92387738 EPRI 1000987 Mechanical Seal Maintenance and Application Guide (2)

Mechanical Seal Maintenance and Application Guide

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WARNING: Please read the License Agreement on the back cover before removing the Wrapping Material.

Technical Report

Plant Maintenance Support Reduced Cost

Equipment Reliability

Mechanical Seal Maintenance and Application Guide 1000987

Final Report, November 2000

EPRI Project Manager M. Pugh

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

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

ORDERING INFORMATION Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box 23205, Pleasant Hill, CA 94523, (800) 313-3774. Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2000 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS This report was produced by Nuclear Maintenance Application Center EPRI 1300 W.T. Harris Boulevard Charlotte, NC 28262 This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Mechanical Seal Maintenance and Application Guide, EPRI, Palo Alto, CA: 2000. 1000987.

iii

REPORT SUMMARY

This guide provides information to personnel involved with the maintenance of mechanical seals, including good maintenance practices, planning, predictive and preventive techniques, and troubleshooting guidance. It provides insight to experienced personnel as well as basic information, guidance, and instructions to personnel assigned to maintain mechanical seals. Background A mechanical seal prevents leakage of pressurized fluid between a rotating shaft and a stationary housing. They are widely used for numerous power plant equipment applications, particularly on pumps of various sizes and pressure ratings. Even though they are capable of providing longterm service, mechanical seals sometimes exhibit unsatisfactory performance, unpredictable failures, and a short life, which can directly affect plant reliability and performance, resulting in costly downtime and outages. Mechanical seal issues rank high in surveys completed by power plant maintenance personnel. Objectives x To help power plant personnel deal with the maintenance and reliability issues of this critical power plant component x

To provide technical information to plant personnel on proper selection and installation of mechanical seals, seal failure modes, and troubleshooting

x

To provide maintenance recommendations for optimizing seal performance and operating life

Approach A detailed review of industry literature, product information, and standards was conducted to establish the state of technology for mechanical seals. Utility and industry personnel were surveyed to determine specific problems and commonly encountered failure mechanisms. Based on all of the information gathered, suitable recommendations were developed for the problems encountered and presented in this report. Results This guide presents a thorough discussion of mechanical seals and provides an in-depth understanding of their design and operation, including expected life and a discussion of proper application and selection. It also provides proper installation methods and guidance on expected failure mechanisms. This guide offers troubleshooting approaches to assist in determining the causes of failure and discusses recommended predictive, preventive, and corrective maintenance practices. The contents of this guide will assist plant personnel in reducing costs and equipment unavailability and in improving equipment reliability and performance.

v

EPRI Perspective Problems with mechanical seals represent a significant reliability impact on rotating equipment. This guide provides power plant maintenance personnel with information to help improve seal performance and component reliability through a better understanding of the operation of mechanical seals and their critical components. It also provides guidelines on investigating and troubleshooting problems that arise during inservice operation and normal planned maintenance activities. Keywords Mechanical seals Maintenance Engineers

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ACKNOWLEDGMENTS This guide was developed by the Nuclear Maintenance Application Center (NMAC) and the following Technical Advisory Group (TAG): Steve Lemberger Bob Mundlapudi Vic Varma Hugh Nixon Steve Rosenau Larry Price Rich Hansen John Montgomery

AEP Amergen Consultant Consumers Energy Duke Energy PG&E UNICOM UNICOM

NMAC and the TAG were supported in this effort by: Kalsi Engineering, Inc. Sugar Land, TX Principal Investigators: M. S. Kalsi P. D. Alvarez

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EPRI Licensed Material

CONTENTS

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

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

1.2

Purpose .................................................................................................................... 1-2

1.3

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

1.4

Highlighting of Key Points ......................................................................................... 1-3

2 GLOSSARY OF TERMS...................................................................................................... 2-1 3 TECHNICAL DESCRIPTION ............................................................................................... 3-1 3.1

Operating Principles and Basic Components of a Mechanical Face Seal .................. 3-1

3.2

Major Design Variations ............................................................................................ 3-8

3.3

Multiple Seals.......................................................................................................... 3-10

3.4

Seal Cartridges ....................................................................................................... 3-12

3.5

Seal Chamber Design and Flushing ........................................................................ 3-15

3.5.1 Seal Arrangements for Abrasive Applications ..................................................... 3-17 3.6

Closing Force.......................................................................................................... 3-17

3.6.1 Balance Ratio ..................................................................................................... 3-18 3.6.2 Pressure Distribution Between the Sealing Faces .............................................. 3-21 3.6.3 Stationary Versus Rotating Seal Balance ........................................................... 3-22 3.7

Pressure Velocity (PV) Parameter and Limit ........................................................... 3-23

3.8

Temperature Considerations and 'T Limit .............................................................. 3-24

3.9

Improved Seal Face Designs .................................................................................. 3-25

3.10 Hydrostatic Seal Design .......................................................................................... 3-28 4 FAILURE MODES AND FUNDAMENTAL MECHANISMS.................................................. 4-1 4.1

Introduction ............................................................................................................... 4-1

4.2

Definition of Seal Failure ........................................................................................... 4-1

4.3

Industry Survey ......................................................................................................... 4-2

4.4

Fundamental Failure Mechanisms............................................................................. 4-3

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4.4.1 PV Limits Exceeded ............................................................................................. 4-4 4.4.2 'T Limits Exceeded, Causing Film Vaporization/Collapse .................................... 4-5 4.4.3 Inadequate Cooling .............................................................................................. 4-6 4.4.4 Transients Causing Excessive Seal Face Coning................................................. 4-6 4.4.5 Operation Away from Best Efficiency Point........................................................... 4-9 4.4.6 Seal Misalignment/Premature Degradation of Primary and Secondary Seals ..... 4-12 4.4.7 Excessive Out-of-Flatness (Warpage) During Operation .................................... 4-15 4.4.8 Seal Faces Too Perfectly Flat to Generate a Film............................................... 4-16 5 APPLICATION AND SELECTION RECOMMENDATIONS ................................................. 5-1 5.1

Introduction ............................................................................................................... 5-1

5.2

Selection Specification .............................................................................................. 5-1

5.3

Selection Data Sheet ................................................................................................ 5-3

5.4

Qualification Testing.................................................................................................. 5-6

6 CONDITION-BASED MONITORING GUIDELINES ............................................................. 6-1 6.1

Introduction ............................................................................................................... 6-1

6.2

Typical Performance Data Logging ........................................................................... 6-2

6.3

Seal Performance Parameters .................................................................................. 6-5

6.4

Instrumentation ......................................................................................................... 6-5

6.4.1 Temperature Gauge ............................................................................................. 6-5 6.4.2 Thermowells ......................................................................................................... 6-6 6.4.3 Pressure Gauges.................................................................................................. 6-6 6.4.4 Alarm, Trip, and Control Switches ........................................................................ 6-6 6.4.5 Pressure Switches................................................................................................ 6-7 6.4.6 Level Switches ..................................................................................................... 6-7 6.4.7 Level Indicators .................................................................................................... 6-8 6.4.8 Flow Indicators ..................................................................................................... 6-8 7 TROUBLESHOOTING TO IDENTIFY CAUSE OF SEAL FAILURE .................................... 7-1 7.1

Introduction ............................................................................................................... 7-1

7.2

Failure Diagnosis ...................................................................................................... 7-1

7.2.1 External Symptoms of Seal Failure....................................................................... 7-2 7.2.2 Checks Before Dismantling .................................................................................. 7-7 7.2.3 Checks During Dismantling .................................................................................. 7-9 7.2.3.1 General Checks............................................................................................. 7-9

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7.2.3.2 Premature Failure Checks ........................................................................... 7-10 7.2.3.3 Mid-Life Failure Checks............................................................................... 7-11 7.3

Visual Seal Examination.......................................................................................... 7-12

8 MAINTENANCE................................................................................................................... 8-1 8.1

Introduction ............................................................................................................... 8-1

8.2

Installation and Operation ......................................................................................... 8-2

8.2.1 Seal Handling and Inspection ............................................................................... 8-2 8.2.1.1 Packaging ..................................................................................................... 8-2 8.2.1.2 Storage.......................................................................................................... 8-3 8.2.1.3 Handling ........................................................................................................ 8-3 8.2.1.4 Physical Checks of Mechanical Seals ........................................................... 8-3 8.2.1.5 Seal Rotating and Stationary Components .................................................... 8-3 8.2.1.6 Seal Faces .................................................................................................... 8-4 8.2.1.7 Gaskets ......................................................................................................... 8-4 8.2.1.8 Spring............................................................................................................ 8-4 8.2.2 Pre-Installation Equipment Checks ....................................................................... 8-4 8.2.2.1 Shaft Straightness ......................................................................................... 8-4 8.2.2.2 Shaft Runout ................................................................................................. 8-5 8.2.2.3 Squareness of Stuffing Box ........................................................................... 8-5 8.2.2.4 Rotational Balance ........................................................................................ 8-6 8.2.2.5 Shaft Bearing Clearances.............................................................................. 8-6 8.2.2.6 Shaft/Sleeve Diameter and Surface Finish .................................................... 8-7 8.2.2.7 Sleeve Hardfacing ......................................................................................... 8-7 8.2.2.8 Sharp Edges ................................................................................................. 8-8 8.2.3 Seal Installation Checks ....................................................................................... 8-8 8.2.3.1 Seal Dimensional Checks.............................................................................. 8-8 8.2.3.2 Seal Cavity Dimensions................................................................................. 8-9 8.2.3.3 Compression Length Tolerance..................................................................... 8-9 8.2.3.4 Auxiliary Glands ............................................................................................ 8-9 8.2.4 Seal Removal ..................................................................................................... 8-10 8.2.4.1 Safety.......................................................................................................... 8-10 8.2.4.2 Failure Evidence.......................................................................................... 8-10 8.2.4.3 Seal Re-use and Inspection......................................................................... 8-10 8.2.5 Startup................................................................................................................ 8-10

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8.2.5.1 Avoid Dry Running ...................................................................................... 8-11 8.2.5.2 Filtration ...................................................................................................... 8-11 8.2.5.3 Venting the Stuffing Box .............................................................................. 8-11 9 REFERENCES AND BIBLIOGRAPHY................................................................................ 9-1 A MECHANICAL SEALS APPLICATION AND MAINTENANCE GUIDE SURVEY................ A-1 B INSPECTION OF SEAL FACES FOR FLATNESS .............................................................B-1 B.1

Optical Principle ........................................................................................................ B-1

B.2

Procedure for Measuring Face Flatness.................................................................... B-2

C TRAINING COURSES.........................................................................................................C-1 D LISTING OF KEY INFORMATION ......................................................................................D-1

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LIST OF FIGURES Figure 3-1 Essential Components of a Mechanical Face Seal................................................. 3-1 Figure 3-2 Multiple Coil Springs .............................................................................................. 3-4 Figure 3-3 Single Coil Springs................................................................................................. 3-4 Figure 3-4 Corrugated Bellows................................................................................................ 3-4 Figure 3-5 Welded Bellows ..................................................................................................... 3-4 Figure 3-6 Rubber Bellows...................................................................................................... 3-5 Figure 3-7 Belleville Washers.................................................................................................. 3-5 Figure 3-8 Rotating Primary Ring - Outside Pressure (or Inside Mounted) .............................. 3-9 Figure 3-9 Rotating Primary Ring - Inside Pressure (or Outside Mounted) .............................. 3-9 Figure 3-10 Stationary Primary Ring - Outside Pressure (or Inside Mounted) ......................... 3-9 Figure 3-11 Stationary Primary Ring - Inside Pressure (or Outside Mounted) ....................... 3-10 Figure 3-12 Back-to-Back Dual Seal ..................................................................................... 3-10 Figure 3-13 Face-to-Face Dual Seal ..................................................................................... 3-11 Figure 3-14 Pressure Stage Tandem Seal ............................................................................ 3-11 Figure 3-15 Single Seal Cartridge ......................................................................................... 3-12 Figure 3-16 Balanced Stator Design Multi-Seal Cartridge Supplied by a Manufacturer for a Main Coolant Pump .................................................................................................... 3-13 Figure 3-17 Seal Stage Details of a Balanced Stator Design Multi-Seal Cartridge Supplied by a Manufacturer for a Main Coolant Pump................................................... 3-14 Figure 3-18 Common Variations in Seal Chamber Design .................................................... 3-15 Figure 3-19 A Typical Flush Plan for a Cooling Seal Chamber .............................................. 3-16 Figure 3-20 Unbalanced, Balanced, and Partially Balanced Seal Designs ............................ 3-19 Figure 3-21 Face Pressure Distribution Due to Hydraulic Pressure and Spring Force........... 3-21 Figure 3-22 Rotating Seal Balance Designs.......................................................................... 3-22 Figure 3-23 Pressure/Temperature Operating Envelope Showing 'T Margin Required for Seal Operation .............................................................................................................. 3-25 Figure 3-24 Seal Face with Thermal Hydrodynamic Grooves for Positive Hydrodynamic Lubrication..................................................................................................................... 3-26 Figure 3-25 Design Options with Hydrodynamic Grooves on the Outer Periphery or Inner Periphery of Seal Face .................................................................................................. 3-27 Figure 3-26 Other Variations in Seal Face Geometry to Enhance Lubrication of the Faces ............................................................................................................................ 3-27 Figure 3-27 Hydrostatic Face Seal Design ............................................................................ 3-29

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Figure 4-1 Lubrication Regimes at Seal Interface Showing Asperity Contact as Lubrication Changes from Full Film to Mixed to Boundary ............................................... 4-5 Figure 4-2 Extremes of Seal Face Distortion (Coning) Due to Thermal and Pressure Effects ............................................................................................................................. 4-7 Figure 4-3 Pressure Distribution Changes Caused by Coning of the Seal Faces (for Outside Pressurized Seal)......................................................................................... 4-8 Figure 4-4 Changes in Seal Contact Area Under Constant Operating Conditions During the Wear-In Process for a Seal With a Hard Face and a Soft Face ................................. 4-9 Figure 4-5 Example of a Wear-In Sequence (Stages 1 through 4) for a Mechanical Seal with a Soft Seal Face....................................................................................................... 4-9 Figure 4-6 Fluid Pumping Action Across the Seal Faces Due to Static Offset and Misalignment ................................................................................................................. 4-11 Figure 4-7 Rotating Balance Seal Wobble Caused by Shaft Tilt ............................................ 4-12 Figure 4-8 Shaft Tilt Accommodated by Stationary Ring Pivot .............................................. 4-14 Figure 4-9 Seal Pumping Caused by Dynamic Offset of Rotating Narrow Face .................... 4-15 Figure 6-1 Seal Data Plot Showing Declining Performance..................................................... 6-4 Figure 8-1 Shaft Straightness Check....................................................................................... 8-5 Figure 8-2 Shaft Runout Measurement ................................................................................... 8-5 Figure 8-3 Stuffing Box Squareness Measurement ................................................................. 8-6 Figure 8-4 Shaft and Impeller Rotational Balance Check ........................................................ 8-6 Figure 8-5 Radial and Axial Bearing Clearance Checks .......................................................... 8-7 Figure 8-6 Measurement of Critical Shaft and Sleeve Diameters ............................................ 8-7 Figure 8-7 Sleeve Hardfacing to Prolong Life .......................................................................... 8-8 Figure 8-8 Lead-In Chamfers to Prevent Secondary Seal Damage During Installation............ 8-8 Figure 8-9 Seal Cavity Dimensional Checks Prior to Installation ............................................. 8-9 Figure B-1 Using an Optical Flat to Determine Seal Face Flatness Light Bands ..................... B-2 Figure B-2 The Viewing Angle Typically Should be 80q to 90q While Checking Flatness Using a Monochromatic Light Source .............................................................................. B-3 Figure B-3 Flat Within One Light Band .................................................................................... B-4 Figure B-4 Bands Bend on One side and Line AB Intersects 3 Bands .................................... B-5 Figure B-5 This Indicates an Egg-Shaped Curvature of 2.5 Light Bands ................................. B-5 Figure B-6 Bands Show a Saddle Shape Out-of-Flat Condition of 3 Light Bands .................... B-6 Figure B-7 Bands Show a Cylindrical-Shaped Part with a 3-Light Band Reading Error ........... B-6 Figure B-8 Band Symmetrical Pattern Indicates a Conical Convex or Concave Part ............... B-6

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LIST OF TABLES Table 2-1 Glossary of Terms................................................................................................... 2-1 Table 3-1 Secondary Seal Properties...................................................................................... 3-3 Table 3-2 Advantages and Disadvantages of Mechanical Face Seal Configurations............... 3-6 Table 3-3 Advantages and Disadvantages of Mechanical Face Seal Springs ......................... 3-8 Table 3-4 Approximate PV Limits psi-ft/min (Mpa-m/sec) for General Seals with Various Combinations of Seal Face Materials and Fluids ........................................................... 3-23 Table 5-1 Seal Application and Selection Guidelines .............................................................. 5-2 Table 6-1 Seal System Log Sheet........................................................................................... 6-3 Table 7-1 External Symptoms of Seal Failure ......................................................................... 7-3 Table 7-2 Checklist of Actions Before Dismantling .................................................................. 7-7 Table 7-3 General Checks During Dismantling........................................................................ 7-9 Table 7-4 Premature Failure Checks During Dismantling ...................................................... 7-10 Table 7-5 Mid-Life Failure Checks During Dismantling.......................................................... 7-11 Table 7-6 Visual Examination: Failure Symptoms Based on Mechanical, Thermal, or Chemical Damage......................................................................................................... 7-13 Table 7-7 Visual Examination: Symptoms, Characteristics, Causes and Remedies .............. 7-14

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1 INTRODUCTION

1.1

Background

In the past, the Nuclear Maintenance Application Center (NMAC) of EPRI has published a number of application and maintenance guides to provide technical guidance to engineers and other plant personnel on mechanical seal equipment and component operation. These have included information on proper selection, installation, and failure mode analysis, and maintenance recommendations designed to optimize equipment operating life. EPRI has conducted and published the following documents relating to equipment seals: x

Guide to Optimized Replacement of Equipment Seals. March 1990 (NP-6731).

x

Shelf Life of Elastomeric Components. 1994 (NP-6608).

x

Main Coolant Pump Seal Maintenance Guide. 1993 (TR-100855).

x

Static Seal Maintenance Guide. 1994 (TR-104749).

x

Centrifugal and Positive Displacement Maintenance Guide. 1997 (TR-107252).

Mechanical seals are widely used in many types of rotating power plant equipment, especially pumps of various sizes and pressure ratings. Even though mechanical seals are capable of providing reliable long-term service with proper consideration to design, application, installation, and maintenance, they still exhibit unsatisfactory performance, short life, and unpredictable (random) failures in some applications. As such, mechanical seals have a significant influence on the reliability of plant equipment. A mechanical seal is a complex assembly of precision-machined components. Design and prediction of mechanical seal performance in a given application requires an in-depth knowledge of all mechanical disciplines: stress/deflection analysis, vibration analysis, heat transfer, fluid mechanics, lubrication, friction and wear, materials, and manufacturing processes. Mechanical seal technology, as well as a fundamental understanding of how such seals work, has evolved and improved significantly over the last two decades. This has been the result of extensive industry-wide research, testing, plant experience, availability of sophisticated analytical tools (for example, computational fluid dynamics analysis and finite element analysis), and advances in manufacturing technology. This has enabled improvements in the performance of mechanical seals in a number of critical applications in nuclear and fossil power plants, petrochemical plants, and other industries.

1-1

EPRI Licensed Material Introduction

1.2

Purpose

The objective of this NMAC Mechanical Seal Maintenance and Application Guide is to provide personnel in nuclear and fossil power plants with: x

An in-depth understanding of the design and operation of mechanical seals

x

Correct selection of mechanical seals for an application

x

Proper installation methods

x

Guidance on failure mechanisms and their causes, including troubleshooting information

x

Guidance on expected seal life under various operating conditions

x

Recommended predictive (diagnostics), preventive, and corrective methods of maintenance to optimize seal life

x

Training material to support personnel training

This guide presents the latest developments in mechanical seal technology and materials. Some of the new seal designs are already in use in industries other than power plants. Their viability in power plant operation was researched and, based on this research, the guide includes recommendations for achieving plant-wide improvements in nuclear and fossil power plants. This NMAC Mechanical Seal Maintenance and Application Guide is a comprehensive, state-ofthe-art text for nuclear and fossil power utility engineers.

1.3

Approach

A detailed review of the available literature was conducted to establish the state of technology in mechanical seals [1-65*]. The objective was to establish the present state of the art regarding: x

The operation of seals

x

Designs offered by the manufacturers

x

Application problems

x

Solutions to address these problems

x

Installation and maintenance recommendations

x

Statistical/failure data

x

Plant experiences

x

Emerging technologies

All relevant technical papers, reports, and publications were reviewed from: x *

The American Society of Mechanical Engineers (ASME) Numerals in brackets denote references listed in Section 9 of this Guide.

1-2


x

The American Society of Lubrication Engineers (ASLE)

x

The British Hydromechanics Research Association (BHRA)

x

The Institution of Mechanical Engineers (IME)

x

Seal manufacturers

The review included both domestic and international mechanical seal manufacturers such as John Crane Company, Chesterton, Borg-Warner, Durametallic, Sealol, AST, Burgmann Seals, Flexibox, Latty International. Significant United States Nuclear Regulatory Commission Generic Communications relating to shaft seal issues were also reviewed and evaluated to develop suitable recommendations for inclusion in this guide. Additionally, a questionnaire was developed as a survey distributed among the nuclear and fossil power utilities to facilitate determination of specific problems and commonly encountered failure modes. The results of the survey were analyzed to determine the root causes of seal failure, to develop troubleshooting, failure diagnosis, installation and maintenance guidelines, and to develop suitable recommendations for this guide. This guide also utilizes relevant data from technical papers, as well as principal investigators’ experience with mechanical seals in the petrochemical, chemical, drilling, and mining industries.

1.4

Highlighting of Key Points

Throughout this guide, key information is summarized in Pop Outs. Pop outs are bold-lettered boxes that succinctly restate information covered in detail in the surrounding text, making the key point easier to locate. The primary intent of a pop out is to emphasize information that will allow individuals to take action for the benefit of the plant. The information included in these pop outs was selected by NMAC personnel and the consultants and utility personnel who prepared and reviewed this guide. The pop outs are organized according to three categories: O&M Costs, Technical, and Human Performance. Each category has an identifying icon, as shown below, to draw attention to it when quickly reviewing the guide. Key O&M Cost Point Emphasizes information that will result in reduced purchase, operating, or maintenance costs. Key Technical Point Targets information that will lead to improved equipment reliability.

1-3


Key Human Performance Point Denotes information that requires personnel action or consideration in order to prevent injury or damage, or ease completion of the task. Appendix D contains a listing of all key points in each category. The listing restates each key point and provides 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 authors believe would benefit their plants.

1-4


2 GLOSSARY OF TERMS

The terminology used to describe the various design features, configurations, applications, installation, and performance of mechanical face seals has evolved over the years. Seal handbooks, manufacturer catalogs, technical papers, and the industry standards for both the United States of America and European countries [3-9] were reviewed to reconcile the differences in definitions and prepare the following comprehensive glossary (Table 2-1) of terms in common use today and adopted in this guide. Table 2-1 Glossary of Terms Term

Definition

Abeyance seal

A non-contacting auxiliary seal that is activated by failure of the primary seal in the case of a single seal, or the outer seal in the case of a double seal.

Abrasive wear

Wear occurring by the mechanical action of an abrasive. Abrasives are substances that are harder than the abraded surface and usually have an angular profile.

Adhesive wear

Wear arising from small-scale local welding at asperities; a common wear mode associated with running in and mild steady state wear.

Anti-rotation pin or device

A device, usually a pin, designed to prevent the stationary seal member from rotating in its mounting.

API 610

API Standard: Centrifugal Pumps for General Refinery Services (8th Ed. in preparation). A specification widely used for heavy duty centrifugal pumps.

API 682

API Standard: Shaft Sealing System for Centrifugal and Rotary Pumps (1st Ed., 1994).

API piping plan

Arrangements recommended in API 610 for connecting auxiliary pipework to the seal chamber.

Asperity

Minute high spot on the seal face resulting from the manufacturing process.

Autobalancing

Alternative term for double balancing (see double balancing).

Auxiliary seal

A seal fitted to the atmospheric side of a quench chamber or secondarycontainment chamber.

Back-to-back seal

A seal configuration consisting of a double seal with the seal rings adjacent to each other, that is, two mechanical seals facing in opposite directions.

Back-up seal

Alternative name for auxiliary seal.

Balance diameter

See note under balance ratio.

2-1

EPRI Licensed Material Glossary of Terms Table 2-1 (cont.) Glossary of Terms Term

Definition

Balance ratio

Balance ratio determines the proportion of the seal chamber pressure that is applied to the faces of a mechanical seal. Mechanical seals are available as both balanced and unbalanced designs. The balance ratio is a ratio of the area subjected to the differential pressure of the fluid to the area between the seal ring faces. Seals are often identified by their balance diameter. The balance diameter, Db, is located between the inside diameter, Di, and outside diameter, Do, of the seal ring contact area. For seals pressurized on the outside diameter:

Balance Ratio

Do 2 D b 2 D o 2 Di 2

For seals pressurized on the inside diameter: D b 2 - Di 2 Balance Ratio D o 2 - Di 2 Note: Balance diameter varies with seal design, but for spring pusher seals under outer diameter (OD) pressure, it is normally the diameter of the sliding contact surface of the inner diameter (ID) of the dynamic O-ring. For spring pusher seals under inner diameter pressure, it is normally the diameter of the sliding contact surface of the outer diameter of the dynamic O-ring. For welded metal bellows type seals, the balance diameter is normally the mean diameter of the bellows but this can vary with pressure. As stated in Diametral Tilt and Leakage of End Face Seals with Convergent Sealing Gaps [26], the balance diameter for the welded bellows is equal to the root mean square average of the bellows OD and ID, that is, Db = [0.5 (OD2 + ID2)]1/2.

2-2

Balanced seal

A mechanical seal arrangement whereby the effect of the hydraulic pressure in the seal chamber on the seal face closing forces has been reduced through seal geometry. Balanced seals have a seal balance ratio of less than 1 (0.65 to 0.85 is typical range).

Balanced sleeve/ secondary seal sleeve

Stationary balance seal designs allow the stationary member to move axially. The secondary seal slides on a sleeve, or insert, called the balance sleeve.

Barrier fluid

A fluid injected between dual mechanical seals to completely isolate the pump process liquid from the environment. Pressure of the barrier fluid is always higher than the process pressure being sealed. (For contrast, see buffer fluid definition.)

Bellows seal

A type of mechanical seal in which one of the faces is mounted on an elastomeric or a flexible metal bellows to provide secondary sealing. Metal bellows, and in some designs elastomeric bellows, also provide spring-type loading to the seal faces.

Blistering

A term used to describe a particular form of damage to carbon-graphite seal faces, usually caused by hydrocarbons.

Boundary lubrication

Condition of lubrication where the seal faces are in solid contact, though separated by adsorbed surface films.


Definition

Bubble point

Mixtures of liquids do not have a clearly defined boiling point. The bubble point is the temperature at which the first bubble is evolved on raising the temperature at constant pressure. The term is most frequently used with mixtures of hydrocarbons.

Buffer fluid

A fluid used as a lubricant or buffer between dual mechanical seals. The fluid is always at a pressure lower than the pump process pressure being sealed. (For contrast, see barrier fluid definition.)

Cartridge seal

A completely self-contained mechanical face seal unit (including seal, gland, sleeve, and mating ring) that is pre-assembled and requires no field adjustments.

Clamp plate

An alternative term for seal plate.

Closing force

Combined hydraulic and spring load acting on the floating seal member in the closing direction.

Coking

The formation of carbonaceous deposits on the atmospheric side of a mechanical seal resulting from the oxidation/polymerization of leakage of organic products.

Compression set

The difference between the thickness of a gasket, or elastomer, or length of a spring, both as supplied and after being subject to compression in service. More specifically, the compression set of an elastomer is defined as:

change in specimen length applied strain x original specimen length Coning

Axisymmetric distortion of the seal faces, causing a rotation of the seal ring crosssection and creating a radial variation in seal film thickness.

Contact pattern

An alternative term for wear track.

Controlled bleedoff (CBO) or staging flow

Staged seal designs use an orifice to bypass a small flow around each seal to reduce pressure to subsequent stages. If the resistance of each orifice device is equal, and the seals are not leaking, the differential pressure across each stage will be equal. This distribution of pressure provides the optimum condition to obtain the maximum seal life.

Controlled leakage seal

Alternative term for hydrostatic seal.

Convergence/ divergence

It is necessary to have an adequate gap at the inner or the outer periphery of the seal faces that is exposed to the pressurized fluid to allow fluid to enter and provide lubrication and cooling. Coning of seal faces can cause the gap to decrease (converging seal faces) or increase (diverging seal faces) in the direction of leakage.

Coolant

A liquid from an external source circulated through a stationary seal member or other separate cooling element to remove heat.

Critical dimensions

Each specific seal design has a unique geometry. In this geometry some dimensions are very important to the successful operation of the seal. Other dimensions, although important, might not have a significant effect as they vary within reasonable values. Dimensions that are very important to the proper operation of the seal are termed critical dimensions. These might be very precise dimensions, such as seal face flatness, or they might have tolerances of 1/16" (.16 cm), such as a spring gap. Generally, critical dimensions are verified and recorded to ensure they are correct.

2-3


Definition

Crystallization

The formation of crystalline solids on the atmospheric side of a mechanical seal resulting from evaporation of product leakage (for example, borated water).

Cyclone separator

Hydrocyclone fitted in a product recirculation line to remove solids.

Dead-ended

Seal arrangement in which there is no product recirculation or injection of flush into the seal chamber.

Degree of balance

The proportion of the face area that is exposed to the low-pressure side of the balance diameter ( = 1 – balance ratio).

Delta T, 'T

The difference between the bulk temperature of the liquid in the seal chamber and the boiling point (or bubble point in the case of mixtures) of this liquid at the pressure in the seal chamber. Also known as the product temperature margin.

Destaging

When individual seal stages leak more than other stages, the differential pressure across the stages that are not leaking increases, and the differential pressure across stages that are leaking decreases. This shift in differential pressures is termed destaging.

Diameter ratio

The ratio (>1) between the outer and inner diameters of the narrower of the seal faces.

Double balancing

A mechanical seal design feature that changes the balance diameter to improve the seal's resistance to operating under reverse pressure. This prevents opening of the inside seal in a double seal upon loss of barrier fluid pressure. (Sometimes called autobalancing.)

Double seal

Restricted in this publication to the arrangement of two mechanical seals in a seal chamber sealing in opposite directions. The seals can be either the back-to-back or face-to-face seal configuration (qv). Note: An alternative usage is to include two seals sealing in the same direction in the category of double seal; in this publication, the latter configuration is referred to as a tandem seal.

2-4

Drain connection

A connection to the quench (or secondary containment) chamber for the collection of liquid.

Drive collar

The part of a cartridge seal that mechanically connects the sleeve to the shaft to transmit rotation and prevent axial movement of the sleeve relative to the shaft.

Drive pin

A device for transmitting torque from the shaft to the rotating seal member.

Dry running

Running with no liquid between the seal faces.

Dual mechanical seal

A seal arrangement using more than one seal in the same seal chamber in any orientation that can utilize either a pressurized barrier fluid or a non-pressurized buffer fluid. (It is also referred to as a double or tandem seal.)

Dynamic secondary seal

A secondary seal in a pusher seal that prevents leakage between the shaft or housing and the floating seal member of a mechanical seal.

Early-life failures

Failures occurring shortly after start-up because of manufacturing or fitting errors; sometimes referred to as infantile mortality.

Elastomer

Non-metallic parts such as O-rings, U-cups, quad-rings, and bellows.


Definition

Erosion

Abrasive wear of a surface by small particles in a gas, vapor or liquid, or droplets of liquid in a gas or vapor (wire-drawing) flowing across it.

Externally mounted seal (also called outside mounted).

An arrangement in which the mechanical seal is mounted outside the pump or sealed vessel so that fewer seal parts are exposed to contact with a corrosive sealed fluid. In this arrangement, the sealed fluid is in contact with the inner diameter of the seal faces.

Face

This term is used in a strict sense to mean the surface of a seal ring at the sealing interface, but is also commonly used for the whole ring, for example, hard face.

Face load

The combined spring and hydraulic load carried between the seal faces before allowing for any fluid pressure in the sealing interface.

Face plate

The primary sealing surface in a hydrostatic seal is a ceramic piece called the faceplate. Some faceplates are stainless steel coated with aluminum oxide and others are silica nitride.

Face width

Half the difference between the outer and inner diameters of the narrower of the seal faces.

Face-to-face seal

A seal configuration consisting of a double seal with the seats adjacent to each other, that is, two mechanical seals facing in opposite directions.

Film thickness

The thickness of the fluid film between the seal faces.

Film transfer

A process by which a film of the material of the soft face is deposited on the hard face.

Fitness testing

Cartridge seals are assembled outside the pump and can be tested to verify the assembly. Normally, a test vessel (with adequate ports, nozzles, gauges, and a flow meter) is used to measure staging pressures and controlled bleed-off flow. Frequently, fitness testers are supplied as skid-mounted assemblies with the required pumps, reservoirs, instrumentation, and connecting piping.

Flashing

A rapid change in fluid state, from liquid to gaseous. In a dynamic seal, this can occur when frictional energy is added to the fluid as the latter passes between the primary sealing faces, or when fluid pressure is reduced below the fluid's vapor pressure because of a pressure drop across the sealing faces. In this publication, the definition of flashing is that vapor pressure is greater than 1 bar (14.5 psia) at pumping temperature.

Flashing hydrocarbon service

Any service that requires vapor suppression by cooling or pressurization to prevent flashing. This category includes all hydrocarbon services where the fluid has a vapor pressure greater than 1 bar (14.5 psia) at pumping temperature.

Flatness

The degree of flatness (peak-to-valley amplitude) of the seal faces, normally expressed in helium light bands (1 helium light band = 11.6 micro-inches (0.29 Pm)).

Flexible graphite

A pure carbon graphite material used for static gaskets in mechanical seal design, both for cryogenic and hot service.

Floating seal member (also called primary ring)

The spring-loaded seal member of a mechanical seal that is allowed limited axial movement to accommodate shaft end float and seal wear.

2-5

EPRI Licensed Material Glossary of Terms Table 2-1 (cont.) Glossary of Terms

2-6

Term

Definition

Fluid film

A film of liquid separating the seal faces, generated by hydrostatic and/or hydrodynamic lubrication.

Fluid film lubrication

Condition of lubrication in which the seal faces are completely separated by a liquid film.

Fluoroelastomer

A type of O-ring material commonly used in mechanical seals, such as Viton.

Flush

A small amount of fluid that is introduced into the seal chamber on the process fluid side in close proximity to the sealing faces and usually used for cooling and lubricating the seal faces and to prevent accumulation of solid particulates.

Flush connection

Connection to the seal chamber to allow circulation of the sealed fluid.

Free length

The unconstrained axial length of a mechanical seal.

Fretting

A combination of corrosion and wear resulting from very small amplitude relative motion. In a mechanical seal, a common example of fretting occurs when the rubbing motion of a secondary seal continually wipes the oxide coating from a shaft or sleeve. The increased surface roughness of fretted surfaces can adversely affect the ability of the floating seal member to track its mating seal ring.

Friction coefficient

Defined in a mechanical seal as the ratio of the friction force at the sealing interface to the closing force.

Gland plate

(Alternative term for seal plate.) An end plate that connects the stationary assembly of a mechanical seal to the seal chamber.

Hang-up

Failure of the secondary dynamic seal to move under the applied spring and hydraulic forces.

Hard face

Seal face manufactured from ceramic, silicon carbide, or metal.

Header tank

An external vessel providing a pressurized barrier fluid to a double seal, either with a static head or with a thermal siphon system.

Heat checking

The formation of fine radial cracks on a hard seal face caused by thermal stresses set up by inadequately lubricated or dry running and quenching by the sealed fluid.

Heat exchanger

A device for cooling a fluid by heat transfer. Heat exchangers might be internal to the pump, or externally mounted and connected with piping spools. Typically, these heat exchangers also cool the water that passes through the pump water bearing. Three types of construction are used for these heat exchangers: a tube-intube, a tube bundle, or a rotating baffle type. Cooling water might be provided from the component cooling water system (CCW).

Hook sleeve

A cylindrical sleeve with a step or hook at the product end placed over the shaft to protect it from wear and corrosion. This step is usually abutted against the impeller to hold it in place with a gasket between the shaft and the step (hook).

Hydraulic balance

Same as balance ratio.

Hydraulic load

The load on the floating seal member resulting from differential pressure between the seal chamber and the low-pressure side of the seal acting on the area of the sealing ring above the balance diameter plus that caused by pressure on the lowpressure side acting on the area of the seal ring below the balance diameter.


Definition

Hydrodynamic lubrication

Fluid-film lubrication in which the pressure in the fluid film is generated by the relative velocity between the seal faces; this can be in either a circumferential or an axial direction.

Hydrodynamic seal

A mechanical seal designed to operate with hydrodynamic lubrication between the seal faces.

Hydrostatic instability

Face separation occurring when hydraulic opening forces exceed the total closing force.

Hydrostatic lubrication

Fluid-film lubrication in which the pressure in the fluid film is generated externally to the seal faces, and is used to maintain separation of the seal faces.

Hydrostatic opening force

The separating force on the seal faces resulting from the hydrostatic pressure between the faces.

Hydrostatic seal

A mechanical seal designed to operate with hydrostatic lubrication between the seal faces. Some seals in use as main coolant pump seals are of hydrostatic film riding taper face design. These seals use large converging gap geometry designed to separate the seal faceplates by introducing pressurized fluid before the pump is rotated.

Icing

Build-up of ice on the outside of a mechanical seal caused by solidification of atmospheric water vapor through evaporative cooling of leakage of a liquid sealed above its atmospheric boiling point.

Inside mounted seal (or internally mounted)

The common arrangement with the mechanical seal mounted inside the pump or sealed vessel. No parts of the seal's flexible element or stationary faces are outside the gland. In this arrangement the sealed liquid is in contact with the outer diameter of the seal faces.

Internal circulating device

A device located in the seal chamber to circulate seal chamber fluid through an internal cooler area or an external cooler barrier/buffer fluid reservoir. Usually referred to as a pumping ring.

L10 life

A statistic used to express the life of a population of mechanical seals; it is the time when 10 percent of the seals have failed.

Lapping

Abrasive machining to achieve a very flat surface is called lapping. It can be performed by hand on a plate or by a lapping machine. A lapping machine rotates a flat surface and the parts being lapped, with respect to each other, using an abrasive as a cutting agent between the two. Abrasives used include diamond compound, aluminum oxide compound, and silicon carbide compound.

Leakage

Sealed fluid loss from the system; it includes non-obvious vapor formed by evaporation, as well as the more obvious liquid emission. Leakage might occur through secondary as well as primary seals.

Leakage rate

The volume of fluid (compressible or incompressible) passing through a seal in a given length of time. For compressible fluids, leakage rate is normally expressed in standard cubic feet per minute (SCFM), and for incompressible fluids, in terms of cubic centimeters per minute.

Light band

Refers to the wavelength of helium light (= 11.6 micro-inches, or 0.29 Pm) used as a measure of the flatness of the seal faces.

2-7


Definition

Mating ring

A disc- or ring-shaped member, mounted either on a shaft sleeve or in a housing, that provides the primary fluid seal when in proximity to the face of an axially adjustable face seal assembly.

Maximum allowable working pressure (MAWP)

The greatest discharge pressure at the specified pumping temperature for which the pump casing is designed.

Maximum dynamic sealing pressure (MDSP)

The highest pressure expected at the seal (or seals) during any specified operating condition and during start-up and shutdown. In determining this pressure, consideration should be given to the maximum suction pressure, the flush pressure, and the effect of clearance changes with the pump.

Maximum static sealing pressure (MSSP)

The highest pressure, excluding pressure encountered during hydrostatic testing, to which the seal (or seals) can be subjected while the pump is shut down.

Main coolant pump (MCP)

The term used to describe a group or family of reactor coolant pumps used in pressurized water reactors, and reactor recirculation pumps used in boiling water reactors, is main coolant pumps (MCP).

Mechanical seal

A device for sealing a rotating shaft whereby the sealing interface is located between a pair of radial faces, one rotating, the other stationary.

Mixed lubrication

Condition of lubrication where the load between the seal faces is partly carried by boundary lubrication and partly by fluid-film lubrication.

Mean time between failures (MTBF)

Mean time between failures. A statistic used to express the life of a population of mechanical seals. It is given mathematically by the expression

MTBF

L1 L 2 . . . L n n

where L1, L2, etc., are the lives of individual seals.

2-8

Neck bush

Closed clearance bush at the inner end of seal chamber to restrict flow of dirty fluid from pump into the seal chamber or to maintain pressure of recirculation flow in seal chamber.

Net closing force

The difference between the total closing force and the hydrostatic opening force.

Non-flashing

A fluid state that does not change to a vapor phase at any operating condition or operating temperature.

Non-flashing hydrocarbon service

This category includes all hydrocarbon services that are predominately all hydrogen and carbon atoms; however, other non-hydrocarbon constituents might be entrained in the stream. A product in this category does not require vapor suppression to prevent transformation from a liquid phase to a vapor phase. For this publication, the definition of non-flashing means that the vapor pressure is less than 1 bar (14.5 psia) at pumping temperature.

Non-hydrocarbon service

This service category includes all services that cannot be defined as containing all hydrogen and carbon molecules. However, some hydrocarbons might be entrained in the fluids. Included in this category are boiler feed water (and other water services), borated water, caustics, acids, amines, and other chemicals commonly used in refinery services.


Definition

Non-pusher type seal

A mechanical seal (usually metal bellows) in which the secondary seal is fixed to the shaft. A bellows seal is an example of a non-pusher seal in which the dynamic secondary seal is eliminated.

Operating length

Axial length of installed mechanical seal.

Optical flat

An optical flat is a precision ground quartz or Pyrex plate. When light waves reflect off the lapped surface through the flat, light bands are visible. The greater the gap between the flat and the lapped surface, the larger the number of light bands. When used with a monochromatic light (emits only one wavelength visible light), the number of light bands can be used to measure the flatness of the lapped parts.

Orifice nipple

A pipe nipple made of solid bar stock with an orifice drilled through it to regulate the flush flow commonly found on Plan 11 systems described in API 682. The nipple should be welded to the discharge casing.

O-ring

Toroidal sealing ring with an O-shaped (circular) cross-section, used as a secondary seal or gasket in both static and dynamic situations.

Outside mounted seal

See externally mounted seal.

Perfluoroelastomer

High temperature, chemical resistant O-ring material such as DuPont Dow Elastomer, Kalrez® or Green Tweed, Chemraz®. This material requires a wider O-ring groove than standard O-ring materials.

Popping

A term used to indicate intermittent leakage of vapor resulting from a rapid change in fluid state from liquid to gaseous and characterized by a popping sound.

Pressure breakdown cells/ staging coils

Staged seal designs in MCPs use an orifice to bypass a small flow around each seal to reduce pressure to subsequent stages. This configuration allows pressure to be evenly distributed at each seal stage. The orifice is usually either a series of small, machined grooves or a coil of small diameter tubing. These breakdown devices are referred to as pressure breakdown cells or staging coils.

Pressure casing

The composite of all stationary pressure-containing parts of the seal, including seal chamber, seal gland, and barrier/buffer fluid chamber (container) and other attached parts, but excluding the stationary and rotating members of the mechanical seal.

Primary seal

Mechanical seals have a rotating seal ring and a stationary seal ring. Fluid sealing occurs at the interface of the rotating ring and the stationary ring. The seal that occurs at this interface is often referred to as the primary seal.

Primary ring

See floating seal member.

Product

The process fluid.

Product recirculation

Circulation of the product through the seal chamber to provide cooling (see recirculation flow, reverse circulation).

Product temperature margin

Alternative name for Delta T, 'T.

Pumping ring

A device fitted inside the seal chamber to circulate the liquid in the seal chamber through an external cooler and/or header tank.

2-9


2-10

Term

Definition

Pusher type seal

A mechanical seal in which the secondary seal (for example, an O-ring, U-cup, plastic wedge ring) is mechanically pushed (and therefore can move) along the shaft or sleeve to compensate for face wear. Bellows are not classified as pusher type seals.

PV factor

A parameter used to express the severity of operating conditions for a mechanical face seal. In this publication, it is defined as the product of the pressure drop across the seal and the mean relative velocity of the seal faces.

Quench

A neutral fluid, usually water or steam, introduced on the atmospheric side of the seal to retard formation of solids or crystallization of dissolved solids that might interfere with seal movement.

Quench chamber

Enclosed space on the atmospheric side of a mechanical seal to which the quench is introduced; normally fitted with an auxiliary seal to prevent excessive leakage to the atmosphere.

RMS or Ra

Root mean square or roughness average – terms used to define surface roughness.

Random failures

Failures occurring during operation, other than early-life failures and those caused by normal wear-out of the seal faces.

Recirculation flow

Flow of the product from the pump discharge through the seal chamber to the back of the pump impeller, or from the back of the pump impeller through the seal chamber to the pump suction.

Recirculation impeller

Many MCPs have external heat exchangers mounted to the pump motor stand. These heat exchangers require the fluid to be pumped from the seal/bearing cavity to the heat exchanger and back. The recirculation impeller is normally a shaftmounted, axial flow-type impeller. Flow rates are normally in the range of 30 to 50 gpm (113 to 189 lpm) for MCPs.

Reverse balancing

Selection of the balance diameter so that a mechanical seal can withstand pressure on the inside diameter of its face rather than on the outside diameter, that is, the reverse of normal outside diameter pressurization. This is of particular use for the inboard seal of a double seal as it puts any solids on the outside diameter of the inboard seal and minimizes clogging.

Reverse circulation

Flow of the product from the back of the pump impeller through the seal chamber to the pump suction to provide cooling of the seal and reduce access of solids to the seal faces.

Rotating balance

A rotating balance seal has the balance diameter Db on the rotating member.

Rotating seal

Mechanical seal in which the floating seal member is mounted on the shaft.

Rotating seal member

The seal member that is mounted on the shaft, either directly or on a sleeve that rotates with the shaft.

Rotation (coning)

Rotation (or conical deformation) of the seal ring cross-section due to torsional ring-type axisymmetrically-distributed load applied by the differential pressure or thermal load.

Seal arrangement

The way in which a seal is mounted in the seal chamber and the method of exercising control over the liquid in the seal chamber, viz, dead-ended, product recirculation (see also API piping plan).

EPRI Licensed Material Glossary of Terms Table 2-1 (cont.) Glossary of Terms Term Seal balance ratio

Definition See balance ratio.

Seal cavity

The seal assembly fits inside the pump between the shaft and housing. The area that the seal fits into is referred to as the seal cavity.

Seal chamber

The region between the shaft and the pump case (housing) into which the shaft seal is installed.

Seal configuration

The design or style of the primary seal (for example, pusher seal, bellows seal, double seal).

Seal envelope

The external dimensions of a mechanical seal.

Seal environment

The physical and chemical conditions prevailing in the seal chamber.

Seal face width

The radial dimension of the sealing face measured from the inside edge to the outside edge.

Seal face(s)

The surfaces of the seal ring and seat in contact with each other.

Seal head

Assembly consisting of primary ring, spring, retainer, set screw, and secondary seal (see Figure 3-1).

Seal injection

Plant designs include MCPs both with and without seal injection. Many seal designers prefer units with seal injection, believing these installations to be more reliable. Seal injection is taken off the charging and volume control system on PWRs and off the control rod drive system for BWRs. Seal injection provides a source of cool filtered water entering the pump seal cavity. Filter sizes typically range from 2 Pm to 20 Pm and the supply temperature is usually between 110°F (43°C) and 120°F (49°C).

Seal plate

A plate that is bolted to the seal chamber and carries the stationary seal member.

Seal reference dimension

A reference mark scribed on the shaft to ensure that a mechanical seal is fitted with the correct operating length.

Seal ring

The floating seal member (sprung seal member) that contacts the mating ring. It can be either the stationary or rotating seal member.

Seal setting

The proper relative position of the rotating portion of the seal to the stationary portion of the seal is necessary to establish the proper seal spring force. The process of establishing this position is termed setting the seal. Some designs do not require any adjustments, only that certain dimensions be measured to confirm the seal setting dimensions. Other designs rely on taking measurements on the assembled seal prior to installation, then establishing the same reference dimension once the seal is installed in the pump.

Seal size

The maximum diameter of the shaft that will pass through the seal, that is, the diameter of the shaft (or shaft sleeve) to which the mechanical seal is fitted. (Alternative definitions based on other dimensions, for example, balance diameter, are also in current use).

Seal springs

Staged seals use coil springs to create closing force at low pressures. The force from the springs must be great enough to overcome the frictional forces from the secondary seal, but not to cause unacceptably high contact pressure when the seal is operating at low pressures.

2-11


2-12

Term

Definition

Seal tooling

Some mechanical face seals require special tools for inspection, assembly, installation, removal, and refurbishment. This collection of special tools is generally referred to as seal tooling. Seal tooling should be carefully controlled to ensure that the tools are not lost or discarded. Attempts to perform seal maintenance with inadequate tooling can result in equipment failures.

Sealant

Alternative term for barrier fluid.

Sealed fluid

Fluid in the seal chamber.

Sealed pressure

Fluid pressure in the seal chamber.

Sealing interface

Contact area between the seal ring and the seat.

Seat

The axially fixed (unsprung) sealing element. It can be either the stationary or rotating seal member.

Secondary containment

An arrangement with a chamber on the atmospheric side of a mechanical seal to contain high leakage consequent on failure. This chamber is normally fitted with an auxiliary seal.

Secondary seal

Seal used to prevent leakage through paths alternative to that between the seal faces. See dynamic and static secondary seals.

Secondary seal land

That part of the shaft or seal sleeve in contact with the dynamic secondary seal.

Service condition

The maximum/minimum temperature and pressure under static or dynamic condition.

Shaft sleeve

A sleeve fitted between the shaft and a mechanical seal to provide a wear-resistant and replaceable secondary seal land. The sleeve is sealed to the shaft with elastomers.

Shelf life

Some mechanical face seal components have a specific shelf life. These parts are usually elastomers that have a shelf life of 5 to 10 years when properly stored. Additionally, lapped parts should always be verified prior to installation. Occasionally, lapped parts will distort over time and need to be relapped prior to installation.

Single seal

A seal arrangement with only one mechanical seal regardless of whether other seal types (for example, throttle bush, lip seal) are included in the seal arrangement.

Slotted seal gland plate

A gland plate with slots instead of holes for the mounting studs.

Soft face

Seal faces manufactured from a relatively softer material (for example, carbongraphite or PTFE) as compared to a harder mating seal face material (for example, tungsten carbide).

Solid length

The axial length of a fully compressed mechanical seal.

Specific load

Face load per unit area of sealing interface.

Spring load

The load on the floating sealing element exerted by the seal spring(s).

Spring pressure

The average seal face pressure due to spring load.


Definition

Stage

Many MCP seals use multiple mechanical seals in series, each seal having a predetermined differential pressure created by the controlled bleed-off. Each individual seal in this style design is termed a stage. Seals of this type of design are termed staged seals.

Start-up torque

The torque transmitted/absorbed by a mechanical seal on start-up.

Static secondary seal

Seal used to prevent leakage between assembled parts that are not subject to relative motion in service, for example, between seal sleeve and shaft, between stationary seal member and seal plate.

Stationary balance

A stationary balance seal has the balance diameter Db on the stationary member.

Stationary seal

Mechanical seal in which the floating seal member is mounted on the seal plate.

Stationary seal member

The seal member that is mounted on the seal plate.

Stationary seal ring

The stationary seal ring is mounted in a supporting piece called a gland, carrier, holder, or ring support. In staged seal designs, the seal ring is generally a soft material, normally carbon graphite. In hydrostatic seals, the stationary member consists of an aluminum oxide or silica nitride faceplate mounted on a ring support.

Stator

Alternative term for stationary seal member of a mechanical seal.

Stuffing box

Alternative name for seal chamber, carried over from soft-packing technology.

Tandem seal

Seal configuration consisting of a pair of mechanical seals mounted in series (that is, two mechanical seals sealing in the same direction).

Thermal stress failure

Alternative term for heat checking.

Throat bushing

A device that forms a restrictive close clearance around the sleeve (or shaft) between the seal and the impeller.

Throttle bush

A close-fitting bush around the shaft to restrict flow; can be used at the inner end of the seal chamber (neck bush) or as an auxiliary seal.

Throttle bushing

A device that forms a restrictive close clearance around the sleeve (or shaft) at the outboard end of a mechanical seal gland.

Total closing force

The sum of the hydraulic load and spring load acting on the floating sealing member to close the seal faces.

Total indicated runout (TIR)

Also known as total indicator reading, is the runout of a diameter or face determined by measurement with a dial indicator. The indicator reading implies an out-of-squareness or an eccentricity equal to half the reading. TIR is measured by securing a dial indicator to either the stationary or rotating component, setting the dial indicator to zero, and then rotating either component.

2-13


Definition

Toxicity rating

Classification of fluid toxicity defined in N. Irving Sax Dangerous Properties of Industrial Materials, 1984. Toxicity Rating: 0 = No harmful effects under normal conditions 1 = Short-term effects that disappear once exposure is removed 2 = May produce both short- and long-term effects, but normally not lethal 3 = May cause death or permanent injury even after short exposure to only small quantities U = Insufficient data available on humans

2-14

Unbalanced seal

A mechanical seal in which the balance ratio is greater than or equal to 1.

U-ring

A "U" section dynamic secondary seal.

Vent connection

A connection to the seal chamber for eliminating gas or vapor from the seal chamber. This is normally accomplished through a gland connection, such as the flush connection.

Volatile hazardous air pollutants (VHAP)

Any compound as defined by Title I, Part A, Section 112 of the National Emission Standards for Hazardous Air Pollutants (Clean Air Act Amendment).

Volatile organic compound (VOC)

Term used by various environmental agencies to designate regulated compounds. Emissions are measured as PPM with a calibrated analyzer.

V-ring

A V section dynamic secondary seal.

Waviness

Deviation of the seal faces from circumferential flatness. Waviness can be present on the faces as manufactured or can develop after running.

Wear track

The wear mark of the narrower seal face on the wider one.

Wedge ring

A wedge-section dynamic secondary seal, usually manufactured from PTFE.

Support surface

Most seal designs provide some type of support surface for the seal rings to control seal ring deflection. Different terminology might be used for these surfaces, such as seat or back seat. In this publication, surfaces controlled to limit seal ring deflections will be referred to as support surfaces.

Thermal barrier

Most MCP designs are insulated from the high Reactor Coolant System (RCS) temperatures by a thermal barrier. The thermal barrier reduces pump cover (or main flange), pump water bearing, and shaft seal cavity temperatures.

Total outflow

The combined flow, consisting of seal leakage and controlled bleed-off, which leaves the seal cavity is referred to as total outflow. This flow rate is the amount of fluid that leaves the seal cavity and is made up with injection or RCS that has been cooled through the seal heat exchanger.

Wear tracking

The mating surfaces of both hydrostatic and hydrodynamic seals operate in close proximity. The faces might either contact or have particulates contact the seal ring faces, resulting in a circular grooving or wear pattern referred to as wear tracking.


3 TECHNICAL DESCRIPTION

3.1

Operating Principles and Basic Components of a Mechanical Face Seal

A mechanical face seal is a dynamic seal that prevents leakage of pressurized fluid between a rotating shaft and a stationary housing. Mechanical face seals are available in a variety of configurations, and their selection depends on the application. However, no matter what the application is, all mechanical face seals operate on the same principle. Basically, the seal is comprised of two rings, either of which rotates relative to the other. One of the rings is usually mounted rigidly and the other is mounted so that it can flex and align axially and angularly with the rigidly mounted ring. Dynamic sealing is achieved at the interface between the two rings, the primary ring and the mating ring. The rings achieve a seal at the interface due to their very high face flatness. Typically, the two rings are made of dissimilar materials. The essential elements of a mechanical face seal are illustrated in Figure 3-1. These elements serve the functions of sealing dynamically and statically, loading the faces, and transmitting rotation to the ring. The essential elements are described below. Advantages and disadvantages of various configurations of these elements are discussed in Table 3-2.

Figure 3-1 Essential Components of a Mechanical Face Seal

Primary Ring: The primary ring is also called a seal ring. The primary ring is the floating seal element that is usually spring-mounted and permits axial and angular alignment in the assembly. Depending on the application requirements, it can be either the rotating member as shown in Figure 3-1 or the stationary member as shown in Figure 3-10. The method in which the primary 3-1

EPRI Licensed Material Technical Description

ring is mounted is dictated by the application requirements because each configuration offers both advantages and disadvantages. The mechanical face seal design or style is defined by the primary ring configuration, that is, rotating primary ring, stationary primary ring, double seal, bellows seal, and so on. Mating Ring: The mating ring is also called a seat or seal seat. The mating ring is the rigidly mounted element and can be installed in the housing as shown in Figure 3-1 or on the shaft as shown in Figure 3-10. Where the mating ring is installed is dependent upon the application requirements and the preferred implementation of the primary ring. Key Technical Point Mechanical face seals come in a variety of configurations, materials, and designs for primary sealing faces, secondary seals, springs, drive mechanisms. Options also include unbalanced or balanced designs, whether the primary seal or the mating seal is rotating, and whether the fluid pressure is on the outside or the inside surface of the seal. Seal design for a given application should be selected after a careful evaluation of trade-offs, as discussed in this section, Section 3. Secondary Seal: Seals used to prevent leakage through paths alternative to that between the seal faces. The secondary seals can be static or dynamic. Static secondary seals prevent leakage between assembled parts that are not subject to relative motion in service, for example, between seal sleeve and shaft, between stationary seal member and housing. Dynamic secondary seals prevent leakage between the shaft or housing and the floating seal member. The type of secondary seal depends on the fluid type, service pressure, and service temperature. Table 3-1 provides the operating temperature limits and properties of materials typically used for secondary seals.

3-2

EPRI Licensed Material Technical Description Table 3-1 Secondary Seal Properties Material

Nitrile

Temp °F

°C

-22 to 248

-30 to 120

Air Permeability

Properties

0.25-1.00

• General purpose • Low cost • Oil resistant • Attacked by ozone

Ethylene Propylene

-58 to 302

-50 to 150

9.6

• Steam, ozone, acid, and alkali resistant

Silicone

-67 to 392

-55 to 200

170-260

• Good at low temperature • Easily damaged • High permeability

Neoprene

-31 to 248

-35 to 120

104

• Weather resistant • Fair oil resistant

Fluoroelastomer

14 to 302

-10 to 150

0.32

• Oil, fuel, chemical resistant

PTFE

-67 to 446

-55 to 230

Polyacrylate

-22 to 347

30 to 175

1.5

• Hot oil and ozone resistant

Epichlorohydrin

-40 to 302

-40 to 150

.015-0.70

• Oil resistant

• Resistant to virtually all fluids

• Low permeability Metal Bellows

High Temperature Fluoroelastomer

-328 to 1202 650

-200 to

12 to 545

-10 to 285

• Positive seal • Chemical resistant 0.32

• Excellent chemical resistant

Spring Springs are used to develop the contact load between the primary ring and the mating ring in the absence of fluid pressure. The amount of face load generated can vary significantly depending on the type of spring selected. The choice includes a single coil spring, multiple coil springs, metal bellows, non-metal bellows, wave or Belleville washer, and magnets (see Figures 3-2 to 3-7). In some cases, such as bellows, the spring can serve both the face-loading function and the secondary sealing function. Advantages and disadvantages of each type of spring are summarized in Table 3-3.

3-3


Figure 3-2 Multiple Coil Springs

Figure 3-3 Single Coil Springs

Figure 3-4 Corrugated Bellows

Figure 3-5 Welded Bellows

3-4


Figure 3-6 Rubber Bellows

Figure 3-7 Belleville Washers

Drive Mechanism: All mechanical face seals require some kind of device to position the primary ring axially and to transmit the rotation of the shaft to the primary ring to ensure that relative motion occurs only at the seal faces. The drive mechanism is designed such that it is not rigidly attached to the primary ring so that it does not prevent self-alignment between the primary ring and the mating ring. The drive mechanism is typically a setscrew, locking collar, key, or wedge ring. In some designs, the secondary seal is used to transmit the torque to the primary ring when sufficient friction can be developed at the secondary seal interface. The drive mechanism is also used to provide torque restraint to the stationary seal if the static secondary seal does not develop sufficient friction to prevent the stationary seal from turning. Seal/Flushing Chamber: An area around the seal is provided to permit heat transfer through the fluid and to allow flushing of contaminants such as abrasive particles or toxic media. In a singleseal configuration, flushing is accomplished by injecting a liquid into the seal chamber at a higher pressure than the sealed product.

3-5

EPRI Licensed Material Technical Description Table 3-2 Advantages and Disadvantages of Mechanical Face Seal Configurations Type of Seal Internallymounted primary seal

• • • • • • •

Externallymounted primary seal Rotating primary seal

• • • • • •

•

Stationary primary seal

• • • • • •

3-6

Balanced seal

•

Unbalanced seal

• • •

Advantages Better cooling - seal surrounded by product Pressure acts to close the seal faces (pressure assisted) Can therefore be used at high pressure Components in compression (preferable to tension) Rotating elements centrifuge particles away from seal face Lower leakage due to centrifugal action Most of the seal is inside machine housing, less space required outside housing Seal leakage containment is simpler Easier to install/replace Easier to inspect Minimizes components in contact with pumped fluid (corrosives, etc.) Centrifugal action keeps particles away from flexible member Generally requires less axial envelope, particularly outside seal chambers Smaller radial section for a given shaft size Generally lower cost Capable of higher speeds Better able to cope with misalignment (particularly angular) Less prone to clogging if leaked product is inside seal chamber Will accept media with higher viscosity Less friction loss due to turbulence of liquids Capable of much higher pressures and/or speeds (enhanced Pressure, Velocity (PV) capability) Smaller envelope, particularly radial No step required on shaft or sleeve Lower cost

Disadvantages • No access for visual inspection • Any repair/replacement is labor intensive

• Subject to environmental contamination and external damage from other environmental factors

EPRI Licensed Material Technical Description Table 3-2 (cont.) Advantages and Disadvantages of Mechanical Face Seal Configurations Type of Seal Non-metal bellows

• • •

Dynamic pusher seal

• • •

Metal bellows

• • • • • • •

Single spring seal

• • • • • •

Multi-spring seal

• • •

• Wave/Belleville Magnetic coupling

Advantages PTFE bellows used in very severe corrosive duties Rubber bellows seal low in cost Eliminates sliding packing (hang-up hysteresis, sleeve wear) More robust Higher pressure/temperature/speed capability Rubber bellows require specially designed components in a variety of materials to cope with different media Less prone to fatigue failure More tolerant to shock and vibration Eliminates sliding packing (hang-up hysteresis, sleeve wear) Can be used at higher temperatures Can be used at higher speeds Inherently balanced without stepping shaft/sleeve More compact (particularly larger sizes) Can be used for a flexible drive Larger section, more robust Better protection against corrosion Less prone to clogging Smaller radial space Low stiffness gives greater axial tolerance on fitting Shorter axial length Rotating seal can tolerate higher speeds Independent of direction of rotation (some single spring designs are also independent) More consistent loading onto face

• Reduces axial length

Disadvantages • Rubber bellows require specially designed components in a variety of materials to cope with different media

• Not suitable for high pressures

• Small axial tolerance • Limited seal face loading • Requires the use of materials that can be magnetized • Reduces the choice of materials suitable for corrosive environments

3-7

EPRI Licensed Material Technical Description Table 3-3 Advantages and Disadvantages of Mechanical Face Seal Springs Type of Spring Single coil

Advantages

Disadvantages

• Corrosion, blockage resistance

• Uneven loading

• Low stress levels

• Requires more axial space

• Low cost

• Difficult to compress as size increases

• Greater axial tolerance

• May unwind/tighten at high speeds Multiple coils

• Less axial space required

• Less corrosion/blockage resistance

• Even face loading

• High stress levels

• Resists high speeds

• More costly

Wave/Belleville washer

• Saves space

• High spring rate

Elastomer bellows

• Also provides secondary seal

• Cannot be used in all fluids

• Relatively inexpensive

• Has temperature limitations

• Provides secondary seal

• Expensive

• Corrosion resistant

• Requires more space than coil springs

Corrugated/welded metal bellows

• Generally high cost

• High temperature • High controlled spring rate

3.2

• Provides little damping to vibration

Major Design Variations

Design variations of the basic mechanical face seal illustrated in Figure 3-1 permit extending the application range and life of the seal. The configuration variation description is based on two primary factors: x

Whether the primary ring is rotating or stationary

x

Location of the pressure relative to the annulus

A combination of these two parameters results in the four configurations illustrated in Figures 38 through 3-11. Figures 3-8 and 3-9 show rotating primary rings where pressure is applied to the outside diameter of the seal and the inside diameter of the seal, respectively. Conversely, Figures 3-10 and 3-11 show a stationary primary ring with pressure on the outside and inside of the seal, respectively. A description of each configuration, with its advantages and disadvantages, is given in Table 3-2. Rotating Primary Ring - Outside Pressure: This configuration (Figure 3-8) is also referred to as a rotating primary ring - inside mounted. In this configuration, the primary ring is mounted on the shaft inside the stuffing box and pressure is applied on the outside diameter of the seal faces. A major advantage of this setup is that the product surrounds the face seals to provide good cooling.

3-8


Figure 3-8 Rotating Primary Ring - Outside Pressure (or Inside Mounted)

Rotating Primary Ring – Inside Pressure: This configuration (Figure 3-9) is also referred to as rotating primary ring - outside mounted. In this configuration, the primary ring is mounted outside the stuffing box and pressure is applied to the inside diameter of the seal faces. These designs are easier to install and inspect than the other configurations. Because the pressure works to push apart the seal faces, this design is not suitable for high pressures.

Figure 3-9 Rotating Primary Ring - Inside Pressure (or Outside Mounted)

Stationary Primary Ring – Outside Pressure: This configuration (Figure 3-10) is also referred to as stationary primary ring - inside mounted. In this configuration, the primary ring is mounted on the housing inside the stuffing box and pressure is applied on the outside diameter of the seal faces. This design offers higher speed capability with ease of inspection. Because the rotating ring does not have multiple parts, this configuration is less susceptible to imbalance.

Figure 3-10 Stationary Primary Ring - Outside Pressure (or Inside Mounted)

3-9


Stationary Primary Ring – Inside Pressure: This configuration (Figure 3-11) is also referred to as stationary primary ring - outside mounted. In this configuration, the primary ring is mounted on the housing inside the stuffing box and pressure is applied on the outside diameter. This design also offers high-speed capability and is less susceptible to imbalance due to a single rotating ring.

Figure 3-11 Stationary Primary Ring - Inside Pressure (or Outside Mounted)

3.3

Multiple Seals

Some applications require the use of multiple seals to provide for flushing or barrier fluids, or pressure staging to deal with higher pressures. Flushing is used to remove contaminants, to cool the faces, or to provide for proper lubrication. This is achieved by installing the seals in a backto-back or face-to-face configuration, as illustrated in Figures 3-12 and 3-13. For cooling and solids/abrasives removal, fluid can be re-circulated from the product side or provided by an external source. In applications where the product has a relatively low vapor pressure, for example, water or hydrocarbons, a barrier fluid with a higher vapor pressure is used to keep the product from vaporizing at the seal interface and to prevent the inboard seal from running dry. If the product is toxic or harmful, a clean barrier fluid is introduced at a higher pressure to minimize toxin release. The outboard seal also provides a back-up in case of failure of the product seal.

Figure 3-12 Back-to-Back Dual Seal

3-10


Figure 3-13 Face-to-Face Dual Seal

Key Technical Point Some applications require the use of multiple seals to provide for flushing or barrier fluids, or pressure staging to deal with higher pressures. Flushing is used to remove contaminants, to cool the faces, or to provide for proper lubrication. Selections include back-to-back, face-to-face double arrangements, and a choice of buffer fluid or barrier fluid, depending upon application. Pressure staging is accomplished by using multiple seals installed in series (shown in Figure 3-14) so that the fluid pressure between any two cavities is limited to the maximum service pressure limit of the mechanical face seal for the particular product fluid. Pressure staging permits isolating very high pressures that cannot be handled by a single mechanical face seal. Pressure staging usually requires the use of an intermediate fluid that is circulated to keep the seals cool. This is because stagnant fluid in the seal cavity is ineffective in removing the heat generated at the sealing interface, which can create hot pockets that cause the seal to malfunction.

Figure 3-14 Pressure Stage Tandem Seal

3-11


3.4

Seal Cartridges

Seal cartridges are pre-assembled mechanical face seal assemblies that contain all of the essential components. Cartridges are used to package mechanical face seals for ease of handling and installation. An example of a single seal cartridge is shown in Figure 3-15. In this arrangement, the primary ring and its associated devices are mounted on a sleeve temporarily attached to the enclosure that holds the mating ring. The assembly provides for proper spring loading and axial positioning of the primary ring and mating ring. After the cartridge is mounted on the housing and the sleeve is secured to the shaft, the temporary attachment device holding the sleeve to the mating ring enclosure is removed.

Figure 3-15 Single Seal Cartridge

Cartridges can be provided with either rotating primary rings or stationary primary rings and with single or multiple mechanical face seals. The schemes for assembling cartridges vary from design to design. Figure 3-16 shows a multi-stage balanced stator design seal cartridge assembly and Figure 3-17 shows details of one of the stages. This seal design is one of the four alternative designs commonly used in a critical application (Main Coolant Pump) in U.S. nuclear power plants [35]. Key O&M Cost Point Seal cartridges are pre-assembled mechanical face seal assemblies that contain all of the essential components. Cartridges are used to package mechanical face seals for ease of handling and installation. Even though material cost is higher, cartridges save money by simplifying maintenance and eliminating installation related failures.

3-12


Figure 3-16 Balanced Stator Design Multi-Seal Cartridge Supplied by a Manufacturer for a Main Coolant Pump [35]

3-13


Figure 3-17 Seal Stage Details of a Balanced Stator Design Multi-Seal Cartridge Supplied by a Manufacturer for a Main Coolant Pump [35]

3-14


3.5

Seal Chamber Design and Flushing

The seal chamber is sometimes referred to as the seal cavity or seal box. Figure 3-18 shows the most common variations in the seal chamber designs in centrifugal pumps. The seal chamber is the cavity where the mechanical face seal resides and is often the same stuffing box chamber that was designed to house conventional soft packing. As such, the chamber provides only limited volume for the fluid to circulate naturally. Lack of circulation leads to hot spots in the face seal, and the stagnant cavity allows solids to settle. To overcome these space limitations, either an alternative seal chamber design can be used or the seal chamber can be equipped with a means to circulate fluid. Depending on the application, the circulated fluid can be the process fluid or an external fluid selected to provide better conditions in which the seal can operate, or to control the release of contaminants.

Figure 3-18 Common Variations in Seal Chamber Design

3-15


Based on research in seal chamber designs [7,48], it is now well established that enlarged seal chambers, and the use of tapered bore chambers, can dramatically lower fluid temperature and seal face temperatures. Wherever the envelope constraints in a given pump application permit, the seal chamber should be enlarged to improve the seal performance/life due to lower temperatures and increased fluid circulation around the seal. The seal chamber design also plays a critical role in obtaining satisfactory performance from mechanical face seals handling abrasive slurries. Key Technical Point Mechanical seals are often installed in the same cavity that is designed to accept conventional packings. This limits the fluid circulation around the seal, leading to high seal temperatures and accumulation of solids. An enlarged seal chamber with tapered bore can dramatically improve fluid circulation, lowering seal temperature and eliminating accumulation of solids. In addition to the chamber design, seal flushing is dictated by application requirements in many cases to achieve satisfactory performance. API Standard 682 describes 17 plans to flush the seal chamber [8]. Selection of the type of plan needed will depend on the process fluid and operating temperature. Fluids having high vapor pressures (for example, hot water, light hydrocarbons, etc.), high temperature, containing abrasives (for example, service water, slurries, etc.), or containing dissolved solids (for example, borated water) are common mechanical seal application problems that can benefit from flushing. The most common API Standard 682 flush plans used with clean process fluids are Plan 11 and Plan 21. Plan 11 is illustrated in Figure 3-19. To control the amount of fluid re-circulated, a throttle bushing is incorporated inboard of the mechanical face seal and a control orifice is installed in the flush line. Flow enters the seal chamber adjacent to the mechanical face seal, flushes the faces, and flows across the seal back into the pump. Plan 21 is similar to Plan 11 except that a cooler is installed in the flush line in series with the control orifice. For contaminated process fluids, strainers/filters can be added to clean the flush fluid.

Figure 3-19 A Typical Flush Plan for a Cooling Seal Chamber

3-16


3.5.1 Seal Arrangements for Abrasive Applications Abrasives will generally cause rapid wear of the faces while excessive heat from the pumped fluid, or as a result of seal friction, will damage the elastomers and distort seal components, causing the seal to leak and fail [49,50,51,55]. The seal should be provided with a clean, relatively cool, abrasive-free flush to lubricate and remove the heat generated by the seal faces and to prevent flashing at the seal faces. A clean liquid from an outside source can be used. However, the resulting contamination of the pumped product by an external source might make this type of flush undesirable. For this reason, a re-circulated or bypass fluid from the liquid being pumped is frequently used. If necessary, this re-circulated flush fluid can be cooled and any abrasive particles removed before it is injected into the seal. When multiple seals, as shown in Figures 3-12, 3-13, or 3-14, are used, a combination of internal and/or external seal flush arrangements can be used. In severe abrasive duty applications (for example, clinker grinder in fossil plants and abrasive slurry handling pumps), mechanical face seals have a history of unreliable performance and short life, even when flushing arrangements are used [50, 51]. This is due to the fact that, in addition to exposure to harsh abrasive particles, seals are exposed to large shaft deflections (both static and dynamic), frequent starts/stops, transients, shock, and vibration, which exceed the capabilities of face seals. Similar sealing problems in downhole drilling applications have been solved by an alternative elastomeric seal design employing hydrodynamic lubrication [52, 53, 54]. This design might be a potential solution to the fossil plant slurry handling equipment and sealing problems where application conditions are unsuitable for mechanical face seals.

3.6

Closing Force

In order for the face seal to function properly, a certain amount of face load is required. Face loading is developed by the energizing springs and by the process of pressure acting on the unbalanced area of the seal. The closing force is the sum of the spring load plus the fluid pressure, multiplied by the unbalanced area, and is expressed as: Fclosing

= =

Spring (Fs) + Hydraulic closing force (Fh) Fs + Af ('P x B + P2)

= = = = = = = =

Pressure drop (P1-P2) Face area Balance ratio Upstream pressure Downstream pressure Closing force Spring force Hydraulic force

Where,

'P Af B P1 P2 Fclosing Fs Fh

3-17


The total closing force, Fclosing, is supported primarily by the fluid film pressure (p) between the seal faces, and the residual force is supported by mechanical asperity contact (pm) between the faces: Fclosing

=

k p Af + pm Af

In this equation, k is a factor that can vary between zero and 1.0, depending upon the actual pressure distribution across the face. k = 0.5 for linear pressure distribution > 0.5 for convex pressure distribution, < 0.5 for concave pressure distribution The value of k depends upon whether the faces are parallel convergent or divergent (Figure 4-3) as further discussed in Section 4.4.4. 3.6.1 Balance Ratio Mechanical face seals can be of an unbalanced design, a fully balanced design, or partially balanced design to reduce the face loading due to hydraulic pressure, as shown in Figure 3-20. The term balanced refers to the case where B < 1.0, or where the average pressure load on the face is less than the sealed pressure. Most mechanical face seals have a balance ratio between 0.65 to 0.85. This range provides reduced face loading while maintaining stability. The seal can become hydraulically unstable or the seal faces can separate under pressure fluctuations if the balance ratio becomes less than 0.65. Seals with a balance ratio greater than 1.0 are termed unbalanced, that is, these seals have an average pressure load on the face that is greater than the sealed pressure. While most seals that operate at high pressure are of the balanced type, many low-pressure seals operate at B > 1.0 because of the convenience of design.

3-18


Figure 3-20 Unbalanced, Balanced, and Partially Balanced Seal Designs

3-19


Key Technical Point Mechanical face seals can be unbalanced, fully balanced, or partially balanced to reduce the face loading due to hydraulic pressure. The term balanced refers to the case where the average pressure load on the face is less than the sealed pressure. Most mechanical face seals have a balance ratio between 0.65 to 0.85. This range provides reduced face loading without potential concern of face parting. The term balance ratio is used to describe the fraction of the fluid pressure that is acting to close the seal faces. It is defined as the ratio of hydraulic loading area to the seal interface area. If a bellows seal is used, the effective sealing diameter must be calculated. Balance ratios are calculated as follows. For externally pressurized seals:

Be

S/4 D 2o D 2b S/4 D o2 D i2

2 D2 b

D o

2

D o

D i2

For internally pressurized seals:

Bi

S/4 D 2b D i2 S/4 D o2 D i2

2 D2 b i 2 2 D o D i

D

For bellows seals, the mean diameter can be used or, alternatively, diameter Dsb is substituted for diameter Db:

D 2bo D 2bi

D sb Where B Be Bi Do Di Db Dsb Dbo Dbi

3-20

2 = = = = = = = = =

Balance ratio (Be or Bi) Balance ratio for externally pressurized seals Balance ratio for internally pressurized seals Outside interface diameter Inside interface diameter Balance diameter Effective sealing diameter for bellows seals Outside diameter of bellows Inside diameter of bellows


3.6.2 Pressure Distribution Between the Sealing Faces In any standard seal design configuration, the hydraulic pressure acts across the seal interface either from the OD to the ID or vice versa. In either case, the fluid film pressure between the faces at the point of action is a maximum that is reduced across the interface to the pressure on the downstream side at the opposite side of the contact area. Although several theories have been advanced that define the pressure gradient across the faces as being either linear, concave, or convex, no one theory has gained general recognition. In fact, the pressure gradient varies during operation due to seal wear and deflections caused by pressure and temperature changes. Whatever the true pressure gradient across the face might be, the film pressure tends to separate the contact faces of the primary seal rings, opposing the closing forces due to the mechanical spring load and the hydraulic pressures acting on the unbalanced area of the seal. However, in most mechanical seal designs, the resultant force from the film pressure does not completely balance the closing forces and the small residual force is supported by the mechanical contact of the asperities on the faces. Key Technical Point Pressure distribution across the seal face width can be linear, concave, or convex and it can change with variations in pressure, temperature, and seal wear. This can affect seal performance (leakage, torque, temperature) during operation. Figure 3-21 shows how the closing force due to spring pressure and hydraulic imbalance is in equilibrium with the pressure. Based on a linearly varying pressure gradient, the seal would be 100 percent balanced when the hydraulic area is one-half the face area. Making the hydraulic area less than half the seal face area would then cause the hydraulic pressure to separate the faces in the absence of spring force.

Figure 3-21 Face Pressure Distribution Due to Hydraulic Pressure and Spring Force

3-21


3.6.3 Stationary Versus Rotating Seal Balance The balance ratio can be affected by the way the pressure area is defined. The same balance ratio can be achieved by two different primary ring and mating ring geometries, depending upon which one of the two faces is the narrower face. If the stationary ring (mating ring) defines the pressure area, as shown in Figure 3-22(a), the face load due to pressure can vary around the circumference if the mating ring is offset radially with respect to the primary ring. The differential pressure area defined by the diameter Do on the mating ring and the shaft diameter Db would be maximum in the direction of the offset and minimum on the opposite side. This circumferential variation in the seal face load exerts moments on the seal faces that can cause vibrations and instability, and affect seal performance. This problem can be eliminated by defining the differential pressure area using the face of the rotating member as shown in Figure 3-22(b). Additional considerations related to primary and secondary seal wear when selecting a rotating balance or stationary balance design are discussed in Section 4.4.6.

Figure 3-22 Rotating Seal Balance Designs

3-22


3.7

Pressure Velocity (PV) Parameter and Limit

The measure of a seal to provide useful service is defined by its PV parameter that, like Journal bearings, is the product of the pressure and the sliding velocity. Two ways are used to define the PV parameter. The first method uses differential pressure multiplied by the average sliding velocity, and the second method uses net face pressure multiplied by the average sliding velocity. The more common method used by mechanical face seal manufacturers and users to rate the PV parameter, is the differential pressure drop method because it can be easily related to seal operating pressure and balance ratio does not need to be known. Table 3-4 provides the PV values (based on differential pressure approach) for materials commonly used in both unbalanced and balanced mechanical face seals. In general, the unbalanced seal design is simpler and less costly, and is the preferred choice if it satisfies the PV limits for a given application. The balanced seal design permits operation under higher pressure and speed combinations but it requires a stepped shaft or stepped sleeve arrangement, which is generally more expensive. If the fluid is clean (free of abrasives/solid particles) and is compatible with the carbon material, the carbon versus the appropriate harder material combination should be selected. For non-clean fluids, both seal faces need to be hard to provide satisfactory wear life. Table 3-4 Approximate PV Limits psi-ft/min (Mpa-m/sec) for General Seals with Various Combinations of Seal Face Materials and Fluids Water and Aqueous Liquids Face Material Combination

Unbalanced

Balanced

Other Liquids Unbalanced

Balanced

Carbon vs. 4

1.45x 10 (3)

4

1.01 x 10 (3.5)

x Stainless steel

1.45 x 10 (0.5)

x Lead bronze

7.23 x 10 (2.5)

x Stellite

7.23 x10 (2.5)

x Alumina

1.01 x 10 (3.5)

3

5

4

2.46 x 10 (8.6)

5

1.45 x 10 (5)

5

6.08 x 10 (21)

5

2.60 x 10 (9)

5

1.22 x 10 (43)

5

1.22 x 10 (43)

5

1.82 x 10 (64)

x Chrome oxide

2.03 x 10 (7)

x Tungsten carbide

2.03 x 10 (7)

x Silicon carbide

2.60 x 10 (9)

5

1.68 x 10 (59)

6

5

1.22 x 10 (43)

5

3.53 x 10 (124)

5

5.35 x 10 (188)

5

1.22 x 10 (43)

5

3.04 x 10 (106)

6

6 6

2.60 x 10 (9)

6

2.60 x 10 (9)

5

2.03 x 10 (7)

6

2.60 x 10 (9)

6 6

Tungsten carbide vs. x Tungsten carbide x Silicon carbide

5

1.30E+10 (4.6) 5

1.74E+10 (6)

7.52 x 10 (26) 1.04 x 10 (36)

6

6

3-23


3.8

Temperature Considerations and 'T Limit

For a mechanical seal to function reliably, a fluid film needs to be maintained between the seal faces. Operation of the seal results in frictional heat generation at the sealing interface, which lowers the fluid viscosity and the load carrying capacity of the liquid film. The load bearing capacity can decrease sufficiently and result in heavy contact between the seal face, causing severe wear or face damage. The frictional heat can also raise the temperature of the liquid film at the sealing interface to such an extent that fluid instantaneously changes its phase from liquid to gaseous under the pressure that is present on the low-pressure side of the seal. This phase change often causes an intermittent banging or popping sound and results in severe face damage and excessive leakage. During seal operation, it is necessary that a stable liquid film be maintained, considering the anticipated increase in temperature ('T) due to the seal friction over the bulk fluid temperature. Figure 3-23 shows how pressure and temperature affect the boiling point of a liquid, and the 'T margin that needs to be maintained between the bulk fluid temperature and the boiling point curve to accommodate the increase in fluid temperature at the sealing interface without causing vaporization. This figure also shows the operating envelope for seal performance defined by the pressure/temperature limits (including the 'T margin), as well as the PV limit. Cooling of the seal chamber (for example, by using one of the flushing arrangements described in Section 3.5) protects against boiling of the fluid, as does an increase in the chamber pressure above the vapor pressure. The most suitable approach to suppress boiling and ensure adequate 'T margin below the limit depends upon the application. Technical performance data regarding the 'T margin should be obtained from seal manufacturers to evaluate and ensure reliable operation in a given application. Key Technical Point For satisfactory performance, the seal design and material selections should satisfy the PV limit and the 'T limit under all operating conditions to ensure that fluid film is maintained between the seal faces. Loss of film can lead to immediate seizure and seal failure.

3-24


Figure 3-23 Pressure/Temperature Operating Envelope Showing 'T Margin Required for Seal Operation

3.9

Improved Seal Face Designs

A fundamental requirement for a mechanical face seal to function reliably is that the faces be separated by a thin fluid film during operation. In practice, a small amount of asperity contact between the faces occurs in most applications, causing a small amount of wear that determines seal life but does not affect seal performance. Under high pressure and high temperature combinations, the film thickness decreases and the asperity contact between the faces increases, which in turn increases seal friction and heat (see Section 4.4.1 for further discussion). This limits the pressure, temperature, and speed performance envelope, as well as, reliability of the conventional flat face mechanical seals. The problem becomes especially severe when sealing hot water and other low lubricity fluids [21-34]. One approach that has proven to be successful for sealing hot water under high pressure and high speeds, as well as for sealing other high-volatility, low-lubricity fluids, is the use of seal face designs that have positive hydrodynamic lubrication features. Figure 3-24 is the first design that became commercially successful and is widely used in critical hot water sealing applications (including Main Coolant Pumps) in many European nuclear power plants and some U.S. nuclear power plants [3]. In this design, the cooling notches or thermal hydrodynamic grooves introduce circumferential waviness of the seal face due to variations in the temperature around the seal circumference.

3-25


Figure 3-24 Seal Face with Thermal Hydrodynamic Grooves for Positive Hydrodynamic Lubrication [3]

The circumferential waviness in conjunction with the relative rotational velocity between the faces introduces a strong hydrodynamic action, higher film pressures, and a thicker film. This is the fundamental mechanism responsible for extending the performance envelope of the seals with hydrodynamic grooves on the seal face. It should be noted that the higher pressure and speed capabilities are achieved at the cost of increased leakage and vulnerability of the seal to ingest debris and unfiltered solid particulates in the fluid. The manufacturer of the specific seal design being considered should be consulted for their recommendations and their experience in similar applications. Prototype qualification testing is strongly recommended for critical service applications. As shown in Figure 3-25, the hydrodynamic grooves can be incorporated on the seal face to pick up fluid from either the outer or the inner periphery, depending upon the application requirements. Figure 3-26 shows several other variations of this basic approach to enhance the lubrication between the seal faces.

3-26


Figure 3-25 Design Options with Hydrodynamic Grooves on the Outer Periphery or Inner Periphery of Seal Face

Figure 3-26 Other Variations in Seal Face Geometry to Enhance Lubrication of the Faces

3-27


Several alternative designs that also maintain a full hydrodynamic film lubrication under high duty application conditions (including transients) have been reported over the years since the successful commercial introduction of the design shown in Figure 3-24. These include eccentric seals for nuclear pumps, optimized grooves face seals, Rayleigh-step floating-ring seals, movingwave mechanical face seals, and polymer seal rings sliding against silicone carbide [37-41, 47].

Key Technical Point Seal designs with special features to enhance lubrication at the sealing interface (for example, hydrodynamic grooves, recesses, or laser-textured surfaces) can extend the pressure, speed, and temperature limits. The tradeoff (for example, higher leakage rate versus increased reliability under transient conditions) should be carefully evaluated during seal selection.

Research in recent years has shown that the newest technology, laser-textured surface designs, are capable of providing the full film lubrication (and therefore long life) without the penalty of excessive leakage associated with the earlier hydrodynamic film seal designs. These include laser-faced entry and return-flow recesses, laser-textured faces with micro-pores that serve as micro-hydrodynamic bearings [42-46]. One of these laser-textured surface designs that has emerged as a promising and commercially viable design was recently introduced by a seal manufacturer [46].

3.10 Hydrostatic Seal Design The hydrostatic seal design is a non-contacting mechanical face seal that permits some controlled flow rate to pass between the faces. As illustrated in Figure 3-27, the seals are designed with a converging taper on the faces to balance the pressure distribution between the back of the seal ring and the seal face. Under no-pressure conditions, the seal faces can come into contact and cause dry running during startup. To prevent dry running, the seal requires that some pressure be applied to the tapered side prior to rotation. The initial pressure ensures that minimum leakage develops and that the seal faces will not contact during startup. Because no rubbing contact occurs in this type of seal, there is virtually no wear. In the Westinghouse configurations used in Main Coolant Pumps, the tapered seal faces are designed to permit a minimum leakage of 0.2 gallons per minute (10 milliliters per second) during startup conditions and a nominal leakage of 3.0 gallons per minute (190 milliliters per second) during normal operation. Filtered seal injection is used to keep particulates from entering the seal cavity. Key Technical Point The hydrostatic seal design is a non-contacting mechanical face seal that permits some controlled flow rate to pass between the faces. To prevent dry running, the seal requires that some pressure be applied to the tapered side prior to rotation.

3-28


In some applications, conventional mechanical face seals contain the leakage past the hydrostatic seal. In this tandem configuration, most of the pressure breakdown occurs as leakage crosses the hydrostatic seal, and the remaining pressure drop is taken across the conventional mechanical face seal. Under normal operation, the mechanical face seal is exposed to a significantly lower pressure drop than the hydrostatic seal. It is typically designed as a backup to the hydrostatic seal to permit a safe shutdown of the system under higher pressure drop, should the hydrostatic seal fail. Hydrostatic seals are available in either a rotating balance design or a stationary balance design. A detailed description of these designs, used in conjunction with hydrodynamic seals, is provided in NMAC TR-100855, Main Coolant Pump Seal Maintenance Guide [35].

Figure 3-27 Hydrostatic Face Seal Design

3-29


4 FAILURE MODES AND FUNDAMENTAL MECHANISMS

4.1

Introduction

The purpose of this section is to describe the failure modes of mechanical face seals and the fundamental mechanisms that are responsible for the failures. A significant amount of research by seal manufacturers, universities, independent research organizations, national laboratories, and seal users has continued over the last four decades to improve fundamental understanding of the mechanisms that cause seal failure, which in turn has led to improvements in design, the selection of an appropriate design for each application, and guidance for installation and maintenance [3, 7, 9, 34, 36]. Industry-specific data were gathered under this project by conducting a utility survey to determine the most common failure modes in the nuclear and fossil power applications. Analyses were then performed to determine all of the significant seal failure mechanisms that are described in this section.

4.2

Definition of Seal Failure

The eventual failure mode of all mechanical face seals is leakage that is considered unacceptable for the seal design/configuration being used. Excessive leakage can cause unacceptable loss of fluid, reduction of pressure, or contamination of the system fluid by the barrier fluid in doubleseal installations. Seal leakage can occur for a variety of reasons and might result from failure at any of several leak paths. The possible leak paths in a typical mechanical face seal are (see Figures 3-1 and 315 for reference): x

Between the seal faces

x

Between the secondary seal and the primary ring

x

Between the secondary seal and the mating ring

x

At the secondary seal in the sleeve (in seal designs employing sleeves)

x

At the secondary seal at the gland plate

While mechanical seal faces require some small level of leakage to function properly, the extent of leakage above this minimum requirement can be from a few drops to a continuous drip. Under normal performance, typical leakage rates from mechanical face seals are in the range of a fraction of ml/hr to a few ml/hr, depending upon seal size, fluid viscosity, pressure, temperature, 4-1

EPRI Licensed Material Failure Modes and Fundamental Mechanisms

and speed. There are no general quantitative criteria for what constitutes seal failure due to excessive leakage. The level of permissible leakage is dependent upon the operating requirements, environmental and safety considerations, and economic considerations. In most clean water systems, quite high leakage rates are often tolerated as long as other functions of the operation are not affected. In general, most premature leakage problems result from improper selection of the seal design and materials, improper use of the seal, and improper installation. Key Technical Point The eventual failure mode of all mechanical face seals is leakage that is considered unacceptable for the seal design/configuration being used. Excessive leakage can cause unacceptable loss of fluid, reduction of pressure, or contamination of the system fluid by the barrier fluid in double seal installations. The level of acceptable leakage is dependent upon the application.

4.3

Industry Survey

Under this EPRI project, an industry survey was conducted to determine the most common failure modes for mechanical seals encountered in the nuclear and fossil power plant applications. A survey questionnaire was sent to all EPRI NMAC and FMAC utility members, both domestic and international. The nuclear utilities included both BWR and PWR plants. Appendix A includes a complete copy of the questionnaire. In addition to the survey results, technical information from many other industry sources was used to identify the most common failure modes and mechanisms responsible for the failures. Based on the above, the following appear to be the most problematical mechanical seal applications: x

Multi-stage centrifugal charging pumps

x

Start-up feedwater pumps

x

Condensate booster pumps

x

Station heat pumps

x

Pumps with mini-flow operation

x

Pumps with variable flow requirements

x

Boric acid system pumps with heat trace lines

This list does not include the main coolant pump seals, which, due to their higher importance, have already been addressed separately in NMAC TR-100855, Main Coolant Pump Seal Maintenance Guide [35].

4-2


It should be noted that the only European nuclear power utility that responded reported no problematical applications. It is conjectured that, like most other European utilities, they are using mechanical seal designs with special features (for example, thermal hydrodynamic grooves or notches on the seal faces as described in Section 3.9) to provide enhanced seal face lubrication. A common denominator in all of these applications is sealing of hot water, which is a lowlubricity/high-volatility liquid that is difficult to seal, especially when high fluid pressures are encountered [21-25]. The problem applications also include operation off the Best Efficiency Point (mini-flow operation, variable flow requirements) and dissolved solids that can crystallize (boric acid application). The most commonly cited reasons (not root causes) for mechanical seal problems encountered at the plants surveyed were: x

Improper installation

x

Improper seal face compression

x

Dirty or abrasive fluids

x

Differences between normal operating conditions and design conditions

x

Excessive axial or radial movement caused by off Best Efficiency Point operation cavitation, out of balance, bent shaft, misalignment, and bad bearings

x

Equipment operating conditions not completely defined

x

Improper design and face seal material selected for the application

x

Pressure and/or temperature transients due to variable system operation

x

Lack of training

4.4

Fundamental Failure Mechanisms

Successful operation of mechanical seals depends upon the development of a thin film of fluid [typically less than 40 micro-inches (1 Pm)] that separates the seal faces during operation, thus keeping the seal wear to a minimum and providing long life [1-6]. It is now well accepted that the fundamental mechanism responsible for generating a fluid film during operation of mechanical seals is hydrodynamic lubrication caused by unavoidable geometrical imperfections, especially waviness of seal faces in the circumferential direction [5,7]. The amount of waviness required to generate hydrodynamic film pressures and keep the faces apart is small, less than 40 micro-inches (1 Pm), and can be caused by manufacturing imperfections, local mechanical distortions due to drive pins/anti-rotation mechanisms, thermal distortions due to non-uniform contact pressure, and wear of the faces during operation.

4-3


To function properly, mechanical seals must maintain a fluid film to provide lubrication, prevent direct rubbing contact, and provide cooling of the seal faces under all operating conditions. Seal failures occur when the film thickness and the film pressure between the seal faces change and become unacceptably low or unacceptably high. This either leads to excessive friction, wear, and heat, causing damage to the seal faces and other seal hardware, or leads to parting of the seal faces. The eventual seal failure mode in both cases is high leakage. The fundamental mechanisms most commonly responsible for seal failures are described below. 4.4.1 PV Limits Exceeded As discussed in Section 3.7, the face loading of the seal faces is dependent upon whether the seal is a balanced or unbalanced design, the degree of balance, the spring force, and the fluid pressure being sealed. For optimum life, the film thickness should be sufficient to completely eliminate asperity contact between the seal faces. As the fluid pressure increases, the film thickness between the seal faces decreases, transitioning from full film lubrication to mixed lubrication, and in extreme cases, to boundary lubrication (Figure 4-1). Under full film operation, all of the seal face load is carried by the fluid pressure generated by hydrodynamic action. Under mixed lubrication, the fluid film pressure still carries a majority of the seal face load; however, the solid contact between the asperities of the mating seal faces carries part of the load. Under a boundary lubrication regime, practically the entire load is carried by direct solid contact and the fluid film carries a negligible amount of the total load. When the asperity contact does occur but is not extensive (as in mixed lubrication), seal life is governed by the wear of the face materials. Seal life can vary from several months to over 3 to 4 years, depending upon the application conditions. When asperity contact becomes extensive, as in boundary lubrication, the seal frictional heat leads to immediate failure. Adverse thermal stress conditions can result from higher pressures as well as from inadequate heat dissipation, and can cause heat checking of the seal faces.

4-4


Figure 4-1 Lubrication Regimes at Seal Interface Showing Asperity Contact as Lubrication Changes from Full Film to Mixed to Boundary

For higher pressures, balanced seals provide the best performance because they reduce the face loads and the asperity contact. However, as the balance ratio is decreased to handle higher pressures, the vulnerability of the seal to parting of the seal faces under fluid pressure/ temperature transients increases. Balance ratios of 0.62 or less should be avoided to prevent face parting. The PV limits for both balanced and unbalanced seals for all commonly used materials are provided in Table 3-4. Key Technical Point For satisfactory performance, the seal design and material selections should satisfy the PV limit and the 'T limit under all operating conditions to ensure that fluid film is maintained between the seal faces. Loss of film can lead to immediate seizure and seal failure.

4.4.2

'T Limits Exceeded, Causing Film Vaporization/Collapse

This is one of the most common causes of seal failure in high pressure, hot water pumps. As discussed in Sections 3.7 and 3.8, sealing of low-lubricity/high-volatility fluids (for example, water, glycol, and light hydrocarbons) is difficult, particularly under higher pressure and speed combinations. If under given operating conditions the liquid film at the seal interface vaporizes, dry rubbing of the seal faces occurs, leading to excessive heat, seal popping, and failure.

4-5


Figure 3-23 in Section 3 shows the 'T margin that needs to be maintained between the bulk fluid temperature and the boiling point curve of the fluid being sealed to accommodate the increase in fluid temperature at the sealing interface without vaporization. Both the PV limits and the 'T margins are frequently challenged and must be respected for successful operation of face seals in high pressure, low-lubricity/high-volatility fluid applications. Increasing the chamber pressure and/or cooling to suppress fluid vaporization can improve seal performance. Approaches discussed in Section 3.9 to improve lubrication of the seal faces can be used to extend the PV and 'T limits of mechanical seals in many applications. 4.4.3 Inadequate Cooling Many mechanical seal chamber dimensions in pumps are based on interchangeability with stuffing box packing arrangement. Often this imposes severe restrictions on the seal design, thus limiting the structural strength of and heat transfer from the seal to the process fluid. The narrow radial clearances between the seal boundary and the seal chamber limits flow of the hightemperature fluid surrounding the seal, resulting in unacceptable thermal distortions and coning of the seal faces. In such cases, isolated pockets of hot fluid in the vicinity of the seal can reach temperatures that are several hundred degrees higher than the process fluid. Excessive coning due to high differential temperatures is often responsible for seal failure as described in Section 4.4.4. As described in Section 3.5, increasing the radial clearance at the seal outside diameter, using enlarged and/or tapered seal chamber designs, incorporating a seal flushing arrangement, or increasing the flow rate of the flushing fluid can significantly reduce the seal temperature. This can provide a dramatic improvement in the performance of the seal in such installations. Key Technical Point Mechanical seals are often installed in the same cavity that is designed to accept conventional packings. This limits the fluid circulation around the seal, leading to high seal temperatures and accumulation of solids. An enlarged seal chamber with tapered bore can dramatically improve fluid circulation, lowering seal temperature and eliminating accumulation of solids. 4.4.4 Transients Causing Excessive Seal Face Coning Thermal stresses and pressures cause deflections of the seal faces (coning) that change the initially parallel fluid film gap between the seal faces to either a convergent or a divergent gap (Figure 4-2). By design, the distortion of the seal faces caused by coning should be limited to less than 40 micro-inches (1 Pm), which is the typical film thickness between the seal faces.

4-6


Figure 4-2 Extremes of Seal Face Distortion (Coning) Due to Thermal and Pressure Effects

A frequent cause of seal failure is coning of seal faces that results in heavy contact at the inside diameter of seal faces during operation (positive coning). Positive coning is caused by thermal distortions due to seal friction and inadequate cooling. Positive coning, if excessive, changes the lubrication regime from full film to mixed or boundary lubrication. This, in turn, increases friction and interfacial temperature and causes rapid wear of the seal faces. Positive coning changes the interfacial film pressure distribution from linear in a parallel face situation to convex or concave pressure distribution, depending upon whether the seal is pressurized on the inside or the outside diameter. Figure 4-3 shows the changes in pressure distribution for an outside pressurized seal. Key Technical Point Thermal distortions of seal faces due to operational transients can cause positive coning (contact on ID) or negative coning (contact on OD) of the seal faces. Coning in excess of film thickness can cause film rupture seizure or face parting, resulting in a large increase in leakage. In extreme cases of positive coning with inside pressurization, fluid leakage past the sealing faces is completely cut off, thus leading to total collapse of the fluid film and immediate failure. In the case of outside pressurization, the increase in film pressure can cause parting of the seal faces.

4-7


Figure 4-3 Pressure Distribution Changes Caused by Coning of the Seal Faces (for Outside Pressurized Seal)

Another cause of seal failure is coning of seal faces that results in contact at the outside diameter of seal faces (negative coning). Negative coning is caused by seal distortion due to pressures, including transients, exceeding acceptable limits. Negative coning causes the pressure distribution between the seal faces to change sufficiently to either overcome the seal closing force, thus causing parting of the seal faces and very high leakage, or to reduce the film thickness, resulting in mixed/boundary lubrication. Key Technical Point Pressure distribution across the seal faces is affected by seal face coning due to changes in pressure and speed as well as the wear-in process. Excessive coning causes seal failure either due to seizure or face parting. Hard face versus soft face material combinations are more tolerant of coning than if both faces are hard.

4-8


In fact, the coning and the wear-in process have complex interactions on seal performance, depending upon the sequence of events (Figures 4-4 and 4-5). The performance is also affected by the ability of one of the faces to wear-in rapidly without causing immediate seal failure (for example, in the case of a carbon face) or by whether both the seal faces are too hard to wear-in rapidly (for example, silicone carbide, tungsten carbide).

Figure 4-4 Changes in Seal Contact Area Under Constant Operating Conditions During the Wear-In Process for a Seal With a Hard Face and a Soft Face

Figure 4-5 Example of a Wear-In Sequence (Stages 1 through 4) for a Mechanical Seal with a Soft Seal Face

4.4.5 Operation Away from Best Efficiency Point Large shaft deflections in pumps due to operation far away from the best efficiency point can cause misalignment and eccentricity between the seal faces during operation. Extensive analytical and experimental research sponsored by NASA has led to a good understanding of how rotor/stator eccentricity and angular misalignment of the faces can create a strong pumping action across the seal faces, over and beyond the hydrodynamic action caused by normal circumferential waviness [5].

4-9


In applications where fluid is present on only one side of the seal, eccentricity can cause high external leakage. In applications where fluid is present on both sides of the seal (for example, in a double seal arrangement with buffer fluid), a high rate of fluid transfer can occur either outwardly (from high-pressure to low-pressure side) or inwardly (from low-pressure to highpressure side). The fluid flow by this mechanism from low-pressure to high-pressure side is called inward pumping. Inward pumping can cause significant mixing of the fluids. When abrasives are present in one of the fluids, inward pumping causes high abrasive wear of the seal faces. These effects can be minimized by controlling the misalignments and eccentricities to an acceptably low level.

Key Technical Point Operation away from Best Efficiency Point (BEP) is a frequent cause of short seal life/seal failures. Off BEP conditions cause large shaft deflections and vibrations resulting in premature degradation of mechanical seals. It is also important to note that the pumping action in a misaligned, eccentric face seal causes the fluid to transfer across the seal interface if the wide seal face is rotating as shown in Figure 4-6(a). Fluid transfer can accelerate abrasive wear of the seal faces, especially in applications where one fluid has solid particulates, for example, service water applications. The effect can be minimized by selecting a seal design in which the narrow face is the rotating element, as shown in Figure 4-6(b).

4-10


Figure 4-6 Fluid Pumping Action Across the Seal Faces Due to Static Offset and Misalignment

4-11


4.4.6 Seal Misalignment/Premature Degradation of Primary and Secondary Seals Mechanical face seal misalignment occurs in all installations, but the severity of the misalignment and the manner in which it is accommodated dictates whether the mechanical face seal will perform satisfactorily in service. Misalignment can be caused by runout of the shaft or face seal due to manufacturing clearances and tolerances or by deflection of the mounting surfaces due to load or temperature. It can be classified in two categories: static misalignment or dynamic misalignment. Both static and dynamic misalignment can reduce the service life of the mechanical face seal by premature degradation of the primary or secondary seals. Static Misalignment: Static misalignment is the condition in which the seal faces run in an eccentric position relative to each other. They remain in that position unless a change in operating conditions upsets their relative positions. The effect of static misalignment is a wear track on the wider face that is offset from its concentric position. If the misalignment remains constant (within limits) after installation, the primary seal faces should function properly and provide normal service life. If the misalignment is the result of load, such as shaft tilt due to side loading as shown in Figure 4-7, then the mechanical face seal will operate satisfactorily until the load is changed. Once the load is changed, a new wear track will need to develop before the mating seal faces again begin to function normally. This condition becomes more severe when the wider face is made of relatively soft material that permits a relatively deep wear track to develop. In most cases, a deep wear track causes face leakage under both static and running conditions.

Figure 4-7 Rotating Balance Seal Wobble Caused by Shaft Tilt

4-12


Key Technical Point Static and dynamic misalignment between seal faces can cause strong fluid pumping action across the faces causing either inward pumping or outward pumping of the product fluid and/or buffer fluid. Leakages under misaligned conditions can be several times the normal leak rate. Static misalignment can also create a condition called pumping-in or pumping-out of the fluid when the rotating face is wider than the stationary face, as already described in Section 4.4.5 and illustrated in Figure 4-6. Pumping is caused by the radial velocity vector that forces fluid in and out of the narrower seal face. This radial vector can be large enough to pump fluid from the lowpressure side to the higher-pressure side. Pumping-in is particularly harmful when the lowpressure side has contaminants. Pumping-out does not usually damage the seal, but only increases the leak rate. As stated earlier, the pumping phenomenon due to static offset can be eliminated by making the rotating face narrower and selecting the softer face material for the narrower face. Static misalignment due to shaft tilt also creates an axial sliding action at the secondary seal location, as shown in Figure 4-7. Premature degradation of the secondary seal area due to fretting/wear can cause seal problems. Dynamic Misalignment: Dynamic misalignment exists when the mechanical face seals have to respond to changes with each revolution. Shaft tilt creates a condition where the seal has to respond dynamically to the change in axial position of the mechanical face seal with every revolution of the shaft. Shaft tilt can create premature failure of the secondary seal and can significantly affect the integrity of the sealing faces. When the secondary seal slides to accommodate shaft tilt (shown in Figure 4-7), it axially sweeps the shaft with each revolution of the shaft and causes the secondary seal and its mating surface to wear. Excessive leakage, especially at high speeds, can also develop if the seal faces cannot dynamically respond to relative axial movement to maintain face contact. Leakage due to shaft tilt can also occur at relatively low speeds if the spring load or pressure do not generate enough face loading, especially when the inside diameter of the seal is pressurized. Problems associated with shaft tilt can be reduced or eliminated by allowing the stationary ring to pivot as shown in Figure 4-8.

Key Technical Point Premature wear of the primary sealing faces and secondary seals, causing excessive leakage when stationary and when running, are also common symptoms of excessive misalignment.

4-13


Figure 4-8 Shaft Tilt Accommodated by Stationary Ring Pivot

Problems caused by dynamic misalignment also occur when the rotating seal face axis is offset from the rotation axis of the shaft. Under this condition, the rotating seal face radially sweeps the stationary face once every revolution as shown in Figure 4-9. This condition exists to some extent in all seals, however, leakage and wear become a problem only when the runout is excessive and the rotating face is narrower than the stationary face. If the narrower rotating face turns with an offset around the axis of revolution, a radial vector is generated that pumps fluid in and out of the narrow face. The problem becomes severe when the product or environment contains abrasives that can be forced between the sealing faces. Leakage due to runout is usually present only during running conditions unless the sealing faces have been damaged.

4-14


Figure 4-9 Seal Pumping Caused by Dynamic Offset of Rotating Narrow Face

Problems associated with dynamic offset are more common when the primary face (which has more components and more potential for imbalance) rotates rather than when the mating ring rotates. Offset problems can also be caused by excessive clearances in the assembly or improper installation. The problem can usually be eliminated by selecting a seal configuration with a rotating mating ring, which can be manufactured to much tighter tolerances to minimize clearances and imbalance. 4.4.7 Excessive Out-of-Flatness (Warpage) During Operation Key Technical Point Mechanical face seals are precision components, requiring the sealing faces to be flat, typically within one light band (11.6 x 10-6 inches) across one-inch width. Too much out-of-flatness can lead to excessive seal leakage. For proper operation without excessive leakage, manufacturers control seal flatness to typically within one light-band per lineal inch. In some cases, the flatness of the seal faces can change considerably during operation due to wear, misalignment, and exposure to high temperatures that continue to age the seal face material. In applications where both faces are made of hard materials (for example, tungsten carbide and silicone carbide), distortions of the seal faces that result in excessive waviness can generate a much higher hydrodynamic pressure than under normal conditions, thus causing a dramatic increase in fluid film thickness and leakage. In such cases, the seal faces typically show no sign of wear or abnormal contact and the problem is only 4-15


recognized by inspecting the seal flatness. Local warpage of several light-bands over a small circumferential part of the seal was observed in a controlled test in which leakage was found to increase by a factor of more than 100 during operation [52]. A more thorough heat treatment and stress relief prior to the final grinding and lapping operation can minimize distortions due to continued aging in operation. 4.4.8 Seal Faces Too Perfectly Flat to Generate a Film As mentioned earlier, mechanical seals function well due to a small, unavoidable circumferential waviness (introduced by manufacturing tolerances or mechanical/thermal loads) that generates hydrodynamic lubricant film pressure at the sealing interface, which prevents direct asperity contact between the faces. Under certain circumstances (fortunately rare), in which the seal faces are lapped too perfectly flat and the seal construction is robust enough to prevent mechanical distortion of the seal faces, the hydrodynamic film pressures are insufficient to separate the faces. This results in direct rubbing and very high friction, causing the seal temperatures to increase rapidly and immediate destruction of the seal. Evidence of high temperatures is also seen in discoloration of the seal hardware. This type of failure was encountered in controlled laboratory tests performed under identical conditions for which a number of tests had been successfully conducted previously [52]. It should be noted that, even though a maximum out-of-flatness criterion has been established by seal manufacturers, there is no minimum flatness requirement to ensure proper operation.

Key Technical Point Conventional mechanical face seals rely on a small amount of waviness, automatically created by face distortions due to mechanical loads, to function properly. Too perfectly flat seal faces on structurally robust seal rings prevent the faces from distorting and developing a fluid film. This results in seal failure due to seizure. Fortunately, this is a rare occurrence.

In conclusion, this section has described in detail all of the significant failure mechanisms that can cause seal failure, either singly or in combination. The insights provided here should be very helpful in following the systematic approach to troubleshooting and diagnosing seal failures in service as outlined in Section 7.

4-16


5 APPLICATION AND SELECTION RECOMMENDATIONS

5.1

Introduction

The mechanical face seal represents a complex design that consists of several single-design components. In order to achieve optimum performance, each of the single design components must be selected to cover the operational requirements. Factors that affect the performance of the seal (and that should be considered when selecting a seal) include: x

Liquid type

x

Liquid temperature during normal and design conditions

x

Liquid pressure during normal and design conditions

x

Rotational speed

x

Radiation exposure

In addition to the above factors, the ease of maintenance is an important consideration in selecting a seal.

5.2

Selection Specification

In most power plants, the system liquid is either water or some type of hydrocarbon. The water might be clean or contain abrasives that can significantly affect seal life if proper flushing is not provided to remove the abrasives from the seal faces. In general, the following recommendations are made depending on the process liquid.

5-1

EPRI Licensed Material Application and Selection Recommendations Table 5-1 Seal Application and Selection Guidelines Application

Typical Construction

Installation Considerations

Water and fuel

The rubber bellows seal is commonly used for water and fuel applications. The seal is relatively inexpensive and typically uses a rotating carbon head and a stationary metal face. To improve life and minimize abrasion, a ceramic face is often used. Tungsten or silicone faces are used in extreme cases. Bellows made from ethylene propylene are used up to 284qF (140qC) with water and water-glycol mixtures. Fluoroelastomers are used for fuels up to a temperature of 302qF (150qC). Faces are typically loaded using a single coil spring

Might require the use of double seals with a clean barrier liquid to prevent vaporization at the seal faces and to provide better lubrication for the seal faces. Borated water, which can crystallize on the seal surfaces, must be externally flushed. Flushing of the interface by direct jetting is mandatory for all liquids with a specific gravity of less than 0.63.

Boiler feed

Demineralized water is a poor lubricant and the face materials must be selected to withstand sparse lubrication. The seals are often sleeve-mounted because the shaft speed might approach 6,000 rpm. Faces are loaded using wave springs, welded springs, or multiple springs.

If the pressure is high, double seals with a clean barrier liquid might be required to stage the pressure drop. The barrier liquid might also be circulated and cooled to remove heat away from the seal.

Mild corrosives

Seals used in mild corrosives usually incorporate PTFE wedge secondary seals to provide the required compatibility with the process liquid. Conventional O-ring and elastomeric bellows seals are also sometimes used provided they do not degrade in service. It is not uncommon to specify asymmetric formed metal bellows for higher temperature applications. Face loading is achieved using multiple springs or metal bellows.

Stainless steel components might be required to prevent corrosion.

Highly

PTFE bellows are typically used in highly corrosive liquids to prevent from escaping into the environment. Asymmetric-formed metal bellows are also available for some applications. The seals are usually externally mounted and have visual wear indicators that signal when the seal must be changed. Dual seals are also often used with a benign barrier liquid to minimize the toxic liquid escaping to the environment. The seal faces are loaded using multiple stainless steel springs or using the metal bellows seals.

Depending on the effects, corrosion might be either beneficial or detrimental. If soft oxides are formed, wear might be reduced as long as the oxide layer is not disturbed. However, free hard oxide particles, floating between the faces, can act as grinders and increase wear. In those instances, flushing with a clean liquid might be required to enhance seal performance and life.

corrosive liquid

5-2

EPRI Licensed Material Application and Selection Recommendations Table 5-1 (cont.) Seal Application and Selection Guidelines Application

Typical Construction

Installation Considerations

Hot hydrocarbons

A wedge seal with multiple springs is used to seal hot hydrocarbons. The wedge is typically made of a high-temperature graphite if high pressure is encountered. Welded metal bellows are used for temperatures up to 572qF (300qC) and pressures up to 290 psi (20 bars). Multiple springs are usually used to load the seal faces unless clogging can occur. When clogging is a problem, then a single coil spring is used.

Clean flushing liquid with lubricating properties are typically required to prevent volatile liquids from vaporization in the vicinity of the seal interface. Vaporization will cause liquid film breakdown and loss of lubrication. Flushing of the interface by direct jetting is mandatory for all liquids with a specific gravity of less than 0.63.

Slurry/dirty process

Seals in slurry applications normally used asymmetrically formed bellows to provide the seal on the primary ring and to load the faces. Bellows are typically made from corrosionresistant materials and have no sharp corners to trap contaminants. The static seals on the stationary ring are usually elastomeric Orings. Hard faced materials are used for the faces to prevent wear caused by the abrasives contained within the slurry.

Clean flushing liquid is typically required to remove abrasives from the seal surfaces. The flushing liquid should be neutral to prevent contamination of the process liquid. Cooling provided by flushing also improves seal life.

Key Technical Point Seal selection requires a detailed and systematic evaluation of all the significant application parameters, for example, fluid type, pressure, temperature, speed, normal operating conditions versus design conditions, radiation exposure, and maintenance. Appropriate data sheets and check lists should be used to ensure a thorough and complete evaluation of suitable alternatives and trade-offs. Prototype qualification tests should be performed for all critical applications.

5.3

Selection Data Sheet

The proper selection of a mechanical face seal requires examination of different areas of the seal installation and operating requirements. The following selection sheet provides guidance on recognizing the critical area that must be identified. This data sheet was developed from the data sheets in API Standard 682. The more detailed data sheet in API 682 can be used in lieu of this abbreviated data sheet. It is expected that the seal manufacturer might need to be contacted to assist in filling out the data sheet.

5-3

EPRI Licensed Material Application and Selection Recommendations

SELECTION DATA SHEET 1. Purchaser Requirements Purchaser Pump service Enquiry Ref Seal mfg

Company Date Plant item no. Ref pump drwg For proposal/purchase Seal installation drwg required, Y/N?

2. Application Details Liquid Seal Size Temperature range Speed range, rpm 3. Supplement Process Data Pump suction pressure Static pressure, max/min Boiling temp at sealed pressure Abrasives Y/N Abrasives concentration Specific gravity of process Auto-ignition temp Corrosive/pH Dry running, Y/N Special operation comments 4. Process Hazard Hazard (state) Toxicity rating 5. Standards Identify applicable compliance standards API ANSI NACE ISO

Shaft/sleeve size Sealed pressure range Rotation CW/CCW

Pump discharge pressure Vapor pressure at process temp Vacuum pressure Abrasives constituents Dissolved solids constituents Viscosity, max/min Max/min ambient temp Carbon dioxide, ppm

Allowable leakage

DIN

Other

6. Type of Installation (circle application selections) Single Double back-to-back Double face-to-face Cartridge Stationary mounted Clean flush can be used Compatible sealant for double seal installation 7. Design Type (circle applicable selection) Rubber bellows O-ring PTFE wedge Metal bellows Unbalanced Balanced Multiple springs Seal materials

5-4

Tandem

PTFE O-ring Single spring


SELECTION DATA SHEET (cont.)

8. Containment Seal in Addition to Item 6 (circle applicable selection) Non spark bushing Lip seal Labyrinth bushing Mechanical Floating labyrinth Standstill Other Maximum temperature Maximum Pressure 9. Auxiliary Fluids Available on Site Water, Y/N Pressure Steam, Y/N Pressure Flush, Y/N Pressure Other, Y/N Pressure

Temperature Temperature Temperature Temperature

10. Auxiliary Equipment to be Provided by Seal Supplier Sealant system per attachment Cooler, type Cyclone separator Filter, type Flow controller Leakage detector type

11. Sealed Equipment Details Pump Make/Model Pump, type Description Horizontal/vertical Axial/Radial split Seal mounted on shaft or sleeve Seals per pump Shaft axial movement Driver (electric motor, steam turbine, engine, etc) Wetted parts materials 12. Material Certification and Performance Test Specify Certification Seal Test (std/spl)

5-5


5.4

Qualification Testing

In some critical service applications, where seal failure is unacceptable from a safety standpoint, or where the economic impact of failure is unacceptable (for example, unscheduled plant shutdowns), seal selection should be verified by appropriate qualification testing. This is especially recommended where the manufacturers cannot provide reference experience for the selected designs from other similar applications. The extent of testing, the key factors to be simulated, and parameters monitored during testing depends upon the criticality of the application and the cost of performing the qualification tests. Guidance is provided in API Standard 682 [8] and in other publications related to mechanical seals [7,56,57], which can be consulted to tailor the qualification testing for a specific application.

5-6


6 CONDITION-BASED MONITORING GUIDELINES

6.1

Introduction

Seal monitoring programs vary greatly from utility to utility, and from site to site. Some of this is the result of different equipment designs, operating philosophies, and different rates of forced outages experienced. Based on survey results, the level of condition monitoring required to develop reliable seal performance data is quite basic except for main coolant pump mechanical face seals. For many plants, condition based monitoring is limited to visual observations with little actual quantification. This section of the guide provides information on how to evaluate seal performance and suggestions for monitoring and data acquisition. The data acquired and tended can be used to assess seal performance and to provide reasonable predictions of the remaining life or operability of a mechanical face seal. The parameters to be trended will be identified, evaluation described, and examples provided. Trouble-shooting problems require good data. Without a trending program, determining the root cause of an operating problem is difficult, if not impossible. Data logging of the various parameters associated with mechanical face seals can be performed in many different ways. The simplest way is to use manual recording, however, sophisticated data-logging systems can also be utilized. Hand logging of data and trending is time consuming, but it is effective in trending most seal performance characteristics over the long term. Required parameters that are routinely trended can be added to the daily or shift logs recorded by the operators. These parameters can then be plotted using standard spreadsheet programs and trends can be maintained and provided to plant personnel as part of the normal system status reports. The major advantages of automated systems are that data can be routinely recorded and downloaded to trending programs, and changes in the frequency of data-logging can be triggered from performance changes. Generally, when analyzing seal performance changes, it is necessary to have data recorded frequently or to have key parameters on continuous recorders. These automated systems are reasonably expensive and, in a time where utilities are being challenged to hold the line on costs, are only appropriate for systems with a relatively high frequency of seal failures. Key O&M Cost Point Seal monitoring programs vary greatly from utility to utility and from site to site due to different equipment designs, operating philosophies, and different rates of forced outages experienced. For many plants, condition-based monitoring is limited to visual observations with little actual quantification, except for main coolant pump mechanical face seals. 6-1

EPRI Licensed Material Condition-Based Monitoring Guidelines

6.2

Typical Performance Data Logging

Data that is typically available for logging includes pressures, temperatures, flows, vibration levels, and, in some cases, speed. The amount of each type of data collected for each seal will depend on the type of seal used and its installation. For example, single seals will require less data collection than double or tandem seal arrangements. The frequency of data logging will vary from system to system based on system conditions and seal operating experience and characteristics. Manual recording might be required only once a day. Automated data-logging systems can acquire data at any frequency, and the frequency can be dynamically adjusted depending on seal performance. A typical log sheet for a multiple seal arrangement and its support system is shown in Table 6-1. An example of pressure being used to trend seal performance is illustrated in Figure 6-1 for a staged seal arrangement. In this example, the lower seal stage differential pressure is plotted against time and a best guess projection is made to predict when the failure limit has been reached. Similar trends can be plotted of temperature in a barrier fluid or loss of barrier fluid in the barrier fluid reservoir. Loss of barrier fluid can be very useful in characterizing seal performance in a corrosive system seal arrangement.

6-2

EPRI Licensed Material Condition-Based Monitoring Guidelines Table 6-1 Seal System Log Sheet Plant

Unit

System

Equip. No.

Date

Time

Recorded By: Seal No. 1

Item

Normal

Minimum

Maximum

Startup

Flow Temperature Differential Pressure Backpressure Frame Vibration level Shaft Vibration level Speed Leakage rate Seal No. 2 Flow Temperature Differential Pressure Backpressure Seal No. 3 Flow Temperature Differential Pressure Backpressure Flush API Plan No.

Fluid type

Flow rate Temperature, inlet Pressure Filtration Quench/Drain API Plan No.

Fluid type

Flow rate Temperature, inlet Temperature, outlet Pressure Filtration

6-3


Figure 6-1 Seal Data Plot Showing Declining Performance (Courtesy of Southern California Edison)

6-4


6.3

Seal Performance Parameters

Other than seal dynamic torque, seal face temperatures and seal face temperature changes are the key measures of the performance of a seal because they characterize what is happening at the seal interface. Seal dynamic torque is almost impossible to measure and is, therefore, not a viable measurement. Temperature is the easiest parameter to measure and, depending on the seal arrangement, temperature measurements can directly characterize seal performance. Usually temperature data in the vicinity of the seal are a measure of the process fluid or support system, especially in seal systems that are flushed or quenched. These temperature measurements tend to mask the actual seal performance and many times fail to provide meaningful data. The more obvious measure of seal performance is leakage, but this method is only viable for single seals or outboard seals of multiple seal arrangements. In systems where only a small leak is acceptable, leakage measurement fails to provide an indication of impending failure. Even within these limitations and short falls, data taken to monitor seal performance can provide a useful tool. These measurements become even more meaningful when tracked over an extended period of time and correlated to seal failure. Parameters such as pressure and flow, which do not directly characterize seal performance but do affect seal performance, become extremely important when predicting when the seal might fail.

Key O&M Cost Point Monitoring and data logging of key performance parameters can serve as very useful tools for trending wear and performance degradation of mechanical seals and preventing unscheduled outages.

6.4

Instrumentation

Seal monitoring can be accomplished with simple and easy-to-implement manual instruments such as temperature and pressure gauges, or with complex computer data-acquisition systems that can initiate controls based on parameter limits. This section describes the manual sensors and switches that are commonly available and used. When used, the sensors should comply with a recognized standard such as API Standard 682. Electronic sensors, such as pressure transducers, thermocouples, etc., should be subject to similar design requirements. The following sections (6.4.1 through 6.4.8) that outline various sensors and switches are based on recommendations contained in the API Standard 682. Deviations from the following recommendations can be made, and other design requirements might be imposed, based on specific needs of the plant. 6.4.1 Temperature Gauge Temperature gauges provide a visual indication of the local temperature. The sensing element is in contact with the liquid being measured.

6-5


Dial temperature gauges should be heavy-duty and corrosion resistant. They should be bimetallic or liquid-filled, with a rigid stem suitable for mounting as needed. Mercury-filled thermometers are not acceptable. Black printing on a white background is standard. Dial temperature gauges should be installed in pipe sections or in tubing runs. The sensing element of temperature gauges should be in the flowing fluid to the depth specified by the gauge manufacturer. Temperature gauges installed in tubing should be a minimum of 1 1/2 inches (38 mm) in diameter and the stem should be a minimum of 2 inches (50 mm) long. All other gauges should be a minimum of 3 1/2 inches (90 mm) in diameter and the stem should be a minimum of 3 inches (75 mm) long. 6.4.2 Thermowells Thermowells provide protection for the sensing element of temperature gauges. Temperature gauges that are in contact with flammable or toxic fluids, or that are located in pressurized or flooded lines, should be furnished with separable threaded solid-bar thermowells made of AISI Standard Type 300 stainless steel or another material more compatible with the liquid as defined by the manufacturer. Thermowells installed in piping should be 1/2 inch-NPT minimum. Thermowell designs and installation should not restrict liquid flow. 6.4.3 Pressure Gauges Pressure gauges provide a visual indication of the pressure and the sensing element is in contact with the liquid being measured. Pressure gauges should conform to ANSI/ASME Standard B.40.1 grade 2A. The gauges should be furnished with AISI Standard Type 316 stainless steel bourdon tubes or other material compatible with the liquid, stainless steel movements, and 1/2-inch NPT male alloy steel connections with wrench flats. Gauges installed in tubing should have 2 1/2-inch (64 mm) diameter dials. Gauges not installed in tubing should have 4 1/2-inch (114 mm) diameter dials. Black printing on a white background is standard for gauges. Gauge range should be selected so that the normal operating pressure is at the middle of the gauge's range. In no case, however, should the maximum reading on the dial be less than the applicable relief valve setting plus 10 percent. 6.4.4 Alarm, Trip, and Control Switches Alarm, trip, and control switches provide a visual or audible signal or control an electric circuit when the preset limit of a sensor has been exceeded. Each alarm switch, each shutdown switch, and each control switch should be furnished in a separate housing located to facilitate inspection and maintenance. Hermetically-sealed, double 6-6


pole, double throw switches, with a minimum rating of 5 amperes at 120 volts AC and 1/2 ampere at 120 volts DC, should be used. Mercury switches should not be used. Unless otherwise specified, electrical switches that open (de-energize) to alarm and close (energize) to trip should be furnished. Alarm and trip switch settings should not be adjustable from outside the housing. Alarm and trip switches should be arranged to permit testing of the control circuit, including when possible, the actuating element, without interfering with normal operation of the equipment. If a shutdown system is being implemented, the need for bypass indication and testing features should be considered. Pressure-sensing elements should be of AISI Standard Type 300 stainless steel. Low-pressure alarms, which are activated by falling pressure, should be equipped with a valved bleed or vent connection to allow controlled depressurization so that the operator can note the alarm set pressure on the associated pressure gauge. High-pressure alarms, which are activated by rising pressure, should be equipped with a valved test connection so that a portable test pump can be used to raise the pressure. All switches sensing the same variable should have reset ranges, such that changing the variable to reset one switch does not activate other switches. 6.4.5 Pressure Switches Pressure switches trip when a pre-set pressure limit has been exceeded. Pressure switches can have low and/or high limit settings. Pressure switches should have over-range protection to the maximum pressure to which the switch can be exposed. Switches exposed to vacuum should have under-range protection to full vacuum. The measuring element and all pressure-containing parts should be AISI Standard Type 316 stainless steel unless the pumped fluid requires the use of alternate materials, as determined by the seal manufacturer. Unless otherwise specified, pressure switches should be bellows or diaphragm. Connections for pressure input should be 1/2-inch NPT. Connection for the air transmission signal should be 1/4-inch NPT. 6.4.6 Level Switches Level switches trip when a pre-set liquid level has been exceeded. Level switches can have low and/or high limit settings. Unless otherwise specified, level switches should be hydrostatic, capacitance, or ultrasonic.

6-7


Be aware that level switches might have a dead band wide enough to activate other switches during re-setting. This is especially true when dealing with the small volumes of barrier fluids associated with dual-seal reservoirs. 6.4.7 Level Indicators Level indicators provide a visual indication of the liquid level and are also used when dealing with small volumes of barrier fluids associated with dual-seal reservoirs. The standard level indicator should be the weld pad reflex design. When specified, an externally mounted, removable, reflex indicator should be furnished instead of the standard weld pad design. 6.4.8 Flow Indicators A flow indicator provides a visual indication of flow rate and, when used, should be a steel body non-restrictive bull's eye. To facilitate viewing of flow through the line, each flow indicator should be installed with its bull's-eye glass in a vertical plane. The diameter of the bull's eye should be at least one-half of the inside diameter of the line in which it is installed and should clearly show the minimum flow.

6-8


7 TROUBLESHOOTING TO IDENTIFY CAUSE OF SEAL FAILURE

Key O&M Cost Point Seal performance is often directly linked to equipment performance and reliability. An in-depth inspection and review of seal failures can improve equipment availability and performance.

7.1

Introduction

A discussion of the fundamental mechanisms responsible for seal failure was presented in Section 4. To improve seal reliability and extend its life in a particular application, a thorough analysis of the cause of failure of a mechanical seal often gives the best indication of action required. This section provides a comprehensive step-by-step troubleshooting approach that can be followed by engineers and operating and maintenance personnel to diagnose seal failures in actual applications. Several excellent sources, including seal manufacturers' published information and seal handbooks, identify causes of seal failure and provide illustrations of failed parts to aid in diagnosis [3,7,11-19]. The troubleshooting approach and tables in this section are based on relevant information for nuclear and fossil power applications from these sources along with the author’s experience in root cause analysis of seal failures. A number of the illustrations and technical notes included in the tables in this section were obtained from John Crane Mechanical Seals and Mechanical Engineering Publications, Ltd., London [7,17]. They have been updated and are used here with permission from these organizations.

7.2

Failure Diagnosis

Seal failure diagnosis is very similar to any other failure investigation and often the best indication of the cause of failure is from visual examination of the seal itself. Once the likely cause of the problem is decided, the available solutions are usually clear. It is very important to keep in mind that evidence of seal failure is an essential element in determining the cause of seal failure and if the evidence is lost there is no way to back track. Therefore, to reduce the risk of losing evidence, it is suggested that a systematic step-by-step approach be followed during the investigation process.

7-1

EPRI Licensed Material Troubleshooting to Identify Cause of Seal Failure

x

Properly document external symptoms of seal failure

x

Perform detailed checks before dismantling

x

Clearly document evidence during dismantling and disassembly

x

Perform detailed visual examinations of seal components

7.2.1 External Symptoms of Seal Failure A useful indication of the cause of a seal problem can often be obtained by analysis of the symptoms experienced in service. These might suggest either the remedy directly or at least the direction of subsequent failure diagnosis. On critical duties, instrumentation might be available to give further assistance, or portable devices can be used for condition checks. Table 7-1 outlines various external symptoms of seal failure and their possible causes, and offers recommendations for managing the symptoms.

7-2

EPRI Licensed Material Troubleshooting to Identify Cause of Seal Failure Table 7-1 External Symptoms of Seal Failure Symptom Seal squeals during operation

Possible Causes Inadequate amount of liquid to lubricate seal faces (Note that not all dry seals squeal.)

Recommendations/Remarks x If not in use, a bypass flush line might be

required. If already in use, the line or associated restrictions, for example, orifices in the gland plate, might need to be enlarged. x If increase in leakage is permissible, use

seal designs with positive hydrodynamic lubrication features, for example, face notches, laser-textured seal faces Carbon dust accumulating on outside of seal area

Seal spits and sputters in operation (often called popping)

Inadequate amount of liquid to lubricate seal faces

See above

Liquid film vaporizing/ flashing between seal faces. In some cases, this leaves a residue that grinds away the carbon-graphite seal ring.

Pressure in seal chamber might be excessively high for the type of seal and the fluid being sealed. See below for actions against vaporization.

Product vaporizing/flashing across the seal faces

Remedial action is aimed at providing a positive liquid condition of the product at all times x Increase seal chamber pressure if it is

possible to remain in seal operating envelope x Check for proper balance design with seal

manufacturer x Change to a seal design not requiring so

much product temperature margin x If not in use, a bypass flush line will be

required x If already in use, the bypass flush line or

associated restrictions might need to be enlarged x Increase cooling of seal faces x Check for seal interface cooling with seal

manufacturer x If increase in leakage is permissible, use

seal designs with positive hydrodynamic lubrication features, for example, face notches, laser-textured seal faces. Note that a review of balance design requires accurate measurement of seal chamber pressure, temperature, and specific gravity of product.

7-3

EPRI Licensed Material Troubleshooting to Identify Cause of Seal Failure Table 7-1 (cont.) External Symptoms of Seal Failure Symptom Seal drips or leaks steadily

Possible Causes

Recommendations/Remarks

If possible, first determine the source of the leakage. Heavy leakage is normally from the faces rather than the O-ring, and so on. Insufficient load on the seal faces Primary seal concerns: x

Faces not flat

x

Faces cracked, chipped, or blistered

x

Distortion of seal faces for thermal or mechanical reasons (usually determined from wear pattern on faces)

Typical corrective actions: x Check for incorrect installation dimensions

or loosening of set screws during operation, permitting axial slippage. x Check for improper seals or material being

used in the application. x Check gland gasket for proper

compression. x Check for gland plate distortion because of

over-torquing of gland bolts (this can cause faces to become distorted). x Clean out any foreign particles between

seal faces. Relap faces or renew. x Check for any installation or similar

damage and renew if necessary. x Check for squareness of stuffing box to

shaft and similar equipment condition concerns. x Ensure pipe strain or machine

misalignment is not causing distortion of seal faces (especially end suction overhung type pumps). x Improve cooling flushing lines.

Secondary seal concerns:

Typical corrective actions

x Secondary seals nicked or

x Renew secondary seals.

scratched during installation x Leakage of liquid under

pump shaft sleeve x Overaged O-ring x Compression set of

secondary seals (hard and brittle) x Chemical attack of

secondary seals (soft and sticky)

7-4

x Check for proper lead in chamfers, burr

removal, and so on. x Check for correct seals with manufacturer. x Check for correct seal materials with

manufacturer.

EPRI Licensed Material Troubleshooting to Identify Cause of Seal Failure Table 7-1 (cont.) External Symptoms of Seal Failure Symptom Seal drips or leaks steadily (cont.)

Pump/shaft vibration

Short seal life

Possible Causes

Recommendations/Remarks

Seal hardware concerns:

Typical corrective actions:

x

Spring failure

x

Renew parts

x

Erosion damage of hardware

x

Check for improved material availability

x

x

Corrosion of drive mechanisms

Modify recirculation flow arrangement to reduce high velocity jets on hardware. Install cyclone separator to remove solids from recirculation flow

x

Misalignment

x

Impeller/shaft system imbalance

x

Cavitation

x

Bearing problems

This will reduce seal life even though leakage might not be immediately apparent.

Equipment mechanically out of line (for example, from undue pipe strain)

See above. In the extreme, this can cause rubbing of the seat on the shaft

Abrasive product (causing excessive seal face wear)

Typical actions are aimed at determining the source of abrasives and preventing them from accumulating at the seal faces x

If abrasives are in suspension, bypass flushing over the seal faces will improve the situation by keeping the abrasive particles moving and so reducing their tendency to settle out or accumulate in the seal area. A cyclone separator is often added to this bypass line (filters give longer term problems unless regularly cleared).

x

When abrasives are forming locally in the seal area, a bypass flush will help introduce the maximum product to the seal cavity at the correct temperature. Abrasives form in the area because of the process liquid cooling down and crystallizing or partly solidifying, or because of local product evaporation.

7-5

EPRI Licensed Material Troubleshooting to Identify Cause of Seal Failure Table 7-1 (cont.) External Symptoms of Seal Failure Symptom Short seal life (cont.)

Seal leaks excessively following a pressure and temperature transient

7-6

Possible Causes Seal running too hot

Recommendations/Remarks x

Check that all cooling lines are connected and operational

x

Check that flow is not obstructed in cooling lines or jackets (for example, from scale formation)

x

Increase the capacity of cooling lines

x

A recirculation or bypass flush line might be necessary

x

Check for possible rubbing of a seal component against the shaft (see also OKUCNKIPOGPV above). Some good points to check are: neck bush clearance, clearance between the rotating seal unit and the seal chamber bore, the bore of the seat, and the seal plate clearance from the sleeve.

Inadequate seal type or seal material for duty.

If there is a concern, advice is readily available from seal manufacturers. Seal material deficiencies might well result in deterioration from corrosion or excessive heat.

Seal wears into a pattern and transients can cause excessive positive or negative coning of the seal faces. Coning changes the film pressure distribution, which can either cause face parting of balanced seals with low balance ratio or cut off the entrance of the lubricant/fluid between the seal faces. Loss of film causes heat damage.

x

Use seal with higher balance ratio if face parting is encountered

x

Control seal environmental temperature by a suitable flushing arrangement

x

Use seal designs with enhanced fluid film lubrication features at the seal faces, for example, cooling notches, hydropads


7.2.2 Checks Before Dismantling In addition to noting any seal failure symptoms, other checks prior to disassembly can be valuable, either directly or to facilitate later diagnosis. Most of these checks are straightforward and are carried out as routine by most engineers. Thus, they are presented as a checklist in Table 7-2 to act as an aide. Key Human Performance Point The importance of maintaining As Found conditions is important to failure mode determinations. Personnel should be instructed to exercise care during the disassembly steps. Table 7-2 Checklist of Actions Before Dismantling Topic

Checklist

Documentation

Take photographs of all key components and subassemblies before and during disassembly

Toxic/hazardous product

In such cases, all necessary precautions are to be observed prior and during assembly. Consult material safety data sheets (MSDS).

Service life of seal

Hours of operation. Duty cycle, stop/starts, and so on.

Process change

Identify any change - often the key to a solution Seal might have been selected on theory of process, not practice Changes in fluid pressure, temperature, or composition Process variation or fluctuation

Background information required

Fluid sealed (including contaminants) Fluid pressure on seal and in system Fluid temperature at seal and in system Fluid flow within the seal chamber Sealed fluid vapor pressure/temperature data Operating shaft speed(s) Special operating conditions Machine assembly drawing Seal assembly drawing Seal design data

Machine vibration

Useful even when not immediately apparent as a symptom Axial and radial bearing housing or shaft vibration Frequency analysis to confirm out-of-balance, misalignment, etc., until machine can be stopped for physical checks

7-7

EPRI Licensed Material Troubleshooting to Identify Cause of Seal Failure Table 7-2 (cont.) Checklist of Actions Before Dismantling Topic Seal leakage pattern

Checklist Safety note: all necessary precautions must be observed during any leakage checks, especially if the fluid is toxic or hazardous. Amount and nature of abnormal leakage? Leakage constant or variable? Leaks when shaft is stationary? Leaks when shaft is rotating? Related to changes of speed, pressure, or temperature of operation?

Possible leakage path(s)

An assembly drawing is of great assistance. If possible, identify source of abnormal leakage while machine is still operating. Inspect exposed machine surfaces for indications of leakage path(s), for example, along shaft, under sleeve, from seal plate gaskets, and so on. This inspection to continue through subsequent equipment and seal dismantling until the leakage path(s) are all found. Typical leakage paths:

Hydrostatic testing

x

Face leakage

x

Secondary seal on primary ring

x

Secondary seal on mating ring

x

Seal/gasket on seal plate(s)

x

Seal/gasket under shaft sleeve

x

Cracked or damaged housing component

If possible, for example with double seals, bench testing of equipment can be a useful method of identifying the leak path. With other seal layouts, a suitable test fixture for subassembly pressure testing might be justifiable if large numbers of seals are being examined.

7-8


7.2.3 Checks During Dismantling For proper diagnosis of seal problems, several checks and observations should be made during the dismantling of a mechanical face seal. These observations are divided into three categories: general, premature failure, and mid-life failure checks, and are given in the checklist Tables 7-3, 7-4, and 7-5. 7.2.3.1

General Checks

Table 7-3 General Checks During Dismantling Topic Seal surfaces

Checklist Avoid disturbing the seal surfaces Avoid wiping or cleaning the faces more than is necessary for safe disassembly Visual examination of seal faces is included in Section 7.3

Dimensional checks

The necessary marks and measurements to determine are: x Seal working length x Squareness of seal faces to shaft axis x Concentricity of seal faces to shaft axis x x

Shaft end play Shaft radial run out, whip and deflection

Possible leakage path(s)

Examination of surfaces as they become exposed for all possible causes of abnormal leakage

Deposits and debris

Examination prior to cleaning for: x Foreign contaminants x x x x

Wear debris Small fragments or chips from broken components Corrosion products Miscellaneous debris/deposits

Seal hang-up

Check for hang-up by flexing the seal slightly above and below its installed working length

Seal sub-assembly cleaning

Avoid removing or obscuring any vital evidence on the seal failure mechanism (especially on the seal faces) Avoid using wire brushes, sharp tools, abrasive cleaners, or powerful solvent cleaning agents (which can attack the elastomeric components)

Packaging

For seal manufacturer examinations/repair: x Many seal makers will personally collect unusual/critical seals for failure diagnosis x x

Packaging needs to be of high standard (as for new seals) Avoid wire mounted identification tags, etc., that can damage parts in transit

7-9


7.2.3.2

Premature Failure Checks

Table 7-4 Premature Failure Checks During Dismantling Topic Seal faces

Checklist Examination for nicks, scratches, and fractures: x

Low power magnification can assist

Examination of non-uniform contact pattern: x

Dirt trapped between the faces

x

Distortion of one or both faces

x

Improperly finished faces

x

See also Appendix B optical flat checking

Examination for thermal distortion:

Secondary seals

Drive mechanism

x

From running dry

x

Heat checks/thermal cracking

x

Pitting, grooving, galling, spalling, blistering, and so on

Examination for : x

Omitted seals

x

Misassembled seals

x

Nicks, extruded, or distorted static seals

x

Score marks from relative rotational movement between secondary seals and mating surface

x

Excessive volume change or compression set

x

Fretting of sealing surfaces at secondary seal positions

Examination for: x

Mis-assembly

x

Mis-indexing

x

Omission

Check for loss of secondary seal interference when used for drive purposes, for example, static seals and bellows Face loading hardware

7-10

Examination for: x

Incorrect type

x

Mis-assembly

x

Mis-indexing

x

Omission


7.2.3.3

Mid-Life Failure Checks

Table 7-5 Mid-Life Failure Checks During Dismantling Topic Seal faces

Checklist Examination for nicks, scratches, and fractures: x

Overall corrosion

x

Leaching

x

Abnormal grooving

x

Erosion damage

x

Excessive pitting, galling, and spalling

x

Thermal damage such as waviness, heat checks, cracks, blisters, deposition of solid material, and overall thermal discoloration

Wear profile check by:

Secondary seals

Drive mechanism

x

Naked eye examination

x

Use of low incidence angle light to highlight features

x

10X magnification, then 50X

x

Measurement to determine the amount of wear

Examination for: x

Extrusion

x

Chemical attack on both seal and its interface surfaces

x

Excessive volume damage

x

Excessive compression set

x

Hardening and cracking

Examination for: x

Failure

x

Excessive wear

x

Check for loss of secondary seal interference when used for drive purposes, for example, static seals and bellows

7-11


7.3

Visual Seal Examination

The symptoms experienced might not be the prime cause of failure. It is often necessary to identify the root cause in order to avoid a recurrence. Once the likely cause of the problem is decided, the available solutions are usually clear. There are cases, however, where further checks are necessary to clarify diagnosis. There are also proven remedies for particular concerns. Therefore, this section notes likely causes, further checks, and proven remedies, as appropriate, for each symptom.

Key Human Performance Point Visual examination is an important element in determining failure mechanisms. Personnel should be attentive during disassembly to be alert for evidence of incipient or chronic failure mechanisms.

As there are a relatively large number of ways a mechanical seal can fail (this section lists 45), it is helpful to group them alpha-numerically, as shown in Table 7-6 below. This split is somewhat arbitrary and several failure modes are caused by a complex mixture of mechanical, thermal, and/or chemical aspects. However, it does show a pattern, which can be helpful when using the subsequent extensive table of common seal, failure modes. Table 7-7 is similarly divided into three parts: seal faced, secondary seals, and seal hardware.

7-12

EPRI Licensed Material Troubleshooting to Identify Cause of Seal Failure Table 7-6 Visual Examination: Failure Symptoms Based on Mechanical, Thermal, or Chemical Damage Contact Pattern Seal faces

Secondary seal

Seal hardware

A1: Proper contact pattern A2: No contact pattern A3: Heavy outside diameter contact A4: Heavy inside diameter contact A5: Wide contact pattern A6: Eccentric contact pattern A7: Contact with one high spot A8: Contact at two or more high spots A9: Contact through 270q A10: Contact at gland bolt locations

Mechanical

Thermal

Chemical

A11: Fracture A12: Scratches and chips A13: Adhesive wear A14: Abrasive wear A15: Grooving and severe wear A16: Erosion of carbon ring

A17: Thermal distress, over 360q A18: Thermal distress over 120q - 180q A19: Thermal distress in patches A20: Coking

A21: Carbon chemical attack A22: Corrosion of metal faces A23: Corrosion of hard faces A24: Flaking and peeling A25: Crystallization A26: Sludging A27: Bonding A28: Blistering

B1: Physical damage B2: Extrusion B3: Excessive torque C1: Physical damage C2: Hardware rubbing C3: Erosion or abrasive wear C4: Drive failure C5: Spring distortion and breakage C6: Seal hang-up C7: Sleeve marking and damage

B4: Hard or cracked elastomer B5: Compression set of elastomer

B6: Elastomer chemical attack B7: Corrosion at secondary seal interfaces

C8: Overheated metal components

C9: Corrosion of seal hardware C10: Excessive deposits

7-13

7-14

A1:

Proper contact pattern

Symptom

the secondary seals and, in this situation, the seal typically drips steadily with the shaft stationary or rotating. warpage of the seal faces due to thermal aging and incomplete stress relief of the seal face material during operation.

x

x

If leakage is present, suspect

Typical contact pattern of a non-leaking seal. Full contact through 360 degrees on the seat surface with little or no measurable wear on either seal ring.

Common Seal Failure Modes – Seal Faces

Characteristics

Table 7-7 Visual Examination: Symptoms, Characteristics, Causes and Remedies

Troubleshooting to Identify Cause of Seal Failure


Check secondary seals for damage, porosity, thermal or chemical attack. Check for compression set of o-ring. Check for correct material with seal manufacturers. Seal hang up (see C6 below). Check face flatness.

x x x x x

Provide lead-in chamfers. Remove burrs. Lubricate secondary seals.

x x x

Pipework distortion. Remedial Action

x

Secondary seals nicked or scratched or installation. If so, renew seals, having checked for proper lead in chamfers, removed burrs, and so on.

x

Leakage is most commonly from secondary seals but in some cases due to excessive waviness of the seal faces due to high temperature exposure during operation. Checks

Causes

Causes/Checks/Remedies

No contact pattern

Heavy outside diameter contact (negative coning or rotation)

A2:

A3:

Symptom

Heavy contact on the sealing ring and the seat at the outside diameter of the sealing plane. Fades away to no visible contact at the inside diameter of the contact pattern. Possible edge chipping on the outside diameter of the sealing ring.

This indicates that the rotary face is not turning against the stationary face.

Characteristics


Slipping of the rotary drive mechanism. Interference of a rotary with a stationary component, for example, seal body with seal chamber bore.

x x

Excessive swell of confined secondary seals. Improper seal face support surface. Entrapment of foreign particles. Thermal effects (usually on ID; see A17-A19 below).

x x x x

7-15

Incorrect lapping, leaving the seal faces not flat.

x

Can also occur from:

Checks

Usually caused by the faces not being flat because of over-pressurization of the seal.

Cause

Improper installation.

x

Possibilities include the following:

Causes



Wide contact pattern

A5:

7-16

Heavy inside diameter contact (positive coning or rotation)

A4:

Symptom


Pump misalignment - this might also cause seal to hang-up on the shaft. Pipe strain. Bearing failure or excessive clearance. Bent shaft. Shaft whirl of large amplitude. Pump cavitation. Pump vibration. Misaligned seat. Pump operation outside specification.

x x x x x x x x

Seal does not leak when shaft is stationary, but leaks steadily when shaft is rotating.

Possibilities include the following: x

Changes of seal material.

x

Cause

Improved cooling of the seal.

x

Remedial Actions

Also can occur from causes listed above under heavy outside diameter contact, A3.

Checks

Typically caused by thermal distortion of seal faces.

Cause


Contact pattern is considerably wider on the seat than the face width of the sealing ring. Possible wear at drive notches if present in sealing ring.

Seal leaks steadily when the shaft is rotating and usually no leakage when the shaft is stationary.

Heavy contact on the sealing ring and the seat at the inside diameter of the sealing plane. Fades away to no visible contact at the outside diameter of the contact pattern. Possible edge chipping on the inside diameter of the sealing ring.

Characteristics


Eccentric contact pattern

Contact with one high spot

A6:

A7:

Symptom

Seal does not leak when shaft is stationary, but leaks steadily when rotating.

Contact pattern on seat through 360° slightly larger than the sealing ring face width. High spot or highly polished area might be present on the seat (for example, opposite a drive pin hole or at location of anti-rotation pin if not correctly assembled into hole). Seat without static seal(s) will rock or move in gland plate or holder. Wear at drive notches if present in sealing ring.

No leakage if the shaft has not contacted the inside diameter of the seat. If seat is damaged, then leakage will occur when the shaft is rotating or stationary.

Eccentric contact pattern on the seat with width of contact equal to sealing ring through 360°. Seat might have contact marks on its internal bore or local cracking (from a shaft rub). No abnormal wear on sealing ring if seat is undamaged.

Characteristics


Check for concentricity between the outside diameter of the shaft sleeve and the inside of the seal chamber.

x

Check that anti-rotation pin is correctly located into seat. Check that anti-rotation pin does not bottom into the seat. Check for correct extension of all drive pins from seal plate. Check for adequate shaft alignment (to avoid it passing through the seal chamber at an angle). Check for piping strain on pump casing.

x x x x

x

7-17

Check that the seal plate surface in contact with the seat is free from nicks/burrs and shows a full pattern when blued with seat.

x

Checks

Mating surfaces are not square.

Cause

Check for correct clearances between the gland plate and the seal chamber.

Check for correct seat design and clearances.

x

x

Checks

7UWCNN[ ECWUGF D[ C OKUCNKIPGF UGCV

Cause



Contact through 270°

A9:

7-18

Contact at two or more high spots

A8:

Symptom


Seal leaks steadily when shaft is rotating or stationary.

Sealing ring shows same symptoms as for mechanical distortion above.

Seal is distorted mechanically giving contact through approximately 270° with the pattern fading away at the low spot.

Seal leaks steadily when the shaft is rotating or stationary.

Sealing ring shows excellent condition after short static and dynamic tests. Possible wire drawing erosion of the sealing ring if it remains stationary. Possible wire brushing erosion if the sealing ring rotates because out-of-flat mating surface allows dirt to enter the seal area.

Seat is distorted mechanically, typically creating two large contact spots - pattern fades away between contact areas.

Characteristics


Check flatness of faces using optical flat. Check squareness of parts used to clamp seat. Check seal chamber face flatness of split case pumps. Check that the seal plate surface in contact with the seat is free from nicks/burrs and shows a full pattern when blued with the seat.

x x x x

Change to a softer gasket material between the seal chamber and the seal plate. Provide full face gasket contact or contact above centerline of bolts to prevent bending of the seal plate.

x x

Check for seal plate distortion because of overtorquing of bolts. Remedial Actions

Seal faces not flat. Check

Cause

Check for seal plate distortion because of over-torquing of bolts.

x

Checks

Seal faces not flat.

Cause


A10: Contact at gland bolt locations

Symptom

Sealing ring in good condition as initial leakage is high, preventing any long-term service life. Seal leaks steadily when the shaft is stationary or rotating.

Seal is distorted mechanically giving high spots at each bolt location.

Characteristics


Provide full face gasket contact or contact above centerline of bolts to prevent bending of the seal plate.

x

7-19

Change to a softer gasket material between the seal chamber and the seal plate.

x

Remedial Actions

Check for seal plate distortion because of overtorquing of bolts.

Check

Seal faces not flat.

Cause



7-20

A11:

Fracture

Symptom


Non-uniform discoloration or partial discoloration of the fracture surface of the presence of wear debris indicates fracture prior to or during seal operation. If no wear debris is present, the fracture probably occurred during disassembly. Fractures caused by excessive face torque generally emanate from one or more points of drive engagement and also show wear or damage on mating drive device. This problem can occur when PTFE O-rings are used to seal a pinned stationary carbon seat without a buffer sleeve over the pin. In this case, it can result in a severe gouge emanating from the pin slot rather than ring fracture. Seal leaks steadily when the shaft is stationary or rotating. When broken parts are well retained the amount of leakage can sometimes be remarkably low.

Broken seal rings or cracked seal rings (if retained in some assembly). Many seal face materials are brittle and relatively thin sections are fragile.

Characteristics


x

x x x

Excessive face torque:

x

Pin sleeve of PTFE not fitted as recommended by seal makers.

Corrosion at seal faces.

Poor lubrication.

Excessive fluid pressure.

Excessive thermal stress from thermal shock or excessive gradients (see Thermal distress, A17-A19 below).

Damage during seal removal and disassembly.

Excessive swell of confined secondary seals.

Excessive hydraulic pressure.

Failure of axial holding devices, excessive fluid pressure, poor lubrication.

Jamming from improper assembly.

Improper seal assembly or installation.

x

Mishandling before or during assembly.

x


Cause


A12:

Scratches and chips

Symptom

Leakage rate depends on the degree of damage and might be reduced when the shaft is stationary.

Chips are usually at seal face edges and severe chipping is similar to that caused by excessive hydraulic distortion.

Scratches and nicks are often erroneously cited as a cause of seal failure and it helps to decide if the scratch was caused before, during, or after operation. If the wear pattern is altered by the scratch, then the scratch occurred before or during operation. If the same scratch extends outside the mating area, it is more likely to have occurred prior to operation. If it does not extend outside the mating area, and is spiral in form relative to the shaft axis and in the direction of rotation, it probably occurred during operation and can be attributed to a particle entering or coming from the seal faces. Scratches that interrupt, but do not alter, the wear pattern, were probably produced after seal operation.

Scratches in the radial direction usually give a leak regardless of depth or width. In other directions, scratches less than 1Pm deep by 25Pm wide do not typically cause extensive leakage.

Characteristics


Edge chipping from slamming together during operation when pump cavitates or fluid vaporizes at seal faces.

Dirt trapped between seal faces.

Mishandling during manufacture, storage, assembly, or installation.

x x x x

x

7-21

(These conditions also cause excessive wear of the drive mechanism.)

Out of square seal faces.

Excessive shaft deflection or whip.

Excessive shaft run out.

Edge chipping can also occur from the following:

Checks

x x

x


Cause



7-22

A13:

Adhesive wear

Symptom


Seal leaks when shaft is rotating. When stationary, the seal might hold or might leak severely.

Excessive adhesive wear leaves typical nonmetallic seal faces heavily worn with a relatively smooth appearance and a minimum of grooving. Severe adhesive wear of metallic faces can lead to scuffing, grooving, and even face seizure.

A combination of mild adhesive/abrasive wear is the normal way seals wear out over a long service life (see proper contact pattern, A1).

Characteristics


Excessive seal contact pressure for the face materials.

x

Check 28 value of seal face materials (this method has its limitations: see Section 3.7).

Check for excessive local temperatures caused by inadequate cooling for the face surface speed.

Improved seal lubricating properties can be achieved by a temperature change. Changing seal face materials. Changing seal balance.

x x x

Remedial Actions

x

x

Degraded seal face conditions. Checks

x

Inadequate lubrication.

x

Cause


A14:

Abrasive wear

Symptom

Seal leaks steadily when shaft is stationary or rotating.

The key clue to abrasive wear is the deposit of solids on the seal faces or adjacent to them. The solids might also result from chemical effects (see A20, A21, A22, A23, A24, A25, A26, A27, and A28 below).

Virtually no wear takes place away from the face contact. Mild abrasive wear from very fine particles gives a wear pattern similar to adhesive wear.

Excessive abrasive wear leaves seal faces severely grooved and even scuffed (both metals and non-metals). Harder faces show regular grooving, while carbon faces tend to wear less evenly with heavy scoring both across the face and in the direction of rotation.

Characteristics


In spite of clean fluid flushing or use of barrier fluid in dual seal arrangement, the seal is causing inward pumping of the abrasive process fluid across the seal faces. The inward pumping phenomenon is caused by large angular misalignments and eccentricities between the seal faces.

x

Introduce a clean flow to the seal from a separate source. Install harder wear-resisting face material, for example, silicon carbide, tungsten carbide. Use double seals. Eliminate excessive misalignment and eccentricities.

x x x x

7-23

Introduce a clean flow to the seal by using filters or cyclone separator.

x

Remedial Actions

Pumped product or flush fluid contains abrasive matter of a sizeable amount to enter between the faces and cause wear.

x

Possibilities include:

Cause



7-24

A15:

Grooving and severe wear

Symptom


with abrasive wear (see A14).

0QVG that the scoring damage can be confused

This is often a start-up problem and the seal drips steadily when the shaft is stationary or rotating.

The sealing ring displays severe, though even, wear throughout 360°, with ITCOQRJQPG UEQTKPI. Soft carbon seal rings possibly have edge chipping. Harder sealing rings, for example, tungsten carbide, have rounded edges. Possible wear at any drive mechanism or notches. Other overheating symptoms might be apparent, for example, hardening and cracking of O-rings.

High wear, even cracking, of the seat with polished circumferential scoring, discoloration, and over-heating symptoms. Metal parts might "blue" with heat of dry running. Even short periods of dry running can form a deep wear groove.

Characteristics


Check for blockage/restriction of circulation line.

x

If a circulation line does not exist, review the need to install one. Increase seal circulation flow. Review operating procedures (see also Section 8.2).

x x x

4GOGFKCN #EVKQPU

Check pump suction flows and filters.

x

x

Check for adequate priming and seal chamber venting.

x

Checks

Dry running because of insufficient or no liquid between the seal faces.

Cause


A16:

Erosion of carbon ring

Symptom

Seal leaks when the shaft is stationary or rotating.

If the carbon is the stationary component, this forms a groove partway across the carbon face adjacent to the circulation inlet on the seal plate. In severe cases, harder face materials such as alumnina can also be eroded in a similar manner.

If the carbon ring is on a rotating component, this results in a sculptured appearance with islands of original mating surface still showing.

Characteristics


Shrouding the seal faces. Injecting the circulation at several points. Methods to reduce abrasive damage as for abrasive wear.

x x x

7-25

Adding a flow controller in circulation line.

x

Remedial Actions

Caused by excessive flow velocity at the seal circulation inlet, the circulation flow containing abrasive materials, or a combination of these.

Cause



7-26

A17: Thermal distress over 360° vaporization

Symptom


Other seal damage can also result, for example, fatigue of metal bellows or wear of shaft/sleeve at secondary seals (called YGFIG GVEJKPI for PTFE wedge designs). In the latter case, carbon pick-up on the secondary seal and wear of the secondary seal (for example, at the nose of the wedge) might be apparent.

The carbon sealing ring shows high wear and possibly light pitting leading to EQOGV trailing. Possible edge chipping of the sealing ring because of opening and closing of the seal faces and also possible wear of any drive notches. Carbon dust deposits on the atmospheric side of the seal and wear/fretting of the shaft/sleeve at the secondary seal (if dynamic) are also symptoms. Seal leaks steadily when shaft is stationary or rotating. The latter usually with sound from flashing or face popping.

High wear or thermally-distressed surface (heat checking) through 360°. This appears as radial surface cracks, sometimes accompanied with circular scoring or discoloration from overheating. If necessary, dye penetrant can help to show up the surface cracks.

Characteristics


x

x

x

x

x

Increased circulation flow assists in marginal situation

Check circulation lines for blockage

Use seal design with enhanced face lubrication features, for example, cooling notches, hydropads, laser textured faces.

Review seal design and seal material selection, for example, use a seal design not requiring so much product temperature margin ('T).

Review options to alter seal chamber pressure; on multiple stage pumps the seal chamber pressure might be taken off another stage to prevent flashing. The seal design will require review to ensure it is then not over-pressurized.

Increase cooling to faces:

Use a narrow face carbon (of the order of 2.5 mm).

Checks/Remedial Actions

Insufficient film thickness.

Cause


Thermal distress over 120 to 180°

Thermal distress in patches

A18:

A19:

Symptom

Seal leaks steadily when shaft is rotating or stationary. Leakage might be in the form of vapor and with sound from flashing or face popping.

High sealing ring wear with possible carbon deposits on the atmospheric side of the seal. Also possible wear at any drive mechanism notches.

CURGTKVKGU

Two, three, four, five, or six hot spots of thermally-distressed or heat-checked surface. These patches are sometimes called VJGTOCN

Seal drips steadily when shaft is rotating or stationary – possible sound from flashing or face popping.

High sealing ring wear with possible carbon deposits on the atmospheric side of the seal. Also possible wear at any drive mechanism notches.

Thermally-distressed (heat-checked) area approximately one-third of the contact pattern. Distressed area 180° from inlet of seal flush with good contact pattern at flush inlet.

Characteristics


Check that seal chamber neck bush clearance is correct.

x

Add a tangential inlet matched to the shaft rotation to aid distribution. See Thermal Distress Over 360° (A17).

x x

Check for seat distortion.

x

Review possibility of seal interface cooling with the seal manufacturer. See Thermal Distress Over 360° (A17).

x x

7-27

Increase cooling of seal faces.

x

Remedial Actions

Check for adequate cooling of seal faces.

x

Sealed liquid vaporizing between the seal faces. Failure from hot spots is more likely to occur on light specific-gravity liquids at high speeds and pressures. Checks

Cause

Add a circumferential flush groove in the gland plate.

x

Remedial Actions

Check for adequate clearances around the seal face to give sufficient face lubrication and cooling.

x

Checks

Sealed liquid vaporizing 180° from the seal flush.

Cause



7-28

A20:

Coking

Symptom


Leakage typically occurs on start-up after a period of shut-down or on standby when solidification of waxes/gums associated with the coke particles takes place. The leakage can in odd cases reduce after a short period of running as these waxes soften.

This usually occurs with hydrocarbon products at high temperatures. It is indicated by failure of the seal to follow up, that is, no sliding action. This can be found after removal of the seal plate during the stripdown for inspection. Coke particles collect on the inside of the sliding member, even to the extent where it can be difficult to remove. In many cases of continuous operation, heat from the product and seal friction can keep the coke and associated waxes and gums reasonably soft and the seal will operate satisfactorily.

Characteristics


The usual approach with hydrocarbons is to fit a permanent low-pressure steam quench on the atmospheric side of the seal to prevent the build-up and solidification of coke and wax particles. An adequately sized drain will both prevent excessive steam pressure and assist particle removal. This quench must be operational before start-up. If not already fitted, a high-temperature lip seal at the back of the seal plate improves quenching efficiency. It also reduces the likelihood of steam entering the bearing housing.

x

x

Minute quantities of leakage carbonizing on the atmospheric side of the seal causing the sliding member to jam and hence not follow up any face wear. Remedial Actions

Cause


A21:

Carbon chemical attack

Symptom


Selective leaching of the impregnant (added to the otherwise porous carbon to make it impervious) results in either increased wear rate or seal face porosity. With this mechanism, a hardness reduction of 5 Shore scleroscope points is typical for carbon-graphite materials. Pressure testing for porosity can also be used to confirm such a problem.

Overall corrosion occurs when it is attacked by highly oxidizing acids or highly concentrated caustic fluids. A hardness reduction of 20 Shore scleroscope points is typical for carbon-graphite materials that have been chemically attacked. In severe cases of this type, seal faces are reduced to sludge.

Essentially, there are two carbon-graphite corrosion modes: overall corrosion and selective leaching.

Area of carbon ring in contact with the product is corrosively attacked, resulting in overall material removal, pitting, porosity, softening, or disintegration.

Characteristics


Overall corrosion.

7-29

Many highly corrosive products, for example, oleum, present a conflict between corrosion and wear resistance of the face materials, which, even with the latest materials, results in a maximum seal life of only a few months.

A corrosion rate of 0.025 mm (0.001 in.) per year is normally quite unacceptable for seals, even though this is satisfactory for most industrial hardware. It is usually, therefore, better to use seal manufacturer data than any non-numerical industrial corrosion data when assessing such a problem.

A change of material – both failure mechanisms require checking the material selection for product compatibility and the original product conditions against the seal selection.

Selective leaching of impregnant. Remedial Actions

x

x

Incompatibility of the carbon with the product, resulting in two failure mechanisms.

Cause



Corrosion of hard faces

A23:

7-30

Corrosion of metal faces

A22:

Symptom



This is commonly the result of leaching of binders or fillers in alumina, tungsten carbide, and silicon carbide. Certain corrosive fluids leach the binders/fillers from these ceramics and, in effect, convert the seal face into a grinding surface. As leaching continues, the ceramic particles eventually become dislodged from the base material and cause abrasive wear of one or both seal faces. Seal face flatness is degraded to the point of seal failure by the resulting voids in the ceramic surface and/or the abrasive damage.


Corrosive attack by the product, sealant, or atmosphere. Corrosion is accelerated because the face is subject to sliding contact wear. Dissimilar materials can also set up an electrolytic corrosive action.

Characteristics


99.5% alumina, silicon-free silicon carbide (sometimes called UKPVGTGF CNRJC), and alloybonded tungsten carbide that withstand such fluids more effectively are now available.

Remedial Actions

Acidic fluids might leach nickel or cobalt binders incorporated in cemented tungsten carbide, again giving the failure characteristics described.

Certain grades of silicon carbide contain free silicon that can be similarly attacked (for example, by hydrofluoric acid).

In typical commercial alumina (75 or 85%), the alumina particles are bonded together by a predominantly silica glass binder. Sealed fluids with a pH greater than 10, or containing hydrofluoric acid, leach out this binder, giving the failure characteristics described.

Cause

This can be analyzed and solved in just the same way as with other mechanical devices.

Many corrosion failure mechanisms such as overall corrosion, intergranular corrosion, stress corrosion cracking, etc., occur in mechanical seals. Remedial Actions

Cause


Flaking and peeling (of hard coatings)

A25: Crystallization

A24:

Symptom

Leakage rates vary widely.

Similar symptoms as for Coking (A20), except that it occurs on various products and conditions. Sometimes the crystals embed in the softer face and rapidly abrade the harder face. Note that as well as from the product, crystals can come from the atmosphere (for example, ice crystals) or from a barrier fluid (for example, hard water deposit).

Seal leakage can escalate quickly and continues when the shaft is stopped.

The failure often starts with slight blistering, then lifting of the coating. Final failure might well be accelerated by abrasive wear of one or both seal faces by hard particles as they become dislodged from the coating.

Stainless steel seal faces are usually plated with a hard-facing of Stellite, ceramic, tungsten carbide, or a variety of other materials.

Characteristics


A defective coating

7-31

The crystals can come from the atmosphere, for example, ice on LPG pump duties, where a nitrogen quench to keep moisture from the seal is one possible approach.

As with coking, the best remedy is a permanent quench to dissolve or disperse the crystals. Examples of quench fluids are hot water, steam, and solvent, according to the product. Again, lip seal improves quench efficiency.

A build-up of crystals from the pumped product giving both high Abrasive Wear Rates (A14) or failure to follow up (Coking, A20, and Seal Hangup, C6). Remedial Actions

Cause

Changing to a solid face material is the usual solution adopted.

The chemical attack might be aggravated by both heat generation at the seal face and the porosity inherent in some coating techniques. Remedial Actions

Chemical attack at the bond between the base metal and the coating. Checks

x

x

Possibilities are:

Causes



7-32

A26:

Sludging

Symptom


Once leakage occurs after start-up, it seldom stops when the pump is stopped again.

Associated with the sealing of high viscosity liquids, particularly acute on pumps sealing hydrocarbon liquids at temperatures above ambient. When shut down, the viscosity of the pumped liquid and the interface film increases as the temperature drops and problems might arise on restarting the pump.

A polished wear track or slight scoring on the hard face. Small cavity holes on the carbon face (from which particles have been pulled). Possible distortion of the drive spring or excessive wear/damage on other drive mechanisms.

Characteristics


Ensure viscosity range of products is within seal capabilities.

x

x

Preheat seal area (for example, low pressure steam to seal chamber jacket/tracing). Preheat seal faces (for example, lowpressure steam quench).

•

•

Supply continuous heat through a heated seal plate.

Such heating to be used for 15 – 30 minutes prior to start-up.

Preheat circulation lines (for example, by steam tracking).

•

To overcome start-up problems:

Check that pump heat is adequate to give product circulation around the seal area under pumping conditions. Remedial Actions

x

x

The shear stresses between the seal faces exceed the rupture strength of the carbon and particles are pulled from the carbon face. This is usually because of a viscosity increase when shut down, but it also occurs when the interface film partially carbonizes from overheating. Checks

Cause


Bonding

Blistering

A27:

A28:

Symptom

Seal leaks steadily when shaft is rotating or stationary.

Normally associated with start-stop applications.

Initially, this failure appears as a shiny bruised effect in the surface and, at a later stage, manifests itself as a crater where the bruise has detached itself from the surface and passed through the seal faces.

Similar phenomenon to Sludging (A26) and Bonding (A27).

Once leakage occurs after start-up, it seldom stops when the pump is stopped again.

The appearance of the seal and other symptoms are similar to that from sludging problems.

Similar phenomenon to Sludging (A26). In this situation, a bond is formed between the two seal faces after the pump has been stationary for a long period. On starting, particles are pulled from the carbon face and leakage occurs.

Characteristics


Operation on an intermediate flushing fluid for a short period between testing and production use.

x

Careful choice of seal materials. Ones with higher thermal conductivity produce less blistering against carbon counterfaces. Certain grades of carbon-graphite are more resistant. Review of start-up procedures.

x

x

7-33

Keeping product viscosity low by heating.

x

Difficult problem to solve; useful approaches include the following:

Remedial Actions

High local heating occurs in a few seconds on start-up, particularly with high viscosity products in high speed, motor-driven pumps operating at high pressure. This heating can cause rapid expansion of liquid that has been absorbed into the seal face surface. This rapid expansion causes high stress which, in extreme cases, exceeds the rupture strength of material.

Cause

Selection of suitable test fluid.

x

The main cause is when a pump is tested on a different liquid to that on which it will operate and a chemical reaction occurs between the test fluid film and the actual product film. Remedial Actions

Cause



Extrusion

B2:

7-34

Physical damage

B1:

Symptom


Flaying or shredding is most common on synthetic rubber rings, whereas a lip is usually formed on Viton or PTFE. Thermoplastic materials, for example, PTFE and Viton, are more susceptible to extrusion at elevated temperatures. Seal leakage might reduce when shaft is stopped.

This can occur with O-rings, wedges, bellows, and other secondary seals. The most common form is O-ring extrusion and this occurs when part of the O-ring is forced through close clearance gaps. Typically, a lip is first formed on the O-ring; it is then cut and, in some cases, peeled off like an outer cover.

Seal drips steadily when shaft is stationary or rotating.

All forms of bellows, rubber, PTFE, and metal, can easily be damaged and the location might not be easy to spot.

Plastic seals, for example, PTFE, possess less elastic self-healing properties than elastomeric secondary seals.

Cuts, scratches, nicks, or tears in O-rings, bellows, wedges, and other secondary seals.


Inadequate installation practice. Presence of dirt. Failure to remove burrs, sharp edges of steps, keyways, holes, etc., and previous set screw indentations prior to seal installation.

x x x

As well as checking the above, other changes can be made, such as fitting a back-up ring, a change of seal design, a change of material, and so on.

Incorrect shaft and/or O-ring groove sizing giving excessive clearance between components. Remedial Actions

Excessive pressure (possibly aggravated by overheating and chemical incompatibility).

x x

Use of excessive force when fitting and assembling components.

x


Cause

Having found the cause, the only usual rectification of the secondary seal damage is renewal.

Bellows damage can also be caused by manufacturing defects–inclusions, incorrect curing, inadequate weld quality, and so on. Remedial Actions

x

Mishandling.

x


Cause

Common Seal Failure Modes – Secondary Seals

Characteristics


B3:

Excessive torque

Symptom

The photograph shows a metal bellows failure (rubber bellows tear in a similar manner). This can be compared with bellows overpressurization, which can also rupture the bellows.

Some secondary seals provide a drive function; exceeding the torque capacity will cause problems. Typically this will either involve (1) rotational movement resulting in wear or ultimate failure of seal from frictional heat developed during sliding contact, or (2) exceeding the structural torque capacity of the device. An example of (1) is rotation of a seat dependent on friction of its O-ring to avoid rotation (no anti-rotation pin). An example of (2) is bellows torsional failure. This can give very large seal leaks.

Characteristics


Bonding of a high viscosity film between the seal faces (A27). On start-up, the bond strength is greater than the design torque capacity of the seal.

7-35

If the cause cannot be rectified using methods referred to under Sludging (A26), Bonding (A27), Adhesive Wear (A13), Grooving and Severe Wear (A15), and Thermal Distress (A17, A18, A19), modified seals with an anti-rotation device appropriate to the problem are available.

High seal face friction, for example, from lack of lubrication. Remedial Actions

x

x


Cause



7-36

B4.

Hard or cracked elastomer

Symptom


Seal drips steadily when shaft is stationary or rotating.

It is important to distinguish between chemical attack and thermal damage to decide on the remedy. Chemical attack is more likely on secondary seal surfaces exposed to the fluid; thermal degradation is more frequently found on surfaces exposed to the atmosphere.

Rubber O-ring hardened and cracked. PTFE Oring discolored blue/black. The portion of the ring nearest the faces is usually the worst. Most commonly a problem with nitrile rubber. Comparative analysis of secondary seals from all locations will reveal whether the thermal condition was local to one secondary seal or an overall excessive temperature.

Characteristics


Relative rotational movement between the secondary seal and the shaft or housing.

x

Check circulation to seal area. Check for dry running, low pump suction flow, sludging, and so on. Ensure any cooling is fully operational. Check product conditions are as originally specified and that O-ring material is suitable.

x x x x

Checks

It is important to identify the source of thermal damage as it might lead to the root cause of the failure. For example, excessive loading of the seal face material could have caused the frictional heat, and changing the O-ring material would not avoid premature future failure of the seal faces.

Heat soak from the seal environment including the shaft and housing.

x

Other possible thermal damage sources are:

Two possibilities: overheating or chemical attack (see also Elastomer Chemical Attack, B6 below). If most or all damage is on secondary seal surfaces that contact a seal face member, excessive frictional heat from the face is the likely cause.

Cause


Compression set of elastomer

Elastomer chemical attack

B5:

B6:

Symptom

Seal face distortion and misalignment caused by swell. Loss of secondary seal interference caused by shrinkage. Shrinkage of seals giving loss of secondary seal drive.

x x x

Leakage rates vary widely.

Leakage might occur from the O-ring being eaten away. It might also appear to have lost its original composition and to be breaking up. Often product-side is badly attacked while nonproduct side has a relatively good appearance.

Extrusion caused by swell.

x

This gives excessive volume change, either swell or shrinkage, which causes a seal failure through one or more of the following:


Although this will occur over a period of time, early changes in section as shown will result in premature failure. Compression set does not involve a significant volume change.

Characteristics


7-37

Specially colored O-rings to assist in identification help to ensure that the correct material is used. However, some coloring additives might have a lower corrosion resistance than the base elastomer.

O-ring/chemical incompatibility charts are available from seal manufacturers. If there is a doubt about a volume change, secondary seal dimensions should be measured in both free and assembled conditions and compared with those specified on assembly drawing. An optical comparator is one useful instrument for such Oring examination.

It is necessary to check the original product conditions against the seal selection and ensure that the O-ring fitted is made of the correct material.

Checks

Chemical attack of elastomer by the product.

Cause

Excessive temperature for the O-ring material. Sometimes caused by incompatibility with fluids.

Cause



7-38

B7:

Corrosion at secondary seal interfaces

Symptom


Crevice (or oxygen concentration cell) corrosion occurs because secondary seals, for example, elastomeric bellows, can trap a small amount of fluid adjacent to the shaft. A good indication of this failure mode is a polished or gas-scrubbed area adjacent to the corroded section that is generated by hydrogen emanating from the crevice, that is, from beneath the bellows.

This gives a subtle leakage path resulting from two different mechanisms; fretting corrosion and crevice corrosion. Fretting corrosion is caused by small relative movements between a secondary seal and its mating surface. The degree of damage is accelerated in the presence of even a slightly aggressive product (for example, water) and is particularly aggravated by the presence of chlorides. The fretting corrosion debris is abrasive and the later stages of attack are assisted by a 3-body abrasive wear mechanism, where debris embeds in the secondary seal and wears the shaft or sleeve.

Characteristics


Excessive shaft deflection – over 0.08 mm (0-.003 in.). Excessive out-of-squareness of seal face to shaft axis – over 0.08 mm (0.003 in) Total Indicated Runout (TIR).

x x

It is common to hardface the sleeve in the secondary seal area to minimize the damage from fretting corrosion. Use a non-pusher seal, for example, a metal bellows seal, to avoid the fretting contact. If crevice corrosion is suspected, then any action to avoid the crevice or provide a corrosion-resistant surface treatment will correct this effect.

x

x x

Fretting corrosion is most common at the dynamic secondary seal under a pusher type seal (for which the above values refer). In a pusher type seal, the secondary seal is pushed along the shaft or sleeve to compensate for wear. Remedial Actions

Excessive shaft end play – over 0.1 mm (0.004 in.).

x

Common contributors to fretting corrosion include

Fretting corrosion is primarily governed by mechanical factors such as equipment condition, seal assembly procedures, and correct materials selection. Checks

Cause


C1:

Physical damage

Symptom

A wide variety of symptoms from chips, minor distortion, nicks in metal bellows, to the example in the picture. In that specific case, care was taken not to damage the faces by placing the seal on its edge. Unfortunately, it was not wedged, and it rolled away and was run over by a forklift truck.

Common Seal Failure Modes – Seal Hardware

Characteristics


Excessive force. Use of incorrect tools, and so on.

x x

7-39

After being careful with the seal faces and secondary seals, the hardware is sometimes damaged by accident.

Insufficient cleanliness.

x

Not observing good fitting practice:

Cause



7-40

C2:

Hardware rubbing

Symptom


In severe cases, the part might be heated to such an extent that it reaches its melting point.

Certain conditions might cause abnormal wear where little should occur, for example, the outer skin of the rotary unit, the shaft (for example, against the stationary seat), the neck bush, and the throttle bush in the back of the seal plate.

Characteristics


Seal chamber too small for rotary unit. Unspigotted stationary unit slips and touches shaft. Non-piloted seal plate touches shaft. Set screws in the rotary unit come loose and contact the seal chamber wall. Pieces of the face break off and jam between the rotary unit and the seal chamber wall. Flush connection lines extend too far into the seal chamber and touch the seal. Single-spring seals might rub the seal chamber wall if broken or over-compressed, or are subjected to high speed. Multiple springs break up and jam between the rotary unit and the seal chamber wall. Product or other seal deposits (see C10 below) might scale up on the seal or on the seal chamber wall. Thermal expansion causing the metal body or other part to expand and, hence, contact the seal chamber wall. Equipment vibration.

x x x x x x

x x

x

x

Pump/motor shaft misalignment.

x x

Bearing failure.

x


Cause


C3:

Erosion or abrasive wear

Symptom

On stationary seal hardware, grooving damage occurs again, often in line with a circulation inlet.

Circular marks on the outside diameter of the rotating seal body – often in line with a circulation inlet.

Characteristics


Making it tangential. Checking this inlet for protrusion into the seal chamber. Flushing with a cleaner fluid. Selecting a smaller outside diameter seal. Boring out the seal chamber.

x x x x x

7-41

Changing the circulation inlet position.

x

Solutions can involve:

Remedial Action

Also caused by wear debris circulating in the seat chamber.

It also can result from the incoming flush containing abrasives and eroding the seal body, especially if the flush pressure differential is too high.

This can be caused by Hardware rubbing (C2).

Cause



7-42

C4:

Drive failure

Symptom


Fatigue failure of metal bellows (an adequate product temperature margin, 'T, is vital as this is often caused by vaporization).

x

(4) Failure of drive screws/collars, for example, set screws cutting into the body.

Wear of drive lugs.

x

x

Wear/fracture of drive pins.

x

This can occur with both the torsional drive devices of rotating components and the antirotation devices of stationary components. Typical examples include:

Characteristics


x x x x x x x x x x

Stick-slip face friction giving seal face vibration.

Equipment vibration.

Excessive shaft deflection.

Excessive shaft run-out.

Seal face out of square with shaft axis.

Excessive seal fluid pressure.

Poor seal face lubrication.

Failure of axial holding device.

Excessive shaft end play.

Jammed seal assembly.


Cause


C5:

Spring distortion and breakage

Symptom

See also Excessive Torque (B3), re: bellows assembly failure.

Typical failure characteristics are radial cracking of the spring section, especially on the inside diameter, straight fracture, wear marks in ends of spring coils and on the sleeve and rotary necks, and build-up of solid contaminants around spring(s), making them ineffective.

All mechanical seals require movement to keep the faces together during changing pump and seal conditions and to compensate for wear. Spring action is obtained by a single coil spring, multiple coil springs, a metal bellows assembly, or a wave spring washer.

Characteristics


7-43

On multi-spring seals, diversion of part of the product circulation through the spring pockets can reduce future build-up of solids.

On multi-spring seals, a build-up of solids around the springs can make some springs ineffective and, hence, cause overload and failure of the others. Remedial Actions

The above and other spring problems are most common on high viscosity duties prone to Sludging (A26) or Bonding (A27).

On many single-spring seals, the drive is unidirectional and the spring should always grip its mating parts. With such seals, reverse rotation or incorrect spring fitting causes the spring to tend to uncoil, slip, distort, crack, or even break.

Checks

These spring devices fail in a variety of ways, for example, corrosion, stress-corrosion and fatigue.

Cause



7-44

C6:

Seal hang-up

Symptom


The sliding assembly movement is typically prevented by a build-up of deposited dissolved solids, corrosion, oxidation, or decomposition products. This possibility is present whenever a pusher type seal (where the secondary seal is pushed along the shaft or sleeve to compensate for wear) is used. See Coking (A20) and Crystallization (A25).

This occurs when the sliding assembly is prevented from following up (by moving axially), thus leaving a gap between the sealing ring and the seat.

Characteristics


Provision of a suitable quench, for example:

x

Use of a seal design in which the secondary seals advance onto clean surfaces can also help. In many cases, mechanical sleeve damage that has occurred will require rectification (including hard facing in the secondary seal area).

x

x

• A suitable coolant quench to prevent thermal decomposition products from forming.

• Oil or similar to prevent the formation of corrosion products.

• Nitrogen to prevent the formation of oxidation products.

• Water to prevent deposition of aqueous dissolved solids.

Use of a non-pusher type seal, for example, metal bellows type.

x

Remedial Actions


C7:

Sleeve marking and damage

Symptom

From fretting corrosion or crevice corrosion between the sleeve and the secondary seal. See Corrosion at Secondary Seal Interfaces (B7). Overall corrosion, usually found on the product side of the sleeve; unless the seal is leaking badly, the atmospheric side is often in good condition. See Corrosion of Seal Hardware (C9).

x

x

When in operation, an increase in shaft eccentricity will increase hydrodynamic action, resulting in thicker fluid film and increased leakage.

Mechanical causes typically give leakage only when running and often leakage disappears when the machine is static.

From mechanical reasons – causes in this section give details.

x

This marking can be divided into three types:

This might well relate to Seal Hang-Up (C6), Coking (A20), or Crystallization (A25). The marking on a sleeve (or shaft if no sleeve is fitted) often gives a useful indication of the cause of seal failure.

Characteristics


If above contact occupies part of the sleeve circumference, this usually suggests an eccentric or gyrating shaft. It is often an indication that external forces are imposing misalignment between the seal faces and causing leakage. A bent shaft often gives rise to two marks diametrically opposite: one on the front landing and one on the opposite rear landing. This bending might be from an outof-balance shaft/rotor assembly. Severe vibration can cause the O-ring landings to contact, resulting in fretting and marking into which foreign matter can lodge, thus causing Seal Hang-Up (C6). Failed bearings can result in either increased vibration or misalignment. Incorrect sleeve manufacture or seal assembly. Lack of hard facing gives excessive wear in secondary seal area of shaft, especially if abrasives are present in the product.

x

x

x

x x x

7-45

If above contact occupies all the sleeve circumference, it is probably caused by a misaligned seal that forces the sealing ring to oscillate relative to the shaft sleeve once per revolution. This is often accompanied by wear on the inside diameter of the secondary seal on the sealing ring.

Contact between O-ring landings on the inside of a rotary seal ring is often caused by an eccentric or misaligned shaft. If landing wear is severe, O-ring extrusion can result.

x

x

Typical causes of sleeve marking:

Cause



Corrosion of seal hardware

C9:

7-46

Overheated metal components

C8:

Symptom


900-1000°F (480-540°C) 1100°F (590°C) 1200°F (650°C)

Brown Blue Black

Corrosion damage is often not present on the atmospheric side of the seal (unless it is leaking badly). Seals can continue to function adequately until quite advanced stages of corrosion.

Corrosive attack results in overall and local loss of metal. Damage characteristics are usually indicative of the corrosion mechanism; these mechanisms are as conventionally experienced in other engineering components.

700-800°F (370-430°C)

Straw Yellow

Typical colors and temperatures that create these colors on stainless steel:

When steel is heated, a color change takes place. This heating causes tempering and, hence, loss of required mechanical properties. This color change might be present generally or related to specific components.

Characteristics


Check for correct processing in manufacture with seal supplier. Review use of any dissimilar metals (electrolytic action).

x x

A change of material. Ground the pump effectively to earth if pitting from electrolysis is suspected.

x x

Remedial Actions

Check material selection against the product and its conditions.

x

This occurs through the various usual mechanisms: overall corrosion, stress-corrosion, electrolytic attack, hydrogen embrittlement, crevice corrosion and fretting corrosion. Checks

Cause

Comparative analysis of parts from all locations will reveal whether the thermal condition was local to one component or an overall excessive temperature.

Checks

Unless caused by Hardware Rubbing (C2), there are usually other components, that is, seal faces or secondary seals that are also damaged and assist diagnosis of the likely cause of excessive heat. Typical reasons are dry running, vaporization, excessive heat soak, and so on.

An easily distinguishable sign of seal trouble.

Cause


C10:

Excessive deposits

Symptom

7-47

In severe cases, a separate clean purge might be required.

Remedial Actions

Inadequate seal chamber circulation to flush out the deposits and stop them from building up.

Deposits from the product, corrosion, etc., build up on the rotary body. This can cause the rotating unit to freeze in the seal chamber. Other concerns, for example, Seal Hang-Up (C6), might well occur first. Cause


Troubleshooting to Identify Cause of Seal Failure Characteristics



8 MAINTENANCE

8.1

Introduction

Seal maintenance programs at most power plants fall within one or more of the following categories: reactive maintenance, preventative maintenance, and predictive maintenance based on condition monitoring. The most cost effective maintenance program should be based on predicted seal performance and its expected life. The least cost effective maintenance program is one based on reactions to failure. Reaction type programs result in unexpected plant shutdowns and reduced plant availability. Except for seals in safety-related and critical applications, most maintenance is performed under the reactive category because of a lack of control of the various factors that lead to premature seal failure and the effort required to perform condition monitoring on such a large population of seals. To prevent reactive maintenance of seals in critical applications, most plants implement some level of preventative or periodic maintenance programs based on experience and manufacturer recommendations. In safety-related installations, seals are maintained periodically, regardless of the condition of the seal to prevent unexpected plant shutdowns. Maintenance in most power plants is performed by maintenance personnel at the plant with assistance from the plant engineer and seal manufacturer on unique problems and specialized processes. In all cases, plant maintenance personnel are responsible for seal removal and installation. However, to maximize their effectiveness, plant engineers and maintenance personnel should not be limited to removal and installation. They should be trained in the proper diagnosis of seal failure and how to correctly address the root cause of a seal failure. It, therefore, becomes particularly important to provide the proper level of training necessary to identify the problem rather than to just maintain the seal. Appendix C lists organizations that provide training classes, short courses, and seminars on the design, selection, operation, maintenance, troubleshooting, and failure diagnosis of mechanical face seals. Key O&M Cost Point The most cost-effective maintenance program should be based on predicted seal performance and its expected life. The least cost-effective maintenance program is one based on reactions to failure. An effective preventative or periodic maintenance program, based on plant experience and manufacturer recommendations, should be implemented to improve plant reliability and prevent unplanned shutdowns.

8-1

EPRI Licensed Material Maintenance

8.2

Installation and Operation

As discussed in Sections 4 and 5, mechanical face seals are relatively precise and complex assemblies that are subject to a variety of failure modes. For reliable operation, mechanical seals require the correct working environment, which demands good engineering, maintenance, and operations practices, and well-written and detailed procedures. Written procedures should be kept current so that, as new information is acquired, it is properly accounted for and implemented into working practice. The following discussion outlines methods that can be utilized by plant engineers and maintenance personnel to improve the chances of obtaining longer life from the seals. The topics covered address: x

Seal handling and inspection

x

Pre-installation equipment checks

x

Seal installation

x

Startup and operation Key Human Performance Point Personnel training is a very important aspect of a mechanical seal maintenance program that is striving to achieve improvements in plant reliability. Comprehensive training courses covering mechanical seal design options, installation, operation, maintenance, troubleshooting, and failure diagnosis are regularly offered by seal manufacturers, universities, and research associates (see Appendix C).

8.2.1 Seal Handling and Inspection This section covers pre-installation checks applicable to the mechanical seal itself and includes seal storage. Checks to be performed on the equipment are discussed in Section 8.2.2. These checks should supplement rather than supercede manufacturer recommendations. These checks should be tempered by plant personnel experience. 8.2.1.1

Packaging Key Human Performance Point Proper storage and handling of seal components is important to seal longevity and performance. Manufacturer’s recommendations should be followed at all times.

8-2


Seal assemblies and spare parts are typically wrapped and boxed. If the package is opened with a knife for inspection, care should be taken to ensure that the faces and elastomeric seals are not cut or scored. If not used, seals should be repackaged in the same manner and returned to their original box, if practical, to ensure that proper labeling and identification is maintained. If the box is unusable, then the replacement box should have proper labeling. 8.2.1.2

Storage

To protect the seals from damage, storage of the seal assemblies and spare parts should be in accordance with the seal manufacturer's recommendations. The storage area should be clean, dry, and adequately warm and ventilated. 8.2.1.3

Handling

Many mechanical seal faces are brittle and fragile and can easily break if dropped. The metal components of a mechanical seal provide the proper restraints and alignment needed for operation. Care should be taken that these components are not damaged. Protect parts from damage wherever possible. Avoid placing a seal face down on any surface, unless it is protected by a clean cloth or similar material. Some parts are prone to attack by common liquids. For example, ethylene propylene rubber is attacked by mineral oil and silicone rubber is attacked by silicone oil. 8.2.1.4

Physical Checks of Mechanical Seals

Obtain specific drawings from the manufacturer. The drawings provide assembly details and key dimensions for fitting and installation. When sufficient information is not available, contact the manufacturer for advice. Technical recommendations and technical information provided with the mechanical seal should be transferred to maintenance procedures for future use. Care should be taken to note any safety/toxicity/industrial hygiene issues. 8.2.1.5

Seal Rotating and Stationary Components

x

Check for physical damage

x

Ensure drive pins and/or spring pins are free to move in the pin holes or slots

x

Check that set screws are free in the threads. Set screws should not be reused because damage to the drive end might have occurred in previous use.

x

Check metal bellows for damage that might cause leakage or improper alignment of the faces.

x

Check secondary seals for nicks or cuts. If the seals need to be replaced, make certain that the replacement seals are of the same type to ensure fluid and temperature compatibility.

8-3


8.2.1.6

Seal Faces

Visually check for nicks or scratches. Face imperfections of any kind can lead to leakage and premature failure of the seal. Detailed inspection of the seal faces for flatness is discussed in Section 8.2.3.1, Seal Dimensional Checks. 8.2.1.7

Gaskets

Check thickness against the manufacturer's specifications. Incorrect gasket thickness can lead to incorrect seal length settings and improper face loading. 8.2.1.8

Spring

Check rotation of spring coil when a single coil is used. The spring coil rotation should be such that shaft rotation tends to tighten the coil. Springs are available in right-hand and left-hand coil rotation. Some springs can be used bi-directionally. 8.2.2 Pre-Installation Equipment Checks Proper equipment function is critical to seal performance and it is recognized that seal life is adversely affected by equipment misalignment and vibration. The following checks can be easily accomplished using good engineering practices and simple measuring instruments. Limits of acceptability on runout provided in this section are general in nature. The seal manufacturer should be contacted for limits applicable to their products. Key Human Performance Point Pre-installation checks are an important element in reliable seal performance. Personnel should perform the steps outlined herein to prevent unsatisfactory seal performance.

8.2.2.1

Shaft Straightness (Figure 8-1)

Shaft straightness is checked with the shaft removed from the equipment. It is mounted between centers to check for runout between the bearing and the shaft or shaft sleeve at the location where the mechanical face seal is installed. Typical runout limits: 0.004 inches (0.1 mm) for speeds d 1,800 rpm 0.002 inches (0.05 mm) for speeds > 1,800 rpm

8-4


Figure 8-1 Shaft Straightness Check

8.2.2.2

Shaft Runout (Figure 8-2)

Shaft runout is checked with the shaft installed in the equipment. Runout is checked at the location where the mechanical face seal is located on the shaft or shaft sleeve, and is accomplished by slowly rotating the shaft against a stationary dial indicator.

Figure 8-2 Shaft Runout Measurement

8.2.2.3

Squareness of Stuffing Box (Figure 8-3)

Squareness of the stuffing box is checked to ensure that angular misalignment does not occur upon installation. Angular misalignment is checked with the equipment completely assembled except for the seals. The measurement is made by mounting a dial indicator on the shaft and then slowly rotating the shaft and dial indicator to measure the runout of the face that controls the angular placement of mating ring. Typical runout limits for wedges, O-rings, and metal bellows seals: 0.003 inches (0.08 mm) for speeds d 1,800 rpm 0.0015 inches (0.04 mm) for speeds > 1,800 rpm Typical runout limits for elastomer and PTFE bellows seals: 0.007 inches (0.18 mm) for speeds d 1,800 rpm 0.0035 inches (0.09 mm) for speeds > 1,800 rpm

8-5


Figure 8-3 Stuffing Box Squareness Measurement

8.2.2.4

Rotational Balance (Figure 8-4)

Rotational balance of the shaft should be checked with the impeller installed as well as other components that normally rotate with the shaft. Excessive out-of-balance can cause premature seal failure. The acceptable amount of out-of-balance is dependent upon the specific application but, in general, the deflection caused by out-of-balance should not exceed the limits defined in 8.2.2.1 and 8.2.2.3 when the shaft is turning at normal operating conditions.

Figure 8-4 Shaft and Impeller Rotational Balance Check

8.2.2.5

Shaft Bearing Clearances (Figure 8-5)

Shaft-to-bearing clearance can allow both radial and axial movement of the shaft. These tests are performed with the shaft installed in the equipment. Radial movement is checked by loading the shaft laterally with a light force so that the shaft does not bend. Axial movement is checked by pulling and pushing the shaft along its axis. Radial movement should be limited to 0.003 inches (0.08 mm) for rolling element bearings. For plain bearings, the movement should not exceed the maximum bearing clearance specified by the manufacturer. Axial movement of the shaft should be limited to 0.003 inches (0.08 mm). If this limit is exceeded, then the face seal load generated by the springs should be checked to ensure that it remains within the manufacturer's recommendation for normal operating conditions. Abnormal operating conditions and stop/start conditions that cause excessive axial movement can lead to reduced seal life. 8-6


Figure 8-5 Radial and Axial Bearing Clearance Checks

8.2.2.6

Shaft/Sleeve Diameter and Surface Finish (Figure 8-6)

The shaft and shaft sleeve should be checked to ensure that the diameter at the seal locations (including secondary seals) is within the seal manufacturer's recommendations. The surface finish under the seal (especially at the secondary seal position) should be free of machine marks, and should have a roughness of less than 25 micro-inches (600 Pm) for static seals and less than 10 micro-inches (250 Pm) for dynamic O-rings and wedge rings. For elastomeric/rubber bellows, the shaft/sleeve surface finish can have fine machined marks but the surface roughness should be limited to 50 micro-inches (1200 Pm).

Figure 8-6 Measurement of Critical Shaft and Sleeve Diameters

8.2.2.7

Sleeve Hardfacing (Figure 8-7)

Sleeves are sometimes hardfaced to prolong their useful life in abrasive service. However, hardfacing should be limited to secondary seal areas and should not extend to the location where the set screws lock the seal to the sleeve. If the set screw lands on the hardfaced surface, the screw grip might be impaired and allow relative movement between the seal and sleeve.

8-7


Figure 8-7 Sleeve Hardfacing to Prolong Life

8.2.2.8

Sharp Edges (Figure 8-8)

Sharp edges are not acceptable where a seal must pass with an interference fit. Sharp edges can occur at shaft steps, keyways, splines, holes, and so on. Sharp edges can cut or nick a soft sealing member and create a leak path. If possible, chamfer the leading edge of the shoulder to allow the seal to slide over it.

Figure 8-8 Lead-In Chamfers to Prevent Secondary Seal Damage During Installation

8.2.3 Seal Installation Checks This section provides some basic step to follow during seal installation and the manufacturer should be contacted for detailed information and recommendations. Some of these steps require some type of measurement. It is therefore important to obtain assembly drawings from the manufacturer. 8.2.3.1

Seal Dimensional Checks

The overall dimensions and critical interface dimensions should be checked against drawings to ensure that the mechanical seal is correct to the drawing. Some check should be made to verify that the seal is able to compress to the correct length. Caution should be taken when compressing metal bellows seals because over-compression might result in yielding of the bellows. If the bellows yield, they will not generate the required load at the installed length.

8-8


Seal faces should be inspected by an optical flat to ensure that they meet the flatness requirements specified by the seal manufacturer. Appendix B describes the typical procedures used to check the seal face flatness and typical examples of out-of-flat conditions. 8.2.3.2

Seal Cavity Dimensions (Figure 8-9)

Seal cavity dimensions should be checked to ensure that proper clearance and alignment will be achieved and to prevent seal damage during installation. Check the seal cavity inside diameters and depths. Visually check for damage of the cavity that might have occurred during previous operation or during disassembly.

Figure 8-9 Seal Cavity Dimensional Checks Prior to Installation

8.2.3.3

Compression Length Tolerance

Interrelated dimensions between the shaft and seal cavity should be checked to ensure proper compression loading of the seal faces. It is important to correctly account for the gasket thickness when calculating the compression of the seal. Do not use previous set screw indention in the shaft/sleeve as a reference point because there can be significant difference in the stacked height of seals, particularly between different manufacturers. It is also important to install the seal so that the set screws do not align with previous indentations that might guide the set screw away from the preferred installation position. 8.2.3.4

Auxiliary Glands

Auxiliary glands should be checked to ensure that fittings do not protrude into the seal cavity and come into contact or affect the performance of the seal. The glands should also be checked to verify that they are clear of obstructions that could prevent proper circulation of the barrier or flushing fluids.

8-9


8.2.4 Seal Removal As discussed in the beginning of this section, seal maintenance programs often occur as a reaction to a seal failure rather than as a planned activity. As a result, seal removals are done at an accelerated pace in order to bring the plant or process back into service. Under this type of condition, special emphasis should be made to ensure that safety and failure evidence are maintained. 8.2.4.1

Safety

Because of their tolerance to a variety of fluids, mechanical face seals are often used in toxic or hazardous processes. To ensure safety of personnel during the removal and handling of the seal and the fluid in the seal cavity, training and written instructions should be provided to clearly identify the type of equipment needed and other safety devices to be utilized during disassembly, handling, and storage. Key Human Performance Point Equipment contents and conditions should be fully known before disassembly to preclude injury.

8.2.4.2

Failure Evidence

As identified in Section 7, the best guide to determining the cause of failure of a seal is often the condition of the seal. It is, therefore, important to properly mark, photograph, and carefully store the seal and other related components for later detailed examinations. It is also recommended that some of the seal cavity fluid be retained because it might also be used to determine the cause of failure. 8.2.4.3

Seal Re-use and Inspection

It is strongly recommended that mechanical face seals not be re-used unless they have been reconditioned to the manufacturer's specifications. The mating faces of mechanical seals develop a wear pattern after an extended period of use and it is almost impossible to reestablish the same relationship after their alignment has been disturbed. Even checking for damage by separating the faces can upset their relationship. The faces should not be separated unless it is absolutely necessary. Whenever possible, inspection of the seals should be limited to visual external inspection only. 8.2.5 Startup Mechanical face seals are precision pieces of equipment. If they are to provide good service, they must be correctly commissioned and operated. The primary aim of a proper startup is to ensure that the seal does not initially run dry. 8-10


Key O&M Cost Point Adherence to manufacturer’s recommendations during start-up and operation is vital to seal longevity and performance.

8.2.5.1

Avoid Dry Running

If barrier or flushing fluids are used, ensure that the seal cavity is properly filled and that there are no leaks. If the fluids in the seal cavity are circulated externally, verify that the equipment is functioning properly and delivering the required flow. Fluids with low vapor pressures should be properly pressurized to ensure that the fluid at the faces does not vaporize when the faces heat up during normal running. 8.2.5.2

Filtration

Dirt and particulate can cause a seal to fail in a very short period of time. Ensure that the seal cavity is completely clean and that the recirculated fluid has been properly filtered. When installing mechanical seals in new piping systems, it might even be necessary to temporarily replace the mechanical face seal with conventional soft packing until the system has been thoroughly flushed of construction and installation debris. 8.2.5.3

Venting the Stuffing Box

The stuffing box should be properly vented to ensure that the seal chamber is completely filled. Never start a mechanical face seal before venting the seal cavity of air and foreign fluids. Ideally, the installation should allow the seal cavity to be vented automatically during pump priming, but, in some installations, it might be possible to flood the pump suction without purging the air trapped in the top portion of the seal cavity. Special attention should be paid to vertical installations where the mechanical face seal is in the uppermost portion of the pressure boundary. Key Human Performance Point Proper venting of seal chamber prior to placing into service is critical to seal performance and longevity.

8-11


9 REFERENCES AND BIBLIOGRAPHY

1. B. S. Nau, “Hydrodynamic Lubrication in Face Seals.” Paper No. E5, 3rd International Conference on Fluid Sealing, BHRA, Cranfield, Bradford, UK (1967). 2. J. G. Pape, “Fundamental Research on a Radial Face Seal,” ASLE Transactions. Vol. 11, No. 4, (October 1968). 3. E. Mayer. Mechanical Seals, 3rd Edition. J. W. Arrowsmith Ltd., Bristol 1969. 4. H. H. Buchter. Industrial Sealing Technology. John Wiley & Sons, Inc., New York 1979. 5. Alan O. Lebeck. Principles and Design of Mechanical Face Seals. John Wiley & Sons, Inc., New York 1991. 6. Handbook of Fluid Sealing, edited by Robert V. Brink, McGraw-Hill, Inc., New York 1993. 7. Mechanical Seal Practice for Improved Performance, edited by Summers-Smith, Mechanical Engineering Publications, Ltd., for The Institution of Mechanical Engineers, London 1988. 8. API Standard 682: Shaft Sealing Systems for Centrifugal and Rotary Pumps, 1st Edition, American Petroleum Institute, Washington, D.C. October 1994. 9. Robert L. Johnson and Karl Schoenherr. Seal Wear, Wear Control Handbook, pp. 727-754, American Society of Mechanical Engineers, 1980. 10. “Seals Flow Code Development – 93,” NASA Conference Publication 10136, Proceedings of a workshop held at the NASA Lewis Research Center, Cleveland, OH (November 3-4, 1993). 11. F. A. Conner and M. T. Thew, “Trends in Mechanical Seal Performance at Three Process Plants in the Oil Industry,” 14th International Conference on Fluid Sealing, Publication 9, BHR Group, Mechanical Engineering Publications Limited, London (1994). 12. D. H. Ahlberg and E. C. Fitch, “Leaking Seals: Causes and Cures,” ASME Paper 79-DE-E-7, 1979. 13. O. von Bertele, “Why Do Seals Fail Unpredictably,” Paper L4, presented at the 10th International Conference on Fluid Sealing, Innsbruck, Austria (April 3-5, 1984). 14. F. K. Orcutt, “An Investigation of the Operation and Failure of Mechanical Face Seals,” presented at the 4th International Conference on Fluid Sealing held in conjunction with the 1969 ASLE Annual Meeting, Philadelphia, PA (1969). 9-1

EPRI Licensed Material References and Bibliography

15. John C. Hudelson, “Dynamic Instability of Undamped Bellows Face Seals in Cryogenic Liquid,” pp. 381-390, ASLE Transactions 9. (1966). 16. J. W. Abar, “Failures of Mechanical Face Seals,” pp. 437-449, Metals Handbook, American Society of Metals, 8th Ed., Vol. 10, (1975). 17. Anon. “Identifying Causes of Seal Leakage,” Crane Packing Company, Form No. S-2031 (1979). 18. Donald L. Berg, “Dynamic Seal Maintenance–Stuffingbox Sealing Considerations,” presented at NMAC 6th Annual Conference and Technical Workshop, Orlando, FL (December 9-11, 1996). 19. Steven Lemberger, “Mechanical Seal Maintenance,” presented at the NMAC 6th Annual Meeting and Workshop, Orlando, FL (December 9-11, 1996). 20. E. Mayer, “High Duty Mechanical Seals for Nuclear Power Stations,” Paper A5, presented at the 5th International Conference on Fluid Sealing, Warwick, Coventry, UK, March 30-April 2, 1971, BHRA Group, Mechanical Engineering Publications Limited, London (1971). 21. H. Laumer and D. Florjancic, “Mechanical Seals for High Pressures and High Circumferential Speeds,” Paper A4, presented at the 5th International Conference on Fluid Sealing, Warwick, Coventry, UK, March 30-April 2, 1971, BHRA Group, Mechanical Engineering Publications Limited, London (1971). 22. W. Schopplein. “Mechanical Seals for Aqueous Media Subject to High Pressures,” Paper E3, presented at the 8th International Conference on Fluid Sealing, University of Durham, UK (September 11-13, 1978). 23. William V. Adams and Peter Lytwyn. “Retrofit of an Unspared Main Boiler Feed Pump to End Face Mechanical Seals,” Paper No. 86-JPGC-Pwr-52, presented at the joint ASME/IEEE Power Generation Conference, Portland, OR (October 19-23, 1986). 24. H-J. Franke, R. Lachmayer, and J. Mosowicz. “Long-Term Tests of Mechanical Seals for Hot Water Application,” 14th International Conference on Fluid Sealing, Publication 9, BHRA Group, Mechanical Engineering Publications Limited, London (1994). 25. J. Nosowicz and A. Eiletz. “Operating Performance of Mechanical Seals for Boiler Feed Pumps,” 15th International Conference on Fluid Sealing, Publication 26, BHR Group, Mechanical Engineering Publications Limited, London (1997). 26. R. Metcalfe, N. E. Pothier, and B. H. Rod. “Diametral Tilt and Leakage of End Face Seals with Convergent Sealing Gaps,” Paper A1, presented at the 8th International Conference on Fluid Sealing, University of Durham, UK (September 11-13, 1978). 27. A. H-C. Marr, R. L. Phelps, and B. Katz. “Loss of Component Cooling Water Capability of a PWR Reactor Coolant Pump,” Paper No. 80-C2/PVP-28, presented at the Century 2 Pressure Vessels & Piping Conference, San Francisco, CA (August 12-15, 1980).

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28. Thomas R. Morton. “Seal Performance from the Manufacturers Viewpoint,” Paper No. 84PVP-115, American Society of Mechanical Engineers, New York, NY, 1984. 29. M. S. Kalsi, T. Horst, H. L. Richter, and M. Hojati. “O-Ring Static Seal Performance at Elevated Temperatures Simulating A Loss of Component Cooling Water Accident,” Paper 87-PVP-5, American Society of Mechanical Engineers, presented at the Pressure Vessel & Piping Conference, San Diego, CA (July 1987). 30. David L. Cummings and Sherman W. Shaw. “Increased Reliability of Reactor Coolant Pump Seals through Retrofit of Proven Technology,” paper presented at the American Nuclear Society Topical Meeting, Myrtle Beach, SC (April 17-20, 1988). 31. Takuya Fujita, et al. “Development of Rotary Shaft Seals for Primary Coolant Pumps for Nuclear Reactors,” Preprint No. 87-TC-3D-1, presented at the STLE/ASME Tribology Conference, San Antonio, TX (October 5-8, 1987). 32. Joseph A. Marsi and Dr. S. Gopalakrishnan, “Full-Scale Station Blackout Test Conducted on Advanced RCP Mechanical Seal,” Nuclear Plant Journal. P. 86 (September-October 1988). 33. Ray Metcalfe, “Canadians Solve Seal Problems,” Nuclear Engineering Internationa. p. 46 (July 1989). 34. T. E. Greene and G. B. Inch. “Evaluation of Shaft Seal Leakage under Station Blackout Conditions for the Reactor–Circulation pumps at Nine Mile Point, Unit One,” presented at Fifth International Workshop on Main Coolant Pumps, Orlando, FL (April 21-24, 1992). 35. Main Coolant Pump Seal Maintenance Guide. Prepared by Quadrex Energy Services for Nuclear Maintenance Application Center: 1993. TR-100855. 36. A. Parmar. “Thermal Distortion Control in Mechanical Seals,” 12th International Conference on Fluid Sealing, BHRA, Cranfield, Bedford, UK (1989). 37. Antonio Artiles, Wilbur Shapiro, and Henry F. Jones. “Design Analysis of Rayleigh-Step Floating-Ring Seals,” Preprint No. 83-LC-38-2, presented at the ASLE/ASME Lubrication Conference, Hartford, CT (October 18-20, 1983). 38. L. A. Young and A. O. Lebeck, “The Design and Testing of Moving-Wave Mechanical Face Seals Under Variable Operating Conditions in Water,” Preprint No. 85-TC-1C-1, presented at the ASLE/ASME Tribology Conference, Atlanta, GA (October 8-10, 1985). 39. J. G. Evans. “New Developments in Bellow Seals for Improved Performance and Reliability,” 14th International Conference on Fluid Sealing, Publication 9, BHR Group, Mechanical Engineering Publications Limited, London (1994). 40. R. Metcalf, T. A. Graham, and W. C. Wong. “Eccentric Seals for Nuclear Pumps,” 14th International Conference on Fluid Sealing, Publication 9, BHR Group, Mechanical Engineering Publications Limited, London (1994).

9-3


41. B. Tournerie, J. Huitric, D. Bonneau, and J. Prene. “Optimization and Performance Prediction of Grooved Face Seals for Gases and Liquids,” 14th International Conference on Fluid Sealing, Publication 9, BHR Group, Mechanical Engineering Publications Limited, London (1994). 42. N. W. Wallace and H. K. Muller. “The Development of Low Friction, Low Leakage Mechanical Seals Using Laser Technology,” 11th International Pump Users Symposium & Short Courses, Houston, TX (March 7-10, 1994). 43. H. K. Muller, C. Schefzik, N. Wallace, and J. Evans. “Laserface Sealing Technology: Analysis and Application,” 15th International Conference on Fluid Sealing, Publication 26, BHR Group, Mechanical Engineering Publications Limited, London (1997). 44. I. Etsion, G. Halperin, and Y. Greenberg. “Increasing Mechanical Seals Life with LaserTextured Seal Faces,” 15th International Conference on Fluid Sealing, Publication 26, BHR Group, Mechanical Engineering Publications Limited, London (1997). 45. B. Antoszewski and J. Rokicki. “Tribology Aspect of the Laser Treatment for Mechanical Seals,” 15th International Conference on Fluid Sealing, Publication 26, BHR Group, Mechanical Engineering Publications Limited, London (1997). 46. Izhak Etsion. Improving Tribological Performance of Mechanical Seals by Laser Surface Texturing, Surface Technologies Ltd., Nesher, Israel, product catalog, 2000. 47. H. K. Muller. “Polymer Seal Rings in Sliding Contact with Silicon Carbide in a Mechanical Seal,” 15th International Conference on Fluid Sealing, Publication 26, BHR Group, Mechanical Engineering Publications Limited, London (1997). 48. N. D. Barnes, R. K. Flitney, and B. S. Nau, “Designing Chambers for Mechanical Seals,” World Pumps. (April 1990). 49. A. I. Golubiev and V. V. Gordeev. “Investigation of Wear in Mechanical Seals in Liquids Containing Abrasive Particles,” Paper B3, 7th International Conference on Fluid Sealing, held at University of Nottingham, England (September 24-26, 1975). 50. David Nolan, “Sorting Out Slurry Pump Seals,” Coal. pp. 86-90 (1988). 51. James S. Budrow, “Seals for Abrasive Slurries,” Chemical Engineering. (September 1, 1986). 52. M. S. Kalsi. “Development of a New High Pressure Rotary Seal for Abrasive Environments,” Proceedings of BHRA 12th International Conference on Fluid Sealing, Paper H2 (May 1989). 53. M. S. Kalsi, W. T. Conroy, L. L. Dietle, and J. D. Gobeli. “A Novel High-Pressure Rotary Shaft Seal Facilitates Innovations in Drilling and Production Equipment,” SPE/ IADC 37627, paper presented at SPE/IADC Drilling Conference, Amsterdam, The Netherlands (March 1997).

9-4


54. M. S. Kalsi. “A Novel High Pressure (up to 5000 psi / 340 Bars) Polymeric Rotary Shaft Seal,” World Tribology Congress, Organized by the Tribology Group of the Institution of Mechanical Engineers, London (September 8-12, 1997). 55. K. C. Wilson, G. R. Addie, A. Sellgren, and R. Clift. Slurry Transport Using Centrifugal Pumps, 2nd Edition. Blackie Academic & Professional, London 1996. 56. R. K. Flitney and B. S. Nau. “Performance Testing of Mechanical Seals,” Fluid Sealing. Kluwer Academic Publishers, pp. 441-466. 57. Denis Buchdahl, Roger Martin, and Jean-Michel Girault. “Mechanical Seals Qualification Procedure of the Main Pumps of Nuclear Power Plants in France,” Fluid Sealing. Kluwer Academic Publishers, pp. 429-439 (1992). NRC Information Notices and Generic Communications 58. USNRC Information Notice 95-42: Commission Decision on the Resolution of Generic Issue 23, Reactor Coolant Pump Seal Failure, September 22, 1995. 59. USNRC Information Notice 87-51: Failure of Low Pressure Safety Injection Pump Due to Seal Problems, October 13, 1987. 60. USNRC Information Notice 96-58: RCP Seal Replacement with Pump on Backseat, October 30, 1996. 61. USNRC GI–23: Reactor Coolant Pump Seal Failures and its Possible Effect on Station Blackout (Generic Letter 91-07). 62. USNRC Information Notice 93-61: Excessive Reactor Coolant Leakage Following a Seal Failure in a Reactor Coolant Pump or Reactor Recirculation Pump, August 9, 1993. 63. USNRC Information Notice 93-84: Determination of Westinghouse Reactor Coolant Pump Seal Failure, October 20, 1993. 64. USNRC Regulatory Issue Summary 2000-02: Closure of Generic Safety Issue 23, Reactor Coolant Pump Seal Failure, February 15, 2000. 65. USNRC Draft Regulatory Guide DG-1008: Reactor Coolant Pump Seals, April 1991.

9-5


A MECHANICAL SEALS APPLICATION AND MAINTENANCE GUIDE SURVEY

This appendix contains the form used to conduct the survey of fossil and nuclear power utilities to determine the most common failure modes, the root causes, and installation and maintenance recommendations in support of the development of this guide.

A-1

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey

EPRI/NMAC Nuclear Maintenance Application Center Mechanical Seals Application and Maintenance Guide Survey

At the direction of the NMAC Steering Committee, NMAC has begun the preparation of an Application and Maintenance Guide for Mechanical Seals used in nuclear power plants. This survey is intended to obtain the most common problems with mechanical seals in use today. Information obtained from this survey will be used in developing a comprehensive and state-of-the-art Guide for the application, use, maintenance, repair, and troubleshooting of problems with mechanical seal. Your participation in this survey is vital to the accuracy and usefulness of this Guide. The Guide is intended to be a single source for utility engineers and maintenance personnel to minimize problems with mechanical seals while extending the number of cycles between seal inspections. In order to evaluate the responses and to make comparisons between utilities to determine successful and unsuccessful practices, besides the responses to the following questions (which can be done by e-mail on this form), the following information is also requested: 1) A copy of your latest procedures for mechanical seal maintenance, repairs, and troubleshooting. 2) Itemization of each individual pump's mechanical seal history since 1/1/90 (Maintenance Rule data is acceptable). This should include any mechanical seal failures and the root cause determination of those failures, corrective actions taken, and the seal inspection reports, even if the maintenance was solely of a routine nature. Please include any mechanical seal leakage trending data available. Please contact us if there is a question about this request. 3) Special problems that the plant may have experienced, and the plant's approach to addressing them. The outcome of each repair or corrective action will be a valuable addition to your response. An important element of a typical NMAC Guide is to involve industry personnel in the review/ comment stages of guide development. Would someone at your facility be willing to participate as a member of our Technical Advisory Group (TAG) which typically involves review/comment of an initial draft and final version of the planned maintenance guide? No Yes Please mail, fax or e-mail responses to:

A-2

Mike Pugh 1300 W. T. Harris Blvd. Charlotte, NC 28262 Fax: 704-547-6035 E-mail: [email protected] Phone: 704-547-6004

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey Page 1 of 7 EPRI/NMAC MECHANICAL SEALS MAINTENANCE GUIDE SURVEY QUESTIONS Contact Name:

Phone: (_____)

Utility:

Fax: (_____)

Plant:

E-Mail:

1) Date of initial plant startup: 2) Number of loops: 3) Plant design:

1 PWR

2

3 BWR

4 or Fossil

4a) Estimated number of all rotary shaft seals in your plant. 4b) Estimated number of mechanical seals in critical applications. 4c) Estimated number of mechanical seals in other applications. 5) Where mechanical face seals are not being used, select the two most important factors for not using them (select two) Cost

Leakage

Unpredictable catastrophic failure potential

Availability

Specialized training & maintenance

Other (explain)

Types of Mechanical Seals, Manufacturers, and Applications in Power Stations 6)

Manufacturers (check all that apply):

6a) Main Coolant Pump Seal: (1) Westinghouse

(3) BWIP

(2) Sulzer Bingham

(4) AECL

6b) Other Mechanical Seal Manufacturers (check all that apply): (1) Crane

(4) Chesterton

(7) Borg-Warner/BWIP

(2) Durametallic

(5) Sealol

(8) AST

(3) Burgmann Seals

(6) Flexibox

(9) Latty International

(10) Other 6c) Most common at your plant (select 3 numbers from list in Question 6b) .

A-3

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey Page 2 of 7 7a) Most common mechanical seal configurations in your plant (2 selections): (1) Inside mounted with rotating seal head (pressure on outside diameter of face) (2) Outside mounted with rotating, externally mounted seal head (pressure on inside diameter of face) (3) Outside mounted with stationary, internally mounted seal head (pressure on inside diameter of face) (4) Inside mounted with stationary, externally mounted seal head (pressure on outside diameter of faces) (5) Cartridge (6) Other (Specify) 7b) Which configurations have the most problems (select 2 from Question 7a) . 8a) Sealed Fluid (select all that apply) Incompressible (1) Clean Water (2) Service Water (3) Oil (4) Hydrocarbon (5) Slurry (6) Other (Specify)

Compressible (6) Air/Nitrogen (7) Steam (8) Other (Specify)

8b) Most common at this location (select 2 numbers from each category in Question 8a) 9) Most common secondary seals (select 2) Elastomeric O-Ring Elastomeric U-Cup Metal Bellows Other (specify)

Elastomeric Chevron Elastomeric Wedge Elastomeric Bellows

10) Select the 3 most common face material combinations from the list below Rotating Face

Stationary Face

Combination 1 Combination 2 Combination 3 (1) Carbon - Graphite (2) Carbon - Babbit (3) Ceramic (4) Nickel - Resist (5) Silicon Carbide (6) Laminated Plastic (7) Teflon (8) Stainless Steel (9) Stellite Hard-Facing on Stainless Steel

A-4

(10) (11) (12) (13) (14) (15) (16) (17) (18)

Aluminum - Bronze Bronze Monel Tungsten Carbide Phosphor - Bronze Carbon-Filled Teflon (nonoxidizing acids) Glass-Filled Teflon (oxidizing acids) Hasteloy A, B, or C Other (specify)

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey Page 3 of 7 Present Failure Rates and Causes 11) Mean time between failure of mechanical seals resulting in leakage less than 6 months 6 to 12 month 12 to 18 months 18 to 36 months 36 to 72 months Other (specify) 12) Symptoms of mechanical seal problems:

Most Least Never Common Common Occurs

Visible or detectable leakage Wear of rotating face Wear of counterface Loss of spring force due to contamination and accumulation of solids Loss of contact force due to spring element relaxation Excessive friction heat Excessive friction torque Loss of coolant/lubricant Corrosion Other (explain)

13a) Causes of mechanical seal problems (select all that apply): • Maintenance installation problem (1) Improper seal face compression (2) Contamination or damage during installation (3) Excessive eccentricity cause by set screw tightening sequence (4) Slippage due to incorrect set screw tip geometry (dog point versus cup point) (5) Slippage due to set screw material being too soft (6) Elastomers not installed correctly (7) Elastomer/lubricant incompatibility (8) Other • Equipment interface/operation problem (9) Mounting surface for seal not square/parallel to shaft (10) Excessive axial or radial movement (off Best Efficiency Point operation, cavitation, out of balance, bent shaft, misalignment, bad bearings, etc.) (11) Other • Manufacturing problem (12) Wrong or improper materials supplied (13) Defects introduced during manufacturing (14) Other

A-5

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey Page 4 of 7 • Application or system problem (15) Incorrect seal selected for the application (e.g., vacuum applications should use a double seal of some sort) (16) Face materials improperly selected for the application (17) Improper environmental controls causing the seal to overheat or allow contaminants (18) Fluid vaporization across seal faces (19) Pressure and/or temperature transients due to variable system operation (20) Equipment operating conditions not completely defined (21) Material chemical attack and corrosion (22) Dirty or abrasive system (23) Product (e.g., crystallized boron) sticks to seal parts and keeps them from moving properly (24) Other 13b) Most common at your plant (select three from list in Question 13a) Inspection and Predictive Maintenance Methods as Related to the Mechanical Seal Condition 14) Frequency of mechanical seal visual inspection (check all that apply and provide number of seals inspected in each category) Monthly seals Quarterly seals Annually seals Every outage seals Over two years (specify period and number of seals) , seals 15) Predictive Maintenance Schedule is based on: Manufacturer Recommendation Plant/Utility Experience Importance of the Equipment to Plant Operation and Plant Output Power Level Importance of the Equipment to Plant Safety Other (specify) 16) Predictive Maintenance Methods Used (select all that apply) Temperature measurement Leakage detection Vibration level None Other (specify) Periodic Preventive Maintenance/Replacement Performed Regardless of the Actual Condition of the Mechanical Seal 17a) Equipment under Periodic Preventive Maintenance (specify or provide list) Safety-Related Critical For Plant Output Balance of the Plant None

A-6

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey Page 5 of 7 17b)

Maintenance/replacement frequency (if none in 17a, skip this question) Every Outage Every Other Outage Other (specify)

Plant-Specific Approaches to Address Mechanical Seal Problems and Maintenance 18) Troubleshooting is performed by: Plant Maintenance Manufacturer Representative Outside Contractor 19) Mechanical seal repairs are performed by: Plant Maintenance; the level of maintenance being: Install only Change O-rings/static seals Change seal faces and finish machine, i.e., grind, lap, inspect Remanufacture complete assembly Manufacturer Representative Outside Contractor 20) Spare parts and inventory (check all applicable options) Spare mechanical seals for high priority equipment are kept at the plant warehouse Spare parts for some key seals for high priority equipment are kept at the plant warehouse Seals and spare parts are stocked by manufacturers and ordered as needed Spare parts are machined from material stock kept at the plant None of the above (explain)

21) Please provide a copy of your data sheet used to specify mechanical seals (if available). Data sheet attached 22) Shaft stiffness criterion used to determine the suitability of a mechanical seal for a given application. 3 4 Shaft deflection at seal L /D ratio Specify value: Other None 23) List 3 applications in which mechanical seal problems continue to be difficult to solve. Application 1: Application 2: Application 3: Provide details of the 3 applications in the following table:

A-7

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey Page 6 of 7 Data Requested

Application 1

Application: safety-related/critical/ balance of the plant Equipment type (pump, agitator, compressor...) Equipment manufacturer Mechanical seal manufacturer (Model No or type if available) Estimated leak rate at failure Fluid (clear water, service water, slurry, ...) Temperature, • F Pressure, psi Speed, rpm Approximate shaft diameter Face material -

Stationary

-

Rotating

Seal design -

Balanced or unbalanced

-

Single, double, or tandem

-

Secondary seals (bellows, elastomers,...)

-

Face loading achieved by single coil, multiple, & Belleville springs; bellows, elastomers, ... Flushing: Process fluid, external source,...

-

Mean time between failures Parameters monitored for predictive maintenance (leakage, temp, pressure, vibration,...) Root cause of failure determined (Yes/No) Provide or attach the root cause Frequency of periodic maintenance if any Attach description of alternative solutions (successful or pursued)

A-8

2

3

EPRI Licensed Material Mechanical Seals Application and Maintenance Guide Survey Page 7 of 7 Training Provided for In-House Maintenance Personnel 24) Mechanical seal training is provided to Equipment engineer All in-house rotating equipment maintenance personnel Only selected group of maintenance personnel No training is provided 25) If training is provided what is the frequency of re-training Every year

Every 3 years

Every 5 years or more

Other

26) Does your plant require contractors to have formal mechanical seal training before commencing repair or replacement work? Yes No

A-9


B INSPECTION OF SEAL FACES FOR FLATNESS

B.1

Optical Principle

Lapped surfaces of seal parts are inspected for flatness using an optical flat and monochromatic light. Light is passed through the optical flat, then reflected off the lapped surface, and back through the optical flat. When a gap exists between the optical flat and lapped surface, the light reflections off the lapped surface and the optical flat interfere with each other, preventing some of the light from passing back through the optical flat. Between the dark bands, the reflections reinforce each other and produce light bands. This phenomenon produces a series of dark and light bands when the optical flat is viewed from above, as shown in Figure B-1. The parallel dark bands form where the change in distance between the flat and lapped surface is one-half the wave length of the light as shown in this figure. An optical flat is made from transparent material, normally quartz or Pyrex, which is very flat. Different size optical flats with different flatness tolerances are available. Typically, seal parts are inspected with an optical flat that is flat within 2 to 5 micro-inches (one micro-inch or 1 P in. is one-millionth (0.000001) of an inch). Optical flats can be flat within the specified tolerance on one or both sides. Single-sided flats are normally adequate for seal inspection. Coating on the flat increases its reflectivity and makes the light bands easier to see. A monochromatic light source emits light of a known wavelength. The most common type is a helium-filled tube that emits orange/yellow light with a wavelength of 23.2 P in. The light bands visible through the optical flat are one-half the total wavelength. Consequently, each band (consisting of one light and one dark band) that is visible represents a gap of 11.6 P in. The actual width of the bands cannot be related to the flatness of the part. The total number of bands seen during inspection is a function of the gap that is created between the flat and the lapped surface, not the flatness. Manufacturers will specify flatness in light bands, normally without regard to the size of the part. A seal part that is required to be flat within 2 light bands has a flatness tolerance of 23.2 P in. over the specified surface.

B-1

EPRI Licensed Material Inspection of Seal Faces for Flatness

Figure B-1 Using an Optical Flat to Determine Seal Face Flatness Light Bands

B.2

Procedure for Measuring Face Flatness

When measuring the flatness of seal parts, the following basic good practices should be used to obtain accurate results. x

The optical flat and lapped surface should be free of dirt or other particles. Parts can be wiped with a lint-free cloth or brushed off with a fine bristle brush prior to setting the flat on the lapped surface.

x

Avoid putting any unnecessary force on the parts being inspected. The tolerances for lapped surfaces are extraordinarily small and exerting unnecessary force on the parts can distort the flatness.

x

The size of the flat needs to be matched to the part. Do not use an optical flat that is much larger and heavier than what is required.

B-2


x

When inspecting carbon seal parts, place the carbon seal ring on a flat surface, such as another flat or a lapped surface, like a carbide seal ring.

x

Perform the inspections in a controlled environment. Changes in temperature and humidity can affect flatness readings.

x

Flatness measurements should only be taken when the part being inspected and the flat are both at a uniform room temperature. For example, if the flat is at room temperature and the part has just been brought in from an uncontrolled cold environment, the warm flat might distort a cold surface.

x

View the optical flat from the correct angle. The flatness reading can be seriously distorted by determining the flatness when viewing the part with too great of incidence angle. Light bands should be determined when looking straight down on the part, as shown in Figure B-2, at a viewing angle of close to 90q. If the flatness reading is taken with a viewing angle of 60q, each light band represents 13.4 P in. instead of 11.6 P in.

Figure B-2 The Viewing Angle Typically Should be 80q to 90q While Checking Flatness Using a Monochromatic Light Source

A procedure for measuring flatness on seal rings and other toroidally shaped lapped seal surfaces is provided below. This method places an air wedge under one side of the flat to help determine if the part is convex or concave, or if it has other out-of-flatness conditions. 1. Place the lapped part under the monochromatic light. If the part is a carbon ring, make sure it is adequately supported. 2. Clean the lapped surface and the optical flat of dust with a lint-free cloth or fine bristle brush. 3. Place the flat on the lapped surface.

B-3


4. Use a piece of lint-free tissue to create an air wedge. Place the tissue between the left side of the lapped surface and the optical flat. Slowly pull the tissue out until the edge of the tissue is at the edge of the lapped surface. The tissue can be manipulated until a light band pattern width that is easy to view is visible. If the tissue wedge is too thick or foreign particles are between the flat and the lapped surface, the light band pattern will be too narrow to read. To check to see if the air wedge is too thick, use light thumb pressure at the air wedge to vary the appearance of the light bands. 5. The light bands are used to determine the degree of flatness. When interference bands are straight, parallel, and equally spaced, the surface is assumed to be flat to within 11.6 P in. 6. Interpretation is carried out noting the number of bands intersected by a straight tangent line, as in the examples shown in Figures B-3 through B-7. Out-of-flatness is measured by multiplying this number by 11.6 P in. It is important to note that, if the bands are inconsistent or missing, it is necessary to draw two imaginary centerlines 90q apart and perpendicular to the axis of the part, and then draw line AB at 45q, connecting the two previous lines (see examples in Figures B-6 and B-7). The procedure used by different seal manufacturers to determine flatness might vary from the procedure above. The relationship to successful performance and flatness measurements should be kept in perspective. If the lapping and measurement techniques provide consistent successful operation, the procedures should not be changed.

Figure B-3 Flat Within One Light Band (The distance x is dependent on the amount of air between the optical flat and the face and does not indicate lack of flatness.)

B-4


Figure B-4 Bands Bend on One side and Line AB Intersects 3 Bands (The face is therefore out-of-flat by 3 light bands or 35 P in.)

Figure B-5 This Indicates an Egg-Shaped Curvature of 2.5 Light Bands (That is, 29 P in. Line AB intersects 2 bands and falls between another 2 at the center of the ring. Line A'B' intersects 2 bands that curve in the opposite direction.)

B-5


Figure B-6 Bands Show a Saddle Shape Out-of-Flat Condition of 3 Light Bands, 35 P in.

Figure B-7 Bands Show a Cylindrical-Shaped Part with a 3-Light Band Reading Error

Figure B-8 Band Symmetrical Pattern Indicates a Conical Convex or Concave Part. (The out-offlatness is measured by the number of bands on the part, that is, 3 bands or 35 P in.)

B-6


C TRAINING COURSES

The following is a listing and description of training materials or courses that are presently known to NMAC that are available for enhancing skills involved with mechanical seals. They are broken down into two major categories. The first category of training, Category A, supports a basic understanding of mechanical seal installation and maintenance practices as well as personnel qualification materials. The second category, Category B, provides a higher level of training that will improve craftsmanship and understanding of seal operation and technology, and also gives a greater insight into performance, problem analysis, and plant implications. NMAC has reviewed these course offerings in limited detail. Reference herein is not intended to be an endorsement of the materials but simply a reference, should additional training information be desired by the membership. CATEGORY A EPRI Maintenance Performance Evaluation Test Bank The Maintenance Proficiency Evaluation Test Bank (MPETB) is a database of validated and reliable task-specific written and performance tests developed by participating utilities following the proven MPE methodology referenced in EPRI technical reports. The database, made available exclusively to utility participants in this project, already contains a large population of task-specific written and performance tests that can be administered to plant or contractor personnel. Currently there are several tests for mechanical seals that are available to participating members. If you would like to find out more about the Mechanical Seals MPEs you can visit the EPRI webpage at http://www.epriweb.com/epriweb2.5/ecd/np/mpe/index.html or contact Loran Maier at 704-547-6152. Annual International Pump Users Symposium and Short Courses Program Texas A&M Turbomachinery Laboratory College Station, Texas 77843-3254 Phone: 979/845-7417 Website: http://turbolab.tamu.edu Contact: Dr. Bailey, Marketing Director Background: The Turbomachinery Laboratory receives inquiries from fluid handling and rotating equipment users who are looking for intensive training opportunities in addition to those currently offered at their symposia. In response to these inquiries, they have initiated a cooperative effort with some exhibiting companies to provide information on their professional development opportunities. These technical training sessions are listed below, by company, with a brief description of each course. C-1

EPRI Licensed Material Training Courses

FLOWSERVE Educational Services Group Pump Training Programs taught at the Learning Resource Center in Dallas Website: www.flowserve.com Course Title: Take the Mystery Out of Pumps and Mechanical Seals Audience: Engineers, supervisors, and craftsmen specializing in Pump and Mechanical Seal Reliability Improvement. Synopsis: This five-day course, split equally between classroom and hands-on learning activities, will provide participants with a strong understanding of centrifugal pumps and mechanical seals. Chesterton Chesterton offers learning via the Internet to allow students to learn at their own pace on a more flexible schedule. Students are provided with immediate feedback on their progress through each course. An index of terms is provided under the Performance Support section. Students can search this section for terms pertinent to their topic area. Course outlines are available at their website: www.activedistancelearning.com/distancelearning/index.asp Course Title: Mechanical Seal Principles I Students will learn each aspect of mechanical seals, including the purpose of a mechanical seal, its component parts, their classifications, its materials of construction, proper operation, environmental controls, and troubleshooting for some basic mechanical packing failures. CATEGORY B Georgia Institute of Technology Paul Weber Space Science and Technology Building on the Georgia Tech Campus Registration: 404/385-3501 Contact: Greg Stenzoski, Marketing Dept. Course Title: Fluid Sealing Technology This four-day, annual course provides an extensive introduction to fluid sealing and is designed to meet the needs of equipment designers, plant and maintenance engineers, and technical sales engineers. This course has been presented at Georgia Tech for the last 11 years. It utilizes the fluid sealing and tribology expertise of both Georgia Tech and the BHR Group (British Hydromechanics Research Group). A sound understanding of the complex factors involved in successful fluid sealing is essential for engineers who specify, design, operate and maintain machinery and mechanical equipment. Seals specialists show how an understanding of basic engineering factors can be used to practical advantage. Fluid sealing technology is based on disciplines as diverse as lubrication, friction, wear, properties of materials, mechanical design, fluid mechanics, and heat transfer. All of these factors are considered in the discussion of different types of seals, seal materials, and sealing applications. Annual International Pump Users Symposium and Short Courses Program See above discussion for background on the below listed courses. C-2


FLOWSERVE Educational Services Group Pump Training Programs taught at the Learning Resource Center in Dallas, TX. Website: www.flowserve.com/ Course Title: Improving Pump, Mechanical Seal, and Systems Reliability Through Maintenance Audience: Pump and mechanical seal craftsmen and technicians. Synopsis: Designed to assist craftsmen in becoming more effective and efficient, and to add value to equipment operation and reliability through thorough maintenance. More than three full days of the five-day course are spent conducting hands-on learning activities. Course Title: Improving Pump, Mechanical Seal, and Systems Reliability Audience: Maintenance engineers, supervisors, and others responsible for reliability improvement will benefit from this course, as will their companies. Synopsis: This weeklong program equips the attendees to identify the root cause of pump failures and apply appropriate corrections. Over 50 failures and 90 corrections are studied utilizing real pumps and mechanical seals, both static and in operation in our six learning labs. Chesterton Chesterton Distance Learning Course Curriculum: Chesterton offers learning via the Internet to allow students to learn at their own pace on a more flexible schedule. Students are provided with immediate feedback on their progress through each course. An index of terms is provided under the Performance Support section. Students can search this section for terms pertinent to their topic area. Course outlines are available at: www.activedistancelearning.com/distancelearning/index.asp Course Title: Mechanical Seal Operation Mechanical seals are designed and engineered differently for specific reasons. Different applications require diverse mechanical seal designs and operational characteristics. This course will describe the different ways that mechanical seals can be designed to operate in order to perform their tasks. Course Title: Common Mechanical Seal Failures To further increase mechanical seal life, we must be able to analyze premature failures. Many of these incidents have symptoms that can tell us what caused it. By examining these failures closely, we can try to eliminate their reoccurrence. This course will identify common mechanical seal failure symptoms and their possible causes. International Conference on Fluid Sealing This conference is held every two to three years and the first conference dates back to April 1961.

C-3


BHR Group Ltd. The Fluid Engineering Centre Cranfield Bedfordshire MK43 0AJ, UK Contact: Mrs. Catherine Cox, The Conference Organizer Tel: 44 (0) 1234 750422 Email: [email protected] Description: This sealing technology forum is the premier event in its field and never fails to provide important and interesting information and new insights into old problems. The aim is to further improve sealing reliability and effectiveness.

C-4


D LISTING OF KEY INFORMATION

The following list provides the location of key Pop Out information in this report. Key O&M Cost Point Emphasizes information that will reduce purchase, operating, or maintenance costs.

Section

Page

Key Point

3.4

3-12

Seal cartridges are pre-assembled mechanical face seal assemblies that contain all of the essential components. Cartridges are used to package mechanical face seals for ease of handling and installation. Even though material cost is higher, cartridges save money by simplifying maintenance and eliminating installation related failures.

6.1

6-1

Seal monitoring programs vary greatly from utility to utility, and from site to site due to different equipment designs, operating philosophies, and different rates of forced outages experienced. For many plants, condition-based monitoring is limited to visual observations with little actual quantification except for main coolant pump mechanical face seals.

6.3

6-5

Monitoring and data logging of key performance parameters can serve as very useful tools for trending wear and performance degradation of mechanical seals and preventing unscheduled outages.

7

7-1

Seal performance is often directly linked to equipment performance and reliability. An in-depth inspection and review of seal failures can improve equipment availability and performance.

8.1

8-1

The most cost-effective maintenance program should be based on predicted seal performance and its expected life. The least cost-effective maintenance program is one based on reactions to failure. An effective preventative or periodic maintenance program, based on plant experience and manufacturer recommendations, should be implemented to improve plant reliability and prevent unplanned shutdowns.

8.2.5

8-11

Adherence to manufacturer’s recommendations during start-up and operation is vital to seal longevity and performance.

D-1

EPRI Licensed Material Listing of Key Information

Key Technical Point Targets information that will lead to improved equipment reliability.

Section

Page

Key Point

3.1

3-2

Mechanical face seals come in a variety of configurations, materials, and designs for primary sealing faces, secondary seals, springs, and drive mechanisms. Options also include unbalanced or balanced designs, whether the primary seal or the mating seal is rotating, and whether the fluid pressure is on the outside or the inside surface of the seal. Seal design for a given application should be selected after a careful evaluation of trade-offs discussed in this section, Section 3.

3.3

3-11

Some applications require the use of multiple seals to provide for flushing or barrier fluids, or pressure staging to deal with higher pressures. Flushing is used to remove contaminants, to cool the faces, or to provide for proper lubrication. Selections include back-to-back, face-to-face double arrangements, and a choice of buffer fluid or barrier fluid, depending upon application.

3.5

3-16

Mechanical seals are often installed in the same cavity that is designed to accept conventional packings. This limits the fluid circulation around the seal, leading to high seal temperatures and accumulation of solids. An enlarged seal chamber with tapered bore can dramatically improve fluid circulation, lowering seal temperature and eliminating accumulation of solids.

3.6.1

3-20

Mechanical face seals can be unbalanced, fully balanced, or partially balanced to reduce the face loading due to hydraulic pressure. The term balanced refers to the case where the average pressure load on the face is less than the sealed pressure. Most mechanical face seals have a balance ratio of between 0.65 to 0.85. This range provides reduced face loading without potential concern of face parting.

3.6.2

3-21

Pressure distribution across the seal face width can be linear, concave, or convex and it can change with variations in pressure, temperature, and seal wear. This can affect seal performance (leakage, torque, temperature) during operation.

3.8

3-24

For satisfactory performance, the seal design and material selections should satisfy the PV limit and the 'T limit under all operating conditions to ensure that fluid film is maintained between the seal faces. Loss of film can lead to immediate seizure and seal failure.

3.9

3-28

Seal designs with special features to enhance lubrication at the sealing interface (for example, hydrodynamic grooves, recesses, or lasertextured surfaces) can extend the pressure, speed, and temperature limits. The trade-off (for example, higher leakage rate versus increased reliability under transient conditions) should be carefully evaluated during seal selection.

D-2

EPRI Licensed Material Listing of Key Information 3.10

3-28

The hydrostatic seal design is a non-contacting mechanical face seal that permits some controlled flow rate to pass between the faces. To prevent dry running, the seal requires that some pressure be applied to the tapered side prior to rotation.

4.2

4-2

The eventual failure mode of all mechanical face seals is leakage that is considered unacceptable for the seal design/configuration being used. Excessive leakage can cause unacceptable loss of fluid, reduction of pressure, or contamination of the system fluid by the barrier fluid in double-seal installations. Level of acceptable leakage is dependent upon the application.

4.4.1

4-5

For satisfactory performance, the seal design and material selections should satisfy the PV limit and the 'T limit under all operating conditions to ensure that fluid film is maintained between the seal faces. Loss of film can lead to immediate seizure and seal failure.

4.4.3

4-6

Mechanical seals are often installed in the same cavity that is designed to accept conventional packings. This limits the fluid circulation around the seal, leading to high seal temperatures and accumulation of solids. An enlarged seal chamber with tapered bore can dramatically improve fluid circulation, lowering seal temperature and eliminating accumulation of solids.

4.4.4

4-7

Thermal distortions of seal faces due to operational transients can cause positive coning (contact on ID) or negative coning (contact on OD) of the seal faces. Coning in excess of film thickness can cause film rupture seizure or face parting, resulting in a large increase in leakage.

4.4.4

4-8

Pressure distribution across the seal faces is affected by seal face coning due to changes in pressure and speed as well as the wear-in process. Excessive coning causes seal failure either due to seizure or face parting. Hard face versus soft face material combinations are more tolerant of coning than if both faces are hard.

4.4.5

4-10

Operation away from Best Efficiency Point (BEP) is a frequent cause of short seal life/seal failures. Off BEP conditions cause large shaft deflections and vibrations resulting in premature degradation of mechanical seals.

4.4.6

4-13

Static and dynamic misalignment between seal faces can cause strong fluid pumping action across the faces causing either inward pumping or outward pumping of the product fluid and/or buffer fluid. Leakages under misaligned conditions can be several times the normal leak rate.

4.4.6

4-13

Premature wear of the primary sealing faces and secondary seals, causing excessive leakage when stationary and when running, are also common symptoms of excessive misalignment.

4.4.7

4-15

Mechanical face seals are precision components, requiring the sealing faces to be flat, typically within one light band (11.6 x 10-6 inches) across one-inch width. Too much out-of-flatness can lead to excessive seal leakage.

D-3


4.4.8

4-16

Conventional mechanical face seals rely on a small amount of waviness automatically created by face distortions due to mechanical loads to function properly. Too perfectly flat seal faces on structurally robust seal rings prevent the faces from distorting and developing a fluid film. This results in seal failure due to seizure. Fortunately, this is a rare occurrence.

5.2

5-3

Seal selection requires a detailed and systematic evaluation of all of the significant application parameters, for example, fluid type, pressure, temperature, speed, normal operating conditions versus design conditions, radiation exposure and maintenance. Appropriate data sheets and check lists should be used to ensure a thorough and complete evaluation of suitable alternatives and trade-offs. Prototype qualification tests should be performed for all critical applications.

D-4


Key Human Performance Point Denotes information that requires personnel action or consideration in order to prevent injury or damage, or ease completion of the task.

Section

Page

Key Point

7.2.2

7-7

The importance of maintaining As Found conditions is important to failure mode determinations. Personnel should be instructed to exercise care during the disassembly steps.

7.3

7-12

Visual examination is an important element in determining failure mechanisms. Personnel should be attentive during disassembly to be alert for evidence of incipient or chronic failure mechanisms.

8.2

8-2

Personnel training is a very important aspect of a mechanical seal maintenance program that is striving to achieve improvements in plant reliability. Comprehensive training courses covering mechanical seal design options, installation, operation, maintenance, troubleshooting, and failure diagnosis are regularly offered by seal manufacturers, universities, and research associates (see Appendix C).

8.2.1.1

8-2

Proper storage and handling of seal components is important to seal longevity and performance. Manufacturer’s recommendations should be followed at all times.

8.2.2

8-4

Pre-installation checks are an important element in reliable seal performance. Personnel should perform the steps outlined herein to prevent unsatisfactory seal performance.

8.2.4.1

8-10

Equipment contents and conditions should be fully known before disassembly to preclude injury.

8.2.5.3

8-11

Proper venting of seal chamber prior to placing into service is critical to seal performance and longevity.

D-5

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92387738 EPRI 1000987 Mechanical Seal Maintenance and Application Guide (2)

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