ML20154A836

Discusses Impact of Prolonged Low Power Operation on Reliability of Check Valves.Accelerated Wear & Failure of Certain Check Valves Could Occur Due to Continued Undesirable Flow Operations

ML20154A836
Person / Time
Site:	Cooper
Issue date:	09/06/1988
From:	Oconnor P Office of Nuclear Reactor Regulation
To:	Trevors G NEBRASKA PUBLIC POWER DISTRICT
References
NUDOCS 8809130106
Download: ML20154A836 (4)

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September 6,1988 Docket No. 50-298 Mr. George A. Trevors, Division Manager - Nuclear Support Nebraska Public Power District Post Office Box 499 Columbus, Nebraska 68601

SUBJECT:

THE IMPACT OF PROLONGED LOW POWER OPERATION ON THE REllABILITY OF CHECK VALVES

Reference:

Letter from M. O. Medford to NRC, "Investigation Report, San Onofre Unit I Water Hamer Event of November 21, 1985," dated April, 1986.

Check valves are used in many systems important to plant safety and reliability.

Failure of check valves can lead to overpressurization of low-pressure systems, water hammer, steam binding, and extensive damage to other components at nuclear power plants.

Recent industry experience showed that accelerated wear and failure of certain check valves could occur due to continued undesirable flow operations. The primary cause of this premature degradation is continuous fluctuation of the check valve disc when it is not firmly held against the backstop by the fluid forces pushing on it. The position of the check valve disc can be determined by a balance of fluid lifting force and the gravitational force acting on the disc and hinge arm weights. Low flow velocities are not sufficient to lift the disc through its full stroke and hold it firmly in a stable position ag31nst a stop. With each minor flow pulsation, the disc then fluctuates causing premature wear of moving parts. This instability if coupled with close proximity upstream flow disturbance such as elbows, tees, control valves and pumps could cause rapid wear and in some cases even failure of the valves.

An analysis (Reference 1) was performed to identify the root causes of check valve failures during the November 21, 1985, water hamer event at San Onofre Nuclear Generation Station, Unit 1.

That analysis concluded that the failure of the five failed check valves was caused by repeated harrenering of the disc stud and the stud nut against the backstop due to insufficient disc lifting force of the low flow rate during reduced power operation. The plant had bean operating at reduced power and the reduced feedwater flow, coupled with a high level of turbu'ince due to increased throttling of the flow control valve, resulted in an amplified oscillation and accelerated degradation. Eventually it led to failures of the Sack valves.

Based on the above discussion, it is reasonable to conclude that certain check valves are vulnerable to damage caused by prolonged reduced flow operation.

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Mr. George A. Trevors,

Therefore, I an. advising you of this concern and requesting that you review your plant configuration for possible impact of prolonged low flow operation on the reliability of check valves.

This letter requires no fon.al response.

Sincerely,

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Paul W. O'Connor, Project Manager Project Directorate - IV Division of Reactor Projects - III, l

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Mr. George A. Trevors -

Therefore, I am advising you of this concern and requesting that you review your plant configuration for possible impact of prolonged low flow operation on the reliability of check valves.

This letter requires no formal response.

Sincerely,

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Paul W. O'Connor Project Manager Project Directorate - IV Division of Reactor Projects - III.

IV, V and Special Projects cc: See next page i

i Mr. George A. Trevors e

Nebraska Public Power District Cooper Nuclear Station cc:

Mr. G. D. Watsen, General Counsel Nebraska Public Power District P. O. Box 499 Columbus, Nebraska 68601 Cooper Nuclear Station ATTN: Mr. Guy R. Horn, Division Manager of Nuclear Operations P. O. Box 98 Brownville, Nebraska 68321 Dennis Grams, Director Nebraska Department of Environmental Control P. O. Box 98922 Lincoln, Nebraska 68509-8922 Mr. ' Larry Bch1 ken, Chairman Nemaha County Board of Conmissioners 2

Nemaha County Courthouse 1824 N Street Auburn, Nebraska 68305 Senior Resident Inspector U.S. Nuclear Regulatory Commission P. O. Box 218 Brownville, Nebraska 68321 Regional Administrator, Reaior, IV U.S. Nuclear Regulatory Coroission 611 Ryan Plaza Drive, Suite 1000 Arlington, Texas 76011 Mr. Harold Borchart, Director Division of Radiological Health i

Department of Health 301 Centennial Mall, South P. O. Box 95007 Lincoln, Nebraska 68509-5007 a

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