ML18038B511

Resolution of Pressurizer Safety Valve Seat Leakage Concerns

ML18038B511
Person / Time
Site:	Browns Ferry, Sequoyah
Issue date:	11/14/1991
From:	Simmons R TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML18038B503	List: ML18038B502 ML18038B504 ML18038B505 ML18038B506 ML18038B507 ML18038B508 ML18038B509 ML18038B510 ML18038B511 ML18038B512 ML20094F131
References
NUDOCS 9511080201
Download: ML18038B511 (22)

Text

SEQUOYMK NUCLEAR PLANT RZSOLUTXON OP PRZSSURXZZR SAPZTY VALVE SEAT LEAKAGE CONCERNS RXCH2QU)

G SXMMONS 11/14/91 951108020i 951103 FDR ADOCK 05000259 p

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SEQUOYAH NUCLEAR PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN TABLEOF CONTENTS 1.0 IYXRODUCTXON.

E.S SIPSMARP OP UEAIIAOE OCCURRENCES SINCE RE)TART 3.0 PAST CORREC'XTVE'CTXON TO RESOLVE CONCERN 3 '

SHORT-TERM CONSIDERATIONS '.

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LONG-TERM CONSIDERATXONS

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LEAKAGE

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4 4.0 RESULTS OF SHORT-XAUMCORRECTIVE ACTIONS..

4 S.S POISON-UP CORRECTUM ACTION LEAXAGECONC'&

5.1 VALVE MODXPICATXON 5.2 SETPOINT INCREASE

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5.3 LEAKAGE ROOT CAUSE UPDATE 5 '

INDEPENDENT REVIEW OP VALVE DESIGN TO RESOLVE 0

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7 6.0 ACIXONS TAKE TO ADDRESS COKXROL ROOM ANNUNCIATION.

7.0

SUMMARY

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8.0 CONCLUSION

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SEQUOYAH NUCLEAR PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN 1.0 IXI'RODUCTXON 2.0 This document provides an update, for t..be L'me period from February

'88 (unit 2 restart) to preset, of efforts being made to resolve seat leakage problems with the pressurizer safety valves.

Efforts to resolve this issu>>

prior to February

'88 were reported in the attached letter from R. Gridley to the U.S.

Nuclear Regulatory Commission dated June 24,

1988, "Setzuoyah Nuclear Plant (SQN)

Performance Testing Of Reactor Relief and Safety Valves".

SOMMART OT IRISOIOR OCCURRENCES SINCE.RESTART Since restart of units 1

and 2

there have been seven identified incidences of pressurizer safety valve seat leakage associated with steam service (drained loop seal).

Two of these leaks were corrected with the valve successfully operating with no detectable leakage until the next refueling outage.

A summary of these leaks follows and Attachment 1

graphically depicts incidences of pressurizer safety valve leakage.

VALVE S N POSIT ON OPERAT G CYCLE SPARE C

RV-1-8010'V-1-8010C A

A A

Cycle 3

Cycle 3

Cycle 4

RV-2-8010B SPARE B SPARE C

RV-1-8010B B

A A

C Cycle 2 Cycle 2

Cycle 3

Cycle 3

3.0 PAST CORREC'ITIVE ACTION TO RESOLVE ~~GE CONCERN S

Because of continued leakage concerns a task force was formed in January, 1989 to identify potential causes of the leakage and recommend appropriate short and long-term corrective actions.

This task force consisted of representatives from Sequoyah Engineering, Secpxoyah Systems, Corporate Maintenance, Corporate Engineering, Gilbert Commonwealth (G/C) and Stone

~ started itrecdxdlkfdy epoo cert stsreups but tpsst attcc pfebkres were corrected with arch)cot tcstspcraesto arsd lhcrtssal bhdl'og cd a strut. 'ge prcssure was dropped sed the valve gagged whib these probkrrss were resohe4 Ho ibrthec katage was dctcctcd duriec the cmajadcr oC the pCvaCiag cydc; howcva, Siasscrebly oC thc vtdvc Amag the outage iedicatcd that sGght kaksge teay have bcca prescer.

~e ~ae wbrts tbe prcssuritcc cocksure hatch was rcreovcd to aihsw access durin unit pecssioe.

Leakage cptit aitcc valve was tcreporsrity sgZcd sed the tete pcraeue sihwcd to rcstsbitisc.

I H

SEQUOYAH NUCLE.M PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN and Webster (SWEC).

Valve sensitivities and corrective action options vere discussed.

The folloving is a list of the options which vere considered:

SENSI TY POSS LE SO O'H NOZZLE LOADS AMBIENT TEMPERATURE INLET GAS MIXTURE RCS HEAT-UP RATE RAPID PRESSURE INCREASE SET PRESSURE Stronger

valve, piping configuration

change, support rearrangement, bellows/flex-hose, ball joints, support the valves off the pressurizer, check for binding, rework the loop seals, relocate the valves, Westinghouse ring

header, cpxantify loading sensitivity through special test at Hyle Operatiogal specification

changes, refrigeration

coils, independent or dedicated HVAC or redesign of existing

HVAC, R&D testing to quantify sensitivity Water snop seal Operational specification change No identified solution, other than slov pressure increase Set at high end of allowable, increase the Technical Specification set pressure The end product of the task force was a set of short and long-term recommendations.

The short-term recommendations addressed several valve sensitivities since a singular root cause for leakage vas not identified and were intended to minimize the possibility of leakage.

The long-term recommendations were to be considered in the event that seat leakage continued.to be a plant concern.

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SHORT-TERH CONSIDERATIONS The following are the actions which vere performed to address the short-term recommendations of the task force:

S1.

Hodifications were implemented for the unit 2

tailpipes to minimize nozzle flange loading.

Modification of the unit 1 tailpipe was placed on hold pending evaluation of the success of the modification for unit 2.

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SEQUOYAH NUCLEAR PLANT RESOLUTXON OF PRESSURXZER SAFETY VALVE SEAT LEAKAGE CONCERN S2 S3.

S4.

Each safety valve was adjusted within the upper half of the acceptable set pressure range in order to provide the maximum margir between operating pressure and valve set pressure.

Wal3c-downs were performed. of the valve piping to assess other causes that may be contributing to lea3cage.

A thermal interference was located and corrected on the unit l, "A" position tailpipe.

Post maintenance acceptance criteria for lea3cage was enhanced to add a

steam seat leakage test following the existing nitrogen seat leakage test.

A study was performed to document the valve positions where leakage had occurred in the past and the serial number of the valve which had lea3ced.

At this time, the leakage appeared to be position related and no clear trend could be identified to indicate a problem with individual valves.

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Sio Maintenance proced~

os were reviewed by Crosby during rework of a valve in order to ensure that appropriate repair techniques vere in place.

The maintenance procedure was enhanced to include comments by Crosby.

Administrative controls were implemented to ensure a

consistent temperature in the pressurizer enclosure during operation.

Pressurizer heat-up rate was administratively limited to 50 degrees F per hour.

An action plan was established to try to correct lea3cage as soon as it was identified (e.g.

gagging of valve, lowering of pressurizer pressure, cooling of the pressurizer enclosure).

All valves were removed during each outage. 'eat leakage testing was performed with any required maintenance performed prior to reinstalling the valves on the unit.

Sll. A preliminary assessment was obtained from Westinghouse on raising or staggering valve set pressure.

The possibility, of operating the plant with two valves available was also discussed.

Sl2. Valve temperatures were measured during normal operation with the "A" position on both units identified, as running significantly hotter than the

SEQUOYAH NUCLEI& PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN other two positions.

Modifications were'nitiated to provide additional cooling to the

."A" position valve.

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LONG-TERM CONSIDERATIONS The following long-term possibilities were identified for future consideration.

L1.

Revise primary loop analysis to raise the set pressure of the pressurizer safety valves.

L2.

Install. Watts Bar valves which have a forged body and are less sensitive to tailpipe loading.

L3.

Install a flexible )oint in the safety valve tailpipe to minimize-tailpipe loading.

L4.

Install Sebim valves (perceived as a better valve design for the application)

L5.

Support valves off pressurizer.

L6.

Relocate, valves and install "halo" header (removes piping loads from valves)

L7.

Return to wet loop seal with smaller water volume.

L8.

Operate with gagged valve (obtain westinghouse Probabilistic Risk Assessment justifying operation at full power with two valves).

L9; Xnitiate HVAC'esign changes and operation procedures to control ambient temperature excursions.

L10. Purchase custom designed valves (investigate alternate vendors and

.expand steam trim user survey).

4.Q MI&3LTSOF SHORT-TERM COMUS CHIVEACTXONS complementation of the short-term actions identified in the previous section of this report met with limited success.

Pressurizer safety valve leakage has not resulted in a forced outage;

however, periodic seat leakage has still been an operational concern.

Leakage of these valves results in increased operator attention and a large amount of rad-waste being processed to cool the pressurizer relief tank.

awhile a

singular root cause of pressurizer safety valve leakage was never clearly pinpointed in the

past, the

PI

SEQUOYAH NUCLE~& PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN combination of corrective actions performed to date has improved seat leakage performance by attempting to address all possible root causes.

5.0 FOLLOVf-UPCOM&CTXVEACTION TO RESOLVZ LEAKAGE CONCXX&l Because of the continued concern with pressurizer safety valve leakage a task force was again established in 1990 consisting of Sequoyah Engineering, Sequoyah Technical Support.',

Sequoyah Maintenance, Corporate Maintenance, and Crosby.

5 ~ 1 VALVE MODIFICATION The task force reviewed the long-term actions from the previous meetings and xeviewed additional options based upon recent operating history and

current, industry technology. It was learned at this time that Crosby was developing a new design for the disc insert which could improve seat leakage performance.

This new. disc insert incorporates a more exaggerated undercut design than the current disc insert and is better able to compensate for thermal effects and othe valve sensitivities that could result in seat leakage.

TVA met.with Crosby to discuss this new disc insert and witne.;.sed a

steam test demonstration of the disc in a pressurizer safety valve similar to Seguoyah's (the test valve was identical except for a

forged instead of a cast body).

After evaluating this valve design change against the previously determined long-term corrective actions it was determined that the disc insert modification provided the best option from a cost/benefit standpoint to resolve the continued seat leakage problem.

However, before committing to implement,.this modification, TVA formed a partnership with Crosby to test the new disc insert design at Hyle Labs in two spare pressurizer safety valves which had never passed a seat leakage test since the test media had been changed from nitrogen to steam.

A portion of the testing also simulated the effects of tailpipe loading and uneven temperature distribution across the valve.

The spare valves passed all testing and were leak tight in excess of 95 percent of their set pressure.

A valve modified with the new disc insert will be installed on unit 1 in the "A" position during the cycle 5 refueling outage scheduled for October of this year.

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SETPOINT INCREASE The possibility of increasing the valve set pressure has also been identified as a creditable solution to the seat leakage problem.

This fix would supplement the

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SEQUOYAH NUCLEM PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN improvement in valve performance provided Sy the new disc insert design. If the disc insert modification by itself does not completely resolve the

leakage, px'oblem, it is believed an increase in set pressure will provide the additional margin between

'operating pressure and set pressure needed to resolve the seat leakage

.ssue.

The study of increasing the valve set pressuru by three percent will be puxsued if necessary based upon a

performance evaluation of the new disc inseC design in the unit 1 "A" position during the next operating cycle.

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LEAKAGE ROOT CAUSE UPDATE'ased upon the results of steam leak testing at Hyle Labs and a history of leakage for valve position versus valve serial number since restart of both

units, some additional insight has been gained regarding the root cause of the continuing seat leakage problems.

Two observations can be made regarding this leakage.

5.3.1 Leakage continues to occur almost every operating cycle in the "A" position for both units.

A significant diffe ence between this position and the other two valve positions is the ambient temperature profile.

The original design of the HVAC duct for the pressurizer enclosure directs the majority of the air flow toward the "B" and "C" valve positions.

Temperatures have been recorded at the valve locations during operation which indicate that the "A" valve has been much

'otter than the other two valves.

Xt can be postulated that. hotter ambient temperatures cause higher valve body temperatures, which can cause additional thexmal growth and distortion with subsequent detrimental effects on the internal valve clearances and disc to seat alignment.

A significant increase in the ambient temperature can also cause a reduction in valve set pressure resulting in a greater potential for seat leakage at normal operating pressure.

The effect of uneven tempexature distribution has recently been reduced by installing a baffle in the existing air duct to direct aix to the "A" valve position.

This modification was implemented on unit 2

during the cycle 4 outage, with no seat leakage identified since unit restart.

The modification has also been implemented on unit 1, but not until the "A" position valve had alxeady started leaking.

Therefore, the value of this modification for unit 1 is still unknown.

5.3.2 Leakage which has occurred at positions other than the "A" position have been on valves which 6

SEQUOYAH NUCLEAR PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN 5.3.3 have subsequently had diffulcultypassing a steam seat leakage test.

Steam seat le+age testing had previously been performed us3..-.g nitrogen, but a

steam test was identifea as a

short-term corrective action to'better ensure sea". tightness at actual system conditions. It is evident from the change in seat leakage testing +c;quirements that the process of heating up a valve can cause thermal effects which result in se~~t leakage.

This type problem cannot be detected by a

nitrogen seat leakage test..

Installation of the nev design disc insert and a careful rework

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. pressurizer safety valves ensuring appropriate guiding clearances is expected to solve the problems with those valves which have had difficulty passing a

steam seat leakage test.

Xt should be noted that the research and development testing of the new disc insert design vhich vas performed by TVA and Crosby was performed on two of these problem valves with satisfactory results.

Another point worth mentioning

~s that the pressure at which seat leakage testing is performed for valves with the nev disc insert design has been increased from 90 to 95 percent of nameplate set pressure.

This more stringent seat leakage test futher ensures seat tightness when installed on the plant by demonstrating that the valve is capable of zero leakage at 5

percent above normal operating pressure.

Further modification of the tailpipes for unit 1 to reduce flange loading is not considered cost effective at this time.

This modification had been performed for unit 2 vith inconclusive results.

Flange loadings based on the current piping analysis are below vendor recommended maximums for both units.

5 4

INDEPENDENT REVIEW OF VALVE DESIGN An independent reviev of the Crosby.pressurizer safety valve design for Sequoyah was soli'cited from Siemens Nuclear Power Services in order to factor European valve design and testing experience into solution of the leakage problem.

Xf leakage continues after the disc insert modification, Siemens has provided possibilities for further modification of the existing valve or replacement vith a qualified valve of a different design that may be more suitable for the application.

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SEQUOYAH NUCLEAR PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN 6-0 ACTIONS TAKE&

TO ADDRESS CORXROL ROOM ANNUNCIATION Leakage of pressurizer safety valves can cause annur.ciation in the main control room of the tailpipe temperature monitoring

- system.

This last occurred for unit 1 on 9/17/91 and for unit 2

on 5/31/91.

In order to clear these annunciators, the pressurizer relief tank (PRT) temperature high alarm setpoint was increased to 155 degrees F on 7/28/91 (unit 1) and 8/7/91 (unit 2) in accordance with temporary alteration control forms 1-91-35-068 and 2-91-36-068, and their associated 10 CFR 50.59 evaluations (including eraluation by westinghouse).

To meet this temperature

limit, the unit 1

PRT was cooled approximately every seven hours.

Separate annunciation for PRT temperature, level and pressure would occur and subsequent operator actions are initiated in accordance vith the annunciator response instructions.

Additionally, a pressurizer back-up heater bank would energize more often to compensate for the increased energy loss.

As a result of the present condition, operators have increased attention to tailpipe temperature indicators.

Permanently installed acoustic monitors ir.. De pressurizer safety valve tailpipes provide annunciation in the control room and light illumination on 0-N-27 in front of th ~ Shift Operating Supervisor desk if a valve opens.

A recorder has been installed to monitor selected pressurizer safety valve activity.

7.0 SUYQvtARY Ia

summary, key actions which have already been completed include:

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Fine tuning and/or modification of tailpipe supports was implemented for both units to minimize safety valve flange loading.

o Administrative controls vere implemented to ensure a

consistent temperature in the pressurizer enclosure."

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An action plan was created to try to correct leakage as soon as it was identified.

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The pressurizer enclosure HVAC duct was modified to provide additional cooling at the "A" valve position.

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Post maintenance testing was enhanced to add a steam seat leakage test following the existing nitrogen seat leakage test.

SEgUOYAH NUCLEAR PLANT RESOLUTION OF PRESSURIZER SAFETY VALVE SEAT LEAKAGE CONCERN

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The pressurizer heat-up rate was administratively limited to 50 degrees,F per hour.

Remaining outstanding action:

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A modified valve which contains a new disc ins rt design will be installed during the unit 1,

cycle 5

outage.

Evaluation of the results of this modification willdictate whether additional valves are modified.

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The feasibility of increasing the safety valve set pressure will be reviewed if.the disc insert modification does not resolve seat leakage concerns.

8.0 CONCLUSION

TVA is continuing to take

" an aggressive approach toward resolving the seat leakage concern with the pressurizer safety valves.

This is evidenced by the level of commitment and funding which has already been dedicated to this effort.

Leadership in solving this industry-wide problem is evident by the partnership formed with Crosby, and the affiliation with Siemens Nuclear Power Service to factor in European valve design and testing experience.

Discussions and meetings with other utilities who are experiencing this problem also are continuing.

Current plans are to further eva'uate the performance of the new disc insert design in the "A" position on unit l.

Upon satisfactory'erformance in the test application plans are t'o implement the modification for the remaining valves, including spares.

Contingency plans are in place to further address the issue in the event that the most recent modification is not successful.

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