How do pneumatic control valves work?

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　　Pneumatic control valves are a very common type among various control valves. They utilize compressed air to generate power, and their simple and easy-to-use nature has made them popular among users. Below, the editor will take you through the principles behind their operation as well as their different classifications. 　　For any machine to function, it needs power; some rely on external fuel as a source of power, while others use energy conversion to generate power. The valves in machines are one such device that utilizes energy conversion to make the machine operate. Among them, a pneumatic valve is a device that uses compressed air to drive machinery, with a sophisticated working principle. Working principle of pneumatic control valves: A pneumatic control valve uses an air source as power, a cylinder as the actuator, and a 4–20mA signal as the driving signal. It relies on accessories such as electrical valve positioners, converters, solenoid valves, and hold-down valves to operate the valve, enabling it to perform regulation in a linear or proportional manner with respect to flow rate. This allows for on/off or proportional control of various process parameters such as flow rate, pressure, and temperature of the fluid in the pipeline. 　　Pneumatic control valves offer advantages such as simple control, rapid response, and intrinsic safety; moreover, when used in flammable and explosive environments, no additional explosion-proof measures are required. 　　Pneumatic control valves typically consist of a pneumatic actuator and a control valve, which are connected and assembled for operation. Pneumatic actuators can be divided into single-acting and double-acting types; single-acting actuators contain a return spring, whereas double-acting actuators do not have such a spring. Among them, the single-acting actuator can automatically return to the initial open or closed position set for the valve in the event of a loss of power or a sudden failure. 　　Pneumatic control valves are divided into two types based on their operating mode: air-open and air-close, which are also known as normally open and normally closed types. 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. Pneumatic control valves operate in two modes: air-open and air-close. Pneumatic control valves come in air-open and air-close types. 　　The Air to Open type means that as the air pressure at the diaphragm head increases, the valve moves in a direction that increases its opening degree; when the maximum input air pressure is reached, the valve is in its fully open state. 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 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 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 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 pneumatically operated valve, as once the gas supply is interrupted, it is more appropriate for the valve to be in a closed position 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 a valve that closes when air supply is interrupted (i.e., FO type 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 desired 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. At this time, other measures can be taken, such as using check valves or installing dedicated air storage cylinders for emergency use, to ensure safety. Valve positioner: A 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 the valve stem is fed back to the valve positioner through mechanical mechanisms, and the valve’s position is transmitted as an electrical signal to the higher-level system. 　　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 on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve. Common actuators include pneumatic actuators and electric actuators, which can be further divided into linear stroke and rotary stroke types. Used to automatically or manually open and close various types of shutters, dampers, etc