Introduction to the main accessories of control valves

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The control valve positioner is a key accessory of the pneumatic actuator; it is used in conjunction with the actuator to improve the accuracy of the valve’s position, overcome the effects of friction on the valve stem and uneven forces from the medium, thereby ensuring that the valve is positioned correctly according to the signals sent by the controller. Positioners are required in the following situations: 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 increase the operating speed of the control valve ； 5. When operating an actuator with a standard signal for a non-standard spring (spring range other than 20–100 KPa) ； 6. For step control ； 7. When causing the valve to operate in reverse (switching between air-shut and air-open modes) ； 8. When it is necessary to change the flow characteristics of the valve (the positioner cam can be adjusted) ； 9. For springless actuator or piston actuator systems, to achieve proportional operation ； 10. When using electrical signals to operate pneumatic actuators, a electro-pneumatic valve positioner must be provided. Solenoid valve: When the system requires program control or two-position control, a solenoid valve is needed. When selecting a solenoid valve, in addition to considering the AC/DC power supply as well as voltage and frequency, it is necessary to pay attention to the relationship between the function of the solenoid valve and that of the control valve; it can be used in either a \"normally open\" or \"normally closed\" configuration. If it is necessary to increase the capacity of the solenoid valve in order to reduce the actuation 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 actuator. Pneumatic relay: A pneumatic relay is a type of power amplifier that enables the transmission of air pressure signals over long distances, eliminating the delays caused by longer signal lines. It is primarily used between field transmitters and control instruments in the central control room, or between regulators and field control valves. Another function of it is to amplify or reduce signals. Converters: Converters are divided into gas-to-electricity converters and electricity-to-gas converters. Their function is to establish a certain relationship between gas and electrical signals, and they are 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 controlling pneumatic actuators with electrical signals. Air filter pressure regulator: An air filter pressure regulator is an accessory used in industrial automation instruments. Its main function is to filter and purify the compressed air coming from air compressors and to maintain the pressure at the desired level. It can be used as a supply source and pressure stabilization device for various pneumatic instruments, solenoid valves, cylinders, spraying equipment, and small pneumatic tools. Self-locking valve (positioning valve): A self-locking valve is a device used to maintain the valve position. In the event of a failure in the air supply, the pneumatic control valve can cut off the air supply signal, thereby keeping the pressure signal in the diaphragm chamber or cylinder at the level it was just before the failure. This ensures that the valve position remains at its value before the failure, thus serving a position-maintaining function. A valve position transmitter is necessary when the control valve is located far away from the control room; it allows for an accurate understanding of the valve’s open or closed position without having to go to the site. This transmitter converts the mechanical displacement of the valve’s opening degree into electrical signals that are sent to the control room. Such signals can be continuous signals reflecting any degree of valve opening, or they can be considered as the reverse action of a valve positioner. Travel switch (feedback device): A device that detects the two extreme positions of a valve’s opening and closing state, and simultaneously sends out a signal; the control room can use this signal to determine the valve’s status and take appropriate actions.