Information Notice No. 85-20: Motor-operated Valve Failures Due to Hammering Effect
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
March 12, 1985
Information Notice No. 85-20: MOTOR-OPERATED VALVE FAILURES DUE TO
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
This information notice is provided to alert recipients of a potentially
significant problem pertaining to motor-operated value failures due to the
hammering that may result when a fully closed (opened) valve continues to
receive a close (open) signal at the valve operator. It is expected that
recipients will review the information for applicability to their facilities
and consider actions, if appropriate, to preclude a similar problem
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:
On March 19, 1984, and September 25, 1984, Commonwealth Edison reported
[licensee event report (LER) 84-003] the failure of a core spray valve to
operate from the control room at the Dresden Nuclear Power Station Unit 2.
The immediate cause of failure was a mechanical failure of the gear housing
of the valve, probably caused by mechanical overloading during operation.
The same LER identified the failure of a second core spray valve. In this
case, the immediate cause of failure was a cracked bearing race and gear
housing. Metallurgical analysis by the licensee indicated that the valve
gear housing had failed as a result of a mechanical overload. In addition,
the valve thermal overload breaker had tripped.
Further investigation by the licensee showed that these mechanical overloads
were the result of the valve being repeatedly hammered closed by the valve
operator. This hammering would continue as long as the valve operator con-
tinued to receive a close demand signal.
March 12, 1985
Page 2 of 3
In general, the sequence of events would be:
1. On receiving a close signal, power would be applied to the valve motor
and the value would begin closing.
2. Once closed, torque would build up arid the torque switch would open
removing power from the valve motor.
3. With the power removed, the valve motor would stop and the torque on the
value would relax.
4. With torque removed, the torque switch would reset.
5. Once the torque switch reset, if a close signal was still present at the
valve motor operator, power would be reapplied to the valve motor and
the valve would be driven further closed.
6. Since the valve was already closed, torque would immediately begin to
build up and the torque switch would open removing power from the valve
The last four steps would then repeat over and over. However, the torque
switch would not stop the motor instantaneously. Thus, the loads would
gradually build up in the valve until either something failed or the close
signal was removed from the valve motor controller. The potential for "valve
hammering" exists if the automatic or manual valve-close demand signal
continues after the torque switch has been activated open. That is, if the
valve full close limit switch is out of calibration. A similar condition
could occur on valve opening, if backseating loads are limited by a torque
Recognizing this condition as a possible common mode failure mechanism for
all valves with this particular valve motor controller logic, the NRC's
Office of Analysis and Evaluation of Operational Data (AEOD) performed a
search of LERs on the Sequence Coding and Search System for the 1983 through
1984 time period (AEOD Engineering Evaluation Report No. AEOD/E501).
Although they did not find any events attributed to hammering, they did find
47 events which had symptoms indicative of the hammering problem. Among
these were failure and damage due to mechanical overloading, overheating of
the valve operator motor, repeated cycling and failure of the starter
contactors, thermal overloading, circuit breaker trips, and valve seat
jamming. From this, AEOD concluded that licensees have not consistently
identified the root cause of motor-operated valve failures, but rather have
only identified the symptomatic ones. (This concept has been previously
addressed in Information Notice No. 82-10, "Following up Symptomatic
Repairs to Assure Resolution of Problem.")
As a part of their corrective action for the valve failures at Dresden, the
licensee plans on modifying the control circuitry of the valves to prevent
this hammering effect. This action will be taken on the valves in both
units. In the interim, caution cards have been put on the valves to warn the
operators not to hold on to the control switch when closing the valves.
March 12, 1985
Page 3 of 3
It is important that any modifications made to the valve's motor controller
logic not adversely affect the valve's safety-related functioning. For
instance, simply locking out a close signal once the torque switch opens
will result in improper valve operation if the valve should experience
momentarily high friction loads from either tight packing or high pressure
differential across the valve.
LER 84-014 submitted by Commonwealth Edison on August 14, 1984, described
the failure of both low pressure coolant injection valves to open at Quad
Cities Nuclear Power Station Unit 1. In 1980 the motor control logic for
these valves was modified to prevent hammering. However, some time later,
when the brakes on the valve motors were removed, it became apparent that
the modification had not eliminated the problem. This is indicative of the
difficulty involved in performing this type of modification.
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 NRC regional office or this
Edward L. Jordan, Director
Division of Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Richard J. Kiessel, IE
Attachment: List of Recently Issued IE Information Notices
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