United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-98: Motor Brakes on Valve Actuator Motors

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                               December 20, 1993


NRC INFORMATION NOTICE 93-98:  MOTOR BRAKES ON VALVE ACTUATOR MOTORS


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential problems with the performance of
motor-operated valves (MOVs) resulting from improper operation of motor brakes
used on valve actuator motors.  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.

Background

Some MOVs have motor brakes installed to help minimize the inertial loads
during valve closure after the control switch has tripped.  Motor brakes also
help prevent MOVs with non-locking gear mechanisms from inadvertently opening. 
One type of motor brake is designed to be electromagnetically released and has
friction pads to slow the motor shaft following control switch trip.  Improper
operation of motor brakes can result in problems with the performance of MOVs.

Description of Circumstances

In Maintenance Update 92-2, Limitorque Corporation informed the nuclear
industry that it had discovered, through operating experience and testing,
that motor brakes do not substantially minimize the thrust load caused by
inertia when closing MOVs.  Limitorque also stated that it did not qualify
motor brakes through its nuclear qualification testing.  Limitorque warned
that voltage variation into the motor brake may render the brake inoperable. 
Limitorque noted that disconnection of the brake leads does not render the
brakes inoperable and specified that the friction pads be removed.  Each of
the following reports involve valves with Limitorque motor operators.

On August 4, 1993, the licensee of Crystal River Nuclear Plant, Unit 3,
reported (Licensee Event Report 93-008) that seven MOVs with motor brakes
installed might receive insufficient voltage to allow the motor brakes to be
released under degraded voltage conditions.  Of these seven MOVs, the licensee

9312150064.

                                                            IN 93-98
                                                            December 20, 1993
                                                            Page 2 of 3


declared a normally closed high-pressure injection suction MOV to be
inoperable on July 6, 1993.  The licensee removed the friction disks and
electrical power connections from this MOV as well as from normally open
borated water storage tank suction MOV.  The licensee tested another of the
seven MOVs to ensure that its motor brake would release under degraded voltage
conditions.  The remaining four MOVs are locked in their safety function
position during plant operation.

On September 14, 1993, the licensee of Millstone Nuclear Power Station,  
Unit 2, notified NRC that it had declared four feedwater supply MOVs with
motor brakes installed inoperable when it determined that the MOVs might
receive insufficient voltage during degraded voltage conditions to allow the
motor brakes to release.  The licensee relies on these MOVs as a backup for
feedwater isolation in the event of a main steam line break.  The licensee
shut down Millstone Unit 2 in accordance with its plant technical
specification requirements.  The licensee subsequently removed the motor
brakes.

On September 28, 1993, the licensee of James A. FitzPatrick Nuclear Power
Plant notified NRC of problems with the operation of motor brakes on two low-
pressure coolant injection MOVs.  The licensee determined that the motor
brakes were undersized to prevent the actuator spring pack from relaxing and
causing the torque switch to restart the actuator motor on a repeated basis. 
Continuous restarting of the actuator motor can cause (1) damage to the motor
from overheating, and (2) excessive stress to the MOV by driving the valve
disk into its seat (sometimes referred to as a "hammering" effect).

Discussion

In Generic Letter (GL) 89-10, "Safety-Related Motor-Operated Valve Testing and
Surveillance," the NRC staff requested nuclear power plant licensees to
confirm the capability of safety-related MOVs to perform their intended
functions by reviewing MOV design bases, verifying MOV switch settings
initially and periodically, testing MOVs under design-basis conditions where
practicable, improving evaluations of MOV failures and necessary corrective
action, and trending MOV problems.  In response to GL 89-10, licensees are
implementing programs to evaluate the design-basis capability of MOVs within
the scope of the generic letter.

As indicated by the discovery of MOV operability problems at Crystal River and
Millstone, some licensees may not be evaluating the minimum voltage required
to ensure the proper operation of motor brakes when determining the design-
basis capability of MOVs with motor brakes installed.  Improper operation of
motor brakes on MOVs could allow the motor to restart repeatedly as found at
FitzPatrick.
.

                                                            IN 93-98
                                                            December 20 1993
                                                            Page 3 of 3


Related Generic Communications

NRC has issued other information notices pertaining to MOVs.  During the past
year, these have included NRC Information Notices 93-74, "High Temperatures
Reduce Limitorque AC Motor Operator Torque;" 93-54, "Motor-Operated Valve
Actuator Thrust Variations Measured With a Torque Thrust Cell and a Strain
Gage;" 93-01, "Accuracy of Motor-Operated Valve Diagnostic Equipment
Manufactured by Liberty Technologies;" 92-83, "Thrust Limits for Limitorque
Actuators and Potential Overstressing of Motor-Operated Valves;" and 92-70,
"Westinghouse Motor-Operated Valve Performance Data Supplied to Nuclear Power
Plant Licensees."

In addition, the hammering effect was specifically addressed in NRC
Information Notice 85-20, "Motor-Operated Valve Failures Due to Hammering
Effect," and its supplement.

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

                              ORIG /S/'D BY BKGRIMES


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

Technical contacts:  P. K. Eapen, RI
                     (215) 337-5150

                     Thomas G. Scarbrough, NRR
                     (301) 504-2794

Attachment:  
List of Recently Issued NRC Information Notices

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Page Last Reviewed/Updated Tuesday, November 12, 2013