United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular No. 77-01, Malfunctions of Limitorque Valve Operators Description of Circumstances


CR77001 

                                                       January 4, 1977 IE 
                                                       Circular No. 77-01 

MALFUNCTIONS OF LIMITORQUE VALVE OPERATORS DESCRIPTION OF CIRCUMSTANCES: 

On October 28, 1976, Portland General Electric Company reported that the two 
motor operated (Limitorque) valves located between the Refueling Water 
Storage Tank and the charging pump suction at the Trojan Nuclear Plant 
failed to open in response to a spurious safety injection (SI) signal. The 
malfunction in both valves resulted from the torque limit switch in the 
opening circuit becoming activated before the valve was fully off its seat. 

The licensee's investigation revealed that in each case the valve had been 
manually closed hard on its seat following a maintenance operation. 

The licensee's investigation of this occurrence revealed difficulty in the 
opening of three additional motor operated (Limitorque) valves in the inlet 
and outlet piping of the Boron Injection Tank. Each of the valves failed to 
open in response to a single actuation of its manual control switch. In each 
case, the cause for failure was attributed to premature activation of the 
valve's torque limit switch. These valves had opened in response to the SI 
signal on October 28, 1976, following which they were closed normally using 
their motor operators. 

Subsequent investigation by the licensee revealed that each of the valves 
which malfunctioned was equipped with a torque limit switch in the opening 
circuit, the actuation of which stops valve motion. The valves are also 
equipped with an adjustable bypass switch which defeats the function of the 
torque limit switch when the valve is being moved from its full open or full 
closed position. Each of the valves which malfunctioned was found to have 
its bypass switch adjusted such that it allowed the limit torque switch to 
be unbypassed and operable in the circuit before the valve was moved from 
its seat. Examination by the licensee revealed similar improper adjustment 
of the bypass switches on several other motor operated valves in safety 
related systems. 

Corrective actions by the licensee included the establishment of procedural 
controls to insure that valves which are manually closed are checked for 
proper operation (by cycling them open with the motor operator) prior to 
their being declared operational. The bypass switches on all similar motor 
operated valves were checked and their position in terms of proper valve 
travel adjusted. 
.

Circular No. 77-01                 -2-                     January 4, 1977 

RECOMMENDED ACTION BY LICENSEES: 

If valves similar to those described are in use in safety related systems, 
you should verify that your procedures contain adequate provisions to insure 
that these valves are adequately checked for operation following maintenance 
or manual closure, and that operating personnel are properly instructed to 
assure conformance with the procedures. You should also verify that measures 
are taken to insure the proper setting of torque limit switches and limit 
switch bypasses, to assure that the bypass function is not negated 
prematurely in either the opening or closing cycle. 

No written response to this Circular is required. If you require additional 
information regarding this matter, contact the Director of the appropriate 
NRC Regional Office. 

Page Last Reviewed/Updated Wednesday, July 09, 2014