Pneumatic control valve is one of the important links of automatic control process instrumentation. The correct selection is essential to the normal operation of the system. The selection of pneumatic control valve is in addition to the commonly used parameters such as pressure, temperature, caliber, material and control valve structure. Need to be implemented, but also pay attention to related accessories. The main accessories of pneumatic control valves are: valve positioner, solenoid valve, valve position feedback, air filter pressure reducing valve, retaining valve, pneumatic relay, etc. The main accessories are described below. The selection principle:
1. Valve positioner
The valve positioner is the main accessory of the pneumatic actuator. It is used in conjunction with the pneumatic actuator to improve the position accuracy of the valve and overcome the influence of the frictional force of the valve stem and the imbalance force of the medium, thereby ensuring that the valve is realized according to the signal from the regulator Position it correctly. The valve positioner is used to convert the 4-20mA control signal into the air pressure signal to the pneumatic actuator, thereby driving 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, and can also be equipped with HART protocol functions.
Two, solenoid valve
When the system needs to achieve program control or two-position control, it needs to be equipped with a solenoid valve. When selecting a solenoid valve, in addition to considering the AC and DC power supply, voltage and frequency, the relationship between the solenoid valve and the regulating valve function type must be paid attention to. It can be equipped with "normally open" or "normally closed".
If you need to increase the capacity of the solenoid valve to shorten the action time, you can use two solenoid valves in parallel or use the solenoid valve as a pilot valve in combination with a large-capacity pneumatic relay.
Three, pneumatic relay
Pneumatic relay is a kind of power amplifier, which can send the air pressure signal to a far place to eliminate the lag caused by the lengthening 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 on-site regulating valve, there is another function to amplify or reduce the signal.
Fourth, the converter
The converter is divided into gas-electric converter and electric-gas converter. Its function is to realize the mutual conversion between gas and electric signal. It is mainly used to control 0～10mA or 4～ 20mA electric signal conversion or 0～100KPa gas signal into 0～10mA or 4～20mA electric signal.
Five, air filter 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 for various pneumatic instruments and solenoid valves. , Air supply and voltage stabilizing device for cylinders, spraying equipment and small pneumatic tools.
Six, self-locking valve (position valve)
Self-locking valve is a device to maintain the valve position. Pneumatic regulating valve, when the air source fails, the device can cut off the air source signal, keeping the pressure signal of the membrane chamber or cylinder in the state immediately before the failure, so that the valve position is also maintained at the position before the failure, to keep in place.
Seven, valve position transmitter
When the regulating valve is far away from the control room, in order to accurately understand the valve switch position without on-site, it is necessary to equip a valve position transmitter, which means that the displacement of the valve opening is converted into an electrical signal and sent to the control room according to a certain rule. The signal can be a continuous signal reflecting any opening of the valve, or it can be regarded as the reverse action of the valve positioner.
8. Travel switch (return device)
The travel switch reflects the two extreme positions of the valve switch and sends out an indication signal at the same time. Based on this signal, the control room can betray the switch state of the valve in order to take corresponding measures.