There are two primary system considerations centering around an emergency operational situation for control valves
I. Loss of instrument air supply (IAS) pressure.
II. Loss of electrical power.
There are four choices as to control valve response to the emergency condition
a.) Fully Close
b.) Fully Open
c.) In Last Position
d.) Continue Throttling
For either fail-safe loss of IAS or loss of electrical power, it is the actuator's benchset range spring that drives the control valve's plug to its fail-safe open or closed position.
I. Loss of Instrument Air Supply Pressure -
a.) Fully Close. The actuator's benchset range spring drives the valve plug closed when loading air pressure goes towards or near 0 psig. (ATO-FC Reverse)
Because IAS piping systems can be extensive (big), the IAS can decay too slowly causing operational problems. In such cases it may be desirable to use a 3-way pilot switching valve to anticipate the eventual loss of IAS and quickly stroke the control valve to its fail-safe position, eliminating the transitory operational effects of the slow decay.