When the pressure is greater than 0.21 barg the stem begins to move. Theoretically, when the pressure is 0.21 barg, 3 psig, the actuator is still in its original position, i.e., the air pressure should just balance the spring pre-load. When this force becomes greater than the spring pre-load, the diaphragm will move up or down, compressing the spring and causing the actuator and valve stems to move up or down as well. Since pressure acts in all directions, then an upward or downward force, the product of pressure and area, will act upon the diaphragm.

Rotary diaphragm pneumatic actuators are the third type of diaphragm actuators and will be discussed later in this chapter. The amount of force produced in this type of actuator depends on three main parameters: air pressure, diaphragm diameter, and spring force. In reverse port diaphragm actuators, the air supply port is located under the diaphragm, so the air supply opens the valve and a stoppage of the air supply leads to closure of the valve. Most of the control valve actuators are reverse-acting, thereby providing a fail-closed (FC) mode of failure. Thus direct-acting diaphragm actuators are suitable for air-to-close (ATC) and fail-open (FO) applications.

Any failure in air supply leads to movement of the stem in an upward direction and the valve opens. The air pressure, which is transferred to the diaphragm, overcomes the spring torque located under the diaphragm, so it pushes the valve stem down and closes the valve. In direct-acting actuators, the air enters the top area of the diaphragm and pushes the diaphragm down. 2.12 compares direct-acting (right side) and reverse-acting (left side) diaphragm actuators used for the control valves. The most common styles for diaphragm actuators are known as “direct-acting” and “reverse-acting.” Fig. Diaphragm actuators are used in all sectors of the oil and gas industry except for subsea. Diaphragm actuators are normally used for control valves, which are a type of globe valve that is typically used to regulate the fluid in order to adjust some process variables, such as pressure, temperature, or flow rate.

Diaphragm actuators are pneumatically operated and they use an air supply from the control system or other sources.

Electric solenoid valve diaphram