What is the principle of a self-acting control valve?

A stand-alone control valve uses the elastic force of its own spring to regulate the pressure difference between the front and back, maintaining this pressure difference at a constant level.

A self-acting control valve is a type of valve that requires no external energy source; it utilizes the pressure changes of the medium being regulated to carry out automatic adjustment. Based on mechanical principles, it directs the controlled medium into an actuator to generate force that drives the valve core element up and down, thereby achieving automatic regulation and stabilizing the pressure behind the valve – making it an energy-saving product. The functional principle of a self-acting control valve is the same as that of a regular control valve; the main difference is that it does not require external power and does not accept control signals from external instruments. The biggest advantage of this product is its ability to operate in areas without electricity or air, with the pressure setting able to be adjusted at will during operation. The main differences between self-acting control valves and pressure reducing valves: 1. Their functions are different; self-acting control valves are designed for regulation, while pressure reducing valves serve solely to reduce pressure ； 2. The pressure relief valve allows for manual adjustment of pressure; if the pressure before the valve fluctuates greatly, adjustments need to be made frequently. A self-acting control valve operates automatically based on a set, objective value, and the regulated pressure can be kept constant ； 3. The pressure relief valve requires manual adjustment of the pressure difference; if the pressure before the valve changes, the pressure after the valve also changes, and it cannot automatically adjust to a fixed pressure. Self-acting control valves can automatically maintain stable back pressure or pressure before the valve ； 4. The main purpose of a self-acting control valve is to maintain pressure stability, whereas the primary function of a pressure reducing valve is to reduce pressure to a certain level or below ； 5. Pressure relief valves have a wider range of adjustment, whereas self-acting control valves can only regulate pressure to a constant value ； 6. Pressure relief valves have a higher adjustment accuracy, typically 0.5, whereas the adjustment accuracy of self-acting control valves is generally 8%-10% ； 7. Self-acting control valves can regulate pressure, differential pressure, temperature, liquid level, flow rate, etc., while pressure reducing valves have a relatively limited function, generally serving only to reduce pressure ； 8. A self-acting control valve can stabilize both the pressure before the valve and the pressure after it, whereas a pressure reducing valve can only regulate the pressure after the valve, serving to reduce that pressure. Self-acting control valves are further divided into direct-acting and indirect-acting types. 1. Direct action: Direct-acting control valves, also known as spring-loaded control valves, have elastic elements in their structure such as springs, diaphragms, or diaphragm-type temperature sensors; they operate on the principle of balancing the elastic force with the feedback signal. 2. Indirect action: In indirect-action control valves, an additional actuator (pilot valve) is used; this actuator amplifies the feedback signal, and then, through the actuating mechanism, it drives the valve disc to move in order to change the degree of valve opening. 1) If it is a pressure control valve, the feedback signal is the outlet pressure of the valve, which is introduced to the actuator through a signal line. 2) If it is a flow control valve, there is a orifice plate (or some other flow-restricting device) at the valve’s outlet; the pressure difference across this orifice plate is used to generate a signal that is fed to the actuator. 3) If it is a temperature control valve, there is a temperature sensor (or thermocouple) at the valve’s outlet; the expansion and contraction of the medium inside this sensor drive the actuator.