Pneumatic control valve is usually composed of pneumatic actuator and control valve connection, installation and commissioning. Pneumatic actuators can be divided into single-acting and double-acting types. Single-acting actuators have a return spring, while double-acting actuators have no return spring . The single-acting actuator can automatically return to the opening or closing state initially set when the valve loses its origin or suddenly fails.

Pneumatic control valve is divided into two types: air-open type and air-close type according to the action form, namely the so-called normally open type and normally closed type. The pneumatic open or close of the pneumatic control valve is usually through the positive and negative action and valve structure of the actuator different assembly methods.

The air-open type (normally closed type) is when the air pressure on the membrane head increases, the valve moves in the direction of increasing the opening degree. When the upper limit of the input air pressure is reached, the valve is fully open. Conversely, when the air pressure decreases, the valve moves in the closing direction, and when there is no air input, the valve is fully closed. Therefore, we usually call the gas-open control valve as a fault-closed valve.

The air-close type (normally open type) operates in the opposite direction to the air-open type. When the air pressure increases, the valve moves in the closing direction; when the air pressure decreases or does not exist, the valve moves in the opening direction until it is fully open. Therefore, we usually call air-closed control valves as fault-opening valves.

The choice of air off and on is based on the safety of process production. When the air source is cut off, the regulator is safe in the closed or open position.

For example, the combustion control of a heating furnace, the control valve is installed on the fuel air pipeline, and the fuel supply is controlled according to the temperature of the furnace or the temperature of the heated material at the furnace outlet. At this time, it is safer to choose a air-open valve, because once the supply is stopped, it is more suitable to close the valve than to open it. If the air supply is interrupted and the fuel valve is fully opened, there is danger of overheating. Another example is a heat exchange equipment cooled by cooling water. The hot materials are cooled by heat exchange with the cooling water in the heat exchanger. The control valve is installed on the cooling water pipe. When the source is interrupted, the control valve should be in the open position to be safer, and a air-closed (ie FO) control valve should be selected.