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Self-acting pressure control valves are divided into three series: self-acting pressure, differential pressure, and flow control valves. Self-acting pressure control valves are classified into two types based on the location of the pressure measurement point: those with the measurement point before the valve, and those with it after the valve. When the pressure measurement point is located before the valve, such valves are used to maintain a constant pressure before the valve ； When the pressure tapping point is located behind the valve, it is used to maintain a constant pressure behind the valve. When the pressures before and after the valve are introduced to both sides of the actuator’s chamber simultaneously, a self-acting differential pressure control valve can maintain a constant pressure across the valve. The differential pressure existing between the two ends of a orifice plate installed in a pipeline can also be applied to both sides of the diaphragm actuator’s chamber, thereby creating a self-acting flow control valve. Alternatively, flow can be detected in some other way, and then flow control can be achieved using a self-acting differential pressure control valve. The pressure of the working medium before the valve, P1, is reduced through throttling after passing through the valve core and seat, thereby becoming the pressure behind the valve, P2. P2 is introduced through control lines into the lower diaphragm chamber of the actuator and acts on the top plate; the forces generated as a result balance each other, determining the relative position of the valve core and the valve seat and thus controlling the pressure behind the valve. As the pressure behind the valve, P2, increases, the force exerted by P2 on the top plate also increases. At this point, the force exerted by the top plate is greater than the reaction force of the spring, causing the valve core to move toward the valve seat until the force from the top plate and the reaction force of the spring are in balance. At this point, the flow area between the valve core and the valve seat decreases, the flow resistance increases, thereby reducing P2 to the set value. Similarly, when the pressure P2 behind the valve decreases, the direction of action is opposite to that described above; this is the working principle of a self-acting (pressure behind the valve) pressure control valve. Valves of this type should generally be installed horizontally in pipelines. The pressure before the valve, P1, which controls the working medium, becomes the pressure after the valve, P2, after passing through the valve core and seat and undergoing throttling. At the same time, P1 is transmitted via control lines to the upper diaphragm chamber of the actuator, where it exerts a force on the top plate; these forces balance each other, determining the relative position of the valve core and seat and thus controlling the pressure before the valve. As the pressure behind the valve, P1, increases, the force exerted by P1 on the top plate also increases. At this point, the force exerted by the top plate is greater than the reaction force of the spring, causing the valve core to move to a position away from it. Until the force exerted by the top plate is balanced by the reaction force of the spring. At this point, the flow area between the valve core and the valve seat increases, reducing the flow resistance, thereby lowering P1 to the set value. Similarly, when the pressure behind the valve P1 decreases, the direction of action is opposite to that described above; this is the working principle of a self-acting (pre-valve) pressure control valve.