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Reliability improvement in electric motor operated valves
Reliability Engineering and System Safety 23 (1988) 159-161
Technical Note Reliability Improvement in Electric Motor Operated Valves
A BSTRA CT This paper contains a reliability analysis of the electric motor operated valves used in various safety related systems of nuclear power plants. The actuation circuit is found to contribute significantly to the probability of failure. An improvement is suggested to reduce this contribution.
INTRODUCTION The electric motor operated valves are used in various safety related systems of nuclear power plants, e.g. emergency core cooling system at NAPP. They are preferred in situations where torque requirements or line sizes are high. The MOVs have an additional feature that they can be reversible. The probability of failure on demand usually quoted for these Valves is ~ 10-a/demand which does not include the contribution from the motor actuation or starter circuit. A typical actuation circuit is as shown in Fig. 1 and comprises of 3~0 supply to the motor from 415V class-II Bus through overload relays R, Y and B and the necessary fuses and contacts. This voltage is stepped down through the 415/110V transformer and supplied to the contactors 42-1, 42-2 whose contacts are wired in the supply to the motor. When relay 3C-1 or 3C-2 (on generation of ECCS C o m m a n d Signal) is energised, the corresponding contacts close which energise 42-1 or 42-2 and in turn, the m o t o r is actuated in the forward or reverse direction to open or close the valve. 159 Reliability Engineering and System Safety 0951-8320/88/$03-50 © 1988 Elsevier Science Publishers Ltd. Printed in Great Britain.
160
A. K. Babar, R. K. Sarq[i V. V. S. Sanyasi Rao R Y
T
A
8
Ei
415 / 110V
I I
,2-2 T T
-T 49
42-1
I I !
"I Fig. I.
Typical actuation circuit.
R E L I A B I L I T Y ANALYSIS Since the motor is off during normal reactor operation, any failure in the actuation circuit would be revealed during testing only. Thus, failure of the following components constitutes the failure of motor actuation and hence the MOV. (i) (ii) (iii) (iv) (v) (vi)
3C-1 or 3C-2 contact fails to close. 42-1 or 42-2 or their contacts fail to close. 42-1 or 42-2 normally closed contacts prematurely open. R, Y, B relays fail to energise. Transformer failure Fuses (4 Nos)
The probability of failure based on a monthly test interval is estimated as 2.5 × 10-3/demand which coupled with the valve becomes 3.5 x 10- 3/demand. Thus, the contribution from the actuation circuit would be very significant and particularly, in a non-redundant configuration. RELIABILITY IMPROVEMENT In the case where an undervoltage relay 27, as shown in Fig. 1, is incorporated and monitored, open failures in the fuses in R and Y phases
Reliability improvement in electric motor operated valves
161
and transformer etc. would be revealed. In addition, it is not essential to provide the overload relays 49-R, Y, B for the protection of the motor. It may be adequate to monitor the winding temperature of the motor for protection through a thermostat contact and shorting the lines in the motor supply circuit. This would reduce the probability of failure of the motor actuation circuit from 2.5 to 0.5 x 10-3/demand. In order to monitor the remaining fuse, an additional 27 Relay can be incorporated but it would add to the cost. However, it is further recommended that during the valve testing, the actuation circuit be also tested along with the mechanical movement of the valve. A. K. Babar, a R. K. Saraf a & V. V. S. Sanyasi Rao b
a Reactor Engineering Division o Health Physics Division, Bhabha Atomic Research Centre, Bombay-400085, India
(Received 12 December 1987)
Technical Note Reliability Improvement in Electric Motor Operated Valves
A BSTRA CT This paper contains a reliability analysis of the electric motor operated valves used in various safety related systems of nuclear power plants. The actuation circuit is found to contribute significantly to the probability of failure. An improvement is suggested to reduce this contribution.
INTRODUCTION The electric motor operated valves are used in various safety related systems of nuclear power plants, e.g. emergency core cooling system at NAPP. They are preferred in situations where torque requirements or line sizes are high. The MOVs have an additional feature that they can be reversible. The probability of failure on demand usually quoted for these Valves is ~ 10-a/demand which does not include the contribution from the motor actuation or starter circuit. A typical actuation circuit is as shown in Fig. 1 and comprises of 3~0 supply to the motor from 415V class-II Bus through overload relays R, Y and B and the necessary fuses and contacts. This voltage is stepped down through the 415/110V transformer and supplied to the contactors 42-1, 42-2 whose contacts are wired in the supply to the motor. When relay 3C-1 or 3C-2 (on generation of ECCS C o m m a n d Signal) is energised, the corresponding contacts close which energise 42-1 or 42-2 and in turn, the m o t o r is actuated in the forward or reverse direction to open or close the valve. 159 Reliability Engineering and System Safety 0951-8320/88/$03-50 © 1988 Elsevier Science Publishers Ltd. Printed in Great Britain.
160
A. K. Babar, R. K. Sarq[i V. V. S. Sanyasi Rao R Y
T
A
8
Ei
415 / 110V
I I
,2-2 T T
-T 49
42-1
I I !
"I Fig. I.
Typical actuation circuit.
R E L I A B I L I T Y ANALYSIS Since the motor is off during normal reactor operation, any failure in the actuation circuit would be revealed during testing only. Thus, failure of the following components constitutes the failure of motor actuation and hence the MOV. (i) (ii) (iii) (iv) (v) (vi)
3C-1 or 3C-2 contact fails to close. 42-1 or 42-2 or their contacts fail to close. 42-1 or 42-2 normally closed contacts prematurely open. R, Y, B relays fail to energise. Transformer failure Fuses (4 Nos)
The probability of failure based on a monthly test interval is estimated as 2.5 × 10-3/demand which coupled with the valve becomes 3.5 x 10- 3/demand. Thus, the contribution from the actuation circuit would be very significant and particularly, in a non-redundant configuration. RELIABILITY IMPROVEMENT In the case where an undervoltage relay 27, as shown in Fig. 1, is incorporated and monitored, open failures in the fuses in R and Y phases
Reliability improvement in electric motor operated valves
161
and transformer etc. would be revealed. In addition, it is not essential to provide the overload relays 49-R, Y, B for the protection of the motor. It may be adequate to monitor the winding temperature of the motor for protection through a thermostat contact and shorting the lines in the motor supply circuit. This would reduce the probability of failure of the motor actuation circuit from 2.5 to 0.5 x 10-3/demand. In order to monitor the remaining fuse, an additional 27 Relay can be incorporated but it would add to the cost. However, it is further recommended that during the valve testing, the actuation circuit be also tested along with the mechanical movement of the valve. A. K. Babar, a R. K. Saraf a & V. V. S. Sanyasi Rao b
a Reactor Engineering Division o Health Physics Division, Bhabha Atomic Research Centre, Bombay-400085, India
(Received 12 December 1987)