ML20153D194
| ML20153D194 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 08/25/1988 |
| From: | Heitner K Office of Nuclear Reactor Regulation |
| To: | Robert Williams PUBLIC SERVICE CO. OF COLORADO |
| References | |
| NUDOCS 8809020092 | |
| Download: ML20153D194 (4) | |
Text
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UNITED STATES y~
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NUCLEAR REGULATORY COMMISSION
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W ASHING TON, D. C. 20655 August 25. 1988 Docket No. 50-267 Mr. k. O. Williams, Jr.
Vice President. Nuclear Operations Public Service Company of Colorado Post Office Box 840 Denver Colorado 80201-0840
Dear Mr. W1111ars:
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 1 Water Hamer Event of November 21, 1985." dated April, 1966.
Check valves are used in rany systems irportant to plant safety and reliability.
Failure of check valves can lead to overpressurization of low-pressure systers, water banner, stear 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 against a stop. With each minor flow pulsation, the disc tnen 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 perfonned 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 hamiering 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 been operating at reduced power and the reduced feedwater flow, coupled with a high level of turbulence due to increased throttling of the flow control valve.
resulted in an amplified oscillation and accelerated degradation. Eventually it led to failures of the check valves.
Based en the above discussion, it is reasonable to conclude that certain check valves are very vulnerable to prolonged reduced flow operation during reduced pcwer operation. This concern may be especially relevant to Fort St. Vrain, t(01 8809020092 880825 I
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Mr. R. O. Williams, Jr.
-P-because of the coisiderable fraction of the tire the plant spends at reduced power. Therefors, I am edvising you of this concern and requesting that you review your plant configuration for possible irrpact of prolonged low power operation on the reliability of chec( valves.
This letter requires no formal response.
Sincerely, Ver.neth L. Heitner, Project Manager Project Directorate - IV I
Division of Reactor Projects - !!!,
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Mr. R. O. Williams, Jr. because of the considerable fraction of the tine the plant spends at redued power. Therefore, I am advising you of this concern and requesting that you review your plant configuration for possible impact of prolonged low power operation on the reliab'11ty of check valves.
This letter requires no formal response.
Sincerely, Kenneth L. Heitner, Project Manager Project Directorate - IV Division of Reactor Projects - !!!,
IV, Y and Special Projects cc: See rot page 4
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Mr. R. O. Williams, Jr.
Public Service Lepany of Colorado Fort St. Vrain l
cc:
Mr. D. W. Warembourg, Manager Albert J. Hazle, Director Nuclear Engineering u..ision Radiation Control Division Peblic Service Company Department of Heaith of Colorado 4210 East lith Avenue
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O. Box 840 Denver, Colorado 80220 Der.ver, Colorado 80201-0840 Mr. Onid Alberstein,14/159A Mr. R. O. Williams, Jr., Acting Manager GA Technologies, Inc.
Nuclear Production Division Post Office Box 85608 Public Service Company of Colorado San Diego, California 92138 16805 Weld County Road 19-1/2 Platteville, Colorado 80651 Mr. H. L. Brey, Manager Nuclear Licensing and Fuel Division Mr. P. F. Tomlinson, Manager Public Service Company of Colorado Quality Assurance Division P. O. Box 840 Public Service Company of Colorado Denver, Colora.do 80201-0840 16805 Weld County Road 19-1/2 Platteville, Colorado 80651 Senior Resident Inspector U.S. ^.dclear Regulatory Comission Mr. R. F. Walker P. O. Box 640 Public Service Company of Colorado Platteville, Colorado 80651 Post Office Box B40 Denver, Colorado 80201-0840 Xelley, Stansfield & 0'Donnell Public Service Company Building Comitment Control Program Room 900 Coordinator 550 15th Street Public Servica Company of Colorado Denver, Colorado 80202 2420 W. 26th Ave. Suite 100-0 Denver, Colorado 80211 Regional Administrator, Region IV U.S. Nuclear Regulatory Comission 611 Ryan Plaza Drive Suite 1000 Arlington, Texas 76011 Chairman, Board of County Comissioners of Weld County, Colorado Greeley, Colorado 80631 Regional Representative Radiation Programs Environmental Protection Agency 1 Denver Place 999 18th Street, Suite 1300 Denver, Colorado 80202-2413
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