Working principle of pneumatic control valve

Pneumatic control valve drives itself by compressed air as power source , cylinder as actuator and with some attachments ,such as electric-pneumatic positioned、converter、 solenoid valve、lock up valve and so on. It realizes the switch quantity or proportional adjustment, and receives the control signal of industrial automation control system to complete regulating the flow 、pressure 、temperature and other various process parameters of pipeline medium. Pneumatic control valve has characteristics of simple controlling, quick response, and intrinsically safe without additional explosion prevention measure.

Pneumatic control valve’s actions are divided into Air to open type and Air to close type. Air to open type is that when the air pressure on membrane head increases, valve moves in the direction of opening increasing; until it reaches to input pressure limit, the valve is in the fully open position. In reverse, when the air pressure is reduced, valve moves in the direction of closing, when there is no input air, the valve is fully closed.

So sometimes, Air to open type is also called Fail to Close/ FC type. Air to close type‘s movement direction is just the reverse of Air to open type. When the air pressure increases, valve moves in the direction of closing; when air pressure decreases or disappears, valve moves in the direction of opening or until fully open. So sometimes it’s called Fail to Open /FO. Pneumatic control valve belongs to which type is according to positive and negative effects of the actuator and different assemble ways of valve structure.

Choosing Air to open or Air to close is according to safety perspective of the process of production. When cut off the air source, in the closed position or open position control valve is safe?

For example, a heating furnace combustion control, the control valve is installed in the fuel pipe, according to the temperature of furnace or the heated material in furnace exit to control the supply of fuel. At this time, Air to open type valve is safer, because once stop the air supply, It’s more suitable in closed position than in fully open position for valve.

If air supply is cut off, fuel valve is fully open, risk will appear because of excessive heating. And such as a cooling heat-exchange equipment with cooling water, hot material in the heat exchanger are cooled when having heat exchange with cooling water, control valve is installed on the cooling water pipe, temperature of heat exchanged materials is used to control the cooling water’s volume, when the air supply is cut off, control valve in open position is more safe, Air to close type is appropriate.

Valve positioner is the main accessory of control valve, it accepts the regulator output signal, and then controls the pneumatic control valve by its output signal, when control valve moves, the displacement of valve stem feedback to valve positioner through mechanical device, valve position condition will be sent o the upper system through the electrical signal .

According to valve positioner structure and working principle ,it can be divided into pneumatic valve positioner, electric-pneumatic valve positioner and smart valve positioner.

      Valve positioner can increase the output power of control valve, reduce the transmission lag of control signal, speed up movement of valve stem, improve the linearity of valve, overcome the friction of valve stem and eliminate the influence of unbalanced force, so as to ensure positioning rightly.

valve positioner

      Actuator has pneumatic actuator, electric actuator, straight stroke and stroke.