Information Notice No. 92-26: Pressure Locking of Motor-Operated Flexible Wedge Gate Valves

                               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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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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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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