What is a Pneumatic Actuator?

Figure 1: Pneumatic actuators for steam systems Ghana Spirax Sarco.

The pneumatic actuator, or the pneumatic control valve actuator, is a device that uses air pressure to drive the opening and closing or regulate the valve. Its actuator mechanism and adjustment mechanism are a unified whole. The valve positioner uses the feedback principle to improve the performance of the actuator, so that the actuator can achieve accurate positioning according to the control signal of the controller.

1. Characteristics of the Pneumatic Actuator

1. In the case of no-load, when the pneumatic actuator inputs the specified air pressure into the cylinder, its action is stable, and there is no jamming and crawling phenomenon.
2. Under the air pressure of 0.6MPa, the output torque or thrust value of the pneumatic device in the opening and closing directions is not less than the value indicated on the nameplate. And its movements are flexible, and there are no permanent deformation of various parts and other abnormal phenomena.
3. When using the working pressure for the sealing test, the amount of air leaking from each side of the back pressure does not exceed (3+0.15D) cm3/min (standard state). The amount of air leaking from the end cover and output shaft does not exceed (3+0.15d) cm3/min.

Figure 2: Pneumatic control valve actuator.

4. When the strength test is carried out with 1.5 times the working pressure, after maintaining the test pressure for 3 minutes, there is no leakage and structural deformation of the cylinder end cover and static sealing parts.
5. As for action life times, when the pneumatic device simulates the action of a pneumatic valve, under the condition of maintaining the output torque or thrust capability in both directions, the opening and closing times of the opening and closing operation are not less than 50,000 times (the opening-closing cycle is one time).
6. For the pneumatic actuator with buffer mechanism, there will be no impact phenomenon when the piston moves to the stroke end position.

2. Types of Pneumatic Actuators

According to different constructions and working modes, pneumatic actuators can be divided into diaphragm, piston, and rack and pinion types.

Figure 3: Comparison of diaphragm, piston, and rack and pinion pneumatic actuator.

2.1 The Diaphragm Pneumatic Actuator

The diaphragm pneumatic actuator can be used as a pusher for general control valves. Its signal pressure acts on the diaphragm to deform it and drive the push rod on the diaphragm to move, which displaces the spool and changes the opening degree of the valve. It has simple structure, low price, easy maintenance and wide applications.

2.2 The Piston Pneumatic Actuator

The pneumatic piston actuator moves a piston in a cylinder to generate thrust. Apparently, the output force of the piston type is much greater than that of the diaphragm type. Piston type is suitable for occasions with large output force, such as large diameter, high-pressure-drop control or pusher of butterfly valve.

2.3 The Rack and Pinion Pneumatic Actuator

The rack and pinion pneumatic actuator delivers a linear torque output throughout its full range of travel. The movement of a piston causes movement of the rack. It has the characteristics of compact structure, beautiful appearance, quick response, stable operation and long service life. All its accessories are applied with advanced anti-corrosion technology, which can adapt to various harsh working conditions. It has good performance in various application fields.

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