Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No: 6835
IN 86-29
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
April 25, 1986
Information Notice No. NO 86-29: EFFECTS OF CHANGING VALVE MOTOR-OPERATOR
SWITCH SETTINGS
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP)
Purpose:
This notice is provided to alert recipients to potential significant safety
effects of changing valve motor-operator switch settings as part of a
program to meet the requirements of Inspection and Enforcement Bulletin
(IEB) 85-03, "Motor-Operated Valve Common Mode Failures During Plant
Transients Due to Improper Switch Settings" Changes to switch settings can
effect valve position indication and signals such as "permissives" to other
equipment It is expected that recipients will review the information for
applicability to their facilities However, suggestions contained in this
notice do not constitute NRC requirements; therefore, no specific action or
written response is required
The information herein is being provided as an early notification of a
possibly significant matter that is still under consideration by the NRC
staff If NRC evaluation so indicates, licensees will be kept informed of
further developments
Description of Circumstances:
On January 23, 1986 the Southern California Edison Company notified NRC
[licensee event report (LER) 85-036] that it had exceeded the cooldown rate
limitation of its Technical Specifications at Unit 3 of the San Onofre
Nuclear Generating Station on December 24, 1985 The licensee failed to
fully followup on the effect of changing the valve motor-operator torque
bypass switch setting as a part of the program to meet the requirements of
IEB 85-03
The plant had entered Mode 3 on December 18, 1985 following their first
refueling outage The plant entered Mode 4 on December 22, 1985 to perform
maintenance on a reactor coolant pump Cooldown continued, and on December
24, 1985 the shutdown cooling system (SDCS) was aligned for warmup At the
time of alignment, the SDCS heat exchangers were thought to be bypassed and
isolated, as indicated by observation of the isolation valve position in the
control room When SDCS flow was aligned, an initial increase in the reactor
coolant
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IN 86-29
April 25, 1986
Page 2 of 3
system (RCS) cooldown rate was expected; however, RCS temperature was found
to decrease at a rate which was considerably greater than expected and which
exceeded the technical specification limitations When the plant operators
recognized the excessive cooldown rate, they attempted to reduce the rate of
RCS temperature decrease by throttling the SDCS loop injection valves to
reduce the SDCS flow rate As part of this process, the control room
switches for the SDCS heat exchanger isolation valves were depressed and
held in the closed position to confirm the existing control room indication
that these valves were indeed in the closed position The RCS cooldown rate
was immediately noted to decrease, thereby indicating SDCS flow had, until
then, been permitted to pass through the SDCS heat exchangers, contrary to
the control room indication With the SDCS heat exchanger isolation valves
truly closed, the RCS cooldown ate was adjusted and maintained within the
limitations of the Technical Specifications
Another limiting condition of the plant's Technical Specifications requires
that two independent emergency core cooling system (ECCS) subsystems be
operarable in Modes 1, 2, and 3 with the pressurizer pressure greater than
or equal to 400 psia) The valve alignment to assure this condition is
verified twice a day by observation of the control room position
indications Subsequent investigation indicated that both of the SDCS heat
exchanger isolation valves could have been as much as 16 percent open while
the control room indication of their position showed closed
The valve motor-operator, torque bypass switches on the SDCS heat exchanger
isolation valves had been adjusted because of concerns raised in IEB 85-03
The motor operators on these valves are protected from overload by torque
switches It was determined that the torque bypass switch had to be
precisely set such that the increased torque required to initially open
valves against high differential pressure would not result in deenergizing
the motor operator However, due to the design of the valve control
circuitry, the torque bypass switch and the valve position indicating limit
switch are on the same position indicating rotor Therefore, when the
position of the rotor was changed to extend the range of the torque bypass
switch, it also affected the closed position indication
The valves involved are throttle, or "jog", valves and are operated by
holding the control switch in the direction of valve travel until the
desired position is reached Both maintenance and operations personnel were
aware of the premature fully closed indication which resulted from the
recent adjustment to the torque bypass switches on the valve motor
operators They planned on compensating for this condition by holding the
valve control switch in the close position for a brief period of time after
the "closed" indication was observed However, the exact time to hold the
switch was not specified in the procedures, and apparently, when the valves
were operated on December 18, 1985, for the surveillance test, they were not
fully closed Subsequent observation of the valve operation by operations
personnel determined that it was necessary to hold the valve switch in the
closed position for at least 15 seconds after the "closed" indication was
observed in the control room This information is being incorporated into
the appropriate operating procedures
IN 86-29
April 25, 1986
Page 3 of 3
Discussion:
This LER points up the importance of fully understanding the effects of
changing any of the valve motor-operator switch settings Even though the
plant operations personnel were aware of the premature valve "closed"
signal, they did not fully appreciate how early this signal was being
produced; hence the operator did not hold the close switch for a long enough
time when they performed the initial valve verification as they entered Mode
3
This problem occurred on a throttle valve that did not have a "seal-in"
feature Valves that do have the "seal-in" feature should continue to torque
closed regardless of the setting of the close position limit switch, and
thus should not experience this particular problem
However, whether or not the valve has the "seal-in" feature, the changing of
the valve motor-operator switch settings could have effects on other aspects
of plant operation, because of the limited number of position rotors
available in the typical valve motor operator For instance, frequently the
valve closed signal is used as a "permissive" signal to other pieces of
equipment Thus, increasing the torque bypass switch setting could result in
the premature starting of some other plant operation--an action which may
not have been fully analyzed with respect to its safety implications
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 Contact: Richard J Kiessel, IE
(301) 492-8119
Attachment:
List of Recently Issued IE Information Notices
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