NUREG/CR-5870

PNL-8051

Results of LWR

Snubber Aging Research

Prepared by

D. P. Brown/LEC

E. V. Werry, D. E. Blahnik/PNL

Lake Engineering Company

Pacific Northwest Laboratory

Prepared for

U.S. Nuclear Regulatory Commission
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NUREG/CR-5870

PNL-8051

RV, R9

Results of LWR

Snubber Aging Research

Manuscript Completed: March 1992

Date Published: May 1992

Prepared by

D. P. Brown, Lake Engineering Company

E. V. Werry, D. E. Blahnik, Pacific Northwest Laboratory

Lake Engineering Company

P.O. Box 296

10 Austin Avenue

Greenville, RI 02828

Under Contract to:

Pacific Northwest Laboratory

Richland, WA 99352

Prepared for

Division of Engineering

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555

NRC FIN B2911
Abstract

This report describes the aging research results and

recommendations for snubbers used in commercial

nuclear power plants. Snubbers are safety-related

devices used to restrain undesirable dynamic loads at

various piping and equipment locations in nuclear

power plants (NPPs). Each snubber must accommodate

a plant's normal thermal movements and must be capable of restraining the maximum off-normal dynamic

loads, such as a seismic event or a transient, postulated

for its specific location. The effects of snubber aging

and the factors that contribute to the degradation of

their safety performance need to be better understood.

Thus, Phase II of Nuclear Plant Aging Research was

conducted to enhance the understanding of snubber

aging and its consequences. Pacific Northwest Laboratory staff and their subcontractors, Lake Engineering

and Wyle Laboratories, visited eight sites (encompassing thirteen plants) to conduct interviews with NPP

staff and to collect data on snubber aging, testing, and

maintenance. The Phase II research methodology, evaluation, results, conclusions, and recommendations are

described in the report. Effective methods for servicelife monitoring of snubbers are included in the

recommendations.

iii NUREG/CR-5870
Contents

Page

Abstract .............................................................................. iii

Summary ........................................................................................ ix

A c k n o wl e d g me n ts...................................................xiii

1 Introduction ......... ; ................................................................. 1

2 Methodology of Snubber Aging Research ............ ............................................. 3

2.1 Methodology ............................................................................. 3

2.1.1 Key Site Selection . ................................................................... 3

2.1.2 In-Plant Research Methodology ....................................................... 3

3 Evaluation of the In-Plant Snubber Research ....................................................... 5

3.1 Review of Available Industry Service Data ................................................... 5

3.1.1 Snubber Performance Versus Failure Mechanisms

and Causes ..... -. . ............................ 5

3.1.2 Aging-Related Versus Nonaging-Related Failures ......................................... 6

3.1.3 Environmental Effects on Elastomeric Seal Degradation Rate ............................. 7

3.1.4 Seals and Leakage .................................................................... 7

3.1.5 Evaluation of Plant Service-Life Monitoring Methods .................................... 9

3.2 Evaluation of Hydraulic Snubber Aging ............................ 10

3.2.1 Aging Factors for Hydraulic Snubbers ........................... 10

3.2.2 l1ansient Loads ..................................................................... 12

3.2.3 Elastomeric Seal Life Evaluation Techniques ............................................ 12

3.2.4 Effects of Compression Set on Low-Pressure Elastomeric Seal Performance ..... ............ 14

3.2.5 Elastomeric Seals Most Affected by Aging . I 16

3.3 Evaluation of Mechanical Snubber Aging .................................................... 17

3.3.1 Aging Factors for Mechanical Snubbers ........................ ............................. 19

3.3.2 Evaluation of Changes in Drag Loads Versus Service Time ................................ 19

3.3.3 Dynamic1flansients .................................................................... 20

4 Conclusions .......................... 21

4.1 Snubber Performance History ......................... 21

v NUREG/CR-5870
Contents

4.2 Service-Life Monitoring ................................................................... 21

4.3 Effects of Compression Set on Low Pressure Elastomeric Seal Performance ...................... 23

4.4 Service Aging of Mechanical Snubbers ....................................................... 23

5 Recommendations ............................................................................. 25

5.1 Service-Life Monitoring Recommendations .................................................. 25

5.1.1 Determination of Snubber Failure or Degradation Causes ................................. 25

5.1.2 Determination and Documentation of Operating Environment ............................ 25

5.1.3 Managing Snubbers in Severe Operating Environments ................................... 27

5.1.4 Augmented Surveillance ................ ............................................. 27

5.1.5 'ftending ............................................................................ 27

5.1.6 'Testing ............................................................................. 28

5.1.7 Seal Leakage Detection and Leakage Rate Determination ................................ 31

5.1.8 Visual Examination .................... .............................................. 31

5.2 Recommendations for the Working Group on Mechanical Equipment Restraints ..... ............ 32

5.2.1 Service-Life Monitoring Recommendations .............................................. 32

5.2.2 Visual Examination Attributes ........................................................ 32

5.23 Failure Grouping and Corrective Action ................................................ 32

5.3 Uniform Snubber Population Classifications by Environment .................................. 33

5.4 Snubber Maintenance Recommendations .................................................... 33

6 Recommendations for Additional Snubber Research ........................................... 35

7 References ........ , 37

Appendix A. Visit Summary ....................................................................... A.1

Appendix B: In-situ Monitoring Methods and Equipment for Snubbers ................................. B.1

Appendix C: Typical Arrhenius Seal Life Extrapolation .............................................. C.1

Appendix D: Effects of Compression Set on Low Pressure Seal Performance ............................ D.1

Appendix E: Evaluation of Drag Force Versus Time Plant D ........................................... E.1

Appendix F: Evaluation of Drag Force Versus Time Plant F ........................................... F.1

Appendix G: Typical Mechanical Failure Causes Plant C .............................................. G.1

Appendix H: Typical Mechanical Snubber Examination Record ........................................ H.1

Appendix I: Sample Mechanical Snubber Failure Evaluation Report .................................. 1.1

Appendix J: Typical Visual Indicators of Snubber Degradation (Photographs) ........................... J.1

Appendix K Recommendations for Subsection ISTD ................................................ K 1

Appendix L: Snubber Maintenance Recommendations ............................................... L.1

NUREG/CR-5870

vi
Contents

Figures

2.1 Scope of snubber in-plant research at eight sites .................................................. 3

3.1 Mechanical snubber failures by category ......................................................... 6

3.2 Hydraulic snubber failures by category ........................................................... 6

3.3 Aging-related versus nonaging-related failures in mechanical snubbers .............................. 7

3.4 Aging-related versus nonaging-related failures in hydraulic snubbers ................................ 7

3.5 Leaking hydraulic snubbers by location in a BWR plant ........................................... 7

3.6 Aging versus nonaging-related seal leakage ...................................................... 8

3.7 Compression set extrapolation . ................................................................. 13

3.8 O-Ring ................................................................................... 15

3.9 O-Spring lip seal ............................................................................. 16

3.10 Quadra spring lip seal . ....................................................................... 16

3.11 U-Cup piston seal . ............................................................................ 16

3.12 Miller piston rod seal . ......................................................................... 16

3.13 Average drag force versus service time for 47 mechanical snubbers .............. .. .................. 20

5.1 Flow chart-typical root cause determination .................................................... 25

5.2 Mechanical snubber with normal drag force trace ................................................. 28

5.3 Mechanical snubber drag force with consistent spikes ............................................. 29

5.4 Mechanical snubber play force with noise ....................................................... 29

5.5 Mechanical snubber drag force with occasional spikes ............................................. 29

5.6 Mechanical snubber with high drag at one end ................................................... 29

Bibles

3.1 Mechanical snubber failures by category ......................................................... 6

3.2 Hydraulic snubber failures by category ........................................................... 6

3.3 Hydraulic snubber leaks versus plant location in a BWR plant ...................................... 7

3.4 Maintenance observations for 15 leaking snubbers at site A ........................................ 8

3.5 1ypical hydraulic snubber failure causes ......................................................... 11

3.6 Maximum measured compression set for various nonleaking seal configurations ...................... 15

3.7 '1ypical problem systems for mechanical snubbers ................................................. 17

3.8 Apical mechanical snubber failure causes ........................................................ 18

5.1 13 pical indicators of snubber degradation ........................................................ 26

vii NUREG/CR-5870
Summary

Snubbers are safety-related devices used to restrain

undesirable dynamic loads at various piping and equipment locations in nuclear power plants (NPPs). Snubber operability is mandated by the Code of Federal

Regulations (CFRs). The CFRs stipulate that systems,

structures, and components (SSCs), e.g., snubbers, shall

be designed to withstand the effects of normal and offnormal dynamic phenomena.l In the mid 1980s, the

U.S. Nuclear Regulatory Commission (NRC) recognized the need to enhance snubber performance

through aging studies and improved service-life monitoring techniques. The NRCs Nuclear Plant Aging

Research (NPAR) Program Plan provided the vehicle

and the logical sponsorship to undertake preliminary

investigations into snubber performance and aging.

Pacific Northwest Laboratory (PNL) and its subcontractors, Lake Engineering (Greenville, Rhode Island) and

Wyle Laboratories (Huntsville, Alabama), performed

the snubber research.

This report describes the Phase II NPAR in-plant aging

research conducted to enhance the understanding of

snubber aging and its consequences. The in-plant aging

research was based on a research plan by Brown et al.2,

which clarified the relationship between snubber aging

and snubber degradation and identified additional information on aging hydraulic and mechanical snubbers that

requires further investigation and analysis.

This report presents snubber aging research, testing and

failure data, and service-life monitoring recommendations that distinguish between aging- and nonagingrelated snubber failures. The graphics, tables, and

supporting text illustrate this distinction. The report

supports the perspective that snubber failures are

closely related to age-related degradation caused by

'Normal dynamic reactions are those associated with thermal expansion and contraction of plant systems during normal startups or

shutdowns. Off- normal dynamic reactions involve loads not associated with normal operations such as postulated seismic events.

2Brown, D. P., G. R. Palmer, E. V Werry, and D. E. Blahnik. 1990.

BasisforSnubberAgingResearch NuclearPlantAgingResearch

Progran. NUREG/CR-5386 (PNL-6911), prepared for the U.S.

Nuclear Regulatory Commission by Pacific Northwest Laboratory,

Richland, Washington.

inservice operational environmental influences, e.g.,

vibration and elevated temperature. Due to the lack of

service-related information pertaining to mechanical

snubbers, special emphasis was placed on gathering such

information for these devices.

The following objectives were developed for the aging

investigation of mechanical and hydraulic snubbers:

* enhance the understanding of how snubbers

degrade due to aging

* enhance the understanding of snubber failure

characteristics

* determine the technical information needed to

improve the level of snubber performance.

'lb meet these objectives, in-plant research was undertaken with cooperating nuclear utilities. Lao

information-gathering methods were used during the

research. The first method included interviews with

plant maintenance and engineering staff. The second

involved analysis of plant operating data, including

maintenance records and inservice testing and examination records. Plant selection was based on several factors, including availability of staff, plant procedures,

snubber types and length of service, and plant types

(BWR and PWR). It should be noted that the in-plant

research was conducted on generic types of snubbers, i.e.

acceleration-limiting mechanicals and lockup/bleed

hydraulics. These generic types constitute a majority of

snubbers installed in U.S. NPPs.

Thirteen plants at eight different sites were visited during a three-month interval. Snubbers used at five of the

sites were primarily mechanical; snubbers at the remaining three sites were primarily hydraulic. In addition to

the site visits, over 70 telephone interviews were conducted with knowledgeable staff at NPPs throughout the

U.S. Snubber "hands-on' research was also conducted at

Lake Engineering's facilities; this work involved the

disassembly, examination, and measurement of mating

parts associated with hydraulic snubber seals.

ix NUREG/CR-5870
Summary

A primary goal of the site visits was to gather information that would fulfill the objectives. The first two

objectives were met through an assessment and evaluation of recent snubber performance history at the sites

visited. By distinguishing between snubber failures

related to service (aging) and failures related to nonservice (nonaging) causes, it was concluded that

approximately half of all snubber failures may be attributed to service-related influences. Service-related

failures are defined as those due to environmental influences, transients, and vibration; nonservice-related failures are defined as those due to other influences such as

installation damage, maintenance deficiencies, manufacturing defects, not related to service time.

All of the environmental influences, including elevated

temperature, vibration, and moisture, can degrade the

performance of mechanical snubbers by increasing drag

and breakaway forces and by changing the activation

acceleration thresholds. Data in one plant indicated an

increasing trend in mechanical snubber drag force with

service time. For hydraulic snubbers, high temperatures

in isolated operating areas can rapidly degrade seal

performance. Radiation probably contributes less significantly to aging than was originally hypothesized.

The research indicates that fluid leakage in hydraulic

snubbers is commonly associated with leaking hydraulic

fittings; however, it was not determined precisely what

percentage of the fitting leakage is caused by this service

environment. Furthermore, the research indicates that

a significant number of seal leaks are attributed to

short-term degradation in high temperature applications. At one BWR plant, the incidence of seal leakage

was higher at elevated temperatures in the drywell than

in other areas of the plant. This finding supports the

premise that seal degradation can be accelerated by

exposure to higher temperatures.

The following recommendations for service-life monitoring guidelines were developed as a result of the inplant research:

It is important to distinguish between servicerelatcd and nonservice-related failures. For this

NUREG/CR-5870

reason, the root cause of snubber failure or degradation should be determined. Diagnostic testing is

useful for this purpose as well as visual evaluation,

particularly during snubber disassembly. Personnel

training in these activities is also recommended.

* Because plant operating environments may differ

from design specifications, general area and

environmental conditions should be monitored.

Depending on the range of environmental stressors

in the plant, it may be practical to establish more

than one service-life population.

* Snubber applications (locations) involving specific

severe environmental influences (e.g., high temperatures, high amplitude vibration) should be isolated and assessed on a case-by-case basis. Such

applications may require in situ monitoring, frequent surveillance, maintenance or snubber

replacement.

* Service life for the general snubber population (i.e.,

snubbers in a moderate environment that are not

subject to short-term degradation) should be established by trending relevant degradation parameters.

Because the primary failure mechanism of concern

for seals is low pressure leakage, snubber seal life

should be primarily based on predicted low pressure

seal performance. Baseline data is essential for

trending.

* "Hands-on' evaluation methods, such as hand stroking, are useful in identifying potential snubber

degradation, particularly degradation caused by

dynamic load transients.

* Evaluation of test parameter time traces obtained

during routine functional tests is useful in identifying performance anomalies that may be indicators

of snubber degradation.

* Test machines used for trending and for diagnostic

tests should be capable of providing a time trace of

load and velocity. Important criteria for test

x
Summary

machines used for trending are accuracy and repeatability. Another important feature, especially for

diagnostic testing, is the ability of a test machine to

vary the magnitude of test parameters such as velocity and load.

Service-life projections based on data from snubbers

exposed to the actual plant operating environment

are generally preferable to analytical service-life

projections.

Overall, the nuclear industry is making progress in snubber inspection, testing, maintenance, and reduction programs. These activities provide a basis for an improved

understanding of snubber performance. Plant staff have

identified severe environments and-have modified the

environment or replaced snubbers with more durable

models. Additionally, more effective and realistic

functional test acceptance limits have been developed to

significantly reduce failure rates. Although many plants

routinely evaluate snubbers for failure causes, the

research, indicates that many plants have yet to

implement formal service-life monitoring programs.

xi NUREG/CR-5870
Acknowledgments

The authors of this report acknowledge the support and

technical guidance of J. P. Vora and J. J. Burns, Jr., of

the Nuclear Regulatory Commission during the course

of this research under the Nuclear Plant Aging Research

(NPAR) Program. We extend our appreciation the

Snubber Utility Group (SNUG) and the ASME Opeiation and Maintenance (ISTD) Working Group on

Mechanical Equipment Restraints for their cooperation

and support throughout the work and for their technical

comments on the research results. We also wish to

thank the nuclear power plant industry, especially those

utilities that provided information and/or participated in

the in-plant research visitations. A special thanks goes

to Glen Palmer, Siemens Nuclear Power Service, Inc.,

for his technical comments and input to this work and

his long-term contributions to snubber research through

his Phase II research efforts and association with the

ASME ISTD Working Group. We also thank M. H.

Kimel, of Wyle Laboratories, who provided strong program management support for the in-plant research and

Mr. John Mucci, of Wyle Laboratories, for his productive site interviews and data collection efforts. We

are grateful for the contributions from the following

staff members of Pacific Northwest Laboratory: A. B.

Johnson, Jr., for Project Management support and technical guidance; S. H. Bush, for his timely peer review

and assistance throughout the research effort; and R. C.

Pedersen for his propitious technical editing of the

report.

xiii NUREG/CR-5870
Definitions

Activation:

Aging:

Aging management:

Aging mechanism:

As-found testing:

Bleed rate:

Breakaway force:

Compression set:

Degradation:

Degradation cause:

Degradation mechanism:

Degradation mode:

Diagnostic testing:

Drag force:

Dynamic seal:

Examination:

External seal:

The change of conditions from passive to active, in which the snubber resists the rapid

displacement of the attached pipe or component

Showing the effects of time or use in the physical characteristics of a snubber.

Engineering, operations, and maintenance activities to control aging degradation and

failures due to aging of snubbers to within acceptable limits.

Process that gradually changes the physical characteristics of a snubber with time or

use.

Testing before conducting any activity that could affect test results (usually applies to

snubber testing after removal of a snubber from service, but before any maintenance

activities are conducted).

(See 'Release rate.')

The minimum applied force required to initiate extension or retraction of the

snubber.

The amount of permanent deformation of a seal expressed as a percentage of the

initial seal deflection.

Immediate or gradual deterioration in the physical characteristics of a snubber, which

could impair performance of any of its design functions.

The circumstances during design, manufacture, or use that have led to degradation.

Physical process that results in degradation.

The manner or state in which a snubber degrades.

Testing to determine the cause or mechanism associated with degradation or failure.

The force required to maintain snubber movement at a low velocity before activation.

A seal used where there is relative motion between the seal and its mating surface.

Visual observation for detecting of improper installation and impaired functional

ability caused by physical damage, leakage, corrosion, or degradation from environmental or operating conditions.

A seal used to isolate the hydraulic system from the surrounding environment.

xv NUREG/CR-5870
Definitions

Failure:

Failure analysis:

Failure mechanism:

Failure mode:

Failure mode group (FMG):

High-amplitude vibration:

High-pressure seal:

Hydraulic snubber:

Inspection:

Internal seal:

Locking velocity:

Low-amplitude vibration:

Low-pressure leakage:

Low-pressure seal:

Mechanical snubber:

Normal operating conditions:

Inability or interruption of ability of a snubber to perform its design function within

acceptance criteria.

Systematic process of determining and documenting the mode, mechanism, causes,

and root cause of the failure of a snubber.

Physical process that results in a failure.

The manner or state in which a snubber fails.

A group of snubbers that have failed and those other snubbers that have potential for

similar failure.

Vibration having an amplitude greater than the mechanical clearances in the

snubber's end attachments.

A seal that provides a sealing function under high-pressure conditions (i.e., greater

than 100 psi).

A restraining device in which load is transmitted through a hydraulic fluid.

Observation or measurement to verify that the physical characteristics of a snubber

conform to acceptance criteria.

A seal used to isolate high-pressure and low-pressure chambers in the snubber.

The extension or retraction velocity of the snubber piston rod at which the control

valve locks, no longer allowing free motion.

Vibration having an amplitude less than the mechanical clearances in the snubber's

end attachments.

Seal leakage when the snubber is not activated (i.e., when the snubber is unlocked and

not pressurized).

A seal that functions under low-pressure conditions (i.e., less than 100 psi).

A mechanical device designed to restrain piping or equipment during abnormal

accelerations and to allow free thermal movement under normal operating conditions.

Operating conditions during reactor startup, operating at power, hot standby, reactor

cooldown, and cold shutdown.

NUREGICR-5870 xvi
Definitions

Position setting:

Post-maintenance testing:

Predictive maintenance:

Preventive maintenance:

Release rate:

Root cause:

Seal life:

Service life:

Service life population:

Static seal:

Stressors:

Surveillance:

Synergistic effects:

Tfending:

Measurement that indicates stroke location as measured from the snubber's fully

retracted position.

Testing after maintenance for component functionality and for verifying correct

maintenance.

A form of preventive maintenance performed periodically or continuously to monitor,

inspect, test, diagnose, or trend a snubber's performance or condition indicators;

results indicate or forecast functional ability or the nature and schedule of planned

maintenance prior to failure.

Periodic, predictive, or planned maintenance performed before failure of a snubber in

order to extend its service life.

The rate of the axial snubber movement under a specified load after the snubber is

activated.

The fundamental reason(s) for an observed condition, which when corrected prevents

its recurrence.

The amount of time that a seal is allowed to remain in service without replacement.

Seal life begins at the time that the seal is installed and continues for a pre-established

period based upon expected performance.

Period from initial operation of a snubber to retirement or overhaul.

A population of snubbers having the same service life.

A seal application where there is no relative motion between the seal and its mating

surface.

Factors that promote degradation.

Observation or measurement of the performance or physical characteristics of a

snubber to verify that it conforms to acceptance criteria.

Changes in the physical properties of a snubber or a subcomponent caused by two or

more stressors interacting so that the total change is different from the changes

caused by each stressor acting independently.

Recording and analyzing in service data with respect to some independent parameter

(usually time or cycles).

xvii NUREG/CR-5870
1 Introduction

Mechanical and hydraulic snubbers are safety-related

devices designed to restrain undesirable dynamic loads

at various piping and equipment locations in nuclear

power plants (NPPs). Snubber operability in Nuclear

Power Plants (NPPs) is mandated by Title 10, Part 50 of

the Code of Federal Regulations (CFRs), Appendix A

and Appendix B. These regulations stipulate that

systems, structures, and components (SSCs), which

includes snubbers, shall be designed to withstand the

effects of normal and off-normal dynamic phenomena.

Each snubber must accommodate normal thermal

movements of plant piping or equipment and be capable

of restraining the maximum off-normal dynamic loads

postulated for its specific location. However, snubbers

are subject to the effects of aging, and the factors that

degrade their safety performance need to be better

understood. This report describes the Phase II NPAR

in-plant aging research conducted to enhance the understanding of snubber aging and to mitigate aging effects.

In the mid 1980s, the U.S. Nuclear Regulatory Commission (NRC) recognized the need to enhance snubber

performance through aging studies and improved

service-life monitoring techniques. The NRC's Nuclear

Plant Aging Research (NPAR) Program Plan, Rev. l,

(USNRC 1987)2 provided the vehicle and the logical

sponsorship to undertake preliminary investigations

into snubber performance and aging.

The NPAR Program Strategy has traditionally specified

a two-phase approach. Phase I of the NPAR snubber

research was undertaken by the Pacific Northwest

Laboratory3 in 1985 and resulted in an initial aging

assessment of snubbers and a snubber reduction evaluation study by Bush et al. (1986). The Phase II snubber

research was conducted by PNL with support from Lake

Engineering (Greenville, Rhode Island) and Wyle

'Normal dynamic reactions are those associated with thermal expansion and contraction of plant systems during normal startups or shutdowns. Off normal dynamic reactions involve loads not associated with

normal operations such as postulated seismic events.

2

The NRC's initial version of the NPAR program plan was issued in

July of 1985.

3Pacific Northwest Laboratory is operated by Battelle Memorial

Institute for the U.S. Department of Energy under Contract

DE-AC06-76RL0 1830.

Laboratories (Huntsville, Alabama). The interim

Phase II study by Brown et al. (1990) resulted in preliminary investigations that further clarified the aging questions and identified additional information on aging that

should be assembled and analyzed for both hydraulic

and mechanical snubbers. Twenty-four nuclear power

utilities were also surveyed for information on snubber

operating experiences.

The preliminary Phase II investigations defined the

snubber in-plant research scope and determined that a

special research emphasis should be placed on mechanical snubbers. The following objectives provide the

scope of the Phase II research described in this report:

* determine how snubbers age and degrade

* define snubber failure characteristics

* determine the technical information needed to

improve snubber service-life performance.

'lb meet these objectives the in-plant research involved:

* reviewing existing service data

* evaluating the effects of compression sct of

hydraulic snubber seals

* developing service-life monitoring guidelines

* improving the understanding of aging in mechanical

snubbers.

The research staff planned and conducted in-plant

research that involved selecting the sites for research,

visiting the sites, and gathering and analyzing data on

snubber performance.

The methodology of the snubber in-plant research is

described in Section 2.0. Sections 3.0,4.0, and 5.0 discuss the results, conclusions, and recommendations

from the in-plant research, respectively. Section 6.0

includes recommendations for additional snubber

research. In addition to supplementing investigations by

1 NUREG/CR-5870
Introduction

Brown et al. (1990), the research findings presented in

this report support key elements of the NPAR program

strategy, including dissemination of technical information, recommendations to improve applicable codes and

standards, guidelines for service-life predictions, and

liaison with industry and NRC staff. Additionally, the

report supports the assumption that snubber failures are

closely related to aging degradation caused by operational environmental influences, e.g., vibration and

elevated temperature.

NUREG/CR-5870 2
2 Methodology of Snubber Aging Research

This section describes the site selection process and the

in-plant research methodology used to determine the

effects of aging on snubber performance.

2.1 Methodology

- testing

- snubber tracking

- snubber trending

Plant selection and the method of on-site visits are discussed in the following sections.

2.1.1 Key Site Selection

Selecting the cooperative key sites was an important

preliminary step of the in-plant research. Key sites are

defined as those plants that participated directly in the

research by providing their facilities and making available appropriate engineering and maintenance staff for

on-site interviews by NPAR staff and subcontractors.

Plants with effective inspection and testing programs

and staff with experience in root cause analysis were

primary candidates.

The following criteria were applied in the site selection

process:

* willingness to participate

* experience in utilizing in situ environmental

monitoring instrumentation.

2.1.2 In-Plant Research Methodology

After the key sites were selected, site visits were planned

and scheduled. 'Technical staff at each plant devoted

two to three days to assist in gathering relevant information during the site visits, and extensive snubber

documentation was made available for review by the

in-plant investigators. A total of thirteen plants at eight

sites (A through G) were visited during a three-month

interval (see Figure 2.1). Five sites were designated as

key sites in evaluating mechanical snubbers. Of these,

three (six plants) are of PWR design and two (four

plants) are of BWR design. lvo sites were designated

as key sites in evaluating hydraulic snubber aging. Both

of these sites (4 plants) are BWR design. One site was

visited for information pertaining to in situ monitoring

of environmental stressors; the plant is a BWR design.

* snubber type, i.e., mechanical or hydraulic

* plant type, i.e., BWR or PWR

* snubber service time

* knowledge and/or available information in the

following areas:

- failure evaluation

- root cause evaluation

- identification of operating environment and

measurement of the level of environmental

stressors

Hydraul c

Snubbers

SiteA

awn

2 Plants

I wn tl~~~LJ

In Situ Monitoring

of Environmental

Stressors

Site at

I Plant

.22 11 Imo!toe]

Figure 2.1 Scope of Snubber In-plant Research at

Eight Sites (A through G)

- effects of environments on snubber

performance

3 NUREG/CR-5870
Methodology

The research methodology used during the site visits

consisted principally of the following:

* on-site interviews with knowledgeable plant

personnel

* review of plant operating history

- snubber tracking databases

- failure evaluation reports

- functional test data

- maintenance practices.

Other methods used during the in-plant research

consisted of the following:

* approximately 70 telephone contacts made with

various operating plant personnel

* specific research involving snubber disassembly,

examination, and measurement of parts. The activity occurred both on-site and off-site. (This activity

was conducted at Lake Engineering Company's,

Rhode Island, facility for the evaluation of compression set of hydraulic snubber seals. This work is discussed further in Section 3.2.3 of this report.)

* use of 'in-house" information available to Wyle

Laboratories, Lake Engineering Company and PNL.

(Personnel from Wyle Laboratories and Lake Engineering Company have many years of experience

working with NPP snubbers and supporting

equipment.)

NUREG/CR-5870 4
3 Evaluation of the In-Plant Snubber Research

In this section, the research data on snubber

performance and failure causes and mechanisms is

evaluated.

3.1 Review of Available Industry

Service Data

A key goal of the plant visits was to evaluate the recent

performance history for both mechanical and hydraulic

snubbers in NPP service. Data on the number of

recorded failures and data on the types of failures and

degradation was also reviewed. An important distinction was made between data on aging-related and

nonaging-related failures; this is addressed in Section 3.1.2. Summary reports for all site visits are

included in Appendix A.

During the site visits, information was obtained from

plant operating records. The Snubber Utility Group

(SNUG) databases and 'in-house" databases were also

reviewed. It should be noted that the majority of

failures for both mechanical and hydraulic snubbers

have been identified by functional testing. The number

of failures found by visual examination, on the other

hand, has been minimal.

From the number of failures evaluated during the site

visits, some correlations can be made between failure

mechanisms and failure causes under comparable service conditions. However, significant differences in plant

design, the lack of precise time-based environmental

data and the effects of more than one environmental

stressor often prevent the use of snubber aging data on a

generic basis. This supports the need for some degree of

plant-specific service-life monitoring.

It should be noted here that approximately 95% of all

mechanical snubbers in NPP service are the acceleration

limiting type; most of the remaining 5% incorporate a

'The first compilation of data on snubber inspections and tests

covering the period 1976 to 1988, issued by SNUG, was made available

by the Nuclear Management and Resources Council (NUMARC) to

the NRC and PNL in January 1989. An internal, unpublished, review

of the data was provided by PNL staff in September of 1989.

velocity limiting feature. Approximately 99% of all

hydraulic snubbers in NPP service utilize a dual mode,

lock-up/bleed type control valve; the remaining 1%

utilize a single mode, velocity limiting design. The aging

research primarily focused on the aging characteristics

of acceleration limiting mechanical snubbers and lockup/bleed type hydraulic snubbers. Much of the information, however, obtained in the research would also be

expected to be characteristic of the remaining snubber

types due to similarities of design features, e.g., ball nut

and screw for mechanicals, seals for hydraulics, and

common environmental stressors.

3.1.1 Snubber Performance Versus Failure

Mechanisms and Causes

Snubber failure causes may be generally categorized into

five groups:

* Deficiencies in installation, handling, and

maintenance

* Environmental influences (e.g., elevated

temperature, moisture, etc.)

* Tfansients (overloading)

* Vibration

* Design or manufacturing deficiencies.

3.1.1.1 Mechanical Snubbers

'Table 3.1 lists by category the number and causes of 357

mechanical snubber failures reported by Sites C, D, E

and F Data supporting these failure causes were

obtained during plant site visits. Thble 3.1 is graphically

illustrated in Figure 3.1. A more detailed discussion of

the effects of aging on mechanical snubbers is provided

in Section 3.3.

3.1.1.2 Hydraulic Snubbers

Of a total of 86 reported hydraulic snubber failures from

plant Sites A and B, the number of failures is listed by

5 NUREG/CR-5870
Evaluation

Table 3.1 Mechanical snubber failures by category Table 3.2 Hydraulic snubber failures by category

Number

of

Category Failures

Installation/mishandling/maintenance 143

Environment 59

Iransients and vibration 94

Manufacturing Defects 52

Unknown 9

Total Number of Failures 357

Number

of

Category Failures

Installation/mishandling/maintenance 16

Environment 26

Transients and vibration 18

Manufacturing Defects 4

Unknown 22

Total Number of Failures 86

140-.

l~Mub, 120

of lot,-

Fniures so

20so

T -40

%of Few..re

14(26%)

ILL'7%1 -20 32(15%)5(1%

-10

6(3%)

i7 - a~~~~~~~~~

2

2&m~

Hunwb' ofu I

0

26 (30%)

5 22(26%)

16(19%) 62%

S.

4(5%)

I0. I - - 0. -

-3M

-20

%of

Failures

-10

4

- UtshtanL. Mg. E nv~ron. VW J Unailovra Defeacts Trans.

Nonaging now"ta Aging Related ?

UW2 S

-an.. an5. Detects

Mnarging Rehated

tTCHnfr. TICS

Trans.

Aging Related

unknewn

A. , S

Figure 3.1 Mechanical snubber failures by category

failure cause category in 'Thble 3.2. This is graphically

illustrated in Figure 3.2.

3.1.2 Aging-Related Versus Nonaging-Related

Failures

As shown in Figure 3.1, for mechanical snubbers,

59 failures were attributed to the environment category

and 94 were attributed to the transients and vibration

category. Thus, approximately 43% (153 out of 357) of

the failures were associated with actual plant service and

are classified as aging-related failures (see Figure 3.3).

Figure 3.2 Hydraulic snubber failures by category

As shown in Figure 3.2, for hydraulic snubbers, 26 failures were attributed to the environment category and

18 were attributed to the transients and vibration category. Plant service influences, therefore, accounted for

approximately 51% of the total number of aging-related

failures (see Figure 3.4).

Note: Data for both hydraulic and mechanical snubbers

reflects failures identified between 1984 and 1990.

The majority of mechanical snubber data reflect

failures identified during the first three orfour

refueling outages. The hydraulic snubber data

reflect failures identified between the tenth and

fifteenth years ofplant operation.

NUREG/CR-5870 6
Evaluation

Table 3.3 Hydraulic snubber leaks versus plant

location in a BWR plant

43% Aging Relaled

55% Managling Related

'Ibtal No. of % of

Snubbers Indicated Indicated

Area Inspected Leaks Leaks

Drywell 263 39 14.8

Remainder of 287 27 9.4

the Plant

2% Unknow

Figure 3.3 Aging-related versus nonaging-related

failures in mechanical snubbers

26% Unknown 23% Nonaging Related

Percentage

of Leaking

Snubbers

1 6.

14.

12.

1 O-'

8 .

6 .

4-'

51% Aging Related 2 L

Figure 3.4 Aging-related versus nonaging-related

failures in hydraulic snubbers

3.1.3 Environmental Effects on Elastomeric

Seal Degradation Rate

The number of leaking snubbers noted in the drywell

during a recent visual examination for one plant at

Site A was compared with those found in the remainder

of the plant. The results are tabulated in Table 3.3. As

indicated in the data, the incidence of leakage was significantly greater for those snubbers installed in the drywell (see Figure 3.5). This indicates that, at least for this

plant, monitoring a separate population (by plant area)

for purposes of establishing seal life may be practical.

3.1.4 Seals and Leakage

The following subsections discuss maintenance practices

and aging phenomena that are associated with hydraulic

fluid leakage.

Iu . L , -, I

Drywell Balance of Plant

Snubber Location

Figure 3.5 Leaking hydraulic snubbers by location in a

BWR plant

3.1.4.1 Aging Versus Non-Aging Related Seal

Degradation

Most cases of hydraulic snubber seal leakage are not

directly attributable to long-term environmental effects.

This conclusion is based, in part, upon data collected in

the research as well as discussions with plant personnel

and the field experience of the authors. This is illustrated by the maintenance observation data presented in

'Tble 3.4. 'TIble 3.4 indicates that from a total of

15 hydraulic snubbers at Site A removed from service

because of significant leakage, there was very little

evidence of seal degradation due to aging (see Figure 3.6). However, most operating plants have identified a limited number of snubbers in isolated areas

(generally involving high temperature) where seal

7 NUREG/CR-5870
Evaluation

Table 3.4 Maintenance observations for 15 leaking

snubbers at Site A

1337 Aging Related

-

Snubber

1

2

Observations

Finger-tight fill plug. No thread sealant.

Elevated temperature affected paint. No

thread sealant. 1/2-diameter tear in the

accumulator piston seal. Particles in the

fluid.

3 No thread sealant. Loose fill plug.

Thread damage. Pinched O-Ring in the

main cylinder head (gland area). Residue

from pinched O-Ring on the main cylinder. Also, residue on cylinder head.

4 No thread sealant. Main cylinder O-Ring

pinched on gland end. Seal was cut during

assembly. Thread damage at fill plug. Fill

plug tight.

5

6

7

No thread sealant. Discoloration of fluid.

Loose fill plug. No thread sealant.

No evidence of reason for fluid loss.

(Bergen original).

86.7% mbonaging Related

Figure 3.6 Aging versus nonaging-related seal leakage

degradation has resulted in leakage in a relatively short

period (1 to 2 operating intervals). Management of

these snubbers generally involves augmented inspections and/or frequent seal replacements.

As noted in Section 3.1.3, a comparison of the

percentage of leaking snubbers observed in the drywell

for Site A with the percentage of leaking snubbers for

the balance of plant indicates a higher incidence of

leakage in the drywell (see Tible 3.3). Operating

temperatures in the upper levels of the drywell for this

plant are known to have exceeded 2200F; short-term

degradation of ethylene propylene (less than two years)

can be expected under such conditions. Operating

temperatures for most plants, on the other hand, are

significantly less than this, i.e., 150'F or less. The higher

incidence of leakage for the Site A drywell snubbers is

probably the result of a combination of seal degradation

and a lower fluid viscosity at elevated temperatures.

A number of plants have implemented programs for

monitoring elastomeric seal degradation in the general

snubber population. Such programs can extend or

reduce seal life based on operating experience. They are

described in more detail in Section 3.2.3 of this report.

3.1.4.2 Thread Seals

8 Loose fill plug. No thread sealant. Main

piston backing ring slight wear.

9 No thread sealant. Loose fill plug.

Thread damage accumulator head.

Pinched main cylinder seal.

10

11

12

13

14

Loose fill plug.

Poor O-Ring installation in accumulator

cap/accumulator cylinder. Loose fill plug.

Loose fill plug. Damaged fill plug threads.

'Ibrn accumulator piston seal.

Loose fill plug. Wear on fill plug hold

threads.

15 No thread sealant. Fill plug tight. Thread seals used with the control valve screws for some

hydraulic snubbers have commonly exhibited low-level

fluid leakage that is generally not sufficient to render

the snubber inoperable between refueling outages.

NUREG/CR-5870 8
Evaluation

It should be noted, however, that, despite the susceptibility of these seals to aging degradation, thread seal

leakage is often the result of seal damage due to

improper installation of the seals or other nonservicerelated degradation. For example, installing snubbers

without using washers between the seal and lock nut

invariably results in damage to the seal.

3.1.4.3 Hydraulic Fitting Leakage

A significant portion of the total number of hydraulic

snubber leakage problems, for the nuclear industry in

general, has been associated with leaking hydraulic

fittings. These fittings are used to connect the control

valve to the hydraulic cylinder and to connect the

reservoir to the control valve.

It is not possible to determine precisely what percentage

of the leaking fittings is directly the result of the service

environment. However, it should be noted that such

fittings are highly susceptible to leakage due to misalignment, damage, and abuse. The SNUG database' indicates that from a total of 247 hydraulic snubbers that

were found to be leaking due to either seal or fitting

leakage, 157 of these (64%) were the result of fitting

leakage.

3.1.5 Evaluation of Plant Service-Life

Monitoring Methods

The NRC Standard Technical Specifications (STS)

(USNRC 1984) require that plants maintain a servicelife monitoring program. The Working Group on

Mechanical Equipment Restraints is developing servicelife monitoring requirements that will likely be included

in Subsection ISTD of the Operations and Maintenance

(OM) Code. A non-mandatory appendix that will provide some guide lines for service-life monitoring is also

being developed by the Working Group.

Formal service-life monitoring programs have not been

established at all NPPs. For some plants, service-life

'The first compilation of data, covering the period 1976 to 1988, on

snubber inspections and tests issued by SNUG, was made available by

the Nuclear Management and Resources Council (NUMARC) to the

NRC and PNL in January 1989. An internal, unpublished, review of

the data was provided by PNL staff in September 1989.

monitoring involves ensuring that only seals and fluid

are replaced in hydraulic snubbers at prescribed intervals. However, it should be noted that most plants have

implemented maintenance practices that have helped to

identify practical methods for a service-life monitoring

program.

3.1.5.1 Realistic Determination of Snubber Failure or

Degradation Causes

In many cases, determining the cause of snubber failure

has been a relatively simple task. In other cases, it has

been impossible to determine the actual cause of failure

or degradation. From interviews with plant personnel

and review of failure evaluation data, it is apparent that

the experience and judgement of personnel involved in

failure evaluations have been critical in identifying

realistic failure causes. In some cases, inspectors have

erroneously identified the failure cause as a result of

either the lack of an in-depth evaluation or inadequate

training.

3.1.5.2 Determination and Documentation of Snubber

Operating Environment

Plants have used a variety of methods to determine the

operating environment. For some plants the defined

snubber operating environment is based on the maximum value of various environmental parameters

described in the plant design specification. However,

the research has determined that, in isolated applications, environmental parameter values can exceed specified design levels. Such applications are generally

plant specific and are often identified by monitoring

snubber degradation.

In many cases, severe operating environments that were

previously unidentified were brought to light by characteristics noted during visual examination, snubber

overhaul, or during failure evaluation. Examples of

such characteristics and the environments to which they

relate are discussed in TIble 5.1.

In some cases, various types of measuring instruments

(see Appendix B) have been used to define the environment precisely. However, such equipment is generally

limited to applications where moderate to severe

9 NUREG/CR-5870
Evaluation

environments are anticipated or where the equipment is

used as a diagnostic aid to identify the cause of service

degradation.

Some plants have obtained localized area temperature

data, for example, at various levels in the drywell, using

portable temperature monitoring devices. In cases of

rapid snubber degradation resulting from high temperatures, precise temperature information has been limited.

In general, more precise information is needed in this

regard to establish practical temperature-time limits.

Area radiation information is typically available from a

plant's radiation protection department. Since there is

little documented evidence of degradation due to radiation (see Section 3.2.1.1), radiation monitoring of specific snubber locations is not common.

Most plants have identified the existence of high amplitude vibration (see Appendix B) from information

obtained during visual examination, testing, or failure

evaluation. Metal filings, darkened hydraulic fluid,

deformed connecting pins, elongated attachment holes,

and fretting of mating parts are all signs of vibration

effects. Some plants have instrumented snubbers in

order to obtain more specific information in this regard

(see Appendix B). In addition to loosening of threaded

fasteners, significant wear of connecting pins and attachment hardware can result from low amplitude vibration

in combination with snubber weight forces. A photo of

a worn clevis pin damaged from low amplitude vibration

is illustrated in Appendix J, Figure J-9.

3.1.5.3 Transients

As with vibration, transients, such as those caused by

water or steam hammer, turbine trip, etc., can induce

loads that are beyond the snubber's design capacity,

often rendering the snubber immediately inoperable.

Some snubbers are exposed to periodic load transients

that are within the rated capacity of the snubber;

however, if such transients are not mitigated, snubbers

might undergo progressive degradation that can also

result in failure.

3.1.5.4 Management of Snubbers Subject to Rapid

Degradation

Many plants have augmented inspection procedures for

evaluating snubbers that are susceptible to rapid

degradation due to a severe operating environment or

dynamic transients. For example, freedom of movement

for snubbers suspected of having experienced a dynamic

transient is often verified by hand stroking or rotation of

the snubber about its spherical end bearings.

3.1.5.5 Other Useful Monitoring Considerations

A number of additional considerations and maintenance

practices identified in the plant research are discussed in

Section 5.4. These are used to form a basis for the

service-life monitoring recommendations in Section 5.1.

3.2 Evaluation of Hydraulic Snubber

Aging

The following subsections discuss the primary aging factors associated with degradation in hydraulic snubber

performance, such as, elevated temperature, vibration

and moisture. Typical failure modes for the majority of

hydraulic snubbers, and associated failure mechanisms

and causes, are shown in Thble 3.5. Separate subsections are also included pertaining to the effects of load

transients, service-life evaluation techniques, and the

effects of compression set on low pressure seal performance. Elastomeric seals most affected by aging are also

discused in a separate subsection.

3.2.1 Aging Factors for Hydraulic Snubbers

3.2.1.1 Radiation

During the course of the snubber aging research, no

cases of snubber degradation were identified that were

specifically attributed to radiation. However, this may

be partially due to the lack of in-depth failure analysis

data. Although the effects of radiation on snubber

degradation are probably less than was originally anticipated, radiation cannot be totally dismissed as a contributor to seal and fluid degradation.

NUREG/CR-5870 10
Evaluation

Taible 3.5 1'ypical hydraulic snubber failure causes

Failure Failure

Mode Mechanism Failure Cause

Low- High fluid Inadequate material control;

Locking viscosity low test temperature

Velocity Gelated fluid Component vibration

Incorrect Inadequate material control;

valve parts improper assembly

Incorrect Field tampering; inadequate

setting calibration

LoW High fluid Inadequate material control;

Bleed viscosity low test temperature

Rate Gelated fluid Component vibration

Incorrect Field tampering; inadequate

setting calibration

Particulate Inadequate parts cleaning

contamination

High- Low fluid Inadequate material control;

Locking viscosity high test temperature

Velocity Air in fluid Inadequate purge; seal

degradation resulting in loss

of fluid

Incorrect setting Inadequate calibration

procedure

Incorrect valve Improper assembly

part

Piston seal Seal degradation

by-pass

High Air in fluid Inadequate purge; seal

Bleed degradation resulting in loss

Rate of fluid

Low fluid Inadequate material control;

viscosity high test temperature

Water in fluid High humidity environment

Laboratory evaluations pertaining to the effects of

gamma radiation on elastomeric seals have been conducted (Barbarin 1977 and Mosca 1977). However, the

results of such evaluations are difficult to apply in

service because of the absence of precise radiation level

information for each snubber location, variations in seal

materials and compounds, and shielding provided by the

snubbers' metallic components. The potentially synergistic effects of radiation in combination with other

environmental stressors have also not been fully

substantiated.

3.2.1.2 Elevated Temperature

Elevated temperature is generally considered to be the

most prevalent degradation stressor for hydraulic snubbers. This is based on the relatively high incidence of

degradation or failure in high temperature applications.

In applications where hydraulic snubbers are subjected

to abnormally high temperatures (above 2500F) significant degradation may result in a relatively short period,

e.g., within one or two operating intervals. In applications involving low temperatures, (less than 120'F)

degradation may progress gradually over a period of ten

or more years.

Specifically, the effects of elevated temperature include

* acceleration of compression set inducement in

elastomeric seals

* accelerated oxidation effects on elastomeric seals,

i.e., cross-linking, resulting in embrittlement and/or

surface fissures.

* deformation of plastic hydraulic reservoirs

* adhesion of dynamic seals to mating surfaces

3.2.1.3 Moisture

Hydraulic snubber degradation due to moisture is generally in the form of corrosion. In some cases, severe

corrosion has resulted in structural failure of springs

used in pressurized reservoirs; however, this problem

was limited to a specific snubber design in a particularly

humid environment and has since been corrected. Some

snubbers have experienced pitting of piston rod plating

in a very humid environment, e.g., in coastal areas.

Regarding hydraulic snubbers subjected to moisture,

internal corrosion resulting in the generation of corrosion products can cause a malfunction of the snubber

control valve. Such degradation has been documented

for a limited number of hydraulic snubbers in high

humidity environments, with reservoirs that are vented

to the atmosphere.

11 NUREG/CR-5870
Evaluation

3.2.1.4 Vibration

High-amplitude vibration, i.e., vibration with an amplitude greater than the clearances in the end attachments,

can result in wear and localized overheating of mating

parts. Such wear can result in particle generation,

potentially effecting control valve performance. The

incidence of seal wear due to vibration is surprisingly

low. Applications involving continuous high- or lowamplitude vibration can result in loosening of threaded

fasteners and/or wear or deformation of clevis pins and

attachment holes (see Appendix J, Figure J.9).

High amplitude vibration has also resulted in deformation of poppets and poppet seats in hydraulic snubber

control valves. However, the incidence of such degradation has been mitigated by the incorporation of

improved materials.

In many cases, extreme high-amplitude vibration can

result in gelation of the hydraulic fluid (see Appendix J,

Figure J.2). The exact cause of this gelation is not

known; however, it is speculated that the gelation is

caused by localized mechanical working of the hydraulic

fluid due to continuous motion of the piston, resulting

in changes in the fluid's physical properties.

3.2.2 fransient Loads

Snubbers are subject to transient dynamic loads due to

abnormal operating conditions such as water hammer,

turbine trip, etc. Such transients occasionally exceed the

rated load capacity of the snubber, in which case significant damage can result, rendering the snubber immediately inoperable. Such damage is typically in the form of

a bent piston rod for compressive loads and sheared

piston/piston rod threads for tensile loads.

Hydraulic snubbers are also subject to transient dynamic

loads that are less than the design capacity of the snubber. Such transients can cause excessive wear of mating

parts, fatigue of structural members, and gelation of

hydraulic fluid.

3.23 Elastomeric Seal Life Evaluation

Techniques

Seal replacement intervals recommended by snubber

manufacturers have generally been conservative due to

the lack of service data at the time of the recommendations. Recent experience with seals manufactured from

environmentally suitable elastomers such as ethylene

propylene and fluorocarbon rubber (Viton)l has

indicated minimal degradation.

Methods used to predict seal life fundamentally involve

either accelerated aging studies or actual inservice data,

these are discussed below.

3.2.3.1 Accelerated-Aging Studies

Seal life for some plants is based on a mathematical

model (Arrhenius) (Gillen 1980) that correlates a given

degradation parameter, e.g., stress relaxation, with a

given environmental stressor, e.g., temperature and

time. Seal life is then monitored based on recorded time

at various operating temperatures. An example of this

approach, used at Site A, is included in Appendix C.

The accelerated-aging model and associated analytical

seal life projections are useful design tools for selecting

optimum materials and designs. However, to predict

seal life from this approach alone--without substantiation by service data--is impractical because of the

number of variables involved. Such variables include:

* seal material or compound

* seal configuration, e.g., O-ring, lip seal, tee seal,

boss seal, thread seal, and spring-energized seal

* seal thickness

* fluid medium

'Viton is a registered tradename of duPont Company.

NUREG/CR-5870 12
Evaluation

* mating surface finish

* initial squeeze.

Furthermore, the accelerated-aging model becomes

overly complicated and unreliable when more than one

environmental stressor is involved, e.g.,

* temperature

* radiation

* moisture

I

2

0

* air exposure

* fluid effects.

3.2.3.2 Evaluation of Seals Removed from Service

Many plants monitor seal life based on data from seals

removed from actual service in the plant. With this

approach, a practical seal life may be projected and

periodically updated for the general snubber population.

Snubbers needing more frequent attention in particularly severe environments may also be identified and

managed on a case-by-case basis.

The most commonly used seal life projection method is

extrapolation of compression set (which is directly

related to seal relaxation). The basis for this approach is

illustrated in Figure 3.7.

For example, for a static seal that had been in service for

seven years with a measured compression set of 50%,

using a compression set limit of 90%, seal life, t2, may be

calculated using the following equation:

t2 = t1 x CL 50 12.6 years

max

where t2 = seal life (in years)

t, = accumulated service time (7 years)

CL = compression set limit (90%)

Cmla = measured compression set value (50%).

Figure 3.7 Compression set extrapolation

3.2.3.3 Other Elastomeric Seal Life Evaluation

Methods

A number of other approaches have been used to substantiate seal life for snubbers. These include seal life

projections based upon available laboratory data and

evaluation of trends in snubber functional test data.

Accurate determination of seal life based on laboratory

test data is difficult. Conclusive laboratory data supporting the long-term effects of temperature aging on

seals is limited. This is primarily due to the difficulties

encountered in simulating the effects of time. Although

some information is available pertaining to the effects of

radiation on elastomeric sealing materials, it is

extremely difficult to apply this information to establish

a practical seal life for snubbers in service (see Section

3.2.1.1).

Since the incorporation of environmentally suitable

elastomers, there has been little evidence of seal failure

when a snubber is activated, i.e., pressurized. Moreover,

the primary aging concern is gradual relaxation of sealing force when the snubber is not activated. Therefore,

a substantiation of seal life based solely on functional

test results is not appropriate.

3.2.3.4 Plant-Specific Seal Life Considerations

Seal life evaluations, for the most part, have been plantor site-specific. Due to the lack of precise environmental data, the potential for combined environmental

13 NUREG/CR-5870
Evaluation

influences and variations in seal compounds and configurations, it is difficult to generically categorize seal

life for hydraulic snubbers (e.g, eight years for an application temperature of 140TF). Another difficulty in

applying generic data is that the levels of environmental

stressors at various snubber locations often differ from

the levels specified in the plant design specifications. (It

should be noted that environmental parameters

included in design specifications are generally specified

as maximum values; actual operating levels may be

lower, or occasionally higher, than the specified value.)

Various plant-specific seal life studies have indicated

variations in seal degradation from one plant to another

with similar design specifications.

Plant-specific seal life evaluations are appropriate for

most hydraulic snubbers. However, it may be generally

stated that seals manufactured from most ethylene propylene compounds, in mild operating environments (low

temperature, low humidity, low radiation level) will

likely exhibit little or no degradation over extended periods of time. More precise plant data is needed, however, in order to quantify seal life in this regard.

Ultimately, seal life should be based on successful of

operating experience in the actual plant environment.

Seal life extension evaluations should be considered as

interim site-specific methods for progressively extending

seal life from current conservative limits.

3.2.4 Effects of Compression Set on LowPressure Elastomeric Seal Performance

The purpose of this portion of the in-plant research was

to verify practical compression set limits for the various

seal configurations that are commonly used in hydraulic

snubbers.

The chief concern regarding aging of hydraulic snubber

seals is relaxation of sealing force under low-pressure

conditions. Compression set is most often used as a

direct indicator of the level of seal relaxation. Seal life

projections are often based on comparison of a predicted compression set level with an established compression set limit.

Compression set, C, may be defined by the following

equation:

W - W1

C=w - W

o s

where WO = original seal thickness

Ws = compressed seal thickness as it is

installed in the seal gland

W, = recovered seal thickness after the seal is

removed from the gland.

Most static seals (seals where there is no relative motion

between mating parts) will still perform adequately

under low pressure conditions at 100% compression set.

For a degree of conservatism, a compression set limit of

90% is typically used for projecting seal life in static

seals used in hydraulic snubbers. For an additional

degree of conservatism, a compression set limit of 80%

is typically used for dynamic seals, i.e., seals where there

is relative motion between mating parts.

Note: As a design tool, when selecting an optimum seal

material based on laboratory-aging simulation,

more conservative limits may be specified, due to

the absence of actual service data.

3.2.4.1 Methodology to Collect Compression Set Data

The basic approach was to obtain nonleaking snubbers

with seals that are expected to have a high level of

compression set. Snubbers with extended service in a

high-temperature environment were priority candidates.

It was anticipated that such snubber samples could be

found in either nonsafety-related applications in nuclear

plants or in fossil fuel plants. Although snubbers in

high-temperature environments were sought for their

higher propensity for compression set, correlating

operating temperatures with compression set is not a

part of this study.

The selected snubbers were then subjected to a preliminary evaluation. The purpose of the preliminary

evaluation was to determine, by measuring compression

NUREG/CR-5870 14
Evaluation

set in one or two representative seals, whether or not

the remaining seals could be expected to have a high

level of compression set.

Snubbers meeting the criteria established in the preliminary evaluation were then subjected to a secondary

evaluation in which compression set was measured for

all seals expected to have high levels of compression set.

Table 3.6 Maximum measured compression set for

various nonleaking seal configurations

Maximum

Measured

Nominal (3) Source

Compression of

Seal Configuration Seal lyrpe Set Data (5)

A total of 24 NPP personnel were contacted either for

candidate snubbers that might be available from the

plant or for the names of fossil fuel plant contacts in the

same utility. Eight major utilities with fossil fuel plants

were also contacted. In addition, two hydraulic snubber

vendors were contacted for snubber candidates available

from their services groups.

O-Ring

O-Spring Lip Seal

Quadra Spring Up Seal

U-Cup Piston Seal(4)

Miller Piston Rod Seal

(Static Portion)

Static

Dynamic

57%

69%

Dynamic 69%

Dynamic 100%

(2)

(1)

(2)

(2)

Static 93% (1)

3.2.4.2 Results of the Compression Set Examinations

A total of only six snubber samples were made available.

TIWo major factors that limited the availability of snubber samples were 1) the limited amount of lead time

allowed to obtain the required snubber samples in NPPs

(snubber availability generally coincides with scheduled

overhauls), and 2) the lack of replacement snubbers in

fossil fuel plants.

Of the six available samples only two snubbers showed

sufficient seal degradation in the preliminary evaluation

to warrant a secondary evaluation. A summary of the

preliminary and secondary evaluations is included in

Appendix D.

3.2.4.3 Additional Data Evaluation

In order to augment the limited amount of data

obtained in the evaluations described above, additional

in-house data were reviewed for maximum recorded

compression set levels for various nonleaking seal

configurations.

3.2.4.4 Summary of Compression Set Evaluation

T'ble 3.6 lists maximum measured values for compression set for various nonleaking seal configurations.

These configurations are illustrated in Figures 3.8

through 3.12. It should be noted that tabulated compression set values by no means reflect a limit for

Note: (1) NPAR study.

(2) In-house data.

(3) Nominal compression set is calculated using nominal

cross-section dimensions for original seal thickness.

(4) Internal seal integrity verified by functional testing.

(5) These data were obtained from 5 different snubbers.

Figure 3.8 O-Ring

compression set for the respective seal configurations.

They reflect only the maximum compression set value

recorded, providing some degree of support for the

typical compression set limits discussed in this section.

is NUREG/CR-5870
Evaluation

WEAR

RINGFigure 3.11 U-Cup piston seal

Figure 3.9 O-Spring lip seal

LIP SEAL

BUSHING

SEAL

Figure 3.12 Miller piston rod seal

Figure 3.10 Quadra spring lip seal

Optimum compression set limits cannot be established

based on the research because of the limited amount of

data presently available. Additional research is required

to accomplish this goal. In view of the limitations

discussed in Section 3.2.4.2, such research should allow

sufficient lead time and provide for replacement snubbers to increase the number of available snubber samples. Future seal data from plants with extended service

will also be useful in further substantiating compression

set limits.

3.2.4.5 Internal Seals

The primary consideration regarding degradation of

internal seals, e.g., piston seals, is progressive relaxation

of sealing force such that locking velocity or bleed rate

might be affected. However, snubbers with U-Cup piston seals with a measured compression set of 100% have

been tested with no observed effect on these parameters.

3.2.5 Elastomeric Seals Most Affected by

Aging

As discussed in Section 3.2.4, low pressure sealing capability is of primary concern for the effects of aging on

hydraulic snubber seals. External seals, i.e., seals that, if

NUREG/CR-5870 16
Evaluation

leaking, would result in loss of fluid from the snubber,

are considered to be the most critical in this regard.

The propensity of seals to age-related degradation may

be characterized as follows:

* In general, the higher the surface area to volume

ratio, the greater the propensity for a seal to take a

set. Seals with a small cross-section thickness,

therefore, are more susceptible to compression set

inducement.

* Seals installed near or on the snubber surface

appear to be more affected by the service environment (in terms of embrittlement and high compression set) than seals that are installed deeper within

the snubber.

* Seals that are exposed to air are prone to degradation due to oxidation, particularly at elevated

temperatures. Although seals also degrade due to

radiation, signigicant effects in this regard have not

been substantiated by the service data.

* Dynamic seals are generally more susceptible to

leakage due to the relatively low initial squeeze that

is characteristic of such seals, continuous changes in

the seal-gland interface, and the potential for wear.

* Thread seals used to seal straight threads on some

snubber models are particularly prone to service

degradation, generally in the form of increased

hardness and high set. It should be noted that

thread seals are installed at the surface of the

snubber and are exposed to air.

3.3 Evaluation of Mechanical Snubber

Aging

The purpose of this evaluation is to develop an

improved understanding of aging of mechanical

snubbers based on recent operating experience.

Systems typically identified as problem safety-related

systems for mechanical snubbers are listed in Table 3.7.

Snubbers used on non-safety-related systems are also

subject to degradation. However, plant data in this

regard are not as comprehensive.

Table 3.7 Typical problem systems for mechanical

snubbers

PWR Plant BWR Plant

* Component Cooling * High-Pressure Core Spray

* Reactor Coolant * Low-Pressure Core Spray

* Safety Injection * Residual Heat Removal

* Steam Generator Blow * Main Steam (particularly

Down between isolation valves and stop

valves)

Snubbers on small piping branching from relatively

large piping (e.g., drain lines and instrumentation lines)

are particularly susceptible to overloading caused by

dynamic transients. Snubbers installed at pipe locations

near connections to rotating equipment are susceptible

to degradation due to vibration.

Until recently, service data on mechanical snubbers was

limited because mechanical snubbers were not used on a

large scale in nuclear plants until the late 1970s. A

significant portion of the data that have been available

pertained to some of the earlier problems encountered

during pre-operational tests and initial ISI. Many of the

reported failures were associated with construction damage, manufacturing defects, and isolated severe environments that have since been corrected.

Failure modes of mechanical snubbers may be grouped

in three basic categories:

* high-drag or high-breakaway force (this includes

frozen snubbers)

* high-acceleration threshold

* low-acceleration threshold.'

Thble 3.8 represents a consolidation of typical mechanical snubber failure causes associated with the three

failure modes described above. A more detailed classification of failure causes, as determined by failure

analysis for Site C, is included in Appendix G.

'Some plants have a lower limit for acceleration. Although a low acceleration by itself is not a major concern, it may indicate a problem

within the snubber that could lead to inoperability.

17 NUREG/CR-5870
Evaluation

hable 3.8 Typical mechanical snubber failure causes

Failure Mode

High Drag Force

Exceeded Maximum

Acceleration Limit

Below Minimum

Acceleration Limit

Failure Mechanism

Bent screw shaft

Inertia mass rubbing against dust cover

Foreign materials on screw shaft

Foreign materials on indicator tube

Cracked thrust bearing

Dry lubricant

Corrosion of torque drum

Corrosion of capstan spring

Rough spots on planetary gears

Thrust bearing fretting

Capstan spring wound too tight

Binding of telescoping members

Loose bearing retainer nut

Telescoping members not concentric

Weld spatter on indicator tube

Bent guide rods

Flaked plating on ball screw

Capstan spring not wound tight enough

Capstan spring not installed correctly

Worn capstan spring

Keeper ring not installed correctly

Excessive lubricant on torque drum

Lubricant on inertia mass

Bent clutch tang

Fractured ball screw shaft

Capstan spring wear

Corrosion of capstan spring

Corrosion of clutch spring

Damaged capstan spring

Damaged thrust bearing

Torque drum retainer bent

Failure Cause

Overload

Overload

Dusty Envir.

Dusty Envir.

Overload

Elev. Temp.

Moisture

Moisture

Handling Damage

Vibration

Mfg. Def.

High Side Load

Mfg./Hand. Def.

Mfg. Def.

Const. Damage

Overload

Mfg. Def.

Mfg. Def.

Mfg. Def.

Vibration

Mfg. Def.

Mfg. Def.

Mfg. Def.

Mfg. Def.

Overload

Vibration

Moisture

Moisture

Mfg. Def.IVibr.

OverloadVibr.

Overload

NUREG/CR-5870 181
Evaluation

Failure causes listed in 'Thble 3.8 may be grouped into

three basic categories:

* service-related, single occurrence

* service-related, progressive degradation

* nonservice-related.

3.3.1 Aging Factors for Mechanical Snubbers

The primary influences associated with progressive

degradation in mechanical snubber performance are

vibration, moderate load transients, elevated

temperature, and moisture.

3.3.1.1 Effects of Vibration on Mechanical Snubbers

Mechanical snubbers, particularly the smaller sizes, are

subject to degradation from high- and low-amplitude

vibration. High-amplitude vibration is vibration having

an amplitude that exceeds the mechanical clearances in

the snubber's end attachments. This type of vibration

can result in localized fretting and wear of mating parts,

such as the lead screw, thrust bearing, capstan spring,

pins, and attachment lugs. It can also result in an

increase in drag force, an increase in mechanical clearances, jamming, and/or an increase in the acceleration

threshold.

Low-amplitude vibration is defined as vibration with an

amplitude less than the mechanical clearances in the

snubber. This type of vibration can lead to loosening of

fasteners and, in combination with the weight of the

snubber, can cause wear of clevis pins and attachments,

resulting in elongated attachment holes. Continuous

high-frequency, low-amplitude vibration can cause

internal wear, which may increase drag force.

3.3.1.2 Elevated Temperature

Elevated temperature has often caused solidification of

lubricants used in mechanical snubbers. This effect

increases friction and results in an increase in drag force.

3.3.1,3 Moisture

Moisture can cause internal corrosion that in turn can

lead to increasing drag force, jamming, and/or a

decrease in the snubber's acceleration threshold as a

result of a build up of rust between the capstan and

capstan spring. Moisture-related corrosion has been a

concern for snubbers installed in a vertical orientation,

whereby water may be trapped in the snubber.

Degradation due to moisture can occur over an

extended period. Degradation can also occur in a relatively short time due to a one-time exposure to large

amounts of moisture, e.g., steam leak. Identification of

the specific cause requires a comprehensive root cause

evaluation.

33.2 Evaluation of Changes in Drag Loads

Versus Service Time

As was discussed in Section 3.3.1.1, progressively

increasing drag force is another consideration. There is

some justification for periodic drag force testing of

representative snubber samples from the general snubber population. An evaluation of the effects of service

time on mechanical snubber drag force is discussed in

the following subsections.

3.3.2.1 Average Drag Force Versus Time - Different

Snubber Samples

For Site D (two plants), data were available for snubbers

of the same size that were tested on the same test

machine. Unfortunately, no snubber had been tested

more than once to date (i.e., there were no repeat tests

for any one snubber). However, it was anticipated that,

if service time significantly affects drag force, then an

average of the measured drag force values for the same

size snubber for successive refueling outages might

indicate such a trend.

Data from both units for five successive refueling outages were reviewed for two sizes of mechanical snubbers.

The data were evaluated for both peak and average drag

'Moderate load transients are defined as frequently occurring load

transients that are less than the rated capacity of the snubber, such as

those occurring during pump start-up.

19 NUREG/CR-5870
Evaluation

force in tension and compression. The number of samples for each size at each Reactor Fuel Outage (RFO)

ranged from 4 to 26.

Associated data and plots of drag force versus time

are included in Appendix E. As may be observed in the

time plots, the results of the evaluation are inconclusive.

3.3.2.2 Average Drag Force Versus Time - Same

Snubber Samples

For Site F, data were available for a total of

47 mechanical snubbers of the same size that had been

tested on at least two and sometimes three occasions

using the same test machine. Peak and average drag

force values for each RFO were plotted versus time (see

Figure 3.13). Associated data and trending plots for

individual snubbers are included in Appendix F. As

with Plant D, both average drag force and peak drag

force for both the tension and compression directions

were evaluated. It should be noted that all snubbers had

been in service for approximately five years before the

initial test data point was obtained.

2-. of Rated Loao

25-

20- a Peak Compreusson

As can be observed from Figure 3.13, there appears to

be a slight trend toward increasing drag force with

service time, at least for one mechanical snubber model

and size at this particular plant. The data support the

need to monitor snubbers in moderate environments,

possibly using a selected number of representative snubbers. However, the magnitude of the drag force value is

generally small, i.e., less than 2% of rated load for all

but four snubber samples, and less than 3% of rated

load for all but one snubber sample.

3.3.3 Dynamic Transients

A significant number of cases of documented degradation or failures in mechanical snubbers have been

associated with dynamic transients. Some transients,

such as those caused by water or steam hammer, can significantly overload a snubber and result in instantaneous

failure. Other, lower load transients such as those due

to sudden valve opening may result in progressive

degradation depending on the number of load cycles.

'Typical failure mechanisms associated with overload

involve fracture of the thrust bearing and/or buckling of

the ball screw or slender attachment hardware. Inability

of the snubber to provide free motion in the passive

mode is often the result of such damage (i.e., jamming

or high drag).

Failure mechanisms associated with lower load transients generally involve wear or local fretting similar to

that resulting from high-amplitude vibration. Such

degradation can result in an increase in snubber drag

force. Many plants have implemented procedures

whereby snubbers that are potentially subject to

transients are identified and evaluated using augmented

inspection methods (Section 3.1.5.4).

10

S

- 1%ol Raled Load Or -- -_- - - -_ - Peak Teson

- 1 Average CoMPMession

3 Average Tenson

I

a,.

' 5 6 7 B

serce Tim. (Yms)

9 10

Figure 3.13 Average drag force versus service time for

47 mechanical snubbers

NUREG/CR-5870 20
4 Conclusions

4.1 Snubber Performance History

For most plants, the greatest number of snubber failures

has been associated with nonservice-related influences,

such as mishandling damage and manufacturing defects

that were detected during initial plant operation. After

identifying isolated severe environments during initial

plant operation, plant personnel have modified the

snubbers' environment, replaced snubbers with more

durable models, or eliminated the snubbers as part of a

snubber reduction program. Snubber functional test

acceptance limits have also been generally broadened.

These methods of snubber management have significantly reduced snubber failure rates.

Aging management for snubbers involves 1) identifying

snubbers susceptible to rapid degradation and minimizing their potential for failure by conducting augmented

inspections or by requiring frequent maintenance or

replacement, and 2) monitoring for progressive degradation in the remaining plant snubbers and scheduling

preventive maintenance accordingly. Many plants have

implemented elastomeric seal life monitoring programs

for hydraulic snubbers. Beyond seal life studies, however, most plants have yet to implement a formal

service-life monitoring program for snubbers.

In general, approximately one-half of all recent snubber

failures for the key plants evaluated have been caused by

aging-related service influences. By contrast, review of

failure evaluation data for one plant indicated that only

25% of the evaluated seal failures were aging related.

The most significant influences resulting in snubber degradation are elevated temperature, vibration, dynamic

transients, and moisture. The effects of radiation, on

the other hand, appear to be significantly less than

originally anticipated. This is probably because of the

relatively low actual radiation levels, the shielding

effects provided by the snubber body, and the frequency

of seal replacements.

'Some plants initially applied manufacturer's production acceptance

limits, which are generally much more narrow than limits required for

snubber operability.

4.2 Service-Life Monitoring

The following conclusions are drawn from the in-plant

snubber research and provide the basis for service-life

monitoring guidelines:

* Many plants utilize an automated database that simplifies tracking and retrieval of pertinent information that may be used for monitoring snubber service life. An example of a snubber data sheet associated with such a system is included in Appendix H.

* Plant data indicate that a significantly large portion

of the total number of snubber failures have

resulted from nonservice-related influences. This

supports the need to distinguish between servicerelated and nonservice-related degradation or failures to ensure that nonservice-related failures are

excluded from the database that is used to monitor

snubber service life.

* Variations in snubber degradation rate due to significant variations in environment from one area in

the plant to another may warrant establishing separate service-life categories for different snubber

populations.

* The primary degradation influences for both

hydraulic and mechanical snubbers are elevated

temperature, vibration, and load transients.

* Moisture can cause corrosion of both internal and

external snubber parts. Such degradation is more

prevalent for mechanical snubbers. External corrosion is easily detected during visual examination

and may be an indicator of internal corrosion.

Methods used to identify internal corrosion include

boroscopic examination, hydraulic fluid analysis,

and snubber disassembly.

* Snubbers are particularly susceptible to service

degradation when installed

21 NUREG/CR-5870
Conclusions

- on small piping branching from large piping

- near high temperature components

- in congested areas with significant personnel

traffic

- near connections to rotating equipment.

* Plant-specific seal life evaluation studies have been

conducted at several plants. Some plants are trending drag force data in a selected number of mechanical snubbers. Realistic service-life projections have

involved the use of trendable degradation parameters that relate to the anticipated degradation

mechanisms.

* Plant systems for which snubber degradation

appears to be prevalent include

- component cooling

- reactor coolant

- safety injection

- steam generator blowdown

- high-pressure core spray

- low-pressure core spray

- residual heat removal

- main steam.

* lypical failure mechanisms and causes are listed in

Tables 3.3 and 3.7.

* Mechanisms and root causes for failure and degradation should be determined (where practical).

This requires experienced personnel.

* Plant operating environments may differ from the

original design specification. General area operating parameters, therefore, should be measured over

time and documented.

* Plants should make a concerted effort to identify

applications involving severe operating environments. Snubber failures in such applications may be

mitigated by augmented surveillance, frequent

maintenance or replacement, retrofit with more

durable snubbers, or by eliminating the snubber in

an approved design review.

* Augmented surveillance may be appropriate for

snubbers in severe operating environments and may

involve attributes that are not normally included

during inservice inspection (IS1) visual examination.

* "Hands-on" surveillance methods may be used in

addition to visual inspections. Such methods can be

used to detect vibration and elevated temperature

and to identify frozen snubbers. They can also be

used to identify anomalies that may indicate the

need for preventive maintenance.

* Evaluation of functional test traces is very useful in

identifying the cause of snubber failure, or in identifying anomalies indicative of impending snubber

failure.

* Diagnostic tests may be used to augment inservice

test data in identifying the cause of snubber failure

or degradation. Snubber test equipment that provides a time trace of test parameters is useful for

evaluating snubber degradation or failure. Variation of test parameters is often necessary for diagnostic testing. 7Test machine accuracy and

repeatability is required for trending.

* Elastomeric seal life evaluations include analytical

methods, e.g. Arrhenius projections, and methods

based on service data. Any seal life evaluation

method for snubbers should be based primarily on

predicting low-pressure seal performance and

should be updated based on service data.

* Monitoring of snubber reservoir fluid level is the

most practical method for verifying fluid leakage.

* Snubber damage due to mishandling or personnel

traffic may be minimized by inspection of such

snubbers just before start-up following an outage.

NUREG/CR-5870 22
Conclusions

* One plant has reported that acceleration thresholds

have decreased for some mechanical snubbers as a

result of internal corrosion that effectively decreases

the clearance between the capstan spring and the

braking surface (See Appendix A). As a result, this

plant has established minimum and maximum

acceptance limits for acceleration threshold. In the

absence of baseline data, however, it is difficult to

determine whether or not the acceleration threshold

has actually decreased or whether it was low to

begin with. In areas where baseline data are available, acceleration threshold may be an effective

parameter for identifying such degradation.

* Critical snubber parts should be identified and

may vary depending on the environmental

stressor involved. Snubber service life should

be based on the part anticipated to have the

shortest life for the primary environmental

stressor.

* Snubber test parameters generally include activation level, release rate, and breakaway or drag

force. These parameters are useful for both ISI

and for service-life monitoring. A clear definition of any parameter should be established by

each plant. This definition should be consistently applied thereafter. Parameter definitions

for ISI purposes may differ from the corresponding definitions used for service-life

monitoring.

4.3 Effects of Compression Set on

Low-Pressure Elastomeric

Seal Performance

Static seals in hydraulic snubbers can seal adequately,

even at a compression set of 100%. However, general

limits used for most seal life evaluations are 90% for

static seals and 80% for dynamic seals. These limits

were substantiated to some degree using compression

set data obtained in this study. However, further

research involving more substantial data is needed in

this area.

4.4 Service Aging of

Mechanical Snubbers

Mechanical snubber performance can be progressively

affected by aging, particularly when snubbers are

exposed to one or more environmental stressors. Performance is related to drag force, breakaway force, and

acceleration threshold. Primary influences affecting

degradation are elevated temperature, vibration, moisture, and dynamic transients.

Snubbers subject to severe environments should be

identified and managed with appropriate preventive

maintenance. Long-term service in moderate operating

environments may also affect snubber performance.

Mechanical snubbers in moderate operating environments should be monitored by testing representative

samples; baseline data are extremely important in this

regard.

23 NUREG/CR-5870
5 Recommendations

5.1 Service-Life Monitoring

Recommendations

The following recommendations for service-life monitoring are based on the results and conclusions of the inplant research activities.

5.1.1 Determination of Snubber Failure or

Degradation Causes

A principal goal of a service-life monitoring program

should be to develop means for separating servicerelated and nonservice-related failures. It is important

that the root cause of snubber failure or degradation

(e.g., snubber overload due to dynamic transient, highamplitude vibration beyond the design capacity of the

snubber, and application temperature exceeding that

specified for continuous use) be identified along with

the failure mode (e.g., high drag force or low activation)

and the failure mechanism (e.g., deformation of the ball

screw shaft or solidification of grease).

Failure evaluation data sheets should include key categories such as failure mode, failure mechanism, failure

cause, environment, service time, abnormal conditions,

visual observations, test data, and test observations. It is

important that personnel involved in failure evaluation

be adequately trained in correctly tracing a failure to its

cause. Failure evaluation data sheets should be

designed and formatted in a manner that encourages

systematic and thorough analysis.

Figure 5.1 illustrates a systematic analysis approach to

root cause failure identification. Thble 5.1 lists typical

irregularities that may be observed during visual examination or during snubber disassembly. The table characterizes features of snubber degradation and may be useful in pinpointing the potential cause.

5.1.2 Determination and Documentation of

Operating Environment

Service-life monitoring takes into consideration the

capability of the various snubber models to endure the

PEure Pailure

Mode Medmanism

I I Pam | i,

I r - i li

I rl-tAI I nFli _ 

Root Cause

i | Getadatio]

r Ngbti;: Raition|

I

l wL L 1 3J1~vto

ri7 F

Inadequate

- Materal

Control

Figure 5.1 Flow chart - typical root cause

determination

full range of plant environments (benign to severe).

Indicators of severe operating conditions can often be

identified during snubber overhauls and other

maintenance-related activities.

Determining specific environmental information often

involves specialized instrumentation and equipment

that would be impractical for use at every snubber location. Such equipment, therefore, should be used in

applications where moderate to severe environments are

anticipated or as a diagnostic aid in determining the

cause of snubber degradation or failure. Various

methods and equipment used to identify or measure

specific environmental parameters are described in Sections 5.1.2.1 through 5.1.2.4. Additional information in

this regard is included in Appendix B.

5.1.2.1 Temperature

Continuous temperature recording devices are available

to indicate the general area temperatures within the

plant (which often vary by elevation) or to measure local

snubber or component temperatures. TRmperaturesensitive tape may be placed directly on the snubber to

25 NUREG/CR-5870
Recommendations

Tible 5.1 Typical indicators of snubber degradation

Observation Potential Indicator of:

Dark hydraulic fluid High-amplitude vibration

Black material on piston rod High-amplitude vibration

Excessive piston and cylinder wear High-amplitude vibration

Worn capstan spring tangs High-amplitude vibration

Fretting High-amplitude vibration

Unsymmetrical wear of clevis pins High-amplitude vibration

Elongation of attachment holes High- or low-amplitude

vibration

Loose fasteners High- or low-amplitude

vibration

Symmetrical wear of clevis pins Low-amplitude vibration

Discoloration of metallic parts High temperature

Embrittled piston rod wiper High temperature

Rod wiper adhered to piston rod High temperature

High seal compression set High temperature

Seal surface cracks High temperature in air

Lack of fluid pigmentation High-radiation level

Corrosion of metallic parts High-humidity/leaking

components

Bent piston rod or attachments Overloading

Changes in cold/hot position Increased drag or

setting jamming

* originally anticipated radiation effects were based

upon a 40-year dose; in actuality, snubber parts that

are sensitive to radiation degradation are replaced

at intervals that are significantly less than 40 years.

Data pertaining to plant radiation levels can generally

be obtained from health physics area surveys. Measurement of radiation levels specifically for service-life

monitoring is not recommended except in evaluating the

cause of snubber degradation in cases where other

causes have been ruled out.

5.1.2.3 Vibration

Vibration may be continuous, in which case snubbers

may degrade in as little time as one operating interval.

Vibration may also be intermittent (e.g., during pump

startup), in which case it may be undetected for long

periods and result in long-term degradation of the

snubber.

The available methods for detecting and measuring

vibration vary from simple visual observation, detection

by feel, portable vibration measuring instrumentation,

and remote vibration measuring equipment. Examples

of some alternatives to detect vibration, along with

actual inservice applications, are described in Appendix A (Site G) and Appendix B.

Snubbers subject to vibration can often be detected by

visual examination. Metal filings, darkened hydraulic

fluid, deformed connecting pins, elongated attachment

holes, and fretting of mating parts are all signs of vibration effects.

5.1.2.4 Transients

As with vibration, the existence of dynamic load transients may often be identified during routine snubber

inspections, augmented inspections, and failure evaluation. Deformed structural members, jammed snubbers,

and deformed internal parts are all potential indicators

of dynamic overloading. In situ devices such as loadmeasuring clevis pins are available for monitoring snubber loads in applications where such transients are suspected (Appendix B).

determine maximum temperature. One shortcoming of

this approach, however, is that a time/temperature profile is not provided. Contact and noncontact temperature measuring devices (e.g., infrared type) are also

available.

5.1.2.2 Radiation

Normal radiation levels of an operating plant do not

usually contribute significantly to snubber degradation.

This is probably due to the following considerations:

* actual in-plant radiation levels are, in most cases,

less than was originally anticipated

* the snubber body provides a significant amount of

shielding

NUREG/CR-5870 26
Recommendations

5.13 Managing Snubbers in Severe Operating

Environments

Significant environmental stressors that can affect snubber performance include overloading, vibration, elevated temperature, moisture, chemicals, and radiation.

Despite the best design and post-startup inspections,

isolated cases of snubbers operating in severe environments may be identified as plant operation continues.

Unfortunately, such applications are often not identified

until the snubbers are functionally tested. This supports

the need for some random functional testing; however,

the extent of functional testing currently required by

technical specifications may not be necessary as plants

gain empirical knowledge pertaining to the plant

operating environments and the associated snubber

capabilities.

Snubber failures in applications involving severe operating environments may be mitigated by conducting augmented inspections, periodic maintenance, periodic

replacement with like kind, retrofitting with snubbers

more suitable for the environment, or eliminating the

snubber by approved engineering analysis methods.

5.1.4 Augmented Surveillance

A number of practices may be used for evaluating snubbers for degradation and for identifying operating environments. Since evaluation methods often do not

employ quantifiable parameters, judgment is required

on the part of the inspector. Experience of inspection

personnel is therefore important.

5.1.4.1 Hand-Stroking

Probably the most common "hands-on' evaluation

method is hand-stroking of mechanical snubbers. This

method is often used to identify snubbers that are damaged or jammed due to transients. In this method, the

inspector removes the connecting pin at one end of the

snubber and slowly strokes the snubber while feeling

and listening for abnormalities such as intermittent or

continuous excess noise or resistance.

Using this method, an experienced inspector can often

identify impending failure. For example, when a

mechanical snubber is hand-stroked, periodic resistance,

accompanied by a chaffing sound for each revolution of

the inertia mass, indicates binding caused by lack of concentricity of rotating parts. Irregular, intermittent noise

and resistance, on the other hand, indicate surface discontinuities on the lead screw.

5.1.4.2 Rotation of Snubbers In Place

Jammed snubbers (i.e., snubbers unable to allow free

thermal motion) may often be identified by attempting

to rotate the snubber about its spherical end attachment

bearings. If the snubber is not free to rotate, it is possible that axial loading exists that is the result of

jamming or premature lockup. It should be noted, however, that this method is most effective for snubbers with

a load capacity of 3,000 lb, or less. Normal friction in

the bearings often prevents rotation of larger sizes.

5.1.4.3 Hand Detection of Vibration

Detection of vibration by placing a hand on the snubber

during operation is a useful technique for evaluating

accessible snubbers.

5.1.4.4 End of Outage Inspection

Just before startup, reinspection of snubbers that are

susceptible to damage due to outage-related activities

will reduce the probability of plant operation with

inoperable snubbers. Future verification that consequent failures were not the result of service-related

influences would otherwise be more difficult.

5.1.5 frending

Note: Trending of test data is discussed further in

Section 5.1.64.

Progressive degradation in the general snubber population (i.e., those snubbers not subject to rapid degradation) should be monitored by trending applicable

degradation parameters for a selected number of snubbers that are representative of the plant operating environment. Such degradation parameters might include

compression set for elastomeric seals (Section 3.2.2) or

drag force for mechanical snubbers (Section 3.3.2).

Some important considerations in this regard are listed

below:

27 NUREG/CR-5870
Recommendations

* The establishment of baseline data is essential for

identifying trends. Data to be used for identifying

trends should be sufficiently accurate to

demonstrate trends.

* l'ending parameters that relate directly to the

anticipated aging failure mode should be used. Such

parameters might include drag force for mechanical

snubbers or elastomeric seal compression set for

hydraulic snubbers.

Note: An important example of inappropriate monitoringparameters is the use offfunctional test data, Le.,

locking velocity and release rate for monitoring or

trending seal degradation. Although functional test

results can be affected to some extent by seal degradation, the primary aging failure mode for snubber seals

(ie., loss of low-pressure seal integrity) would not be

reflected in functional test data.

* Acceleration threshold (activation level) in

acceleration-limiting mechanical snubbers is a

potentially trendable parameter that may indicate

internal snubber degradation. A decreasing acceleration threshold may indicate internal corrosion

(Section 4.2) or increased friction between the

inertia mass and its spindle. An increasing acceleration threshold may indicate weakening of the

capstan spring tangs as a result of wear or a decrease

in friction between the capstan spring and its braking surface.

* Although changes in active hydraulic snubber

parameters [i.e., locking velocity and bleed (release)

rate] can indicate snubber degradation, these

parameters are not considered practical trending

parameters for monitoring progressive degradation.

* Activation Level: Locking velocity for poppet-valve

hydraulic snubbers. Acceleration threshold for

acceleration-limiting mechanical snubbers.

* Release Rate: Snubber velocity at a given load.

* Drag Force: Snubber resistance load at a given

stroke velocity.

* Breakaway Force: Force required to initiate snubber motion.

These parameters are also useful in identifying potential

degradation or in determining the cause of snubber

failure.

5.1.6.1 Evaluation of Inservice Test Results

Since existing IST plans are statistically based on the

number of failures, test results are often evaluated on

only a pass/fail basis. Most test machines, however, provide a continuous trace of load and velocity for both

activation tests and drag force tests. Such traces often

contain information useful in identifying snubber degradation. For example, during mechanical snubber drag

force testing, such characteristics as the number of load

spikes, consistency of load spikes, duration of load

spikes, noise, variations in drag force with stroke position, and directional sensitivity are all useful in identifying potential snubber degradation or impending snubber

failure (see Figures 5.2 through 5.6). For hydraulic

snubbers, traces can be used to identify air in the snubber or a clogged bleed orifice.

TNTh I'l

1X

* Reservoir fluid level is the most appropriate parameter for monitoring snubber fluid leakage.

fV\A/\-I\J\AAAAIV\J\ 1 a

VVVWI"VVV_\VV VV\J

5.1.6 Testing

The following functional test parameters are normally

measured during inservice testing (IST):

It

rl"rsnl ml PwlUMACTHM

Figure 5.2 Mechanical snubber with normal drag force

trace

NUREG/CR-5870 28
Recommendations

TUIs'O, TEMSIOa

Is

US

a

!I

Sa

i. vvvvwvvW

DbSPIAlCACt

c01lrPsEsSIOl

Figure 53 Mechanical snubber drag force with

consistent spikes

COKPRESSIOW

Figure 5.5 Mechanical snubber drag force with

occasional spikes

TPSION ?PtFS tof

II

It

I--

DbIPMACtMI1T Di StIAMCFJTM

COMMMSf OlmH ornPrRSSION

Figure 5.4 Mechanical snubber drag force with

noise

Figure 5.6 Mechanical snubber with high drag

at one end

5.1.6.2 As-Found Testing 5.1.6.3 Diagnostic Testing

As with IST results, a considerable amount of information can be obtained by conducting post-service functional tests on snubbers removed from service. In fact,

as a general rule, such tests are recommended any time a

snubber is removed from service, regardless of whether

or not the snubber is to be reinstalled.

Diagnostic tests (see Section 7.0) are specifically

designed to obtain useful information about the condition of a particular snubber, beyond what may be

available from routine IST or as-found tests. For failures, diagnostic tests are often helpful in identifying the

failure mechanism before disassembling the snubber.

29 NUREG/CR-5870
Recommendations

Repeat tests are helpful in determining the repeatability

of a given anomaly. For example, a load spike observed

during a mechanical snubber drag force test that repeats

in the same location for several tests would indicate the

existence of a surface imperfection at one point on the

lead screw. However, a spike that does not repeat in the

same location or does not repeat at all indicates particulate contamination, e.g., dirt or sand.

It may sometimes be desirable to vary test parameters

such as applied load, drag-force velocity, or test time

duration in order to observe the effect on snubber performance. For example, extending the time duration for

a hydraulic snubber bleed rate test is a good method for

identifying bleed orifice blockage as the cause of an

observed low bleed rate.

5.1.6.4 'rendingTLIst Results

Tfrending is a useful tool for monitoring progressive

snubber degradation. If test data are to be used for

trending, the following should be considered in addition

to those considerations listed in Section 5.15:

* Because the prevalent failure mode is failure to

allow free thermal motion, a potential trending

parameter for mechanical snubbers is drag force.

This is supported by test data obtained that suggest

an increase in drag force with service time for

mechanical snubbers.1

* It is important that test data to be used for trending

are consistently obtained using the same type of test

machine, under the same test conditions. Ideally,

the data from the same snubber should be used for

comparison purposes.

* Administrative limits for functional test results are

intended to ensure replacement or repair of a given

snubber before failure. However, it is important to

have a reasonable indication that the selected test

parameter is progressing toward the failure limit.

Overly restrictive administrative limits can have the

negative effect of limiting the amount of data available for trending. They can also encourage replacement of reliable snubbers.

If test data are obtained for a different set of snubbers at

each refueling outage, then IST results are not appropriate for trending. Similarly, if snubbers are tested on different types of test machines, then test data are generally

not adequate for identifying trends.

Another important consideration involves defining the

test parameter. For example, for ISI purposes, drag

force may be defined as the highest (peak) resistance

force when stroking the snubber from end to end. On

the other hand, for trending, drag force may best be

measured as an root mean square (RMS) average of the

measured resistance force throughout the stroke range.

A test parameter, therefore, must be defined both from

the standpoint of snubber operability (i.e., for ISI) and

from the standpoint of service-life monitoring (i.e., for

trending).

5.1.6.5 Test Equipment

Note: Test machines are discussed here in terms of their

application for service-life monitoring.

The types of snubber test equipment currently used in

the industry vary considerably. Some provide only a

single value for a given test parameter such as load or

velocity, while others provide a continuous trace of the

parameter versus time. The operation of some test

equipment is totally manual, while others are fully automated. In general, it is recommended that functional

test equipment be provided with a data acquisition system (either analog or digital) that is capable of providing a continuous trace of load and velocity versus time

for the duration of the test. As discussed in Section 5.1.6.1, information from such traces is useful in

detecting degradation and identifying failure

mechanisms.

Some test equipment is of a "go/no-go" nature, in which

the snubber is determined to be either operable or

inoperable; however, this method neither accounts for

variations in test results nor measures exact values for

the given test parameter. Such equipment is not useful

for test interpretation, diagnostic testing, or trending.

Snubbers that fail functional tests using this equipment

are often retested using a more accurate test machine.

'Such a trend is yet to be fully substantiated.

NUREG/CR-5870 30
Recommendations

Many test machines are totally automatic. Automatic

test machines may be advantageous for ISI because

operator bias is minimized. Thus, data from automatic

testers are generally acceptable for trending purposes.

However, for diagnostic testing, the test operator should

be able to vary the level of various test parameters for

exploratory purposes.

It should be noted that whenever a snubber is tested in a

different type of test machine than the one previously

used, a number of new variables are introduced that may

complicate the identification of trends. Such variables

include test control methods and parameters, data

acquisition systems, etc. For this reason, trending tests

are most effectively conducted using the same test

machine as well as the same test methods.

5.1.7 External Seal Leakage Detection and

Leakage Rate Determination

Minor seal leakage is common for many snubber types

and applications. A number of influences can cause seal

leakage. A leaking snubber, however, does not necessarily imply inoperability nor does it necessarily require

immediate snubber overhaul.

Measurement and trending of reservoir fluid level is

probably the most practical approach to monitoring for

external seal leakage. For this reason, reservoir fluid

level should be recorded whenever fluid is added. An

example of this method is discussed in Appendix A for

Site A

The location of seal leakage in many cases may be obvious by visual observation. However, in some cases the

precise location of the seal leakage may require a time

consuming follow-up evaluation.

It should be noted that in many cases, seal leakage can

be the result of improper snubber assembly, defective

parts, etc. A practical method for checking for seal

leakage following snubber overhaul is to place the

snubber on an absorbent (paper) pad where it can be

observed for a period of time before installation.

5.1.8 Visual Examination

Snubbers are normally visually examined during each

refueling outage in compliance with technical specification ISI requirements. However, the intent of these

examinations is to identify characteristics that might

indicate snubber inoperability. Several visual examination attributes included in ISI visual examinations are

not related to service degradation. Service-life monitoring examinations may be conducted at the same time as

those required for ISI or separately. However, qualification of personnel for such examinations is critical (see

Appendix L).

Visual characteristics that would provide information in

regard to service degradation are listed below. These

snubber attributes may be used to define a visual examination checklist for service-life monitoring.

* deformed structural member or piston rod

* loose or missing threaded fasteners

* cold or hot position varies from specified value

* evidence of corrosion

* evidence of solid deposits (e.g., boric acid) from

leaking components

* loss of hydraulic fluid since previous visual

examination

* metal filings on or in the vicinity of the snubber

* observed fluid leakage

* evidence of significant dark (i.e., black or dark

brown) material deposit on piston rod

* rod wiper adhered to piston rod

* abnormal color of hydraulic fluid

* wear or deformation of clevis pins

31 NUREG/CR-5870
Recommendations

* elongation of attachment holes

* evidence of wear on support cylinder

* cracked or deformed fluid reservoir

* evidence of foreign material (e.g., water, solid particles, etc.) in hydraulic fluid

* discoloration of metallic parts due to elevated

temperature.

5.2 Recommendations for the Working

Group on Mechanical Equipment

Restraints

Based upon the results of the NPAR research, a number

of suggestions are made in regard to the OM Code, Section IST, Subsection ISTD, Part 4 (ASME 1990). These

suggestions are to be recommended to the Working

Group on Mechanical Equipment Restraints for consideration in the next revision of Subsection ISTD.

In general, recommendations pertain to service-life

monitoring, visual examination attributes, and failure

grouping. A detailed discussion of these recommendations is included in Appendix K The recommendations

are summarized in the following sections.

5.2.1 Service-Life Monitoring

Recommendations

Service-life monitoring recommendations proposed for

consideration in Subsection ISTD are generally based

upon the recommendations discussed in Section 5.1.

Specifically, they include:

* determination of snubber failure causes

* determination and documentation of the snubber

operating environment

* evaluation of inservice test results

* diagnostic testing

* as-found testing

* trending

* augmented surveillance methods

* establishment of service-life categories.

5.2.2 Visual Examination Attributes

'Typical visual examination attributes that may be used

to update the Subsection ISTD, Appendix B, "Dynamic

Restraint Examination Checklist Items," are suggested

for consideration by the Working Group on Mechanical

Equipment Restraints. Recommended attributes are

listed separately in the following categories:

* preservice examination attributes only

* preservice and inservice examination attributes

* service-life monitoring examination attributes.

5.2.3 Failure Grouping and Corrective Action

Subsection ISTD currently requires that any snubber

that fails to meet functional test acceptance criteria be

classified into one of the following failure mode groups

(FMGs):

* design/manufacturing

* application induced

* maintenancelrepair/installation

* transient dynamic event

* isolated

* unexplained.

Depending upon the failure mode group, various corrective actions may apply. Recommendations are made in

the following areas pertaining to failure grouping and

associated corrective action.

NUREG/CR-5870 32
Recommendations

5.2.3.1 Terminology

It is recommended that the following terms (as defined

in Section 7.0) pertaining to aging be incorporated into

the proposed ISTD 8.0, Service-Life Monitoring:

* failure mode

* failure mechanism

* failure cause

* root cause.

5.2.3.2 Isolated Failure Mode Group

It is recommended that the isolated FMG be eliminated.

5.2.3.3 Failure Categories

Based upon the results of root cause evaluations, it is

recommended that snubbers with a similar propensity

for failure be grouped together to facilitate corrective

action. How such root failure-cause groups are defined

involves judgment on the part of the owner; a failure

group should be defined after a failure has been identified. However, for purposes of determining follow-up

action in Subsection ISTD, it is suggested that failures

need only be identified as service related, nonservice

related, or unexplained.

5.2.3.4 Replacement or Modified Snubbers

It is suggested that some flexibility be provided in

ISTD 1.11.1 to allow for continued use of existing snubber models in cases where more compatible models are

not available.

5.3 Uniform Snubber Population

Classifications by Environment

Many utilities have elected to pre-group snubbers based

on design differences for purposes of ISI. This approach

presupposes that failure causes will be associated only

with snubbers in the predefined group.

Another method of pre-grouping that is commonly used

for ISI purposes is to categorize snubbers as either

accessible or inaccessible and to examine them as separate populations. This approach is generally the result

of practical considerations in order to minimize inspection activities during refueling outages, rather than having anything to do with susceptibility to degradation.

From the standpoint of service-life monitoring, a primary consideration for pre-grouping should be based on

anticipated variations in service life. Snubbers in isolated severe environments, e.g., those with high temperatures and vibration operating conditions, should be

separated from general population and managed on a

case-by-case basis. For plants with general environmental extremes, such as temperature, it may be desirable to group the general snubber population into two or

more subpopulations with separate service lives.

5.4 Snubber Maintenance

Recommendations

A number of maintenance recommendations were identified during the NPAR in-plant research. Recommendations generally are associated with the service-life

monitoring guidelines discussed in Section 5.1. A

detailed discussion of maintenance recommendations is

presented in Appendix L Specific topics in this regard

are listed below:

* General Maintenance Practices

* Identification of the Operating Environment

* Snubbers Prone to Rapid Degradation in Severe

Environments

* Failure Evaluation

* Failure Grouping

* Modification of Operating Environment

* Snubber Elimination

* Augmented Inspections

* Snubber Maintenance Frequency

* Ttending

33 NUREG/CR-5870
Recommendations

* Baseline Data

* Snubber Data Base

* Functional Test Equipment

* Spare Snubber Rotation

* Personnel Qualification

* As-Found Evaluation

* Coordination and Communication

* Replacement Parts and Materials

NUREG/CR-5870 34
6 Recommendations for Additional Snubber Research

Although compression set limits currently in use are

supported by limited data available from this in-plant

research, additional compression set studies should be

undertaken to strengthen the credibility of the compression set limits and to establish optimal compression set

limits. As plants accumulate service time, such data will

probably become available.

This research has identified environmental conditions

that are the most significant aging factors for snubbers.

Development of service-life monitoring programs that

ascertain realistic humidity, vibration, and temperature

conditions are therefore encouraged within the industry.

As plants implement service-life monitoring programs, a

significant amount of additional service data will

become available regarding these critical environmental

influences and the associated age-related degradation of

snubbers. Additional research would be required to

evaluate this information.

Results of the work reported here should be transferred

to industry in an active and assertive manner. A

workshop/presentation developed from the in-plant

research is suggested. The workshop would be presented to engineering, quality assurance (QA), and

maintenance staff at the plant sites. The workshop

should consist of two separate presentations, one for

engineering and maintenance management staff and the

other for the staff who perform the work; e.g., craft

supervisors and craft workers.

35 NUREG/CR-5870
7 References

Barbarin, R. January 1977. Selecting Elastomeric

Seals for Nuclear Service.' Power Engineering Magazine.

Penn Well Corporation, Tulsa, Oklahoma.

Brown, D. P., G. R. Palmer, E. V Werry, and D. E.

Blahnik. 1990. BasisforSnubberAgingResearch:

NuclearPlantAgingResearch Program. NUREG/

CR-5386 (PNL-691 1), prepared for the U.S. Nuclear

Regulatory Commission by Pacific Northwest Laboratory, Richland, Washington.

Bush, S. H., P. G. Heasler, and R. E. Dodge. 1986.

Aging and Service Wear of Hydraulic and Mechanical

Snubbers Used on Safety-Related Piping and Components

of Nuclear Power Plants. NUREGICR-4279

(PNL-5479), prepared for the U.S. Nuclear Regulatory

Commission by Pacific Northwest Laboratory, Richland,

Washington.

Gillen, K T, et al. 1980. PredictingLife Expectancy and

SimulatingAge of Complex Equipment UsingAccelerated

Aging Techniques. NUREG/CR-1466 (SAND-79-1561),

prepared for the U.S. Nuclear Regulatory Commission

by Sandia National Laboratories, Albuquerque, New

Mexico.

Mosca, F. July 1977. Radiation and Environmental Tests

of Seal and Fluid Materials for Snubbers. IT Grinnell

Corporation, Providence, Rhode Island.

U.S. Nuclear Regulatory Commission. November 1984.

Technical Specifications for Snubbers. Generic

Letter 84-13, Washington, D.C.

U.S. Nuclear Regulatory Commission. 1987. Nuclear

Plant Aging Research (NPAR) Program Plan,

NUREG-1144, Rev. 1., Washington, D.C.

Code for Operation and Maintenance, Rules for

Inservice Testing of Light-Water Reactor Power Plants,

Subsection ISTD Part 4, Inservice Testing of Dynamic

Restraints (Snubbers) in Light-Water Reactor Power

Plants. 1990. American Society of Mechanical Engineers (ASME), New York.

37 NUREG/CR-5870
Appendix A

Plant Visit Summary Reports
Appendix A

Plant Visit Summary Reports

This Appendix includes trip reports for eight key sites

(one each from site A through G) visited as part of the

NPAR snubber aging study. Plants are assigned letter

codes from A through H. Information pertaining to

hydraulic snubbers was obtained from Plants A and B.

Information pertaining to mechanical snubbers was

obtained from Plants C, D, E, F, and H. Information

pertaining to in situ monitoring of environmental

stressors was obtained from Plant G.

A.1 NUREG/CR-5870
Appendix A

I N T E R 0 F F I C E M E M 0

Plant A

TO: D. P. Brown DATE: August 17, 1990

FROM: S. Cole UE:

The following are notes from my visit to the plant.

1. Notified resident NRC Inspector of upcoming plant visit and meetings.

Notification date: 7/31190.

2. Arrived on site 8/8/90 and was escorted by the snubber engineer. In depth

discussion took place with the snubber engineer, and the snubber

maintenance foreman.

3. General Plant Information

A. The plant consists of two mid-sized boiling water reactors. The

plant began commercial operation in the mid-1970's.

B. Each unit has over 600 Bergeii-Paterson snubbers of which about 550

are safety related (most of the snubbers are the M77 model). In

addition to these, the plant has approximately 350 hydraulic snubbers

available as spares for both units.

D. The plant utilizes a seal life with several separate populations

based upon accelerated aging tests and modified on a regular basis by

temperature for those snubbers in the drywell.

4. Plant has instituted (starting in 1986) a comprehensive maintenance

program which includes the following items:

o Filtering of snubber fluid immediately prior to use with a 5 micron

filter. (Plant found it hard to keep pre-filtered fluid clean over

long periods of time.)

o Sand blasting (glass beads) to bare metal of all metalic parts.

o Complete deconning of all metalic parts using a freon blaster.

o Replacement (rather than repair) of most questionable parts.

o Automatic replacement of poppet springs and piston rings.

o Use of the same trained personnel to rebuild snubber each cycle.

o All rebuilds take place in a clean' room used only for snubbers.

NUREG/CR-5870 AL2
Appendix A

5. Plant is retrofitting snubber poppets to a higher bleed rate poppet.

This never poppet still allows the snubber to meet bleed rate limits as

defined in the plant technical specifications while moving the actual

bleed rate more toward the middle of the allowable range (the plant.

suffered a significant number of functional failures due to low bleed

rates in the past.)

6. Due to a combination of the changing of the poppets, improved maintenance

and changing of the functional test procedure to allow 1/2" bleed testing

as opposed to full stroke bleed testing the number of functional failures

has fallen dramatically.

7. Fluid Leakage

A. Fluid leakage (as indicated by low reservoir level) during visual

examinations for the last three Unit 2 outages can be summarized as

follows:

Refuel

Outage

7

8

9

Number

Inspected

Total

Leaking

Significant

Leaks

581

553

553

50

49

59

( 8.9%)

( 8.9%)

(10.7%)

12 (2.1%)

5 (0.9%)

6 (1.1%)

OVERALL AVERAGE: 9.4% 1.4%

Note: The number of leaking snubbers is not the actual total number

but rather those snubbers whose total indicated fluid has fallen

below a given amount (5 3/4) since the last outage. Significant

leaks are those who's total indicated fluid has fallen below

3 1/4.

B. Those snubbers with significant leakage were disassembled and

inspected giving the following results.

Refuel

Outage

Number

Significant

Leaks

Aging

Related

Non-Aging

Related

7 12 2 (16.6Z)

8 5 2 (40.0%)

TOTAL 17 4 (23.5%)

Failure data was unavailable for those

leakage from refuel outage number 9.

10 (83.3%)

3 (60.0%)

13 (76.1%)

Note: snubbers with significant

A3 NUREG/CR-5870
Appendix A

C. A third category of leaking snubbers are those snubbers which show

visual signs of leakage but still have an acceptable reservoir fluid

level. These snubbers can-be considered to either have a-very slow

leakage rate or only started to leak just prior to their inspection.

Refuel Total Visual Indicated

Outage Leaks Leaks Leaks

8 66 17 (25.7%) 49 (74.2%)

9 87 22 (25. %) 65 (74.7%)

OVERALL AVERAGE: 25.4% 74.5%

Note: This data is not available from refuel outage no. 7.

8. Critical Snubber Parts

Snubber parts must frequently be replaced during rebuild (excluding those

parts automatically replaced) include: piston, cylinder tubes and piston

rods.

A. Pistons and Cylinder Tubes

These parts are most frequently replaced due to scoring of the piston

and inner cylinder tube surface. This condition appears to be caused

by vibration, with side loading being a significant degradation

accelerator. This condition is considered to be aging related.

B. Piston Rods

Piston rod degradation most often consists of dings and scratches.

These dings and scratches are most often caused by human error.

(Metal to metal contact with the piston rod). This condition is not

aging related.

9. Snubber Fluid Degradation

A. The plant has a significant number of snubbers, which when

disassembled, yield darkened fluid with globules of a black

grease-like substance. This effect is typical of snubbers that have

been subjected to extensive high amplitude vibration.

It should be noted that darkened fluid and particulate contamination

was found in a significant percentage of the low bleed snubbers that

were disassembled and inspected during RFO 7.

It should also be noted that due to changes in bleed rate testing

criteria, many of these snubbers would no longer be considered

functional failures.

NUREG/CR-5870 A-4
Appendix A

B. Chemical Analysis

Fluid was chemically analyzed from 10 snubbers found to have darkened

fluid following the most recent outage. Chemical analysis revealed

that the black globules were composed of silicon rather than grease.

Prior to being analyzed each sample was graded for visual clarity.

Analysis revealed that those samples which were visually darker/had

more black globules, and had a higher content of iron in the fluid.

The iron is attributed to wear products resulting from wear of the

cylinder tube and piston due to vibration.

Note: No particle counts where made from the fluid samples during

analysis.

10. Snubber Trending

The plant recently set up a computer based trending system that allows

sorting searching and seeking of any snubber attribute by any other. The

snubber engineer used this system to search snubbers with indicated

leakage vs. snubber location (i.e. drywell and balance of plant).

Total # of Indicated Z of Indicated

Area Snubbers Leaks Leaks

Drywell 263 39 14.8%

Balance of Plant 287 27 9.4%

Thus snubbers located in the drywell showed a leakage rate 57.4% greater

than those in the balance of the plant. It should be noted that

temperatures in the drywell are significantly higher than anywhere else in

the plant. Some correlation between plant operating temperature and seal

degradation rate is, therefore, supported by this data.

11. Review of plant failure evaluations shows piston and cylinder tube scoring

(possibly accelerated by side loading) to be the single largest failure

cause. Scoring also leads to metalic particles in the fluid which can

shorten seal life and foul the bleed poppets.

One unexpected failure cause that cropped up a significant number of times

was fouling of the poppet by pieces of lint. The frequency of this

failure cause has fallen off dramatically since the plant's adoption of

improved maintenance procedures.

A5 NUREG/CR-5870
Appendix A

CONCLUSIONS

It is my opinion that the plant has a very effective hydraulic snubber

program. The utility appears to not only strive to meet the intent of the

Codes, Standards and Technical Specifications applicable to hydraulic snubbers

but often to surpass them.

Areas where I believe the plant to be particularly effective in the handling

of their snubber program include:

o The rebuilding of snubbers in a "clean room' atmosphere to prevent

the inclusion of foreign material in the snubber.

o The use of a 5 micron filter to filter snubber fluid prior to use.

o The plant having a flexible snubber trending system up and running.

o The adjusting of snubber seal life up or down on a regular basis

depending on ambient temperature.

o The use of the same personnel to perform snubber testing and

rebuilding each cycle. This is facilitated by the smooth transition

of personnel into and out of the snubber group coupled with a

relatively low turnover rate.

Areas wnere I consider there may be room for improvement in the plant's

snubber program include:

o The plant's reliance on area temperature monitors to track snubber

environments rather than performing specific environmental surveys.

o The need to go more in depth on root caue evaluations. This is

particularly important since there exists a significant number of the

same types of failures whose root causes have been listed in the past

as unknown.

SMC:aca:1125A

NUREG/CR-5870 A6
Appendix A

Plant B

L A K E E N G INEERING C O M P A N Y

O F F I C E M E M E R A N D U M

TO: D. P. Brown DATE: July 16, 1990

FROM: S. M. Cole RE:

The following are notes from my visit to the plant:

1. Notified resident NRC Inspector of upcoming plant visit and meetings.

Notification date: 7/3/90.

2.- Arrived on site-719/90 and was escorted by the snubber engineer. In depth

discussions took place with the current snubber engineer and the previous

snubber engineer.

3. General Plant Information

A. The Plant is a mid-sized boiling water reactor which began commercial

operations in the mid-1970's.

B. Plant was originally supplied with approximately 350 hydraulic

snubbers, all of which were Grinnell Figuie 200's.

C. Due to agressive and on-going snubber reduction programs, the plant

now has 120 hydraulic snubbers (all Grinnell Figure 200). Of these,

64 are safety related. All hydraulic snubbers are accessible.

D. In 1990, the plant began 18-month refuel cycles; prior to 1990, the

plant was on 12-month cycles.

E. Seal life is based upon an accelerated aging study that correlates

service time with operating temperature.

4. Snubber failures and rebuilds by outage:

Year Failures Rebuilds

1986 7(1) 84

1987 0 12

1988 2 27-

1989 0 10

1990 0 app. 40(2)

Note: (1) Current plant theory is that these failures were caused by

incorrect test procedures (i.e. too high ramp rate, etc.) rather

than actual snubber failures.

(2) The reason for the high number of rebuilds in 1990 was to

prevent expiration of shelf life on seal kits in the warehouse.

A7 NUREG/CR-5870
Appendix A

5. The plant has conducted specific area temperature surveys for all

hydraulic snubbers with the exception of those located near the ceiling or

the Heater Bay. This area may be the harshest environment in the plant

consisting of temperatures exceeding 150OF and a possibly significant

amount of vibration.

A. The highest specific temperature surveyed was 125 0F.

B. Looking into resurveying snubber environments in more detail

(possibly including vibration measurements) because of changing

environments due to construction (outages).

6. A group of 15 snubbers in the Heater Bay regularly are found to have

darkened fluid. The darkened fluid often has globules of a black

grease-like substance dispersed throughout.

A. These snubbers are visually inspected on a more frequent basis than

required by preventative maintenance procedures.

3. Upon evidence of fluid darkening, snubbers are rebuilt as a matter of

course.

C. No failure or root cause exams or fluid testing performed because

these are all non-safety related snubbers. (Plans to fluid test

these snubbers in future).

D. Per former snubber engineer, if these snubbers are left in place

after the fluid turns dark, the fluid will thicken and congeal.

7; Plant has ain extremely large percentage of spare snubbers to installed

snubbers. Total number of spares is about 80.

A. Spare snubbers are rotated to ensure those spares that have the

oldest rebuild dates are installed first.

B. Usually rebuilds number of snubbers required for replacement plus

about 50% for emergencies.

C. Extra snubbers may be rebuilt when the warehouse has seal kits

nearing the end of their shelf life to avoid having to dispose of the

kits.

8. Root cause evaluations indicate the plant has several snubbers that

regularly fail due to being stepped on (broken reservoirs and connecting

tubing). These are safety related snubbers, which per tech spec, require

failure and root cause evaluation. Even though being stepped on is a

non-service related root cause, due to the frequency of this problem the

plant is considering treating these snubbers as if they were in a severe

environment.

NUREG/CR-5870 A-8
Appendix A

9. Failure evaluations are only performed if they have an operability

related failure of a tech spec (safety related) snubber.

A. Root cause evaluations are done automatically as part of failure

evaluations.

B. Recent revisions to plant procedures require the taking of photos

during failure evaluations to aid in the documentation of failure

causes and modes.

10. In the opinions of both snubber engineers, the plant experiences few to

no age related snubber failures unless the snubbers are located in harsh

environments.

11. For the past three years (3 cycles), the plant has performed all

hydraulic snubber testing in-house (both ISI and functional).

A. The plant also rebuilds hydraulic snubbers in-house.

B. In process of buying Bergen-Paterson MK IV test machine (have been

leasing this model) with upgraded computer and printer.

C. It is their belief that doing rebuilds and testing in-house allows

them tighter control of the snubber life cycle.

12. The plant currently uses two mainframe computer programs (utility owned

and specific) to track snubbers.

A. The programs are reasonably comprehensive and include tracking of the

following fields: serial number, model, make, CIC (mark number),

last test date, installed date, maintenance history, class and

comments.

B. The single biggest problem with this system is that it takes

approximately six months to update fol owing an outage.

C. Currently setting up a new system to allow better trending of snubber

characteristics than current systems allow.

- New system will allow search, sort and seek of any field by any

other fields.

- New system should be operational in about six months.

1.3. The plant has found the most critical (prone to failure) parts of

Grinnell snubbers to be thread seals and tubing connections. A

significant percentage of leaking snubbers has been traced to thread

seals. In the past, tubing connections were a significant source of

leakage in the snubbers. The plant replaced these connections with

Swagelock fittings to minimize this source of leakage.

A.9 NUREG/CR-5870
Appendix A

14. Specific concerns of plant personnel:

A. Hot and cold setting of snubbers need to be reverified on a

semi-regular basis due to changing plant conditions.

B. Drawings must always be kept updated.

C. Some plants were built to one spec and then changed to another

farther into their life (i.e. Section III vs. 3 31.7). This makes it

difficult to rotate snubbers.

D. Side-loading resulting from the use of long extension pieces may

accelerate snubber degradation caused by vibration.

NUREG/CR-5870 A.10
Appendix A

CONCLUSIONS

It is my opinion that the plant has an effective snubber program. There is

evidence that the utility is dedicated to not only meeting the intent of the

Codes, Standards and Technical Specifications applicable to hydraulic

snubbers, but to surpassing them when possible.

Areas where I believe the plant to be particularly effective in the handling

of their snubber program include:

o The use of the same personnel to perform rebuilds and visual inspections

each outage. This is facilitated by the smooth transition of in-going

and out-going snubber personnel coupled with a relatively low turnover

rate.

o The performing of an environmental survey to define snubber environments.

o Augmented inspection of snubbers in harsh environments.

o Photographic documentation during failure evaluations.

o Utility leasing/buying their own state of the art snubber test machine.

o Good snubber tracking system up and running.

o Currently setting up a system to allow the trending of snubbers.

Areas where I consider there may be room for improvement in the plant's

snubber program include:

o Performing more frequent and more in depth failure evaluations.

o Finding and correcting the root cause of failing non-safety related

snubbers.

o Performing specific environmental surveys for all snubbers.

o Minimizing the time needed to update the computerized snubber database

following outages.

SMC:aca:ll0A -

A.11 NUREG/CR-5870
Appendix A

Plant C

Notes from investigation and information reviewed

during the visit and subsequent discussion with Plant Personnel

which includes; (Tech Staff), (Tech

Staf0) and (Tech Staff) and myself.

1. (NRC Sr. Resident Inspector) was notified of

the impending visit to the plant and subsequent meetings. - Ext. 2490, called in on 7/3/90 at 10:30 AM.

2. Arrived at the plant on July 5, 1990, AM and was escorted by

with whom I met with for the rest of the

day.

General Plant Information:

a. There are 2 BWR Units of M.W. sizes - Unit 1 went

into operation in and Unit 2 went into operation

in . Both units have been through refueling

outages with #2 unit just completed this past spring.

b. The units were constructed with P.S.A snubbers ranging

from PSA 1/4 through PSA-100. The total population for

the two units at start was 2384 which was divided as

1244 each for Unit 2 and 1140 each for Unit 1. Of this

population a total of 2265 snubbers were tested by the

end of the first refueling outage for both units, with

the remaining snubbers being deleted and 3 being

exempted from ISI.

c. The testing of all snubbers was precipitated by the

high percentage failure rate encountered during 1st

R.O. for Unit #1 which began and continued

through . During this first outage 104

snubbers had failed in Unit 1 and 53 snubbers would

fail in the first Unit 2 refueling course which started

on Jan. and continued through April . During

the second R.O. for both units, a massive snubber

reduction program had brought down the total population

from 2384 to a total of 487 snubbers. This process was

completed with full calculation review and necessary

design changes.

d. During units 2 refueling outage all snubbers in service

were again tested and a total of 8 snubbers for unit 1

and 18 for Unit 2 failed the functional test.

e. Failures are categorized by 1) Installation and

Handling Deficiencies, - 2) Environmental Failure, -)

Failure '&to transient or vibration load, - 4)

Manufacturing defects - Snubbers failed in categories 2

& 4 were relatively few in number, representing less

than 1% of the total population by group sizing (1/4 &

1/2) small and (1 through 100) large. Failure

attributed to Category t 3, which indicated transient

or vibration or both, were given greatest attention as

NUREG/CR-5870 A-12
Appendix A

2

these were the ones most likely to fail again. Aside

from the system operating low level vibration

(amplitude and frequency), some systems had special

problems requiring engineering review, such systems

were:

1) High Pressure Core Spray ( ) venting relocation

(Failure #5)

2) Low Pressure Core Spray ( ) High Point Venting

(11, 12, 50, 59)

3) Residual Heat Removal ( ) suction lines from

vessel to RHR Pump (71.79).

4) Residual Heat Removal (RHR) discharge lines outside

primary containment. (Failure 58,60,61, 66, 74, 82 for

A Loop), (Failure 62, 63, 64, 72 for B Loop).

All the above systems required engineering evaluation and or

plant modification in order to eliminate transient load and

low level vibration causing snubber failure.

f. Other snubbers failed in various system where small

drain and vent lines were connected to large lines and

forces induced from the large piping, although not

having detrimental effect on the larger snubbers

directly connected to the main system, they were

effecting the smaller supports by induced vibration and

transients. The system was modified to avoid future

failure.

3) A 100% testing of the 238 snubbers left in service in Unit I

second R.O. and a total of 8 snubbers failed to meet

functional test limits. A tear-down and inspection was

conducted to determine the cause of failure which were

categorized as follows:

a. 6 each due to installation and handling deficiencies,

2 each manufacturing defects.

It is evident that the snubber reduction program through

elimination and modification was proved very effective

considering the failure of the 8 units during the second

R.O. were attributed to causes other than fatigue related.

4) During the third R.O. the eight snubbers which had failed in

the previous outage were retested, in addition a 10% test

sample was implemented with the snubbers divided into two

groups:

1) Small, consisting of PSA 1/4 and PSA 1/2 (3 tested),

2) Larger, consisting of PSA-1 through PSA-100 (22 tested) -

There were two failures in this surveillance, one was from

the retest group and one PSA 1/4 from the sample plan.

A.13 NUREG/CR-5870
Appendix A

Therefore, 3 additional snubbers were tested withou-.

failure. There was no failure in the large group.

It is evident that failure rates have decreased drastically

by addressing system loads, transients, environment,

location and other factors contributing to past failures as

experienced during 1 R.O. -

The classification of primary causes of failure and

consequent evaluation is an effective method of separating

snubber units which would have a repeatable failure as it is

caused by environment and/or load, - Other failures

attributed to handling or installation deficiencies,

manufacturing defects, are considered random failure and are

addressed separately. Safety analysis and necessary

corrective actions were performed for each individual

failure and at a system level.

5) The same events were noted and addressed for Unit 2 with

comparable results. The number of failures are somewhat

different, however the investigation and corrective actions

were conducted with the same diligence and thoroughness.

The failures occurred as follows:

1 R.O. - 53 total failures with 19 each in Category 1, 6

each in Category 2, 23 each in category 3 and 5 each in

category 4.

2 R.O. - 18 total failures with 16 each in category 1 and 2

each in category 4.

3 R.O. - No failure recorded

6) An overall view of failures for the

Unit 1 & 2 listed by failure cause and sizes (small &

large):

1 R.O. - 157 total failures of which 74 each were small 1/4

& 1/2, 83 each were large 1 through 100.

2 R.O. - 26 total failures of which 6 a were small, 20 ea

were large.

3 R.O.- 2 total failures, of which both were small snubbers

(PSA-l/4)

Further grouping by failure cause as defined by plant

engineering:

Category 1: Installation and Handling Deficiencies

Total failures 100 units (46 large bore, 54 small

bore). These failures ranged from poor handling,

storage or installation practices.

NUREG/CR-5870 A 14
Appendix A

Category 2:

Category 3:

Category 4:

4

Environmental Failures

Total failures 17 units (1 large bore, 16 small

bore)

These failures appeared divided into two groups,

a) small snubbers failed due to grease drying out

and causing excessive drag, b) units corroded

internally due to being sprayed or dripped upon by

water while in service.

System Transient and/or vibration loads

Total failures 4i units (40 large bores, 9 small

bores) These failures are attributed primary to

system transient loads during service, such as

pump start-stop, valve opening - closing, causing

snubber internals to degrade. The failed parts

are listed as a) Thrust bearing races damaged or

broken, b) Capstan spring damaged, c) ball

screw bent or stripped.

Manufacturing Defects

Total Failures 19 units ( 14 large bore, 5 small

bore) These failures are attributed to assembling

or manufacturing problems existing as unit was

reviewed from PSA - The most common being improper

installation of capstan spring (to loose or to

tight) and excessive grease. - Since most of these

snubbers failed activation limits, these defects

are not noted with hand stroke.

stated that since PSA performed a functional test

on representative number for each size of snubber,

it is very possible to install a defective unit as

new snubbers are not, functionally tested before

installation.

There was also a very informative meeting between

myself and and

of the plant technical staff who

have been involved with the snubber program for an

extended period. - The following is a summary of

their personal views and concerns based on passed

experience.

It is commonly agreed that continuous and/or

periodic low level (amplitude and frequency)

vibration has the most damaging effect on

mechanical snubbers. This vibration may be

induced by components change of state, like valves

opening and closing, pumps starting and stopping,

or it may be inherent to system design, as process

flow. personnel have aggressively

attacked the problems concerning support system in

eliminating or minimizing failures by evaluating

and resolving root cause. For example replacing

snubbers with rigid support by verifying

calculation, modifying systems in order to

A.15 NUREG/CR-5870
Appendix A

5

eliminate transients, revising systems ISI

procedures, moving snubber supports to more

effective and less damaging location. Also

considering the replacement of some mechanical

snubbers with lisega hydraulic as no other options

were available. Heat shields are used in

localized high temperature and additional chiller

installed to lower ambient temperature in the

upper part of the containment dome, changing

procedure for venting system during test and the

training of personnel in the handling of snubbers.

8) Standard Procedure is employed in snubber

inspection which include visual, hand stroking,

performance testing and failure evaluation, If a

snubber fails, it is disassembled, inspected,

identify failed part or parts and determine root

cause. A report is written with the above

findings which also includes system, location and

conditions that may effect snubber performance.

Where performance test results are marginal, good

work practice is employed. Spikes of high

amplitude, however of short duration may be

considered acceptable. Grease has been the cause

of high drag where the temperature is above 1400F.

It has been noted that where the temperature for a

component reached 400°F and sustained for 36

hours, the adjacent snubber failed because of

grease failure. A snubber also had a marginal

performance test after being subjected to 3000F

for 48 hours.

Where temperature and vibration are suspected for

the failures thermocouples and accelerometers are

utilized in evaluating working conditions and

environment.

NUREG/CR-5870 A.16
Appendix A

6

Conclusion:

It is of my opinion that the personnel at have a

very effective ongoing surveillance and testing snubber program.

This is evident in the tremendous reduction in snubber failure

since the first R. 0.. This was accomplished through

investigation of component, system, environment and failure

analysis. Resolution utilizing analysis by the architect

'engineer support calculation by the A&E and special testing by

the system engineer were used to determine if system

modification, snubber relocation or support redesign was

warranted, which also led to a very extensive snubber reduction

program. Personnel training and effective work practices were

.also implemented in order to eliminate or reduce failures due to

handling.

Due to grease failure, I would think a qualification test may be

warranted to determine lubricant requirements classification of

snubbers by system, size and environment may facilitate the

possibility of necessary augmented inspection for specific

application.

e Laboratories

Euntsville, Alabama 35807

(205) 837-4411 ext. 583

A.17 NUREG/CR-5870
Appendix A

Plant D

BATTELLE NPAR RESEARCH

MEETnNG NOTE - S///90

Attachmet 1

Page 1

John Mucci and David Brown met with

csisg plant data on mechanical snubbers (Objectie IB).

for evaluation of

Unit 1went on rme inl 19 Unit 2went on line in19 .Brekdownofsnubbers isas follows:

Unit 1: Hydraulic

Mechanical

TOTAL

Unit 2: Hydraulic

Mechan;cal

TOTALI

- 889 (Grinell)

- *~9 (Padcic Scitific)

s573

115 (Grinnell)

- lS71 (Pacific Scientific)

1686

Plant has limited failure evaluation dat. Just initiated failure evaluation (toot cause analysis) 2 years ago.

Plant has some snubbers located on the top of the stcam generators (gh temperature area) that are buried

in insulation and that have consistently jammed (discovered by hand stroking).

So far, for both units, there are approximately 11 mechanical snubbers that have failed functional tests. They

have had numerous others that have failed hand stroking (conduced for all snubbers on hgh energy lines).

Plan has had no failed hydraulics.

Carent drag acceptanc- limits are 2%e (administratie) and 3% (alure limit). For acceleration the limit is

o.5g.

Plant has no PSA 1/4s (those that they did have, have been rcplaced with PSA 1/Zs).

to provide a breakdown of the snubber population by siz

is the snubber guy at

that they encountered on PSA 1/1s.

He should be contacted in regard to a previous high failure rate

Reviewed some of their failure analysis sheets. Plant has a tendency to attribute failures to snubber overload.

This may be reasonably valid since failures were discovered by hand stroking on high enery lines ic. lines

known to have transients).

They have recerly implemented a plan whereby any snubbers that are removed as a part of their snubber

reduction proga are futnctionally tested to obtain a test bench mark. They also hand stroke any remaining

saubbers on that system. Any snubbers that are used to replace snubbers that failed the hand stroke, are also

funciornally tested to obtain a bcnch mark. Rplaceent snubbers would also get tested again during the nex

outage This data is used for trending

NUREG/CR-5870 A.18
Appendix A

BATTELLE NPAR RESEARCH

MEETING NOTES - 5/1/90

Attaehment 1

Page 2

Currently, their service life monitoring program just involves hand stroking, but will eventually involve bench

testing. Servic: life monito g is currently limited only to those systems to which they previously had problcs.

They w phase in systms that they antidipate they may have problems on.

During their last outage, they hand stroked all. snubbers on high energy lines (Le those lines known to have

TheirTe^chnicI Specifcations allow them to exclude hand stroked failures froro having to be considered as ISI

faiures. Hownver, they haven t had any that have coincided with their functional test sample. Had they, they

would have osidcred tdem to be ISI failures eveu though they are not obligated to do this.

The following aon itms for the Battelce evaluation were established

o For valuation of drag force trcwdS.

- Evaluate data for szes 1/2 and 1

* M ccsure drag force for various amus of service time (exclude data fromn test plots

incting abnormal results such as uneven load patters).

- Record serial no, size, unit no, tens, drag. comp. drag, service time (RFO no.), and

location C to provide environmental information).

O Get functional test printouts for all functional test failures for which a failure evaluation was

condcted (both nits).

O Try to okmei functional test printouts (from previous tests) for snubbers that failed the hand

strok evaluation.

o Get a copy of faiure evaluation format.

A.19 A.19 ~~~~~NUREG/CR-
Appendix A

Plant E

TRIP REPORT

John Mucci 8/23/90

Notes from Investigation and Information reviewed during the visit and

subsequent discussion with of the Engineering Staff assigned to

support systems.

-General Plant Information

a. There are two units of 1150 M.W. sizes - Unit #1 went into operation in

and has been through refueling outages. Unit #2 went into operation

in , and is now going through its third refueling outage.

b. The units were constructed with mostly PSA ranging from 1/4 KIP to 100

K[P. The total opulation for the two units are 1720 all mechanical with

hydraulic (8 each) ESEGA used on the steam generator for Unit #1, and a

total of 1100 mechanical with (8 each) hydraulic LESEGA for Unit #2.

There has been a removal or replacement of 120 snubbers in Unit #1

through a snubber reduction program. During the first 3 RO a total of over

1000 snubbers had been tested through ISI for Unit #1 with a total of 85

failures during performance test. There were also over 500 tested in Unit #2

with a total of 69 failures during the first two refueling outages.

c. Failures are categorized by

1 Environment

2 Overloading

3 Handling or mishandling

4 Manufacturing Defects

5 Vibration

6 Unknown

As for percentage of the total failures, Cat#1 with 20%o, Cat #2 with 17%, Cat #3

with 280%ro, Cat #4 with 22%o, Cat #5 with 8% and Cat #6 with 5%o. This would

account for 28% as aging related, 67% as non-aging related and 5% unknown.

d. As for systems having the most failures we can identify and classify them as

follows:

1) Component Cooling System ( ) 2) Reactor Cooling System ( )

3) Safety Injection System() 4) Steam Generator Blowdown Recycle ( )

The above systems exhibited the highest snubber failures with approxdmately 55% of

the total with the remaining 45% being distributed among more than 20 other

systems which experienced snubber failures.

1) The snubber reduction program is not in full swing as result of timing

and priority. They have also purchased some Teledyne Load Pins

with which to monitor continuous or transient loads in separate

systems with overload failures. They have also replaced some PSA

with corrosion problems, with A/D because of unit construction.

NUREG/CR-5870 A-20
Appendix A

2) They have a formal failure analysis program addressing each failure

with respect to failure mode, failure cause and corrective action

They had a photo of a snubber destroyed by overload, and also a

metallurgical analysis of identifying material stress which is included

in the data collected.

3) Failures have occurred on all systems, however, the steam generator

blowdown system has had a higher number of failures than any other

system. Supports on primary system have less failures due to

contamination or'leaky components as in general leaks are less likely

to develop and when they develop they are addressed with higher

priority.

4) The plant has a tracking program in their computer system called

ASIS, which addresses snubbers and support systems. They have

divided the snubbers in three groups, Sma Medium, and Large and

includes all relevant data associated with each unit. The system is not

complete, as much of the necessary data has not been entered

addressing past test data, failure causes, failure mechanism, ambient

environment, which would be used eventually to establish service

trend. It is the opinion of snubber personnel that because of basic

design and lay out differences which exist between plants, a standard

approach to service life monitoring would not be entirely practical as

each plant would have their own peculiarity associated with system

design. and operation, however, some basic surveillance and

monitoring standards may be decided.

5) They identified the most critical snubber arts as the screw shaft,

thrust bearing as the items which exhibit loa related failures.

6) It is of the opinion of the snubber personnel that the test machine

should have the automatic capability to test snubbers, however, it

should allow the operator to vary input parameters without

complicated procedure. This would allow the evaluation of snubber

performance at levels different than the preset Tech. Spec.

requirements.

A.21 NUREG/CR-5870
Appendix A

Plant F

TRIP REP0RT -

John Mucci

Job 17131

A discussion was conducted between J. Mucci of Wyle and

Snubber Engineering Central Support.

o The plant has been in operation since the early 80's

however, the testing program for snubbers was initiated

during the forth refueling outage (2R4) which took

place in April

o Since then all the snubbers have been tested twice and

many have been tested more than twice.

o This effort was concentrated on Unit #2 Mechanical

Snubbers and in particular, PSA-1, which had an

original population of 60 ea., however, several have

been since replaced with AD-501 in cases where the

inherent design was more suitable.

o has also replaced all of their PSA-1/4 and PSA-1/2

with AD-41, AD-43 and AD-71R because of a high failure

rate of the original PSA caused in most part by misshandling during inspection and testing.

o A total of 47 exam data was collected all from Unit #2

as Unit #1 only had a total population of 3 in PSA-1

Model.

o All data was reviewed and 16 exams were selected for

trending based on in-service-time. Data from these

exams were grouped together, separated by tension run,

compression run, which included average and peak values

- graphs have been created in order to identify any

possible trend or trends.

o The exams used in the evaluation are from snubbers

installed at different locations in the plant, inside

and outside containment, at elevations from 370' to

420' with temperature ranging from 1400 F to 90°F.

These temperatures are not considered accurate as we

were unsuccessful locating operating temperature ranges

for Unit #2. A temperature study was conducted of Unit

41 in 1987 for justification for continued operation,

and it is generally assumed that temperatures for Unit

#2 are approximately 200 F lower at respective

locations. It is noted however, that varying

temperatures do not reflect major changes in service

performance.

NUREG/CR-5870 A-22
Appendix A

O F F I C E X E MO& R A N D U M

Plant G

TO: File RE: Visit at for

discussion of in-situ monitoring equipment

FROM: D. Brown DATE: August 24, 1990

On 8/17/90, I visited and met with the following

personnel:

- ISI

- Nuclear Engineering

We discussed vibration problems they had experienced on the mainsteam

bypass piping. Vibration frequency was approximately 400 Hz. and was actually

the result of radial pulsations of the pipe wall. The vibration was causing

localized cracking in the vicinity of interval attachment lugs used for the

snubber pipe clamp. Vibration was monitored using strain gauges; data was

retrived remotely. The vibration problem was, for the most part, resolved by

increasing the pipe wall thickness and by installation of two multiple orifice

plates within the pipe. Vibration amplitude was reduced by a factor of 10.

One remaining strut that was located upstream of the orifice plates

remained subject to high amplitude vibration. The strut was instrumented with

strain gauges for monitoring load; substantial loads were documented.

LVDT's (linear variable differential transformers) generally are not

acceptable for monitoring vibration. They are, however, useful for measuring

thermal displacement and have been used in this respect at this plant.

Vibration can also be monitored using portable vibration monitoring

instrumentation. Two devices that were discussed in this regard are:

o Askania hand-held vibration monitoring equipment.

This instrument has a circular chart that provides direct readings of

vibration amplitude. Frequency can be determined based on chart

speed.

o B & K hand-held accelerometer

This instrument has the capability of providing velocity and

displacement data vs. time integrating by acceleration

Both of the above described instruments are acceptable for monitoring

accessible systems, but must be hard-wired for use with remote data retrieval

systems for inaccessible systems.

A.23 NUREG/CR-5870
Appendix A

The plant had also experienced transients in the HPSI system where they

were experiencing back flow through the check valves. Other types of

transients they have experienced are caused by turbine trip, SRV venting, pump

start, etc.

Force measuring pins are probably the most practical way to measure axial

loads on snubbers and struts due to system transients. Such equipment was

particularly useful for axial supports on the mainsteam piping system for

straight runs of pipe.

In general, due to environmental considerations such as heat, wear, etc.,

monitoring instrumentation is not readily practical for continuous use on

various systems. Load pins had to be considered as temporary modifications of

supports.

They had also experienced problems with high frequency vibration

(approximately 800 Hz.) on mainsteam piping with a secondary vibration at

10 Hz. The high frequency vibration appeared to be due to pulsating radial

pipe expansion and contraction similar to that experienced on the mainsteam

bypass system. The 10 Hz. vibration appeared to involve gross movement of the

piping system as opposed to the pulsating wall; in this case, they were able

to use a spring loaded LVDT to measure vibration. In their opinion, the low

frequency vibration was the result of small pressure pulses within the reactor

which were also monitored at 10 Hz.

In their opinion, accelerometers are not really a good choice of

instrumentation for measuring piping or support response to dynamic

transients. However, their experience has been positive in using these

devices for measuring steady state vibration.

Snubbers installed on severely vibrating piping systems have, for the most

part, been removed as part of a snubber reduction program. However, two

50 KIP snubbers remain and are continuously degraded. These snubbers are

monitored and replaced frequently.

Plant personnel provided an extensive amount of back-up data including

isometrics, instrumentation specifications, etc., that apply to the subject of

this memo.

NUREG/CR-5870 AL24
Appendix A

Plant H

, Baltele

Pacific Northwest Laboratories

Batlteile Boulevard

P.O. Box 999

Richianu. Washing 7 3 

August 2, ~~~~~~~~~~~~~~~~ ~Telephone 6091 August 2, 1990

Dave Brown

Lake Engineering Company

P.O. Box 296

10 Austin Avenue

Greenville, RI 02828

Subject: Snubber Test Data from

Dear Dave,

Attached is the test data that we received from . Also included is a

portion of the plant's Technical Specifications, snubber acceptance criteria

and the validator acceptance criteria.

The test data does show a number of repeat tests for various sizes. The

results shown are for a validator, not a test machine.

I have reviewed most of the data; however I haven't completed a summary for

all sizes. For the repeat tests of 1, 2, and 3 year intervals, the sizes ...1/2, 3, and 10 (all PSAs), according to my analysis, show the following

results:

* for the PSA size 1/2...out of a total of 18 repeat tests,

5 indicate an increase in drag

8 indicate an improvement or less drag

2 indicate the same status

3 snubbers failed

* for the PSA size 3...out of a total of 6 repeat tests,

4 indicate an increase in drag

2 indicate an improvement or less drag

0 failures

* for the PSA size 10...out of a tctal of 25 repeat tests,

12 indicate an increase in drag

13 indicate an improvement or less drag

0 failures

Twenoydlvyean of scem h r DOt Amd ADe nV t,

A.25 NUREG/CR-5870
Appendix A

August 2, 1990

Dave Brown

Page 2

Also included in the data are 1/4, 1, and 35 size snubbers, I haven't

summarized all of these at this time. For the 1/2s, 3s, and 10s the test data

is not conclusive in showing increasing drag over time, i.e., the 10s and 1/2s

show improvement over time. If you have any questions give me a call.

Sincerely,

Senior Development Engineer

Energy Sciences Department

EVW/cdr

Enclosures

cc: Mike Kimel

Scott Cole w/o enclosures

John Mucci w/o enclosures

Don Blahnik-w/o enclosures

NUREG/CR-5870 A-26
Appendix B

In Situ Monitoring Methods and Equipment for Snubbers
Appendix B

In Situ Monitoring Methods and Equipment for Snubbers

This section includes a discussion and examples of in

situ monitoring methods and equipment for snubbers.

Methods and equipment are available to monitor snubbers periodically or continuously for service stressors

and degradation. Snubbers can be monitored either

individually or collectively on a system basis. In situ

monitoring is commonly used to confirm design loads

and to help analyze problem snubbers located in severe

environments. Stressors commonly monitored are load,

vibration, and temperature. Snubber stroke position

can also be monitored to verify thermal movements; displacement transducers are often used for this purpose.

Load Monitoring

Snubber loads can be monitored when the calculated

design loads are to be verified or when piping and equipment adjustments need to be made. Loads can also be

monitored on specific snubbers where overloading or

excessive drag force is suspected. Snubbers placed in

locations subject to water and steam hammer or flow

stratification can be monitored for excessive loading.

Loads are commonly measured using shear pin transducers that replace the snubber clevis pins (Figure B.1)

during the monitoring procedure. The shear pin transducer uses a strain gage element that is sensitive to

shear loads (Figure B.2). Bi-axial shear pin transducers

are available where two components of the load need to

be measured. The measurements can be read on a real

time or recorded basis.

Temperature Monitoring

Snubbers subject to a potentially high temperature

environment can be evaluated by monitoring the snubber's local temperature. The monitoring data can

identify applications that require augmented

surveillance or environmental modifications. The data

can also help improve service-life predictions in such

applications.

Temperatures can be accurately monitored, even in difficult access areas, using portable, non-contact infrared

temperature measurement tools on a spot or continuous

basis. Where practical, thermocouples and RTDs can be

used to provide continuous data on the snubber at the

heat source. Thermometers are used for measuring

temperatures where access is possible. Thmperaturesensing tape can be used to register peak temperatures

on the snubber during operations. A shortcoming of the

latter two alternatives, however, is that a continuous

record is not provided.

Vibration Monitoring

Where vibration is suspected, snubbers can be monitored using state-of-the-art vibration monitoring equipment. Characterization of these conditions can help

mitigate vibration effects through corrective action, such

as system modification.

Various remote and local vibration monitoring equipment is available. Hand-held instruments are easier to

use, but are limited to use only in those locations that

are accessible during plant operation. Remote monitoring of vibration is another alternative; this approach

generally involves instrumenting the pipe or snubber

with accelerometers. Typical acceleration monitoring

instrumentation is shown in Figure B.3. Acceleration

location isometrics are shown in Figure B.4. Frequency

and amplitude are the most important parameters for

characterizing vibration.

Note: Displacement transducers are generally not

practical for use in monitoring snubber vibration due to

their lack of adequate response.

B.1 NUREG/CR-5870
z

m

00

Locking Ring

Snubber

Base Bracket

Load Sensing

Clovis Pin

Anti-Rotation Plato

Figure B.1 Load sensing clevis pin
Appendix B

Base

Front

&

Back

Place Two (2) - 2 Element 90° Tee Rosettes at

Mid Span of Strut 180° Apart

A - Front Axial

B - Front Poisson

C - Back Axial

D - Back Poisson

Strain Gage Bridge Connection

R9106142.1

Figure B.2 Stain gage element for load monitoring of snubbers

13.3 NUREG/CR-5870
z

c-)

00xi

TO

)4FfADAC DIATA ACUTSUTT'Ot AN R fUMTON SY!T E

Figure B.3 Acceleration monitoring instrumentation scheme
a

8

6;

A, 2 r a?4 ~ R4~/9 36• - 52 "A4VM .Srr411A4tr.e .--- " - - -- I.

/-O~~~~~~~~~~~e or rr

M Figure B.4 ¶Ipical acceleration location Isometric

0 > PO 

00 a

0o C~~~~~~~~~~~~~~~ ~~~~~~~w
z

00

C0

TO

Alos r#

r 0

a

I.7

-j

X.-

-I

: 414w Srzt4,

Covr4AIMEvr fv r&4roASv

Figure B.4 (Continued)
Xv-Iawf /VoaxxtjS

44ry

I

,DrAu t

-J

z

(J'

00

?5

0

/lf1AW -S TA-i

7., ,"OA

Figure B.4 (Continued)
Appendix B

TRANS'-NTw vI3RATION DATA

Test Mode: Turbine Scop Valve Trip

Systa: Main Saais

r , I I

I Sz,:qSOz I

I ID* I

I A-009 I

I A-010 I

I A-011 I

I A-012 I

I A-013 I

I A-OIl I

i A-015 I

I A-016 II

I7-00 1.4

17-002 1*

1 F-003 I

I 7-OOAM

1 F-007 1

I 7-008 I

I 7-4009 I

i 7-010 I I F-aos 12

i I F-013 72Ii

I 7-013

I P-005 i

LEVEL I 1 LEVIL 2

ALLIO'WA3LX zmcn

(Yils,lbsj (.fils,

os *si) I lbs, vsi)I

MA I MA

NA MA I _ MA _ I

MA I XA t

NA ! MA

MA I A I

MA 1 !A I

MA I A I

15300 1 13900 1

33200 30100

36400 1 33100 1

38900 ! 35300 _

17400 1 15800 1

33000 1 3A500

19800 I 18000 1

37100 1 33700 1

81400 1 74000 1

57300 1 52100 1

NA I NA I

MA.ZLMUM =10 -1

P!AK READ IMG*"4

(Xils, lb., 1

,si)

772-~~~

47.7 1 0. 1

Z~~i ~I

1--t. 1,4

. Z 1 1 I

ZlOr; I

1 bG I

I

till I

oSL. (> I

I I

WITR13 L!.V!LT

ALLCWI3LZ

_(ye/Mo)

MA

NA

NA

A -

IA -

MA

MA -

M-RA

I -

I -

I =

11

1

-I

-I

-I

-I

-I

I1

(Yes/Mo)

RA

MA

NA

MA

MA

MA

RA

21

-I

.1

~1

-I

~1

. _ ,

- _ -

'am'-S: *A - 7 -

7 -

Accelerometer, 7ibration in mils i.?*--,ok

Force, Static Forcs in lbs

PSessUre, Static Pressure in psi

*Xazi:uz positive or negative deviation from initial, baseline value.

MA - Not Applicable - HX-surteen:t for iuloaretion cUly.

NUREG/CR-5870 B.8
Appendix B

TRANSIENT V1I3RTION DATA

Test Mode: Turbine Stop Valve Trip

System Main Steam

I

L

L

L

I.

I.

I.

L

III.

I.

I.

1.

I LZVEL I 1 LVEL 2 XMA xM ZRO - W 1

SENSOR ALLOWA3US I EXPEC PEAK RMDING*lI j

D* (Mils) I (Xils) I (MlS) I

D-001 I N I NA I 31_ I I7_

D-002 1 NA I RA I 2Y 1

D-0031 RA XA I (O I

D-004 I XA I RA I &/LE7 I

D-005 I NA I NA 1 121 I

D-006 I ! N AI I 

D-007I A I A I ,

D-0081 NA 1 NA 3

D-009 I A NA 1

D-010 NA I NA

D-011 NA I NA -70

D-012 NA I NA I .

D-013I RA I NA I "

D-OI 402 1 366 1 i

D-015 I NA I NA 101

D-0161 NA 1 NA I I

D-017 404 1 367 1 1

A-0021 NA I IA I

A-0021 NA R A

A-003I NA I A I 24

A-004 1 A NA I I

A-005I RA NA I 3S3 I

A-006 1 A 1 NA I 1 i

A-0071 N I NA I Z7

A-0081 1;A I NA I 9 3

.HIN LEVEL

=LLWA3LE-

(Yes/No)

KA

KA

RA

NA

NA

XA

NA

NA

KA

NA

KA

KA

RA

NA

KA

RA

NA

KA

KA

NA

NA

2&A

Ii

~1

.1

.1

.1

~1

.1

~1

I

WITHIN LEVEL

Z3'EC3"D

(Tes/No)

KA

KA

1UA

NA

KA

RANA

NA

NA

RA

NA

NA

RA

NA

NA

RA

HA

RA

NA

NA

RA

VAs

2J

z -I

-I

-J

I1

I1

-I

I1

I

-I

-I

IROES: *A - Accelerometer, Vibration in ails sero-peak

D - Displacement. Vibration in ails zero-peAk

**Maximum positive or negative deviation from initial, baseline value.

NA - Not Applicable - Mesauvereats for Infomation only.

B.9 NUREG/CR-5870
Appendix B

Other Monitoring Methods and

Equipment

Other methods and equipment are available for evaluating specific snubber characteristics that may relate to

service degradation.

* Fluid particle analysis equipment can be used to

identify and to establish the density and size of

solid particles in the hydraulic fluid. Such

equipment is particularly useful for evaluating

potential degradation in snubbers that are left

in place for extended periods, e g., snubbers that

are not removed for refurbishment or functional

testing. Particle contamination can result from

internal corrosion products or from wear due to

vibration.

* A boroscope is a useful device for internal visual

examination of snubbers. This apparatus can be

used to detect evidence of wear or other internal

damage resulting from service.

* Spectrographic analysis equipment is often used to

identify the source of contaminants in snubbers.

* Aerometers are used to determine the level of

entrained and dissolved air in the hydraulic fluid.

* Various types of equipment are also available for

determining the moisture content of hydraulic fluid.

* Acoustic emissions technology offers some potential

for in situ monitoring (e.g., vibration) of snubbers.

This method should be evaluated further.

NUREG/CR-5870 B.10
Appendix C

9I pical Arrhenius Seal Life Extrapolation (Plant A)
Appendix C

typical Arrhenius Seal Life Extrapolation (Plant A)

This Appendix generically illustrates an empirically

derived seal life curve reflecting an Arrhenius type

relationship between seal life and temperature. A

mathematical representation in this regard is also

included.

C.1 C.' ~~~~~NUREG/CR-
Appendix C

Empirically Derived Seal Life Curve

. . ... .- f . -... _ : . -

_ _- _ _ _ -- ---

CD

LU.

LLI

LU.

CA

LU

-i- -. _ __ A_

- --- _ _ _ _ _- - -- '--

F 7~~~~~~ I-- ~II ' ! _ _ _ _ _ _ _ _ _ _ * J z I- -- :-- ~

I I I

-- -- ~ - - - - - -.- - I. -

9-_

- .- =. . - -  --.-

_ . I A z -I

______ _ _ _..z -. zz z r zz i ~ -

, , ___ .= _ __ = _ e =.

.. I _ Jo__

=- =

I ---r- - -7- ~ -~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~Z1.i71=

_____________ --- - --- - -r-- - --- --- -,-- ----- .-.-------- , -____ -

z I -_ _, _ _____-__

------ ---- 1 ------

I

1 I

S--l-et-4 I-.,

3 . .11 ,, 11 ... h

TEMPERATURE OF

In (L) = mTn + b

L = life

m = constant

T = Temperature

al = constant

b = constant

NUREG/CR-5870 C.2
Appendix D

Effects of Compression Set on Low Pressure Seal Performance
Appendix D

Effects of Compression Set on Low Pressure Seal Performance

This Appendix contains procedures and results for

evaluation of seals removed from nonleaking snubbers

that were anticipated to have high compression set. The

goal of this evaluation was to obtain data that could be

used to substantiate compression set limits, based upon

low pressure performance, for various configurations of

elastomeric seals. Inspection data sheets are included to

show snubber conditions and as-found seal dimensions.

Due to the limited amount of data in this regard, optimum compression set limits could not be totally

confirmed.

DA1 NUREG/CR-5870
Appendix D

PRELIMINARY SEAL EVALUATION SUMMARY

COMPRESSION SET LIMITS STUDY

Preliminary evaluations were performed on six snubbers with no visible signs of

leakage. For the preliminary evaluation, easily accessible face seals were chosen. These are located between the valve body and reservoir on Bergen-Paterson

snubbers and between valve body and cylinder cap on Grinnell model PH-74 snubbers.

To justify a secondary evaluation, a minimum compression set level of 45Z was

established. Only two of the six preliminary evaluations yielded a high enough

level of compression set to justify secondary evaluations. The results of the

preliminary evaluations were as follows:

Sample

Number

Snubber

Type

Plant

Type Comments

1 Bergen-Paterson

2 Grinnell PH-74

3 Grinnell PH-74

4 Bergen-Paterson

5 Bergen-Paterson

6 Bergen-Paterson

Fossil This snubber had been in service for a

relatively short time period, reportedly

in a severe environment. Evaluation did

not, however, result in compression set

levels high enough to justify continued

evaluation.

Fossil This snubber had been in service for

approximately 15 years. This snubber had

been shipped with no fluid and was removed

from further consideration.

Fossil This snubber had been in service for

approximately 15 years under high

temperature (a) conditions. Visual

inspection revealed no signs of leakage.

Compression set evaluation resulted in

levels that were high enough to justify a

secondary evaluation.

Fossil This snubber had a long service life in a

relatively moderate environment. Visual

inspection revealed no signs of leakage.

Preliminary evaluation did not justify a

secondary evaluation.

Nuclear This snubber had a short service life in a

high temperature (a) environment. Visual

inspection revealed no signs of leakage.

Preliminary evaluation did not justify a

secondary evaluation.

Nuclear Moderate service life (6 years) in a high

temperature (a). No leakage. Secondary

evaluation conducted.

(a) Greater than 150 0F.

NUREG/CR-5870 D.2
Appendix D

SECONDARY COMPRESSION SET EVALUATION

1.0 BACKGROUND

This procedure delineates the methods used to measure compression set in

seals removed from snubbers selected for compression set evaluation for

the NPAR Snubber Aging Study.

2.0 EVALUATION PARAMETERS

Note: Values for the below described parameters are included in Table 3

for Grinnell snubbers and Table 4 for Bergen-Paterson snubbers.

Where more than one measurement was obtained for given seal or

gland, average values are listed.

2.1 Original Seal Thickness

Since original seal thicknesses (WO) were not measured, thickness

values are based upon manufacturer's information (nominal values).

2.2 Recovered Seal Thickness

Recovered seal thickness (W1) is defined as the post-service

thickness of the seal after removal from the gland.

2.3 Compressed Thickness (Simply Compressed Seals)

Compressed thickness (W.) i4 the thickness of the seal when it is

installed in the gland.. The compressed thickness for each simply

compressed seal was determined from snubber dimensional data obtained

during the evaluation. Specific equations used in this regard are

listed below for each seal.

Note: Dimensional variables are identified in Figures 1 through 4

and Table 1 for Grinnell snubbers and in Figures 5

through 7 and Table 2 for Bergen-Paterson snubbers.

D.3 NUREG/CR-5870
Appendix D

2.0 EVALUATION PARAMETERS

2.3 Compressed Thickness (Simply Compressed Seals) (Cont'd)

2.3.1 Compressed Thickness (Bergen-Paterson Snubbers)

2.3.1.1 Item 3 - Reservoir Piston Seal

I - H

Ws ' 2

2.3.1.2 Item 4 - Valve/Reservoir Mounting Seal

ws ' J

2.3.1.3 Item 13 - Piston Rod Seal

S - R

Ws M 2

2.3.2 Compressed Thickness (Grinnell Snubbers)

2.3.2.1 Item 1 - Piston Rod Seal

Ws = H

2.3.2.2 Item 2 - Piston Seal

P - Q

WS - 2

2.3.2.3 Item U - Valve Mounting Seal

Ws ' F

NUREG/CR-5870 D.4
Appendix D

2 .0 EVALUATION PARAMETERS

2.4 Compression Set

Compression set is defined as the amount of permanent deformation

expressed as a percentage of the initial seal compression.

Compression set (C) was calculated using the following formula:

W - W

C - W -W 100 0 8

where WO - initial seal thickness

Wl recovered seal thickness after removal from the

gland

Ws ' gland width (or compressed thickness of the seal

when installed)

2.5 Instrumentation and Test Equipment

Dimensional data was obtained using calibrated measuring devices*

such as micrometers, depth gages, calipers, feeler gages, etc.

D.5 NUREG/CR-5870
Appendix D

TABLE 1

LIST OF SEALS AND SEAL RELATED PARTS

FOR GRINNELL MODEL PH-74 SNUBBER WITH MILLER CYLINDER

Part Quantity Per

Number Part Snubber

1 Miller Piston Rod Seal 1

2 Miller Piston Seal 2

3 Mil er Cylinder Tube End Seal 2

4 Reservoir Tube End Seal 2

5 Valve Barrel Seal (External) 2

6 Valve Barrel Seal (Internal) 2

7 Fitting Seal 4

8 Reservoir Connecting Tube Boss Seal (PH-74 only) 2

9 Valve Body Plug Seal (PH-74 only) 1

10 Miller Fill Plug Seal (PH-74 only) 2

11 Valve Mounting Seal (PH-74 only) 2

12 Thread Seal - Valve Connecting Tube (P1-74 only) 2

13 Thread Seal - Locking Velocity Screw 2

14 Thread Seal - Bleed Rate Screw (PH-74 only) 2

18 Miller Rod Bushing 1

19 Miller Cylinder Tube 1

20 Reservoir Tube 1

21 Reservoir End Cap 2

22 Valve Barrel 2

23 Valve/Reservoir Conn. Tube (PH-74 only) 1

26 Miller Piston/Piston Rod Assembly 1

27 Valve Body 1

28 Valve Block (PH-74 only) 1

29 Miller Cylinder Head 1

30 Miller Cylinder Cap 1

31 PH-74 Tube Fitting 2

NUREG/CR-5870 D.6
AppendblxD

TABLE 2

LIST OF SEAL RELATED PARTS

Part Quantity Per

Number Part Snubber

1 Reservoir Bleed-off Screw Seal 1

2 Reservoir Tube End Seal 1

3 Reservoir Piston Seal 1

4 Valve/Reservoir Mounting Seal 2

5 Connector Tube/Stop Plug Boss Seal 2

6 Port Plug Seal (2)

7 Relief Valve Plug Internal Seal 1

8 Relief Valve Plug Boss Seal 1 (1)

9 Back-up Ring 1

10 Connector Tube Sliding Seal 1

11 Cylinder Tube End Seal I

12 Allemite Fitting Seal 1

13 Piston Rod Seal 1

14 Reservoir Bleed-off Screw 1

15 Reservoir Tube 1

16 Reservoir Cap 1

17 Reservoir Piston 1

18 Control Valve Body 1

19 Poppet Stop Plug 1

20 Port Plug 1

21 Relief Valve Plug 4

22 Connector Tube 1

23 Cylinder Read 1

24 Piston 1

25 Piston Rod 1

26 Rod Bearing 1

27 Cylinder Cap 1

28 Cylinder Tube 1

29 Reservoir Head 1

NOTE: (1) Manifold configuration only

(2) Quantity varies depending upon snubber size and configuration

D.7 NUREG/CR-5870
Apppendix D

-CYLINDER

PORT

/---% %-DETAIL G

FIGURE 1

GRINNELL P11-74 VALVE AND RESERVOIR ASSEMBLY

NUREG/CR-5870 D.8.
Appendix D

-VALVE PORT (TYP)

(MODEL PH-74 ONL

..-- E I "I, 7, T R O1 ROD WIPER

voo

~__ \ _ ~ _ _" _ - , >

Y ) \SE \DETAIL K

FIGURE 2

GRINNELL MILLER CYLINDER ASSEMBLY

D.9 D.9 ~~~~~NUREG/CR-
Appendix D

BUSHING

SECTION A-A

DETAIL A

FIGURE 3

GRINNELL DETAILS - SIIEET I

NUREG/CR-5870 D.10 D1
Appendix D

DETAIL K

FIGURE 4

GRINNELL DETAILS - SHEET 2

D.11 D. 11 ~~~~NUREG/CR-
z

DET.C DET. D

( i s _ BID E T~~~~~~~~~~~~~~~~~~~~E. I

0 WIPER

I / ~PISTON t //

4 4 ~~PISTON ROD y8

FIGURE 5

BERGEN-PATERSON GENERAL ASSEMBLY
Appendix D

7

I(DIA)

I

DETAIL D

DETAIL E

FIGURE 6

BERGEN-PATERSON DETAILS - SHEET 1

D.13 NUREG/CR-5870
Appendix D

R (DIA){

f 13A

DETAIL K_

FIGURE 7

BERGEN-PATERSON DETAILS - SHEET 2

NUREG/CR-5870 D.14
Appendix D

TABLE 3

GRINNELL EVALUATION PARAMETERS

Seal No.

1

2

11

Wo

(inches)

.128

.330

.070

Wi

(inches)

.103

.276

.061

Ws

(inches)

.101

.250

.050

Compression Set

(X)

93Z

69%

52%

TABLE 4

BERGEN-PATERSON EVALUATION PARAMETERS

Seal No.

3

Wo

(inches)

.312

Wi

(inches)

.276

.098

.273

Ws

(inches)

.250

.078

.250

Compression Set

(X)

50%

20%

63%

4

13

.103

.312

D.15 NUREG/CR-5870
Appendix D

DATA SHEET 1

GRINNELL

Page 1 of 1

Snubber Sample Number: 3

Size (Bore X Stroke): PR -7g

Configuration: rMi;ller

GENERAL COMMENTS:

a 5n, e in 3

r ~~e j

4

h yie-A I -" ) 4tfr

II

O o nj s5 ~r, s wJ, An1ajec ~e C P l keAIa i a

Ins p. by: s J i i L . . . Date: 7/3 /90

NUREG/CR-5870 D.16 D1
Appendix D

DATA SHEET 2 Page 1 of 1

GRINNELL

Snubber Sample Number: 3

Model: 'PW-74

Cylinder Type: MIl-r

POST-SERVICE SEAL DIMENSIONAL DATA

Ref.

Part

No.

2

1

Radial/ Loc.

Face Code

PI

N/A

F

B

Sequence

Number

1

2

3

4

I

2

3

4

Meas. (Wl)

(inches)

.a 765

-a 70

.a go ..272

.16t a

.0 bo

.0 by1

D 1.1

.rOa

.16A

/n-s

.Mg

Comments

11 A 1

2

3

4 -

COMMENTS:

Insp. by: ____ Date: 7/3__/9 _

D.17 NUREG/CR-5870
Appendix D

DATA SHEET 3 Page 1 of 1

GRINN*ELL

Snubber Sample Number: ,

Model: PH-7'f

Cylinder Type: M:IvPOST-DISASSEMBLY PART DIMENSIONAL DATA

Part

No.

18

Loc.

Code Dim.

Sequence

Number

Meas. (WI)

(inches) Comments

H 1

2

3

4

.101

.101

19 P L4 001

A

B

T

T

I

2

3

4

1

2

3

4

-11

VA- i

26 B Q

27 F I

2

3

4

,D fis

.6nIff

.D49

.0-s0

COMMENTS:

Insp. by: :_ Date: 7/5/__

NUREG/CR-5870 D.18
Appendix D

DA'A SHEET 4 Page 1 of 1

BERGEN-P.VrERsON

Snubber Sample Number: to

Size (Bore X Stroke): 30

Configuration: M,;cAI A

GENERAL COMMENTS:

* . IiI!LvI - -7/ 9IC p .. ,-7~4~fq~(%l~A!

N Nn V~Ldo W 4 leha k&3

o 1;, very cIr -. 41 bI~ck 566t.It1. #

C uj -egej! in c4Ii-.%ise-4.6~e Avime iJ -. A

11 t . ):,40P AvA 4, of ~~d ' -

- ~AIki O4.6v 4~Al" ~ ~

T-4 skn.,O 4~~~~,,+ +06v- tjp~~~~-

Insp. by: _ Date: 7/1q9L)

D).19 NUREG/CR-5870
Appendix D

DATA SHEET 5 Page 1 of 1

BERGEN-PATERSON

Snubber Sample Number: ,

Size (Bore X Stroke): ,n

Configuration: M0;;,,L1I

POST-SERVICE SEAL DIMENSIONAL DATA

Ref.

Part

No.

3

4

Radial/

Face

R

F

Loc. Sequence

Code Number

- 1

2

3

4

Meas. (WI)

Cinches)

.ns

.2a77

jlft

,Ta7

.O6n

.rMs

.M%-

,rA7

Comments

A 1

2

3

4

-

13 R 1

2

3

4

all

-am

COMMENTS:

Insp. by: _ H Date: 7/ISI/O

NUREGICR-5870 D.20
Appendix D

DATA SHEET 6 Page 1 of 1

BERGEN-PATERSON

Snubber Sample Number: t,

Size (KIPS): 3c

Configuration: MA ;c Id

POST-DISASSEMBLY PART DIMENSIONAL DATA

Part Loc.

No. Code Dim.

Sequence

Number

1

Meas. (Wl)

(inches)

3.q14

3.Ct qq

Comments

15 I

17 H

18 A J 1

2

3

4

.Dfl

nil7

23 S

25 R

COMMENTS:

Insp. by: Date: 7/i/40

D.21 NUJREG/CR-5870
Appendix E

Evaluation of Drag Force Versus Time (Plant D)
Appendix E

Evaluation of Drag Force Versus Time (Plant D)

This Appendix includes drag force data obtained from

Plant D for two sizes of mechanical snubbers. The data

were used in an attempt to correlate drag force with

time. Al snubbers were tested on the same test

machine. No snubbers were tested on more than one

occasion. Average drag force (i.e., average of the drag

force values for several snubbers) is plotted versus

service time. No trends are evident.

E.1 NUREG/CR-5870
Appendix E

iable EA. Historical drag force data for snubbers at Plant D, Unit I (PWR)

Average T Average C Peak T Peak C

t

1984

PSA-L1

1984

PSA-1

I

1985

PSA-1/2

PSA-1

I

4.33

3.24

2.16

1.62

2.2

1.7

1.1

1.9

2.0

2.2

2.24

3.75

5.0

5.0

3.1

4.21

2.0

2.2

4.5

1.3

3.75

4.85

3.0

3.08

2.75

2.70

2.72

4.33

2.48

2.7

1.62

2.2

1.1

2.0

1.65

2.2

2.7

2.30

6.3

5.0

3.75

5.0

5.01

1.7

2.2

6.5

1.4

4.65

2.65

2.55

3.09

5.0

2.5

3.75

8.0

5.0

3.24

2.37

3.3

2.5

2.2

2.7

3.2

3.25

3.57

7.5

10.0

10.0

7.5

8.75

2.66

3.3

9.0

2.8

4.7

10.8

4.9

5.45

9.2

6.25

7.72

7.0

3.2

3.6

2.4

3.3

2.0

3.0

2.7

3.8

4.3

3.53

12.5

11.5

8.0

10.0

10.5

2.2

3.3

13.0

2.65

6.0

4.8

6.5

5.50

10.0

6.0

8.0

Note: numbers in bold indicate averages.

T = tension drag force (lb).

C = compression drag force (lb).

NUREG/CR-5870 E.2
Appendix E

Table El (Continued)

Average T Average C Peak T Peak C

2.0 2.5 4.4 4.5

1.65 1.75 3.25 3.20

2.0 2.2 3.50 3.75

7.7 10.25 12.0 15.5

3.1 3.0 4.8 4.3

1.60 2.05 3.85 3.90

1986 3.10 2.10 5.50 3.50

PSA-1/2 1.60 1.90 3.20 3.3

2.10 3.3 3.3 5.1

1.70 1.8 3.0 3.0

1.60 2.0 2.2 3.0

1.75 1.0 3.25 2.0

0.9 0.9 1.80 1.5

1 2.37 2.67 4.20 4.35

1 3.75 2.2 6.5 6.2

3.3 2.5 6.25 7.0

2.0 5.0 6.25 7.0

PSA-1 5.0 4.0 11.5 8.0

2.5 3.0 5.0 8.75

3.75 3.75 8.75 8.75

3.38 3.40 7.37 8.53

1.3 1.6 2.0 2.75

1.25 1.6 2.25 2.2

1.2 1.7 1.95 2.7

1.1 1.1 2.0 2.15

1987 1.7 1.85 3.0 1.0

PSA-1/2 1.25 .9 2.16 1.0

2.3 1.6 3.85 3.40

6.5 7.8 8.2 9.75

1.5 1.1 2.1 1.6

1.6 2.6 2.6 3.2

l 1.97 2.18 3.01 3.26

PSA-1 3.3 3.0 6.3 7.0

E.3 NUREG/CR-5870
Appendix E

lTble E.1 (Continued)

Average T Average C Peak T Peak C

2.89 2.11 5.55 4.12

4.60 2.35 6.95 4.80

3.37 3.10 6.5 6.15

2.45 2.75 6.0 5.62

1988 2.95 2.75 6.10 4.45

PSA-1/2 2.37 2.75 4.40 4.35

1.78 2.45 4.20 4.65

3.30 1.92 6.25 3.92

2.76 5.20 5.10 7.05

2.95 2.82 5.67 5.01

I 5.56 4.55 10.95 11.67

5.79 5.71 13.27 12.96

6.75 4.15 11.02 8.66

PSA-1 5.17 6.53 11.17 15.99

6.24 9.42 13.52 22.98

7.63 5.64 13.30 13.30

5.63 5.36 12.12 11.55

7.25 5.91 13.42 12.75

1 6.25 5.90 12.35 13.73

f

7.40 5.20 13.0 11.0

t 5.20 4.50 8.30 7.80

1989 3.75 2.75 6.0 3.75

PSA-1/2 5.25 3.25 7.0 5.40

1.60 1.10 3.10 1.90

0.85 1.25 1.75 2.10

2.0 2.25 3.50 4.25

1 3.72 2.90 6.09 5.17

2.5 2.75 5.0 7.50

PSA-1 9.0 2.50 17.5 6.25

5.0 6.25 10.0 12.0

5.5 3.83 10.8 8.58

NUREG/CR-5870 E.4
Appendix E

UNIT 1

PLANT D PSA-1/2

Aon

4 i; Drag otAverage Tensoun

2 ___ Average Drag Compression

INAL 1984 1985 1986 1987 1988 1989

Figure F-l PSA-1/2 drag force versus service time Unit 1, Plant D

E.5 NUREG/CR-5870
Appendix E

UNIT I

PLANT 0 PSA-1

I I

I l I X i i

0

.-

Figure E.2 PSA-1 drag force versus service time

NUREG/CR-5870 E.6
Appendix E

Table E.2 H1istorical drag force data for snubbers at Plant D, Unit 2 (PWR)

1985

PSA-1/2

(26)

1985

PSA-1

(15)

Average T

2.00

1.56

1.25

1.60

3.20

1.50

2.25

1.90

1.80

2.70

2.20

4.85

1.68

1.95

2.20

2.60

1.70

3.40

2.00

2.00

3.10

2.10

3.40

1.00

1.80

1.15

2.05

&20

7.50

6.00

3.50

5.00

5.50

3.00

3.75

6.00

4.00

11.00

4,85

6.00

3,50

F.43

Average C

1.60

1.00

1.75

1.40

2.70

3.00

2.45

1.80

1.45

1.60

1.75

1.60

1.75

2.85

1.45

2.75

1.45

3.24

2.10

3.70

3.00

1.70

3.60

1.10

1.70

1.40

2.07

9.50

3.00

4.00

3.75

5.20

4.50

3.75

3.95

2.50

4.50

8.50

11.00

3.50

4.50

6.01

Peak T

3.20

2.20

3.20

3.25

5.40

3.25

3.25

3.10

3.25

3.75

3.75

9.30

3.20

3.00

3.50

5.90

2.70

5.40

2.70

4.30

3.85

3.25

5.65

2.15

2.90

2.25

3.75

15.00

17.50

12.50

10.00

11.00

11.50

7.50

10.00

13.00

17.50

18.50

10.50

12.00

8.50

12.50

Peak C

3.25

2.10

3.75

2.35

5.30

6.50

3.25

3.00

2.50

2.60

3.25

2.90

3.25

4.20

2.65

4.60

2.25

5.20

3.10

7.58

4.35

2.40

7.10

2.16

2.70

2.20

3.6

26.00

12.50

10.00

8.75

11.50

11.25

11.00

12.00

11.00

12.50

17.50

24.00

15.00

11.50

13.90

T = tension drag force(lb).

C = compression drag force (lb).

E.7 NUREG/CR-5870
Appendix E

Table E.2 (Continued)

Average T Average C Peak T Peak C

1.30 1.30 2.40 3.00

1986 1.10 1.50 2.00 3.20

1.05 2.00 2.20 3.25

1.20 1.85 2.00 2.65

1.70 2.40 3.25 3.75

1.85 1.10 3.85 2.40

1.10 1.60 1.60 2.50

1.55 4.80 4.00 13.00

PSA-112 2.20 2.70 3.50 5.40

(19) 1.00 1.60 1.75 3.00

1.20 1.65 2.45 2.75

1.10 .90 2.05 1.55

1.35 1.95 2.55 2.35

3.40 2.75 4.75 3.85

1.20 1.20 2.20 1.95

0.95 1.45 1.65 2.20

2.35 1.65 3.70 2.60

2.25 1.00 3.20 3.25

1.62 1.95 2.87 3.60

3.45 3.25 6.90 7.00

4.25 6.30 11.00 14.00

PSA-1 3.75 2.50 6.25 6.75

(6) 3.40 4.45 8.25 9.00

3.50 3.60 7.50 11.25

5.10 3.75 16.0 15.0

3.90 3.97 9.31 10.5

1.70 1.60 3.00 4.80

2.00 2.85 3.50 4.30

2.40 3.50 6.50 4.50

3.30 1.40 2.40 8.50

1.60 5.40 7.00 4.32

1987 2.50 3.15 7.40 3.00

PSA-1/2 1.00 2.25 4.80 2.65

(14) 1.90 3.25 2.70 2.4

2.40 3.00 3.00 4.3

3.25 2.20 4.30 6.4

1.60 4.80 5.40 2.2

4.40 2.15 3.20 10.0

3.00 2.00 3.60 4.7

3.20 1.55 2.20 4.2

2.44 2.80 4.21 4.5

5.00 4.00 10.00 8.75

3.75 2.75 7.00 6.50

NUREG/CR-5870 ES8
Appendix E

Table E.2 (Continued)

Average T Average C Peak T Peak C

PSA-1 5.00 3.00 11.00 7.50

(6) 2.50 3.00 6.00 7.50

2.50 3.75 5.10 8.75

3.00 3.70 8.20 20.0

3.62 3.36 7.88 9.8

2.62 6.07 6.10 8.69

3.07 4.08 5.58 7.44

1.65 1.66 2.85 5.70

2.92 2.63 5.44 6.73

5.03 4.26 7.33 6.94

1.73 3.33 3.08 6.40

3.55 2.05 5.38 3.63

1988 1A3 3.05 2.96 5.11

(21) 3.65 2.48 6.06 5.50

4.26 2.64 5.64 6.37

8.62 4.74 16.30 8.63

PSA-1/2 2.53 1.85 3.75 3.08

4.39 4.72 11.20 12.70

3.69 2.47 6.25 4.95

1.38 2.90 2.92 4.41

4.49 3.85 7.55 9.43

3.72 1.91 6.01 4.54

3.74 1.81 6.20 3.76

3.99 2.30 6.14 5.52

1.66 3.14 3.95 6.74

2.54 2.27 5.29 4.30

3.36 3.05 6.0 6.21

6.19 5.67 11.00 11.96

4.69 5.99 10.24 10.34

5.61 5.01 11.92 10.15

6.42 5.63 13.93 12.64

6.43 4.51 14.63 11.99

PSA-1 3.68 6.08 6.95 12.87

(11) 16.68 9.12 37.13 21.52

11.64 7.90 22.74 15.68

9.76 10.18 18.59 18.30

6.85 6.18 15.71 12.00

6.49 8.45 21.29 23.97

7.67 6.79 16.74 14.66

4.20 2.70 8.66 4.32

1.90 1.20 3.70 3.25

4.32 3.25 6.80 7.00

0.85 3.25 2.00 4.85

3.25 3.25 5.10 5.40

4.60 5.40 7.60 7.60

E.9 NUREG/CR-5870
Appendix E

TIhble E.2 (Continued)

PSA-112

1989

(16)

PSA-1

(11)

Average T

1.10

1.70

2.30

1.20

2.00

2.25

1.35

3.10

1.70

1.15

2.31

2.00

2.00

5.00

3.75

5.30

3.00

4.45

3.75

3.75

2.50

3.75

3.56

Average C

1.10

1.20

2.00

1.60

1.00

3.10

1.40

3.20

1.95

1.65

2.32

2.25

2.00

4.50

3.10

3.90

5.00

3.20

3.00

2.50

3.20

5.00

3.42

Peak T Peak C

2.00

2.20

4.32

2.25

3.25

4.05

2.70

5.40

3.25

2.20

4.21

7.00

5.00

11.00

8.50

18.00

7.50

7.00

8.50

8.55

7.50

10.50

8.91

1.60

1.80

3.30

3.00

2.00

6.10

2.50

4.30

3.25

2.30

3.

5.10

4.90

12.00

7.00

11.00

10.50

7.00

7.00

6.20

7.00

9.25

7.96

NUREG/CR-5870 E.10
Appendix E

I

; i

I

I

i I . I I ,

I I

V 

%a . I

1 I

I

i I

I

i

I

i

I

j

I

F

i

L

I

I

1; I I

.i

i

I

I -

r

UNIT 2

PLANT D PSA-1/2

: .I - { -- 1 i

-7*

I

i 45

12

M

SC=

10

9

8

7

6

S

4

3

2

1

I i

Ii I j.i1I1iti...4' I .1..-

__ ___ I I

>o: I i I z ._

Peak Drag (Average) Tension

!.. - _ . ! Average Peak Compression

Average Drag Compression

IX Average Drag Tension

_ i

. -

_ _

° 1985 1986t 1987 1988 1989

Figure E3 PSA-1/2 drag force versus service time Unit 2, Plant D

E.ll NUREG/CR-5870
Appendix E

UNIT 2

PLANT D PSA-1

I8

17 i

i 1s

,s_ 14 d_

Figure E.4 PSA-1 drag force versus service time Unit 2, Plant D

NUREG/CR-5870 E.12 E1
Appendix F

Evaluation of Drag Force Versus Time (Plant F)
Appendix F

Evaluation of Drag Force Versus Time (Plant F)

This Appendix contains drag force data from site F for

one size of mechanical snubber. The data were used in

an attempt to correlate drag force with service time. All

snubbers were tested using the same type of test

machine. All snubbers were tested on a minimum of

two separate occasions (refueling outages). Average

drag force (average for all snubbers at each refueling

outage) is plotted versus service time. Individual snubber drag force is also plotted versus service time for five

typical snubbers, samples A through E. Some trend

toward increasing drag force with service time is

observed.

El1 NUREG/CR-5870
Appendix F

2% OF RATED LOAD 30-

25-.

- 20 X PEAK COMPRESSION

so ,/ 75- 1% OF RATED LOAD

0 -- - - ----- X PEAK TENSION

.- -: . X AVERAGE COMPRESSION

a10- - AXXX AVERAGE TENSION

5- -

5. 6 7 18 9 10

SERVICE TIME (YRS.)

Figure F.I Drag force vs. service time, Plant F average of all samples

NUREG/CR-5870 F.2
Appendix F

en

m

-J

w

0:

L

cl:

COMPRESSION

X -.XAVERAGE COMPRESSION

- - - -XEA TENSION

X x A E R G E E N I O

Xr XAVERAGE TENSION

SERVICE TIME (YRS.)

Figure F.2 Drug force vs. service time, Plant F sample A (# 188)

F.3 E3 ~~~~~~NUREO/CR-5870
Appendix F

PEAK COMPRESSION

en

m

03

-J

w

0

LL

QD

0:

7'l7

7

7,

"- AVERAGE COMPRESSION

_a PEAK TENSION

X / w~~~~~>AVERAGE TENSION

Xz

X

SERVICE TIME (YRS.)

Figure F.3 Drag force vs. service time, Plant F sample B (# 96)

NUREG/CR-5870 F.4
Appendix F

30- 2% OF RATED LOAD

25

20-

m

.4 a< PEAK COMPRESSION

0 15-l1% OF RATED LOAD

IL X _X PEAK TENSION

(. X__ __

.- ~-=X-- X AVERAGE COMPRESSION

a10-_

X~~~

xAVERAGE TENSION

5- x-- -I

l !6 17 le 19 l

SERVICE TIME (YRS.)

Figure F.4 Drag force vs. service time, Plant F sample C (# 62)

F.S F5 ~~~~~~NUREG/CR
Appendix F

2% OF RATED LOAD

-, 20

(n

an

-j

a 15.

0

o(9

a 10-

_y PEAK COMPRESSION

1% OF RATED LOAD

>to PEAK TENSION

_X--X AVERAGE COMPRESSION

X~~ B

X _< AVERAGE TENSION

x

x

SERVICE TIME (YRS.)

Figure E5 Drag force vs. service time, Plant F sample D (# 13)

NUREG/CR-5870 F,6
Appendix F

2% OF RATED LOAD

v" Pr,

m

w d PEi

0: 15- '- AVE

oL 1 OF RATED LOAD

X/

10 / / AVE

/ ~~X

x

5- x

5 6 7 8 9 Ii

SERVICE TIME (YRS.)

Figure F6 Drag force vs. service time, Plant F sample E (# 15)

AK COMPRESSION

AK TENSION

:RAGE TENSION

F.7 NUREG/CR-5870
Appendix F

Table 1 Historical drag force data, Plant F

Exam

Number

7

2R4(a)

Av. T 1.6 + 7.0 - 4.3

Av. C 12.9 + 15.2 - 14.1

Pk. T 6.0 + 12.5 - 9.3

Pk. C 18.0 + 20.0 - 19.0

8 Av.

Av.

Pk.

Pk.

13 Av.

Av.

Pk.

Pk.

T

C

T

C

T

C

T

C

5.5 + 3.9 -

10.1 + 10.9 -

10.0 + 6.1 -

14.0 + 14.0 -

3.1 + . 8-

6.6 + 7.4 -

6.0 + 4.2 -

13.0 + 13.0 -

4.7

10.5

8.0

14.0

2.0

7.0

5.1

13.0

2R5

3.7 + 7.8 - 6.8

15.1 + 12.4 - 13.8

8.4 + 11.3 - 9.8

23.3 + 19.3 - 21.3

4.0 + 5.3 - 4.7

8.9 + 8.1 - 8.5

11.6 + 10.0 - 10.8

15.1 + 13.9 - 14.5

1.9 + 5.2 - 3.6

9.4 + 7.6 - 8.5

5.3 + 8.5 - 7.9

14.2 + 12.0 - 13.1

2R6 2R7

4.8 + 9.8 - 7.3

11.1 + 8.1 - 9.6

12.7 + 21.6 - 17.2

20.5 + 17.2 - 18.8

8.3 + 4.1 - 6.2

8.8 + 11.8 - 10.3

11.8 + 10.2 - 11.0

14.2 + 17.8 - 16.0

14 Av.

Av.

Pk.

Pk.

T

C

T

C

1.6 + 3.5 -

.4 + 4.7 -

6.2 + 7.0 -

6.0 + 10.0 -

2.6

2.6

6.6

8.0

7.5 + 5.7 - 6.6

7.3 + 6.2 - 6.8

12.6 + 10.4 - 11.5

12.8 + 14.6 - 13.7

15 Av.

Av.

Pk.

Pk.

T

C

T

C

4.7 + 5.9 -

2.7 + 2.0 -

9.5 + 13.0 -

9.0 + 8.0 -

5.3

2.4

11.3

8.5

8.1 + 7.1 - 7.6

9.6 + 8.5 - 9.1

16.9 + 16.1 - 16.5

16.6 + 14.6 - 15.5

9.71 + 10.74 - 10.2

15.52 + 16.76 - 16.1

15.66 + 17.01 - 16.3

20.28 + 21.30 - 20.8

16 Av.

Av.

Pk.

Pk.

17 Av.

Av.

Pk.

Pk.

18 Av.

Av.

Pk.

Pk.

C

C

T

C

T

C

T

C

T

C

T

C

4.3 + 3.9 -

.8+ .4 -

10.0 + 9.0 -

5.0 + 6.0 -

5.1 + 3.1 -

3.1 + 5.5 -

10.0 + 6.6 -

8.0 + 10.0 -

2.0 + 1.2 -

12.5 + 12.9 -

6.0 + 7.0 -

19.0 + 17.5 -

4.1

.6

9.5

5.5

4.1

4.3

8.3

9.0

1.6

12.7

6.5

18.3

4.0 +

10.4 +

8.5 +

16.0 +

6.3 + 7.3 -

10.1 + 9.6 -

9.9 + 10.9 -

16.9 + 17.2 -

6.2 + 6.4 -

10.8 + 8.5 -

10.8 + 10.3 -

17.8 + 14.3 -

6.8

9.8

10.4

17.1

6.3

9.7

10.6

16.1

24 Av. T

Av. C

Pk. T

Pk. C

26 Av. T

Av. C

Pk. T

Pk. C

9.0 + 9.0

9.4 + 11.7

32.0 + 35.0

30.0 + 35.0

7.0 + 11.3

7.0 + 15.2

19.0 + 24.0

22.0 + 28.0

C,)

SO

a,

9.3 + 7.5 -

.6 + 10.2 -

14.7 + 10.9 -

9.4 + 21.1 -

8.4

5.4

12.8

15.3

14.8 + 9.2 - 12.0

14.2 + 4.7 - 9.6

23.0 + 16.3 - 19.8

29.3 + 22.5 - 25.9

4.2 + 3.4 - 3.8

10.1 + 9.5 - 9.8

10.8 + 11.6 - 11.2

17.1 + 17.1 - 17.1

8.4 + 21.5 - 14.9

15.5 + 15.1 - 15.3

13.3 + 26.3 - 19.8

23.7 + 22.7 - 23.2

(a) 2 = unit 2 (PWR), R = refueling outage number, T = tension drag force (lbs.), C = compression drag force (lbs.)

NUREG/CR-5870 F.8
Appendix F

Table 1 (Continued)

Exam

Number 2R4(a) 2R5 2R6 2R7

42 Av. T 6.2 + 5.1 - 5.6

Av.

Pk.

Pk.

43 Av.

Av.

Pk.

Pk.

44 Av.

Av.

Pk.

Pk.

45 Av.

Av.

Pk.

Pk.

46 Av.

Av.

Pk.

Pk.

47 Av.

Av.

Pk.

Pk.

62 Av.

Av.

Pk.

Pk.

C

T

C

T

C

T

C

T

C

T

C

4.7 + 5.9

11.0 + 12.0

10.0 + 12.0

-

T

C

T

C

T

C

T

C

T

C

T

C

T

C

T

C

3.9

7.4

7.0

13.0

4.7

10.1

7.5

15.0

2.8

9.8

6.0

14.0

5.1

4.7

10.0

24.0

4.3

12.9

10.0

18.0

5.9

7.8

14.0

12.2

+ 5.1 -

+ 9.0 -

+

+ .4-

+ 10.9 -

+ 5.0 -

+ 16.0 -

+ 4.3 -

+ 10.1 -

+ 8.0 -

+ 15.0 -

+ 4.3 -

+ 3.9 -

+ 9.0 -

+ 18.0 -

+ 3.5 -

+ 12.9 -

+ 10.0 -

+ 18.0 -

+ 3.5 -

+ 4.3 -

+ 11.0 -

+ 8.5 -

5.3

11.5

11.0

4.5

7.4

8.0

13.0

2.6

10.5

6.3

15.5

3.6

9.9

7.0

14.5

4.7

4.3

9.5

21.0

3.9

12.9

10.0

18.0

4.7

6.1

12.5

10.4

3.5 + 7.3 -

7.3 + 6.6 -

8.8 + 12.8 -

12.4 + 11.8 -

4.8 + 5.1 -

8.0 + 9.4 -

9.1 + 8.3 -

13.8 + 14.6 -

8.5 + 4.7 - 6.6

10.4 + 12.0 - 11.2

11.7 + 8.2 - 9.9

17.0 + 17.8 - 17.4

5.5

6.9

10.8

12.1

4.9

8.7

8.7

14.1

3.3 +

6.6 +

7.5 +

12.9 +

10.4

14.1

14.2

27.1

2.0

9.4

7.6

16.8

2.8

10.5

10.8

17.3

+

+

+

+

+

+

+

+

+

+

+

+

3.1

9.9

9.6

15.9

5.3

8.9

9.1

17.0

5.1

9.9

10.1

17.7

3.7

6.3

12.3

11.2

3.2

7.8

8.6

14.4

7.9

11.5

11.6

22.0

3.5

9.7

8.8

17.2

3.3

8.4

11.6

14.3

5.22 + 5.90 - 5.5

10.44 + 12.20 - 11.3

13.22 + 14.91 - 14.1

17.86 + 17.86 - 17.86

88 Av.

Av.

Pk.

Pk.

89 Av.

Av.

Pk.

Pk.

T

C

T

C

T

C

T

C

6.2

2.7

12.0

15.0

5.1

14.0

28.0

22.0

+ 5.5 - 5.8

+ 7.4 _ 5.1

+ 12.0 - 12.0

+ 24.0 - 14.5

+ 3.9

+ 7.4

+ 24.0

+ 16.0

4.5

10.7

26.0

19.0

wn

C,

3.5

5.5

10.3

17.5

15.6

6.5

55.0

25.2

15.1

21.3

27.9

32.5

+ 2.7

+ 5.0

+ 11.1

+ 30.7

+ 16.1

+ 17.7

+ 49.9

+ 37.3

+ 10.6

+ 28.2

+ 25.9

+ 45.6

3.1

5.3

10.7

24.1

15.8

12.1

52.5

31.3

12.8

24.8

26.9

39.0

90 Av. T 17.9 + 8.6

Av. C 20.3 + 21.1

Pk. T >30.0 + >30.0

Pk. C >30.0 + >30.0

(a) 2 = unit 2 (PWR), R = refueling outage number, T = tension drag force (lbs.), C = compression drag force (lbs.)

F.9 NUREG/CR-5870
Appendix F

Table 1 (Continued)

Exam

Number 2R4MO) 2R5

95 Av.

Av.

Pk.

Pk.

T

C

T

C

2.3 +

8.6 +

10.0 +

13.0 +

3.9

11.3

11.5

16.0

- 3.1

- 9.9

- 10.8

- 14.5

4.5 + 5.5

8.2 + 4.2

11.4 + 14.2

17.3 + 8.8

- 5.0

- 6.2

- 12.8

- 13.1

2R6

4.7 + 5.3 - 5.0

6.8 + 9.1 - 8.0

15.1 + 15.8 - 15.4

14.5 + 18.1 - 16.3

2R7

96 Av.

Av.

Pk.

Pk.

99 Av.

Av.

Pk.

Pk.

108 Av.

Av.

Pk.

Pk.

109 Av.

Av.

Pk.

Pk.

110 Av.

Av.

Pk.

Pk.

111 Av.

Av.

Pk.

Pk.

112 Av.

Av.

Pk.

Pk.

113 Av.

Av.

Pk.

Pk.

116 Av.

Av.

Pk.

Pk.

3.1 +

9.0 +

7.0 +

13.0 +

7.8 +

7.0 +

20.0 +

17.0 +

.4 +

8.2 +

4.0 +

14.0 +

2.3 +

13.7 +

7.0 +

20.0 +

5.5 +

9.4 +

9.0 +

14.0 +

4.3 +

7.8 +

8.0 +

14.0 +

4.7 +

.4 +

8.0 +

6.0 +

3.5 +

3.9 +

10.0 +

14.0 +

3.5 +

1.6 +

6.0 +

6.0 +

.39 -

5.1 -

6.0 -

10.0 -

7.6-

7.6 -

23.0 -

26.0 -

2.0 -

9.0 -

7.0 -

14.0 -

3.5 -

14.0 -

8.0 -

20.0 -

5.1 -

7.0 -

10.0 -

12.0 -

2.7 -

9.4 -

8.0 -

14.0 -

6.6

12.1

>30.0

19.0

- 3.5

- 7.0

- 6.5

- 11.5

- 7.7

- 7.3

- 21.5

- 21.5

- 1.2

- 8.6

- 5.5

- 14.0

- 2.9

- 13.9

- 7.5

- 20.0

- 5.3

- 8.2

- 9.5

- 13.0

- 3.5

- 8.6

- 8.0

- 14.0

3.4 + 8.8

12.2 + 13.4

7.1 + 11.4

13.3 + 17.0

15.5 + 25.4

31.1 + 43.7

39.7 + 49.8

50.0 + 50.0

4.8 + 3.4

5.9 + 7.9

8.1 + 6.5

11.3 + 13.1

10.1 + 10.9

14.9 + 15.1

15.1 + 18.0

21.2 + 24.0

10.3 + 6.3

11.9 + 9.8

15.7 + 11.9

18.8 + 15.5

8.8 + 8.6

10.1 + 9.0

14.8 + 16.0

16.0 + 17.2

2.5 +

7.9 +

5.7 +

12.2 +

11.5 + 4.8

10.9 + 13.6

16.7 + 9.3

19.5 + 21.3

3.1 + 3.4

7.3 + 5.4

6.5 + 7.6

11.7 + 11.6

- 6.1

- 12.8

- 9.3

- 15.2

8.5 + 5.4 - 7.0

15.8 + 14.0 - 14.9

12.9 + 10.4 - 11.7

22.1 + 20.7 - 21.4

Cn

U,

8.3 + 7.1

7.1 + 9.8

15.3 + 13.7

21.0 + 20.4

Changed to AD-153

9.3 + 11.6 - 10.4

15.3 + 11.1 - 13.2

20.2 + 22.1 - 21.2

29.3 + 22.8 - 26.0

- 10.5

- 15.0

- 16.5

- 22.6

Changed to AD-151

- 8.3

- 10.9

- 13.8

- 17.2

7.98 +

10.35 +

12.52 +

17.40 +

6.20 +

6.56 +

12.66 +

11.39 +

8.52 - 8.2

5.27 - 7.8

13.13 - 12.8

10.05 - 13.7

4.73 - 5.5

5.61 - 6,1

8.69 - 10.7

9.37 - 10.3

- 8.7

- 9.5

- 15.4

- 16.6

X

13.3 + 13.2

12.2 + 11.8

20.1 + 19.2

18.3 + 16.9

Changed

to

AD-71R

2.0 - 2.8

2.0 - 3.0

8.0 - 9.0

9.0 - 11.5

CA

1-u

Changed to AD-71R

3.5 -

2.0 -

6.0 -

7.0 -

3.5

1.8

6.0

6.5

- 3.3

- 6.4

- 7.0

- 11.6

(a) 2 = unit 2 (PWR), R = refueling outage number, T = tension drag force (lbs.), C = compression drag force (lbs.)

NUREG/CR-5870 F.10
Appendix F

Bible 1 (Continued)

Exam

Number 2R4(')

117 Av.

Av.

Pk.

Pk.

141 Av.

Av.

Pk.

Pk.

142 Av.

Av.

Pk.

Pk.

T

C

T

C

C

C

T

C

I

C

T

C

5.1 + 4.3 - 4.7

1.2 + 5.9 - 3.5

8.0 + 8.0 - 8.0

9.0 + 11.0 - 10.0

3.1 + 3.5 -

.8+ 2.0

7.0 + 7.0 O

'30.0 + >30.0

7.0 + 4.7 - 5.8

8.6 + 3.1 - 5.8

12.0 + 10.0 - 11.0

17.0 + 10.0 - 13.5

2R5

1.8 + 3.2 - 2.5

4.3 + 9.6 - 7.0

5.6 + 7.1 - 6.4

9.1 + 15.0 - 12.0

2.3 + 5.7 - 4.0

5.8 + 6.4 - 6.1

8.7 + 9.1 - 8.9

10.0 + 10.7 - 10.4

13.2 + 4.5 - 8.8

14.8 + 16.2 - 15.5

19.6 + 13.6 - 16.6

21.4 + 23.7 - 22.6

2R6 2R7

144 Av.

Av.

Pk.

Pk.

T

C

T

C

3.9 + 2.3 - 3.1

.4 + 3.1 - 1.8

7.0 + 6.0 - 6.5

4.0 + 8.0 - 6.0

5.9 + 12.8 - 9.4

20.6 + 16.3 - 18.4

17.2 + 23.7 - 20.4

29.3 + 23.5 - 26.4

8.1 + 6.6

12.5 + 9.6 O

15.7 + 18.0 c

17.9 + 18.5

4.56 + 4.81 - 4.7

7.93 + 2.51 - 5.2

8.59 + 11.05 - 9.8

10.93 + 9.69 - 10.3

146 Av.

Av.

Pk.

Pk.

147 Av.

Av.

Pk.

Pk.

153 Av.

Av.

Pk.

Pk.

182 Av.

Av.

Pk.

Pk.

2.3 + 1.2 - 1.8

2.3 + 4.7 - 3.5

8.0 + 5.5 - 6.8

7.0 + 10.0 - 8.5

5.5 + 3.5 - 4.5

7.4 + 9.4 - 8.4

9.0 + 8.0 - 8.5

12.0 + 13.0 - 12.5

2.0 + 2.7 - 2.4

10.9 + 8.6 - 9:8

8.0 + 8.0 - 8.0

18.0 + 18.0 - 18.0

9.0 + 6.2 - 7.6

10.1 + 8.6 - 9.3

16.0 + 15.0 - 15.5

16.0 + 16.0 - 16.0

4.4 + 9.3 - 6.9

10.1 + 7.7 - 8.9

7.9 + 13.0 - 10.5

15.7 + 13.4 - 14.5

5.1 + 5.6 - 5.4

4.0 + 13.3 - 8.6

11.1 + 9.9 - 10.5

11.5 + 19.5 - 15.5

3.3 + 1.8 - 2.5

9.5 + 9.1 - 9.3

7.0 + 6.3 - 6.6

16.4 + 16.0 - 16.2

13.9

16.9

41.8

32.5

11.0 + 11.9

10.0 + 13.8

45.9 + 49.9

27.9 + 130.0

Replaced with AD-153

184 Av.

Av.

Pk.

Pk.

188 Av.

Av.

Pk.

Pk.

T

C

T

C

T

C

T

C

7.8 + 1.2 - 4.5

10.5 + 15.6 - 13.0

13.0 + 7.0 - 10.0

14.0 + 21.0 - 17.5

6.2 + 2.7 - 4.5

2.3 + 5.5 - 3.9

10.0 + 10.0 - 10.0

8.0 + 15.0 - 11.5

10.1 + 6.2 - 8.1

16.0 + 12.8 - 14.4

17.2 + 14.3 - 15.6

23.0 + 22.0 - 22.6

2.9 + 7.8 -

9.9 + 14.3 -

11.8 + 9.3 -

15.3 + 15.1 -

5.4

12.1

10.5

15.2

8.6 + 4.6 - 6.5

7.2 + 10.0 - 8.6

12.8 + 8.1 - 10.4

13.2 + 16.6 - 14.9

3.44 + 3.44 - 3.4

10.30 + 12.54 - 11.4

9.71 + 9.71 - 9.7

16.54 + 17.45 - 17.0

(a) 2 - unit 2 (PWR), R = refueling outage number, T - tension drag force (lbs.), C = compression drag force (lbs.)

F.11 NUREG/CR-5870
Appendix F

Table 1 (Continued)

EXam

Number 2R4(a)

192 Av. T

Av. C

Pk. T

Pk. C

194 Av. T

Av. C

Pk. T

Pk. C

195 Av. T

Av. C

Pk. T

Pk. C

202 Av. T

Av. C

Pk. T

Pk. C

205 Av. T

Av. C

Pk. T

Pk. C

7.0 + 7.8 - 7.4

18.7 + 18.7 - 18.7

14.0 + 28.0 - 21.0

26.0 + 29.0 - 27.5

4.7 + 10.1 - 7.4

3.5 + 3.5 - 3.5

11.0 + 17.0 - 14.0

12.0 + 16.0 - 14.0

2.7 + .39 - 1.5

6.2 + 5.9 - 6.0

6.0 + 3.0 - 4.5

17.0 + 13.0 - 15.0

3.1 + 16.6

12.1 + 10.5 n

18.0 + >30.0

22.0 + 18.0

5.9 + 4.3 - 10.1

6.2 + 5.9 - 6.0

10.0 + 10.0 - 10.0

12.0 + 10.0 - 11.0

2R5

.8+ 1.8 - 1.3

11.8 + 14.6 - 13.2

14.2 + 7.3 - 10.7

19.2 + 25.3 - 22.3

4.4 + 7.1 - 5.8

14.1 + 9.6 - 11.8

10.6 + 14.0 - 12.3

37.7 + 30.9 - 34.4

6.2 + 5.8 - 6.0

12.2 + 9.6 - 10.9

11.7 + 10.4 - 11.1

25.7 + 16.2 - 21.0

6.7 + 4.8 - 5.7

4.2 + 6.3 - 5.3

11.9 + 9.3 - 10.6

10.8 + 11.7 - 11.2

6.4 + 11.3 - 8.9

14.5 + 14.8 - 14.6

13.7 + 16.9 - 15.3

19.9 + 20.7 - 20.3

2R6 2R7

13.74 + 10.86 - 12.3

20.52 + 10.52 - 15.5

24.20 + 17.62 - 20.9

27.91 + 45.80 - 36.8

9.52 + 10.35 - 9.9

9.98 + 10.0 - 10.0

18.74 + 17.54 - 18.1

17.52 + 17.30 - 17.4

7.59 + 5.83 - 6.7

12.88 + 10.37 - 11.6

12.03 + 10.69 - 11.3

17.98 + 16.62 - 17.3

3.2 + 3.0 -

7.7 + 5.5 -

11.6 + 7.4 -

13.2 + 10.7 -

7.7 + 9.5 -

6.9 + 10.5 -

12.2 + 14.5 -

13.3 + 16.3 -

3.1

6.6

9.5

11.9

8.6

8.7

13.3

14.8

244 Av. T

Av. C

Pk. T

Pk. C

245 Av. T

Av. C

Pk. T

Pk. C

3.9 + 9.0 - 6.5

7.8 + 12.5 - 10.1

8.0 + 14.0 - 11.0

18.0 + 18.0 - 18.0

3.5 + 6.6 - 10.0

12.9 + 14.0 - 13.5

7.5 + 12.0 - 9.8

18.0 + 22.0 - 20.0

8.7 + 10.7 - 9.7

14.7 + 11.9 - 13.3

12.7 + 14.8 - 13.7

22.5 + 21.6 - 22.0

7.8 + 5.2 - 6.5

12.4 + 3.1 - 7.7

11.9 + 8.9 - 10.4

18.8 + 9.7 - 14.4

4.1 + 6.6

7.9 + 5.1 S

9.3 + 10.0 E

16.2 + 10.4

2.3 + 3.5

4.5 + 4.4 n

5.5 + 6.9 T

10.5 + 11.3

Replaced with AD-S01

Replaced with AD-S01

247 Av. T

Av. C

Pk. T

Pk. C

3.1 + 1.6 - 2.3

4.7 + 3.9 - 4.3

12.0 + 8.0 - 10.0

10.0 + 10.0 - 10.0

8.3 + 6.1 - 7.2

5.8 + 9.5 - 7.6

12.1 + 10.5 - 11.3

15.3 + 14.9 - 15.1

13.7 + 6.2 - 10.0

7.5 + 7.5 - 7.5

19.5 + 13.4 - 11.4

15.5 + 16.0 - 15.8

248 Av. T

Av. C

Pk. T

Pk. C

254 Av. T

Av. C

Pk. T

Pk. C

1.6 + 3.1 - 2.4

3.1 + 1.2 - 2.2

8.0 + 10.0 - 9.0

10.0 + 8.0 - 9.0

4.3 - 4.3

5.5 + 9.8 - 7.7

8.0 - 8.0

16.0 + 14.0 - 15.0

6.0 + 3.2 - 4.6

8.7 + 5.1 - 6.9

15.2 + 9.6 - 12.4

15.3 + 13.8 - 14.5

6.4 + 8.4 - 7.4

8.7 + 9.0 - 8.8

22.5 + 30.2 - 26.3

33.3 + 25.4 - 29.3

(a) 2 = unit 2 (PWR), R = refueling outage number, T = tension drag force (lbs.), C = compression drag force (lbs.)

NUREG/CR-5870 F.12
Appendix F

hbIe 1 (Continued)

Exam

Number 2R4(a)

255 Av. T

Av. C

Pk. T

Pk. C

269 Av. T

Av. C

Pk. T

Pk. C

3.5 + 3.1 - 3.3

9.4 + 8.2 - 8.8

10.0 + 10.0 - 10.0

17.0 + 16.0 - 16.5

9.8 + 6.6 - 8.2

1.2 + 6.6 - 3.9

20.0 + 15.0 - 17.5

7.0 + 15.0 - 11.0

2R5

4.4 + 9.0 - 6.7

11.8 + 15.2 - 13.5

10.3 + 16.4 - 13.4

18.3 + 23.2 - 20.7

8.7 + 3.3 - 6.0

16.8 + 10.5 - 13.6

27.6 + 13.5 - 20.5

24.2 + 18.3 - 21.2

2R6 2R7

273 Av. T

Av. C

Pk. T

Pk. C

3.9 + 5.5 - 4.7

2.0 + 3.9 - 3.0

8.0 + 11.0 - 9.5

13.0 + 12.0 - 12.5

2.1 + 4.4 - 3.3

11.6 + 9.1 - 10.3

5.9 + 8.5 - 7.2

18.1 + 15.6 - 16.8

4.1 + 2.7

5.8 + 6.0 g'

7.0 + 7.1 "

10.7 + 12.2

Replaced with AD-Sal

293 Av. T

Av. C

Pk. T

Pk. C

295 Av. T

Av. C

Pk. T

Pk. C

5.5 + 7.8 - 6.6

14.8 + 14.0 - 14.4

10.0 + 13.0 - 11.5

20.0 + 21.0 - 20.5

2.3 + 3.1 - 2.7

7.4 + 10.1 - 8.7

9.0 + 8.0 - 8.5

14.0 + 16.0 - 15.0

4.2 + 6.2 - 5.2

16.5 + 12.6 - 14.6

9.3 + 12.7 - 11.0

27.2 + 43.9 - 35.6

7.3 + 4.4 - 5.9

17.1 + 10.0 - 13.5

.2 + 4.9 - 2.5

6.6 + 12.7 - 9.7

393 Av. T

Av. C

Pk. T

Pk. C

4.1 + 5.1 - 4.6

14.1 + 6.7 - 10.4

9.4 + 10.1 - 9.7

19.8 + 12.8 - 16.3

12.27 + 6.78 - 9.5

16.25 + 8.32 - 12.3

17.59 + 11.91 - 14.7

21.20 + 12.47 - 16.8

(a) 2 = unit 2 (PWR), R = refueling outage number, T = tension drag force (lbs.), C = compression drag force (lbs.)

F. 13 NUREG/CR-5870
Appendix G

Typical Mechanical Snubber Failure Causes (Plant C)
Appendix G

Typical Mechanical Snubber Failure Causes (Plant C)

This Appendix contains a list of mechanical snubber

failure causes documented by plant personnel at

Plant C Failure causes are categorized in accordance

with the following functional test failure categories:

* high drag force

* exceeded maximum acceleration limit

* below minimum acceleration limit.

G.1 01 ~~~~~~NUREG/CR
Appendix G

TYPICAL MECHANICAL SNUBBER FAILURE CAUSES Page 1

PLANT C

HIGH DRAG FORCE

o Lubricant dried out internally. Unit exposed to radiant heat from

reactor recirculating pump.

o Significant corrosion of torque drum-and Capstan Spring. Torque

carrier screw shaft bend caused when Unit tried to stroke under

thermal expansion.

o Weld slag spattered on indicating tube caused binding during stroke.

o Guide rods and bearing assembly bent. Unit improperly twisted with

wrench to align pad eyes during installation.

o Capstan Spring ears outside of clutch window. Unit improperly pulled

apart while dust cover snap ring was loosened to align Unit pad eye

during installation.

o Internally corroded. Installed in normally dry area of primary

containment. May have been improperly stored during initial plant

construction.

o Repetitive vibratory loads on Unit caused balls in thrust bearing to

cut groove in its races.

o Capstan Spring wound too tight by manufacturer. Rubbed on Unit

cylinder during normal stroke instead of spinning freely caused

breaking action of Unit.

o Unit mounted vertically and housing became filled with water.

Internals severely corroded. Poor protection during maintenance

activities in the area.

o Telescoping members binding due to high side load of unit.

o Rough spots on planetary gears and shaft of unit due to poor handling

of large snubber.

o Bearing retainer nut became loose which occured due to handling or

was loose at fabrication.

o Fouling of snubber internals with dirt and metal filing causing

binding.

o Screw shaft sheared In two places. Unit overloaded due to system

transient.

o Poor machining of inner and outer telescoping members at factory.

Pieces not concentric, causing a rub on one side.

NUREG/CR-5870 G.2
Appendix 0

TYPICAL MECHANICAL SNUBBER FAILURE CAUSES Page 2

PLANT C

HIGH DRAG FORCE (Cont'd)

o Slight bend in screw shaft near inertia areas. Inertia mass rubbing

inside of dust cover.

o Dirt and dust on screw shaft thereby restricting movement.

o Adhesive on indicating tube causing binding of telescoping member.

(Adhesive was left from tape used to cover unit with plastic during

plant construction).

o Damaged inner race of thrust bearing (cracked in several places) due

to transient overload forces.

EXCEEDED MAXIMUM ACCELERATION LIMIT

o Spring not wound tightly enough at factory. Would not tighten

against cylinder at required acceleration.

o Capstan Spring not properly installed. Spring ears outside clutch

window, unit could not activate.

o Small retaining parts of snubber internals were loose, Capstan Spring

worn when unit was rattled during service checks.

o Improper assembly of internals did not allow Capstan Spring to

tighten fully to activate Unit.

o Manufacturer's defect. Keeper ring not installed properly.

o Snubber ws damaged during handling. Stroked too hard causing it to

lock.

o Dirt between inertia mass and lead screw caused mass to slip during

activation.

o Capstan Spring not properly placed in unit at factory. Spring ears

outside clutch window so unit could not activate.

o Dried dirt and grease in torque drum and inertia mass area, causing

high acceleration.

G.3 NUREG/CR-5870
Appendix G

TYPICAL MECHANICAL SNUBBER FAILURE CAUSES Page 3

PLANT C

BELOW MINIMUM ACCELERATION LIMIT

o Capstan Spring installed crooked at factory. Bound up on dust cover

during cycling.

o Dirt and grit was caked on seat area of Capstan Spring, causing it to

activate too low.

o Snubber inner thrust bearing race chipped, torque drum retainer

bent. Subjected to frequent transients within design limits.

o Excessive grease placed in inertia mass area at factory causing

slippage of internal parts during activation test.

o Severe corrosion of Capstan and clutch spring area. Onit was leaked

upon by damaged pump seal.

NUREG/CR-5870 GA
Appendix H

Iypical Mechanical Snubber Examination Record
Appendix H

I rpical Mechanical Snubber Examination Record

This Appendix Contains a typical mechanical snubber

examination record. This record was not obtained from

one of the key study plants.

H.1 NUREG/CR-5870
Appendix H

Snubber Examination Record

.e~~~~~~~~~~fl~~~~~snmnis-ssn _ n .ss n e a -

Ex!a Number .89

Hanger Number .

Comoonent Number.

Serial Number . . 36291

Size . PSA I

Rated Load.

Design Tre ... MSML

Functional Type . .

Date Seals Reolaced /

Date Nev Seals Due. I

Date Installed. . . 02/21

Vertical Orientation: 90

Location Description:

114

150

211

LI

Orientation to Pipe

4igh Vibration? :

Service Temperature

Humidity.

Radiation Condition

Accessibility Code.

Building/Root .

Elevation.

Orowell Zone.

Grid Row.

Grid Column.

Drawing Number.

90

N

135

DRY

9

4220

113

10

I'N 18

E R

150 Number. . I-P-AS-277

System. . AS

Loo.

Line Number. . I-AB-ICCA-038

U-S Number . 6. 58111

Scaffold Required? : N

Safety Class . . : A-0

Nuclear Class. .

Hot Piston Set . . . 2 9116

Cold Piston Set. . : 1 7/16

Movement . .. * : 118

Last FT Date . . . 09/22/19

Last FT Result . . . P

Perform Visual Exam? Y Date Performed: 09/20/89 Result: S Examiner:

Uork Order/NOA 1: Date Initiated: I / Date Resolved: / Failure Code:

Description:

Resolution:

Acceotance Code: Date Re-Examined: 10/11/89 Result: S Examiner:

runctional Test Code: F Date Performed: 09/22/89 Result: P Examiner:

Date As-Left Test Performed: / I Result: Examiner:

As Found As Left

Mechanical Snubber Acceotance Tension Compression Acceptance Tension Compression

Breakaway Drag - Extenced (L8F) 1:81 6.44

Breakaway Drag - Plidstroke (LSF) 0.00 0.00

Breakaway Drag - Retracted (LF) 0.00 0.00

Initial Running Drag . . . (LEF) 1.39 5.12

Acceleration Limit . . . . (6s) 0.0073 0.007t

Final Running Drag . . . . (LBF) 4.67 4.78

Load Achieved. . .LF) 215 217

Uork Order/NOA t: Date Initiated: I I Date Resolved: 1 I Feilure Code:

Descriotion:

Resolution:

Acceotance Code: Failure Analysis Code:

Snubber Reolacement Code:

Replacement Exam Number:

Date Installed: I I

Incoming Snubber from Scares? N

0 Component Number: Serial Number:

Disposition of Removed Snubber:

Size:

Uork Package Reviewed and Acceoted: Y Reviewer:

NUREG/CR-5870 H.2
Appendix I

Sample Mechanical Snubber Failure Evaluation Report
Appendix I

Sample Mechanical Snubber Failure Evaluation Report

This Appendix contains a failure evaluation report that

reflects comprehensive evaluation of a mechanical snubber that was found to be frozen in service. The report

reflects the methods used to identify and verify the problem and to determine the cause of failure.

This Appendix also includes a failure evaluation report

associated with failure of two mechanical snubbers due

to overload in the compression direction.

1.1 NUREG/CR-5870
Appendix I

SYNOPSIS OF EVENTS

The following is a brief recount of the events surrounding

the failure of high temperature snubber Serial No.

Der the requirements of Technical Specification

a VT-3 examination was oerformed on support no.

on 10/18/86. The exam, sited

two conditions: the insulation for the reactor vessel head

appeared to be binding the snubber extension tube, and paint

on the lower ball bushing was hindering free rotation in the

cone of action. Condition Report (CR) was written to

evaluate these findings.

While reviewing the CR. Design Engineering found the field

setting to be over 1 1/2 " off the design setting. MR,

was generated to address this additional concern. At this

time the operability of the snubber was not believed to have

been compromised, and on 10/28/86 Maintenance Work Order

(MWO) was written to correct the conditions noted in

the field.

On 11/04/86, MR. was work released to change the

snubber setting in the field. CR was written on

11/05/86 as a means of documenting the events prior to the

discovery of the incorrect field setting.

On 11/06/86. Design Engineering was informed 'hat the line to

which the suject snubber was attached shifted over 1 1/2

when the clamo was removed. This was the first indication

that the snubber was damaged. FCN 1 to MR was then

generated to document the snubber failure analysis (but was

later cancelled as not required). and FCN 2 was generated to

replace the failed snubber with one already installed just a

few feet away.

was brought to the site on 11/09/86 to perform the

snubber failure analysis. The cause of failure was determined

to be from foreign matter lodged in the screw sha-t assembly.

NUREG/CR-5870 L2
Appendix I

ILLUST16RATIONS

;N j-Hllwx - --a9 ___ N -- I

-A st _SI

2ZL= a= L-,

VS r Womb Z"iM r 1/ _-.)61 PI/39

1. SX or Wr

2. 7 3 M _S G 

l

_

r =FICIAL WVORK CC

IjT j: 7r

JuN^ E 5 98a6

STAr 4C C.C C : -1 _ r c, _

I rI XAF VQ '-*---.) .s-.Vv.yF-.- iP-V - a7)@-<--r .9v vo Fr.1-te .- -

_

. _

_ _ _ _ _

STEPS

T. 4. I

7.'.'

7 .(..4

1.41-t LLS

&Aq.ru

AM

*,5AA

7.&.6

7.4. 7

7.&.I

7.'r.,9

R~IB

A.f

1.3 NUREG/CR-5870
Appendix I

VISUAL INSPECTION VT-3 INSPECTION REPORT

*-NO. DATE: ICOMPONENT ID COMPONENT S/N VT EXAMINER: |INSP.INI

IC/ St j <;c ' SIGNATURE:

PROCEDURE NO. 'ORAWJING fO. jSIs. CODE LOCATION:

.1'#Ay %ZedidV.

ILLUMINATION VERIFIED: EXANINATICN METHOD: NSTE USED

Ej SAT r7 UNSAT >OIRECT r REMOTE _IPi

ID ATT. - ATT.

I TEN CODE ATTRIBUTE INSPECTED RESULTS CODE RECORDABLE CONOtTIONS/COMENTS

NO. __ N-NA.S-SAT.R-Recordable S,2._11

1. T13 PREREQUISITES S

2. T13 IDENTIFICATION

3 . T13 |ALIGNMENT

.4. | T13 |PHYSICAL DAMAGE

5 T13 _ END ATTACHMENT

6. T13 CLAMPS

7. T13 WELDING

3. I T13 INTERNALS

9. - T13 SLIDING SURFACES

. 5IT13 FIELD SET DIMENSIONS S RECORD: , |

''4

p4.O~ ~ ~ ~ ~ ~ ~ ~ ~ ~~r C~ ~~~~~~~~~~~84,9

N. 3

NUREG/CR-5870 I.4
DATE : 11/09/GG

U.5n KLB'

0.4tlK1t;

0.311 KLBUS.

O.( l KLB(I

O,10 .KLB'-

I I I I I

o~~~~~~n_~~~~~

- -- I - _____

I a I[P.t

21 IPMRUtINfNG CRAG TEST

SHL,5!&ER TYPE ----------------------PSA-3

SERtAL NUMBER ------------------ L.O~N - N--------------------------

RATED LOAD ------------------------- 60D0 I.BS.

ACCEPTABLE DRAG FORCE -------------- 6 LBS

STROKE DIRECTION------------------- TENSION

AMBIENT TF.I1P ---------------------- ' DEGREES F.

DRAG FORCE : AUERAGE-------------- 296.4 LB,./ PEAK-- 319

TEST RESULTS ACCEPTABLE (YES/NO) : 11O

TEST OPERATOR:

I -

LBS.

- -- 1- - -~~~~~~~~

I00

0

aO WPt.

4tl IPfL

so IP"

7
Appendix I

Subject: Preliminary Report of Test and Failure Analysis Performed on PSA High

Temperature Snubber S/N , November 9 and 10, 1986, at the

Nuclear Station

1. On November 9 and 10, 1986. Mr. and Mr. , both

of , assisted Station personnel In a test and

disassembly for failure analysis of the subject PSA High Temperature Snubber. The

snubber had been found In a locked-up, fully compressed position; and, when

removed, could not be hand stroked. x determination was made to perform an as

found breakaway test at the locked-up position, modified such that a load over 600

lbs-force (10% of full rated load) would not be exceeded. This limit, 10 times the

minimum drag, was imposed to avoid potential further damage. The test was

conducted and no movement- was noted at a load of less than 400 lbs-force. A

decision was made that no additional load would be applied and that the snubber

would be disassembled for failure analysis.

2. Disassembly was performed by station personnel using both the station procedure

and the Pacific Scientific Document No. PS-193, Rev. 4, March 1986. As each step

was performed, components were inspected by Mr. . All steps through

step AB of paragraph 4-3 of PS-193 were performed, except that the anti-rotation

key was not removed, per step Z. The ball screw assembly was not removed from

the ball screw shaft. Photographs were taken throughout the disassembly.

3. The following unusual conditions were noted during disassembly:

a. All parts were noted to be in good condition with very little signs of wear at

bearing or other moving contact areas. Except for the locked-up ball screw

shaft, all moving parts appeared to function normally. There were no bright

metal contact points on the capstan spring and spring tangs.

b. The fine white powder and residual grease usually found in PSA snubbers was

not evident. Instead, a black powder was found which was believed to be

carbonized grease resulting from the high temperature application. This black

powder could be wiped from surfaces and was not built up except on the

bearing surfaces for the inertia mass. The inertia mass did not bind on these

bearing surfaces.

c. The telescoping cylinder was observed to have three bands of discoloration on

its circumference. These bands did not exhibit signs of heavy wear or pitting and were only visible over an arc of about 120 degrees. The longitudinal

NUREG/CR-5870 1.6
Appendix I

spacing of these bands was measured. It was found that relative to the

location of the lip on the support cylinder tube, the three bands corresponded

to the snubber at cold, hot, and as found settings. The bands appeared to be

the result of a steady side-load on the snubber.

d. The snubber did not show evidence of unlocking until the ball screw assembly

was removed from the telescoping tube. When the end plug was removed, care was taken to empty the tube on a clean cloth. This was also accomplished

while unstaking the telescoping cylinder from the ball screw assembly.

Several particles of grit were noted on the cloth. Two bright metal particles

appeared to come from the unstaking process. The other particles and were

not easily identified as to source. Upon Inspection a similar particle was noted

inside the telescoping tube. That particle did not jar loose and was left in

place. In service the snubber was mounted In a vertical position with the end

plug up. In this position the snubber would be susceptible to lock-up in tension

if a particle in the tube had fallen into the ball screw.

e. When free of all other assemblies, the ball screw would move hesitatingly up

and down the ball screw shaft under its own weight. A slight touch would teinitiate motion. The shaft appeared satisfactory, but lacked signs of movement and wear. The ball screw appeared satisfactory, but would not sustain

motion for more than two or three revolutions up or down the shaft under its

own weight. As the ball screw was worked from end to end, this condition

improved, but not to the extent of free motion from end to end.

f. Prior to testing and disassembly, plant personnel had indicated that the

hanging position of the snubber was in an axially displaced position to one side,

such that spherical bearing movement would be required. They further

indicated that the spherical bearing on the housing end was frozen by paint,

and that this bearing motion had been freed only after deconning had removed

the paint. Evidence of this paint in and around the bearing was noted.

Conclusions reached from the above and other conditions found are as follows:

a. The snubber had been subjected to very little motion and apparently no

vibration. All parts appeared in good condition.

b. Lack of lubricant did not appear to restrict motion of the inertia mass, thrust

bearing, or other moving parts, except for the ball screw and shaft assembly.

c. The bands on the telescoping tube are clear evidence of side loading at the

three positions of extension noted by measurement. This evidence of side

loading is supported by the frozen spherical bearing.

d. Evidence of some grit in the telescoping tube was found after some shock to

the assemblies during removal of the end cap, and unstaking the ball screw

assembly. Other evidence of grit was found on the tubes inside wall. The ball

screw was locked until these disassembly steps were performed. As the

snubber was hung vertically, any grit in the tube could have fallen into the ball

screw.

1.7 NUREG/CR-5870
Appendix I

e. The screws securing the indicating tube to the end plug were found finger

tight. Likewise, the set screws below were found finger tight. The end plug

was misaligned approximately 10 degrees from the rear housing, indicating possible readjustment or removal during installation for alignment purposes.

f. The ball screw assembly appeared reasonably free in full length action only

after removal from the telescoping tube. The noted hesitation in the travel of

the ball screw on the screw shaft may have caused lock-up; however, only a

light touch was required to initiate continued motion.

OVERALL CONCLUSION

Most probable cause of lock-up was from grit wedged in the ball screw assembly. This

cause is not considered generic to the high temperature type snubber.

RECOMMENDATIONS

1. Hand stroking other similarly installed snubbers would Indicate no lock-up condition

exists and probably less than minimum running drag of 60 lbs-force. Based upon the

excellent condition noted in the disassembled snubber, a high probability edsts these

snubbers are fully functional.

2. Although evidence of side loading was found, this did not appear to be the cause of

lock-up. However, spherical bearings should be kept free from paint, etc., to allow

complete freedom of motion.

3. The loose screws in the end plug showed no evidence of "lock tite," and were In fact

barely finger tight. This, plus the visible realignment of the end plug indicates a

possible removal at one time, and a source of particle contamination. PSA

installation instructions should be followed.

4. The ball screw and shaft assembly should be replaced and the snubber rebuilt per

PS-193 (with special instructions regarding parts and grease required for the high

temperature snubber obtained from Pacific Scientific Co.). The removed ball screw

and shaft should be returned to Pacific Scientific Co. for further analysis.

NUREG/CR-5870 1.8
Appendix I

January 19. 1987

The cause of failure of ball-screw shafts from two system snubbers

is attributed to sudden loading of the snubber resulting in buckling of the

shafts. The fracture mode of the precipitation hardened stainless steel

shafts is combined ductile and brittle fracture. Seams and.cracking at the

crowns of the threads are attributed to thread rolling during the

manufacturing process and are not related to the snubber failures.

Confirming chemical analysis shows the ball-screw shaft material to be 17-4

PH stainless steel.

INTRODUCTION

The failed snubbers are Pacific Scientific size 1 snubbers with a load

rating of 1500 pounds. The service location of the snubbers was the

auxiliary steam line for the Unit 1 auxiliary feedwater turbine. Failure

of the snubbers was detected during routine surveillance required by

technical specifications.

MACRO-EXAMINATION

The damaged ball screw shafts and capstan springs are shown in Figures 1

and-2. The shaft from sample 405-I fractured at two locations while 405-II

fractured at a single location. The ball screw shafts plastically deformed

adjacent to the fracture locations by bending prior to fracture. A tensile

shear lip and compressive shear lip on both shafts indicated the shafts

buckled under compressive loading. The specific location and size of the

shear lips varied from fracture to fracture. The major portion of the

fracture face possessed a shiny, brittle appearance (Figure 3). The key

between the ball screw shaft and capstan spring housing was twisted out of

the key slot in both snubbers indicating the shaft experienced a sudden

torsional load. Seams formed during the thread rolling process are visible

at the crowns of the threads.

I.9 NUREG/CR-5870
Appendix I

i

I

I

II

I ON-S

i

FIGURE 1 As received view of snubber ball-screw shaft and torque transfer

drum. Shaft failed in buckling as the result of a sudden

compressive load. Fracture occurred'at two locations. Ma-661.

*405

(FOom

FIGURE 2 Second snubber ball-screw shaft which failed by buckling.

Close Inspection of capstan spring housing shows the key to the

ball-screw shaft twisted out of the key slot. Ma-662.

NUREG/CR-5870 1.10
Appendix I

The chemical analysis of the shaft material is consistent with that of 17-4

precipitation hardened stainless steel.

CONCLUSIONS

The ball-screw shafts of the Pacific Scientific size 1 snubber failed in

buckling as a result of a sudden compressive overload force applied externally

to the snubber. The keys between the ball-screw shaft and capstan spring

housing were twisted out of the key slot by a sudden application of torque.

The fracture faces are characterized by a combination of tensile and compressive fracture indicative of buckling. The material is in the hardened

condition and the chemistry appears to be correct. The load required to

buckle the ball-screw shaft has not been determined.

If the Metallurgy Lab can be of further assistance,.please call.

1.11 NUREG/CR-5870
Appendix I

I LLUSTRATIONS

x1o ewr -OM

:XI Y_%QWD =c= Ty

c.wr v.

C.ofjl LA

mm= a= -L.

r.:i- wqI Lo

As=*~ MM- ~ too 

_um VCM ___m L<_ i

I

j. z

L =FI FI I A X L WORK CC

1 ~~~~~~JUN'" 19 S86

,@E. Li~~~Iij S~~TA rl^i -CC:dos' t

-it5R

-.. - . - -

Imi":004 -., @sLt v r E-- X

TEPS

7.4.L

7.'.:

7.'.'

7,6.I.

EN1-4r1 kLs

MUA8

7.6.6

7.'..

7.4.9

L~4!Th1U

NUREG/CR-5870 I.12
Appendix J

ITypical Visual Indicators of Snubber Degradation
Appendix J

Typical Visual Indicators of Snubber Degradation

This Appendix contains photographs of snubbers and snubber parts associated with various types of degradation.

J.1 NUREG/CR-5870
Appendix J

."t'~

Itbw _.w

I

Figure J.1 Mechanical snubber telescoping cylinder (with side loading marks

from support cylinder in hot, cold, and as-found position)

Figure J.2 Gelated, blackened hydraulic fluid (snubber

subjected to high amplitude vibration)

NUREGICR-5870 J.2
Appendix J

Figure J.3 Extensive piston wear (snubber subjected

to high amplitude vibration)

Figure J.4 Typical used o-ring lip seal (removed from snubber with

7 years service in moderate operating environment)

J.3 NUREG/CR-5870
Appendix 1

_ _

Figure J35 Section photograph - o-ring with extensive set (left side) (removed from

snubber with 1-1/2 years service In high temperature environment > 250° F)

W - A= ~~~~~A

Figure J.6 Embrittled rod wiper (removed from snubber with 1-112 years

service in high temperature environment, > 250° F)

NUREG/CR-5870 J.4

X
Appendix J

Figure J.7 Embrittled o-ring with surface cracks (removed from snubber with

1-V2 years of service in high temperature environment > 250 F)

Figure J.8 Section photograph - damaged thread seal (right side) (seal

damaged due to failure to utilize flat washer with nut)

J.5 NUREG/CR-5870
Appendix J

Figure J.9 Uniformly worn clevis pin (snubber subjected

to continuous, low amplitude vibration)

NUREG/CR-5870 J.6
Appendix K

Recommendations for Subsection ISTD
Appendix K

Recommendations for Subsection ISTD

This appendix includes recommendations identified

from the NPAR In-plant Research for the Operations

and Maintenance (OM) Part 4 Code, Section IST,

Subsection ISTD, Examination and Performance

Testing of Nuclear Power Plant Dynamic Restraints

(Snubbers).

K1I NUREG/CR-5870
Contents

Recommendations for Subsection ISTD 1...............................

K1

1 NPAR Snubber Aging Research Recommendations for ISTD ........................................ K3

2 Service-Life Monitoring Recommendations ................. ...................................... K3

2.1 Determination of Snubber Failure or Degradation Causes ...................................... K3

2.2 Determination and Documentation of the Operating Environment .............................. K3

2.2.1 Temperature ....................................................................... K 4

2.2.2 Radiation ......................................................................... K4

2.2.3 Vibration ....................................................................... K4

2.2.4 'lansients ......................................................................... K5

2.3 Inservice 'Tsting Results ................................................................... K5

2.4 Diagnostic Testing ...................................................................... KS5

2.5 As-Found Testing ....................................................................... K5

2.6 'lending . ...................................................................... K5

2.7 Augmented Surveillance Methods . .......................................................... K6

2.8 Service-Life Categories .................................................................... K6

3 Visual Examination Attributes ................................................................... K6

3.1 Recommended Examination Checklist Attributes (Pre-Service Examination Only) ..... ........... K6

3.2 Recommended Examination Checklist Attributes (Pre-Service and Inservice Examination) ..... .... K6

3.3 Recommended Examination Checklist Attributes (Service-Life Monitoring) ...... ................ K7

4 Failure Grouping and Corrective Action ..................... ..................................... K7

4.1 Definitions ........................................................................ K7

4.2 Elimination of the Isolated Failure Mode Group .............................................. K8

4.3 Distinction Between Service-Related and Nonservice-Related Failures ........................... K8

4.4 Replacement or Modified Snubbers ................... ...................................... K9

NUREG/CR-5870 K-2
Appendix K

1 NPAR Snubber Aging Research Recommendations for ISTD

As a result of information obtained during the NPAR

Snubber Thsk research, recommendations are made in

the three following areas for the next revision of the

ASME OM Code, Subsection ISTD:

* Service-Life Monitoring

* Visual Examination Attributes

* Failure Grouping and Corrective Action.

2 Service-Life Monitoring Recommendations

Service-life monitoring recommendations were developed from the results of the NPAR research. Major

recommendations are highlighted below for consideration in the next revision of ISTD (Section 8.0 and

Appendix F).

It should be noted that suggestions pertaining to

service-life monitoring include a number of recommendations for testing in addition to that specified in ISTD,

Section 7.0. Such testing includes trending tests, diagnostic tests, and post-service as-found tests. If such tests

are included in ISTD, a statement should be included to

specify that the results of such tests will not require testing of additional snubber samples in accordance with

ISTD Section 7.9 or 7.12.

2.1 Determination of Snubber Failure

or Degradation Causes

A service-life monitoring program will be most effective

if it distinguishes between service-related and

nonservice-related failures. It is important that the root

cause of snubber failure or degradation (e.g., dynamic

transient, vibration, excessive temperature, etc.) be identified along with the failure mode (e.g., high drag force,

low activation, etc.) and the failure mechanism (e.g.,

deformation of screen shaft, solidification of grease,

etc.).

It is suggested that failure evaluation data sheets provide key information, including failure mode, failure

mechanism, failure cause, environment, service time,

abnormal conditions, visual observations, test data, test

observations, etc.

For effective determination of failure cause and root

cause, it is important that personnel involved in failure

evaluation have adequate experience. Failure evaluation data sheets should not be formatted in a manner

that might lead the examiner to a potentially incorrect

failure cause.

Thble 1 lists typical irregularities that may be observed

during visual examination or during snubber disassembly. These items characterize features of snubber degradation and may be useful to pinpoint the potential

cause.

2.2 Determination and Documentation

of the Operating Environment

Service-life monitoring techniques take into consideration the capability of the various snubber models to

endure the full range of plant environments (benign to

severe). Previously unidentified severe environments

may often be identified by root cause evaluation of

failed or degraded snubbers. Information regarding the

snubber endurance capability is often obtained from

operating experience (i.e., from failure data or by monitoring degradation).

Determination of specific environmental information

often involves specialized instrumentation and equipment that would be impractical for use at every snubber

location. The use of such equipment, therefore, would

be most practical for those applications where moderate

to severe environments are anticipated or as a diagnostic

aid in determining the cause of snubber degradation.

K3 NUREG/CR-5870
Appendix K

Table K.1 'ypical indicators of snubber degradation

Indicator

Dark hydraulic fluid

Black material deposit on rod

Excessive piston and cylinder wear

Worn capstan spring tangs

Localized ball screw fretting

Unsymmetrical wear of clevis pins

Elongation of attachment holes

Loose fasteners

Symmetrical wear on clevis pins

Discoloration of metallic parts

Hardened piston rod wiper

Rod wiper adhered to piston rod

High compression set

Cracked seal

Lack of fluid pigmentation

Corrosion of metallic parts

Bent piston rod or attachments

Changes in cold/hot position setting

Possible Cause

High amplitude vibration

High amplitude vibration

High amplitude vibration

High amplitude vibration

High amplitude vibration

High amplitude vibration

High and/or low amplitude vibration

High and/or low amplitude vibration

Low amplitude vibration

Excessive temperature

Excessive temperature

Excessive temperature

Excessive temperature

Excessive temperature

High radiation level

High humidity/leaking components

Overloading

Increased drag or jamming

Various methods and equipment used for measurement

of specific environmental parameters are described

below.

2.2.1 Lbmperature

Temperature-sensitive tape is useful for identifying

hot spots. Ideally, however, to monitor environmental

temperatures, a time/temperature profile is most useful.

Chart recorders or digital data acquisition systems (e.g.,

bit loggers, computers, etc.) may be used for this

purpose.

2.2.2 Radiation

Normal radiation levels in operating plants do not

usually contribute significantly to snubber degradation.

Pertinent data pertaining to plant radiation levels can

generally be obtained from health physics area surveys.

Measurement of radiation levels specifically for servicelife monitoring does not seem justified, except in cases

of snubber degradation where other potential causes

have been ruled out.

2.23 Vibration

A number of methods and equipment for detecting and

measuring vibration are available. They vary from

simple visual observation, detection by "hands on"

NUREG/CR-5870 K4
Appendix K

inspection, portable vibration measuring instrumentation, and remote vibration measuring equipment.

Vibration can often be identified during routine snubber

visual examination or during failure evaluation. Characteristics such as metal filings, darkened hydraulic fluid,

deformed connecting pins, elongated attachment holes,

and fretting of mating parts are indicators of vibration.

2.2.4 'lransients

As with vibration, dynamic load transients that exceed

the snubber load capacity may often be identified during

routine inspections (e.g., observation of deformed structural members), augmented inspections (e.g., during

hand-stroking of the snubber), and failure evaluation

(e.g., deformed internal parts). In situ devices, such as

load measuring clevis pins, are also available to monitor

snubber load in applications where such transients are

suspected.

2.3 Inservice Testing Results

Evaluation of time traces (e.g., load and velocity)

obtained during routine functional testing is useful in

identifying degradation that could lead to functional

failure if not corrected. For example, an unusual

number of load spikes may indicate the presence of dirt

or other solid particles in the snubber; a single load

spike may indicate local fretting of the ball screw,

Follow-up diagnostic tests (see below) are useful for

further evaluating such anomalies.

2.4 Diagnostic Testing

Diagnostic tests may be used to obtain information

beyond that available from routine functional test data.

Results from diagnostic testing can be helpful in identifying the failure or degradation mechanism. For example, a progressive decrease in the "bleed' velocity of a

hydraulic snubber during a sustained load can be indfca.

tive of particulate contamination of the hydraulic fluid.

Tbst equipment used for diagnostic tests should be

configured to allow the application of various levels of

controlled test parameters such as load and velocity,

2.5 As-Found Testing

Considerable information can be obtained by conducting post-service tests on snubbers removed from service.

2.6 lnending

Bfending is a useful tool to monitor progressive snubber

degradation. MIending has a number of important

considerations:

* The establishment of baseline data is essential for

trending.

* 'Bending data should be sufficiently accurate so that

trends may be identified.

* 'Rending parameters should relate directly to the

anticipated aging failure mode. Such parameters

include, but are not limited to, drag force for

mechanical snubbers and seal compression set for

hydraulic snubbers.

Note: An important example of inappropriate monitoring parameters is the use offunctional test data for

monitoring or trending seal degradation. Although

seal degradation can affect functional test results to

some extent, loss of low pressure sealing integrity--the

primary aging failure mode for snubber seals--would

not be reflected in functional test data.

* If test data are to be used for trending, it is recommended that the data be obtained consistently by

the same type of test machine, under the same test

conditions. Ideally, the same snubber should be

used. Snubbers selected for trending should be

representative of the service environment related to

the snubber population to be monitored.

* Reservoir fluid level is the most appropriate

trending parameter for monitoring snubber leakage.

'Rends in average drag force are generally more

detectable than for peak drag force.

* A number of plants have established administrative

limits for functional test parameters in order to

prompt the replacement or repair of a given snubber

K5 NUREG/CR-5870
Appendix K

before failure. This approach assumes that the

parameter in question (e.g., drag force) is progressing toward the failure limit, which may or may not

be the case. It is therefore recommended that

administrative limits be established at a level that is

outside the range of normal variations for the given

parameter. Premature replacement or maintenance

can increase the probability of snubber failure by

introducing potential maintenance or manufacturing defects and reduce the potential benefits of

the trending analysis.

2.7 Augmented Surveillance Methods

Various 'hands-on' methods may be used to identify

snubber degradation and to detect severe environmental

conditions. These include hand stroking for verification

of free movement, rotation of the snubber about its

spherical bearings as a check for jamming, hand detection of vibration, and hand detection of high

temperature.

2.8 Service-Life Categories

Depending on the significance of environmental

extremes from one area in the plant to another, separate

and distinct service-life populations may be practical.

For example, it may be practical to establish a separate

service-life population for snubbers in the upper level of

the drywell for some BWR plants, due to relatively high

temperatures in that area that may result in more rapid

seal degradation. On the other hand, isolated applications involving very severe environments (eg., steam

tunnel, pressurizer cubical, etc.), should be managed

separately on a case-by-case basis.

3 Visual Examination Attributes

Many attributes that should be included in snubber preservice examinations need not be checked again during

inservice examination. Snubber characteristics that are

potential indicators for inoperability, e.g., empty reservoir, missing clevis pin, etc., are normally evaluated during inservice inspection (ISI). For service-life monitoring, characteristics that relate more to degradation prior

to failure are emphasized. It is, therefore,

recommended that Appendix B (Recommended

Examination Checklist Items) be divided into three

basic checklists: one for pre-service examination only,

another for inservice and pre-service examination, and

another for service-life monitoring.

3.1 Recommended Examination

Checklist Attributes (Pre-Service

Examination Only)

It is recommended that snubbers be visually examined

for the following unacceptable attributes during preservice examination only:

snubber installed with preset locking screws (used

for shipment only)

* snubber installed in wrong location

* protective coverings or shipping plugs not removed

* snubber freedom of movement impaired by

interference with adjacent equipment

* other one-time pre-service checks recommended by

the manufacturer.

3.2 Recommended Examination

Checklist Attributes (Pre-Service

and Inservice Examination)

Visual examination attributes that may indicate snubber

inoperability during pre-service inservice examinations

are listed below:

* non-pressurized reservoir oriented such that

hydraulic fluid cannot gravitate to snubber

* severe corrosion or solid deposits that could impair

snubber performance

NUREG/CR-5870 K6
Appendix K

* inadequate swing clearance

* paint on piston rod (could cause a frozen condition)

* permanent deformation (e.g. bending) of the

snubber or its structural attachments

* inadequate reservoir fluid level

* clevis pin not installed

* weld arc strikes, weld slag, adhesive, or other

deposits on piston rod or support cylinder (could

cause a frozen condition)

* loose or missing fasteners

* cold or hot position setting varies from specified

value

* spherical bearing not fully engaged in attachment

lug

3.3 Recommended Examination

Checklist Attributes (Service-Life

Monitoring)

'Pypical attributes that should be noted for service-life

monitoring purposes are as follows:

* evidence of corrosion

* evidence of solid deposits (e.g., boric acid) from

leaking components

* loss of hydraulic fluid since previous visual

examination

* metal filings on or in the vicinity of the snubber

* observed fluid leakage

* evidence of significant dark (i.e., black or dark

brown) material deposit on piston rod

* rod wiper adhered to piston rod

* abnormal color of hydraulic fluid

* wear or deformation of clevis pins

* elongation of attachment holes

* evidence of wear on support cylinder

* cracked or deformed fluid reservoir

* evidence of foreign material (e.g., water, solid

particles, etc.) in hydraulic fluid

* discoloration of metallic parts due to heat.

4 Failure Grouping and Corrective Action

Subsection ISTD currently requires that any snubber

that fails to meet functional test acceptance criteria be

classified into one of six Failure Mode Groups (FMGs)

(see Figure K1). Depending on the FMG, various corrective action alternatives may apply.

The following recommendations pertain to the classification of failures and follow-up corrective action.

4.1 Definitions

It is recommended that the following definitions be

included in Subsection ISTD. These definitions are

consistent with those proposed by the Technical

Committee on Common Aging Terminology (CAT):

* Failure Mode: The manner in which the snubber

failed (e.g., high drag force, high acceleration, high

bleed rate, low locking velocity, etc.).

K.7 NUREG/CR-5870
Appendix K

A or C A, B, or C C D E

A:

B:

C:

D:

E:

Replace all FMG snubbers with compatible snubbers; no additional testing.

Change the environment; no additional testing.

Additional testing in FMG.

Test or stroke all FMG snubbers; no additional testing.

Continue testing in sampling plan. I

Figure K1 Current failure categorization and additional testing requirements

* Failure Mechanism: The physical process that

resulted in failure (e.g., deformation of screw shaft,

thermally induced compression set, etc.).

* Failure Cause: The circumstances during design,

manufacture, or use which led to failure (e.g., excessive temperature, defective plating process, vibration, side loading, etc.).

* Root Cause: The fundamental reason(s) for failure

which, when corrected, prevents its recurrence.

4.2 Elimination of the Isolated Failure

Mode Group

Isolated failures should be identified by root cause

evaluation. However, due to their singular nature, such

failures cannot represent a group. It is therefore

recommended that the isolated failure mode group be

eliminated.

4.3 Distinction Between ServiceRelated and Nonservice-Related

Failures

A snubber failure that is associated with a manufacturing or design deficiency could nonetheless be service

related. For example, the root cause for a seal failure

resulting from the inadvertent use of a seal material that

is less resistant to heat than the material specified by the

manufacturer may be identified as inadequate material

control (a manufacturing-related failure cause).

Although the seal may not have endured for as long as

one manufactured from the specified material, it is likely

that it did provide some amount of service prior to failure. The option should be allowed, therefore, for continued use of additional snubbers that may utilize this

material, provided that the environment is adjusted to

be compatible with the seal material.

NUREG/CR-5870 K-8
Appendix K

Snubbers with a potential for failure from the same root

cause should be assigned to the same root cause group

(RCG) in order to take steps that would reduce potential failure during future operation. However, in order

to establish additional testing requirements (ISTD), it

need only be determined whether or not the failure is

service-related, nonservice-related, or unexplained (see

Figure K2). Therefore, it is probably not necessary to

pre-establish failure cause groups in Subsection ISTD.

Categorization using the snubber-grouping plan in Figure 2 would distinguish between service-related and

nonservice-related failures. This is important for two

reasons:

1. To monitor the rate of occurrence of service-related

failures.

2. lb provide the option to modify the environment

for all snubbers subject to service-related failures

without having to test additional snubbers. (This

option is currently allowed for t applicationsinduced" failures only.)

Resulting data would facilitate the compilation of useful

failure statistics, both plant-specific and for the industry

in general, and would allow flexibility in establishing

various RCGs in an industry data base without concern

over conflict with ISTD.1

By comparing Figures K1 and K2, it can be seen that

additional testing requirements associated with the proposed classification system are consistent with those

currently in the ISTD standard. One change, however,

is that the option to replace, modify, or repair all

snubbers in the RCG (failed and unfailed) without

requiring additional testing would be allowed for all

failures. This option was previously allowed only for

design, manufacturing, and application-induced failures.

4.4 Replacement or Modified Snubbers

Snubbers are occasionally subject to operating environments for which they have not been qualified. Such

environments include dynamic load transients, high

amplitude vibration, high temperature, etc. Paragraph

ISTD 1.11.1 of the standard requires that replacement

or modified snubbers have a proven suitability for the

1The Snubber Utility Group (SNUG) has encountered difficulties in

establishing failure categories for the SNUG data base due to potential

inconsistencies with FMGs currently included in ISTD.

I Nonservice-Related Unexplained

I I

A, B, C, or D A or C E

A:

B:

C:

0:

E:

Replace all RCG snubbers with compatible snubbers; no additional testing.

Change the environment; no additional testing.

Additional testing in RCG.

Test or stroke all transient event snubbers; no additional testing.

Continue testing in sampling plan.

Figure K.2 Proposed failure categorization and corrective action

K.9 K.9 ~~~~~NUREG/CR-5870
Appendix K

application or environment. Because environmentally

compatible snubbers are not available for all such applications, utilities often have no alternative but to continue to use the same snubber model or another

unqualified model.

It is suggested, therefore, that some flexibility be

provided in ISMD 1.11.1 that would allow for continued

use of existing snubber models in such cases. For example, requirements for augmented inspections for these

applications would ensure that snubbers would be

replaced or maintained before failure.

NUREG/CR-5870 K10
Appendix L

Snubber Maintenance Recommendations
Appendix L

Snubber Maintenance Recommendations

This appendix includes a number of recommendations pertaining to snubber maintenance that were developed during

the NPAR research.

L.1 NUREG/CR-5870
Contents

Snubber Maintenance Recommendations ................. .............................. L.1

1 General Maintenance Practices ...................................... L.4

2 Identification of the Operating Environment .......... ............................ L.4

3 Snubbers Prone to Rapid Degradation in Severe Environments ...................................... L.4

4 Failure Evaluation ...................................... L.5

5 Failure Grouping ...................................... L.5

6 Modification of the Operating Environment ...................................... L.6

7 Snubber Elimination ...................................... L.6

8 Augmented Inspections ............................................... L6

8.1 Hand Stroking ............................................... L.6

8.2 Rotation in Place ............................................... L.6

8.3 Augmented Testing ................................. L.6

8.4 Measurement of Degradation Stressors ................................. L.7

8.5 Boroscopic Evaluation ................................. L.7

8.6 Fluid Analysis ........................................ L.7

9 Snubber Maintenance Frequency .................................. L.7

10 'Rending ........................................ L.7

10.1 Evaluating Bends in Test Results .................................. L.7

10.2 Parameters for Rending ................................. L.7

10.3 Reservoir Fluid Level .................................. L.7

10.4 Administrative Acceptance Limits ................................. L.8

10.5 Baseline Data ........................................ L8

11 Baseline Data .................................. L.8

11.1 Baseline Test Data ................................. L.8

11.2 Baseline Seal Data ................................. L.8

12 Snubber Data Base ................................. L.9

13 Functional Test Equipment ................................. L9

13.1 Diagnostic Tsting ................................. L.9

NUREG/CR-5870 L-2
13.2 hfending ........................... I 9

13.3 Verification of Degradation .. 9

14 Spare Snubber Rotation ...... L9............ L9

15 Personnel Qualification .L10

16 As-Found Evaluation. L.10

17 Coordination and Communication .. 10

18 Replacement Parts and Materials. Li

L.3 NUREG/CR-5870
Appendix L

1 General Maintenance Practices

In general, the effects of aging on snubber performance

may be mitigated through the implementation of sound

maintenance practices, including an effective service-life

monitoring program. The goal of snubber aging management should be to ensure that snubbers are removed

from service or maintained prior to failure. In this

regard, if a snubber is suspected of being in a failed

condition, it should not be repaired, modified, or altered

before determining its operability, in compliance with

current inservice inspection (IS1) requirements. However, this does not include snubbers not suspected of

being in a failed condition that are subjected to preventive maintenance (e.g., the addition of hydraulic fluid to

a low fluid reservoir).

2 Identification of the Operating Environment

Stressors of primary concern for snubbers are heat,

vibration, transient loads, and moisture. Measurement

of temperature and humidity levels in various plant

areas is recommended; such information is useful in predicting snubber service life and for identifying possible

service-life populations. However, some judgement

must be used in deciding the number and location of

areas to be monitored.

Measurement of environmental parameters for specific

snubber locations should be limited to applications for

which severe operating conditions are suspected or used

as a diagnostic aid to verify the cause of degradation or

failure. Characteristics noted during visual examination

or testing of degraded snubbers are often helpful in

identifying the existence of severe operating environments. A number of instruments and techniques are

available to measure environmental parameters, including temperature-sensitive tape, thermocouples, portable

area temperature and humidity monitors, hand-held and

remote vibration measuring equipment, load measuring

clevis pins, strain gauges, etc. Various data acquisition

systems are available for use in obtaining a time profile

for environmental parameters.

3 Snubbers Prone to Rapid Degradation in Severe Environments

Snubbers in isolated applications that are prone to rapid

degradation, i.e, within two to three operating cycles,

should be identified and managed on a case-by-case

basis. Such applications frequently involve small capacity snubbers installed on piping that is attached to large

capacity, high energy systems. Snubbers with particularly long extension pieces appear to be vulnerable to

such degradation. Environmental stressors that can

cause rapid degradation include high amplitude

vibration, dynamic transients, and high temperature,

e.g., above 250F. Such applications may require augmented inspections (Section 8.0) or maintenance or frequent replacement with new or refurbished snubbers.

Other alternatives include replacement with snubber

models or materials that are more compatible with the

environment, e.g., Viton seals for high temperature

applications, modification of the environment (Section 6.0), or elimination of the requirement for a

snubber at that location (Section 7.0).

NUREG/CR-5870 L.4
Appendix L

4 Failure Evaluation

Determination of the cause of snubber failures or degradation is essential for adequate management of snubber

aging. Separation of service-related and nonservicerelated degradation and failures is necessary to ensure

that nonservice-related degradation is not included in

the data base used to monitor snubber aging. Also

important is identification of the root cause in order

that conditions resulting in the degradation or failure

may be rectified.

Various techniques and considerations are recommended for evaluating and documenting failure or

degradation causes. These include:

pre-identification of logical steps to be used in the

evaluation

* visual examination

* personnel training

* diagnostic testing (Section 13.1)

* photographic documentation of observations

* metallographic analysis

* hydraulic fluid analysis (Section 8.6)

* chemical analysis

* physical property analysis.

5 Failure Grouping

Failure grouping is a useful method for ensuring that all

snubbers subject to a particular root cause of failure are

identified and managed accordingly. Implementation of

this method involves using insight and judgement to

optimize its effectiveness.

Failure groups should be based upon the root cause of

failure, not necessarily the simple cause. It may be

determined, for example, that the failure cause for a

given snubber was high temperature, and that the root

cause was a local steam leak associated with a valve having a particular manufacturing defect. The failure group

should include only those snubbers that are subject to

degradation from that root cause, i.e., degradation due

to leakage from that valve or other valves having potential for the same manufacturing defect. Elimination of

the root cause, e.g., repair of defective valves, and evaluation of all snubbers in the group would justify elimination of the failure group as well as any augmented

inspection requirements. However, this would not

justify the elimination of special requirements for other

snubbers that may be subject to high temperature

degradation resulting from another root cause.

For the above reasons, failure groups should not be

established prior to the identification of a failure and its

subsequent root cause analysis.

L5 NUREG/CR-5870
Appendix L

6 Modification of the Operating Environment

One method for minimizing snubber degradation in a

severe operating environment is to modify that environment. Examples of this approach involve the following:

* use of heat shields for high temperature

applications

* improved plant ventilation or cooling

* system modification to reduce vibration or dynamic

transients

* use of protective boots to minimize corrosion.

* repair of leaking components in order to reduce the

humidity or corrosives

7 Snubber Elimination

Snubber applications involving severe operating environments and snubber models that are particularly susceptible to service degradation should be priority targets

in a snubber elimination program. This supports the

need for close coordination between maintenance and

engineering personnel (Section 17.0).

8 Augmented Inspections

Augmented inspections, i.e., inspections in addition to

that required during routine ISI, are recommended for

snubbers in severe operating environments (Section 3.0)

and for snubbers susceptible to rapid degradation. A

number of techniques and hardware that are useful for

conducting such inspections are described in the following subsections.

8.1 Hand Stroking

Removal of one clevis pin and hand stroking a mechanical snubber while listening and feeling for abnormalities

is a useful method for identifying degradation that could

lead to eventual snubber failure. This method requires

some judgement on the part of the inspector, supporting

the need for experienced personnel and adequate

training (Section 15.0). This method is also particularly

useful for identifying degradation due to dynamic

transients.

8.2 Rotation in Place

A useful method for evaluating snubbers that are suspected of being locked-up (or jammed) is to attempt to

rotate the snubber about its longitudinal axis within the

physical limits allowed by the clevis attachments. If the

snubber can be rotated freely, the probability is low that

the snubber is locked, since lockup during thermal

motion of the piping or equipment would have resulted

in a significant axial load, prohibiting free rotation of

spherical end bearings. This method is most effective

for snubbers with a load capacity of 3,000 lb or less.

8.3 Augmented Testing

Snubber degradation may often be detected by evaluating functional test traces. Diagnostic testing (Section 13.1) in which controlled test parameters are varied

is also useful in this regard.

NUREG/CR-5870 L.6
Appendix L

8.4 Measurement of Degradation

Stressors

Various degradation stressors such as operating environment, dynamic transients, etc., may be monitored

using various types of instrumentation or devices (Section 2.0). Such methods are particularly useful for

monitoring snubbers subject to severe operating

conditions.

8.5 Boroscopic Evaluation

A boroscope is sometimes useful for internal inspection

of hydraulic snubbers. Such inspections may reveal, for

example, significant wear of internal parts or the existence of solid particles caused by wear or internal

corrosion.

8.6 Fluid Analysis

Microscopic evaluation of hydraulic fluid samples can

identify the existence of solid contaminants and can

often be used to identify the source of such particles,

e.g., particles generated during machining, corrosion

products, wear products, etc. Moisture evaluation of the

fluid is also useful for evaluating fluid from snubbers

with vented reservoirs in high humidity environments.

9 Snubber Maintenance Frequency

Scheduled maintenance should be based on realistic

considerations in regard to progressive degradation. It

should be noted that frequent, unnecessary maintenance, e.g., seal replacements, can actually increase the

probability of snubber failure due to damage or errors.

Maintenance frequency should be based on an assessment of the degradation rate for the general snubber

population in the plant, i.e., those snubbers in the

normal plant operating environment, excluding isolated

snubbers in severe environments (Section 2.0). If degradation data from snubbers in service are to be used to

establish the scheduled maintenance frequency, then

such data should be obtained from snubbers exposed to

operating environments that represent the environmental extremes for the general population of snubbers,

e.g.; snubbers from the upper level of the drywell etc. In

some cases, depending upon environmental variations in

the plant, it may be practical to establish more than one

service-life population.

10 'Frending

10.1 Evaluating lnends in Test Results

In general, evaluations to identify trends in snubber

degradation should not rely on functional test data

obtained during ISI. Such data is often not useful for

identifying trends because more than one data point for

the same snubber is often not available (i.e. the same

snubbers are not tested during each outage. For this

reason, if test data, e.g., snubber drag force, are to be

used for trending, representative snubbers should be

selected and tested periodically, using the same test

machine. For any trending evaluation, the establishment of baseline data (Section 11.0) is extremely

important. Such tests should be conducted separately

from routine ISI tests.

10.2 Parameters for Thending

Parameters to be used for trending should relate to the

anticipated degradation mechanism, e.g. compression

set in seals or drag force for mechanical snubbers.

10.3 Reservoir Fluid Level

Reservoir fluid level for snubbers with a pressurized

reservoir can be easily determined by observing the fluid

L.7 NUREG/CR-5870
Appendix L

level indicator. However, for vented reservoirs used on

many hydraulic snubbers the fluid level may not be

easily quantified. Verification of progressive fluid loss is

therefore difficult A method to verify fluid loss from

one inspection to the next is to mark the fluid level on

the reservoir and to compare the level with this mark

during subsequent inspections.

10.4 Administrative Acceptance Limits

they should not be too restrictive such that snubbers

that are not progressively degrading are unnecessarily

replaced or refurbished.

10.5 Baseline Data

If degradation parameters, e.g. snubber drag force, are

to be monitored for trends, the establishment of baseline data is essential (Section 11.0).

If administrative functional test acceptance limits are

used to identify snubbers to be replaced or overhauled,

11 Baseline Data

For trends to be identified, more than one data point for

a given parameter is required with respect to time (or

number of cycles). Therefore, attempts to identify

trends based on a single inservice data point, e.g. drag

force, without the existence of baseline (initial) values,

require unnecessarily conservative assumptions. Two

examples in this regard are the assumption that a measured drag force value of 2% indicates that drag forces is

increasing, and 2) the assumption, in calculating compression set, that the initial seal thickness was the

maximum value allowed by the manufacturer.

11.1 Baseline Test Data

It is recommended that baseline test data for snubber

activation parameters (locking velocity, release rate,

acceleration threshold, etc.) and drag force (for

mechanical snubbers) be obtained for plant snubbers,

whenever possible. Ideally, this data should be consistently obtained under the same test conditions, using the

same test machine (Section 13.2). Baseline data would

be available for comparison with inservice data for identifying degradation trends.

11.2 Baseline Seal Data

Premeasurement of the section thickness of replacement

seals before their installation in the snubber will provide

baseline data that may be used for a more accurate (i.e.,

less conservative) evaluation of seal life at a future date.

Similarly, baseline data for seal hardness would also be

useful.

Data from plant seal life evaluations have indicated

variations in seal degradation for various plants and for

different seals in the same snubber in the same plant.

Unless the seal compound is documented, it is impossible to determine whether or not such variations are the

result of differences in seal materials.

Documentation of seal compounds for replacement

seals will provide a basis for evaluating the performance

characteristics of various seal compounds.

NUREGICR-5870 1_8
Appendix L

12 Snubber Data Base

An automated snubber data base is strongly recommended for managing snubber service data, documenting snubber installation locations, and documenting

maintenance activities. It is important that such data

bases be updated regularly.

13 Functional Test Equipment

It is recommended that a snubber test machine be maintained at the plant, in addition to any test equipment

that might be temporarily located at the plant during

ISI. The plant test machine should be available for

diagnostic testing, trending, and verification of

degradation.

13.1 Diagnostic Testing

Diagnostic tests are extremely useful for verifying test

results obtained during ISI and for determining the

mechanisms associated with any failures or degradation.

For diagnostic testing, the test machine should be capable of producing a time-related trace of test parameters,

e.g., velocity and load. Manual operation of the tester is

also important as well as the ability to vary the level of

controlled test parameters.

13.2 dTending

snubbers. Separate trending tests should be conducted

using the same test machine that was used to establish

baseline data (Section 11.0) for the selected snubbers.

The test machine should have some automated capability to minimize operator influence during trending tests.

Since average drag force is generally more meaningful in

identifying trends, a data management system capable of

calculating average drag force would be useful.

13.3 Verification of Degradation

The operability of snubbers for which degradation or

failure is suspected, e.g., unacceptable snubbers identified during visual examinations, may be evaluated by

functional testing. The availability of an in-house

test machine will allow for expeditious evaluation, in

lieu of shipping the snubbers off-site or postponement

of testing until ISI tests are conducted.

ISI data is generally not usable for identifying degradation trends, such as increasing drag force in mechanical

14 Spare Snubber Rotation

Replacement (rotation) of snubbers removed from

service with spare snubbers is useful in reducing the

need for snubber refurbishment during refueling outages. The following suggestions pertain to spare

snubber rotation:

* Jidgement should be used for snubbers with a significant amount of remaining service life that are

removed for reasons other than maintenance, e.g.,

for functional testing. Arbitrary replacement of

such snubbers will reduce the amount of service data

L.9 L.9 ~~~~~NUREG/CR
Appendix L

that will be available for use in the service life data

base, and possibly increase the probability of snubber failure due to the introduction of maintenance

or manufacturing deficiencies that may exist in the

replacement snubbers.

Seal life begins when the seals are installed. Therefore, in order to optimize service life, seal replacements for spare hydraulic snubbers should be scheduled so that the period of time between seal

replacement and anticipated snubber installation is

* A data base for tracking the various installed

locations for each snubber should be maintained

(Section 12.0).

* If snubbers with remaining service life are removed

and then reinstalled without refurbishment, every

effort should be made to install the snubbers in the

same locations from which they were removed. The

cause of failure or degradation of snubbers that

were installed in various plant locations would

otherwise be extremely difficult to determine.

15 Personnel Qualification

A key maintenance consideration in managing snubber

aging is the qualification of maintenance personnel.

Adequate training is obviously important, but since

good maintenance practice involves considerable judgement, experience is of equal importance. Steps should

therefore be taken to minimize turnover of experienced

maintenance personnel.

As a minimum, training in the following areas is

recommended:

* snubber testing, diagnostic testing, testing for

trends, and recognition of test anomalies

* snubber visual examination and recognition of

meaningful visual anomalies

* snubber rebuilding and recognition of meaningful

anomalies during overhaul

* determination of failure or degradation causes

(Section 4.0)

* snubber handling, installation, and storage

procedures.

16 As-Found Evaluation

As-found evaluations may provide information that is

useful in identifying and managing degradation that

might otherwise have gone undetected. It is recommended that snubbers removed from service be visually

examined and functionally tested before reinstallation in

the plant and before performing any maintenance.

Hand stroking of mechanical snubbers is also

recommended.

17 Coordination and Communication

Continuous coordination between ISI, maintenance,

engineering, quality assurance, and engineering staff is

important. For example:

* Coordination with ISI staff can optimize service

data for both ISI and service-life monitoring use.

NUREG/CR-5870 LIO
Appendix L

* Coordination with engineering staff can ensure

elimination of problem applications in a snubber

reduction program.

* Communication with operations staff will aid in the

identification of dynamic transients.

18 Replacement Parts and Materials

The use of replacement parts that are most resistant to

degradation due to the service environment are recommended. Examples here include:

* use of temperature-resistant lubricants

* use of temperature-resistant seal materials, e.g.,

Viton

* use of corrosion-resistant materials for snubbers in

high humidity environments.

L.11 NUREG/CR-5870
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NRC FORM 335 Up. NUCLEAR REGULATORY COMMtSSION t . REPORT NUMBER t2 991 led At NF C Adet Vol., SAW., hv.,

NRCM 1102 It Drum Numoeq, if ans.l

3201, 3202 BIBLIOGRAPHIC DATA SHEET

{9w instacrions on tXc snrscJ NUREG/CR-5870

_ I PNL-8051 2. TITLE AND SUBTITLE

Results of LWR Snubber Aging Research 3. DATEREPORTPUELISHED

114ONTN | YeAR

May ' 1992

.FIN OR GRANT NUMBER

B2911

S. AUTHOR4S)6. TYPE OF REPORT

D. P. Brown/Lake Engineering Company Technical

E. V. Werry, D. E. Blahnik/Pacific Northwest Laboratory 7. PERIOD COVEREO neclxwve Ogress

NRC FORM 335 t24t91
THIS DOCUMENT WAS PRINTED USING RECYCLED PAPER
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