United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-20, Supplement 1: Motor-operated Valve Failures Due to Hammering Effect

                                              SSINS No.:  6835         
                                            IN 85-20, Supplement 1 

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                           WASHINGTON, D.C. 20555 

                                May 14, 1985 

Information Notice No. 85-20 SUPPLEMENT 1:   MOTOR-OPERATED VALVE 
                                                FAILURES DUE TO HAMMERING 
                                                EFFECT 

Addressees: 

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

Purpose: 

This supplement to Information Notice No. (IN) 85-20 is provided to alert 
recipients of additional information on a potentially significant problem 
pertaining to motor-operated valve failures caused by 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 require ments; therefore, no specific action or written response is 
required. 

Background: 

IN 85-20 described a number of valve failures at the Dresden Nuclear Power 
Station Unit 2 and Quad Cities Nuclear Power Station Unit 1 sites. It identi
fied a process by which the relaxation of torque on a closed valve would 
lead to repeated attempts to further close the valve as long as the valve 
operator continued to receive a valve-close demand signal. Such a continuing 
signal would occur if the plant operator held the control switch in the 
closed posi tion or an emergency signal (i.e. , containment isolation, etc.) 
was present. 

Discussion: 

Subsequent conversations with several utilities and Limitorque Corporation, 
the manufacturer of the motor operators used on the valves identified in the
information notice, have clarified the conditions under which this hammering
effect can occur. The majority of valve motor operators manufactured by 
Limitorque use a self-locking worm-and-worm gear to drive the valve stem. In
such installations, when the valve is fully closed, the Belleville washer is
compressed and the torque switch opens, stopping the motor. The geometry of 
the worm gear tooth form prevents the worm from moving when the motor stops.
Thus, the torque is maintained on the valve, the torque switch remains open,
and hammering is prevented. 

8505130038  

.

                                                 IN 85-20, Supplement 1 
                                                 May 14, 1985   
                                                 Page 2 of 2 

However, Limitorque also produces an actuator which uses a low-ratio worm 
gear that is not self-locking. This is generally associated with high speed 
valve applications. In this type of actuator, the worm may be repositioned 
by the energy stored in the compressed Belleville washers, which are located
at one end of the worm. Because the operator essentially uses the position 
of the worm as an indication of torque, this movement of the worm closes the
torque switch contacts. Thus, if a valve-close demand signal is still pres-
ent, the motor will restart and attempt to further close the valve. 

Table 1 provides a list of worm gear ratios that are not self-locking. Con-
versations with Commonwealth Edison confirmed that the valves identified in 
the information notice had high-speed operators on them. 

Conversations with several utilities who were previously aware of this 
problem indicated that their normal design practice was to close on torque 
unless the valve had a high-speed operator. In such cases, they closed on 
valve position. However, now the current philosophy is to use a combination 
of torque and position to ensure closure while preventing hammering. 
Limitorque Corporation indicated that actuators without self-locking ratios 
and with a motor brake, generally do not experience this hammer effect. As 
noted in the information notice, Quad Cities was not aware of this in their 
change to the operator logic until after they removed the motor brake from 
the valves. 

No specific action or written response is required by this information 
notice supplement. 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 Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  R. J. Kiessel, IE 
                    (301) 492-8119 

Attachments: 
1.  Table 1, "Worm Gear Ratios Which Are Not Self-locking" 
2.  List of Recently Issued IE Information Notices  
.

                                                 Attachment  
                                                 IN 85-20, Supplement 1 
                                                 May 14, 1985  
                                                 Page 1 of 1 

            TABLE 1 WORM GEAR RATIOS WHICH ARE NOT SELF-LOCKING 

          MODEL 
          SMB OR SB           WORM GEAR 
          SIZE                RATIO 

          000                 18 2/3 : 1 
          00                  19     : 1 
          0                   18 2/3 : 1 
          1                   14.5   : 1 
          2                   13.3   : 1 
          3                   10.3   : 1 
                              16     : 1 
          4                   12 2/3 : 1
                              19     : 1 
          5                   none 

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