Installation principles of pneumatic control valve

Pneumatic control valve is one of the industrial process control instruments widely used in petroleum, chemical, electric power, metallurgy and other industrial enterprises. In chemical production, the control valve is indispensable in the control system. It is an important part of the industrial automation system, just like the hands and feet of the production process automation.

Valve Positioner

The valve positioner is the main accessory of the regulating valve, and is used in conjunction 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 is actuated, the displacement of the valve stem 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. According to its structural form and working principle, the 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 control valve, reduce the transmission lag of the control signal, speed up the movement speed of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem and eliminate the influence of unbalanced force, thereby ensuring the correct positioning of the control valve.

The actuators are divided into pneumatic actuators and electric actuators, and they are divided into linear actuators and angular actuators. They are used to automatically or manually open and close various types of gates, wind panels, etc.

Installation principles of pneumatic control valve:

(1) The installation position of the pneumatic control valve must be at a certain height from the ground, and a certain amount of space must be left above and below the valve to facilitate disassembly and repair of the valve. For control valves equipped with pneumatic valve positioners and hand wheels, it must be ensured that operation, observation and adjustment are convenient.

(2) The regulating valve should be installed on a horizontal pipeline and vertically connected to the pipeline. Generally, it should be supported under the valve to ensure stability and reliability. For special occasions, when the regulating valve needs to be installed horizontally on a vertical pipeline, the regulating valve should also be supported (except for small-diameter regulating valves). During installation, avoid adding additional stress to the regulating valve).

(3) The working environment temperature of the regulating valve should be between (-30 and +60) and the relative humidity should not exceed 95%95%.

(4) There should be a straight pipe section before and after the regulating valve, with a length of not less than 10 times the pipe diameter (10D) to avoid the straight pipe section of the valve being too short and affecting the flow characteristics.

(5) When the diameter of the regulating valve is different from that of the process pipeline, a reducer should be used for connection. When installing a small-diameter regulating valve, a threaded connection can be used. The fluid direction arrow on the valve body should be consistent with the fluid direction.

(6) A bypass pipeline should be set up to facilitate switching or manual operation, so that the regulating valve can be inspected and repaired without stopping the machine.

(7) Before installing the regulating valve, foreign matter in the pipeline, such as dirt, welding slag, etc., must be thoroughly removed.

Pneumatic control valve working mode:

The air-open type (normally closed type) is that when the air pressure on the diaphragm head increases, the valve moves in the direction of increasing the opening, and when the upper limit of the input air 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. Usually we call the air-open type control valve a fault-closed valve.

The action direction of the air-to-close type (normally open type) is just opposite to that of the air-to-open type. When the air pressure increases, the valve moves in the closing direction; when the air pressure decreases or disappears, the valve moves in the opening direction or fully opens. However, we usually call the air-to-close type control valve a fail-open type valve.