Source: Thermal Control Circle
An overview of pneumatic actuators
Pneumatic actuator is an actuator that uses air pressure to drive the opening and closing or adjust the valve, also known as pneumatic actuator or pneumatic device, but it is generally called pneumatic actuator. The actuator and adjusting mechanism of the pneumatic actuator are a unified whole, and the actuator has a membrane type, piston type, fork type and rack and pinion type.
The piston type has a long stroke and is suitable for applications that require high thrust, while the diaphragm type has a small stroke and can only drive the stem directly. The fork pneumatic actuator has the characteristics of large torque, small space, and the torque curve is more in line with the torque curve of the valve, but it is not very beautiful, and is commonly used in the valve with large torque. The rack and pinion pneumatic actuator has the advantages of simple structure, stable and reliable action, and safety and explosion-proof, and has a wide range of applications in power plants, chemical industry, oil refining and other production processes with high safety requirements.
Second, the working principle of pneumatic actuators
1. Working principle diagram of double-acting pneumatic actuator
When the air source pressure enters the middle cavity between two pistons of cylinder from the air port (2), make the two pistons move to the direction of both ends of the cylinder separately, and the air of the air cavity at both ends is discharged through the air port (4), and make the two piston racks synchronously drive the output shaft (gear) to rotate in the counterclockwise direction simultaneously. On the contrary, when the air source pressure enters the air cavity at both ends of the cylinder from the air port (4), the two pistons are made to move to the middle direction of the cylinder, and the air of the middle air cavity is discharged through the air port (2), and the two piston racks are synchronously driven to drive the output shaft (gear) to rotate in the clockwise direction simultaneously. (If the piston is mounted in the opposite direction, the output shaft will rotate in the opposite direction.)
2. Single-acting pneumatic actuator working principle diagram
When the air source pressure enters the middle cavity between the two pistons of the cylinder from the air port (2), the two pistons are separated and move in the direction of the two ends of the cylinder, the springs at both ends are forced to compress, and the air of the air cavity at both ends is discharged through the air port (4), and the two piston racks are synchronously driven to rotate the output shaft (gear) in the counterclockwise direction simultaneously. After the air source pressure is reversed by the solenoid valve, the two pistons of the cylinder move to the middle direction under the elastic force of the spring, and the air of the middle air cavity is discharged from the air port (2), and the two piston racks are synchronously driven to rotate the output shaft (gear) in the clockwise direction. (If the piston is mounted in the opposite direction, the output shaft will rotate in the opposite direction when the spring is returned.)
3. Classification of pneumatic actuators
1. Membrane actuator
Diaphragm actuators are the most commonly used, which can be used as a pushing device for general control valves to form pneumatic diaphragm actuators. The signal pressure p of the pneumatic diaphragm actuator acts on the diaphragm to deform it, drives the push rod on the diaphragm to move, and displaces the valve core, thereby changing the opening of the valve. It has simple structure, cheap price, easy maintenance and wide application.
Pneumatic membrane actuators are available in two forms: positive and reactive.
When the signal pressure from the controller or valve positioner increases, the downward action of the valve stem is called the positive action actuator, and when the signal pressure increases, the upward action of the valve stem is called the reaction actuator. The signal pressure of the positive-acting actuator is the membrane air chamber above the corrugated diaphragm, and the signal pressure of the reaction actuator is the membrane air chamber below the corrugated diaphragm. By replacing individual parts, the two can be retrofitted with each other.
2. Piston actuator
The pneumatic piston actuator causes the piston to move in the cylinder to generate thrust, and obviously, the output force of the piston type is much greater than that of the diaphragm type. Therefore, the diaphragm type is suitable for applications with small output force and high precision, and the piston type is suitable for applications with large output force, such as large diameter, high pressure drop control or butterfly valve pushing device. In addition to the diaphragm type and piston type, there is also a long-stroke actuator, which has a long stroke and a large torque, which is suitable for the occasion of output angular displacement and high torque.
The standard for the signal received by the pneumatic actuator is 0.02 to 0.1MPa.
The main components of the pneumatic piston actuator are cylinders, pistons, and push rods. The piston in the cylinder moves with the change in the pressure difference between the two sides of the cylinder. According to the characteristics, it can be divided into two types: proportional and two-digit. The two-position type pushes the piston from the high-pressure side to the low-pressure side according to the amount of 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 and the signal pressure are proportional.
3. Rack and pinion actuator
Rack and pinion type (double piston rack and pinion type) pneumatic actuator has the characteristics of compact structure, beautiful appearance, fast response, stable operation and long service life. All fittings are treated with the most advanced anti-corrosion treatment technology and can adapt to a variety of harsh working conditions. Its actuators with high and low temperatures and various special strokes have good performance in various applications.
Fourth, the selection of pneumatic actuators
Before selecting a pneumatic actuator, check the valve torque. And increase the safety value in the torque, water vapor or non-lubricating liquid medium by 25% safety value, non-lubricating slurry liquid medium increase 30% safety value.
The selected valve torque is 210NM, the air source pressure is only 5bar, the medium is non-lubricated water vapor, taking into account the safety factor, increase the safety value value by 25%, that is, 262NM, according to the double-acting output torque table to find the corresponding torque value when the air source pressure is 5bar. 277NM should be selected, and the model is POADA300.
Pneumatic actuator torque table
5. Performance characteristics of pneumatic actuators
1. The rated output force or torque of the pneumatic device shall comply with the provisions of GB/T12222 and GB/T12223, and the above is a membrane actuator;
2. In the case of no load, the cylinder should be input according to the specified air pressure, and its action should be stable, without jamming and crawling;
3. Under the air pressure of 0.6MPa, the output torque or thrust of the pneumatic device in the opening and closing directions should not be less than the value indicated on the pneumatic device signage, and the action should be flexible, and permanent deformation and other abnormal phenomena are not allowed in each part;
4. When the maximum working pressure is used for the seal test, the amount of air leaked from the respective back pressure side shall not exceed (3+0.15D)cm3/min (standard state);The amount of air leaked from the end cover and output shaft shall not exceed (3+0.15d)cm3/min;
5. The strength test is carried out with 1.5 times the maximum working pressure, and after maintaining the test pressure for 3min, the end cover and static sealing part of the cylinder block are not allowed to leak and structural deformation;
6. The number of action life, the pneumatic device simulates the action of the pneumatic valve, and the opening and closing times of the opening and closing operation should not be less than 50,000 times (the opening and closing cycle is one) under the condition of maintaining the output torque or thrust capacity in both directions;
7. Pneumatic device with buffer mechanism, when the piston moves to the end of the stroke position, the impact phenomenon is not allowed.