Factors for selecting control valves

Factors for selecting control valves: Control valves, also known as regulating valves, are one of the main types of actuators. They receive control signals from a control unit and use power to adjust the flow rate of fluids. A control valve generally consists of an actuator and a valve. Based on the power source used by their associated actuators, control valves can be classified into three types: pneumatic, electric, and hydraulic. Pneumatic control valves use compressed air as a power source, electric control valves use electricity as the power source, while electro-hydraulic control valves utilize the pressure of a liquid medium such as oil. Additionally, depending on their functions and characteristics, there are also 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 for control valves; common ones include straight-through single-seat, straight-through double-seat, angle-type, diaphragm-type, low-flow, three-way, eccentric rotary, 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 temperature and pressure changes. (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 as well as 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 is to determine the maximum output force and the rotational 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 oil refineries. Selection of the operating mode of control valves: The operating mode of control valves is a consideration only when selecting pneumatic actuators; it is determined by the combination of the forward and reverse actions of the actuator along with those of the valve. 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 considered from three aspects: a) process production safety ； b) Characteristics of the medium ； c) Ensure product quality and minimize economic losses.