Introduction to the main accessories of the regulating valve

　　The valve positioner is a major accessory for pneumatic actuators. It is used with pneumatic actuators to improve valve positioning accuracy and overcome the effects of valve stem friction and media imbalance, so that the valve is accurately positioned according to the signal from the regulator.

　　A positioner should be used under the following circumstances:

　　1. When the medium pressure is high and the pressure difference is large;

　　2. When the diameter of the control valve is large (DN>100);

　　3. High-temperature or low-temperature control valves;

　　4. When it is necessary to improve the operating speed of the control valve;

　　5. When using a standard signal to operate a non-standard spring actuator (spring range outside 20～100KPa);

　　6. When used for range control;

　　7. When the valve needs to perform reverse action (interconversion of air-to-close and air-to-open types);

　　8. When it is necessary to change the flow characteristics of the valve (the positioner cam can be changed);

　　9. Springless actuator

　　or piston actuator, to achieve proportional action;

　　10. When using an electrical signal to operate a pneumatic actuator, an electro-pneumatic valve positioner must be used.

　　Solenoid valve:

　　When the system needs to achieve program control or two-position control, a solenoid valve needs to be used. When selecting a solenoid valve, in addition to considering AC/DC power supply and voltage/frequency, the relationship between the solenoid valve and the control valve action type must be noted. "Normally open" or "normally closed" types 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 and combined with a large-capacity pneumatic booster.

　　Pneumatic relay:

　　A pneumatic relay is a power amplifier that can send pneumatic signals to remote locations, eliminating the lag caused by lengthening the signal lines. It is mainly used between field transmitters and central control room regulating instruments, or between regulators and field control valves. Another function is to amplify or reduce signals.

　　Converter:

　　Converters are divided into pneumatic-to-electric converters and electric-to-pneumatic converters. Their function is to achieve a certain relationship between pneumatic and electric signals. They are mainly used to convert 0～10mA or 4～20mA electrical signals or 0～100KPa pneumatic signals into 0～10mA or 4～20mA electrical signals when using electrical signals to operate pneumatic actuators.

　　Air filter pressure regulator:

　　The air filter pressure regulator 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 to the required value. It can be used as an air source and pressure regulating device for various pneumatic instruments, solenoid valves, cylinders, spraying equipment, and small pneumatic tools.

　　Self-locking valve (position holding valve):

　　The self-locking valve is a device that maintains the valve position. When the air source of the pneumatic control valve fails, this device can cut off the air source signal, keeping the pressure signal in the diaphragm chamber or cylinder at the state just before the failure, thus maintaining the valve position at the position before the failure, playing a position holding role.

　　Valve position transmitter

　　When the control valve is far from the control room, in order to understand the valve's on/off position without going to the site, a valve position transmitter must be equipped. The displacement of the valve opening mechanism is 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.

　　Limit switch (feedback device)

　　The limit switch reflects the two extreme positions of the valve's on/off state and sends an indication signal at the same time. The control room can judge the on/off state of the valve based on this signal and take corresponding measures.