United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-37: Sheared Pinion Gear-To-Shaft Keys in Limitorque Motor Actuators

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

                                May 24, 1990


Information Notice No. 90-37:  SHEARED PINION GEAR-TO-SHAFT KEYS IN 
                                   LIMITORQUE MOTOR ACTUATORS


Addressees:

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose:

This information notice alerts addressees to potential problems related to 
sheared pinion gear-to-shaft keys in Limitorque motor actuators.  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 do not 
constitute NRC requirements; therefore, no specific action or written 
response is required.

Description of Circumstances:

On April 27, 1990, the Public Service Electric & Gas Company (PSE&G) 
submitted a 10 CFR Part 21 report to the NRC regarding a materials 
misapplication in valve motor actuators supplied by the Limitorque 
Corporation at its Salem Nuclear Power Generating Station, Units 1 and 2.  
PSE&G reported that it had discovered that the pinion gear-to-shaft keys in 
the motor actuators of six service water valves (three per unit) were 
sheared or severely deformed.  These keys had failed under conditions of 
normal operation, and one of them showed significant deformation after only 
15 valve cycles.  The valve units use high-speed (3600 rpm) Limitorque SMB-0 
actuators with 25 ft-lb of torque that operate 30-inch Jamesbury butterfly 
valves.  At Salem, this type of valve is only used for the fast isolation 
(closing within 10 seconds) of non-safety thermal loads in the turbine 
building from the safety-related service water system load during accident 
conditions.  These keys failed after the licensee had replaced the original 
valves with the subject Jamesbury valves in a recent upgrade program during 
the last fuel cycle.  The original valves manufactured by the Henry Pratt 
Company had Limitorque SMB-1 actuators.  

The failure identified by PSE&G indicated that the failed keys had wedged 
between the motor shafts and the pinion gears at the key-slots.  These 
wedged keys had allowed the valves to operate under low-flow conditions.  
However, during normal flow conditions, the failure of these keys could 
prevent the valve units from operating reliably.  PSE&G realized the 
magnitude of the 




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problem after performing a thorough inspection following a second valve 
failure during service water valve testing.  The licensee did not initially 
realize the magnitude of the problem because even during normal flow 
conditions, the valve appeared to operate adequately, masking the potential 
for valve failure.

Discussion:

In the past, the NRC issued Information Notices 88-84, "Defective Motor 
Shaft Keys in Limitorque Motor Actuators;" 85-67, "Valve-Shaft-to-Actuator 
Key May Fall Out of Place When Mounted Below Horizontal Axis;" and 81-08, 
"Repetitive Failures of Limitorque Operator SMB-4 Motor-to-Shaft Key," 
regarding the failure of keys in the actuators of motor-operated valves.  
These earlier failures resulted either from use of improper materials or 
incorrect installations, whereas, the failure reported by PSE&G resulted 
from an apparent misapplication of material.

Although the licensee is continuing its investigation, the preliminary 
results indicate that the failures resulted from the impact loading on the 
soft keyway assemblies during fast valve closures.  The licensee performed a 
hardness check of a failed key and of 38 other keys in stock supplied by 
Limitorque.  The inspection confirmed that the material was within the 
ASTM-1018 range for material hardness that was specified by Limitorque.  The 
licensee also inspected all 14 actuators supplied by Limitorque under the 
same purchase order and found the remaining 8 actuators had no similar 
problems.  These 8 actuators are used for valves that are not required to 
close within 10 seconds and are not subject to the same impact loading 
experienced in the failed service water system valves.  

PSE&G decided to replace the failed keys in the 6 valve actuators with keys 
made from a harder material, such as ASTM-4140 stainless steel.  The 
licensee tested one valve with a key made from this new material with a 
hardness of 290 (Brinell) and found no apparent wear after it was operated 
for 60 valve cycles.      

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 NRR project 
manager.




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  Peter C. Wen, NRR
                     (301) 492-1172

                     Paul D. Swetland, RI
                     (215) 337-5114

Attachment:  List of Recently Issued NRC Information Notices
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