Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 86-93
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
November 3, 1986
Information Notice No. 86-93: IEB 85-03 EVALUATION OF MOTOR-OPERATORS
IDENTIFIES IMPROPER TORQUE SWITCH SETTINGS
Addressees:
All nuclear power reactor facilities holding an operating license or a
construction permit.
Purpose:
This notice is provided to alert recipients of a potentially significant
safety problem discovered while performing the evaluation requested by IE
Bulletin 85-03, "Motor-Operated Valve Common Mode Failures During Plant
Transients Due to improper Switch Settings,"(IEB 85-03). It is expect view
the information for applicability to their facilities and consider actions,
if appropriate, to preclude similar problems from occurring at their
facilities. However, suggestions contained in this notice do not constitute
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
As a result of followup on IEB 85-03, Duke Power Company (DPC) discovered
problems with valves operated by Rotork valve actuators at McGuire Nuclear
Station. Specifically, the problem involved valves for which the factory-set
torque switch settings had been previously changed at the plant site using a
generic correlation between actuator torque output and torque switch
setting. This could cause valve actuator motors to switch off before the
valves complete their travel. Arbitrarily raising the torque switch setting
to its maximum may result in damage to the valve and/or motor especially
since thermal overload protection has been eliminated in many applications.
Based upon this information, DPC has declared safety systems inoperable and
shut down McGuire Units 1 and 2.
Discussion:
The vendor states that whenever the factory torque switch setting is changed
in the field, an individual calibration curve or a bench test is required to
accurately determine torque output.
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IN 86-93
November 3, 1986
Page 2 of 3
According to information recently provided by the vendor, torque switch
settings of 1, 2, 3, 4, and 5, do not always correspond to rated torque
output values between 40 and 100 percent as was used based on general
information available several years ago. Tests and analytical evaluations by
the licensee now confirm that the correlation used by the licensee was
incorrect for some actuators. For example, evaluation of several valve
actuator certificates for the same model revealed that the actual torque
output with a switch setting of "1" varied through a range of 11 to 55
percent of maximum actuator torque output. The review did not indicate a
variation in maximum output at the number 5 setting.
Analysis of two valves installed in the normal charging path, which would be
required to close during safety injection, indicated that they may not be
able to do so under differential pressure conditions which could exist
following a loss-of-coolant accident. Although the as-found switch setting
agreed with the design setting determined by DPC,the application of output
torque values being linear between 40 and 100 percent was not correct for
all actuators. Preliminary data indicates that up to 41 nuclear units may
have some Rotork valve actuators. It is not known whether or not these are
used in safety related applications.
The above example specifically deals with improper setting of the torque
switches and illustrates the need for exercising extreme care in the setting
of motor-operator switches because all types of switches must be set
properly to ensure that the valves will function properly when needed. In
fact, the specific event that prompted the issuance of IEB 85-03 was caused
by improperly set torque bypass switches. Improperly pet thermal overload
switches recently (October 22, 1986) rendered the high-pressure coolant
injection system inoperable at the Hope Creek Nuclear Station Unit 1. Both
of these instances involved actuators manufactured by a company other than
Rotork.
In addition, care must be taken to insure that all of the ramifications of
changes to any of the motor-operator switches are fully understood. For
instance, Information Notice No. 86-29, "Effects of Changing Valve
Motor-Operator Switch Settings, describes how the changing of the limit
switches on certain motor-operated valves resulted in a control room
indication that the valves were closed when, in fact, they were partially
open. This led to an excessive cooldown rate in the reactor coolant system
at San Onofre Nuclear Generating Station Unit 3.
The information herein is being provided as an early notification of a
possibly significant matter that is still under consideration by the NRC
staff. Recipients should review the information for possible applicability
to their facilities. If NRC evaluation so indicates, further licensee
actions may be requested.
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IN 86-93
November 3, 1986
Page 3 of 3
No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office.or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contacts: George Schnebli, RII
(404) 331-5582
Richard J. Kiessel, IE
(301) 492-8119
Attachment: List of Recently Issued IE Information Notices