United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-41: Consideration of the Stem Rejection Load in Calculation of Required Valve Thrust

                              UNITED STATES
                         WASHINGTON, D.C.  20555

                              May 29, 1992

                               IN CALCULATION OF REQUIRED VALVE THRUST


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


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the fact that a valve vendor (Anchor Darling
Valve Co.) did not include the stem rejection load in its calculation of
the minimum thrust requirements for some of its motor-operated valves
(MOVs).  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 February 12, 1992, the General Electric Company (GE) notified the NRC
by letter that it had found instances in which Anchor Darling had not
included the stem rejection load in its calculation of the minimum thrust
requirements for some motor-operated gate valves.  GE indicated that
Anchor Darling representatives had stated that their standard practice was
not to include the stem rejection load if it was less than 20 percent of
the disk force.  Further, when the stem rejection load exceeded 20 percent
of the disk force, they would only include the excess above 20 percent. 
GE stated that the licensees of boiling water reactor (BWR) plants were
including the stem rejection load in their minimum thrust calculations in
their responses to Generic Letter (GL) 89-10, "Safety-Related Motor-
Operated Valve Testing and Surveillance." 

On March 18, 1992, the Commonwealth Edison Company notified the NRC by
letter of a similar concern.

During a telephone conversation with the NRC, Anchor-Darling stated that
in December 1980 they began including the stem rejection load in
calculations of the minimum thrust requirements for all motor-operated
gate valves.  


                                                            IN 92-41
                                                            May 29, 1992
                                                            Page 2 of 2


A primary reason for developing GL 89-10 was the concern that valve
vendors had underestimated the thrust required to operate their valves
under design-basis conditions.  This concern has been verified for some
valves under certain differential pressure and flow conditions through
operating events, motor-operated valve research, and licensee testing as
part of their GL 89-10 programs.  As licensees have obtained additional
information from their own MOV tests and other sources, they have found
that some MOVs required upgrading to ensure their design-basis operabil-

Valve vendors typically have predicted the minimum thrust required to
operate valves by calculating the sum of the differential pressure load
(the product of the valve factor, the differential pressure, and the disk
flow area), the stem rejection load (the product of the system pressure
and the stem area), and the packing load.  In its Gate and Globe Valve
Operator Selection Procedure SEL-3 (February 26, 1979), the actuator
manufacturer (Limitorque Corporation) states that stem rejection load may
be neglected if system pressure is below 500 pounds per square inch. 
However, Limitorque also states that further consideration may be
necessary where the running load (the sum of stem rejection load and
packing load) constitutes more than one-third of the total required

The fact that Anchor Darling did not consider stem rejection load before
December 1980 provides additional evidence that thrust requirements for
some MOVs may be underestimated.  This condition may also apply to other
valve vendors in that they may have not included the stem rejection load
in their minimum thrust calculation.  The implementation of GL 89-10
programs by licensees on a prompt basis will likely resolve this issue. 

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 (NRR) project manager.

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical contacts:  Thomas G. Scarbrough, NRR
                     (301) 504-2794
                     Edmund J. Sullivan, Jr., NRR
                     (301) 504-3266

Attachment:  List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Tuesday, November 12, 2013