Common actuators are of two types: diaphragm type and piston type. Among them, the membrane-type actuator is the most commonly used, it can be used as the push device of the general control valve to form a pneumatic membrane-type actuator. The signal pressure p of the pneumatic membrane actuator acts on the diaphragm to deform it, drive the push rod on the diaphragm to move, and cause the valve core to displace, thereby changing the opening of the valve. It is simple in structure, cheap in price, convenient in maintenance, and widely used. The pneumatic piston actuator makes the piston move in the cylinder to generate thrust. Obviously, the output force of the piston type is much greater than that of the film type. Therefore, the diaphragm type is suitable for occasions with small output and high precision; the piston type is suitable for occasions with large output force, such as large diameter, high-pressure drop control, or pusher of the butterfly valve. In addition to the film type and piston type, there is also a long-stroke actuator, which has a long stroke and large torque, and is suitable for occasions with output angular displacement and large torque. The signal standard received by the pneumatic actuator is 0.02 to 0.1MPa.
Pneumatic membrane actuators are available in both positive and negative roles. When the signal pressure from the controller or the valve positioner increases, the downward action of the valve stem is called a positive-acting actuator; when the signal pressure increases, the valve stem moves upwards is called a reaction-acting actuator. The signal pressure of the positive-acting actuator is passed into the membrane air chamber above the corrugated diaphragm; the signal pressure of the reverse-acting actuator is passed into the membrane air chamber below the corrugated diaphragm. The two can be retrofitted to each other by replacing individual parts.
The main components of the pneumatic piston actuator are cylinder, piston, and push rod. The piston in the cylinder moves with the pressure difference between the two sides of the cylinder. According to the characteristics, it is divided into two types: proportional type and two-position type. In the two-position type, the piston is pushed from the high-pressure side to the low-pressure side according to the magnitude of the operating pressure on both sides of the input piston. The proportional type is based on the two-position type with a valve positioner so that the displacement of the push rod is proportional to the signal pressure.
