What is the difference between a Single acting and double acting valve?

Answer 1

Normally a control valve is referred to by its 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 a valve as fail open or fail close, the valve actuator has to be spring loaded. So by having the spring on top or bottom of the actuator piston will we determine if it will be an FO or an FC valve. This kind of valve is also called single acting valve 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) that the spring might in certain instances be too weak to push the valve into the fail position quick enough, due to the back pressure from the process, and can cause damage to the plant or even an explosion.

To ensure that the valve will go to the fail position, we install a double action positioner with two outputs. One goes to the top of the actuator and one to the bottom. This is also very helpful in having very accurate and stable control on a high flow line since the pressure from the positioner does the actual control unlike spring controlling one way and positioner controlling the other way as is in the single acting control valves. It is also solving the problem that the valve will now be forced into the fail position by the spring as well as the positioner supply pressure during an emergency.

In shutdown valves (open/close ESDVs) the same is true and sometimes at critical and high pressure points we use hydraulics instead of pneumatic as the double acting agent to make sure the valve will close during the emergency. So to summarize the double action in ESD and control valve, it suffices to say that this double action is there just to make sure that the valve will do what it was designed for. Call it an extra fail safe if you want. It might not be needed in theory since a single acting valve should do the trick just as well, but in practice you are at times very glad that you did it especially if you look at the kind of pressures the valves are working on. With those kind of flows and pressures you don't want to leave anything to chance.