Failures of Nut-Locking Devices in Check Valves
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 19, 1993
NRC INFORMATION NOTICE 93-16: FAILURES OF NUT-LOCKING DEVICES IN CHECK VALVES
All holders of operating licenses or construction permits for nuclear power
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems found with the nut-locking devices in
certain check valves. 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
Trojan Nuclear Plant
On December 20, 1991, the Trojan Nuclear Plant was in mode 5, cold shutdown,
for a refueling outage. Portland General Electric Company (the licensee) was
conducting routine inservice testing of the residual heat removal (RHR) pumps.
While testing the A RHR pump, operators noticed that indicated flow was lower
than expected and the B RHR pump was rotating backwards, indicating the B RHR
pump discharge check valve was leaking.
On January 21, 1992, the licensee inspected the B RHR pump discharge check
valve. The licensee discovered the nut, washer, and lock wire which fasten
the disk to the swing arm (see diagram, Attachment 1) were missing. On
January 28, 1992, the licensee inspected the A RHR pump discharge check valve.
The licensee discovered the nut was loose and the lock wire was broken,
although the disk nut and washer were in place.
The licensee reported this event to the NRC, as a condition that alone could
have prevented the fulfillment of the safety function of systems needed to
mitigate the consequences of an accident. The licensee based this
determination on the potential scenario of the discharge check valve disks
for both trains separating from the swing arms, falling into the valve bodies,
and impeding RHR flow.
February 19, 1993
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Vogtle Electric Generating Plant, Unit 1
On April 2, 1992, the Vogtle Electric Generating Plant, Unit 1, was operating
at 100 percent power. Georgia Power Company (the licensee) reduced power to
65 percent when the internals of the "B" feedwater (FW) pump discharge check
valve failed. The licensee inspected the check valve and discovered the
valve disk and hanger assembly were not in place. Subsequently, the licensee
inspected the downstream piping and found the disk wedged in a reducer section
of the pipe. The licensee retrieved two hangar capscrews and a portion of the
hanger capscrew locking device from a downstream FW heater.
Trojan Nuclear Plant
The Trojan Nuclear Plant licensee determined the root cause of the check valve
failures to be inadequate design of the valve disk nut locking device.
Inadequate disk nut torquing had allowed the nut to rotate slightly back and
forth. The free motion at the nut/lock wire interface induced low stress,
high cycle fatigue and eventual failure of the lock wire. Nut motion had been
caused by flow induced vibration and by normal operational actuation movement
of the valve internal components.
The licensee believes this problem is limited to valves actuated frequently or
to cases where the associated piping has undesirable flow characteristics.
The valves in question are Copes-Vulcan Model 8C74 valves. The licensee
believes this failure mechanism is not limited to Model 8C74 valves, as
similar nut locking device design (i.e., thin gauge lock wire) exist in other
model Copes-Vulcan check valves. Licensee repair of the check valves
consisted of torquing the disk retaining nut and replacing the lock wire with
a 1/8-inch cotter pin.
The licensee examined five other Copes-Vulcan valves but no other lock wires
had failed. In addition, the licensee torqued all retaining nuts and replaced
all lock wires with cotter pins on the valves examined.
The licensee modified its check valve inspection program to require
documentation of the type and condition of all valve retaining nut locking
devices, including the extent to which retaining nut rotation was limited by
the locking device.
February 19, 1993
Page 3 of 4
Vogtle Electric Generating Plant
The Vogtle Unit 1 licensee determined the root cause of the check valve
failure to be inadequate procedural guidance on check valve maintenance.
Procedural deficiencies identified included lack of specifications for
(1) capscrew torque requirements; (2) the correct number of hangar capscrews
to be used; and (3) the correct use and/or reuse of hangar capscrew locking
devices. The hangar capscrew locking device, shown in Attachment 2, is a
stainless steel plate used to prevent the inadvertent loosening of the hanger
capscrews. The locking devices undergo plastic deformation during
installation. Reuse of the locking devices results in work hardening of the
material, making it more susceptible to unacceptable levels of cracking. As a
result of procedural deficiencies, some valves were reassembled without
locking devices, or with used locking devices. Locking device failure can
allow the hanger capscrews to loosen and the hanger to separate from the body
of the valve.
Valve failure may also have been partially caused by the use of an incorrect
number of capscrews. The valve body is drilled and tapped to accept three
capscrews. The locking device and hanger shims are predrilled for use with
three capscrews. The disk hangers (also illustrated on Attachment 1) removed
from the Vogtle Unit 1 check valves were drilled for use with only two
capscrews. Both Unit 1 check valves had previously undergone hanger
replacement, as evidenced by warehouse records. The disk hangers supplied by
Pacific Valve, as stock spares, are blanks which must be drilled at the time
of installation to ensure proper disk/seat alignment. The licensee believes
Pacific Valve may have originally supplied check valves with disk hangers
drilled for use with only two capscrews. This belief was based on an
inspection of a Vogtle Unit 2 check valve. The original disk hanger for the
pre-assembled valve was drilled for use with only two capscrews.
The licensee is revising its check valve maintenance procedure to ensure the
hanger capscrews are adequately torqued and the required locking device is
correctly installed. In addition, Pacific Valve Engineering is being asked to
specify the correct number of capscrews required for integrity of the
hangar/body bolted joint, and to identify any differences between design and
Related Generic Communication
In Information Notice 81-35, "Check Valve Failures," the NRC discussed an
event similar to the failure at Trojan involving Anchor Darling check valves.
The Anchor Darling check valve failures were attributed to the valve being
assembled with lock wire used as the retaining nut locking device, rather than
a locking pin as shown on the design drawing.
February 19, 1993
Page 4 of 4
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.
ORIGINAL SIGNED BY
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Eric J. Benner, NRR
Francis Grubelich, NRR
1. Copes-Vulcan Check Valve Internals
2. Copes-Vulcan Hanger Capscrew Locking Device
3. List of Recently Issued NRC Information Notices.
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