Specific principle of pneumatic control valve

Specific principle of pneumatic control valve

Pneumatic control valve action is divided into air on type and air off 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 fully open. Conversely, when the air pressure decreases, the valve acts in the direction of closing. When there is no input air, the valve is fully closed. Therefore, sometimes air to open valves are also called fault closed valves (fail to close FC). The action direction of air to close type is just opposite to that of air 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 acts in the opening direction or fully opens. Therefore, it is sometimes called fault open type (fail to open fo). The pneumatic opening or closing of pneumatic control valve is usually realized through the positive and negative action of actuator and different assembly methods of valve state structure.

The selection of gas on and gas off is considered from the perspective of process production safety. When the air source is cut off, is the regulating valve in the closed safety position or the open safety 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 temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this time, it is safer to select the air on 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, it will cause danger of excessive heating. Another example is a heat exchange equipment cooled by cooling water. The hot material is cooled by heat exchange with the cooling water in the heat exchanger. The regulating valve is installed on the cooling water pipe to control the cooling water volume with the material temperature after heat exchange. When the air source is interrupted, the regulating valve should be in the open position, which is safer. It is better to select the air closed (i.e. fo) regulating valve.

The air open type is changed to air closed type or the air closed type is changed to air open type. If the regulating valve is equipped with an intelligent valve positioner, it can be easily switched to each other on site.

However, in some cases, the valve is not expected to be in the fully open or fully closed position in case of failure, and the operation is not allowed. Instead, it is expected to remain in the original position before gas cut-off in case of failure. At this time, some other measures can be taken, such as using position holding valve or setting special air reservoir for accident.

Valve positioner 

The valve positioner is the main accessory of the regulating valve, which is greatly matched with the pneumatic regulating valve. It receives the output signal of the regulator, and then uses its output signal to control the pneumatic regulating valve. When the regulating valve acts, the displacement of the valve rod is fed back to the valve positioner through the mechanical device, and the valve position status is transmitted to the upper system through the electrical signal.

According to its structure and working principle, valve positioner can be divided into pneumatic valve positioner, electro-pneumatic valve positioner and intelligent valve positioner.

The valve positioner can increase the output power of the regulating valve, reduce the transmission lag of the regulating signal, accelerate the moving speed of the valve stem, improve the linearity of the valve, overcome the friction force of the valve stem and eliminate the influence of the unbalanced force, so as to ensure the correct positioning of the regulating valve.

Common actuators are divided into pneumatic actuators and electric actuators, including straight stroke and angular stroke. It is used to open and close all kinds of cutting doors and air panels automatically and manually.