A pneumatic actuator is an executive device that uses air pressure to drive the opening, closing or regulation of valves. It is also called a pneumatic actuator mechanism or pneumatic device, but it is generally commonly referred to as a pneumatic head. The actuator and regulating mechanism of a pneumatic actuator are an integrated whole, and its actuators include diaphragm type, piston type, fork type and rack and pinion type.

Diaphragm Type Pneumatic Actuator

The diaphragm type actuator is commonly used. It can be used as a driving device for general control valves to form a pneumatic diaphragm actuator. The signal pressure p of the pneumatic diaphragm actuator acts on the diaphragm, causing it to deform, driving the push rod on the diaphragm to move, making the valve core displace, thereby changing the opening of the valve. It has a simple structure, low price and convenient maintenance, and is widely used. Pneumatic diaphragm actuators have two forms: direct-acting and reverse-acting.

When the signal pressure from the controller or valve positioner increases, the valve stem moving downward is called a direct-acting actuator; when the signal pressure increases, the valve stem moving upward is called a reverse-acting actuator. The signal pressure of the direct-acting actuator is introduced into the diaphragm air chamber above the corrugated diaphragm; the signal pressure of the reverse-acting actuator is introduced into the diaphragm air chamber below the corrugated diaphragm. The two can be modified into each other by replacing individual parts.

Piston Type  Pneumatic Actuator

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 diaphragm type. Therefore, the diaphragm type is suitable for occasions with small output force and high precision; the piston type is suitable for occasions with large output force, such as the driving device of large-diameter, high-pressure drop control or butterfly valves. In addition to the diaphragm type and piston type, there is also a long-stroke actuator, which has a long stroke and large torque, and is suitable for occasions requiring angular displacement and large torque.

The standard signal received by the pneumatic actuator is 0.02 to 0.1MPa. The main components of the pneumatic piston actuator are the cylinder, piston and push rod. The piston in the cylinder moves with the change of the pressure difference on both sides of the cylinder. It is divided into proportional type and two-position type according to characteristics. The two-position type pushes the piston from the high-pressure side to the low-pressure side according to the magnitude of the operating pressure input on both sides of the piston. The proportional type is based on the two-position type with a valve positioner added, so that the push rod displacement is proportional to the signal pressure.

Rack and Pinion Type Pneumatic Actuator

The rack and pinion (double piston rack type) pneumatic actuator has the characteristics of compact structure, beautiful appearance, quick response, stable operation and long service life. All accessories adopt anti-corrosion treatment technology, which can adapt to various harsh working conditions. Its high and low temperature and various special stroke actuators have good performance in various application fields.

The piston type has a long stroke and is suitable for occasions requiring large thrust; while the diaphragm type has a small stroke and can only directly drive the valve stem. The fork type pneumatic actuator has the characteristics of large torque, small space and torque curve more in line with the torque curve of the valve, but it is not very beautiful; it is often used on valves with large torque. The rack and pinion type pneumatic actuator has the advantages of simple structure, stable and reliable operation, and safety and explosion-proof. It is widely used in production processes with high safety requirements such as power plants, chemical industry and oil refining.