Classification and introduction of pneumatic actuators

The common actuators are membrane type and piston type. The membrane actuator is the most commonly used one. It can be used as the driving device of the general control valve to form a pneumatic membrane 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 displacement of the valve core, thus changing the opening of the valve. It has the advantages of simple structure, low price, convenient maintenance and wide application. 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 membrane type is suitable for small output and high precision occasions; the piston type is suitable for large output occasions, such as large caliber, high pressure drop control or butterfly valve driving device. Besides membrane type and piston type, there is also a long travel actuator, which has long travel and large torque, and is suitable for output angular displacement and large torque occasions. Signal standards received by pneumatic actuators range from 0.02 to 0.1 MPa.

Pneumatic actuator

The pneumatic film actuator has two forms: positive and negative. When the signal pressure from the controller or valve positioner increases, the downward action of the stem is called a positive actuator; when the signal pressure increases, the upward action of the stem is called a reaction actuator. The signal pressure of the positive actuator is to enter the film chamber above the corrugated diaphragm, while the signal pressure of the reaction actuator is to enter the film chamber below the corrugated diaphragm. By replacing individual parts, the two can be altered to each other.

Pneumatic actuator

The main components of pneumatic piston actuator are cylinder, piston and push rod. The piston in the cylinder moves with the change of the pressure difference between the two sides of the cylinder. According to the characteristics, it can be divided into two types: proportional type and two-way type. According to the operating pressure on both sides of the input piston, the piston is pushed from the high pressure side to the low pressure side. Proportional type is based on two-way valve positioner, so that the push rod displacement and signal pressure proportional relationship.

Function of electric valve positioner

Pneumatic actuator

(1) Used in the important regulating system with high quality requirement to improve the positioning accuracy and reliability of the regulating valve.

(2) When the pressure difference between the two ends of the valve is large (p > 1MPa). By increasing the pressure of the air source, the output force of the actuator is increased to overcome the unbalanced force of the liquid on the valve core and reduce the travel error.

(3) When the regulated medium is high temperature, high pressure, low temperature, toxic, flammable and explosive, in order to prevent external leakage, packing is often pressed very tightly, so the friction between the stem and packing is large. At this time, the time lag can be overcome by positioner.

(4) When the adjusted medium is viscous fluid or contains solid suspended matter, the resistance of the medium to the valve stem movement can be overcome by using a positioner.

(5) For large caliber (Dg > 100mm) control valves to increase the output thrust of the actuator.

Pneumatic actuator

(6) When the distance between the regulator and the actuator is more than 60 m, the positioner can overcome the transmission delay of the control signal and improve the reaction speed of the valve.

(7) To improve the flow characteristics of the control valve.

(8) When one regulator controls two actuators to implement split-range control, two locators can be used to receive low-input signals and high-input signals respectively. Then one actuator operates at low-range and the other at high-level, which constitutes split-range regulation.