ML20024H647

Special Rept:On 910227,svc Water MOVs Associated W/ Recirculation Spray Heat Exchangers Exceeded Torgue Limits & Stopped in Mid Position While Being Opened During Test.Event Reported on Voluntary Basis to Provide Info to NRC

ML20024H647
Person / Time
Site:	North Anna
Issue date:	05/31/1991
From:	Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
91-229, NUDOCS 9106070242
Download: ML20024H647 (5)

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Viitoixl41:1.1 T 1111C A Fil I'OW 1:14 C mt l'.W Y Il lt'li blO N il, Yl lt O IN I A UllU Gl May 31, 1991 U.S. Nuclear Regulatory Commission Serial No.91-229 Attention: Document Control Desk NAPS /WCHiTAH:R7a Washington, D.C. 20555 Docket No. 50-338 License No. NPF4 Gentlemen:

VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNIT 1 REVERSE INSTALLATION OF SAFETY RELATED MOTOR OPERATED OFFSET DISC BUTTERFLY VALVES On February 27,1991, three service water motor operated valves (MOVs) associated with the recirculation spray heat exchangers (RSHXs), exceeded their torque limits an'd .

stopped (torqued out) in mid position while being opened during a special test. The MOVs had been installed in a reversed orientation which resulted in et abnormally high torque required to open them under the high Service Water flow conditions present during the special test.

No significant safety consequences resulted from this event because if a problem had occurred during a Design Basis Accident, the reversed valves and the correct valves on the affected RSHXs could have been reclosed. With the high flow, high differential pressure condition removed, the reversed valves could then be re-opened first without torquing out. The correctly installed valves could then be opened. Researching the event determined that offset disc butterfly valves will seat closed even if installed in a reversed orientation. Therefore, the health and safety of the public nad not been adversely affected.

This Special Report has been reviewed by the Station Nuclear Safety and Operating Committee and will be provided to the Management Safety Review Committee. The event is being reported on a voluntary basis to provide operating experience information to the NRC and nuclear industry. Details of the event and the evaluation are attached.

Should you have any questions regarding this report, please contact us.

Very truly yoti s,

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): hy W. L. Stef/6r(t Senior Vrce-President - Nuclear Attachment lj i

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Docket No. 50-338 Sorial No.91-229 Page 2 of 2 pc: U.S. Nuclear Regulatory Commission Reg!on 11 101 Marietta Street, N.W.

Suite 2900 Atlanta, Georgia 30323 Mr. M. S. Lesser NRC Senior Resident inspector North Anna Powcr Station

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1[QRTil AHMA POWER _ STATION UNIT 1 REVERSE _1HSIALLAI1Q1LQE._ SAFETY 13ELAIEQ MQlOR OPERATED OFFSET DISC DUTTERFLY VALVES On February 27,1991, throo servico water motor operated valves (MOVs) associated with the racirculation spray heat exchangers (RSHXs), exceeded their torque limits and stopped (torquod out) in mid position while being opened during a special test.

This event is being reported on a voluntary basis to provido operating experience information to the NRC and nuclear industry.

The special test was performed to provide MOV performanco information in response to NRC Gonoric Letter 8910 which requires flow and pressure tests under design basis conditions. The testing of the RSHX isolation valvos assumes inadvertent operator action. The worst caso inadvertent action fur the SW RSHX isolation valves occurs when an operator mistakenly shuts and then attempts to re open the valves during the recovery phase of the design basis event. Thorofore, the valvos (Allis-Chalmers 16" 150 lb. MOVs Model Number YV.01887 R) woro subjected to flow rates approximately 50% higher than would bo expected during an actual containment depressurization actuation ovent (CDA).

During the ovaluation of the torque out problem, the ydvo Sol 0Ill0AliandbD0h, by R. W. Zappe, was reviewed to gain more information about oliset disc butterfly valves.

This review revealed that, for offset disc buttorily valves, the required opening torque is dependent on whether the flow is toward the shaft or toward the disc (soo Figure 1, below). If flow is toward the disc, the initial torquo when opening the valvo is the Elnute_.1 e s A\\\\\ % NN\\;s %  %

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Atachmont t'ago 1 of 3

Docket Ma. 50 338 SenatNo 91 ??9 1

maximam the valve experiences. However if flow is toward the shaft, the initial torque value is about the same but it increases as the valve opens reaching a maximum value near mid position (see Figure 2). The Allis Chalmers " installation, operation, and maintenance manual" for "Streamsoal butterfly valves" states that *150 psi rated valves may be normally installed with the flow in either direction." The oifferential i pressures, measured during the special test were approximately 75 psid, i l

EJgure 2 l ly.DICRLonenino_lotgues_loLQIlset Disc Buttedy_Yalvss  !

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4k B curve a: off se.' disc, ibw towards shaft Curve 6: off set disc, Ibw tonards disc OPENING i

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y closed open it was not readily apparent that the valves had been installed backwards because valve orientation can only be determined by the slight offset of the valve stem. An examination of the RSHX isolation MOVs indicated that three valves had been installed with the flow towards the shaft which caused a significantly increasing torque as the valve opened. Coupled with the abnormally high differential pressure resulting from the required test conditions, this high torque caused the torque limit switches to stop the valve movement prior to the MOVs reaching full open position. Research determined that offset disc butterfly valves will seat closed even it installed in a reversed orientation. However in this application, there is a potential for offset disc butterfly v1lve actuators to experience an overthrust condition in the open direction which could damage the actuator if it is not designed for, or protected from this torque.

l When performing calculations to predict torques required to operate butterfly valves (AWWA C504), care should be taken to ensure the correct coefficients of dynamic torque are used. The relationship of the coefficient of dynamic torque and disc angle varies depending upon the shape of the disc and how it is mounted on the shaft.

Characteristic curves showing this relationship can be obtained from the valve manufacturer. If coefficients for symmetrical valves are used in calculations for offset disc butterfly valves, the result will be a non conservative opening torque prediction Attachment Page 2 of 3

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Dcdot Mo. 50 338 senal Mo. 91 220 when flow is toward the shaft side of the disc. A review of the North Anna RSHX isolation MOV calculations showed that the calculations did not account for the specific disc typo installed in this application.

On February 28 and March 1,1991, the three MOVs woro removed and proporly reinstalled. Post maintenance testing and SW flow balancing to the RSHXs was performed and the service water isolation valvos for the RSHXs performed as expected. In addition, a partial repetition of the special test was performed to determino prcssure differentials for the MOVs which had boon rotated 180 degrees.

During the test, no mid cycle high torque valvo problems occurred. The RSHX service water isolation valvos on unit 2 woro inspected and found to bo installed correctly.

No significant safety consequences resulted from this event because a CDA functional test performed at the beginning of the refueling outago demonstrated that the SW valves had boon operational prior to the outage and would havo opened to provide flow to the RSHXs under actual CDA conditions, in addition, the SW system flow had boon balanced during the refueling outage, prior to the special tost, which confirmed correct MOV operation through the full range of movement. In actual Design Basis Accident (DBA) conditions, the RSHX Inlet and outlet valves will open simultaneously and would not be subjected to high difforontial pressures that caused the problems during the special test.

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