ML20050C291

Updated LER 79-034/03L-1:on 790918,motor Operated Isolation Valve AFW-V2-16A in Auxiliary Feedwater Line to Steam Generator a Failed to Open During Monthly Periodic Test. Caused by Power Supply Breaker Trip on Previous Closure

ML20050C291
Person / Time
Site:	Robinson
Issue date:	03/25/1982
From:	Cox H CAROLINA POWER & LIGHT CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML20050C286	List: ML20050C285 ML20050C291
References
LER-79-034-03L, LER-79-34-3L, NUDOCS 8204080378
Download: ML20050C291 (3)

Text

R S. NUCtEAR REMATORMOMMISSION

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g 0"" 38" Update Report - Previous Report Date Was 9/18/79

, i.lCENSEE EVENT REPORT CONTROL ELOCX: l 1

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(PLEASE PRINT OR TYPE ALL REQUl%ED INFORMATION) l@l67 CAT b8l@

0 1 l 7 8 [$!ClHlBlRl2l@150 9 LICENSEE CODE 14 l0 l- l0 l0

, LeCf l0 l0 l0 l- l0 l0 l@4 NSE NUMBER 2') 26 l1 LICENSE l1 l1TYPE l1 JO CON'T O t I 8 7n"c' Ql 60 0 l 695 l 0 l 0DOCK l 0ETlNUMBER 2 l 6 l 1 @l to 069l 9 11 l8 l 7 l 9 l@l 075 l 3 REPORT EVENT DATE 14 l 2 l 5DATE l 8 l 280l@

EVENT DESCRIPTION AND PROBABLE CONSEQUENCES h o 2 Mhile performing the monthly pericole test of the AFW System, the motor operated iso- l O 3 llation valve in the AFW line to "A" S/G, AFW-V2-16A, failed to open. The valve was l o 4 l declared inoperable at 1900 hours0.022 days <br />0.528 hours <br />0.00314 weeks <br />7.2295e-4 months <br />. This resulted in operation in a degraded mode l 10151 IPermitted by T.S. 3.4.3 and constitutes a reportable occurrence per T.S. 6.9.2.b.2. l O s [Throughout the event botn main feed pumps and three auxiliary feedwater pumps were l IO ITl lavailable. Also, V2-16A was manually operable if required. Therefore, there was no l O R l threat to either plant or public safety. (LER 79-32) (LER 79-33). l 7 8 9 80 SYSTEM CAUSE CAUSE COMP. VALVE CODE CODE SUBCODE COMPONENT CODE SU8 CODE SUBCODE 7

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33 34 35 2 40 48 42 43 44 47 CAUSE DESCRIPTION AND CORRECTIVE ACTIONS h i o lThe operator on V2-16A failed to open due to the operator power supply breaker having g i i l tripped on the previous closure. The breaker was reset, and the valve declared g i 2 loperable at 2d32 bours after Iuriins been stroked several times with no problems. The  ;

i a iroot canae 6t the fai3ure was subsequebtly determined to be a worn gear in the valve g

M 'loperator, The valve has bsen replaced,'and no further corrective action is necessary. ;

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7 8 9 Bo STA  % POWER' OTHERSTATUS i o Y DISCOVERY DESCRIPTION t 5 W h (0 l9 l6 l h l N/A l W @l Operator Observation l ACTIVITY C TENT net.E AsEn OF RELE ASE AMOUNT OF ACTIVITY LOCATION OF RELEASE 7

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'e SUPPLEMENTAL INFORMATION FOR

~ LICENSEE EVENT REPORT 79-34R1

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1. Cause Description and Analysis e, a The valve failures occurred when the motor driven auxiliary feedwater pumpa were started and AFW-V2-16A failed to open. This failure occurred initially when the AFW pumps were being used to maintain steam generator level with the reactor at zero power on 9/5/79 and

, , again during the performance of the monthly periodic test of the AFW

-f c System on 9/6/79 and 9/18/79.

Immediate investigation in each case revealed that the thermal over-current relay in the power supply breaker to the valve operator had been' tripped. This overcurrent relay provides for motor protection.

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, , Additional investigations, performed subsequent to the 9/18/79

' failure, traced the overcurrent_ trip to a torque switch designed to de-energize the operator after the valve has closed. The torque switch is' contacted by'a worm which operated against a spring l mechanism when valve stem movement is restricted. Specifically, motor torque is converted to worm translation which is resisted by the spring. When valve stem travel stops, sufficient torque results in~enough worm translation to contact the torque switch at a given setpoint. In this case, it was determined that the torque' switch was not being contacted on every valve closure. Since the valve was closed but power was not de-energized from the motor, the motor operated in a locked rotor condition until the thermal overcurrent trip operated. Subsequent investigation of the operator internals revealed a badly worn worm gear. The excessive wear of the worm gear is believed to have prevented the transmission of sufficient torque through the worm to cause the. torque switch to be tripped during. valve closure. The excessive worm gear wear is believed to have occurred from overstressing of the gear on previous occasions when the valve stuck closed from thermal binding as a result of system backleakage.

.The main feed pumps and both motor driven auxiliary feedwater pumps were available to supply steam generator feedwater throughout the 9/5/79 and 9/6/79 occurrences. Throughout the 9/18/79 occurrence, the' steam driven auxiliary feedwater pump was also available.

Throughout each event, the AFW-V2-16A was manually operable if required. There was, therefore, no threat to either plant or public safety during.any of these occurrences.

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2. Corrective Action Following the 9/5/79 and 9/6/79 failures, the thermal overcurrent relay in the power supply breaker was reset, and the valve was stroked several times. The failure would not repeat, and the valve was declared operable. Following the 9/18/79 failure, investigations revealed that the torque switch was n it being contacted. The thermal overcurrent relay was reset, and the torque switch setting was re-duced as temporary corrective action. The proper operation of the valve with the reduced torque switch setting was verified by stroking.

Following the inveatigation of the valve operator internals, which revealed the excessive worm gear wear, the worn worm gear was re-plac.ed and the operator was returned to service in satisfactory condition.

3. Corrective Action To Prevent Further Oct.urrence In order to eliminate system backleakage and thermal binding problems in the Auxiliary Feedwater System motor operated valves, the three motor operated valves in the motor driven train (V2-16A, B, and C) and the three motor operated valves in the steam driven train (V2-14A, B, and C) have been replaced with double disk gate valves. The original valves were solid wedge gate valves which were far more susceptible to this problem. These valves were replaced during the 1980 refueling outage, and no further problems have been experienced with these valves to date. No further corrective action is deemed necessary.

This revision to LER 79-34 also supplements and closes LER 79-32 and 79-33.

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