Answer / carl_ellis

The timer forces the on-off pneumatic control to drive open
or closed based on the timer's output state.

The timer's output relay activates or deactivates a 3 way
solenoid valve that supplies air to the valve's actuator.

In one timer state, the solenoid valve passes supply air to
the actuator driving the valve to its 'not-failed' state.

In the other timer state, the solenoid stops the air supply
and dumps the existing actuator pressure to atmosphere, letting
the valve assume its 'failed' state, whether open or closed.

Carl Ellis
Measure First

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