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A control valve, also known as a regulating valve, receives a control signal from a control unit and uses power to adjust the flow rate of the fluid. A control valve generally consists of an actuator and a valve. Based on the power source used by their actuators, control valves can be divided into three types: pneumatic control valves, which use compressed air as a power source; electric control valves, which use electricity as a power source; and electro-hydraulic control valves, which use the pressure of a liquid medium such as oil as a power source. Additionally, depending on their functions and characteristics, there are also hydraulic control valves, solenoid valves, electronic control valves, intelligent control valves, and fieldbus-type control valves. Selection of the valve body type for control valves: There are many types of valve bodies available for control valves; common ones include straight-through single-seat, straight-through double-seat, angle-type, diaphragm-type, low-flow, three-way, eccentric rotating, butterfly, sleeve-type, and ball-type. When making a specific selection, the following factors can be taken into consideration: (1) The shape and structure of the valve core are mainly determined by factors such as the desired flow characteristics and unbalanced forces. (2) Wear resistance: When the fluid medium is a suspension containing high concentrations of abrasive particles, the internal materials of the valve must be hard. (3) Corrosion resistance: Since the medium is corrosive, valves with a simple structure should be preferred. (4) Temperature and pressure of the medium: When the temperature and pressure of the medium are high and subject to significant fluctuations, valves should be selected whose valve core and seat materials are less affected by such changes in temperature and pressure. (5) Preventing flashing and cavitation: Flashing and cavitation occur only in liquid media. In actual production processes, flashing and cavitation can cause vibrations and noise, thereby reducing the service life of valves; therefore, when selecting valves, it is necessary to prevent flashing and cavitation from occurring. Selection of the control valve actuator: For the control valve to function properly, the actuator used must be capable of generating sufficient output force to ensure a tight seal and the proper opening of the valve. Double-acting pneumatic, hydraulic, and electric actuators generally do not have a return spring. The magnitude of the acting force is independent of its direction of movement; therefore, the key to selecting an actuator lies in determining the maximum output force and the torque of the motor. For single-acting pneumatic actuators, the output force is related to the valve opening, and the force acting on the control valve also affects its operating characteristics; therefore, it is necessary to establish a force balance across the entire range of valve openings. Determination of actuator type: After determining the output force of the actuator, the appropriate actuator is selected based on the requirements of the operating environment. When explosion protection is required on-site, pneumatic actuators should be selected. From an energy-saving perspective, electric actuators should be preferred as much as possible. If high adjustment accuracy is required, a hydraulic actuator can be selected. Such as the speed control of turbines in power plants, and the temperature control of reactors in catalytic units in refineries. Selection of the operating mode of the control valve: The operating mode of a control valve is a consideration only when selecting a pneumatic actuator; it is determined by the combination of the forward and reverse actions of the actuator along with those of the valve itself. There are 4 combination types: positive-positive (air-shut type), positive-negative (air-open type), negative-positive (air-open type), and negative-negative (air-shut type). The operation modes of the control valves resulting from these four combinations are air-open and air-shut. The selection of the operating mode for control valves is primarily based on three factors: a) process safety; b) properties of the medium; c) ensuring product quality while minimizing economic losses