Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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