Information Notice No. 90-37: Sheared Pinion Gear-To-Shaft Keys in Limitorque Motor Actuators
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
All holders of operating licenses or construction permits for nuclear power
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
May 24, 1990
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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.
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
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Peter C. Wen, NRR
Paul D. Swetland, RI
Attachment: List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015