Information Notice No. 85-20: Motor-operated Valve Failures Due to Hammering Effect

                                                         SSINS No.:  6835 
                                                            IN 85-20  

                               UNITED STATES 
                       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 
                                  HAMMERING EFFECT 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or 
construction permit (CP). 

Purpose: 

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. 




8503070477 
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                                                           IN 85-20  
                                                           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 
    motor.  

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 
switch. 

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. 

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                                                           IN 85-20 
                                                           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 
office. 



                                   Edward L. Jordan, Director 
                                   Division of 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