Information Notice No. 93-97: Failures of Yokes Installed on Walworth Gate and Globe Valves

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                               December 17, 1993

                               AND GLOBE VALVES


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 the potential for failure of the yokes installed
on Walworth gate and globe valves.  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.


The actuator of each motor-operated valve (MOV) is supported by a yoke.  The
valve yoke maintains the actuator in position and prevents linear and
rotational movement.  The Walworth valve yoke is made of cast carbon steel and
has a plate which supports the motor operator and yoke arms that attach to the
valve body.

Description of Circumstances

On October 1, 1993, during a refueling outage, the licensee of Peach Bottom
Atomic Power Station, Unit 3, notified the NRC resident inspector that it had
discovered cracks in the yokes of several valves manufactured by Walworth. 
The cracks appeared to have originated in the transition between the plate and
yoke arms.  Attachment 1 is a sketch of a typical Walworth valve yoke showing
the general location of the majority of the cracks.

In response to finding these cracks, the licensee inspected those MOVs in
Peach Bottom Atomic Power Station, Unit 2, that were accessible during
operation and found similar cracks in similar places.  As a precaution, the
licensee shut down Unit 2 in order to inspect all Walworth valve yokes.

The licensee found cracks in 9 of 62 Walworth valves of the pressure seal
design with semicircular yoke legs.  The licensee also inspected valves of
other designs and confirmed that the cracking was limited to this type of 


                                                            IN 93-97
                                                            December 17, 1993
                                                            Page 2 of 3

valve.  All of the cracked valves were safety-related components.  The sizes
of the cracked valves ranged from 4 inches to 24 inches.

The licensee determined that the cracking appeared to be due to a combination
of three factors:  (1) a yoke design which concentrated stress in the area of 
the cracks, (2) insufficient bolt torque, which increased the stress, and 
(3) existing casting defects.  The licensee also stated that the failure mode
appeared to be fatigue. 

The licensee repaired the cracks by welding in accordance with the American
Society of Mechanical Engineers Boiler and Pressure Vessel Code.  When
reinstalling yokes on valves that were disassembled for repair, the licensee
torqued the valve operator mounting fasteners with sufficient preload to
reduce joint flexing during valve operation.  

Cracks in Walworth valve yokes have been reported twice before; however, these
reports pertained to welded assemblies rather than to cast assemblies.

On July 8, 1988, the licensee of the Pilgrim Nuclear Power Station, Unit 1,
notified the NRC (Licensee Event Report 88-017 and Revision 1 on July 3, 1989)
of a crack in the yoke flange of an 18-inch Walworth pressure seal globe
valve.  The 270-degree crack originated in the flange-to-yoke leg weld joint. 
The licensee attributed the cause of the cracking to (1) high peak and
discontinuity stresses in the flange-to-yoke weld, (2) low strength flange
material, (3) numerous areas of poor fusion in the weld, and (4) higher-than-
expected motor operator thrusts.

During construction, on October 1, 1973, the licensee of the Peach Bottom
Atomic Power Station, Units 2 and 3, had notified NRC [10 CFR 50.55(e) report]
of a crack in the weld between the yoke and the motor operator mounting plate
of a 12-inch Walworth globe valve.  The licensee stated that the area had
cracked because the effective size of the fillet weld throat had been reduced
by the unauthorized installation of shims between the yoke legs and the
mounting plate.  A copy of this report was attached to NRC Bulletin 74-01,
"Valve Deficiencies."


In Generic Letter (GL) 89-10, "Safety-Related Motor-Operated Valve Testing and
Surveillance," the NRC staff asked nuclear power plant licensees to confirm
the capability of safety-related MOVs to perform their intended functions by
reviewing MOV design bases, verifying MOV switch settings initially and
periodically, testing MOVs under design-basis conditions where practicable,
improving evaluations of MOV failures and necessary corrective action, and
trending MOV problems.  In response to GL 89-10, licensees are implementing
programs to evaluate the design-basis capability of MOVs within the scope of
the generic letter, including the inspection of MOVs for deficiencies and


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Cracks in the valve yoke can result in the inability of the MOV to perform its
safety function.  For example, if the actuator is allowed to rotate or move
linearly by a small amount, the stem can bind, decreasing efficiency in the
conversion of torque to thrust and causing the actuator torque switch to trip
prematurely.  More severe movement could render the actuator unable to open or
close the valve.

For valve yoke designs similar to those of the Walworth valves at Peach
Bottom, the force delivered by the motor actuator may bend the support plate
slightly and cause cracks to develop or propagate at the junction of the plate
and yoke arms.  Loose bolts will induce greater stresses in the yoke mounting
flange area, which could greatly reduce the fatigue life of the yoke.  This
problem could result in common-mode failures affecting both redundant and
diverse safety systems.

Although this notice addresses only the cracking of yokes on valves
manufactured by Walworth, designs of other valve manufacturers may be
susceptible to the development of such cracks.  In addition to direct stress-
related failures, the events above reveal that the use of high thrust settings
can induce fatigue-related failures of MOVs.

Related Generic Communications

NRC has issued other generic communications on overstressing of MOVs.  The
most recent of these is NRC Information Notice 92-83, "Thrust Limits for
Limitorque Actuators and Potential Overstressing of Motor-Operated Valves."

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:  P. K. Eapen, NRC RI        Thomas G. Scarbrough, NRR
                     (215) 337-5150             (301) 504-2794

                     Neal K. Hunemuller, NRR
                     (301) 504-1152

1.  Typical Walworth Valve Yoke
2.  List of Recently Issued NRC Information Notices

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