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ASME SUBSECTION ISTD RECOMMENDATIONS

BASED UPON NPAR SNUBBER AGING

RESEARCH RESULTS

D. P. Brown(a)

D. E. Blahnik

November 1991

Submitted to the

ASME Subsection ISTD Working Group

at the December 3, 1991 Meeting in

St. Petersburg Beach, Florida

Work supported by the

U.S. Nuclear Regulatory Commission

under Contract DE-AC06-76RL0 1830

NRC FIN B2911

Pacific Northwest Laboratory

Richland, Washington 99352

(a) Lake Engineering Company

Greenville, RI 02828
CONTENTS

ABSTRACT ............................................................... 1

SERVICE-LIFE MONITORING RECOMMENDATIONS ................................ 2

DETERMINATION OF SNUBBER FAILURE OR

DEGRADATION CAUSES ............. .................................. 2

DETERMINATION AND DOCUMENTATION OF

THE OPERATING ENVIRONMENT ........................................ 4

Temperature .............. .................................. 4

Radiation ................ .................................. 4

Vibration ................ .................................. 5

Transients ............... .................................. 5

INSERVICE TESTING RESULTS .............................. . 5

DIAGNOSTIC TESTING .............................. . 5

AS-FOUND TESTING ................................................. 6

TRENDING ............................... 6

AUGMENTED SURVEILLANCE METHODS ........ ............. . 7

SERVICE-LIFE CATEGORIES .............................. . 7

VISUAL EXAMINATION ATTRIBUTES ................ 8

RECOMMENDED EXAMINATION CHECKLIST ATTRIBUTES

(PRE-SERVICE EXAMINATION ONLY) ........ ............ . 8

RECOMMENDED EXAMINATION CHECKLIST ATTRIBUTES

(PRE-SERVICE AND INSERVICE EXAMINATION) ................. . 8

RECOMMENDED EXAMINATION CHECKLIST ATTRIBUTES

(SERVICE-LIFE MONITORING) ........................................ 9

FAILURE GROUPING AND CORRECTIVE ACTION ................................. 10

DEFINITIONS ...................................................... 10

ELIMINATION OF ISOLATED FAILURES ................ ................. 10

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DISTINCTION BETWEEN SERVICE-RELATED AND

NONSERVICE-RELATED FAILURES ...................................... 12

REPLACEMENT OR-MODIFIED SNUBBERS ................................. 13

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ABSTRACT

As a result of information obtained during the U.S. Nuclear Regulatory

Commission's (NRC) Nuclear Plant Aging Research (NPAR) Program, Snubber Task

research, recommendations are made in the three following areas for the next

revision of the American Society of Mechanical Engineers (ASME) Operations and

Maintenance (OM) Code, Subsection ISTD:

* Service-Life Monitoring

* Visual Examination Attributes

* Failure Grouping and Corrective Action.

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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 the ASME OM Code, Subsection 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 those

requirements 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.

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 screw or ball shaft, solidification of grease,

etc.). This information provides the basis to take effective countermeasures.

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.

It is important that personnel involved in failure evaluation have adequate experience and training to identify failures and trace them to their

root causes. Failure evaluation data sheets should not be formatted in a

manner that might lead the examiner to a potentially incorrect failure cause.

Table 1 lists typical irregularities that may be observed during visual

examination or during snubber disassembly that characterize features of

snubber degradation and may be useful to pinpoint the potential cause.

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TABLE 1. Typical Indicators of Snubber Degradation

Indicator

Dark hydraulic fluid

Gelation of 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 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

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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.

Various methods and equipment used for measurement of specific environmental parameters are described below.

Temperature

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.) are valuable for this purpose, but

obviously must be applied selectively to monitor more severe conditions in

relatively large snubber populations.

Radiation

Normal radiation levels in operating plants do not usually contribute

significantly to snubber degradation. Data pertaining to plant radiation

levels can generally be obtained from health physics area surveys. Measurement of radiation levels specifically for service-life monitoring does not

seem justified, except in cases of snubber degradation where other potential

causes have been ruled out, and where radiation levels may be abnormally high.

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Vibration

A number of methods and equipment for detecting and measuring vibration

are available. They include simple visual observation, detection by "hands

on" 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.

Transients

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 handstroking 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.

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 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.

DIAGNOSTIC TESTING

Diagnostic tests may be used to obtain information beyond that available

from routine functional test data. Such information may 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 indicative of particulate contamination of the hydraulic fluid.

Test equipment used for diagnostic tests should be configured to allow the

S
application of various levels of controlled test parameters, such as load and

velocity.

AS-FOUND TESTING

Considerable information can be obtained by conducting post-service

tests on snubbers removed from service. Such "as-found" tests can identify

snubber degradation before failure occurs and can also identify previously

unidentified severe operating environments.

TRENDING

Trending is a useful tool to monitor progressive snubber degradation.

To develop meaningful trending data, a number of important factors should be

considered:

* The establishment of baseline data is essential for trending.

* Trending data should be sufficiently accurate so that trends may be

identified.

* Trending 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 of functional 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 using the same test machine, under the

same test conditions. Ideally, the same snubber should be tested.

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.

* Trends in average drag force are generally more detectable and meaningful 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

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snubber 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.

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.

Methods and equipment can be used to monitor snubbers periodically or

continuously for service stressors and degradation. The snubbers can be

monitored either individually or on a system basis. In situ monitoring can be

used to confirm design loads and to help in the analysis of problem snubbers

located in severe environments. Stressors commonly monitored are load,

vibration, and temperature. Snubber stroke positions can also be monitored to

verify thermal movements.

SERVICE-LIFE CATEGORIES

Depending upon 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 servicelife population for snubbers in the upper level of the dry well 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 (e.g., steam tunnel, pressurizer cubical,

etc.) should be managed separately on a case-by-case basis.

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VISUAL EXAMINATION ATTRIBUTES

Many attributes that should be included in snubber pre-service examinations need not be checked again during inservice examination. Snubber

characteristics that are potential indicators of inoperability, e.g., empty

reservoir, missing clevis pin, etc., are normally evaluated during 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 preservice examination, and another for service-life monitoring.

RECOMMENDED EXAMINATION CHECKLIST ATTRIBUTES (PRE-SERVICE EXAMINATION ONLY)

It is recommended that snubbers be visually examined for the following

unacceptable attributes during pre-service 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.

RECOMMENDED EXAMINATION CHECKLIST ATTRIBUTES (PRE-SERVICE AND INSERVICE

EXAMINATION)

Visual examination attributes that may indicate snubber inoperability

during pre-service and 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

* inadequate swing clearance

* paint on piston rod (could cause a frozen condition)

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* deformed structural attachment or piston rod

* 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 on approved

drawing

* spherical bearing not fully engaged in attachment lug.

RECOMMENDED EXAMINATION CHECKLIST ATTRIBUTES (SERVICE-LIFE MONITORING)

Typical 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

* evidence of snubber used as a stepping support

* cracked or deformed fluid reservoir

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t . .

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

hydraulic fluid

* discoloration of metallic parts due to heat.

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 1). Depending on the FMG, various alternatives for

corrective action may apply.

The following recommendations pertain to the classification of failures

and follow-up corrective action.

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.).

* Failure Mechanism: The physical process which resulted in failure

(e.g., deformation of screw shaft, thermally-induced compression set,

etc.).

l 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.

ELIMINATION OF ISOLATED FAILURES

By definition, isolated failures can not represent a group. It is,

therefore, recommended that the isolated FMG be eliminated. Isolated failures

will be addressed in the root cause category.

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. . I

IIsolated Uneplaine

A or C A, B, or C C D E

A: Replace all FMG snubbers with compatible snubbers; no additional testing.

B: Change the environment; no additional testing.

C: Additional testing in FMG.

D: Test or stroke all FMG snubbers; no additional testing.

E: Continue testing in sampling plan.

FIGURE 1. Current Failure Categorization and Corrective Action
DISTINCTION BETWEEN SERVICE-RELATED 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 which results 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.

Snubbers with a potential for failure from the same root cause should be

assigned to the same root cause groups (RCG), in order to take steps that

would reduce the potential for failure during future operation. However, in

order to determine which ISTD followup(a) actions are applicable, it need

only be determined whether or not the failure is service-related, nonservicerelated, or unexplained (see Figure 2). 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. to provide the additional followup activity to modify the environment

for all snubbers subject to service-related failures. (This is

currently allowed for "applications induced" failures only.)

(a) Currently referred to in ISI as "corrective action."

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A, B, C, or D A or C E

A: Replace all RCG snubbers with compatible snubbers; no additional

testing.

B: Change the environment; no additional testing.

C: Additional testing in RCG.

D: Test or stroke all transient event snubbers; no additional testing.

E: Continue testing in sampling plan.

FIGURE 2. Proposed Failure Categorization and Corrective Action

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 RCG cause categories in an industry

data base without concern over conflict with ISTD.(al

By comparing Figures 1 and 2, it can be seen that followup actions

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 without requiring additional testing would be allowed for all failures. This option was previously

allowed only in the maintenance/repair/installation failures category.

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

(a) The 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.

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ISTD 1.11.1 of the standard requires that replacement or modified snubbers

have a proven suitability for the 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

ISTD 1.11.1 that would allow for continued use of existing snubber models in

such cases. Conduction of augmented inspections for these applications would

increase the probability that snubbers would be replaced or maintained prior

to failure.

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