Page 231 - Root Cause Failure Analysis

Control Valves 219

noid’s coil and the actuation triggers a movement of the main valve’s control mecha-
nism. As soon as the main valve’s position is changed, the energy to the solenoid coil
is shut off.

This operating characteristic of solenoid actuators is important. For example, a nor-
mally closed valve that uses a solenoid actuation can be open only when the solenoid
is energized. As soon as the electrical energy is removed from the solenoid’s coil, the
valve returns to the closed position. The reverse is true of a normally open valve. The
main valve remains open, except when the solenoid is energized.

The combination of primary and secondary actuators varies with the specific applica-
tion. Secondary actuators can be another solenoid or any of the other actuator types
that have been previously discussed.