Mejora en el desempeño de empaques para válvulas

Valve Packing Performance Improvement

Sealing Technology & Plant Leakage Reduction Series

Technical Report

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WARNING:Please read the License Agreementon the back cover before removingthe Wrapping Material.

EPRI Project Manager M. Bridges

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

Valve Packing Performance Improvement Sealing Technology & Plant Leakage Reduction Series 1000923

Final Report, March 2002

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

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

Dominion Engineering, Inc.

ORDERING INFORMATION

Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax).

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc.

Copyright 2002 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS

This report was prepared by

Dominion Engineering, Inc. 6862 Elm Street McLean, VA 22101

Principal Authors S. Hunt, Dominion Engineering, Inc. K. Hart, PPL Electric Utilities Corp.

This report describes research sponsored by EPRI.

This report is a corporate document that should be cited in the literature in the following manner:

Valve Packing Performance Improvement: Sealing Technology & Plant Leakage Reduction Series, EPRI, Palo Alto, CA: 2002. 1000923.

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REPORT SUMMARY

Valve Packing Performance Improvement is the seventh in a series of training modules addressing leakage at nuclear power plants. The first six modules in this series address:

Leakage management programs Assembling bolted joints with spiral-wound gaskets Preload requirements for bolted joints with spiral-wound gaskets Lube oil system leakage mitigation Leakage reduction from threaded joints Leakage reduction from bolted joints with sheet gaskets Background Leakage from valve packing was identified as a major concern by participants in the EPRI Fluid Sealing Technology Program Working Group. This group has provided the technical guidance for research into the causes of, and solutions to, valve packing leakage.

Objectives To provide maintenance personnel, work planners, engineers, quality control (QC) personnel,

and plant management with an understanding of the causes of leakage from valve packing

To provide maintenance personnel, work planners, engineers, QC personnel, and plant management with alternative approaches available to reduce this leakage

To develop cost-effective, plant-specific programs to reduce leakage from these joints Note: Because many different valve designs and conditions are encountered in the field, it is recognized that a guide of this type cannot cover all situations. Users must work closely with experienced and qualified packing suppliers to select and apply the best products for difficult applications.

Approach EPRI Fluid Sealing Technology Program documents are provided in two parts. The first part is a technical guide that is directed toward engineers, craft supervisors, and trainers. This technical guide provides an in-depth review of the causes of leakage and the basis for the EPRI-recommended approach to reducing valve packing leaks. The second part consists of training materials including viewgraphs for classroom instruction and information on the use of a Valve Packing Performance Demonstration Unit to demonstrate the principles presented in the classroom.

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Results Upon completion of the training:

Engineers should be prepared to work with valve packing suppliers to select the best products for each application, to update plant maintenance procedures to reflect the latest knowledge, and to assess the root cause of leaks from valve packing.

Work planners should be able to provide better information in work packages. Maintenance technicians should have a better understanding of the key factors that led to

high-integrity packing and how to apply this knowledge in the field.

QC personnel should have the knowledge to determine which of the assembly practices warrant the most attention.

Plant management should have a better overall perspective of issues associated with leakage from valve packing.

EPRI Perspective EPRI views this series of reports pertaining to plant leakage reduction as an important and needed contribution to the state of the art with respect to plant maintenance practices and operation and maintenance (O&M) cost reduction. Because mitigation of leakage from valve packing is important to supporting many plant applications, this document within the series is of significant potential benefit to members concerned with improving plant safety, operability, and availability, while reducing associated O&M costs.

Keywords Maintenance Leakage Valves Valve packing Packing Live load

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EPRI Licensed Material

PREFACE

In 1995, EPRI initiated a training program to help utilities reduce leakage from bolted joints with spiral-wound gaskets. The current version of these training materials is reported in EPRI TR-111472, Assembling Bolted Connections Using Spiral-Wound Gaskets [11]. This training consists of classroom instruction in the fundamentals of gasketed joints and hands-on instruction on an EPRI-patented Bolting Performance Demonstration Unit. As of June 2001, this EPRI training program had been conducted at more than 30 nuclear plants and also at several fossil electric plants, government facilities, and industrial plants.

In 1998, the leakage reduction program was expanded to cover other aspects of external leakage from piping systems and components at nuclear plants. This work is coordinated by a utility Fluid Sealing Technology Program working group. Current documents published under this program cover:

Leakage management programs Assembling bolted joints with spiral-wound gaskets Preload requirements for bolted joints with spiral-wound gaskets Lube oil system leakage mitigation Leakage reduction from threaded joints Leakage reduction from bolted joints with sheet gaskets This document describes the results of work to determine the causes of leakage from valve packing and cost-effective solutions to this leakage. This work included the following activities:

Identification of the types of valve packing available Identification of the causes of valve packing leakage Development of a program to reduce valve packing leakage Preparation of training materials, including a Valve Packing Performance Demonstration

Unit, to demonstrate the key factors required to develop high valve packing integrity

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CONTENTS

1 BACKGROUND......................................................................................................................1-1 1.1 Objective......................................................................................................................1-1 1.2 Target Audience ..........................................................................................................1-1 1.3 Relationship to Existing Plant Procedures...................................................................1-2 1.4 Valve Packing Performance Demonstration Unit.........................................................1-2 1.5 Summary .....................................................................................................................1-3

2 REVIEW OF VALVE PACKING CONCEPTS ........................................................................2-1 2.1 Alternative Methods for Sealing Valve Stems..............................................................2-1 2.2 Valve Packing Concepts..............................................................................................2-2 2.3 Summary .....................................................................................................................2-4

3 VALVE PACKING LEAKAGE EXPERIENCE........................................................................3-1 3.1 Boric Acid Corrosion Guidebook..................................................................................3-1 3.2 Review of Plant Maintenance Records ........................................................................3-2 3.3 NUREG/CR-6582 ........................................................................................................3-4 3.4 Summary .....................................................................................................................3-6

4 VALVE PACKING CONFIGURATIONS AND PRODUCTS...................................................4-1 4.1 Valve Packing Technology Evolution...........................................................................4-1

4.1.1 Braided Asbestos Packing (Figure 4-1.a) ...........................................................4-3 4.1.2 Braided Non-Asbestos Packing (Figure 4-1.b)....................................................4-3 4.1.3 Die-Formed Flexible Graphite Packing With Braid End Rings (Figure 4-1.c).......................................................................................................4-4 4.1.4 Engineered Die-Formed Flexible Graphite Packing System (Figure 4-1.d) ........4-5

4.2 Components in Die-Formed Flexible Graphite Packing System..................................4-5 4.2.1 Die-Formed Flexible Graphite Packing Rings .....................................................4-6 4.2.2 Anti-Extrusion Rings..........................................................................................4-10 4.2.3 Spacers.............................................................................................................4-11

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4.2.4 Bushings ...........................................................................................................4-12 4.2.5 Lantern Rings....................................................................................................4-12 4.2.6 Packing Washers ..............................................................................................4-12 4.2.7 Cushion Rings...................................................................................................4-13 4.2.8 Junk Rings ........................................................................................................4-13 4.2.9 Gland Bolting and Hardened Steel Washers ....................................................4-13 4.2.10 Live Loading ..................................................................................................4-14

4.3 Components in Braid Ring Packing System ..............................................................4-15 4.3.1 Braided Asbestos..............................................................................................4-15 4.3.2 Braided PTFE....................................................................................................4-15 4.3.3 Braided Graphite Fiber Yarn .............................................................................4-16 4.3.4 Braided Carbon Fiber Yarn ...............................................................................4-16 4.3.5 Braided Flexible Graphite Tape ........................................................................4-16

4.4 Commercial Products.................................................................................................4-16 4.5 Summary ...................................................................................................................4-19

5 CAUSES OF VALVE PACKING LEAKS ...............................................................................5-1 5.1 Low Gland Stress ........................................................................................................5-1 5.2 Packing Consolidation .................................................................................................5-2 5.3 Dimensions and Clearances........................................................................................5-5 5.4 Stem and Stuffing Box Surface Finish .........................................................................5-6 5.5 Stem and Stuffing Box Corrosion.................................................................................5-7 5.6 Stem Misalignment ......................................................................................................5-8 5.7 Stem Thermal Taper ..................................................................................................5-10 5.8 Product Misapplication...............................................................................................5-11 5.9 Aging..........................................................................................................................5-13 5.10 Functional Failure Due to Excessive Friction ........................................................5-13 5.11 Summary...............................................................................................................5-14

6 SELECTING PACKING CONFIGURATIONS AND PRODUCTS ..........................................6-1 6.1 Overview of Packing Options.......................................................................................6-1 6.2 Packing Coding System...............................................................................................6-2 6.3 Suggested Packing Configurations..............................................................................6-2

6.3.1 Basic Die-Formed Flexible Graphite Packing (BX000XSW) ...............................6-2 6.3.2 Low-Friction Packing...........................................................................................6-3

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6.3.3 All Braid Non-Asbestos Packing (BZZZZSW) .....................................................6-4 6.4 Other Packing Configurations......................................................................................6-4 6.5 Inventory Optimization .................................................................................................6-4 6.6 Evaluating Packing Suppliers and Products ................................................................6-5

6.6.1 Vendor Support ...................................................................................................6-5 6.6.2 Product Qualification Testing ..............................................................................6-5

6.7 Summary .....................................................................................................................6-5

7 GLAND PRELOAD.................................................................................................................7-1 7.1 Gland Preload Stress and Required Sealing Force .....................................................7-1 7.2 Achieving Preload by Torquing....................................................................................7-2

7.2.1 Determining Appropriate Torque.........................................................................7-2 7.2.2 Ensuring That Applied Torque Results in the Desired Preload...........................7-3 7.2.3 Providing for Consolidation .................................................................................7-5

7.3 Stem Friction................................................................................................................7-5 7.4 Live Loading ................................................................................................................7-7 7.5 Diagnostics ................................................................................................................7-11 7.6 Gland Design .............................................................................................................7-12

7.6.1 Gland and Follower Design One- vs. Two-Piece ...........................................7-12 7.6.2 Gland Length.....................................................................................................7-13 7.6.3 Gland-to-Stuffing Box Clearance ......................................................................7-13 7.6.4 Gland Machining Practices ...............................................................................7-14 7.6.5 Gland Materials .................................................................................................7-14

7.7 Summary ...................................................................................................................7-14

8 DEMONSTRATIONS USING VALVE PACKING PERFORMANCE DEMONSTRATION UNIT..........................................................................................................8-1

8.1 Valve Packing Performance Demonstration Unit.........................................................8-1 8.2 Initial EPRI VPDU Experiments ...................................................................................8-4 8.3 Summary .....................................................................................................................8-7

9 VALVE PACKING INSTALLATION PROCEDURES.............................................................9-1 9.1 Valve Packing Procedure ............................................................................................9-1

9.1.1 Remove Old Packing ..........................................................................................9-1 9.1.2 Inspect Valve.......................................................................................................9-2 9.1.3 Prepare Bolting ...................................................................................................9-2

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9.1.4 Obtain New Packing Components ......................................................................9-3 9.1.5 Install Packing.....................................................................................................9-3 9.1.6 Consolidate Packing ...........................................................................................9-4 9.1.7 Apply Live Loading if Used..................................................................................9-4 9.1.8 Periodic Retorquing.............................................................................................9-5

9.2 Overview of Key Installation Steps ..............................................................................9-6 9.3 Valve Packing Tips ......................................................................................................9-6 9.4 Summary .....................................................................................................................9-8

10 VALVE PACKING IMPROVEMENT PROGRAMS.............................................................10-1 10.1 Establishing Program Goals..................................................................................10-1 10.2 Selecting a Packing Supplier ................................................................................10-2 10.3 Selecting a Packing Product Strategy...................................................................10-2 10.4 Creating a Database of All Valves ........................................................................10-3 10.5 Prioritizing Valves for Rework ...............................................................................10-3 10.6 Providing Training .................................................................................................10-4 10.7 Work Planning.......................................................................................................10-5 10.8 Technical Support .................................................................................................10-5 10.9 Monitoring and Periodic Retorquing......................................................................10-5 10.10 Root Cause Analysis and Feedback .....................................................................10-8 10.11 Summary...............................................................................................................10-8

11 OVERVIEW.........................................................................................................................11-1 11.1 Technical Fundamentals .......................................................................................11-1 11.2 Programmatic Fundamentals ................................................................................11-2

12 REFERENCES ...................................................................................................................12-1

A TRAINING SLIDES ............................................................................................................... A-1

B SAMPLE VALVE PACKING SURVEILLANCE AND RETORQUING PROGRAM.............. B-1

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LIST OF FIGURES

Figure 1-1 Valve Packing Performance Demonstration Unit .....................................................1-2 Figure 2-1 Alternative Methods for Sealing Valve Stems ..........................................................2-1 Figure 2-2 Idealized Model of Valve Packing.............................................................................2-3 Figure 3-1 Sources of Leaks Causing Reported Boric Acid Corrosion ......................................3-2 Figure 3-2 Recent EPRI Analysis of Sources of Leaks at Six Plants.........................................3-4 Figure 3-3 Sources of Leaks per NUREG/CR-6582 ..................................................................3-5 Figure 3-4 Number of Valve Packing Leaks per Year per NUREG/CR-6582 ............................3-6 Figure 4-1 Evolution of Valve Packing Systems ........................................................................4-2 Figure 4-2 Typical Component Parts of Engineered Die-Formed Packing System ...................4-6 Figure 4-3 Design and Performance of Wedge-Type Packing ..................................................4-9 Figure 4-4 Damage to Graphite Spacer Caused by Angled Surface at Bottom of Stuffing

Box ...................................................................................................................................4-13 Figure 4-5 Typical High-Pressure Valve Gland Bolting Without Live Load..............................4-14 Figure 5-1 Consolidation of Die-Formed Flexible Graphite Packing..........................................5-2 Figure 5-2 Packing Consolidation ..............................................................................................5-3 Figure 5-3 Effect of Consolidation on Gland Load .....................................................................5-4 Figure 5-4 Running Tolerances and Clearances (MSS SP-120) ...............................................5-5 Figure 5-5 Stuffing Box Tolerances (MSS SP-120) ...................................................................5-6 Figure 5-6 Stem Tolerances (MSS SP-120) ..............................................................................5-6 Figure 5-7 Stem Corrosion Test Fixture (MSS SP-121) ............................................................5-8 Figure 5-8 Typical Effects of Stem Misalignment.......................................................................5-9 Figure 5-9 Use of Bushings to Mitigate Stem Misalignment ....................................................5-10 Figure 5-10 Effect of Stem Thermal Taper on Packing Consolidation.....................................5-11 Figure 5-11 Decomposition Rate of PTFE at Elevated Temperature.......................................5-13 Figure 7-1 Galling on Follower Caused by Lack of Hardened Steel Washers ...........................7-4 Figure 7-2 Galling of Soft Steel Washers Provided by Valve Supplier ......................................7-4 Figure 7-3 Live Loading Using Belleville Washers.....................................................................7-7 Figure 7-4 Alternative Live Loading Configurations ...................................................................7-8 Figure 7-5 Effect of Live Loading on Valve Gland Stress ..........................................................7-9 Figure 7-6 Live Loading Spring Packs .....................................................................................7-10 Figure 7-7 Potential Gland Follower Improvements.................................................................7-13 Figure 8-1 Valve Packing Performance Demonstration Unit (VPDU) ........................................8-2

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Figure 8-2 VPDU Packing Assembly and Valve Stems .............................................................8-3 Figure 9-1 Summary of Key Valve Packing Points ....................................................................9-6 Figure 9-2 Use of Shims to Ensure Gland Alignment ................................................................9-8

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LIST OF TABLES

Table 3-1 Evaluation of Leakage-Related Work Orders for Two Sites (19951999) ................3-3 Table 4-1 Valve Packing Products Commercial Products Survey ........................................4-17 Table 8-1 Initial VPDU Experimental Results ............................................................................8-6 Table 8-2 Summary of Initial VPDU Experimental Results ........................................................8-7 Table 10-1 Valve Retorque Schedule Used at BWR Plant With Reported Small Number

of Leaks............................................................................................................................10-7 Table B-1 Sample Valve Packing Surveillance and Repacking Program (BWR Plant) ........... B-2

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1 BACKGROUND

This section provides background information regarding training objectives, target audience, and the basis for focusing attention on leakage from valve packing.

2 1.1 Objective

The objective of this training is to help nuclear plant personnel reduce the number and severity of leaks from valve packing. This objective will be accomplished by providing a better understanding of the key factors that contribute to valve packing integrity. These factors include: Icons

represent the accompanying course slides, which can be found in Appendix A.

How valve packing works Types of packing available and where they should be used Required packing gland loads Role of consolidation in long-term packing performance Importance of live loading for valves in cyclic duty applications Major causes of packing leaks Valve packing installation guidelines The training also provides hands-on experiments to reinforce the understanding of key factors that result in high valve packing integrity.

1.2 Target Audience

The training is directed toward a broad audience of plant maintenance technicians, work planners, maintenance engineers, systems engineers, and quality control (QC) personnel. The training is intended to help these individuals reduce leakage from valve packing through improved knowledge of the factors that contribute to valve packing integrity. The training can also serve as a useful refresher for plant engineers responsible for preparing procedures and for troubleshooting leaking equipment.

1-1

EPRI Licensed Material Background

1.3 Relationship to Existing Plant Procedures

The training is not intended to supersede existing plant procedures, but rather to better support them. Personnel should continue to follow approved plant procedures until those procedures have been changed.

1.4 Valve Packing Performance Demonstration Unit

Experience with previous EPRI leakage reduction training programs has confirmed the importance of providing an appropriate level of hands-on training to reinforce fundamentals. This is especially important in a mature industry where personnel are often reluctant to change from past practices unless the limitations of these practices, and the benefits of the proposed change, can be clearly demonstrated.

A Valve Packing Performance Demonstration Unit (VPDU) was developed to demonstrate key valve packing principles. The VPDU is shown in Figure 1-1. Section 8 of this report provides a more complete description of the VPDU and the results of the initial experiments performed using the VPDU.

Figure 1-1 Valve Packing Performance Demonstration Unit

1-2


Background

1.5 Summary The objective of this training is to assist plants in reducing leaks from

valve packing by providing a better understanding of the factors that contribute to high joint integrity and a suggested packing procedure.

Training is directed toward engineers, maintenance technicians, and work planners.

Information presented must not supersede plant procedures but can be used to improve procedures.

Classroom instruction is supplemented by experiments using the VPDU.

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2 REVIEW OF VALVE PACKING CONCEPTS

This section briefly reviews several methods that can be used to seal valve stems and describes how valve packing functions to produce a seal.

5 2.1 Alternative Methods for Sealing Valve Stems

6 As shown in Figure 2-1, valve stems can be packed or hermetically sealed. Packing can include packing rings, chevron or V packing, and o-ring seals. Stems can be hermetically sealed by metallic bellows or diaphragms.

Figure 2-1 Alternative Methods for Sealing Valve Stems [1]

2-1

EPRI Licensed Material Review of Valve Packing Concepts

Packing rings are used for most applications in nuclear plants. Rings of flexible packing material are inserted into the annulus between the valve stem and stuffing box and are then compressed axially by a gland follower. The axial load on the flexible packing forces the packing to expand outward and to create radial seals to the stem and stuffing box.

Chevron, or V packing, is designed to be installed using the same approach as packing rings, but to produce higher radial forces, and theoretically a better seal, for the same amount of axial force.

O-rings provide low leakage and low friction, but the elastomeric materials used in o-rings typically have a limited temperature capability and often contain chemicals such as halogens that preclude their use in many nuclear applications. However, o-rings are used successfully in many instrument and pneumatic system applications.

Metallic bellows and diaphragms produce a hermetic seal that results in the lowest achievable leak rates. However, metal diaphragms are limited to low-stroke applications, and it is difficult to achieve a long stroke with high system pressures using metallic bellows.

The majority of valves in commercial nuclear plants are sealed by packing rings. This relatively low-cost solution can accommodate large valve strokes at high system temperatures and pressures. In addition, when properly matched to the application and properly installed, packing rings will provide a reliable low-leakage seal. This report focuses on commonly encountered packing ring valve stem seals.

7 2.2 Valve Packing Concepts

Figure 2-2 shows how packing rings function to seal a valve:

Packing is inserted between the valve stem and the stuffing box in the valve bonnet

An axial preload force is applied to the packing by a gland that is in turn loaded by torquing the gland nuts

The axial force on the packing causes the packing to expand both inward and outward, creating radial seals at both the stem and the stuffing box

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Review of Valve Packing Concepts

Figure 2-2 Idealized Model of Valve Packing

Several important technical issues can affect packing performance. These include:

The packing should ideally include rings of die-formed flexible graphite material that provide a tighter seal than most braided materials.

Braided materials that are intended for use as seals, rather than only as anti-extrusion rings, include blocking materials to fill up the voids between the braided strands. Some examples of blocking materials are graphite, polytetrafluoroethylene (PTFE), and lubricants.

Because packing materials are flexible and can flow under load, provisions must be made to keep the material from extruding between the stem and gland, between the gland and the stuffing box, and between the stem and bonnet. Many braid-type packing materials can accommodate the normal clearances between the stem and gland and between the gland and stuffing box without adding anti-extrusion rings. However, die-formed flexible graphite has less strength and typically requires the addition of anti-extrusion rings.

Because all packing materials contain voids, they must be properly compressed (consolidated) during installation. If the material is not properly compressed during assembly, the axial load on the gland will decrease as the valve is stroked.

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EPRI Licensed Material Review of Valve Packing Concepts

The axial load on the gland decreases over time due to wear, or loss of braid ring blocking material. Ideally, there should be some compliance in the packing that can accommodate small amounts of consolidation, wear, stem imperfections, and stem thermal taper.

Loss of gland force due to service-induced packing consolidation, wear, or loss of blocking material can be compensated for by periodically retorquing the packing or by installing Belleville spring washers between the gland follower and gland nuts.

The radial force between the packing and valve stem results in friction that must be overcome during valve operation. High axial preload forces, large depths of packing in the stuffing box, and high-friction packing materials all contribute to high packing friction.

2.3 Summary Most valve stems in nuclear plants are sealed by packing rings. When packing rings are loaded axially by a gland, the packing material

expands both inward and outward to develop radial seals at the valve stem and the stuffing box.

Packing should ideally include die-formed flexible graphite rings with inherently low leakage relative to most braided packing.

Packing must be properly consolidated during assembly to prevent an abnormally high rate of loss of axial force during service.

Packing should be retorqued periodically to compensate for small amounts of service-induced consolidation and wear.

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3 VALVE PACKING LEAKAGE EXPERIENCE

This section briefly reviews industry experience with leakage from valve packing. The description is brief because valve packing leakage represents an ongoing source of significant operation and maintenance (O&M) expenditures at most plants and has been described at length in previous Sealing Technology & Plant Leakage Reduction Series reports.

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3.1 Boric Acid Corrosion Guidebook

EPRI TR-104748, Boric Acid Corrosion Guidebook [2], includes a survey of the causes of boric acid leakage incidents reported before the summer of 1994. Sources for the information include Nuclear Power Experience reports, U.S. Nuclear Regulatory Commission (NUREG) reports, U.S. Nuclear Regulatory Commission (NRC) Information Notices, NRC Inspection and Enforcement Bulletins, NRC Generic Letters, and NRC Public Documents Room files. Figure 3-1, excerpted from the Boric Acid Corrosion Guidebook [2], shows that valve packing leaks represented approximately 20% of the identified sources. This was the second leading cause of leaks behind gaskets that comprised approximately 45% of the total identified sources. A 2001 revision to this report shows similar conclusions [3].

3-1

EPRI Licensed Material Valve Packing Leakage Experience

Figure 3-1 Sources of Leaks Causing Reported Boric Acid Corrosion [2]

3.2 Review of Plant Maintenance Records

As part of the Fluid Sealing Technology Program, EPRI visited several sites to collect information regarding the source of leakage-related maintenance work orders. Table 3-1 shows the total number of leakage-related work orders at two plant sites over the five-year period 19951999, the number of work orders that indicated valve packing as the source of the leaks, and the percentage of work orders that related to valve packing. These data confirm that valve packing remains a major source of leakage-related work orders in terms of absolute numbers and percentage of total leaks.

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3-2


Valve Packing Leakage Experience

Table 3-1 Evaluation of Leakage-Related Work Orders for Two Sites (19951999) [3]

Source of Leak

Total WO's

Plant A

Total WO's

Plant B

Average WO's

per Year per Unit

Percent Average WO Man-Hours for

Plant B

Flanged Joints and Gaskets

850 189 69 23% 32

Pipe and Tube Fittings

500 359 57 19% 31

Valve Packing 663 563 82 27% 16

Seals 302 220 35 12% 24

Other 263 620 59 20% 69

Total 2,578 1,951 302 100% 38*

Valve Packing Was Single Largest

Source of Leaks From 1995 Through 1999

*This total is generated from more information than is presented in this table.

3-3


Recent analysis by EPRI of the source of leaks at six plants is shown in Figure 3-2. These data indicate that valve packing is the single largest identifiable source of leaks.

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Figure 3-2 Recent EPRI Analysis of Sources of Leaks at Six Plants

12 3.3 NUREG/CR-6582

The Idaho National Engineering and Environmental Laboratory (INEL) performed a study in 1997 to determine the causes of primary system leaks reported to the NRC via Licensee Event Reports from 1985 through the third quarter of 1996 [4]. This includes leakage events that occurred during hot shutdown, hot standby, startup, and power operation, except for those through steam generator tubes. Figure 3-3, excerpted from this study, shows that valve packing was one of the two main reportable causes of leaks.

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3-4


Valve Packing Leakage Experience

Figure 3-3 Sources of Leaks per NUREG/CR-6582 [4]

Of the 199 reportable leak events during the study period, approximately 60% (121 events) occurred during the first four years and the remaining 40% occurred during the last eight years. One reason for the significantly lower rate over the last eight years was the elimination of reportable leakage from valve packing, illustrated in Figure 3-4, which is also excerpted from the INEL report. However, some primary system valve packing leaks have not been included in the INEL survey report.

3-5


Figure 3-4 Number of Valve Packing Leaks per Year per NUREG/CR-6582 [4]

3.4 Summary Valve packing has been a major source of leakage in pressurized water

reactor (PWR) plant primary systems.

Recent operating experience at six plants visited by EPRI has shown that valve packing represents a significant percentage of the total recorded leaks at these plants.

A study conducted by INEL for the NRC in 1997 showed that valve packing was a major source of primary system leakage prior to approximately 1992. Since that time, significant progress appears to have been made in reducing leakage from primary system valves.

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4 VALVE PACKING CONFIGURATIONS AND PRODUCTS

This section briefly describes the evolution of valve packing and the basic valve packing configurations and products in widespread use.

15 4.1 Valve Packing Technology Evolution

Valve packing has evolved significantly over the past thirty years. Several key stages in this evolution are illustrated in Figure 4-1. A review of the evolutionary stages provides an overview of how packing designs were changed to improve performance.

4-1

EPRI Licensed Material Valve Packing Configurations and Products

Figure 4-1 Evolution of Valve Packing Systems

The evolution of packing designs was gradual, with several different types of packing commonly in use at any point in time. With the exception of the braided asbestos packing in Figure 4-1.a, all of these configurations are used to some extent in plants today.

4-2


Valve Packing Configurations and Products

4.1.1 Braided Asbestos Packing (Figure 4-1.a) 16

Until the mid-1970s, braided asbestos was the most widely used valve packing product. It was not perfect, but users were willing to accept and work around its limitations. The main limitations were poor packing life and the need for periodic retorquing of gland bolts. These limitations were accommodated by periodic retorquing, periodic repacking, use of improved blocking materials,1 and use of double sets of packing with a leakoff port to monitor for and collect leakage from the primary seal. A main source of problems was that many packing rings were typically used on the theory that the greater the length of packing, the better the seal. Beginning in the mid-1970s, the potential health hazards of asbestos became better known and became an increasingly significant limitation.

In the 1970s, Atomic Energy of Canada Ltd. (AECL), at their Chalk River Nuclear Laboratories in Chalk River, Ontario, Canada, began researching problems associated with leakage from asbestos packing. The incentive for this research was that the Canadian Deuterium Uranium reactors use heavy water as a primary system coolant, and leakage of this coolant was costly. The AECL research increased the understanding of asbestos packing and highlighted the value of live loading packing with Belleville washers to compensate for volume loss and consolidation that occurs over time due to stroking and loss of blocking material (see Section 5 of this report for a further description of the causes of consolidation). This effort also resulted in increased use of double-packed valves with monitored leakoff ports.

16 4.1.2 Braided Non-Asbestos Packing (Figure 4-1.b)

Starting in the mid-1970s, interest in replacing the potentially hazardous asbestos led to the search for asbestos replacements. The main alternatives at this point were braided packing woven from graphite, PTFE, or other fibers. These materials could easily be substituted for the original asbestos packing. Braided PTFE packing could be used for low-temperature water applications. Braided graphite could be used for higher temperature applications and where PTFE was not acceptable due to its potential to cause cracking of certain corrosion-resistant materials.

Braided graphite, PTFE, and other fiber-type packing rings worked well for many applications, and are still used in some lower temperature and pressure nuclear plant applications. However, the braided materials remain prone to consolidation and loss of blocking material over time. These 1 Blocking material is used to fill in gaps between the fiber strands in braided packing

and thereby produces a better seal for a given gland pressure. Graphite, PTFE, and lubricants are examples of blocking materials. If low-friction materials are used for blocking, then the valve stem friction forces can also be reduced.

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factors lead to the need for periodic retorquing, especially for applications that involve significant valve stroking. Consolidation during operation was partially addressed by reducing the number of packing rings to four to five so that all rings would be more uniformly compressed.

4.1.3 Die-Formed Flexible Graphite Packing With Braid End Rings (Figure 4-1.c) 17

In the early 1980s, several research and development (R&D) programs were initiated to identify the root causes of problems with braided packing and to develop improvements that would provide long packing life without the need for frequent retorquing. This work was performed by EPRI, Chalk River Nuclear Laboratories, several valve manufacturers and packing suppliers, and others. Key results of the EPRI sponsored research are reported in the following documents:

NP-2560 Valve Stem Packing Improvement Study, August 1982 [5]

NP-2455 Valve Stem Packing Improvements, February 1986 [6]

NP-5697 Valve Packing Improvements, May 1988 [7]

This research led to several findings including:

Reduction of the packing height to minimize the effects of consolidation

Increased use of die-formed flexible graphite packing rings in lieu of braided rings for the primary seal

The importance of consolidating packing during assembly The importance of stem condition on friction and wear The need for packing to remain compliant to accommodate conditions

such as thermally induced taper in the valve stem

The benefit of live loading in accommodating in-service consolidation without the need for periodic retorquing

These findings are described at greater detail in Section 5.

The result of this work is the configuration of die-formed flexible graphite packing rings with graphite braid rings above and below the die-formed rings to prevent extrusion of the die-formed flexible graphite rings at the gaps between the valve stem and mating parts.

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4.1.4 Engineered Die-Formed Flexible Graphite Packing System (Figure 4-1.d) 17

Research on valve packing continued throughout the 1980s and 1990s and led to the development of an engineered valve packing system. This system consists of:

Die-formed flexible graphite rings Higher-density graphite or composite anti-extrusion rings Graphite spacers to reduce the packing height Graphite bushings to keep the stem centered for cases involving lateral

stem loads

Cushion rings/washers to prevent cracking of spacers at the bottom of the gland

The use of Belleville washer packs to provide a live load on the gland Some vendors offer additional features such as the use of wedge-shaped, die-formed rings to increase the radial sealing stress per unit axial compressive stress and the addition of thin wafers of PTFE between packing rings to provide reduced friction with little volume of PTFE material.

18 4.2 Components in Die-Formed Flexible Graphite Packing System

19 Figure 4-2 shows a typical modern nuclear plant valve packing system using die-formed flexible graphite packing rings. The major component parts are described in Sections 4.2.1 through 4.2.10 of this report. Some of the parts such as live loading are not used for all applications.

20

21

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Figure 4-2 Typical Component Parts of Engineered Die-Formed Packing System

4.2.1 Die-Formed Flexible Graphite Packing Rings

Die-formed flexible graphite packing rings form the heart of most modern nuclear plant valve packing. Laboratory and field experience has confirmed the superior properties of this material.2 While experience has demonstrated that sealing takes place over a single ring, most valve packing sets have two or three flexible graphite rings to provide a backup and to ensure that there is no leakage past the split in the ring required for installation.

Several advantages of die-formed expanded graphite packing rings were identified by the EPRI valve packing improvement study in the 1980s [7]: 2 Experiments using the EPRI VPDU (see Section 8) show how adding a single die-

formed flexible graphite packing ring significantly reduces the rate of leakage of nitrogen gas relative to the leakage for braid ring packing only.

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Low coefficient of friction (less than 0.1) Self lubricating No binders, fillers, or resins Flexible, yet exhibit minimal cold or warm flow characteristics Corrosion resistance Very low rate of aging Excellent resistance to temperature changes High anisotropy of electrical and thermal conductivity Capable of 5,432F (3,000C) temperatures in reducing or inert media Capable of -328F to +932F (-200C to +500C) temperatures in

oxidizing media

No asbestos Chemically resistant and can operate in fluids with pH of 1 to 14 Available in nuclear grades with low levels of halogens and heavy

metals and leachable chlorides of less than 50 ppm

Available as ribbon, laminated rings, or die-formed rings Die-formed flexible graphite also has the highest radiation resistance of all packing materials and has a coefficient of thermal expansion that results in little differential thermal expansion of the graphite relative to the valve stem and body materials.

In the 1970s, Union Carbide introduced a Grafoil expanded flexible graphite to provide superior sealing without the use of asbestos. Flexible graphite is a high-purity graphite material that has been chemically treated to form a compound with and between the layers of graphite structure. This material is heated rapidly to produce an eighty-fold expansion in size relative to the raw flake graphite material. This expanded material is then molded or calendered into sheet form with a density of approximately 70 lb/ft3 (1,120 kg/m3) or approximately half of the theoretical 140 lb/ft3 (2,240 kg/m3) of solid graphite [8].

Die-formed rings are manufactured from flexible graphite ribbon that is wrapped around a mandrel and then compressed in a die to form a single packing ring. The amount of material used and the compression force applied determines the density of the manufactured rings. Ring density is typically in the range of 90100 lb/ft3 (1,4401,600 kg/m3) or 6570% of the theoretical density of solid graphite.

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Die-formed rings are typically manufactured slightly undersize to ensure that they will fit without being damaged during installation. Typical tolerances for die-formed packing rings are +0.005/-0.000 in. (+0.013/-0.000 cm) on the inside diameter and +0.000/-0.010 in. (+0.000/-0.025 cm) on the outside diameter. Die-formed rings are typically cut after forming to create a butt or angled overlap.

22 Die-formed rings typically have a square cross section, but the cross section can also be a reduced-height rectangular or a wedge shape. The EPRI valve packing improvement study showed that wedge-shape packing such as shown in Figure 4-3 can develop a seal under lower axial gland stresses than square packing. However, subsequent qualification tests and field experience have demonstrated that square packing can also provide reliable sealing at acceptable gland preload stresses.

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Figure 4-3 Design and Performance of Wedge-Type Packing [7]

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Laboratory R&D and product qualification tests have demonstrated that die-formed flexible graphite packing rings form an excellent low-friction seal against typical valve stem materials. The two main weaknesses of flexible graphite for this application are that the material must be contained because it has little inherent resistance to extrusion under high gland pressures and that it can cause corrosion of the stems.

Flexible graphite for valve packing applications is manufactured with small amounts of corrosion inhibitor distributed uniformly throughout the material. The inhibitors are proprietary and their effect on stem materials must be validated by testing. Section 5.5 of this report describes test procedures recommended by the Manufacturers Standardization Society (MSS) to assess the effect of packing materials on stem corrosion.

Additionally, die-formed flexible graphite packing is described in Section 4.3 of the EPRI valve packing improvement study [7].

4.2.2 Anti-Extrusion Rings

Die-formed flexible graphite packing rings have little inherent strength and tend to extrude under axial loading into gaps between the stem and body, stem and gland, and gland and body. Three methods are typically used to prevent this extrusion.

Braided Anti-Extrusion Rings Rings of braided material can be installed at both ends of the die-formed flexible graphite rings to prevent extrusion and to help wipe the stem. These rings need not have any blocking agents because they do not act as a seal.

Braided seal rings with blocking agents and lubrication can also serve as anti-extrusion rings. These materials can act to prevent extrusion of the die-formed flexible graphite rings, provide some additional sealing capability, and produce less friction.

Alternative types of braid rings are described in Section 4.3 of this report.

Composite Anti-Extrusion Rings Composite anti-extrusion rings are manufactured from fiber material in a resin matrix. The dimensions of these rings are such that there is only a small clearance at the three possible extrusion locations. These rings have high strength so that they can accommodate axial compressive stresses without applying high radial loads to the stem. Accordingly, these rings resist extrusion without increasing friction.

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Composite anti-extrusion rings must be manufactured to close tolerances so that the gaps between the rings and the stem and stuffing box are tight enough to prevent extrusion of the die-formed flexible graphite material yet not so tight that the rings cannot be installed. The need for tighter tolerances also applies to the stem and stuffing box. If the stem and stuffing box are not within the specified tolerances for stock composite anti-extrusion rings, then custom-fit rings must be procured to ensure proper functioning.

The most detrimental conditions for composite anti-extrusion rings are applications involving vibration and misalignment. For these applications, a braided anti-extrusion ring may be more appropriate.

High-Density Expanded Graphite Anti-Extrusion Rings Anti-extrusion rings can be manufactured using expanded graphite material compressed to a higher density than the die-formed flexible graphite sealing rings. This higher-density material is more resistant to extrusion than normal packing rings, not as sensitive to tolerances as composite rings, and has a stem friction in between the braided and composite rings.

Plants typically use both braided and composite anti-extrusion rings. Braid rings are often selected for routine static applications and composite rings for critical applications where there is repeated cycling and friction must be minimized. For cases where composite anti-extrusion rings are used, consideration should be given to applying live loading. The packing system must have some compliance to maintain a radial sealing load on the stem in the presence of small displacements (for example, thermal taper of the valve stem) and the composite rings have significantly less compliance than braid rings. The live loading restores compliance to the system.

4.2.3 Spacers

A main conclusion from early R&D work on valve packing was that sealing takes place over a relatively short length of packing, dispelling the belief that a long length of packing was required to ensure a good seal. In fact, as described in Section 5.2, long lengths of packing increase the difficulty of achieving consolidation during assembly and increase the amount of consolidation that occurs in service. Therefore, most modern packing is based on short packing lengths, with any additional space in the stuffing box filled by a spacer.

Spacers also act to center the stem in the stuffing box and to accommodate side loads so that they are not imposed on the packing.

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Spacers are typically manufactured from high-density graphite. This material is capable of accommodating high gland loads yet not scoring the stem if contact occurs. The spacers are typically split in half longitudinally to facilitate assembly, and tapped holes are provided at one end to aid in removal.

4.2.4 Bushings

In some cases, the valve stem does not run through the centerline of the gland/stuffing box due to misalignment, stem bowing, or side loads that occur when the valve is not mounted vertically. This can lead to non-uniform loading of the packing or to the stem contacting the gland and subsequently being damaged as the valve is stroked. For these cases, a bushing can be installed at the gland end of the packing set to center the stem and prevent metal-to-metal contact. Bushings are typically made from the same high-density graphite material as spacers.

Because bushings can be helpful in many applications, excess length in the stuffing box should be filled with equal lengths of spacer below the packing and bushing above the packing.

4.2.5 Lantern Rings

Many valves manufactured before the development of improved packing were provided with two sets of packing separated by a rigid lantern ring. A leakage monitoring port was typically used to check for leakage past the innermost seal.

Given the advancements in packing technology, and the resultant lower leakage and higher reliability resulting from the use of die-formed flexible graphite packing, there is less need for the double-packed valves and lantern rings. In many of these cases, the lantern ring can be removed and replaced by a longer spacer. However, intermediate monitoring and a leakoff function is still required for some applications. Modern practice in these cases is to use graphite lantern rings to avoid the possibility of valve stem damage.

4.2.6 Packing Washers

Washers made from flexible graphite sheet material are sometimes used to aid in assembly of the packing. These washers have the same mechanical properties as the die-formed packing rings but are relatively thin.

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4.2.7 Cushion Rings

As shown in Figure 4-4, graphite spacers can fracture if installed in a gland that is not flat and square at the bottom. Flexible washers or graphite braid rings can be used to provide a cushion to prevent fracture of the spacers. Braid rings provide more cushioning, but their greater height will result in more friction and potential for service-induced consolidation.

Figure 4-4 Damage to Graphite Spacer Caused by Angled Surface at Bottom of Stuffing Box [10]

4.2.8 Junk Rings

If the bottom of the gland is machined on an angle, it is generally necessary to install a metal junk ring or hardened graphite ring machined to the bottom angle to provide a flat and square surface for the packing assembly.

Figure 4-4 shows damage that has occurred to a graphite spacer as a result of high gland preload applied to a valve where the bottom of the stuffing box was not machined flat and there was no junk ring or cushion ring to even out the compressive stress.

4.2.9 Gland Bolting and Hardened Steel Washers

Gland bolting has been a major source of packing leaks. Specifically, if the torque applied to the gland nuts does not result in the desired gland stress on the packing, the packing will not be properly consolidated. As described in Sections 5.1 and 7.2.2, it is important for the bolting to be in good condition and to be well lubricated on the threads and the nut-to-follower rubbing surface. In addition, there should be a hardened steel washer between the nut and gland follower surface to minimize friction

23

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that prevents the torque from producing the desired gland preload force. Figure 4-5 shows gland bolting for a typical high-pressure valve with hardened steel washers to minimize damage and high parasitic friction at the point where the gland nut turns against the follower surface.

Figure 4-5 Typical High-Pressure Valve Gland Bolting Without Live Load [10]

4.2.10 Live Loading

The final component of a modern packing system is live loading to compensate for in-service consolidation and to provide compliance for cases involving composite anti-extrusion rings. This loading is most often provided by a stack of Belleville (spring) washers installed between the follower and nut as shown in Figure 4-2. Spring packs must be properly engineered to provide the required gland force and displacement capability. Live loading is described in greater detail in Section 7.4 of this report.

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4.3 Components in Braid Ring Packing System

For some applications, utilities use braid rings in lieu of die-formed flexible graphite rings and anti-extrusion rings. Cost and convenience are the primary reasons for selecting braid rings. However, this configuration is not generally as good as one using die-formed flexible graphite because the braid rings are more susceptible to consolidation, hardening, and loss of blocking material over time.

A valve packed with braid rings can have all of the other features of a valve packed with die-formed flexible graphite rings such as spacers, bushings, packing washers, junk rings, and live loading.

Braided packing consists of a braided material and a blocking agent to seal leaks between adjacent fibers. Braided packing can also include corrosion inhibitors and lubricity additives such as PTFE, graphite powder, or oil. The main product forms are described in Sections 4.3.1 through 4.3.5.

4.3.1 Braided Asbestos

Braided asbestos packing is no longer installed in valves due to the health hazard posed by the asbestos fibers and the generally poor performance resulting from loss of the blocking agents and resultant hardening over time.

4.3.2 Braided PTFE

Braided PTFE is used for low-temperature (


4.3.3 Braided Graphite Fiber Yarn

Graphite fiber yarn can be woven into packing rings. The packing is typically provided with blocking agents and lubricants to improve sealing and reduce friction. PTFE is particularly useful as a blocking agent because it reduces friction, but might not be acceptable for all applications for the reasons noted in Section 4.3.2. Graphite fibers are much larger in diameter than asbestos fibers and are susceptible to fracturing at the extrusion gap for gland pressures higher than 46 ksi (2842 MPa). However, for most nuclear plant applications, it should not be necessary to use gland pressures greater than 4 ksi (28 MPa); and braided graphite has been used successfully in these applications.

4.3.4 Braided Carbon Fiber Yarn

Carbon fibers have greater strength than graphite fibers, are smaller in diameter, and are less susceptible to fracture at high gland stresses. As in the case of graphite fiber yarns, blocking agents and lubricants are added to improve sealability and reduce friction.

4.3.5 Braided Flexible Graphite Tape

Flexible graphite tape can be formed into large strands and braided into square packing. The flexible graphite tape strands can be wrapped with small-diameter, corrosion-resistant wire to increase extrusion resistance. Blocking agents and lubricants can also be added to improve sealability and reduce friction. While the fine wire wrap improves resistance to extrusion, it has the potential to score the stem, especially under high-cycle applications.

In summary, several types of braided packing have been used successfully in nuclear plant applications. The main disadvantages relative to die-formed flexible graphite packing are that braided-type products have greater consolidation in service and are susceptible to loss or degradation of the blocking materials over time. For these reasons, braided-type packing typically requires more frequent retorquing and more frequent replacement to achieve the same level of leak tightness as die-formed flexible graphite packing.

24 4.4 Commercial Products

25 Valve packing is provided by valve manufacturers and independent packing suppliers. Table 4-1 provides a summary of products offered by three leading packing suppliers. Similar products are offered by other suppliers. Because products are continually being improved, users should contact suppliers for the most current information.

26

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Table 4-1 Valve Packing Products Commercial Products Survey

Products Offered by Major Suppliers

Product Type Alternative Products

Positive Features Negative Features Argo Chesterton Garlock

Die-Formed Graphite Packing Rings

Square or Rectangular Rings

Excellent sealing Low friction Low consolidation

Cost and increased inventory Tolerance sensitivity Increased installation effort

6300 J 5200 5300

GRAPH-LOCK Rings

Note: All die-formed rings are susceptible to extrusion

Wedge-Shaped Rings

Excellent sealing (low gland stress) Lowest friction (low gland stress) Low consolidation


5800 9000-EVSPincludes various end ring types

Graphite Yarn Ring Low cost Low halogen Not tolerance sensitive

Consolidation Increased friction Gland stress limited to 46 ksi

524 OneOne-CI

G-700

High-Density Die-Formed Graphite

Low friction Low consolidation


5600 GRAPH-LOCKRings

Anti-Extrusion Rings

Composite Ring Graphite/Resin

Lowest friction Low consolidation

High cost and increased inventory Tolerance sensitivity Increased installation effort

6000 N/A

Spacers and Bushings

Solid Graphite Rings Split for Assembly

Contribute to better seal Contribute to lower friction Bushings take side loads

Increased cost and inventory Increased installation effort Bushings tolerance sensitive

5005 std-density5010 high-density

5100 4525

Washers Graphite SheetRings

Low cost Aids in assembly Cushions bottom of spacer

Extra cost and inventory Increased installation effort

6200 N N/A

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Table 4-1 (cont.) Valve Packing Products Commercial Products Survey

Products Offered by Major Suppliers

Product Type Alternative Products

Positive Features Negative Features Argo Chesterton Garlock

Braided Packing Rings

Asbestos Yarn Graphite/Mica Filler

Not applicable since no longer being installed

Health hazard Hardening in service

N/A N/A N/A

Graphite Fiber Yarn Graphite BA/Lube

Low cost Low halogen Not tolerance sensitive

Increased friction w/o PTFE Gland stress limited to 46 ksi

7300 N/A

Graphite Fiber Yarn PTFE BA/Lube

Low cost Low friction Not tolerance sensitive

Small amounts of PTFE Gland stress limited to 46 ksi

525 G-200

Carbon Fiber Yarn Graphite BA/Lube


Increased friction w/o PTFE 526 N/A

Carbon Fiber Yarn PTFE BA/Lube


Small amounts of PTFE 5000 #98

Graphite Tape High Temp BA


Increased friction w/o PTFE Needs braid end ring for >1200 psi

7000 1400 GRAPH-LOCKTape

Graphite Tape Wire Reinforced


Increased friction w/o PTFE Potential leakage Wire reinforcement

7300-I 1601 13981399

Note: All braid-type packing is susceptible to consolidation

Graphite Tape Wire Reinforced PTFE BA/Lube


Small amounts of PTFE Wire reinforcement

7301-I 1600 1303-FEP

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4.5 Summary Valve packing has evolved over the years from braided asbestos

packing rings, to non-asbestos braid rings, to die-formed flexible graphite rings with braid ring backup, to fully engineered die-formed flexible graphite packing systems.

All of the available systems are currently in use for replacements except for asbestos braid rings.

The best approach for most nuclear plant valves is to use die-formed flexible graphite packing rings with appropriate (braided or composite) anti-extrusion rings.

It is not necessary to use a fully engineered die-formed flexible graphite packing system with composite anti-extrusion rings and live loading for all applications. In many cases, simple die-formed flexible graphite packing with braid type anti-extrusion rings will provide acceptable service.

A packing system can contain many parts including braid rings, die-formed flexible graphite rings, anti-extrusion rings, spacers, bushings, lantern rings, washers, junk rings and live loading. Selection of the best assembly of parts for given applications is described in Section 6 of this report.

A summary table of commercial products from three suppliers is included to illustrate the range of products offered. Users are encouraged to solicit up-to-date information from these and other suppliers regarding available products.

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5 CAUSES OF VALVE PACKING LEAKS

This section describes the main causes of leaks from valve packing and the measures that can be taken to mitigate these causes. 28

A common misconception is that the majority of packing leaks are caused by the material simply wearing out. This often leads to the conclusion that valve packing leakage is inevitable. However, experience with well-executed valve packing programs has demonstrated that properly packed valves do not leak and do not require frequent retorquing or repacking. It is, therefore, important for engineers and maintenance technicians to search for and to resolve the actual root causes of leaks rather than just to repack leaking valves using previously used methods.

5.1 Low Gland Stress 29 The gland stress must be high enough to create and maintain the seal under operating conditions. The required gland stress is described in Section 7. For most nuclear plant applications using die-formed flexible graphite packing rings, the gland preload stress should be 3,0004,000 psi (2128 MPa) to ensure proper initial consolidation and to provide margin for the inevitable loss in preload over time due to service-induced consolidation and wear.

Gland preload is typically applied by torquing gland nuts. However, the amount of load on the gland produced by the torque depends on factors including the applied torque, friction between the nut and gland stud threads, friction between the nut and follower, surface condition of the threads and the nut/follower interface, and the type of lubricant used. In this regard, gland bolting behaves essentially the same as flange bolting. Work in EPRI TR-111472, Assembling Bolted Connections Using Spiral-Wound Gaskets [11], another document in the Sealing Technology & Plant Leakage Reduction Series, shows that the actual bolt load can be as little as one-third of the desired load for a given torque depending on these factors. The actual load is rarely higher than the desired load.

The keys to achieving the desired gland preload stress are typically to have:

A suitably high target stress 5-1

EPRI Licensed Material Causes of Valve Packing Leaks

Clean studs/nuts Smooth contact surfaces between the nut and follower Good lubrication at the threads and between the nut and follower Proper consolidation as described in Sections 5.2 and 7.2.3

30 5.2 Packing Consolidation

By design, valve packing is compliant to allow it to fit to imperfections in the stem and stuffing box during assembly and to follow small displacements during valve stroking. Braid ring-type packing must typically be compressed 2550% by volume during assembly to close up voids. Figure 5-1 shows that typical 90 lb/ft3 (1,440 kg/m3) initial density die-formed graphite packing must be compressed 1520% by volume during assembly. 31

Figure 5-1 Consolidation of Die-Formed Flexible Graphite Packing [7]

If the packing were frictionless, a gland stress of 4,000 psi (28 MPa) would be sufficient to compress most packing materials to the desired density. However, as shown in Figure 5-2, friction between the packing, stem, and stuffing box can result in parts of the packing not being properly

5-2


Causes of Valve Packing Leaks

compressed. As shown in this figure, the top of the packing that is loaded by the gland is subjected to the desired compressive stress while the bottom of the packing has a lower compressive stress due to friction at the stem and stuffing box wall. During operation, thermal cycling and valve stroking will result in the decrease of compressive stress at the initially higher stress locations, and the increase of compressive stress at the initially lower stress locations. With time, the packing will shake down to an equilibrium stress that is less than the initial target compressive stress. It is said that the packing has consolidated in service and the resultant lower gland stresses may result in leakage.

32

33

Figure 5-2 Packing Consolidation [7]

Service-induced packing consolidation is obviously a more significant factor in applications involving:

Deep stuffing boxes Braided-type packing that requires greater initial compression to fill up

voids and has the potential for loss of blocking material over time

High-friction packing Rough stem and stuffing boxes surfaces that contribute to high friction The main solutions to service-induced consolidation are:

Use packing materials that require less consolidation (die-formed flexible graphite sealing rings with composite anti-extrusion rings).

Use lower-friction coefficient packing materials (PTFE-filled, anti-extrusion braid rings in lieu of unfilled graphite anti-extrusion braid rings where practical).

Reduce the packing height by using bushings or spacers, minimizing the number of seal and anti-extrusion rings, and using rectangular (reduced-height) die-formed flexible graphite packing rings.

5-3


Ensure that the valve stem and stuffing box finish are smooth so that unnecessary friction is not created.

Ensure that the bolting materials are in good condition and lubricated, and, preferably, include a hardened steel washer between the nut and follower so that the specified torque results in the desired gland preload force.

Repeatedly stroke the valve stem and retorque the gland nuts until there is no further gland nut rotation for the specified final torque. At this point, the packing should be uniformly compressed and there should be little relaxation of gland load during service. Figure 5-3 illustrates the beneficial effect of initial consolidation. This can require many cycles of stroking and torquing the gland nuts.

34

Figure 5-3 Effect of Consolidation on Gland Load [7]

Finally, for problem valves, valves that are in cyclic service, or critical valves that cannot be retorqued during plant operation, live loading (typically Belleville washers) can be used to maintain the desired compressive load on the packing and thereby compensate for service-induced consolidation or wear.

5-4



5.3 Dimensions and Clearances 35 Because packing is designed to be compliant and therefore able to accommodate imperfections in valve parts and small displacements during operation, excessive clearances can cause problems.

36

Excessive gaps between the stem and gland, gland and stuffing box, and between the stem and stuffing box can allow the packing material to extrude into the gaps between the mating parts, especially under cyclic motions. Extrusion can result in loss of gland preload stress, accelerated packing wear, or in wedging of the stem with resultant increased friction.

The MSS Standard Practices (SP) 120, Flexible Graphite Packing System for Rising Stem Steel Valves (Design Requirements) [12], provides suggested tolerances for each of the main parts that affect performance of valve packing. Figures 5-4 through 5-6 show suggested clearances and tolerances for the stuffing box, stem, and gland. If problems are encountered, the clearances and tolerances should be reviewed. Solutions include replacing out-of-spec parts or using an alternative packing system that better accommodates the as-found conditions. Packing suppliers should be consulted for the best solution for difficult or important cases.

37

38

39

Figure 5-4 Running Tolerances and Clearances (MSS SP-120) [12]

5-5


Figure 5-5 Stuffing Box Tolerances (MSS SP-120) [12]

Figure 5-6 Stem Tolerances (MSS SP-120) [12]

5.4 Stem and Stuffing Box Surface Finish

The stem surface must be smooth to create a good seal, to allow for proper initial packing consolidation, and to minimize packing wear and friction. Guidance regarding stem condition includes:

40

MSS SP-120 specifies that the stem surface should have a finish of 32 inch (0.81 meter) root mean square (RMS) or better and should be free of scratches, pits, or voids deeper than 0.002 in. (0.051 mm)

Union Carbide states that a surface finish of 32 inch (0.81 meter) RMS is acceptable for hand-operated valves but that a surface finish of 16 inch (0.41 meter) RMS is better for control valves [8]

5-6



Fisher Controls Monograph 38 reports that their control valve stems are polished to a surface finish of 4 inch (0.10 meter) RMS or better [13]

In summary, the stem surface finish is important, and smoother finishes are typically desirable for control valves that involve large numbers of operating stroke cycles. Surface finishes can be checked in the field using a comparator.

The stuffing box surface finish is also important in achieving proper packing consolidation. While not specified by MSS SP-120, a surface finish of 32 inch (0.81 meter) RMS should be achievable in the stuffing box. MSS SP-120 does specify that the stuffing box inside surface should be free of scratches, pits, or voids deeper than 0.006 in. (0.152 mm).

41

5.5 Stem and Stuffing Box Corrosion 42 Corrosion of the stem and stuffing box surfaces is detrimental to packing consolidation and service-induced wear. There is significant potential for corrosion using graphitic packing materials that are cathodic with respect to the metal parts and are electrically conductive.

MSS SP-120 specifies that flexible graphite packing shall contain a dispersed passive corrosion inhibitor, a dispersed embedded active corrosion inhibitor, or a suitable combination of corrosion inhibitors to minimize the potential for stem pitting. MSS SP-121, Qualification Testing Methods for Stem Packing for Rising Stem Steel Valves [14], specifies that corrosion tests shall be performed on packing systems. Figure 5-7 shows the specified corrosion test specimens. Test conditions and acceptance criteria are specified in MSS SP-121.

43

5-7


Figure 5-7 Stem Corrosion Test Fixture (MSS SP-121) [14]

Corrosion has been a significant concern with valves and valve parts stored in warehouses where there is moisture, or in locations where packing is wet due to water trapped during valve hydrostatic testing. Some utilities have avoided these conditions by removing the packing during storage. Installing new packing when the valve is placed in service will ensure that the new valve is packed to the latest plant standards.

44 5.6 Stem Misalignment

Several adverse conditions can occur if the valve stem is misaligned. These include:

Contact between the stem and gland can cause friction, wear, and degradation of the stem surface finish

Lateral loads on the packing can potentially cause corrosion, and under worst case conditions, open up a gap between the stem and packing

Wedging of the gland on the stem so that the gland force is not applied to the packing (this is often a problem for valves with short glands)

Typical effects of stem misalignment are illustrated in Figure 5-8. Stem misalignment can result from several causes including a bent stem, a misaligned stem, side loads on the stem (often occurs with valves mounted off-vertically), and the stuffing box walls not being perpendicular to the bonnet flange surface.

5-8



Figure 5-8 Typical Effects of Stem Misalignment [10]

45 In most cases, the solution to misalignment is to ensure that the parts (for example, stuffing box, stem, and gland) meet previously noted dimensional tolerances. It can be desirable to include graphite bushings at the top and bottom of the packing set as illustrated in Figure 5-9. The bushings act to keep the stem centered in the packing and to take side loads.

5-9


Figure 5-9 Use of Bushings to Mitigate Stem Misalignment

5.7 Stem Thermal Taper 46

Thermal expansion of valve stems can lead to packing leaks as illustrated in Figure 5-10. Specifically, the portion of the stem deep in the valve can operate at a higher temperature than the portion of the stem in contact with the packing. As the valve stem is retracted, the hotter portion of the stem, which will be slightly larger in diameter due to the higher temperature, is pulled up into the packing region where it expands the packing outward. This outward displacement of the packing produces further packing consolidation. When the valve stem cools, or the stem is stroked back into the valve, the packing might not follow the resultant reduction in stem diameter. This can produce a leak.

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5-10



Figure 5-10 Effect of Stem Thermal Taper on Packing Consolidation

There are several possible solutions to this problem. These include:

Use packing with a greater amount of compliance to accommodate small amounts of radial expansion. For example, use braided anti-extrusion rings rather than composite anti-extrusion rings.

Provide live loading (Belleville springs) that acts to keep the packing loaded at all times.

5.8 Product Misapplication 48 Many packing leaks result from misapplication of packing products. Sections 4 and 6 of this report provide information regarding the main types of products that are available and the best applications for these products. Several examples of problems that can result from misapplication are:

Virgin PTFE packing has a coefficient of thermal expansion more than six times that of typical stainless steel valve parts. As a result, the packing expands more than the valve body as the valve heats up causing high loads in the packing and possible extrusion through gaps between the gland and stem, gland and stuffing box, and between the stem and stuffing box. Extrusion can lead to loss of compressive load on the packing after cooldown and the loss of sealing function. This suggests that virgin PTFE should be limited to lower temperature applications.

5-11


Virgin PTFE has very little creep resistance at temperatures over 100F (37.78C) so that it is susceptible to extrusion between the stem and gland, gland and stuffing box, and stem and stuffing box. Microcellular or expanded microcellular PTFE has much better creep resistance than virgin PTFE.

PTFE begins to decompose at temperatures of approximately 600F (315.56C) as shown in Figure 5-11 [13]. Given the significant coefficient of thermal expansion and the potential for material degradation, pure PTFE packing should generally be limited to 200300F (93.33148.89C). However, higher allowable temperatures have been reported for packing involving PTFE as a blocking material in graphite braid packing. In these cases, the graphite braid resists the higher temperatures and the PTFE acts solely to block leakage and ensure low friction.

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PTFE materials are limited to an integrated radiation exposure of 104105 rads [9]. This factor eliminates PTFE packing from consideration at high radiation locations.

Graphite fibers tend to fracture at gland stresses of 4,0006,000 psi (2841 MPa). However, because most nuclear plant packing can be limited to 4,000 psi (28 MPa), this should not pose a practical concern. It is reported that carbon fibers can accommodate higher gland stresses without fracturing.

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Graphite bushings or spacers used to reduce packing height and to resist lateral stem loads are hard materials that can crack if not properly supported (see Figure 4-4). This is a special concern if the bottom surface of the stuffing box is not flat. Solutions for problems at this location include installation of: 1) a braid ring to cushion the spacer, 2) die-formed flexible graphite washers between the sp

Mejora en el desempeño de empaques para válvulas

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