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Hi, your industrial control assistant is online! I was surprised to find that the article I posted last time about the differences between control valves and regulating valves has been read over 10,000 times. I’m really happy; as long as I can provide everyone with useful and interesting information, I don’t feel tired at all. Since everyone seems interested in some basic knowledge about control valves, today the little assistant has summarized some fundamental aspects of control valves for you. Let’s take a look together. 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 the actuator associated with them, control valves can be divided into pneumatic control valves, electric control valves, and hydraulic control valves. Additionally, according to their functions and characteristics, they can be classified into those with linear characteristics, equal percentage characteristics, and parabolic characteristics. There are many types of valve bodies for control valves; the commonly used ones include straight-through single-seat, straight-through double-seat, angle, diaphragm, low-flow, three-way, eccentric rotary, butterfly, sleeve, ball, and others. Working principle of pneumatic control valves: Pneumatic control valves use compressed air as the power source, with cylinders serving as actuators. They rely on accessories such as electrical valve positioners, converters, solenoid valves, and hold-down valves to operate the valve, enabling on-off or proportional control. By receiving control signals from industrial automation systems, they adjust various process parameters of the fluid flowing through the pipeline, such as flow rate, pressure, and temperature. Pneumatic control valves are characterized by simple control, fast response, and intrinsic safety, eliminating the need for additional explosion-proof measures. Working principle of self-acting control valves: Self-acting control valves are used to regulate process parameters such as fluid flow rate, pressure, temperature, and liquid level in the field of industrial automation process control. Based on the control signals in the automation system, the opening degree of the valve is automatically adjusted, thereby enabling the regulation of the flow rate, pressure, temperature, and level of the medium. I. Working principle of the self-acting temperature control valve (heating type): The temperature control valve operates based on the principles of the incompressibility of liquids and thermal expansion and contraction. It is a self-acting temperature control valve for heating; when the temperature of the object being controlled is below the set temperature, the liquid inside the thermal bulb contracts, which reduces the force acting on the actuator rod. Under the action of the spring force, the valve element opens, increasing the flow rate of heating media such as steam and hot oil, thereby raising the temperature of the object being controlled. Once the temperature reaches the set value, the valve closes. After it closes, the temperature of the object being controlled drops, and the valve opens again, allowing the heating media to flow back into the heat exchanger and raise the temperature once more, thus maintaining a constant temperature for the object being controlled. The valve opening degree is related to the difference between the actual temperature of the controlled object and the set temperature. II. Working principle of self-acting temperature control valves (cooling type): The working principle of self-acting temperature control valves used for cooling can be similar to that of those used for heating; the only difference is that the valve element opens and closes under the action of the actuator and spring force, in contrast to temperature rise valves. A cold medium flows through the valve body, and these valves are primarily used for temperature control in cooling systems. III. Working principle of the self-acting flow control valve: After the medium to be controlled enters the valve, the pressure P1 in front of the valve is transmitted to the lower diaphragm chamber through the control pipeline. The pressure Ps, which results from throttling by the throttle valve, is sent to the upper diaphragm chamber. The difference between P1 and Ps, namely △Ps = P1 – Ps, is referred to as the effective pressure. The difference between the thrust generated by P1 on the diaphragm and the thrust generated by Ps on the diaphragm, balanced against the spring force, determines the relative position of the valve core and the valve seat, thereby determining the flow rate through the valve. As the flow rate passing through the valve increases, that is, as △Ps increases, P1 and Ps act on the lower and upper diaphragm chambers respectively, causing the valve element to move toward the valve seat. This changes the flow area between the valve element and the valve seat, resulting in an increase in Ps. The increased value of Ps exerts a thrust on the diaphragm; this thrust, combined with the spring force, balances the thrust exerted by P1 on the diaphragm at the new position, thereby achieving control over the flow rate. Conversely, the same applies. That’s it for today. If there’s anything else you’d like to know, feel free to leave a message. 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