United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-21: Potential Failure of Motor-Operated Butterfly Valves to Operate Because Valve Seat Friction was Underestimated

                                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
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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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