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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 operate the valve, enabling on-off or proportional control. It receives control signals from industrial automation systems in order to regulate various process parameters of the fluid flowing through the pipeline, such as flow rate, pressure, and temperature. Pneumatic control valves are characterized by simple control, fast response, and the need for no additional explosion-proof measures. Classification and Applications of Pneumatic Control Valves 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; when no air is supplied, the valve is fully closed. 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 the air pressure decreases or is absent, the valve moves in the opening direction 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 a pneumatic control valve is usually achieved through the opposing 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 production process. Is it safer for the control valve to be in the closed position or the open position when the air supply is cut off? For example, in the combustion control of a heating furnace, a control valve is installed on the fuel pipeline to regulate 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 advisable. Changing from air-open to air-close, or from air-close to air-open, is easy to accomplish on-site if the control valve is equipped with an intelligent valve positioner. However, there are also situations where it is not desirable for the valve to be in the fully open or fully closed position in case of a failure; such operation is not allowed, and instead it is preferred that the valve remain in its original position before the air supply was cut off in the event of a failure. At this time, other measures can be taken, such as using hold-down valves or installing dedicated air storage cylinders for emergency use, to ensure safety. Precautions for using pneumatic control valves: 1. These valves should be stored in a dry indoor environment, and both ends of the pipeline must be sealed off. Stacking for storage is not allowed. 2. Control valves that are to be stored for an extended period should be inspected regularly to remove dirt; anti-rust oil should be applied to all moving parts and machined surfaces to prevent rusting. 3. This valve should be installed on a horizontal pipeline and must be installed vertically. The valve stem is upward. 4. The required components must be installed in the direction of medium flow indicated by the arrows in the diagram