ML20058F499
| ML20058F499 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 11/24/1993 |
| From: | Marcus G Office of Nuclear Reactor Regulation |
| To: | Harry B PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| References | |
| IEIN-93-097, IEIN-93-97, NUDOCS 9312080152 | |
| Download: ML20058F499 (4) | |
Text
.
bl'~///c277 Nove h 24, 1993 y
Mr. Bruce Harry, Manager Engineering i
Crane Company 104 North Chicago Street Joliet, Illinois 60431
Dear Mr. Harry:
SUBJECT:
REQUEST FOR COMMENTS ON PRGP0 SED INFORMATION NOTICE TITLED " FAILURES OF YOKES INSTALLED ON WALWORTH GATE AND GLOBE VALVES" The U. S. Nuclear Regulator Commission is preparing an information notice (IN) pertaining to the recent cracking of motor-operated valve yokes at the Peach Bottom Atomic Power Station Please review the enclosed draft IN to ensure that the technic.al information is correct and return any comments you may have as soon as possible.
Your cooperation is appreciated.
If no comments are received by December 3, 1993, we will assume the technical info lation in the IN is correct.
If you have any questions regarding this issue, please contact Richard J. Kiessel of my staff at (301) 504-2840.
Sincerely, original signed by Gail H. Marcus, Chief 0100.4 Generic Communications Branch Division of Operating Reactor Support Office of Nuclear Reactor Regulation
Enclosures:
As Stated cc w/ enclosures:
DISTRIBUTION FJMiraglia, NRR w/ enclosure LJCallan, NRP. w/ enclosure BKGrimes, NRR w/ enclosure AEchaffee, NRR w/ enclosure CIGrimes, NRR w/ enclosure GHMarcus, NRR w/ enclosure SHWeiss, NRR w/ enclosure PKEapan, Region I w/ enclosure NRHunemuller, NRR w/ enclosure RJKiessel, NRR w/ enclosure TGScarbrough, NRR w/ enclosure DORS r/f w/ enclosure 0GCB r/f w/ enclosure RJKiessel r/f w/ enclosure Central Files w/ enclosure PDR w/ enclosure OFFICE _
OGCB/ DORS /NRR C:0GCB/ DORS /NRR
[/ [//[
GHMarcus [rOU NAME RJKiessel l
DATE ll/fN93 11/2'(/93 DOCUMENT hAME:
letter.3b2 NBC HIE CfNTH C9PY 9312000 g % $$7i PDR ADO PDR Q
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1 UNITED STATES MM3 NUCLEAR REGULATORY COMMISSION WMM 0FFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C.
20555 November 24, 1993 NRC INFORMATION NOTICE 93-XX:
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, sugge::tions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required.
Backaround 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.
DescriDtion of CircumstanC_R1 C
On October 1, 1993, the licensee of Peach Bottom Atomic Power Station, Unit 3, notified the NRC resident inspactor that it had discovered cracks in the yokes' of several valves manufactureG
' Walworth during its ongoing refueling outage.
The cracks appeared tt originate in the transition between the plate and yoke arms.
In response to finding these cracks the licensee inspected those MOVs in Peach Bottom Atomic Power Station, ait 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.
r 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 valve. All of the cracked valves were safety-related components. The sizes of the cracked valves ranged from 4 inchr to 24 inches.
The licensee determined that the cracking appeared to be due to a combination of three factors: (1) a marginal design resulting in high stress in the area DRAFT
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DRAFT of' the cracks, (2) insufficient bolt torque, which increased the stress, and (3) existing manufacturing defects. The licensee also stated that the failure mode appeared to be fatigue rather than a rapid failure mechanism.
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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 keep the joints from separating. The licensee stated that this would minimize the stress in the yoke area affected by the cracking.
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, l
notified 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.
On October 1, 1973, the licensee of the Peach Bottom Atomic Power Station, Units 2 and 3, 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 license'e 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-0perated Valve Tes.m 'nd Surveillance," the NRC staff asked nuclear power plant licensees to confit.
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.
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 i
prematurely. More severe movement could render the actuator unable to open or close the valve.
For valves like 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 may exacerbate the problem. This problem could result in common-mode failures affecting both redundant and diverse safety systems.
DRAFT
DRAFT Although this notice only addresses the cracking of yokes on values manufactured by Walworth, designs of other valve manufacturers may be susceptible to the development of such cracks.
For example, the Walworth design is now owned by Crane Company.
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-0perated Valves."
This information notice r'equires 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.
i Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts:
P. K. Eapen, NRC Region I (215) 337-5150 Neal K. Hunemuller, NRR (301) 504-1152 Thomas G. Scarbrough, NRR (301) 504-2794
Attachment:
List of Recently Issued NRC Information Notices DRAFT
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