Pneumatic control valve is one of the important links of automatic control process instruments. Correct selection is crucial to the normal operation of the system. Besides the common parameters such as pressure, temperature, diameter, material and structural form of the control valve, the selection of pneumatic control valve should also pay attention to the relevant accessories. The main accessories of pneumatic control valve include: valve positioner, solenoid valve, valve position feedback, air filter pressure reducing valve, position retaining valve The selection principles of main accessories are described as follows:

1、 Valve positioner

The valve positioner is the main accessory of the pneumatic actuator. It is used together with the pneumatic actuator to improve the position accuracy of the valve, overcome the influence of the friction force of the valve rod and the unbalanced force of the medium, and ensure that the valve can be correctly positioned according to the signal from the regulator. The valve positioner is used to convert 4-20mA control signal into air pressure signal to pneumatic actuator to drive the regulating valve for regulating function. Valve positioners are divided into mechanical positioners and intelligent positioners. Intelligent positioners have higher control accuracy and lower air consumption than mechanical positioners. In addition, they can also have HART protocol function.

2、 Solenoid valve

When the system needs to realize program control or two-position control, it needs to be equipped with solenoid valve. When selecting the solenoid valve, in addition to considering the AC and DC power supply, voltage and frequency, attention must be paid to the relationship between the solenoid valve and the action type of the regulating valve, which can be matched with "normally open" or "normally closed".

If it is required to increase the capacity of the solenoid valve to shorten the action time, two solenoid valves can be used in parallel or the solenoid valve can be used as a pilot valve in combination with a large-capacity pneumatic relay.

3、 Pneumatic relay

The pneumatic relay is a power amplifier, which can send the air pressure signal to a far place and eliminate the lag caused by the extension of the signal pipeline. It is mainly used between the field transmitter and the regulating instrument in the central control room, or between the regulator and the field regulating valve. Another function is to amplify or reduce the signal.

4、 Converter

The converter is divided into pneumatic-electric converter and electro-pneumatic converter. Its function is to realize the mutual conversion of certain relationship between pneumatic and electrical signals. It is mainly used to convert 0 ～ 10mA or 4 ～ 20mA electrical signals or 0 ～ 100KPa gas signals into 0 ～ 10mA or 4 ～ 20mA electrical signals when the pneumatic actuator is operated by electrical signals.

5、 Air filtration pressure reducing valve

The air filter pressure reducing valve is an accessory in industrial automation instruments. Its main function is to filter and purify the compressed air from the air compressor and stabilize the pressure at the required value. It can be used as the air supply source and pressure stabilizing device for various pneumatic instruments, solenoid valves, cylinders, spraying equipment and small pneumatic tools.

6、 Self-locking valve

The self-locking valve is a device to maintain the valve position. When the air source of the pneumatic control valve fails, the device can cut off the air source signal and keep the pressure signal of the membrane chamber or cylinder in the state immediately before the failure, so that the valve position can also be maintained in the position before the failure, which plays a role in maintaining the position.

7、 Valve position transmitter

When the regulating valve is far away from the control room, in order to accurately understand the opening and closing position of the valve without going to the site, a valve position transmitter shall be equipped, that is, the displacement of the valve opening mechanism shall be converted into an electrical signal according to a certain rule and sent to the control room. This signal can be a continuous signal reflecting any opening of the valve, or it can be considered as the reverse action of the valve positioner.

8、 Travel switch (annunciator)

The travel switch is a device that reflects the two positions of the valve switch and sends an indication signal at the same time. The control room can break the switch status of the valve according to this signal so as to take corresponding measures.