United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-51: Inappropriate Greasing of Double Shielded Motor Bearings


July 15, 1994



All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for failure of safety-related
equipment as a result of inappropriate greasing of double shielded motor
bearings.  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 are not NRC requirements; therefore, no specific action or written
response is required.

Description of Circumstances

On January 20 and January 21, 1994, two different emergency diesel generator
fuel oil transfer pumps failed to start at the North Anna Power Station when
the motor breakers tripped on overload during testing.  On January 22, 1994,
the licensee tested the remaining six diesel fuel oil transfer pumps, and two
more of the pump motor breakers tripped on thermal overload.  These two pumps
supply the same diesel generator.  Consequently this diesel generator was
declared inoperable.

There are eight fuel oil transfer pumps at North Anna; two redundant pumps
supply each of the station four emergency diesel generators from underground
fuel oil storage tanks.  The transfer pumps are located in an unheated
building near the fuel oil tanks.  This building is continuously supplied with
outside air to reduce the oil fumes.  As a result, the temperature in the
building was about the same as that of the outdoor air, which was abnormally
cold: -15C [5F].  The licensee installed temporary heaters in the building
and, after a short warmup period, successfully tested all of the transfer


The licensee investigation of the problem showed that the failures to start
the pumps were caused by excessive grease in the motor bearings, which became
very stiff at the abnormally low temperatures.  This caused abnormally high
starting loads that prevented the motor from accelerating past the low

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                                        July 15, 1994
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rotational speeds that cause the motor to draw high starting currents.  The
sustained high starting currents caused the motor power supply breaker thermal
overload devices to trip.

This determination was confirmed by placing the motors in a freezer for
2.5 hours at  -14C [6F] and applying power to them.  At this temperature
some of the motors drew 6 amperes when the rated 460 volts were applied to the
motor terminals, compared to the rated full load current of 0.75 amperes.
These motors continued to run at a slow speed in the start-up current range.
When tested at the same temperature after the bearings were replaced, the
motors ran at normal speed and drew only 0.4 amperes.

The motor bearings are of the double shielded type, which are supplied by
several different manufacturers, and normally require no greasing after they
are installed.  However, station records showed that 7 of the 8 motors had
been greased every 18 months since 1986.  The old bearings were found to be
completely full of grease.  According to the bearing vendors, a high grease
content inside the bearing shields could not only cause high starting loads,
but could also cause the bearing to overheat and solidify the grease under
normal operating conditions.

Related Generic Communications

NRC Information Notice 88-12, "Overgreasing of Motor Bearings," April 12,

NRC Information Notice 93-26 and Supplement 1, "Grease Solidification Causes
Molded-Case Circuit Breaker Failure To Close," April 7, 1993, and
January 31, 1994.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation project manager.

/s/'d by BKGrimes

                        Brian K. Grimes, Director
                        Division of Operating Reactor Support
                        Office of Nuclear Reactor Regulation

Technical contact:  Rick McWhorter, RII
                    (703) 894-5421

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