05000341/LER-1997-002, :on 970216,HPCI Sys Pump Discharge Valve Failed to Open.Caused by Failure of Pinion Gear Setscrew.Maint Procedure Controlling Work on Valve Mos Revised,Motor Shaft for Valve E4150-F006,spot Drilled & New Setscrew Installed
| ML20137C075 | |
| Person / Time | |
|---|---|
| Site: | Fermi  |
| Issue date: | 03/18/1997 |
| From: | Wittschen R DETROIT EDISON CO. |
| To: | |
| Shared Package | |
| ML20137C064 | List: |
| References | |
| LER-97-002, LER-97-2, NUDOCS 9703240148 | |
| Download: ML20137C075 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3411997002R00 - NRC Website | |
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LICENSEE EVENT REPORT (LER) i F AC,luTv haME Op IKX:kf T MMitE R (2)
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On February 16,1997, while the plant was in a forced outage, the High Pressure Coolant Injection (HPCI) system pump discharge valve (E4150-F006) failed to open when the open push-button was depressed as part of a routine surveillance test. The plant was shutdown at the time and HPCI was not required to be operable. An investigation of the failure was initiated.
The Limitorque valve motor was inspected. The pinion gear on the motor shaft was found to be loose. It had slipped axially along the shaft causing it to disengage from the drive gear. The setscrew was not holding the pinion gear to the shaft although the setscrew was properly secured with lockwire. The cause of the event was determined to be a failure of the pinion gear setscrew.
The motor shaft was spot drilled to provide a seat for the setscrew and a new setscrew was installed in the E4150-F006 valve. The work history for all other Limitorque motor operated valves (MOVs) was reviewed. Other Limitorque MOVs, potentially susceptible to this type failure were identified and a plan for inspection and replacement of the setscrews has been developed.
9703240148 970318 PDR ADOCK 05000341 PDR S
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Initial Plant Condition:
Operational Condition:
4 (Cold Shutdown)
Reactor Power:
0 Percent Reactor Pressure:
O psig Reactor Temperature:
127 degrees Fahrenheit Descriotion of the Event:
l On Febmary 16,1997, while the plant was in a forced outage, the High Pressure Coolant Injection (HPCI)[BJ] system pump discharge valve [BJ)[20] (E4150-F006) failed to open when demanded by a manual signal while performing shutdown surveillance 24.202.05, "HPCI System Cold Shutdown Valve Operability Test." Inspection of the valve found the motor pinion gear displaced from its normal position on the motor shan such that it no longer engaged the worm shaft helical gear.. The pinion gear setscrew was positively secured by the installed lockwire, but the setscrew tip appeared worn. The motor shan to pinion gear key was properly staked in place. No damage or unusual wear was obsened
. on the key or gear teeth. No other unusual conditions were obsened. This valve was last successfully 1
1 operated on November 15,1996.
This event was reported in accordance with 10CFR50.72(b)(2)(iii)(D) on Febmary 16,1997, at 1657 hours0.0192 days <br />0.46 hours <br />0.00274 weeks <br />6.304885e-4 months <br />.
Cause of the Event
During operation of Limitorque motor operated valves (MOVs) the motor torque is transferred from the motor shan to the worm shaft via a helical gearset. The gear teeth are on an angle such that when running, the powered pinion gear imparts an axial thrust to the worm shan gear. The setscrew is designed to absorb this axial thrust and prevent axial movement of the pinion gear along the motor shaft. The shan key prevents rotational movement between the motor shan and the pinion gear.
Examination of the motor shan indicated that the setscrew had originally been firmly embedded into the shaft, but had worn over time such that it was unable to resist the axial forces created when the MOV was operated.
The cause of the event was failure of the setscrew. The vendor, Limitorque, was aware of similar failures and in 1989 issued a Maintenance Letter addressing the motor pinion gear installation. This letter includes a recommendation that the motor shan be spot drilled to ensure an adequate set of the setscrew. The spot drilling for pinion gear setscrews is being systematically implemented at Fermi 2, but work on valve E4150-F006 had not yet been performed; this is considered to be a contributing factor to the failure.
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i LICENSEE EVENT REPORT (LER) TEXT CONTINUATION P ACLITY DeAME til DOCKET # amber (2; LER DamMA #&
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l"l 4 0 0 2 TEXT (17; A historical search for pinion gear setscrew failures identified two other instances. In one case the setscrew had backed out, and in the other, the setscrew was crossthreaded.
The original motor on valve E4150-F006 had been replaced in 1988 with a larger size motor. During that modification the pinion gear was removed from the old motor and reinstalled on the new motor. A review of maintenance records indicated that the shaft key and setscrew have not been rewerked since that time.
Evaluation of the setscrew determined that it was appropriate for the application. The setscrew was well secured in place by the lockwire. Based on the depth of the indentation, it was determined that the setscrew was properly embedded on the motor shan. A considerable amount ofwear damage on the setscrew tip was observed and the socket head was deformed. It was concluded that much of the damage occurred when the setscrew was originally tightened.
l The failure mechanism was determined to be that the setscrew tip had deformed to the extent that it could no longer provide sutTicient restraining force to overcome the axial forces acting on the pinion gear when the valve was operated. It was determined that the setscrew was overtorqued during the initial installation which resulted in the significant deformation to the setscrew tip. This reduced the effectiveness of the setscrew. Valve operation and vibrations can, over time, degrade or wear the contact between the setscrew and shan. Differential movement between the two caused the setscrew to wear.
Based on this information, the root cause of the event is considered to be that the installed setscrew was overtorqued which caused excessive deformation and reduced its ability to lock the pinion gear to the motor shaft.
Analysis of the Event
The HPCI subsystem of the Emergency Core Cooling System (ECCS) is designed to mitigate the effec:s of a small break Loss of Coolant Accident (LOCA) while the reactor is still at pressure. The temperature and pressure transients that result from a small break LOCA are normally not of the severity necessary to cause fuel damage given the other ECCS systems are available to mitigate the event. If HPCI failed to operate, the remaining ECCS systems and the Reactor Core Isolation Cooling System (RCIC)[BN] would have been available to fulfill the safety function. Therefore, the health and safety of the public were not adversely affected by this event.
8 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION
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Corrective Actions
The maintenance procedure controlling work on valve motor operators was revised to enhance guidance with respect to proper setscrew installation. For valve E4150-F006, the motor shaft was spot drilled, a new setscrew was installed and secured in place in the pinion gear.
Other Limitorque MOVs were reviewed for susceptibility to this failure mechanism. It wasjudged that motor pinion gear installations would be inspected and reworked as necessary. Limitorque guidance for installation of the pinion gear setscrew was reviewed and factored into plant staff experience with Limitorque to create an enhanced inspection procedure. This includes spot drilling the motor shaft and positive securing of the setscrew. An MOV prioritization was established based on information obtained from the Generic Letter 89-10 MOV program, current plant configuration, and insights from the Maintenance Rule and the Probabilistic Risk Assessment model. Based on this prioritization, safety related and balance of plant (BOP) MOVs which have been evaluated as being risk significant in the overall perspective will be inspected and repaired, as necessary, prior to plant restart. The remaining MOVs will be similarly evaluated for future inspection and repair.
AdstitionalInformation:
A.
Failed Components HPCI Pump Discharge Valve (E4150-F006)
Component:
HPCI Pump Discharge Valve (E4150-F006)
Description
Motor Operated Valve Manufacturer:
Limitorque Model Number:
SB-3 B.
Previous LERs on Similar Problems LER 93-002 addressed a failure of HPCI to start on a simulated automatic injection signal during performance of r, surveillance. As an initial condition for the surveillance, the HPCI pump i
discharge valve (E4150-F006) was deenergized and rendered inoperable. The root cause of the I
failure was determined to be loss of supply voltage to the governor control system, therefore, the failure mechanism was different than this LER.
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