ML13330B517
| ML13330B517 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/30/1989 |
| From: | Nandy F SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 8904060002 | |
| Download: ML13330B517 (4) | |
Text
Southern California Edison Company P. 0 BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD. CALIFORNIA 91770 F. R NANDY TELEPHONE MANAGER OF NUCLEAR LICENSING March 30, 1989 (818) 302-1896 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D. C. 20555 Gentlemen:
Subject:
Docket No. 50-206 Root Cause Determination of Bearing Rotation Problem in Standby Diesel Generator No. 1 San Onofre Nuclear Generating Station Unit 1 In response to a request by the NRC staff during a recent telephone conversation, enclosed is a report describing the chronological history, root cause determination and corrective actions for the connecting rod bearing rotation problem, as it relates to standby diesel generator No. 1 at San Onofre Unit 1. Standby diesel generator No. 2 does not have this problem.
If more information is needed, please contact me.
Very truly yours, Enclosure cc: 3. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3 8904060002 890330 PDR ADOCK 000O206
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g Enclosure CHRONOLOGICAL HISTORY AND ROOT CAUSE DETERMINATION OF DIESEL GENERATOR CONNECTING ROD BEARING ROTATION PROBLEM
- 1. Statement of Problem The connecting rod bearings of standby diesel generator Nos. 1 and 2-of San Onofre Unit 1 are made of cast aluminum and are designed to remain securely in place (no rotation) during operation of the engine. On February 5, 1989, the Nos. 5 and 8 connecting rod bearings for diesel generator (DG) No. 1 were determined to have rotated following DG No. 1 test runs after engine overhaul.
Bearing rotation for connecting rod Nos.5 and 8 had occurred on several previous occasions, but was not considered to be a significant problem, as explained below.
- 2. Chronological History of Problem The following is a chronological history of the problem obtained from station Quality Assurance records and other documentation.
March 1982:
During engine disassembly and inspection it was discovered that connecting rod bearing Nos. 5 and 8 of DG No. 1 had rotated. The cause was determined to be loss of bearing crush or interference fit between the bearing shells and the connecting rod. The bearing rotation was counterclockwise while engine rotation is clockwise. It was therefore concluded that bearing rotation occurred during engine rollback following engine shutdown or during engine barring in the counterclockwise direction. It was further concluded after discussion with the manufacturer (Transamerica Delaval, Inc.) that "bearing rotation does not present an operational problem. Bearing rotation will not result in loss of lubrication. The bearing locating dowels are made of soft aluminum to prevent shaft galling in the event they are broken."
The bearing shells and the broken dowel pins for bearing Nos. 5 and 8 were replaced. (The remaining connecting rod bearing shells were replaced about this time as part of piston modification).
August 1984:
During engine disassembly and inspection, it was discovered that rod bearing Nos. 5 and 8 had rotated. The bearing shells and dowel pins for Nos. 5 and 8 were replaced.
December 1985:
As part of OR/QR inspection, it was discovered that rod bearing Nos. 5 and 8 had again rotated. Extensive field measurements were performed which established that the rod box bore, dowel pin hole size, location, center, perpendicularity, depth and crankpin journal size of rod Nos. 5 and 8 were within the same ranges as for rods that had not experienced rotated bearings. In addition, rod Nos. 4 and 9 were swapped with rod Nos. 5 and 8 respectively to determine if the rotation, should it occur again, was a function of bearing location or the rods. Again, engine operability was not believed to be a problem based on the conclusion cited above.
July 1988:
Inspection of rod bearing No. 8 revealed that it had rotated. (Rod bearing No. 5 was not available for inspection.)
No. 8 bearing shell and dowel were replaced. No. 8 rod bore was "roughed" to provide greater resistance to rotation.
Enclosure page 2
-2 January-February 1989:
During connecting rod inspections, it was discovered that rod bearing No. 5 had rotated, whereas rod bearing No. 8 had not rotated. The bearing shell and dowel for No. 5 were replaced.
Bearing Nos. 5 and 8 positions were marked so that bearing rotation could be determined via the inspection covers. It was discovered after a 15 minute run that rod bearing No. 5 had rotated. After a one hour run, rod bearing No. 8 was also found to have rotated.
After extensive investigation, it was concluded that the initial rotation of the bearing shells was the result of inadequate clamping force between the upper and lower shells. The rotation would be arrested when galling of the bearing shell back increased the coefficient of friction such that combined with the burnishing effect of running and reduced rotational torque, the net effect is to lock the shell in place. Based on that evaluation, a solid core dowel was installed for bearing shell Nos. 5 and 8 and the friction between the shells and rod boxes was increased by dressing.the shell backs and the connecting rod bearing bore with abrasive paper. After ten engine runs, neither bearing No. 5 nor No. 8 had rotated. However, on February 21, 1989, after 28 runs, bearing No. 5 had again rotated. Corrective actions have been taken as indicated in the following section on corrective actions.
- 3. Root Cause Determination Following the discovery on February 21, 1989 that bearing No. 5 had again rotated, the following additional investigative steps were performed:
- 1) The upper and lower crankcase through bolts were verified to be properly torqued.
- 2) One engine base bolt was disassembled and the initial installation and the torqueing completed during Cycle 10 refueling were verified. All DG No. 1 engine base bolts were ultrasonically tested for indications of cracking.
- 3) The main oil supply lines to connecting rod journal Nos. 5 and 8 were verified to be free of obstructions.
- 4) Rod journal Nos. 5 and 8 were checked to determine if the journals were truly cylindrical.
This check revealed high spots opposite low spots on both journals and high spots at the edges on journal No. 8. The uneven shapes of journal Nos. 5 and 8 would result in smaller oil clearances at the high spots. This condition could easily have gone undetected during previous dimensional checks using a two point micrometer.
The root cause of the connecting rod bearing shell rotation has been identified to be irregularities in the journal at rod bearing location Nos. 5 and 8, resulting in "high" spots and a reduction in oil clearances. When the engine stops with position Nos. 5 and 8 in compression, oil could be "squeezed' out at the high spot such that the bearing shell can grab the journal on engine rollback.
Enclosure page 3
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- 4. Corrective Actions Connecting rod journal Nos. 5 and 8 have been dressed to a nominal diameter of 12.998 inches to remove high spots. This will provide an additional.001 inch bearing-to-shaft clearance which will result in additional cooling due to increased oil flow, but will not affect the engine operating characteristics, as the clearances are well within the specified tolerances.
Rod bearing Nos. 5 and 8 were replaced with standard size bearings and a standard hollow core locator dowel was used. The bearings were checked for rotation after each of four consecutive engine runs performed under various loading conditions and of various durations from 15 minutes up to 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />.
By these checks, it was confirmed that neither bearing had rotated. Additional inspections will be performed once every three months during the next plant operating cycle to verify that rod bearing Nos. 5 and 8 have not rotated.
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