While selecting the control valve, one most important parameter is to select the failure mode of the valve when it loses its source of power.
For electrical valve, it stays in its last position when it losses power supply.
For hydraulic and pneumatic type actuator, a valve comes with spring have a capability of fail-safe in case of air loss.
The fail-safe mode in hydraulic and pneumatic is depended on both actuator type and valve body type.
For sliding stem valve it contains two possibilities,
In sliding stem valve with direct-acting action, with an increase in pressure pushes down the valve plug.
In a reverse-acting case with increases in pressure, stem lifts up.
Based on both action such type of valve has two possibilities in fail-safe condition, one is Air to open(ATO) or Air to close(ATC).
Now let's discuss each type of valve body action with its fail-safe action mode from the photograph.
1. DIRECT ACTING - AIR TO OPEN AND AIR TO CLOSE MODE.
Have a look at the below figure to see how valve with direct-acting action looks like. As you can see in the left side figure air supply is fed through downside. Increase in pressure will lift the valve. Such type of valve is known Air-to-open. In case of air fail valve due to spring return action is done by spring, the mode fails to closed.
On the right-hand side, the air supply is fed through the top side, with an increase in air supply it closes the valve, in the case of air failure spring take it back in actual position known as a fail-open mode.
Here, the action is direct-acting because plug has space of the area of the valve body. so, It can only move up-down in that area.
2. REVERSE ACTING - AIR TO OPEN AND AIR TO CLOSE MODE.
Now have a look into below image of the reverse-acting body. Here, on the left-hand side air supply is fed through the bottom side which lifts the valve plug.
so that plug assembly which is inserted in the valve body will move up blocking the flow. This type of valve is called air-to-close, if the air is failed due to spring it returns to its previous position, this called fail-open mode.
On right-side in the red circle, you can see plug is not seated there, so with an increase in air supply will push the plug down making valve open as flow pass-through hole located on it, which is not visible here in the image.
If air fails the valve, returns to its previous position which is called fail to closed mode.
Here in both action, there is a difference in valve body design, which is an important consideration while selecting control valve. To have a knowledge about all above-explained point is worth to remember it, make it easier for you to select the valve.