Information Notice No. 88-12: Overgreasing of Electric Motor Bearings
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 12, 1988
Information Notice No. 88-12: OVERGREASING OF ELECTRIC MOTOR BEARINGS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to problems
caused by accumulation of grease on electric motors used to drive rotating
equipment in nuclear power plants. It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems. However, suggestions contained in
this information notice do not constitute NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances:
NRC personnel observed accumulations of grease on the air vent screens of
electric motors used for driving such rotating equipment as fans and pumps at
the Millstone and Calvert Cliffs nuclear power plants. The grease appeared to
have come from overgreasing of the electric motor bearings. Grease was forced
out of the bearing seals, onto the stator windings and rotor, from where it
either fell or was thrown onto the inside of the motor housing. Because of
these observations, the NRC began an investigation into problems that have
been caused in the past, or could be caused in the future, by the over-greasing
of electric motor bearings.
Several electric motor overgreasing events were identified that have led to
failures of nuclear plant components:
� At a nuclear plant in 1975, overgreasing of the motor bearings caused the
failure of the containment cooler fans when a bearing failed and seized.
� At Susquehanna Unit 1, on July 24, 1984, excessive grease buildup in the
electric motor of a motor generator set caused the set to shut down and
resulted in the spurious emergency actuations of the standby gas
treatment system and the control room emergency outside air system (LER
387/84-036).
8804060370
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� At Palisades Unit 1, overgreasing of the motor bearings caused the
containment cooler fans to fail on three different occasions: On July 7,
1986, overgreasing of the motor shaft bearings caused the bearings to
skid in-stead of roll. This caused the motor to overheat and
automatically shut down. On August 8, 1986, and again on August 11,
1986, overgreasing caused the motor bearings to deteriorate which, in
turn, caused the motor and fan to vibrate, and bolts on the equipment to
loosen and fail (LER 255/86-031).
In addition to these overgreasing events, a number of cases have been reported
in which excessive motor bearing grease caused problems that were caught
before the systems actually failed. For example, at a nuclear plant in 1978,
excessive grease in a motor bearing in the service-water system caused the
bearing to become abnormally hot and noisy. At another nuclear plant in 1986,
grease from a bearing in the component cooling pump motor ran into the motor,
causing it to smoke.
Discussion:
The NRC staff has solicited technical information and operating experience on
the problems caused by the overgreasing of electric motor bearings from motor
and bearing manufacturers, as well as from other licensees. Their responses
are summarized in this discussion.
Two basic mechanisms involving overgreasing of bearings can lead to the
failure of electric motors:
(1) The excess grease will leak out from the bearing seal and be deposited on
the stator and rotor windings. The grease deposits create a thermal
barrier between the windings and their cooling air, leading to an
increase in motor temperature. For motors designed to use outside air to
cool the windings, the temperature is increased still more by the buildup
of dirt, dust, and other foreign materials that adhere to the grease. It
is estimated that for every 10�F rise in motor temperature, the
insulation life of the windings is reduced by half. The deterioration of
the winding insulation then causes the motor to "short out".
(2) The excess grease causes a "churning" action of the ball bearings within
their housing, increasing the temperature of the grease and bearings. As
the temperature approaches the upper functional design limit of the
grease, viscous or hard residues form, causing a breakdown of the lubri-
cating characteristics of the grease. The bearing temperature then rises
higher, and eventually causes a reduction in bearing hardness and a sub-
sequent breakdown of the bearing. A rule of thumb, based on certain
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manufacturers' tests, is that grease life is halved for every 25�F
increase in temperature and is doubled for every 25�F decrease in
temperature.
High bearing temperature may also cause the inner race of the bearing to
expand and slip on the rotor shaft. The expansion and slippage of the
bearing's inner race leads to a misalignment between the rotor shaft and
the bearing; this misalignment can cause contact between the rotor and
stator. Once contact occurs, the windings short and burn out.
One frequent cause of excess grease problems is the failure to provide an exit
path for the displaced grease when new grease is added. This can result from
the failure to remove the grease drain plug, or from failure to unblock drain
lines that have become plugged by hardened grease or dirt. Another frequent
cause is the addition of too much grease. The correct amount must be deter-
mined for each situation. Bearing manufacturers generally recommend filling
the motor bearing void space from one-third to one-half full of grease. How-
ever, the optimum amount can vary from 20% to 80% full, depending on such
factors as the application of the motor, the environment of the motor, the
retainer design, and the type of grease.
The following actions can be taken to correct or prevent motor overgreasing
and related problems:
(1) Review motor lubrication procedures to ensure that they identify the type
and quantity of grease to use, the specific fill and drain nozzles to
uncap, and the length of time motors should be run with drain plugs off
after greasing the bearings.
(2) To prevent foreign materials from contaminating the grease, ensure that
grease containers are covered during periods of storage and that nozzles
and grease fittings are cleaned.
(3) Determine the optimum quantity and correct type of grease required for
each motor by examining the manufacturer's recommendations and by
monitoring the behavior of grease added to motors.
(4) Consider using prelubricated sealed bearings in applications where re-
lubrication is difficult, where contaminants can adulterate the grease,
or where overgreasing might damage safety systems.
It should be noted that the vast majority of the reported lubrication-related
incidents have been the result of underlubrication rather than
overlubrication. This notice is not intended to discourage the adequate
lubrication of motor bearings, but to prevent overgreasing problems by
encouraging a thorough understanding of correct lubrication procedures.
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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical
contact listed below or the Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: James A. Prell, RI
(215) 337-5108
Donald C. Kirkpatrick, NRR
(301) 492-1152
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 88-12
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Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-11 Potential Loss of Motor 4/7/88 All holders of OLs
Control Center and/or or CPs for nuclear
Switchboard Function Due power reactors.
to Faulty Tie Bolts
88-10 Materials Licensees: Lack 3/28/88 All NRC licensees
of Management Controls Over authorized to use
Licensed Programs byproduct
material.
87-44, Thimble Tube Thinning in 3/28/88 All holders of OLs
Supp. 1 Westinghouse Reactors or CPs for nuclear
power reactors
that employ a
Westinghouse NSSS.
88-09 Reduced Reliability of 3/18/88 All holders of OLs
Steam-Driven Auxiliary or CPs for nuclear
Feedwater Pumps Caused power reactors.
by Instability of Woodward
PG-PL Governors
88-08 Chemical Reactions with 3/14/88 All NRC licensees
Radioactive Waste generating or pro-
Solidification Agents cessing low level
radioactive waste.
88-07 Inadvertent Transfer of 3/7/88 All NRC broad
Licensed Material to licensees and
Uncontrolled Locations licensees
authorized to
possess byproduct
material as sealed
sources in
teletherapy units
or "self-
contained"
irradiators.
88-06 Foreign Objects in Steam 2/29/88 All holders of OLs
Generators or CPs for PWRs.
88-05 Fire in Annunciator Control 2/11/88 All holders of OLs
Cabinets or CPs for nuclear
power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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