Pneumatic control valve is one of the important link of the automation control process of instrument, the right selection is very important to the normal operation of the system, pneumatic control valve structure selection in addition to the pressure, temperature, diameter, material and other regulator structure commonly used parameters need to be implemented, but also pay attention to the related accessories, pneumatic control valve of the main accessories: control valve positioner, solenoid valve, control valve position feedback, air filtering pressure reducing valve, valve, pneumatic relay etc, the following describe principle of type selection of pneumatic control valve main accessories:

1.Pneumatic control valve positioner
Valve positioner is the main accessory of pneumatic actuator, which is used with pneumatic actuator to improve valve position accuracy, overcome the influence of valve stem friction and medium unbalanced force, so as to ensure that the valve is positioned correctly according to the signal from regulator.The valve positioner is used to convert control signals of 4-20ma into pneumatic signals for pneumatic actuators to drive the regulator for regulating functions.Valve positioner is divided into mechanical positioner and intelligent positioner. The intelligent positioner has higher control accuracy and lower gas consumption than mechanical positioner. In addition, it can also carry HART protocol function.
 
2.Solenoid valve
When the system needs to implement program control or two control, it needs to be equipped with electromagnetic valve.When selecting solenoid valve, in addition to the ac, dc power supply and voltage, frequency, must pay attention to the relationship between the solenoid valve and regulating valve action type, can be used with "normally open" or "normally closed".
If it is required to increase the capacity of the solenoid valve to shorten the operation time, two solenoid valves can be used side by side or a combination of the solenoid valve as a pilot valve and a large capacity pneumatic relay can be used.
 
3.Pneumatic relay
Pneumatic relay of Pneumatic control valve is a kind of power amplifier, it can send the pressure signal to the far place, eliminate the delay caused by the extension of signal pipeline, it is mainly used for the field transmitter and the central control room adjusting instruments, or between the regulator and the field regulating valve, there is also a function of amplifying or reducing the signal.
 
4.Converter
The converter of Pneumatic control valve is divided into gas-electric converter and electro-gas converter. Its function is to realize the conversion between gas and electric signals in a certain relationship. It is mainly used to convert 0 ~ 10mA or 4 ~ 20mA telecommunications signals or 0 ~ 100KPa gas signals into 0 ~ 10mA or 4 ~ 20mA telecommunications signals when operating pneumatic actuators by electric signals.
 
5.Air filter pressure reducing valve
The air filter pressure reducing valve is a kind of accessory in the industrial automation instrument, its main function is to filter and purify the compressed air from the air compressor and stabilize the pressure on the required value, which can be used for all kinds of pneumatic instruments, electromagnetic valve, cylinder, spraying equipment and small pneumatic tools supply air source and pressure stabilizer.

6. Self-locking valve (positioning valve)
Self - locking valve is a device to keep Pneumatic control valve position.When the air source fault occurs, the device can cut off the air source signal, so that the pressure signal of the membrane chamber or cylinder is kept in the state of the instantaneous before the fault. In this way, the Pneumatic control valve position is also maintained in the position before the fault, which plays a role in maintaining the position.
 
7.Valve position transmitter
When control valve is far away from the control room for less than the scene can accurately understand the switch position, will be equipped with Pneumatic control valve position transmitter, the pneumatic control valve opening mechanism displacement, converted to electrical signal to the control room according to certain rule, the signal can be reflect the continuous signal of any valve opening, can also be considered to be the pneumatic  valve positioner inverse action.
 
8. Travel switch (echo)
The travel switch reflects the two extreme positions of the valve switch and simultaneously sends a device indicating the signal, which can be used in the control room to reverse the pneumatic control valve's switch status and take appropriate measures.