ML20141K774

Part 21 Rept Re Auxiliary Feedwater Check Valve Failure. Concluded Thermal Gradient Conditions Created by Flowing Cold Water Through Hot Valve Created Rapid Cooldown of Seat Ring,Allowing Ring to Displace

ML20141K774
Person / Time
Site:	Beaver Valley
Issue date:	04/24/1997
From:	DUQUESNE LIGHT CO.
To:	NRC
References
REF-PT21-97-331 NUDOCS 9705300003
Download: ML20141K774 (2)

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P21 4 7-33-l Attachment to NRC Form 361. Event Notification Worksheet

& Initlal Notification 10CFR Part 21 - BVPS Unit 2 Auxiliary Feedwater Check Valve Falture During the March 19,1997 Beaver Valley Power Station (BVPS) Unit 2 trip (previously d~---ted in -

Licensee Event Report 197 005 00, dated April 14,1997), Auxiliary Feedwater (AFW) anomalies were observed. The AFW flow through the "B" steam generater was lower (150 vs. 280 GPM) than expected.

Flow through the "A" and "C" steam generators was as expected. The performance of all three AFW l Pumps was normal for the trip conditions. Subsequent inspection of the "B" steam generator check valve (2FWE-100) revealed that the seat ring had partially moved into the flow stream, decreasing the available opening for flow to pass through the valve. The three Unit 2 AFW check valves were shipped to the vendor's facility for further examination and analysis. The resulting investigation concluded that the thermal gradient conditions created by flowing cold water through the hot valve created a rapid cooldown l of the seat ring, allowing it to displace. All three of the subject check valves were modt5ed to prevent reoccurrence. The valves were then shipped back to the site and remstalled.

j The check valves are normally held shut by steam generator pressure. Failed check valve 2FWE 100, is

Touted in close proximity to the main feedwater header, and is at approximately 430 degrees F. The other

! two check valves are below 300 degrees F. The differences in the temperatures are attributed to the distance and location of the valves p respect to the mam feedwater header. Dunng a reactor trip, AFW l at approximately 60 degrees F is ir$scted. It is estimated that it takes approximately 5 seconds for the seat to cool down, whereas the massive valve body stays relatively hot. It appears that the valve seat loosened

! because of cold water passing through the valve. The massive valve retained its shape, whereas the seat shrunk. This reta:ive shrinkage allowed the seat to displace and move into the flow stream.

An extent of condition evaluation has shown that other Enenoch nozzle check valves of this design in l service at Unit 2 are not subject to thermal gradients of s; f5cient magrutade to induce the condition

! observed for 2FWE 100. Unit I does not have Enertech nozzle check valves.

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! A similar failure of AFW check valve 2FWE 100 would have resulted in a reduction of AFW flow to the l "B" steam generator during a postulated design basis accident. The reduction in flow caused by the defect l would have resulted in AFW Gows less than analyred for the Unit 2 Accident Analysis. Therefore, for the l postulated accidents, the ability to provide adequate AFW cooling would be adversely affected and the system may not.have performed its safety function.

An evaluation of this event, completed on April 24,1997, has determined that a substantial safety hazard could be created as the result of the identafled valve defect and that it is, therefore, reponable pursuant to the requirements of 10CFR Part 21.

l Component

Description:

The component is a nozzle check valve intended for use with water service.

l Supplier.

Enertech

( (BW/IP) l 2930 Birch Street Brea, CA 92621 l

i Enertech "4" Nozzle Check Valve, ANSI Class 600, Type DRV-Z j n Valve Body - Dwg. # PD96227, ASME SA105 -

l Seat - Dwg. # PB96233 ASTM A479 Type 316

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