDY VAN SAUN Millsのトラブルシューテ-ィングの図 ......................... 17-7 テーブル17-4 RILEY POWERのチューブミルのトラブルシューテ-ィングの図 .................... 17-9 テーブル17-5STEIN Industrieミルのトラブルシューテ-ィング............................................ 17-10 テーブル18-1粒子カウントのレンジ番号 ............................................................................... 18-5 テーブル18-2油の追加パッケージのエレメント .................................................................... 18-9 テーブル18-3潤滑油プログラムのためのKENNEDY VAN SAUN Mill構成 ........................... 18-13 テーブル18-4 Majuba発電所の状態監視の値........................................................................ 18-16 テーブル18-5 Majuba発電所で状態監視されたパラメータ条件 ........................................... 18-19 テーブル18-6 Majuba発電所の状態監視用の計器 ................................................................ 18-20 テーブル18-7 Majuba発電所の状態監視スケジュール ......................................................... 18-24 テーブル19-1外部ミルの点検タスク ...................................................................................... 19-4 テーブル19-2内部点検タスク ................................................................................................. 19-6 テーブル19-3分類機の点検タスク .......................................................................................... 19-8 テーブル19-4ミル駆動機構トレインの点検タスク ................................................................. 19-9 テーブル19-5シェルおよびトラニオンライナー取替タスク ................................................ 19-10 テーブル19-6トラニオンベアリングインサート取替タスク ................................................ 19-12 テーブル19-7ガースギヤ取替タスク .................................................................................... 19-14 テーブル19-8変速機の改造タスク ........................................................................................ 19-19 テーブル19-9装置の潤滑油リスト ........................................................................................ 19-23 テーブル19-10ピニオン、ガースギヤおよび潤滑油システムのための点検タスク .............. 19-25 テーブル19-11変速機のワームギヤの逆転タスク ................................................................ 19-26 テーブル19-12摩耗ライナーの取替 ...................................................................................... 19-32 テーブル19-13減速変速機油容量.......................................................................................... 19-33 テーブル19-14三重速度減速変速機の分解タスク ................................................................ 19-37
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テーブル19-15クラッチの点検タスク .................................................................................. 19-40
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テーブル19-16チェーン修理タスク ...................................................................................... 19-41 テーブル19-17運転されたスプロケットの取り外しおよび再設置タスク............................. 19-42 テーブル19-18ドライブシャフト取替タスク........................................................................ 19-44 テーブル19-19熱交換器の点検タスク .................................................................................. 19-46 テーブル19-20潤滑油の熱交換器の取替タスク .................................................................... 19-47 テーブル19-21分類機予防保全タスク .................................................................................. 19-48 テーブル19-22潤滑油スケジュール ...................................................................................... 19-49 テーブル19-23RILEY POWERの推薦予備品 ........................................................................ 19-50 テーブル19-24送り装置の7000作動時間間隔点検タスク ..................................................... 19-53 テーブル19-25予防保全タスク ............................................................................................. 19-54 テーブル19-26 5500作動時間間隔の点検タスク .................................................................. 19-59 テーブル19-27 18ヶ月の暫時点検タスク ............................................................................. 19-60 テーブル19-28 9年の汎用分解検査のための点検タスク ...................................................... 19-61 テーブル19-29ピニオンおよびガースギヤの取替タスク...................................................... 19-61 テーブル20-1ボール/チューブミルのため故障位置、劣化メカニズムおよびPM戦略20-4 テーブル20-2ボール/チューブミルのためのPMタスク及び劣化メカニズム ........................ 20-10 テーブル20-3ボール/チューブミルのためのPMテンプレート.............................................. 20-15
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RESUMEN DEL REPORTE Objetivos • Para determinar las prácticas de mantenimiento preventivas y proféticas para los molinos de bola/tubo •
Para asistir al personal de mantenimiento de la planta en la identificación y la resolución de problemas de equipos de molinos de bola/tubo
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CONTENIDOS
1 INTRODUCCIÓN .................................................................................................................. 1-1 1.1 Antecedentes ................................................................................................................. 1-1 1.2 Aproximación ................................................................................................................. 1-2 1.3 Organización ................................................................................................................. 1-3 1.4 Puntos Dominantes ....................................................................................................... 1-4 2 GLOSARIO ............................................................................................................................ 2-1 3 USO DE SISTEMAS ............................................................................................................ 3-1 3.1 Sistema de Manejo del Carbón ..................................................................................... 3-1 3.2 Características del Carbón ............................................................................................ 3-5 3.3 Molinos de Pulverizador de Carbón .............................................................................. 3-7 3.4 Reglas Ambientales ...................................................................................................... 3-9 4 DESCRIPCIÓN TÉCNICA- GENERAL ................................................................................ 4-1 5 DESCRIPCIÓN TÉCNICA-ALLIS CHALMERS
................................................................. 5-1
5.1 Cajas de Entrada/Salida ................................................................................................ 5-4 5.2 Granada, Forros, y Bolas Rotadoras ............................................................................. 5-7 5.3 Cojinetes del Muñón ...................................................................................................... 5-9 5.4 Clasificador ................................................................................................................. 5-15 5.5 Motor Impulsador ........................................................................................................ 5-17 5.6 Unidad de la Caja de Engranajes ................................................................................ 5-23 5.7 Engranaje de la Circunferencia y Árbol del Piñón ....................................................... 5-29 5.8 Sistema de Condicionamiento del Molino Poder-Sonic .............................................. 5-33 5.9 Lista de Especificación técnica ................................................................................... 5-35 6 DESCRIPCIÓN TECNICA DE RODADOR FOSTER
......................................................... 6-1
6.1 Asamblea del Transportador ......................................................................................... 6-4
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6.1.1 Asamblea de Apoyo del Transportador ................................................................. 6-6 6.2 Asamblea del Tambor ................................................................................................... 6-8 6.2.1 Forros de Doble-Agite .......................................................................................... 6-10 6.2.2 Puertas de Acceso Doble-Talla, Doble-Agita ....................................................... 6-12 6.2.3 Barras del Vuelo .................................................................................................. 6-12 6.2.4 Bolas que Esmerilan ............................................................................................ 6-12 6.3 Cojinete y Sello del Árbol del Transportador ............................................................... 6-13 6.4 Cojinete del Muñón y Sellos Principales del Polvo ..................................................... 6-14 6.5 Engranaje .................................................................................................................... 6-16 6.5.1 Cojinetes del Piñón .............................................................................................. 6-17 6.6 Tubo del Muñón .......................................................................................................... 6-18 6.7 Clasificador ................................................................................................................. 6-20 6.7.1 Amortiguador de Rechazo del Clasificador ......................................................... 6-20 6.7.2 Clasificador Ajustable de la Pala ......................................................................... 6-21 6.7.3 Clasificador Tipo-M............................................................................................... 6-23 6.7.4 Clasificador Dinámico .......................................................................................... 6-24 6.8 Ventiladores ................................................................................................................ 6-27 6.9 Sistemas Lubricantes .................................................................................................. 6-28 6.9.1 Sistema Lubricante de Cardwell .......................................................................... 6-28 6.9.2 Sistema lubricante de Farval ............................................................................... 6-30 7 DESCRIPCION TECNICA DE KENNEDY VAN SAUN ........................................................ 7-1 8 DESCRIPCIÓN TECNICA - RILEY POWER INC.................................................................. 8-1 8.1 Descripción General ...................................................................................................... 8-1 8.2 Componentes del Sistema ............................................................................................ 8-4 8.2.1 Alimentador ........................................................................................................... 8-5 8.2.2 Trituradora-Secante ............................................................................................... 8-6 8.2.3 Tambor o Barril Rotante ........................................................................................ 8-7 8.2.4 Bolas que Esmerilan .............................................................................................. 8-8 8.2.5 Clasificador ............................................................................................................ 8-9 8.2.6 Válvulas de Cierre ............................................................................................... 8-10 8.2.7 Caja de Engranajes que Reducen la Velocidad .................................................. 8-10 8.2.8 Embrague ............................................................................................................ 8-12
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8.3 Sistemas ..................................................................................................................... 8-15 8.3.1 Sistema de Aire Primario ..................................................................................... 8-15 8.3.2 Sistema de Sello de Aire ..................................................................................... 8-16 8.4 Modificaciones ............................................................................................................. 8-17 8.4.1 Adición de la Válvula del Chorrito ........................................................................ 8-17 8.4.2 Reajuste del Sello de Aire del Muñón ................................................................. 8-18 8.4.3 Mejora de Condicionamiento del Sistema del Molino .......................................... 8-19 8.4.4 Conversión hidrodinámica del Cojinete del Zapato de la Placa .......................... 8-19 9 DESCRIPCIÓN TECNICA- STEIN INDUSTRIES ................................................................. 9-1 9.1 Descripción General ...................................................................................................... 9-1 9.2 Sistemas ....................................................................................................................... 9-4 9.2.1 Sistema de la Entrada del Carbón ......................................................................... 9-5 9.2.2 Sistema de Aire Primario ....................................................................................... 9-5 9.2.3 Sistema del Sello de Aire ...................................................................................... 9-6 9.2.4 Sistemas de Aceite de Lubricación ....................................................................... 9-7 9.2.5 Sistema de Impulsión .......................................................................................... 9-12 9.2.6 Sistema de la Purga ............................................................................................ 9-13 9.2.7 Sistema del Cargamento de la Bola .................................................................... 9-13 10 OPERACIÓN Y SEGURIDAD– ALLIS-CHALMERS ....................................................... 10-1 10.1 Operaciones .............................................................................................................. 10-1 10.2 Protección Contra los Incendios ................................................................................ 10-2 11 OPERACIÓN Y SEGURIDAD-RODADOR FOSTER ....................................................... 11-1 11.1 Operación General .................................................................................................... 11-1 11.2 Indicaciones de la Operación .................................................................................... 11-3 11.3 Procedimientos de Lanzamiento ............................................................................... 11-4 11.4 Sistema de Detección de Fuego ............................................................................... 11-6 11.5 Protección Contra los Incendios ................................................................................ 11-7 12 OPERACIÓN Y SEGURIDAD– KENNEDY VAN SAUN .................................................. 12-1 12.1 Cambios de Carga .................................................................................................... 12-1 12.2 Apoyo de Gasolina y Aceite ...................................................................................... 12-2 12.3 Tubo de combustible bloqueado ................................................................................ 12-3
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12.4 Carbón Mojado .......................................................................................................... 12-4
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12.5 El Estropeado del Molino .......................................................................................... 12-5 12.6 Lanzamiento río ......................................................................................................... 12-6 13 OPERACIÓN Y SEGURIDAD – RILEY POWER INC…………………. ............................ 13-1 13.1 Operación general ..................................................................................................... 13-1 13.2 Sistema de Mando .................................................................................................... 13-2 13.3 Manejo de Carga de la Bola ...................................................................................... 13-7 13.4 Operación Primaria del Amortiguador del Aire .......................................................... 13-8 13.5 Sistema de Sello de Aire .......................................................................................... 13-11 13.6 Calibración del Alimentador .................................................................................... 13-12 13.7 Fijaciones de la Instrumentación ............................................................................. 13-13 13.8 Detección de Fuego ................................................................................................ 13-15 13.9 Protección Contra los Incendios .............................................................................. 13-17 14 OPERACON Y SEGURIDAD– STEIN INDUSTRIES ........................................................ 14-1 14.1 Periodo de Parada .................................................................................................... 14-1 14.2 Protección Contra los Incendios ................................................................................ 14-2 15 FUNCIONAMIENTO ......................................................................................................... 15-1 15.1 Fineza ....................................................................................................................... 15-1 15.2 Habilidad de Esmerilar .............................................................................................. 15-2 15.3 Humedad ................................................................................................................... 15-2 15.4 Capacidad ................................................................................................................. 15-3 16 MODOS DE FALLO ......................................................................................................... 16-1 16.1 Excoriación ................................................................................................................ 16-1 16.2 Erosión ...................................................................................................................... 16-3 16.3 Componentes fallados .............................................................................................. 16-4 17 LOCALIZANDOR DE AVERÍAS ...................................................................................... 17-1 17.1 Allis-Chalmers ........................................................................................................... 17-1 17.2 Rodador Foster ......................................................................................................... 17-5 17.3 Kennedy Van Saun ................................................................................................... 17-7 17.4 Power Riley Inc........................................................................................................... 17-9 17.5 Industrias Stein ........................................................................................................ 17-10
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18 MANTENIMIENTO PROFÉTICO....................................................................................... 18-1 18.1 Análisis de la Vibración ............................................................................................. 18-3 18.2 Análisis del Aceite ..................................................................................................... 18-3 18.3 Acondicionado Basado en Mantenimiento Kennedy Van Saun ............................... 18-12 18.4 Acondicionado Basado en Mantenimiento Stein Industrie ....................................... 18-15 19 MANTENIMIENTO PREVENTIVO .................................................................................... 19-1 19.1 Allis-Chalmers ........................................................................................................... 19-1 19.1.1 Consideraciones de Inspección ......................................................................... 19-2 19.1.2 Inspección Externa del Molino ........................................................................... 19-3 19.1.3 Inspección Interna del Molino ............................................................................ 19-6 19.1.4 Inspección del Clasificador ................................................................................ 19-7 19.1.5 Inspección del Tren de Expulsión....................................................................... 19-9 19.1.6 Forro de la Granada y del Muñón .................................................................... 19-10 19.1.7 Repuesto de la Pieza Inserta en el Cojinete del Muñón .................................. 19-11 19.1.8 Repuesto del Engranaje de la Circunferencia ................................................. 19-13 19.1.9 Reconstrucción de la Caja de Engranajes ...................................................... 19-18 19.1.10 Equipo Misceláneo ........................................................................................ 19-22 19.1.11 Lista del Equipo de Lubricación ..................................................................... 19-22 19.2 Kennedy Van Saun ................................................................................................. 19-24 19.2.1 Piñón, Engranaje de la Circunferencia, y Sistema Lubricante ........................ 19-25 19.2.2 Invirtiendo el Engranaje del Tornillo Sin Fin .................................................... 19-25 19.3 Riley Power Inc......................................................................................................... 19-27 19.3.1 Alimentador ..................................................................................................... 19-27 19.3.2 Trituradora-Secante ......................................................................................... 19-28 19.3.3 Entrada/Sellos de la Caja y del Aire de la Salida ............................................ 19-31 19.3.4 Forros del Molino ............................................................................................. 19-31 19.3.5 Carga de la Bola .............................................................................................. 19-32 19.3.6 Caja de Engranajes Reductora de Velocidad .................................................. 19-33 19.3.7Tren de Impulso................................................................................................. 19-38 19.3.8 Eje del Motor ................................................................................................... 19-44 19.3.9 Cambiador de Calor en la Lubricación ............................................................ 19-46 19.3.10 Clasificador .................................................................................................... 19-47
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19.3.11 Ventilador y Canalización Primaria del Aire .................................................. 19-48
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19.3.12 Válvulas de Cierre del Carbón ....................................................................... 19-49 19.3.13 Horario de la Lubricación ............................................................................... 19-49 19.3.14 Piezas de Repuesto ...................................................................................... 19-50 19.3.15 Inspecciones del Mantenimiento Preventivo del Ejemplo ............................. 19-51 19.4 Industrias Stein ........................................................................................................ 19-51 19.4.1 Tareas del mantenimiento preventivo ............................................................. 19-53 19.4.2 Tareas de la Inspección .................................................................................. 19-58 19.4.3 Repuesto del piñón y del Engranaje de la Circunferencia ............................... 19-61 20 BASES DEL MANTENIMIENTO PREVENTIVO ............................................................... 20-1 20.1 Fondo ........................................................................................................................ 20-1 20.2 Situaciones de la Falla, Mecanismos de la Degradación, y Estrategias del P.M. ..... 20-2 20.3 Tareas del P.M. y sus Mecanismos de la Degradación ............................................ 20-9 20.4 Patrón del P.M. ........................................................................................................ 20-14 21 REFERENCIAS ................................................................................................................ 21-1 A PUNTOS DOMINANTES .................................................................................................... A-1
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LISTA DE FIGURAS Figura 3-1 Un Diagrama Típico del Sistema de Tramitación del Carbón de Descargar a la Instalación .................................................................................................................... 3-2 Figura 3-2 Un Diagrama Típico del Sistema de Tramitación del Carbón de la Instalación a las Casamatas de la Unidad ......................................................................................... 3-3 Figura 3-3 Fomenta el Molino de la Bola del Rodador ............................................................ 3-8 Figura 3-4 Combustible Limita la Evolución del Nitrógeno a NOx ......................................... 3-10 Figura 5-1 Allis-Chalmers/el Esquema del Molino del Tubo .................................................... 5-2 Figura 5-2Caja de Salida/Entrada ............................................................................................ 5-4 Figura 5-3 Colocación del Sello en el Molino........................................................................... 5-6 Figura 5-4 Tolva de la Carga de la Bola ................................................................................... 5-8 Figura 5-5 El Muñón que Soporta el Sistema Lubricante de Baja Presión ........................... 5-10 Figura 5-6 Bomba Manual que Alza Con el Gato de Alta Presión .......................................... 5-13 Figura 5-7 Clasificador Estático .............................................................................................. 5-15 Figura 5-8 Motor Impulsor del Molino .................................................................................... 5-17 Figura 5-9 Salvo Unidad del Engranaje ................................................................................. 5-19 Figura 5-10 Salvo el Acoplamiento ........................................................................................ 5-21 Figura 5-11 Salvo Freno ........................................................................................................ 5-22 Figura 5-12 Unidad de la Caja de Engranajes ...................................................................... 5-23 Figura 5-13 Acoplamiento del Rendimiento de la Caja de Engranajes .................................. 5-25 Figura 5-14 Sistema de Lubricación de la Caja de Engranajes .............................................. 5-26 Figura 5-15 Lubricación de los Dientes del Engranaje ........................................................... 5-27 Figura 5-16 Escape del Molino ............................................................................................... 5-30 Figura 5-17 Sistema Lubricante del Engranaje de la Circunferencia...................................... 5-31 Figura 5-18 Sistema de Condicionamiento del Molino Poder-Sonic....................................... 5-33 Figura 6-1 Diagrama del Molino de la Bola del Rodador ......................................................... 6-1 Figura 6-2 Organigrama del Aire/Carbón.................................................................................. 6-2 Figura 6-3 asamblea del transportador de la cinta del vuelo de .............................................. 6-5 Figura 6-4 Asambleas de Apoyo del Muelle de la Cinta del Vuelo……………………………….6-6 Figura 6-5 Asamblea de Apoyo del Transportador ................................................................... 6-6 Figura 6-6 Retiro del Transportador con Ocho-Habló Diseño ................................................. 6-8 Figura 6-7 Termina la Asamblea del Tambor .......................................................................... 6-9
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Figura 6-8 Fin de la Fundición .............................................................................................. 6-10
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Figura 6-9 Figuras de Doble Agite………………………………………………………………….6-11 Figura 6-10 Puerta de Acceso Doble-Talla, Doble-Agita ...................................................... 6-12 Figura 6-11 Regímenes del Desgaste de la Bola ................................................................... 6-13 Figura 6-12 Modificación del Cojinete del Transportador ....................................................... 6-13 Figura 6-13 Conjunto de Cojinetes Original ............................................................................ 6-14 Figura 6-14 Nuevo Diseño de Cojinete ................................................................................... 6-15 Figura 6-15 Sello del Muñón ................................................................................................... 6-16 Figura 6-16 Asamblea del Piñón y Asamblea del Engranaje de Bull ..................................... 6-17 Figura 6-17 Cojinetes de Rodillo............................................................................................. 6-18 Figura 6-18 Tubo Clasificador del Muñón ............................................................................... 6-19 Figura 6-19 Clasificador Original Tipo Desfile ....................................................................... 6-20 Figura 6-20 Amortiguadores del Rechazo del Clasificador..................................................... 6-21 Figura 6-21 Clasificador Ajustable .......................................................................................... 6-22 Figura 6-22 Comparación del Clasificador.............................................................................. 6-22 Figura 6-23 Mejoría Ajustable de la Fineza del Clasificador.................................................. 6-23 Figura 6-24 Clasificador Tipo M .............................................................................................. 6-24 Figura 6-25 Clasificador Dinámico .......................................................................................... 6-25 Figura 6-26 Diagrama del Ventilador ...................................................................................... 6-27 Figura 6-27 Ventilador Araña .................................................................................................. 6-28 Figura 6-28 Sistema Lubricante Cardwell .............................................................................. 6-29 Figura 6-29 Sistema Lubricante Farval .................................................................................. 6-30 Figura 7-1 Sistema de Molino Kennedy Van Saun .................................................................. 7-2 Figura 7-2 Componentes del Molino Kennedy Van Saun ......................................................... 7-3 Figura 8-1 Bolas de Poder Riley/Sistema Chain-Driven del Molino del Tubo……...……………8-2 Figura 8-2 Bolas de Poder Riley/Sistema del Molino del Tubo……………………………………8-3 Figura 8-3 Alimentador Tipo Tambor ........................................................................................ 8-5 Figura 8-4 Trituradora-Secante................................................................................................. 8-6 Figura 8-5 Molino con la Impulsión del Engranaje del piñón/Anillo Fijado ............................... 8-8 Figura 8-6 Clasificador .............................................................................................................. 8-9 Figura 8-7 Caja de Engranajes Reductor de Velocidad.......................................................... 8-11 Figura 8-8 Asamblea de Embrague ........................................................................................ 8-13 Figura 8-9 Sello del Rotor ...................................................................................................... 8-15 Figura 8-10 Sistema de Mando del Aire del Embrague .......................................................... 8-15 Figura 8-11 Nuevo Diseño de la Válvula del Chorrito ............................................................. 8-18 Figura 8-12 Diseños del Sello del Aire del Muñón .................................................................. 8-19 Figura 8-13 Cojinete Hidrodinámico del Zapato de la Placa................................................... 8-20 Figura 8-14 Cojinete de Tracción............................................................................................ 8-21 Figura 9-1 Molino del Tubo de Stein Industrie .......................................................................... 9-2
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Figura 9-2 Opinión Detallada del Molino del Tubo de Stein Industrie ...................................... 9-3
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Figura 9-3 Circulación de Aire del Molino ................................................................................. 9-6 Figura 9-4 Sistema de Aceite Lubricador de Alta y Baja Presión ............................................. 9-9 Figura 10-1 Unidad Movible de la Botella del Gas.................................................................. 10-3 Figura 10-2 Sistema de Protección Contra el Fuego por Dióxido de Carbono Permanentemente Instalado ........................................................................................... 10-4 Figura 11-1 Sensor de Temprana Detección de Fuego.......................................................... 11-7 Figura 12-1 Operación de Carbón Mojado ............................................................................. 12-4 Figura 13-1 Diagrama de Operación de Riley Power ............................................................. 13-4 Figura 3-12 Poder del Motor Comparado con Carga del Producto ....................................... 13-5 Figura 13-3 Parámetros del Molino Usando Mando de la Carga del Producto ...................... 13-5 Figura 13-4 Efectos de la Carga Reducida de la Bola ........................................................... 13-6 Figura 13-5 Localización de la Boquilla del Sistema de Aerosol de Agua ............................ 13-18 Figura 16-1 Componente de Frecuencia de Falla del Pulverizador ...................................... 16-1 Figura 18-1 Detección Temprana de Fallas en el Pulverizador ............................................. 18-2 Figura 19-1 Reductor de Único-Engranaje del Eslabón-Cinturón......................................... 19-34 Figura 19-2 Reductor de Doble-Engranaje del Eslabón-Cinturón ........................................ 19-35 Figura 19-3 Reductor de Triple-Engranaje del Eslabón-Cinturón ....................................... 19-36
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LISTA DE TABLAS Tabla 1-1 Factores de Conversión............................................................................................ 1-2 Tabla 5-1 Condiciones de la Alarma para el Sistema de Condicionamiento del Molino a Poder-Sonic ................................................................................................................... 5-34 Tabla 5-2 Datos de los Molinos de Allis-Chalmers ................................................................ 5-35 Tabla 9-1 Valores Normales de la Presión del aceite de Lubricación .................................... 9-11 Tabla 9-2 Sistemas de Lubricantes del Molino ....................................................................... 9-12 Tabla 10-1 Tareas del Reemplazo de Bola ............................................................................ 10-2 Tabla 11-1 Cerrando un Lado de un Molino de Doble-Extremo ............................................ 11-2 Tabla 11-2 Operación Normal Verifica ................................................................................... 11-4 Tabla 11-3 Preparación Inicial del Molino para el Arranque ................................................... 11-4 Tabla 11-4 Carga el Molino para el Lanzamiento .................................................................. 11-5 Tabla 11-5 Colocando el Molino en Servicio ......................................................................... 11-6 Tabla 11-6 Procedimientos para el Extintor en el Molino Foster del Rodador ....................... 11-8 Tabla 11-7 Procedimientos del Paro de Emergencia para el Molino Foster del Rodador ..... 11-9 Tabla 11-8 Recomendación de los Procedimientos para Cerrar el Molino Foster del Rodador Lleno de Carbón ............................................................................................... 11-9 Tabla 12-1 Tareas para Estropear el Molino ......................................................................... 12-5 Tabla 12-2 Tareas de Arranque Frío ...................................................................................... 12-6 Tabla 13-1 Tareas de la Calibración del Alimentador de Carbón ......................................... 13-12 Tabla 13-2 Parámetros de la Instrumentación del Molino .................................................... 13-13 Tabla 13-3 Parámetros Típicos del Equipo........................................................................... 13-14 Tabla 13-4 Situaciones del Sensor de Temperatura de la Detección de Fuego .................. 13-16 Tabla 15-1 Dimensiones Estándar de la Pantalla ................................................................... 15-1 Tabla 16-1 Coeficientes Abrasivos del Desgaste ................................................................... 16-3 Tabla 16-2 Componentes de la Falla de Allis-Chalmers ........................................................ 16-4 Tabla 17-1 Localizador de Averías para los Molinos de Allis-Chalmers ................................ 17-1 Tabla 17-2 Carta de Localización de Averías para los Molinos Foster de Rodador .............. 17-5 Tabla 17-3 Carta de Localización de Averías para Molinos Kennedy Van Saun ................... 17-7 Tabla 17-4 Carta de Localización de Averías para los Molinos de Bola de Poder de Riley ... 17-9 Tabla 17-5 Localización de Averías para los Molinos de Stein Industrie ............................. 17-10 Tabla 18-1 Conteo de Partículas de Alcance de Números ..................................................... 18-5
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Tabla 18-2 Elementos en Empaques de Adictivos de Aceite ................................................ 18-9
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Tabla 18-3 Componentes de Molinos Kennedy Van Saun para el Programa de Lubricación ................................................................................................................... 18-13 Tabla 18-4 Vigilancia Basada-Acondicionada de Válvulas en la Central Eléctrica de Majuba ......................................................................................................................... 18-16 Tabla 18-5 Condiciones Vigiladas en la Central Eléctrica de Majuba .................................. 18-19 Tabla 18-6 Instrumentación de Vigilancia de Condiciones para la Central Eléctrica de Majuba .......................................................................................................................... 18-20 Tabla 18-7 Vigilancia de Condición Programada en la Central Eléctrica de Majuba............ 18-24 Tabla 19-1 Tareas Externas de Inspección del Molino .......................................................... 19-4 Tabla 19-2 Tareas de Inspección Internas ............................................................................. 19-6 Tabla 19-3 Tareas de Inspección del Clasificador .................................................................. 19-8 Tabla 19-4 Tareas de Inspección del Tren de Impulsión del Molino ...................................... 19-9 Tabla 19-5 Tareas de Reemplazo del Forro de la Granada y del Muñón............................. 19-10 Tabla 19-6 Tareas del Reemplazo de la Pieza Inserta en el Cojinete del Muñón ................ 19-12 Tabla 19-7 Tareas del Reemplazo del Engranaje de la Circunferencia ............................... 19-14 Tabla 19-8 Tareas de Reconstrucción de la Caja de Engranajes ........................................ 19-19 Tabla 19-9 Lista de Lubricación de Equipos ......................................................................... 19-23 Tabla 19-10 Tareas de Inspección para el Piñón, el Engranaje de la Circunferencia, y los Sistemas Lubricantes ............................................................................................. 19-25 Tabla 19-11 Tareas para Invertir el Engranaje de Tornillo sin fin de la Caja de Engranajes ................................................................................................................... 19-26 Tabla 19-12 Repuesto del Forro del Desgaste ..................................................................... 19-32 Tabla 19-13 Capacidades de Aceite de la Caja de Engranajes del Reductor de Velocidad ...................................................................................................................... 19-33 Tabla 19-14 Tareas de Desmontaje de la Caja de Engranajes del Reductor de la TripleVelocidad ...................................................................................................................... 19-37 Tabla 19-15 Tareas de la Inspección del Embrague ............................................................ 19-40 Tabla 19-16 Tareas de Reparación de Cadena.................................................................... 19-41 Tabla 19-17 Tareas de Retiro y de la Reinstalación del Piñón Impulsor .............................. 19-42 Tabla 19-18 Tareas de Reemplazo del Eje del Motor .......................................................... 19-44 Tabla 19-19 Tareas de Inspección del Cambiador de Calor ............................................... 19-46 Tabla 19-20 Tareas de Reemplazo Aceites Lubricantes del Cambiador de Calor ............... 19-47 Tabla 19-21 Tareas de Mantenimiento Preventivo del Clasificador ..................................... 19-48 Tabla 19-22 Horario de Lubricación...................................................................................... 19-49 Tabla 19-23 Piezas de Repuesto Recomendadas por Riley Power ..................................... 19-50 Tabla 19-24 Tareas de Inspección del Alimentador para el Intervalo de Operación de 7000 Horas.................................................................................................................... 19-53 Tabla 19-25 Tareas de Mantenimiento Preventivo ............................................................... 19-54 Tabla 19-26 Tareas de Inspección para el Intervalo de Operación de 5500 Horas.............. 19-59 Tabla 19-27 Tareas de Inspección para la Inspección del Interino de 18 Meses ................ 19-60
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Translated Table of Contents
Tabla 19-28 Tareas de Inspección para el Reacondicionamiento General de 9 Años ........ 19-61
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Translated Table of Contents
Tabla 19-29 Tareas del Reemplazo para el Piñón y el Engranaje de Circunferencia ......... 19-61 Tabla 20-1 Localizaciones de Fallas, Mecanismos de Degradación, y Estrategias de PM para Molinos de Tubo/Bola ............................................................................................. 20-4 Tabla 20-2 Tareas de P.M. y sus Mecanismos de Degradación para los Molinos de Bola/Tubo ..................................................................................................................... 20-10 Tabla 20-3 Patrón de PM para el Molino de Bola/Tubo ....................................................... 20-15
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