Source：Time：2021/07/19Views：418
The regulating 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 and overcome the influence of stem friction and medium unbalanced force, so as to ensure the correct positioning of the valve according to the signal from the regulator.
The positioner shall be equipped in the following cases:
1. When the medium pressure is high and the pressure difference is large;
2. When the diameter of the regulating valve is large (DN > 100);
3. High or low temperature regulating valve;
4. When it is necessary to improve the action speed of the regulating valve;
5. When operating the actuator of non-standard spring with standard signal (spring range beyond 20 ~ 100KPA);
6. When used for split range control;
7. When the valve realizes reverse action (air close and air open switch each other);
8. When the flow characteristics of the valve need to be changed (the positioner cam can be changed);
9. Spring less actuator,Mechanism or piston actuator, to achieve proportional action;
10. When operating the pneumatic actuator with electrical signal, the power distribution - pneumatic valve positioner must be used.
Solenoid valve:
When the system needs to realize program control or two position control, it needs to be equipped with solenoid valve. When selecting solenoid valve, in addition to considering AC and DC power supply, voltage and frequency, attention must be paid to the relationship between the action type of solenoid valve and regulating valve, and "normally open type" or "normally closed type" can be used.
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 high-capacity pneumatic relay.
Pneumatic relay:
Pneumatic relay is a kind of power amplifier, which can send the air pressure signal to a distant 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.
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 signal or 0 ~ 100KPA pneumatic signal into 0 ~ 10mA or 4 ~ 20mA electrical signal when operating the pneumatic actuator with electrical signal.
Air filter pressure reducing valve:
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 all kinds of pneumatic instruments, solenoid valves, cylinders, spraying equipment and small pneumatic tools.
Self locking valve (holding valve):
Self locking valve is a device to maintain the valve position. When the air source fails, the device can cut off the air source signal to keep the pressure signal of the membrane chamber or cylinder in the state immediately before the failure, so as to maintain the valve position at the position before the failure and play the role of position protection.
Valve position transmitter
When the control 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 mechanism displacement of the valve opening shall be converted into an electrical signal according to a certain law and sent to the control room. This signal can be a continuous signal reflecting any opening of the valve or the reverse action of the valve positioner.
Travel switch (annunciator)
The travel switch is a device that reflects the two extreme positions of the valve switch and sends an indication signal at the same time. According to this signal, the control room can turn off the switching state of the valve in order to take corresponding measures.