Fail position, abbreviated as FP, is the term used to describe how an actuator reacts when there is a loss of power, loss of signal or similar event. There are three different fail scenarios, each has its own consequences and safety effects.
Fail Open, abbreviated as FO, is the term where a valve opens at loss of signal. This scenario might be chosen to prevent overpressure in the event of a blocked line or in case of a catastrophic failure.
When sizing a pneumatic actuator this is indicated as "spring to open" or "air to close."
Fail Closed, abbreviated as FC, means the valve will closed when the signal is interrupted. An example of this would be block valves closing to isolate a steam injection well on loss of power. Because uncontrolled steam is dangerous and potentially lethal, automatic valve closure might be required.
When sizing a pneumatic actuator this is indicated as "spring to close" or "air to open."
Fail in Place
Fail in Place might apply to a ball valve or a control valve that does not react on loss of power or signal. This would be used where the process cannot be shut down or where it is undesired to do so.