Non standard customized pneumatic actuator Pneumatic valve manufacturer overview Pneumatic actuator - principle, classification, model selection
1、 Overview of pneumatic actuators
Pneumatic actuator is an actuator driven by air pressure to open and close or adjust valves, also known as pneumatic actuator or pneumatic device, but generally called pneumatic head. The actuating mechanism and regulating mechanism of the pneumatic actuator are a unified whole, and its actuating mechanism includes membrane, piston, fork and gear rack.
The piston type has a long stroke, which is suitable for occasions requiring large thrust; The diaphragm type has small stroke and can only directly drive the valve stem. Fork type pneumatic actuator has the characteristics of large torque, small space, torque curve more in line with the valve torque curve, but it is not beautiful; Commonly used for high torque valves. The rack and pinion pneumatic actuator has the advantages of simple structure, stable and reliable operation, safety and explosion-proof. It is widely used in power plants, chemical industry, oil refining and other production processes with high safety requirements.
2、 Working principle of pneumatic actuator
1. Working principle diagram of double acting pneumatic actuator
When the air source pressure enters the middle chamber between the two pistons of the cylinder from the air port (2), the two pistons are separated to both ends of the cylinder, the air in the air chambers at both ends is discharged through the air port (4), and the two piston teeth drive the output shaft (gear) to rotate counterclockwise. On the contrary, when the air supply pressure enters the air chambers at both ends of the cylinder from the air port (4), the two pistons move towards the middle of the cylinder, the air in the middle air chamber is discharged through the air port (2), and the two piston racks synchronously drive the output shaft (gear) to rotate clockwise. (If the piston is installed in the opposite direction, the output shaft will rotate in the opposite direction)
2. Working principle diagram of single acting pneumatic actuator
When the air source pressure enters the middle chamber between the two pistons of the cylinder from the air port (2), the two pistons are separated to both ends of the cylinder, forcing the springs at both ends to compress. The air in the air chambers at both ends is discharged through the air port (4), and the two piston bars simultaneously drive the output shaft (gear) to rotate counterclockwise. After the air source pressure is changed through the solenoid valve, the two pistons of the cylinder move towards the middle under the elasticity of the spring, and the air in the middle air chamber is discharged from the air port (2), so that the two piston frames synchronously drive the output shaft (gear) to rotate clockwise. (If the piston is installed in the opposite direction, the output shaft will rotate in the opposite direction when the spring returns.).
3、 Classification of pneumatic actuators
1. Membrane actuator
The membrane actuator is commonly used, which can be used as the pushing device of general control valves to form a pneumatic membrane actuator. The signal pressure P of the pneumatic membrane actuator acts on the diaphragm to deform it, driving the push rod on the diaphragm to move, shifting the valve core, thus changing the opening of the valve. The utility model has the advantages of simple structure, low price, convenient maintenance and wide application.
Pneumatic membrane actuators have two forms: positive and negative effects.
When the signal pressure of the controller or valve positioner increases, the valve stem moves downward, which is called positive actuator; When the signal pressure increases, the valve rod moves upward, which is called the reverse actuator. The signal pressure of the positive acting actuator passes through the membrane air chamber above the corrugated membrane; The signal pressure of reaction actuator passes through the membrane air chamber below the corrugated membrane. By replacing individual parts, the two can be retrofitted to each other.
2. Piston actuator
The pneumatic piston actuator moves the piston in the cylinder to generate thrust. Obviously, the output strength of the piston is far greater than that of the membrane type. Therefore, the film type is suitable for the occasions with small output force and high precision; The piston type is suitable for occasions with large output force, such as large diameter, high pressure drop control or butterfly valve drive device. In addition to membrane type and piston type, there is also a long stroke actuator with long stroke and large torque, which is suitable for the occasion of output angular displacement and large torque.
The signal standard received by the pneumatic actuator is 0.02 to 0.1MPa.
Cylinder, piston and push rod are the main components of pneumatic piston actuator. The piston in the cylinder moves with the change of the pressure difference between the two sides of the cylinder. It is divided into proportional type and two digit type according to characteristics. 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 a valve positioner based on two position type to make the displacement of push rod proportional to the signal pressure.
3. Rack and pinion actuator
Gear rack (double piston rack) pneumatic actuator is compact in structure, beautiful in appearance, fast in response, stable in operation and long in service life. All accessories adopt more advanced anti-corrosion technology, which can adapt to various harsh working conditions. High and low temperature actuators and actuators with various special strokes perform well in various application fields.
4、 Selection of Pneumatic Actuator
Before selecting the pneumatic actuator, please confirm the valve torque. The safety value of torque shall be increased, and the safety value of water vapor or non lubricating liquid medium shall be increased by 25%; The safety value of non lubricating slurry medium is increased by 30%.
When the valve torque is 210 NM, the air source pressure is only 5 bar. Considering the safety factors, the medium is non lubricating steam, and the safety value increases by 25%, that is 262NM. Find the corresponding torque value of air source pressure 5 bar according to the dual function output torque table. 277NM shall be selected and the model shall be POADA300.
▼ Pneumatic actuator torque table
5、 Performance characteristics of pneumatic actuator
1. The rated output force or torque of the pneumatic device shall meet the requirements of GB/T12222 and GB/T1223. It is a membrane actuator;
2. Under no-load condition, input the specified air pressure into the cylinder, and the action shall be stable without jamming and creeping;
3. Under the air pressure of 0.6MPa, the output torque or thrust of the pneumatic device in both directions shall be opened and closed. The value shall not be less than the value marked on the sign of the pneumatic device. The action shall be flexible, and there shall be no ********* deformation and other abnormal phenomena on all parts;
4. When the sealing test is carried out under a large working pressure, the air quantity leaked at each back pressure side shall not exceed (3)+0.15D) cm3/min (standard state); The amount of air leaking from the end cover of the output shaft shall not exceed (3)+0.15d) cm3/min;
5. The strength test shall be conducted with 1.5 times greater working pressure. After the test pressure is maintained for 3min, the cylinder end cover and static sealing parts shall be free of leakage and structural deformation;
6. For the action life time, the pneumatic device simulates the action of the pneumatic valve. When maintaining the output torque or thrust capacity in both directions, the number of opening and closing operations shall not be less than 50000 times (one opening and closing cycle);
7. When the piston moves to the end of the stroke, the pneumatic device with buffer mechanism is not allowed to impact.
