Information Notice 1992-25, Pressure Locking of Motor-Operated Flexible Wedge Gate Valves

Pressure Locking of Motor-Operated Flexible Wedge Gate Valves

ML031200604
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Crane
Issue date:	04/02/1992
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-92-026, NUDOCS 9203270028
Download: ML031200604 (5)

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

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 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> after the test was completed, a control room operator

attempted to open the inboard valve. The valve actuator energized for approxi- mately 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 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, IE Information Notice 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.

IN 92-26 April 2, 1992 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.

XA-

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

Attachment 1

IN 92-26

April 2, 1992

. Pressure Locking

Flexible-Wedge Gate Valve

Figure 1

Attachment 2-:

a-- IN 92-26"

April 2, 1992i LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information

Date of

Notice No.

Subject

Issuance

Issued to

92-25

92-24

92-23

92-22

92-21

Potential Weakness in

Licensee Procedures for A

Loss of the Refueling

Cavity Water

Distributor Modification to

Certain Commercial-Grade

Agastat Electrical Relays

Results of Validation Test- ing of Motor-Operated Valve

Diagnostic Equipment

Criminal Prosecution and

Conviction of Wrongdoing

Committed by A Commercial-

Grade Valve Supplier

Spent Fuel Pool Reactivity

Calculations

Inadequate Local Leak Rate

Testing

Misapplication of Potter &

Brumfield MDR Rotary Relays

Potential for Loss of Re- mote Shutdown Capability

during A Control Room Fire

NRC Inspections of Pro- grams being Developed at

Nuclear Power Plants in

Response to Generic

Letter 89-10

03/31/92

03/30/92

03/27/92

03/24/92

03/24/92

All holders of OLs or CPs

for nuclear power reactors.

All holders of OLs or CPs

for nuclear power reactors.

All holders of OLs or CPs

for nuclear power reactors

and all vendors of motor- operated valve (MOV) diag- nostic equipment.

All holders of OLs or CPs

for nuclear power reactors.

All

for

holders of OLs or CPs

nuclear power reactors.

92-20

92-19

03/03/92

03/02/92

02/28/92

02/26/92

All holders of OLs or CPs

for nuclear power reactors.

All holders of OLs or CPs

for nuclear power reactors.

All holders of OLs or CPs

for nuclear power reactors.

All holders of OLs or CPs

for nuclear power reactors.

92-18

92-17 OL = Operating License

CP = Construction Permit