Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 2, 1992
NRC INFORMATION NOTICE 92-26: PRESSURE LOCKING OF MOTOR-OPERATED
FLEXIBLE WEDGE GATE VALVES
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 describe a mechanism by which flexible wedge gate valves could
become inoperable because of pressure locking. 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
In July 1991, the New York Power Authority, licensee for the James A.
Fitzpatrick Nuclear Power Plant, was performing a 2100 psig hydrostatic test
of the piping in the low pressure coolant injection (LPCI) system. The
piping being tested was between the inboard and outboard LPCI injection
valves. The inboard valve is a motor-operated 24-inch flexible wedge gate
valve manufactured by William Powell Company. Upon completing the test, the
licensee depressurized the piping between the valves and filled and vented
the system to return it to service. About 10 hours after the test was
completed, a control room operator attempted to open the inboard valve. The
valve actuator energized for approximately 30 seconds, after which the motor
actuator circuit breaker tripped. (The valve normal stroke time is about 120
seconds.) The licensee determined the root cause of the actuator motor
failure to be pressure trapped between the wedges of the flexible wedge gate
valve. This phenomenon is known as "pressure locking" (see Figure 1). The
licensee determined that other flexible wedge gate valves at its plant are
susceptible to this failure mechanism.
The licensee identified a concern that the flexible wedge gate valves could
become pressure locked during normal plant operation and may not function
during an accident. For example, if a check valve is exposed to high
reactor coolant pressure and is in series with a gate valve, the gate valve
can become pressure locked in the following manner. The coolant can leak
past the check valve and, over time, the pressure in the piping between the
check valve and gate valve can increase. Eventually, the pressurized side
of the flexible disk moves slightly away from its seat, allowing fluid to
enter the bonnet cavity.
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IN 92-26
April 2, 1992
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With time, the bonnet cavity pressure and pipe pressure will tend to
equalize at reactor coolant pressure. If an accident such as a
loss-of-coolant-accident (LOCA) occurred, the pressure in the piping would
be rapidly reduced. The pressurized side of the disk would move back
against its seat, trapping high pressure fluid in the bonnet cavity. If no
internal or external path is provided to equalize the pressure in the
bonnet, the valve may become pressure locked.
Prior to the event at Fitzpatrick, the plant had experienced problems with
pressure locking of double disk gate valves in 1988, but did not recognize
the potential problems with flexible wedge gate valves. The licensee
believed that its flexible wedge gate valves would not become pressure
locked because any pressure trapped in the valve bonnet cavity would cause
the wedge to compress and would allow the pressure within the bonnet cavity
to decrease. This proved not to be the case.
In taking corrective action, the licensee modified the valve by providing a
vent path to release the high pressure between the disks and in the bonnet
cavity.
On October 18, 1991, the Pennsylvania Power and Light Company, licensee for
the Susquehanna Steam Electric Station, Units 1 and 2, similarly reported
that some of its motor-operated flexible wedge gate valves were susceptible
to this pressure locking phenomenon. The licensee for Susquehanna also
provided a pressure relief path from the valve cavities to prevent an
internally pressurized condition.
Discussion
The recent reports for the Fitzpatrick and Susquehanna plants indicate that
previous operating experience feedback has not been completely effective in
addressing the problem of hydraulic locking at all plants.
The potential for valve operability problems caused by excessive pressure in
the valve bonnet has been known for many years in the nuclear industry. The
NRC documented the problem in 1977 in NRC IE Circular 77-05, "Liquid
Entrapment in Valve Bonnets." In 1981, Information Notice No. 81-31,
"Failure of Safety Injection Valves to Operate Against Differential
Pressure," was issued based on a 1981 San Onofre event involving the Safety
Injection MOVs. In July of 1984, AEOD issued a study on the pressure
locking phenomenon, AEOD/S402, "Pressure Locking of Flexible-Disk Wedge-Type
Gate Valves." Again, in September 1988, the NRC issued Information Notice
88-72, "Inadequacies in the Design of DC Motor-Operated Valves." This
information notice discussed both pressure locking and thermal binding of
gate valves, but was primarily concerned with valve operator problems.
Once a valve has been identified as susceptible to hydraulic locking, all
safety functions performed by that valve need to be carefully evaluated to
determine the appropriate solution. Changing the size of the operator or
using a different valve design may be appropriate. Providing a vent path
may be an acceptable solution, but only after a careful review of all the
functions of the valve. For example, a vent path will not be acceptable if
leaktightness in both directions is required.
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IN 92-26
April 2, 1992
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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
the technical contact 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 contact: T. Greene, NRR
(301) 504-1175
Attachments:
1. Figure 1, Pressure Locking Flexible-Wedge Gate Valve
2. List of Recently Issued NRC Information Notices
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