1, open air (air
To Open) The regulating valve moves in the direction of increasing opening when the air pressure on the membrane head increases, and fully opens when the upper limit of input air pressure is reached. 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 air-operated valves are also called fail-closed valves.
Turn off FC).
2, airtight type (air
To close) the direction of action is just the opposite of the open air type. 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. When there is no air input, the valve is fully open. Therefore, it is sometimes called a Fail to open.
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.
3. The choice of air opening and air closing is based on the safety of process production. When the air supply is cut off, it is safe whether the regulating valve is 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. It is recommended to select the air-tight (FO) control valve. "