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Pneumatic control valves use compressed air as the power source, air cylinders as actuators, and accessories such as electrical valve positioners, converters, solenoid valves, and retaining valves to drive the valves. They achieve on-off or proportional regulation, and receive control signals from industrial automation control systems to adjust various process parameters such as flow, pressure, and temperature of the pipeline medium. Pneumatic control valves are characterized by simple control, rapid response, and intrinsic safety, without the need for additional explosion-proof measures.
Pneumatic regulating valves usually consist of connecting, installing, and debugging pneumatic actuators and regulating valves. Pneumatic actuators can be divided into single acting and double acting types, with a return spring inside the single acting actuator and no return spring inside the double acting actuator. The single acting actuator can automatically return to the opening or closing state initially set by the valve in case of loss of origin or sudden failure.
Pneumatic regulating valves can be divided into two types according to their action forms, namely, the so-called normally open type and the normally closed type. The pneumatic opening or closing of pneumatic regulating valves is usually achieved through the positive and negative effects of the actuator and different assembly methods of the valve structure.
Action mode of pneumatic control valve:
Air open type (normally closed type) refers to when the air pressure on the membrane head increases, the valve acts in the direction of increasing the opening degree. When the upper limit of input air pressure is reached, the valve is in a fully open state. Conversely, when the air pressure decreases, the valve moves towards the closed direction, and when there is no air input, the valve is fully closed. Generally speaking, we call the air opening type control valve a fault closing type valve.
The action direction of the air close type (normally open type) is just opposite to the air open type. When the air pressure increases, the valve moves towards the closing direction; When the air pressure decreases or does not exist, the valve moves towards the opening direction or fully opens. Generally speaking, we call an air shutoff regulating valve a fail open valve.
The selection of gas on/off is considered based on the safety perspective of process production. When the air supply is cut off, is the regulating valve in the closed position safe or the open position safe.
For example, for the combustion control of a heating furnace, a regulating valve is installed on the fuel gas pipeline to control the supply of fuel based on 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 choose an air-opening valve because once the air supply is stopped, it is more appropriate for the valve to be closed than fully open. If the air supply is interrupted and the fuel valve is fully opened, excessive heating may be dangerous. Another example is a heat exchange device cooled by cooling water. The hot material is cooled by exchanging heat with the cooling water in the heat exchanger. The regulating valve is installed on the cooling water pipe, and the temperature of the material after heat exchange is used to control the cooling water volume. When the air source is interrupted, the regulating valve should be in the open position, which is safer. It is advisable to select an air shutoff (FO) regulating valve.
Valve positioner
The valve positioner is the main accessory of the regulating valve and is greatly used in conjunction with the pneumatic regulating valve. It receives the output signal from 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 a mechanical device, and the valve position status is transmitted to the upper system through an electrical signal. Valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners according to their structural forms and working principles.
The valve positioner can increase the output power of the regulating valve, reduce the transmission delay of the regulating signal, accelerate the movement speed of the valve stem, improve the linearity of the valve, overcome the friction force of the valve stem, and eliminate the impact of unbalanced forces, thereby ensuring the correct positioning of the regulating valve.
The actuator is divided into pneumatic actuator and electric actuator, with straight stroke and angular stroke. It is used to automatically and manually open and close various cutting doors, wind panels, etc.
Installation principle of pneumatic control valve
(1) The installation position of the pneumatic control valve requires a certain height from the ground, and a certain space should be left above and below the valve to facilitate valve disassembly and repair. For regulating valves equipped with pneumatic valve positioners and hand wheels, it is necessary to ensure convenient operation, observation, and adjustment.
(2) The regulating valve should be installed on a horizontal pipe and perpendicular to the pipe from top to bottom. Generally, it should be supported under the valve to ensure stability and reliability. For special occasions, when it is necessary to install the regulating valve horizontally on a vertical pipe, the regulating valve should also be supported (except for small diameter regulating valves). During installation, it is necessary to avoid bringing additional stress to the regulating valve.
(3) The operating ambient temperature of the regulating valve should be (- 30 to+60) with a relative humidity of not more than 95% and 95%, and a relative humidity of not more than 95%.
(4) There should be straight pipe sections at the front and rear positions of the regulating valve, with a length not less than 10 times the pipe diameter (10D), to avoid the flow characteristics being affected by the short straight pipe sections of the valve.
(5) If the diameter of the regulating valve is different from that of the process pipe, a reducer pipe should be used for connection. When installing small diameter regulating valves, threaded connections can be used. The fluid direction arrow on the valve body should be consistent with the fluid direction.
(6) Bypass piping is to be provided. The purpose is to facilitate switching or manual operation, allowing maintenance of the regulating valve without stopping the machine.
(7) Before installing the regulating valve, thoroughly remove foreign matters in the pipeline, such as dirt, welding slag, etc
