Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 22, 1990
Information Notice No. 90-21: POTENTIAL FAILURE OF MOTOR-OPERATED
BUTTERFLY VALVES TO OPERATE BECAUSE
VALVE SEAT FRICTION WAS UNDERESTIMATED
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to the potential for
motor-operated butterfly valves to fail to open on an electrical signal as a
result of friction forces exerted on the valve seats that exceed the values
assumed when selecting the motor actuators and setting the torque switches.
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:
In October 1988, at Catawba Nuclear Station Unit 1, a motor-operated
butterfly valve in the service water system failed to open under high
differential pressure conditions. Following the valve failure, the licensee
concluded that the valve manufacturer, BIF/General Signal Corporation, had
underestimated the degree to which the material used in the valve seat would
harden with age (the responsibility for these valves has been transferred to
Paul-Munroe Enertech). This underestimation of the age hardening had led
the manufacturer to assume valve seat friction forces that were less than
the actual friction forces in the installed valve. To overcome the
larger-than-anticipated friction forces, the licensee's engineering staff
recommended the open torque switch for 56 butterfly valves be reset to the
maximum allowable value. These valves are required to open to satisfy their
safety function and were supplied by this manufacturer to Catawba Units 1
and 2. The systems in which these valves are located include the component
cooling water system, service water system, and various ventilation systems.
By July 26, 1989, the torque switch adjustments were completed at Catawba
Units 1 and 2. After reviewing the final settings, the licensee's
engineering staff determined that the actuators for three butterfly valves
in the component
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IN 90-21
March 22, 1990
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cooling water system might not be able to overcome the friction forces
resulting from maximum seat hardening. On December 13, 1989, the licensee
determined that the failure of these BIF/General Signal motor-operated
valves (MOVs) could cause a loss of cooling water to residual heat removal
system heat exchangers. To resolve the concern regarding the operability of
these BIF/General Signal valves, a torque switch bypass was installed on two
of the actuators to allow full motor capability during opening. The third
actuator was considered to be operable because of a recent replacement of
the seat material, but an open torque switch bypass will be installed in
that actuator at a later date. This situation is described in detail in
Catawba Licensee Event Report 89-29, dated January 15, 1990.
Discussion:
The underestimation of the friction forces that occur as a result of age
hardening of the seat material could lead to the common mode failure of a
large number of motor-operated butterfly valves to open on an electrical
signal. In the Catawba case, the licensee determined that the torque
switches of 56 valve actuators in several important plant systems required
adjustments. A database search shows 12 other reactor units having a
combined total of approximately 300 butterfly valves from this manufacturer.
The affected valves are located in such plant systems as high pressure
coolant injection, service water, and standby gas treatment.
In addition to the BIF/General Signal valves, motor-operated butterfly
valves supplied by other manufacturers might fail to operate properly if the
manufacturers underestimated friction forces during the selection of the
motor actuators and the trip setpoints for the torque switches. For
example, in Information Notice 88-94, dated December 2, 1988, "Potentially
Undersized Valve Actuators," the staff stated that past inaccuracies in the
method used to predict valve friction forces had led to the potential for
undersized actuators on certain motor-operated butterfly valves manufactured
by Fisher Controls International. As a result, the concern with regard to
the effect of various friction losses on the operability of motor-operated
butterfly valves is applicable to all such valves.
On June 28, 1989, the NRC issued Generic Letter 89-10, "Safety-Related
Motor-Operated Valve Testing and Surveillance," to all holders of nuclear
power plant operating licenses and construction permits. In the generic
letter, the NRC staff requested that the addressees establish a program to
provide for the testing, inspection, and maintenance of safety-related MOVs
and certain other MOVs in safety-related systems. The scope of Generic
Letter 89-10 includes motor-operated butterfly valves in safety-related
systems. One of the factors contributing to the need for the generic letter
was the uncertainty in the analytical techniques used by licensees and valve
vendors in selecting motor actuators for valves and setting their torque
switches. The potential failure of butterfly valves discussed in this
information notice is evidence of that uncertainty.
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IN 90-21
March 22, 1990
Page 3 of 3
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please call one
of the technical contacts listed below or the appropriate NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Thomas G. Scarbrough, NRR
(301) 492-0916
John W. Thompson, NRR
(301) 492-1175
Attachment: List of Recently Issued NRC Information Notices
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