ML20059D033
| ML20059D033 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 08/21/1993 |
| From: | Rueger G PACIFIC GAS & ELECTRIC CO. |
| To: | |
| References | |
| OLA-2-I-MFP-132, NUDOCS 9401060334 | |
| Download: ML20059D033 (8) | |
Text
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.wc August 28, 1992
'93 0CT 28 PS :49 PG&E Letter No. DCL-92-189 U.S. Nuclear Regulatory Commission 2.'
ATTN:
Document Control Desk Washington, D.C.
20555 d'
Re: Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-82 Diablo Canycii Units 1 and e Licensee Event Report.-91-021-00 1
failure of Motor Pinion Keys in Limitorque SMB-3-80 Motor Operators due to inadequate Design of Material i
Gentlemen:
l PG&E is submitting the enclosed voluntary Licensee Event Report (LER) concerning the failure of motor pinion keys ;n Limitorque model SMB-3-80 motor operators. This LER is submitted for information purposes only as described in Item 19 of Supplement I to NUREG-1022.
This event has in no way affected the health and safety of the public.
Sincerely, l
( ~ G ~, * * *) lll 5 c) '
- (m Gregory M. Rueger cc: Ann P. Hodgdon John B. Martin Philip J. Morrill Harry Rood CPUC Diablo Distribution' INPO l
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FAILURE OF MOTOR PINION KEYS IN LIMITORQUE W18-3-80 MOTOR OPERATORS DUE TO ma i.,
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X k0 aesua:' ties This voluntary LER is submitted for information purposes only as described in item 19 of Supplement 1 to NUREG-1022.
On September 16, 1991, due to a miscommunication resulting from inadequate procedures, the motor operator for cold leg isolation valve SI-2-8809B electrically engaged while the valve was being manually stroked open during post-modification testing.
The motor operator electrically engaged and closed the valve ("short stroking").
Investigation determined that this electrical "short stroking" of the valve caused the motor pinion key to shear.
On October 3, 1991, other safety-related Unit 2 SMB-3-80 motor operators were inspected.
The motor operators for accumulator discharge isolation valves SI-2-8808B and SI-2-88080 were identified as having failed keys similar to the failed key identified in S1-2-8809B.
Further investigation revealed small cracks emanating from both corners of the keyway on the SI-2-8809B motor shaft.
The root cause of the sheared motor pinion key was that the key material was inadequate.
The key was supplied with the motor operator by the vendor.
The key material is considered outdated but still acceptable according to the vendor's design.
Corrective actions to prevent recurrence include:
(1) replacing the motor pinion keys with stronger material on safety-related SMB-3-80 motor operators; and (2) revising operating and maintenance procedures to prevent valve "short stroking."
1031S/85K
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 195228 ucw,.- m
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Plant Conditions Unit I was in Mode 1 (Power Operation) at 100 percent power.
1 Unit 2 was in Mode 5 (Cold Shutdown) at 0 percent power.
- 11. Descriotion of Event A.
Summary:
On September 16, 1991, during post-modification testing, a sheared motor pinion key was identified in the motor operator for cold leg isolation valve (BP)(20) SI-2-8809B.
Subsequent inspections identified similar failed keys in the motor operators for accumulator (BP)(ACC) discharge isolation valves (BP)(20) SI-2-8808B and 51-2-88080, and also small cracks emanating from both corners of the keyway on the SI-2-8809B motor shaft.
B.
Background:
51-2-8809B is the residual heat removal (RHR) heat exchanger (BP)(HX) outlet valve to reactor coolant system (RCS) (AB) cold legs 3 and 4.
The valve was not required to be in service at the time the key sheared.
The operator for 51-2-88098 is a Limitorque SMB-3-80, high speed (3380 RPM), 80 ft-lb start torque /16 ft-lb running torque (80/16) operator. The motor pinion key connects the motor drive shaft to the pinion drive gear.
The pinion gear drives the valve stem to stroke the valve.
51-2-8808B is the 2-2 accumulator discharge isolation valve, and S1-2-8808D is the 2-4 accumulator discharge isolation valve.
These valves have Limitorque SMB-3-80 80/32 ft-lb operators.
C.
Event
Description:
On September 16, 1991, post-modification testing was being performed on valve SI-2-8809B following the implementation of a wiring design change.
The valve was successfully electrically stroked several times.
The motor operator was then locally placed in manual so that a manual stroke test of the valve could be performed.
However, the three position switch (BP)(HS) in the control room (NA) was incorrectly positioned to the close position because the electrical maintenance technician did not request that the position switch in the control room be set to neutral. As the valve was being manually opened, the spring pack relaxed, which allowed the close torque switch to indicate that the valve was no longer fully closed. The motor operator then electrically engaged and attempted to close the valve further.
This electrical 1031S/85K
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 195228
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engagement of the motor operator during. the manual operation ("short stroking") resulted in application of a force higher than usual, but still within design limits, to the motor pinion key.
Further attempts to electrically stroke the valve were unsuccessful.
The motor operator was inspected, and it was determined that the motor pinion key had sheared.
The sheared key allowed the motor drive shaft to rotate within the pinion gear, thus preventing the valve from opening.
Although it was initially believed that the sheared key was the result of the one-time short stroking event, the other Unit 2 safety-related SMB-3-80 high start torque (80 ft-lb) operators were also inspected, with the following results.
On October 3, 1991, the SMB-3-80 motor operators on valves SI-2-8808B and S1-2-88080 were identified as having sheared keys.
However, the motor operators were still capable of stroking the valves due to friction caused by contact between the motor drive snaft and the pinion gear, and the valves were determined to have been operable.
SI-2-8808B and SI-2-8808D were last successfully stroke tested on March 22, 1991, with no pressure differential across the valves.
These same conditions would exist during an accident.
The inspections of the other Unit 2 SMB-3-80 operators with high speed motors did not identify any other sheared keys.
The corresponding motor pinion keys in Unit I cannot be inspected with the unit at power; however, the pinion keys are scheduled to be inspected and rcplaced during the next refueling outage.
On October 16, 1991, and in March 1992, PG&E received information from another nuclear power plant that had experienced similar Limitorque motor operator key shearing problems.
In addition to sheared keys, that plant noted that motor shaft deformation and cracking may occur.
Following receipt of this information, inspection of the SI-2-8809B motor shaft identified small cracks emanating from both corners of the keyway.
On April 29, 1992, metallurgical analysis of the SI-2-8809B sheared key indicated that it was made of a low carbon, resulfurized and leaded steel, such as ASTM A29, Grade 12L13.
The key from the SI-2-8809A operator (which exhibited some plastic deformation but had not sheared) was made of a carbon steel similar to ASTM A29, Grade 1018.
Both of these materials have a lower hardness, and therefore a lower shear strength, than ASTM A29, Grade 4140 material, which the vendor currently recommends for use in SMB-3 motor pinion keys.
D.
Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
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1031S/85K l
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 195228 FO;3kI15 e64ME (1)
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E.
Dates and Approximate Times for Major Occurrences:
1.
September 16, 1991:
Event / Discovery Date.
51-2-88098 was identified as having a sheared motor pinion key.
2.
October 3, 1991:
51-2-8808B and-SI-2-8808D were identified as having sheared keys.
3.
March / April 1992:
Small cracks were-identified on the motor shaft from SI-2-8809B.
F.
Other Systems or Secondary Functions Affected:
None.
G.
Method of Discovery:
The sheared key was identified by utility electrical maintenance personnel during the performance of scheduled post-maintenance testing.
H.
Operator Actions:
None.
I.
Safety System Responses:
None.
111.
Cause of the Event a.
Immediate Cause:
The immediate cause of the failore of the keys for SI-2-8809B, SI-2-8808B, and SI-2-8808D was the application of a force to the keys exceeding the yield point of the material, thereby causing the keys to shear.
B.
Root Cause:
The root cause of the sheared motor pinion key was that the key material was inadequate.
The key'was supplied with the motor operator by the vendor. The key material is considered outdated but still acceptable according to the vendor's design.
The root cause of the motor shaft keyway cracking was also determined to be the key material.
Deformation of the key allows increased clearance between the key and the shaft.
This increased clearance allows higher impact loading due to a " hammer blow"-type effect, and also shifts the 1031S/85K
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 195223 '.
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This causes the shaft to fracture at the sharp keyway corners.
C.
Contributory Cause:
1.
A contributory cause of the event was short stroking (due to electrical engagement of the operator) while the valve was being manually operated. The cause of the short stroking was failure of the operating and maintenance procedures to specify that the control room switch for valve 51-2-88098 be placed in neutral prior to manually operating the valve.
IV. Analysis of the Event 51-2-8809B:
Valve SI-2-88098 is the RHR heat exchanger outlet isolation valve to cold legs 3 and 4.
The safety function of the valve is to be open during the cold leg recirculation phase of recovery from a loss-of-coolant accident (LOCA) to provide a flowpath from the RHR heat exchanger back into the RCS.
During normal plant operation, valve SI-2-88098 is normally open, with electrical power removed from the operator.
Valve 51-2-8809B is required to close during a transfer from cold leg recirculation to hot leg recirculation.
1 Investigation indicates that the valve had stroked properly prior to the inadvertent electrical operation of the valve while being manually operated.
The force applied to the operator under this "short stroking" condition is greater than that applied during normal operation or accident conditions.
The valve would not be manually engaged during accident conditions.
Recent testing of SI-2-8809B pursuant to NRC Generic letter 89-10 indicates that the initial application of force to the key in the closing direction is approximately 1/10 of the force when seating or unseating the valve.
The force when seating or unseating the valve is greater because the valve disc is torqued into its seat when closed, but is not torqued into its backseat when opened.
The key is most likely to fail during the initial " hammer blow" of force in the opening direction. However, if the key were to fail in the closing direction, it would fail when the valve disc was seated but not fully torqued shut.
Nevertheless, the full seating force on this valve would not be required for the valve to perform its safety function of diverting flow from the cold leg to the hot leg.
Further, this safety function does not i
require the valve to open and close repeatedly.
l Metallurgical analysis of the key indicates that the key failed due to shear i
overload.
Since the key did not fail due to fatigue, additional cycling of the valve would not have sheared the key unless excessive force was applied l
to the key.
l 1031S/85K
,w LICENSEE EVENT REPORT (LER) TEXT CONTINUATION -
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DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 91 0l2l1 0l0 6 l"l7 flat (3P; Since the key is expected to fail when operating the valve from the closed-to-open direction only during excessive force conditions, the key would not be expected to shear during operation under accident conditions in the open-to-closed direction and therefore would perform its intended safety function.
Therefore, the failure of the key in valve SI-2-8809B did not adversely affect the health and safety of the public.
SI-2-8808B and 51-2-8808D:
Valves SI-2-8808B and SI-2-8808D are the accumulator discharge isolation valves for accumulators 2-2 and'2-4.
These valves are nonnally open, motor operated valves.
During normal operation, power to the motor operator is removed to prevent inadvertent closure of the valves.
The safety function of the valves is to be open to allow the accumulators to discharge into the RCS if the RCS pressure drops below approximately 600 psia.
Closure of these valves is not a required safety function.
Although the keys on valves SI-2-8808B and 51-2-8808D were found to be sheared, the valves were still capable of stroking. The valves had been successfully stroked during their last stroke test on March 22, 1991.
Inspections of the motor shaft and pinion key assemblies showed.that while the keys had indeed failed, there was mechanical interference between the motor drive shaft and the pinion gear that stopped the shaft from slipping in the pinion gear.
Because valves SI-2-8808B and SI-2-8808D were in the proper position to perform their safety function and were capable of being stroked, the-function of the valves was not impaired.
In addition, failure of the keys would not have caused the valves to close.
Therefore, the failure of the keys in valves SI-2-8808B and SI-2-8808D did not adversely affect the health and safety of the public.
V.
Corrective Actions A.
Immediate Corrective Actions:
1.
Following the discovery of the SI-2-8809B key failure, the'other safety-related Unit 2 SMB-3-80 motor operators were inspected to determine if a similar problem existed.
As discussed above, SI-2-8808B and SI-2-8808D were found to have similar sheared keys.
The failed components were replaced, and no other key failures were discovered on Unit 2.
B.
Corrective Actions to Prevent Recurrence:
1.
The motor pinion keys on the Unit I high speed SMB ^ 'O ft-10 motor operators (i.e., those on the 8808 and 8809 valves, 11 be replaced with 4140 steel during the next refueling outage.
1031S/85K
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 19522S i
FA:lLITY NAM ( [])
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DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 91
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Operating procedures will be revised to prevent valve short stroking.
3.
Maintenance procedures will be revised to prevent valve short stroking.
VI. Additional Information A.
Failed Components:
Limitorque motor operator model SMB-3-80 failed.
B.
Previous Similar LERs:
None.
10315/85K