Information Notice No. 94-61: Corrosion of William Powell Gate Valve Disc Holders
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 25, 1994
NRC INFORMATION NOTICE 94-61: CORROSION OF WILLIAM POWELL GATE
VALVE DISC HOLDERS
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 related to the corrosion of carbon
steel disc holders in William Powell gate valves ordered with stainless steel
internal components. 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 December 1991, Farley Nuclear Plant maintenance personnel replaced a gate
valve stem, disc and disc holder assembly for the "Service Water (SWS) to
Component Cooling Water (CCW) System Room Cooler." The stem assembly was
replaced because the holder and disc separated from the valve stem as a result
of severe corrosion on the boss edge (the "ears") of the disc holder.
Although the valve stem moved in the open direction when the valve was
operated, the detached valve disc and holder remained in the closed position
and obstructed SWS flow to the CCW system room cooler.
Southern Nuclear Operating Company (the licensee) and Farley personnel
evaluated the cause of the failure and determined that the carbon steel disc
holder had become corroded. The disc holder is mounted between the stainless
steel stem and disc and is submersed in service water (an electrolytic
solution). This situation subjected the disc holder to localized galvanic
corrosion. The use of a carbon steel disc holder may have been a
misapplication of internal valve component material.
In July 1992, the maintenance/engineering support group (MESG) at Farley
issued a memorandum to Southern Nuclear management describing concerns with
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August 25, 1994
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the disc holders in stainless steel gate valves supplied by William Powell.
In a telephone conversation with MESG, the Vice President of Engineering and
Quality Control at William Powell stated that the valve disc holders might be
stainless steel, carbon steel, or another (alloy) material.
The William Powell company identified the as-shipped disc holder material for
each valve based on specific valve identification, serial, and drawing numbers
supplied by Farley.
In July 1992, the licensee reviewed information supplied by William Powell and
found 22 SWS valves (11 per unit) that were supplied with carbon steel disc
holders and that the licensee considered to be susceptible to the form of
corrosion discovered in December 1991. The licensee had removed 4 of the 22
valves from the system for other Farley plant modifications. The remaining 18
valves were all 25 cm (10-inch) gate valves, used in normal and emergency
containment cooler discharge isolation valve applications. These containment
coolers are necessary to control the pressure inside containment for a
loss-of-coolant accident (LOCA) or a main steam line break.
During a Unit 1 outage in 1992 and a Unit 2 outage in 1993, the licensee
inspected the remaining 18 susceptible valves for evidence of corrosion on the
valve disc holders. Nine of the valves showed little or no signs of galvanic
corrosion or disc holder deformation. Upon further investigation, the
licensee noted that these valve assemblies had been reworked between 1989 to
1991. All nine valves were rebuilt after surveillance tests revealed
excessive stem rotation. The licensee rebuilt the valves using upgraded
William Powell Company valve stem, disc, and disc holder assembly packages in
which the disc holders were made of stainless steel.
The remaining nine valves contained the original valve internal assemblies.
The disc holders in these valves showed varying degrees of deterioration and
galvanic corrosion. The licensee repaired the internal valve components
during the outages.
Southern Nuclear Operating Company determined that the Alabama Power Company,
the original plant licensee, ordered stainless steel valves specifically for
the SWS. The original bid clearly stated that these valves would be used in
such an environment. It appears that the licensee was unaware that the valves
contained carbon steel disc holders. Unit 1 valves had been in service
approximately 14 years, and Unit 2 valves had been in service about 9 years
before the problems noted in 1991. To date only one valve, the aforementioned
room cooler valve, has failed and been considered inoperable.
The corrosion problem can not be detected through external visual inspection
or valve performance until the valve fails completely. Valve disc holders
that are not severely corroded will continue to function normally. The only
way to find the problem before a complete failure is to remove the valves from
service, disassemble them, and inspect the valve internal components.
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August 25, 1994
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This problem is not necessarily limited to SWS environments. The operating
parts of any valve subjected to similar electrolytic conditions could be
susceptible to corrosion if improper materials are installed. The experience
at Farley indicates that even valves specifically purchased for use in raw
water cooling systems may contain inappropriate materials.
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 CIGrimes/for
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Michael J. Morgan, RII
(205) 899-3386 or 3387
Frank Grubelich, NRR
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