Normally a control valve is refer to by it's fail position. This means "what position will the valve move to should the supply air or control signal to the valve falls away". This is important to safe guard the process at various places so some valves will be fail open and some fail close. In order to have valve as a fail open or close the valve the actuator have to be spring loaded. So by having the spring on top or bottom of the actuator piston, will determine if it will be a FO or FC valve. This kind of valve is also called single action since it will only have one output from its positioner to either the top or bottom of the actuator. The positioner on the valve is also setup as a single acting positioner since it will only give a single action to the actuator, the reverse action will be done by the spring. The problem with this setup is that it is possible that the process might be so strong or the pressure so high (during a blow down or ESD shutdown in the plant) th