In designing a processing facility, we usually use control valve to manipulates the temperature, pressure, level, or fluid flow rate in process system. The selection of control valve, which is air-to-open or air-to-close, is base on safety consideration. In case of power loss or air failure , control valve should move in safe position (fail-safe).
Fail position is the term used to describe how control valve will react when there is loss of power. I read there are three different fail positions.
Fail Open (FO) or Air-to-Close (AOC)
Fail open means when instrument air is loss, the control valve will be in open position. It is because the return spring provides more opposing force than the diminishing instrument air applied to the diaphragm. When sizing a pneumatic actuator, this is indicated as “spring-to-open” or “air-to-close”.
Fail Closed or Air-to-Open
Fail closed means when instrument air is loss, the control valve will be in closed position. It is because the return spring provides more opposing force than the decreasing instrument air applied to the diaphragm. When sizing a pneumatic actuator, this is indicated as “spring-to-close” or “air-to-open”.
Fail In Place
Actuator without spring or return mechanism usually fail in the last position (fail in place) prior to loss of power.
When to Use Fail Open, Fail Closed, or Fail In Place?
The answer is depend of process system. The system is very unique and may be different from one to another. When we will select fail-safe position, it is better to understand process and safety consideration. However guides below will help you to select fail position of control valve.
When to use fail close position?
- Preventing overflowing a tank
- Control valve discharging wastewater should be fail closed to prevent release of toxic material contained in wastewater
When to use fail open position?
- Control valve controlling cooling water flow rate to condenser should be fail open so that remaining hot fluid could be cooled by the cold water thus preventing overheating and damaging critical component
- To prevent pressure build up in tank due to pressure relief valve failure, we use control valve in fail open position.
- Control valve installed at minimum flow line should be fail open position