Fail safe positions for control valve
It is always important for process engineers to finalise a fail safe position for a Control valve so that the system will undergo safe shutdown during instrument air failure.
Fail safe positions are the final position of the control valve during failure of instrument Air and it can be “Fail open” OR “Fail Close”. Air fail to open means “Fail open (FO)” and Air fail to close means “fail close (FC)”.
Sometimes it can be FLC (fail last closed) and FLO (Fail last open). Fail last close means fail in last position and close slowly. Fail last open means fail in last position and open slowly. These are used during special requirements e.g. To avoid sudden thermal shock in Reactor/Furnaces.
Process engineers shall have thorough understanding of process/operation to finalise the fail safe position of control valve during P&ID development. Typically Cooling water control valves, Pressure control valve and Pump min circulation control valves are provided with FO position to safeguard plant equipment from over pressurisation and rest of the control valves will be FC.