United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-55: Problems with Copes-Vulcan Pressurizer Power-Operated Relief Valves

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                                August 4, 1994


NRC INFORMATION NOTICE 94-55:  PROBLEMS WITH COPES-VULCAN PRESSURIZER POWER-
                               OPERATED RELIEF 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 alert addressees to problems involving cracking of plug material,
severe wear of plugs and cages, and a problem with the misalignment and
galling of a stem in Copes-Vulcan power-operated relief valves (PORVs).  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

On April 7, 1994, at the Salem Generating Station, Unit 1, a reactor trip was
followed by two automatic actuations of the safety injection (SI) system.  The
continued injection of water from the safety injection system filled the
pressurizer steam space with subcooled water and, without the normal
pressurizer steam space to dampen pressure excursions, resulted in repeated
actuation of the plant PORVs to limit reactor coolant system pressure.  Salem
Unit 1 has two pressurizer PORVs, each of which is actuated through separate
automatic controls.  During the event, one PORV (1PR-2) cycled at least
200 times, and the other PORV (1PR-1) cycled at least 100 times.  The fluid
inlet conditions to the valves were liquid water at a temperature and pressure
of approximately 290�C (550�F) and 16,000 kPa (2300 psia).  The PORVs are 
2-inch, air-operated valves manufactured by Copes-Vulcan that have a plug and
cage-type internal trim design.  (See Figure 1 for a detail of the valve
trim.)

Discussion

The Copes-Vulcan PORV design used at Salem Unit 1 has a plug which is guided
by a cage; close clearances exist between the outside diameter of the plug and
the inside diameter of the cage.  When the valve opens, fluid in the system
flows from under the plug through several equally spaced ports in the cage, 

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and then to the valve outlet.  When the valve closes, the plug seats against a
machined surface of the cage.  The cage is positioned on a gasket in a close-
clearance counterbore in the valve body.  The stem transmits the motive force
from the air actuator to the plug and is threaded into the plug.  A steep
taper section on the stem just above the threaded section produces a wedging
action at the relatively thin pinning boss.  The valve stems are made of
Type 316 austenitic stainless steel, and the plug and cage are both made of
Type 420 hardened martensitic stainless steel.

Following the event of April 7, Public Service Electric and Gas Company (the
licensee) removed both sets of PORV internal components from Unit 1 for
inspection.  The licensee inspected the internal components and discovered
three deficiencies:  (1) scoring in the plug and cage area (both valves),
(2) axial cracking on the pinning boss through which the anti-rotation roll
pin passes (both valves), and (3) galling on the stem where it passed through
the bonnet (1PR-2 only).  The scoring on the 1PR-2 plug and cage was more
severe than on 1PR-1.  

The licensee believes the scoring found on the plug and cage of 1PR-2 was the
result of out-of-tolerance machining of the inside diameter of the cage and
the increased thickness caused by deposition of material as the scoring
occurred.  In the area of the scoring, the inside diameter of the cage of this
valve was between 0.038 to 0.046 mm (1.5 to 1.8 mils) less than that allowed
by the tolerance provided by the manufacturer.  The inside diameter of the
cage of the 1PR-1 PORV was within the allowable tolerance.  The fewer number
of operational cycles together with the dimensional conformance of the 1PR-1
internal components may have caused less scoring in 1PR-1 than in 1PR-2.

The licensee determined that the cracks in the pinning boss were caused by
intergranular stress corrosion cracking (IGSCC).  The extent of cracking was
similar for both 1PR-1 and 1PR-2, extending from the top of the pinning boss,
above the anti-rotation pinhole, and continuing down to just below the top of
the plug.  Metallographic examinations of the cracked regions showed that the
cracking followed intergranular morphology, and the results of fractographic
examinations confirmed that cracks initiated at the top and bottom surfaces of
the pinholes and progressed axially outward from the anti-rotation pinhole in
the pinning boss.  There was no evidence that fatigue contributed to the
failure.  Other factors that could promote IGSCC include the preload stresses
applied when the valve stem and plug are assembled together by the
manufacturer and stresses due to differential thermal expansion.  The licensee
found similar cracking, though less prominent than the cracking in 1PR-1 and
1PR-2, on internal components of valves maintained in the warehouse as new
spare valves.  

The licensee performed stress and fracture mechanics analyses to evaluate the
stress condition in the valve plug and to assess the potential for additional
crack growth.  These analyses indicated that differential thermal expansion of
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                                                            August 4, 1994
                                                            Page 3 of 4


the stem and plug materials causes significant stresses in the pinning boss. 
In addition, because of the steep taper wedging action in the stem-to-plug
assembly, high stress concentrations are found in the vicinity of the anti-
rotation pin hole of the plug boss.  The licensee determined that continued
crack growth further into the much heavier plug itself is possible, if the
plug is left in service.

To prevent further problems with the internal components made of Type 420
stainless steel, the licensee has installed plugs made of Type 316 stainless
steel overlaid with Stellite and cages made of 17-4 PH stainless steel in the
Unit 1 PORVs.  The design of the replacement plug eliminates the pinning boss
used for the Type 420 stainless steel plug, thus eliminating the tendency for
crack formation in the thin boss section.  The stem is now pinned to a thick
section of the plug rather than through the relatively thin boss, and the plug
height has been increased (to account for the elimination of the boss) to
provide the same stroke length as before.  The licensee determined that the
Unit 2 PORVs did not require modification because their plugs and cages are
made of 17-4 PH stainless steel.

The manufacturer has determined that Salem is the only nuclear power facility
that installed the Type 420 stainless steel internal components in Copes-
Vulcan pressurizer PORVs.  However, the manufacturer believes there may be
other Copes-Vulcan valves in nuclear service utilizing Type 420 material for
the internal components and these components could experience similar
problems.

The licensee determined that the galling of the Type 316 stainless steel stem
in 1PR-2 was caused by the tight clearance between the stem and the bonnet
stem guide, together with a misalignment of valve internals that occurred
during field assembly.  If the galling had continued, the stem could have
become bound to the bonnet, preventing the valve from responding to the air
operator.  

The licensee has revised the installation procedures for the internal
components and the bonnet to reduce the possibility of misalignment.  The
revised procedures require periodically stroking the valve by hand during the
assembly process to ensure smooth movement and to improve stem and plug
assembly centering.  Because the clearances between the plug and cage and
between the stem and bonnet are very small, the manufacturer recommends
manually stroking the stem and plug before and after bonnet torquing and after
installing packing regardless of the material used in the valve internal
components.

In summary, this information notice raises questions with respect to:  (1) the
use of Type 420 stainless steel in this or similar valve applications,
(2) valve component misalignment during field assembly, (3) out-of-tolerance
machining on Copes-Vulcan PORVs, and (4) high stress concentrations because of
the design of the internal components of Copes-Vulcan PORVs.
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                                                            August 4, 1994
                                                            Page 4 of 4


Related Generic Communications

The NRC issued Bulletin 89-02, "Stress Corrosion Cracking of High-Hardness
Type 410 Stainless Steel Internal Preloaded Bolting in Anchor Darling Model
S350W Swing Check Valves or Valves of Similar Design," on July 19, 1989,
following the discovery of IGSCC of Type 410 martensitic stainless steel, a
material similar to Type 420 martensitic stainless steel.

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 BKGrimes


                                   Brian K. Grimes, Director
                                   Division of Operating Reactor Support
                                   Office of Nuclear Reactor Regulation


Technical Contacts:  Charles G. Hammer, NRR           James A. Davis, NRR
                     (301) 504-2791                   (301) 504-2713

                     Harold I. Gregg, RI              Cheryl D. Beardslee, RI
                     (610) 337-5295                   (610) 337-5084
 

Attachments:
1.  Figure 1, Copes-Vulcan Power-Operated
    Relief Valve Trim Detail

Page Last Reviewed/Updated Friday, November 15, 2013