ML20141K771
| ML20141K771 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 03/10/1997 |
| From: | Galeazzi D, Nuding M External (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20141K769 | List: |
| References | |
| REF-PT21-97-220 NUDOCS 9705300002 | |
| Download: ML20141K771 (12) | |
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.. Fax:9194461134 Mar 11 '97 12:07 P.02 i Elm
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ENGINE SYSTEMS, INC. ; __
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'w Report No.10CFR21-0075 MARCH,10,1997 l i 10CFR21 REPORTING OF DEFECTS AND NON-COMPLIANCE l
COMPONENT: Parsons Peebles/ Electric Products Generators with Modified Bearing Insulation SYSTEM: Standby diesel generators CONCLUSION: Defect is Reportable in Accordance with 10CFR21 pREON
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PREPARED BY: DATE: Donald D. Galeazzi l Engineering Manager l REVIEWED BY: DATE: Michael Nuding Quality Assurance Manager 9705300002 970311 PDR ADOCK 05000266 S PDR
. . Fax:9194461134 Mar 11 '97 12:07 P.03 REPORT NO,10CFR21-0075
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PAGE: 10F 5 i. i
SUMMARY
j Engine Systems, Inc. (ESl) received notification on 3/3/g7 from Wisconsin Electric (WE), Point Beach nuclear plant about generator bearing failures on their EDG G-03. l 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 ESl) 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, NEl 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 3 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 l 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 , i i outer race and bearing housing. Generators utilizing the original insulation design with i insulation between the generator shaft and the bearing inner race are n_p! suspect i because contact between the bearing outer race and the bearing retainer ring will not S provide a path for circulating currents.
I . Fax:9194461134 Mar 11 '97 12:09 P.04 , REPORT NO.10CFR21-0075' ' PAGE: 2 OF 5 t i
- . CUSTOMERS AFFECTED
! Wisconsin Electric is the only user to report this type of problem. The WE generators j 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 i i being notified and are included on the list below. Generators with the original bearing j insulation design (insulation between shaft and bearing inner race) are n_o_t affected by i this notification. 1-j 1 USER WITH PARSONS PEEBLES/ ELECTRIC PRODUCTS GENERATORS . Dornestic Foreion i . ' Duke Power /Oconee Almaraz, Spain ! FP&L/St. Lucie CNV l Cofrentes I Knolls Atomic Power Laboratory Kou Sheng l KRSKO l
- MP&L/ Grand Gulf NYPA/Fitzpatrick Laguna Verde (HPCS) l Laguna Verde )
Portland G.E. i Puget Sound /Skagit' Zorita, Spain ! TVA/Sequoyah TVA/ Watts Bar l Wisconsin Electric / Point Beach I l l 1 l' 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 diebel generator or by normal vibration during operation. In any case, the potential i 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. i
. Fax:9194461134 Mar 11 '97 12:10 P.05 REPORT NO.10CFR21-0075 PAGE: 30F5 i
CORRECTIVE ACTION: . 1. Wisconsin Electric has implemented a modification to add insulation to the outer l . spacer ring. See Attachment 2. l
- 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 j 1983 and was addressed by the NRC in 1986 (IN 86-26); therefore, we assume any i 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 I 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 j probably been in service for many years without experiencing bearing problems so i 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 nqt affected by this notification. Users with non-Parsons Peebles/ Electric Products generators are also no_t affected by this notification.
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, Fax:9194461134 Mar 11 '97 12:11 P.06 i REPORT NO.10CFR21-0075 PAGE: 4 OF 5 1
I l i l l i l INNER l SPACER RING l g q INSULATED OIL SLINGER \ h E HUB ASSEMBLY
\ l CONTACT SURFACE )
! / OUTER E _ .Ir on e eaf Edi3
/ SPACER RING l x *i
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3 i If l J - BEARING OUTER i RACE ASSEMBLY I inn
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umb== % ejip *i LOCKING WASHEF 1 .
& NUT ASSEMBLY SHAFT - BEARING l lNNER RACE 1
1 SLIP RING END l I l ATTACHMENT 1 Figure showing contact path for bearing i i
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I - . Fax:9194461134 ' Mar 11 '97 12:11- P.07 J l ' REPORT NO.10CFR21-0075 PAGE: 5 OF 5 i i l i l l l l l l l I ATTACHMENT 2 Wisconsin Electric corrective action (6 pages attached) 1 i I j 1 I J i l l l l
, Fax:9194461134 Mar 11 '97 12:12 P.08 MR 97 Ol8'A Final Design Description I
in response to Condition Report 97-0583, additional bearing insulation will be installed on the EDG G-03 generator outboard bearing outer cover spacer rmg The bearing design allows the bearing outer race to I 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 (approxima:cly 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" Boaring travel is limited by the oil slinger nng on the 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 rirg limits the bearing outer race travel to 0.110", This design will not change this travel distance within tolerance. The spacer ring will be 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 mQqum tolerance of spacer ring cut back (0.060"), the travel distancu would be limited to 0.070" whirk ryresents 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 minimt:m 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 a!! awed. If the spacer ring did not exist, the bearing outer race could travel until contacting the inner race lock washer. The outer race travel distance between this lock washer and the oil slinger nng is no less than 0 200" Therefore by limiting the outer race travel distance to no greater than 0. !?0", the possibility of the outer race contacting the inner race lock washer is eliminated. The new insulator will only contact the bearmg outer race and not the bearing cage since the rotatmg beanng cage is recessed from the outer bearing race. The new insulation wit! be m accordance with SK MR 97 018-1 and 2. The insulation will be made or NEM A Grade OPO 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 tne cover plate spacer nqs with a friction ih. An ap rovgagesive (The Dexter Corporation's Epoxi Patch, iC White) will also be used, tr$ 5 , ar, wT1 also act to fill any surface imperfections in the specer 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, dt.ted 3FEB94 shown the details of the modification performed on the beunng insulation during EDG 003 and G04 refurbishment before initial installation under MR 91 116. This document shows two 1/8" layers of Scotchply type 1009-36 crossply insulation provides the msulation 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 I/8" thick. This mittal modification did not account for the beanng outer race free floatmg design. The dielectric strength of the Scotchply type 1009 36 is t00 VPM for 1/16" thickness of material. The die!cetric strength of the NEMA GPO-3 Polyester / Glass Mat Sheet Laminate material 1.5 450 VPM for 1/16" thickness of matenal. Therefore the Polyester /G! ass Mat Sheet Laminate matenal offers only 70% of the insulation value of the Scotchply material. However, with 1/16" nf matenal, the Polyester / Glass-Met insul4 tion is rated for 28,000 volts. The thickness of the insulator is greater than 0.060" and will provide upproximately 28,000 volts ofinsulation value. Page i of 2 _. . ,. . w,a u.m,m vma u rm e at AAt-An-mW
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- . Fax:9194461134 Mar 11 '97 12:13 P.09 l ,
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MR 97-018'A Final Design Description i Magnetic dissymmetry in the generator produce shaft voltages which in turn can produce shaft currents Per EPR.1 Power Plant Electrical Reference Swies Volume 1, Electric Generators, typical shaft voltages are e 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 bearms since this will provide an insulation level (10,000V) that is orders of l magnhude greater than a (consersatively high) maximum espected shaR voltage (100 V). The new msulation 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 esposed to tensile or shear forces which exceed its ratings. The worst case forces will be due to the outer race working hself to the end ofits free travel and coming in contact with the msulator. Since the bearms outer race is free floating and sufficient float distance is prevtded in the desi 6n as daecribed above, the worst case forces are well below the material cornpressive strength of 35.000 psi Therefore the material he adequate structurel strength for the application. The additional weight of the insulators to the generator will have no affect on the seismic qualification of the EDO sets since the insalator weight is negligible. The new insulation material and cpery adhesive will be QA qualified. MKW Power Systems. Inc. (now Engine Systems, Inc.) was the vencor that performed the original diesel generator refusishment including the bearing insulation modification They have reviewed and concurred with the design describcd above sacr consultmg with their sub comractor Sr. inter Elecnic. Post installation testing (PMT) will consist of a generator alignment check tollowed by a 4 hour run of the EDG per TS-83. During the four hour run, generator vibration will be verified to be within ac.:eptAble limits.
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