Free Download of Reference Book on Basic Operation and Function of Control Valves

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.