United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-31: Motor-Operated Valve Failure Caused by Stem Protector Pipe Interference

                          UNITED STATES
                  NUCLEAR REGULATORY COMMISSION
               OFFICE OF NUCLEAR REACTOR REGULATION
                   WASHINGTON, D.C.  20555-0001

                          August 9, 1995


NRC INFORMATION NOTICE 95-31:  MOTOR-OPERATED VALVE FAILURE CAUSED
                               BY STEM PROTECTOR PIPE INTERFERENCE


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 of motor-operated valve (MOV) failures caused by the stem protector pipe
interfering with stem nut rotation.  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

A stem protector pipe may be attached to an MOV through the housing cover to prevent debris
from entering the stem/actuator interface area (see Figure 1).  To keep the stem protector pipe
from interfering with actuator operation (specifically rotation of the stem nut and its locknut), the
threads on the pipe may be restricted to a certain length.  Another option is to stake the threads
on the stem protector pipe at a specific location.  If neither of these precautions is taken, the
stem protector pipe may thread sufficiently into the actuator housing to interfere with the
rotation of the stem nut locknut.  Additional torque may be required to operate the valve, which
may cause the torque switch to trip prematurely, motor thermal overload devices to activate, or
the motor to be damaged on high torque demand.

Description of Circumstances

On February 11, 1995, a residual heat removal (RHR) MOV at Cooper Nuclear Station failed to
close on demand while RHR trains were being switched.  Licensee reviews showed that the
stem protector pipe on the valve actuator had threaded into the MOV housing and interfered
with the stem nut rotation.  The motor was damaged during an attempted opening stroke.  The
replacement stem protector pipe was manufactured by the licensee.  The root cause apparently
was that a replacement stem protector pipe was not constructed to the same tolerance levels
as the original.  Specifically, the length of threaded portion of the protector pipe was too long
when manufactured.  The extended threads allowed the pipe to be threaded to the point where
it interfered with 


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the stem nut locknut.  Also, actions (such as staking) were not taken to prevent threading of the
stem protector pipe into the valve actuator housing.  Three other valves were found to have
stem protector pipes with extended threads that might have interfered with actuator operation. 
The licensee long-term solution was to stake the extended threads in all of the stem protector
pipes of safety-related MOVs.

In addition to the Cooper event, similar failures have occurred at four other plants as reported in
the Nuclear Plant Reliability Data System.  These failures are discussed below.

On May 24, 1992, an RHR pump discharge valve failed to close on demand.  The licensee
review showed the stem protector pipe had been threaded too far into the operator and had
interfered with actuator operation.  This also caused a gear to crack.

On August 14, 1991, an RHR suppression pool full flow discharge isolation valve failed to fully
close on demand.  The cause of the failure was that the stem protector pipe had interfered with
the stem locknut.  Two years earlier on January 23, 1989, the plant had experienced the same
failure of the RHR suppression pool full flow discharge isolation valve; the problem was
corrected by removing approximately one inch of threads from the stem protector pipe.  After
the second failure, the licensee solution was to stake the threads of the stem protector pipe.  

On March 18, 1988, a low-pressure core spray pump suction valve would not stroke manually
or electrically.  The licensee analysis indicated that the stem protector pipe had been threaded
too far into the actuator.

On March 6, 1987, an RHR outboard containment isolation valve tripped on thermal overload
while being stroked open.  The valve was still operable by hand.  The licensee review showed
that the stem protector pipe had been inserted too far and had interfered with the actuator.

Discussion

The NRC staff noted that the failure occurred at Cooper Nuclear Station because the stem
protector pipe was manufactured with an extensive thread length.  The licensee constructed
and installed its own stem protector pipe without taking adequate precautions to prevent the
protector pipe from interfering with stem nut rotation.  This incident and the earlier problems
indicate that such MOV failures are a recurring problem.  The trend toward "in house"
manufacture of components could result in an increase in this type of failure.  The licensee
could have prevented this type of failure by manufacturing the stem pipe protector with the
same tolerance levels as those made at the factory or adequately staking the stem protector
pipe during the installation process.

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                                                                   August 9, 1995
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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 contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

                              /s/'d by DMCrutchfield


                              Dennis M. Crutchfield, Director
                              Division of Reactor Program Management
                              Office of Nuclear Reactor Regulation

Technical contacts:  Thomas G. Scarbrough, NRR         Glenn T. Dentel, NRR
                     (301) 415-2794                    (301) 415-1321

                     Thomas Greene, NRR
                     (301) 414-1175

Attachments:  
1.  Figure 1:  Valve Actuator and Protector Pipe 
2.  List of Recently Issued NRC Information Notices



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