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98

Information Notice No. 93­98: 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

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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.

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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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