There are two modes of action of pneumatic control valve:
1, gas open type (normally closed type) is when the air pressure on the membrane head increases, the valve to increase the opening direction of action, when the upper limit of the input pressure is reached, the valve is in a fully open state. Conversely, when the air pressure decreases, the valve moves in the closing direction, and when there is no input air, the valve is fully closed. Gu usually we call the gas-open regulating valve a fail-closed valve. The choice of gas opening and closing is considered according to the safety point of view of process production. When the gas supply is cut off, the regulating valve is in the closed position or the open position is safe.

2. The action direction of air-off type (normally open type) is exactly opposite to that of air-on type. When the air pressure increases, the valve to close the direction of action; when the air pressure is reduced or not, the valve to open the direction or fully open. Gu usually we call the gas-closed regulating valve a failure-open valve.

Pneumatic control valve installation position, from the ground requirements have a certain height, the valve up and down to leave a certain space, in order to carry out the valve disassembly and repair, electric control valve for pneumatic valve positioner and hand wheel control valve, must ensure operation, observation and adjustment convenience.

Regulating valve should be installed in the horizontal pipe, and up and down with the pipe vertical, generally to be supported under the valve, to ensure stable and reliable, for special occasions, need to regulate the level of installed in the vertical pipeline, should also be the control valve support (except small diameter control valve), installation, to avoid bringing additional stress to the control valve).

The accurate and normal operation of the pneumatic control valve is of great significance to ensure the normal operation and safe production of the process device. Therefore, it is necessary to strengthen the maintenance of pneumatic control valve.
　
Key inspection parts during maintenance:
Check the inner wall of the body: in high pressure and corrosive media occasions, the inner wall of the valve body, diaphragm valve diaphragm often by the impact of the medium and corrosion, must focus on checking the pressure and corrosion resistance;
Check the valve seat: due to the infiltration of the medium during work, the internal surface of the thread used to fix the valve seat is susceptible to corrosion and the valve seat is loose;
Check the valve core: the valve core is one of the movable parts of the control valve, the erosion of the medium is more serious, the maintenance should carefully check whether the valve core parts are corroded, wear, especially in the case of high pressure difference, the wear of the valve core cavitation caused by cavitation is more serious. The damaged valve core shall be replaced;
Check the sealing packing: check whether the packing asbestos rope is dry, such as the use of PTFE packing, should pay attention to check whether the aging and its mating surface is damaged; check whether the rubber film in the actuator is aging, whether there is cracking phenomenon.

Routine maintenance of pneumatic control valve:
When the control valve using graphite asbestos as filler, about three months should be added on the filler lubricating oil, in order to ensure that the control valve flexible and easy to use. If the packing pressure cap is found to be very low, the packing should be supplemented. If the PTFE dry packing is found to be hardened, it should be replaced in time. Attention should be paid to the operation of the regulating valve during the patrol inspection, and whether the output of the valve position indicator and the regulator are consistent. For regulating valves with positioners, the air source should be checked frequently, and problems found should be handled in time. The sanitation of the regulating valve and the integrity and the components should be kept easy to be kept.