Part 21 Report - 1997-221

 
 

ACCESSION #: 9705300002



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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.



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