05000352/LER-2011-003
| Limerick Generating Station, Unit 1 | |
| Event date: | 06-22-2011 |
|---|---|
| Report date: | 08-22-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3522011003R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event. The Unit 1 high pressure coolant injection (HPCI) system (EIIS:BJ) was available and post maintenance testing (PMT) was in progress to restore the system to operable status following planned maintenance activities.
Description of the Event
On Wednesday, June 22, 2011, Limerick Unit 1 was operating at 100% power. The PMT for a planned system outage was in progress and required performance of the two-year HPCI Pump Comprehensive Test (ST-6-055-231-1). At the completion of this test the HPCI control valve (FV-056-111) (EIIS:V) failed to close as expected when the auxiliary oil pump was secured. A subsequent inspection of the control valve assembly identified damage to the No.1 control valve sub-assembly that was preventing full travel of the control valve lifting beam.
Unit 1 HPCI was previously declared inoperable and unavailable on Sunday, June 19, 2011, at 2300 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.7515e-4 months <br />, for a planned system outage window. The HPCI system was restored to available status on Tuesday, June 21, 2011, at 1614 hours0.0187 days <br />0.448 hours <br />0.00267 weeks <br />6.14127e-4 months <br />. The HPCI system was secured following the PMT on Wednesday, June 22, 2011, at 0345 hours0.00399 days <br />0.0958 hours <br />5.704365e-4 weeks <br />1.312725e-4 months <br />.
Shortly following turbine (EIIS:TRB) shutdown, failure of the control valve to fully close was observed when the auxiliary oil pump was secured. The HPCI system was considered inoperable, but available until a clearance was applied to support troubleshooting and inspection of the control valve assembly and hydraulic system.
The HPCI turbine control valve was repaired and the HPCI system was restored to available status on Monday, June 27, 2011 at 0449 hours0.0052 days <br />0.125 hours <br />7.423942e-4 weeks <br />1.708445e-4 months <br />.
The HPCI System was restored to operable status on Monday, June 27, 2011 at 2124 hours0.0246 days <br />0.59 hours <br />0.00351 weeks <br />8.08182e-4 months <br /> following completion of the PMT.
An 8-hour NRC ENS notification was required by 10CFR50.72(b)(3)(v)(D) for a condition that could have prevented fulfillment of the HPCI safety function. The ENS notification (#46979) was completed on Friday, June 23, 2011, at 17:39 ET. This event involved a condition that could have prevented fulfillment of the HPCI safety function. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(v)(D).
V RC FORM 366A (10-2010)
Analysis of the Event
There was no actual safety consequence associated with this event.
The potential safety consequences of this event were minimal. There was no event that required HPCI operation during the affected period. In addition, the RCIC System remained operable during the period when HPCI was unavailable for control valve repair.
Disassembly and inspection of the control valve assembly identified the following damage to the No. 1 control valve:
1) Pilot venturi valve stem cap was cracked, but intact.
2) Anti-rotation pin provided to prevent rotation of the pilot venturi valve stem relative to the lifting beam was broken or worn to the degree that engagement to the pilot venturi valve stem slot was lost.
3) Anti-rotation pin provided to prevent rotation of the pilot venturi valve relative to the venturi valve guide was in place and functional, but excessive clearances were evident allowing rotation between the parts over a limited arc.
4) Restricted movement (binding) of the pilot venturi valve relative to the venturi valve guide was evident at certain angular orientations.
An extent of condition inspection on Unit 2 HPCI system was performed to inspect the No. 1 control valve assembly. This inspection identified the following damage to the valve:
1) Anti-rotation pin that is installed in the lifting beam to prevent valve stem rotation for the purpose of minimizing wear between the parts was found to be broken or worn to the degree that engagement with the valve stem slot was lost.
2) The remaining parts were found to exhibit minor evidence of wear, but were fully functional.
Cause of the Event
Failure of the HPCI turbine control valve to fully close was caused by binding between the pilot venturi valve and the venturi valve guide that did not permit the valve assembly to float freely and self align at certain angular orientations. Failure of the lifting beam to venturi valve stem anti-rotation pin initiated accelerated wear of the valve assembly parts, which led to intermittent binding of the parts during turbine operation and ultimately fatigue failure of the pilot venturi valve stem.
V RC FORM 366A (10-2010) Corrective Action Completed The Unit 1 HPCI turbine control valve No. 1 valve sub-assembly was reworked to replace all damaged parts.
The Unit 2 HPCI control valve was inspected and identified as HPCI system was restored to operable status.
Corrective Action Planned The procedure for HPCI turbine inspection (M-C-756-001) will be revised to direct inspection of the No. 1 valve sub-assembly during each HPCI turbine minor inspection.
The procedure for HPCI turbine inspection (M-C-756-001) will be revised to require disassembly of the HPCI turbine control valve assembly for detailed inspection of all valve sub-assemblies including non-destructive examination (NDE) of the valve stem to cap transition areas every major turbine inspection.
The results of the completed failure analysis will be reviewed and any additional corrective actions identified.
Previous Similar Occurrences There were no previous similar occurrences regarding HPCI control valve failure in the prior three years.
Component data:
System:
Component:
Manufacturer:
Model:
High Pressure Coolant Injection FV-056-111 HPCI Turbine Control Valve T129 Terry Turbine Turbine Type CCS V RC FORM 366A (10-2010)