Part 21 Report - 1997-221

ACCESSION #: 9705300002 NOTE: This text document was processed from a scanned version or an electronic submittal and has been processed as received. Some tables, figures, strikeouts, redlines, and enclosures may not have been included with this submittal, or have been omitted due to ASCII text conversion limitations. In order to view this document in its entirety, you may wish to use the NUDOCS microfiche in addition to the electronic text. ENGINE SYSTEMS, INC. Report No. 10CFR21-0075 MARCH, 10, 1997 10CFR21 REPORTING OF DEFECTS AND NON-COMPLIANCE COMPONENT: Parsons Peebles/Electric Products Generators with Modified Bearing Insulation SYSTEM: Standby diesel generators CONCLUSION: Defect is Reportable in Accordance with 10CFR21 PREPARED BY: DATE: Donald D. Galeazzi Engineering Manager REVIEWED BY: DATE: Michael Nuding Quality Assurance Manager REPORT NO, 10CFR21-0075 PAGE: 1 OF 5 SUMMARY: Engine Systems, Inc. (ESI) received notification on 3/3197 from Wisconsin Electric (WE), Point Beach nuclear plant about generator bearing failures on their EDG G-03. Investigation by WE revealed that the bearing failures were caused by circulating currents through the generator shaft. The generator bearing insulation was previously modified by MKW Power Systems, Inc. (now ESI) as a result of an insulation problem identified in NRC Information Notice No. 86-26 (originally reported by Power Systems in their report no. 10CFR21-0017). The WE EDG's are single engine arrangements. Modification of the generator bearing insulation was performed in accordance with a procedure developed by the generator manufacturer, NEI Peebles-Electric Products, specifically for the WE generators. The original bearing insulation design placed insulation between the generator shaft and the inner race of the bearing. On some machines, the bearing had rotated on the shaft and destroyed the insulation. The reduction in insulation thickness allowed the shaft to drop slightly and rub on the bearing housing. To prevent this, the insulation design was modified by removing the insulation between the shaft and the bearing and applying insulation between the bearing outer race and the bearing housing. The opposite drive end generator bearing (the one that is insulated) is a two-piece design. This means that the inner race can be removed from the outer race/bearing/cage assembly. The inner race is mounted on the shaft using an interference fit and a locking washer/nut assembly. The outer race is mounted in an insulated hub with a clearance fit. Bearing retaining rings are mounted on each side of the bearing race to limit the allowable travel of the race/bearing/cage assembly. Once the generator is assembled the outer race/bearing/cage of the opposite drive end bearing was not expected to move relative to the insulated bearing housing because of the low coefficient of friction between the rolling elements and the inner race. Upon inspection, WE determined the bearing outer race had actually moved sufficiently to contact the outer retaining ring. This ring is not insulated from the generator frame and contact with the bearing outer race effectively shorted out the insulated hub and provided a path for the circulating currents. Over time, these circulating currents permanently damaged the bearing assembly. COMPONENT: Parsons Peebles/Electric Products generators with bearing insulation between bearing outer race and bearing housing. Generators utilizing the original insulation design with insulation between the generator shaft and the bearing inner race are not suspect because contact between the bearing outer race and the bearing retainer ring will not provide a path for circulating currents. REPORT NO. 10CFR21-0075 PAGE: 2 OF 5 CUSTOMERS AFFECTED: Wisconsin Electric is the only user to report this type of problem. The WE generators are also the most recent ones to have the bearing insulation modified. ESI does not have an accurate list of users who have had their generator bearing insulation modified; therefore, all users with Parsons Peebles/Electric Products generators are being notified and are included on the list below. Generators with the original bearing insulation design (insulation between shaft and bearing inner race) are not affected by this notification. USER WITH PARSONS PEEBLES/ELECTRIC PRODUCTS GENERATORS Domestic Foreign Duke Power/Oconee Almaraz, Spain FP&L/St. Lucie CNV Cofrentes Knolls Atomic Power Laboratory Kou Sheng MP&L/Grand Gulf KRSKO NYPA/Fitzpatrick Laguna Verde (HPCS) Portland G.E. Laguna Verde Puget Sound/Skagit Zorita, Spain TVA/Sequoyah TVA/Watts Bar Wisconsin Electric/Point Beach DEFECT: Design of the bearing insulation modification did not include provisions to insulate the bearing spacer ring. The bearing outer race/bearing/cage assembly is free to move relative to the inner race; if sufficient movement occurs, the outer race can contact the spacer ring and provide a path for circulating shaft currents. This effectively defeats the bearing insulation and can result in generator bearing failure. See Attachment 1. Cause of outer race movement is unknown. Thermal growth results in a small amount of movement, but is not enough to overcome the clearance between the outer race and the space ring (nominal clearance has been measured at 0.070'). Only one of the two generators at Wisconsin Electric has had enough movement to contact the outer spacer ring (farthest from engine). Movement could have occurred during shipment of the diesel generator or by normal vibration during operation. In any case, the potential exists for movement and therefore insulation of the outer spacer ring should be evaluated to prevent contact with the bearing outer race. The inner spacer ring is not of concern because movement of the outer race is limited by the oil slinger and therefore it is not possible for the outer race to contact the inner spacer ring. REPORT NO. 10CFR21-0075 PAGE: 3 OF 5 CORRECTIVE ACTION: 1. Wisconsin Electric has implemented a modification to add insulation to the outer spacer ring. See Attachment 2. 2. Users with Parsons Peebles/Electric Products generators that have had the generator bearing insulation modification performed should inspect the generator bearings for circulating currents and consider the above corrective action implemented by Wisconsin Electric. The bearing insulation problem first surfaced in 1983 and was addressed by the NRC in 1986 (IN 86-26); therefore, we assume any bearing insulation modifications were made approximately 10 years ago. The WE generators are an exception to this because the units had been in storage, then purchased by WE, bearing insulation modified in 1992 and placed into service in 1994. With the exception of WE, any generators with the bearing modification have probably been in service for many years without experiencing bearing problems so the probability is small that a bearing has contacted the spacer ring; therefore, bearing inspection can be performed at the users convenience. Users with Parsons Peebles/Electric Products generators that have the original bearing insulation design (insulation on shaft) are not affected by this notification. Users with non-Parsons Peebles/Electric Products generators are also not affected by this notification. REPORT NO. 10CFR21-0075 PAGE: 4 OF 5 ATTACHMENT 1 "Figure showing contact path for bearing" omitted. REPORT NO. 10CFR21-0075 PAGE: 5 OF 5 ATTACHMENT 2 Wisconsin Electric corrective action (6 pages attached) MR 97-018*A Final Design Description In response to Condition Report 97-0583, additional bearing insulation will be installed on the EDG G-03 generator outboard bearing outer cover spacer ring. The bearing design allows the bearing outer race to float freely along the axis of the generator shaft and to come into contact with the outer cover spacer ring. The outer cover spacer ring is grounded through the generator housing. Field measurements show the outboard bearing inner cover spacer ring will have adequate clearance (approximately 3/8") from the bearing outer race such that no insulation is required, The bearing is designed to have a total axial travel of 1/4". Bearing travel is limited by the oil slinger ring on tile engine side of the bearing and by the cover spacer ring on the outboard side. Field measurements show the distance between the slinger ring and spacer ring limits the bearing outer race travel to 0.110". This design will not change this travel distance within tolerance. The spacer ring will bc cut back 0.090" +/- 0.030" such that it extends 2.542" (nominal) from the cover face. The thickness of the insulation will be 0.080" +/= 0.020". At maximum tolerance of the insulation thickness (0.100") and minimum tolerance of spacer ring cut back (0.060"), the travel distance would be limited to 0.070" which represents the minimum travel that would be allowed. With the G03 generator shaft length of 6', the maximum expected thermal growth is approximately 0.006" so the minimum travel distance of 0.070" is acceptable. At minimum tolerance of the insulation thickness (0.060") and maximum tolerance of spacer ring cut back (0.120"), the travel distance would be limited to 0.170" which represents the maximum travel that would be allowed. If the spacer ring did not exist, the bearing outer race could travel until contracting the inner race look washer. The outer race travel distance between this lock washer and the oil Slinger ring is no less than 0.200". Therefore by limiting the outer race travel distance to no greater than 0.170", the possibility of the outer race contacting he inner race lock washer is eliminated. The new insulator will only contact the bearing outer rate and not the bearing cage since the rotating bearing cage is recessed from the outer bearing race. The new insulation will be in accordance with SK-MR-97-018-1 and 2. The insulation will be made of NEMA Grade GPO-3 Polyester/Glass-Mat Sheet Laminate. The insulation will provide adequate isolation between the outboard bearing outer race and the generator housing so as to prevent any induced voltages from setting up a current path to ground and causing premature bearing degradation. The insulation will be attached to the cover plate spacer ring with a friction fit. An approved adhesive (The Dexter Corporations's Epoxi-Patch, 1C White) will also be used, though not required, and will also act to fill any surface imperfections in the spacer ring. The adhesive provides an acceptable bond for metal and glass, and is suitable for the high temperature (less than 185 degrees F), oil environment of the bearing enclosure. MKW Document No. 6090-TR-02, dated 3FEB94 shows the details of the modification performed on the bearing insulation during EDG G03 and G04 refurbishment before initial installation under MR 91-116. This document shows two 1/8" layers of Scotchply type 1009-36 crossly insulation provides the insulation between the bearing outer race and the generator housing. The outer layer was subsequently machined as part of the original modification and therefore the outer layer is less than 1/8" thick. This initial modification did not account for the bearing outer race free floating design. The dielectric strength of the Scotchply type 1009-36 is 650 VPM for 1/16" thickness of material. The dielectric strength of the NEMA GPO-3 Polyester/Glass-Mat Sheet Laminate material is 450 VPM for 1/16" thickness of material. Therefore the Polyester/Glass-Mat Sheet Laminate material offers only 70% of the insulation value of the insulation value of the Scotchply material, However, with 1/16" of material, the Polyester/Glass-Mat insulation is rated for 28,000 volts. The thickness of the insulator is greater than 0.060" and will provide approximately 29,000 volts of insulation value. Page 1 of 2 MR 97-018*A Final Design Description Magnetic dissymmetry in the generator produce shaft voltages which in turn can produce shaft currents Per EPRI Power Plant Electrical Reference Series Volume 1, Electric Generators, typical shaft voltages are measured in the milli-volt range but may reach magnitudes of 10's of volts. A minimum insulation thickness of 1/16" of the polyester material will be adequate to prevent any circulating currents from flowing through the generator bearing since this will provide an insulation level (10,000V) that is orders of magnitude greater than a (conservatively high) maximum expected shaft voltage (100 V). The new insulation will be installed in a hot oil environment. The insulation material data sheet shows the dielectric strength is increased from 450 VPM to 550 VPM when placed in oil rather than air. Therefore the dielectric properties of the polyester/glass-mat laminate (GPO-3) is greater when in a oil environment. The material has a 120 degrees C (248 degrees F) rating which provides adequate margin above the 185 degree F generator bearing alarm setpoint. The epoxy adhesive is specifically designed for this type of application. The new insulation material will not be exposed to tensile or shear forces which exceed its ratings. The worst case forces will be due to the outer race working itself to the end of its free travel and coming in contact with the insulator. Since the bearing outer race is free floating, and sufficient float distance is provided in the design as described above, the worst case forces are well below the material compressive strength of 35,000 psi. Therefore the material has adequate structural strength for the application. The additional weight of the insulators to the generator will have no affect on the seismic qualification of the EDG sets since the insulator weight is negligible. The new insulation material and epoxy adhesive will be QA qualified. MKW Power Systems, Inc. (now Engine Systems, Inc.) was the vendor that performed the original diesel generator refurbishment including the bearing insulation modification. They have reviewed and concurred with the design described above after consulting with their sub-contractor Sumter Electric. Post installation testing (PMT) will consist of a generator alignment check followed by a 4 hour run of the EDG per TS-83. During the four hour run, generator vibration will be verified to be within acceptable limits. Page 2 of 2 Figures [3 pages] omitted. Table "PROPERTY PROFILE" omitted. *** END OF DOCUMENT ***

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