Structure and working principle of pneumatic control valve

Pneumatic control valves can be divided into two types: air-open type and air-closed type. Air-open type means that when the air pressure on the membrane head increases, the valve moves in the direction of increasing opening, and when the upper limit of input air pressure is reached, the valve is in a fully open state. On the contrary, when the air pressure decreases, the valve moves in the closing direction, and when there is no air input, the valve is completely closed. Therefore, sometimes gas-operated valves are also called unable to close FC. The action direction of closed air is just the opposite to that of closed air. When the air pressure increases, the valve moves in the closing direction; When the air pressure decreases or does not decrease, the valve will be opened or fully opened. ThereFOre, it is sometimes called unable to open fo. The air opening or closing of pneumatic control valve is usually realized by the positive and negative actions of actuator and different assembly methods of valve state structure. The selection of gas switch is based on the safety of process production. When the air supply is cut off, is it safe for the regulating valve to be in the closed position or the open position? For example, for the combustion control of the heating furnace, a regulating valve is installed on the gas pipeline to control the fuel supply according to the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. It is safer to use pneumatic valve at this time, because once the gas supply is stopped, it is more appropriate to close the valve than to fully open it. If the air supply is interrupted and the fuel valve is fully opened, there will be danger of overheating. Another example is a heat exchange device cooled by cooling water. The hot material is cooled by heat exchange with cooling water in a heat exchanger. The regulating valve is installed on the cooling water pipe, and the cooling water quantity is controlled by the temperature of the material after heat exchange. When the gas supply is interrupted, the regulating valve should be in the open position, which is safer, and it is advisable to choose the gas closed (FO) regulating valve. If the air-opening type is changed to the air-closing type or the air-closing type is changed to the air-opening type, if the control valve is equipped with an intelligent valve positioner, it can be easily switched on site. However, on some occasions, the valve is not expected to be in the fully open or closed position, so it is not allowed to operate, but it is hoped that it will remain in its original position before the gas is cut off. At this time, some other measures can be taken, such as using a position-retaining valve or setting up a special gas storage tank for emergencies. Valve positioner Valve positioner is the main accessory of control valve, which has great matching with pneumatic control valve. It receives the output signal of the regulator, and then controls the pneumatic regulating valve with its output signal. When the regulating valve acts, the displacement of the valve stem is fed back to the valve positioner through the mechanical device, and the valve position is transmitted to the upper system through the electrical signal. Valve positioner can be divided into pneumatic valve positioner, electro-pneumatic valve positioner and intelligent valve positioner according to its structure and working principle. The invention can increase the output power of the regulating valve, reduce the transmission lag of the regulating signal, accelerate the moving speed of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem and eliminate the influence of unbalanced force, thus ensuring the correct positioning of the regulating valve. Commonly used actuators are divided into pneumatic actuators and electric actuators, which are divided into straight stroke and angular stroke. Used for automatically and manually opening and closing various cutting doors, wind shields, etc.