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Hi, this is the Industrial Control Assistant. In the automatic control of modern factories, control valves play a very important role, as the production in such factories depends on the proper distribution and control of the fluids flowing through them. These controls, whether it is the exchange of energy, the reduction of pressure, or simply the feeding of material into a container, all require certain final control elements to be implemented. Control valves function to provide variable resistance in pipelines. There are many types of valve bodies for control valves; the commonly used ones include straight-through single-seat, straight-through double-seat, angle type, diaphragm type, low-flow type, three-way type, eccentric rotary type, butterfly type, sleeve type, ball type, and so on. When making a specific selection, the following considerations can be taken into account: (1) The shape and structure of the valve core are primarily determined by factors such as the desired flow characteristics and the 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 whose valve cores and seat materials are less affected by such changes should be selected. A radiator should be installed when the temperature is ≥250°C. (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. To ensure the proper operation of a control valve, its actuator must be capable of generating sufficient output force to maintain a high level of sealing and to enable the valve to open. 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. After the output force of the actuator is determined, the appropriate actuator is selected based on the requirements of the operating environment. When explosion protection is required at the 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. The mode of operation of a control valve is relevant only when selecting a pneumatic actuator; it is determined by the combination of the forward and reverse actions of the actuator and 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. That’s all for the little tips in this issue. The little assistant will continue to share more tips related to industrial control – stay tuned!