Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
*** END OF DOCUMENT ***
