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97

Information Notice No. 93­97: Failures of Yokes Installed on Walworth

Gate and Globe Valves

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

December 17, 1993

NRC INFORMATION NOTICE 93‐97: FAILURES OF YOKES INSTALLED ON WALWORTH GATE

AND GLOBE 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 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.

Background

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

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IN 93‐97

December 17, 1993

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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."

Discussion

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

defects.

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IN 93‐97

December 17, 1993
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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

Attachments:

1. Typical Walworth Valve Yoke

2. List of Recently Issued NRC Information Notices

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