The working principle of the control valve is simply realized by compressed air. In practical application, it is of great significance for manufacturers to understand the working principle of the pneumatic control valve.

Pneumatic control valve is one of the industrial process control instruments widely used in petroleum, chemical, electric power, metallurgy and other industrial enterprises. Pneumatic control valve is usually formed after connection, installation and commissioning of pneumatic actuator, valve, positioner, etc.

The action of pneumatic control valve is divided into air open type and air close type. Air to Open means that when the air pressure on the diaphragm head increases, the valve acts in the direction of increasing the opening. When the upper limit of input air pressure is reached, the valve is in full open state. Conversely, when the air pressure decreases, the valve acts in the closing direction. When no air is input, the valve is fully closed.

Therefore, sometimes the air opening valve is also called Fail to Close FC. The action direction of the air to close type is just opposite to that of the air to open type. When the air pressure increases, the valve acts in the closing direction; When the air pressure decreases or does not exist, the valve shall be turned to the opening direction or fully opened. Therefore, it is sometimes called Fail to Open FO. The air opening or air closing of the pneumatic control valve is usually realized by the positive and negative action of the actuator and different assembly methods of the valve state structure.

The selection of gas on/off is based on the safety of process production. When the air source is cut off, is the regulating valve safe in the closed position or the open position?

For example, for the combustion control of a heating furnace, the regulating valve is installed on the fuel gas pipeline to control the fuel supply according to the furnace temperature or the temperature of the heated material at the outlet of the heating furnace. At this time, it is safer to choose the air to open valve, because once the air supply stops, it is more appropriate for the valve to be closed than to be fully open.

If the air source is interrupted and the fuel valve is fully opened, excessive heating may occur. Another example is a heat exchange equipment that is cooled by cooling water. The hot materials exchange heat with the cooling water in the heat exchanger and are cooled. The regulating valve is installed on the cooling water pipe. The cooling water volume is controlled by the material temperature after heat exchange. When the air source is interrupted, the regulating valve should be in the open position, which is safer. The air closed (i.e. FO) regulating valve should be selected.