The working principle of the pneumatic control valve

　　The pneumatic control valve is to use compressed air as the power source, the cylinder as the actuator, and with the help of electrical valve positioners, converters, solenoid valves, position retention valves and other accessories to drive the valve, realize the switching or proportional adjustment, and receive the control signal of the industrial automation control system to complete the adjustment of various process parameters such as flow, pressure, temperature and so on. Pneumatic control valves are characterized by simple control, fast response, and no additional explosion-proof measures are required.

Classification and application of pneumatic control valves

　　There are two types of pneumatic control valve operation: air open type and air closed type. Air to Open means that when the air pressure on the membrane head increases, the valve moves in the direction of increasing the opening, and when the upper limit of the input air pressure is reached, the valve is in a fully open state. Conversely, when the air pressure decreases, the valve moves in the direction of closing, and when no air is being fed, the valve is fully closed. Therefore, sometimes the air-open valve is also called the Fail to Close FC type. Air to Close moves in the opposite direction to Air Open. When the air pressure increases, the valve acts in the direction of closing; When the air pressure decreases or is absent, the valve is open or fully open. Therefore, it is sometimes referred to as Fail to Open FO. The air opening or closing of the pneumatic control valve is usually realized by the positive and negative action of the actuator and the different assembly methods of the valve state structure.

　　The choice of gas switch is considered from the perspective of safety in process production. When the air supply is cut off, is the control valve safe in the closed position or in the open position? For example, in the combustion control of a furnace, a regulating valve is installed on the fuel gas pipeline to control the supply of fuel according to the temperature of the furnace chamber or the temperature of the material being heated at the furnace outlet. In this case, it is safer to use an air-opening valve, because once the air supply stops, it is more appropriate for the valve to be closed than to be fully open. If the gas supply is interrupted and the fuel valve is fully open, it can make overheating dangerous. Another example is a heat exchange equipment cooled by cooling water, the hot material is cooled by heat exchange with cooling water in the heat exchanger, the regulating valve is installed on the cooling water pipe, and the cooling water is controlled by the temperature of the material after heat exchange, and when the air source is interrupted, the regulating valve should be in the opening position to be safer, and the gas closed (i.e. FO) regulating valve should be selected.

　　If the control valve is equipped with an intelligent valve positioner, it can be easily switched between them on site.

　　However, there are also some occasions when the valve is not expected to be in the fully open or fully closed position during the failure, and the operation is not allowed, but it is hoped that the valve will remain in the original position before the gas cut-off when the fault occurs. At this time, some other measures can be taken, such as the use of position retention valves or the installation of special accident air storage cylinders and other facilities to ensure this.

Precautions for the use of pneumatic control valves

　　1. This valve should be stored in a dry room, and both ends of the passage must be blocked. It is not allowed to be stored in stacks

　　2. The control valve for long-term storage should be checked regularly to remove dirt, and the moving parts and processing surfaces should be coated with anti-rust oil to prevent rust.

　　3. This valve should be installed on the horizontal pipeline, and the vertical installation is mandatory. The stem is up.

　　4. It is mandatory to install the medium flow direction indicated by the arrow in the figure