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Working principle of pneumatic control valves: A pneumatic control valve uses compressed air as its power source, with a cylinder serving as the actuator. It relies on accessories such as electrical valve positioners, converters, solenoid valves, and hold-down valves to drive the valve, enabling on-off or proportional control. It receives control signals from industrial automation systems in order to regulate various process parameters such as the flow rate, pressure, and temperature of the medium flowing through the pipeline. Pneumatic control valves are characterized by simple control, fast response, and intrinsic safety, eliminating the need for additional explosion-proof measures. Pneumatic control valves operate in two modes: air-open and air-close. The Air to Open type operates such that as the air pressure at the diaphragm head increases, the valve opens further; when the maximum input air pressure is reached, the valve is in its fully open position. Conversely, as the air pressure decreases, the valve moves in the direction of closure, and when no air is supplied, the valve closes completely. Therefore, sometimes air-operated valves are also referred to as fail-to-close (FC) valves. The operating direction of the air-to-close type is exactly the opposite of that of the air-to-open type. As air pressure increases, the valve moves in the direction of closure; when air pressure decreases or is absent, the valve moves in the direction of opening until it is fully open. Therefore, it is sometimes also referred to as Fail to Open (FO). The air-open or air-close operation of pneumatic control valves is usually achieved through the opposite actions of the actuator and different assembly methods of the valve structure. The choice between gas on and gas off is determined from the perspective of safety in the manufacturing process. Is it safer for the control valve to be in the closed position or the open position when the gas supply is cut off? For example, in the combustion control of a heating furnace, the control valve is installed on the fuel pipeline, and it regulates the fuel supply based on the temperature of the furnace chamber or the temperature of the material being heated at the outlet of the furnace. At this time, it is safer to use a gas-operated valve, as once the gas supply is interrupted, it is more appropriate for the valve to be closed rather than fully open. If the gas supply is interrupted with the fuel valve fully open, it can lead to dangerous overheating. Another example is a heat exchange device cooled by cooling water; the hot material exchanges heat with the cooling water inside the heat exchanger to be cooled. A control valve is installed on the cooling water pipe, and the amount of cooling water is regulated based on the temperature of the material after heat exchange. In the event of a disruption in the air supply, it is safer for the control valve to be in the open position, so an air-operated shut-off (i.e., FO) type control valve is recommended. The valve positioner is a key accessory for control valves, used in conjunction with pneumatic control valves. It receives the output signal from the controller and uses this signal to control the pneumatic control valve. When the control valve moves, the displacement of its stem is fed back to the valve positioner through mechanical mechanisms, and the valve’s position is transmitted to the higher-level system via electrical signals. Based on their structural design and working principles, valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners. Valve positioners can increase the output capacity of control valves, reduce the delay in the transmission of control signals, speed up the movement of the valve stem, improve the linearity of the valve, overcome the frictional forces acting on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve.