Difference in pressure control before and after a self-acting pressure regulating valve
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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 pre-valve and post-valve types depending on the location of the pressure sampling point; when the sampling point is located before the valve, it is used to maintain a constant pressure before the valve ； When the pressure measurement point is located after the valve, it is used to maintain a constant pressure behind the valve. When the pressures before and after the valve are applied 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 between the two sides of a orifice plate installed in a pipeline can also be applied to both sides of the diaphragm actuator’s chamber to create a self-acting flow control valve; alternatively, flow can be detected and then regulated using a self-acting differential pressure control valve. 1. Pressure control before the valve The pressure P1 of the working medium before the valve is reduced through throttling by the valve core and valve seat, resulting in the pressure P2 after the valve. At the same time, P1 is delivered through control lines to the upper diaphragm chamber of the actuator, acting on the top plate; the force generated thereby balances the reaction force of the spring, which in turn determines the relative position of the valve core and the valve seat and controls the pressure before the valve. As the pressure before 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 away from the valve seat until the force from the top plate balances the reaction force of the spring. At this point, the flow area between the valve core and the valve seat increases, the flow resistance decreases, thereby reducing P1 to the set value. Similarly, when the pressure P1 in front of the valve decreases, the direction of action is opposite to that described above; this is the working principle of a self-acting (pressure in front of the valve) pressure control valve. When it is necessary to change the set value of the pressure P1 in front of the valve, the adjusting nut can be adjusted. 2. Pressure control after the valve The pressure P1 of the working medium before the valve decreases due to throttling as it passes through the valve core and seat, resulting in the pressure P2 after the valve. P2 is delivered through control lines to the lower diaphragm chamber of the actuator, acting on the top disc; the force generated thereby balances the reaction force of the spring, determining the relative position of the valve spool 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. When it is necessary to change the set value of the pressure P2 behind the valve, the adjusting nut can be adjusted.
A self-acting pressure control valve uses changes in the pressure of the medium being controlled to drive the valve, thereby achieving pressure regulation. Depending on the pressure tapping point, self-acting pressure control valves can be divided into two types: pre-valve pressure control and post-valve pressure control. 1. Pressure control before the valve: In pressure control before the valve, the valve adjusts its opening degree by sensing the pressure changes (P1) ahead of the valve, in order to maintain a constant pressure there. When the pressure before the valve exceeds the set value, the increased pressure acts on the upper diaphragm chamber of the actuator; this increased force causes the valve core to move in the open direction, thereby increasing the flow area and reducing the pressure drop across the valve, until the pressure before the valve drops back to the set value. Conversely, when the pressure before the valve is below the set value, the spring force takes precedence; the valve core moves in the closing direction, reducing the flow area and increasing the pressure drop across the valve, thereby raising the pressure before the valve to the set value. The set pressure can be adjusted by changing the spring preload. 2. Back-pressure control: In back-pressure control, the valve adjusts its opening degree based on the changes in the pressure behind the valve (P2), in order to maintain a constant back-pressure. When the pressure behind the valve exceeds the set value, the increased pressure pushes the diaphragm in the actuator downward, compressing the spring and causing the valve element to move in a closing direction. This reduces the flow area and increases the resistance, thereby lowering the pressure behind the valve back to the set value. Conversely, when the pressure behind the valve is below the set value, the spring force pushes the valve core in the direction of opening, increasing the flow area and reducing resistance, thereby raising the pressure behind the valve to the set value. Similarly, the set pressure can be changed by adjusting the spring preload. The main advantage of self-acting control valves is that they do not require any external power supply; they operate entirely based on the pressure of the medium itself. They are widely used in various industrial processes to maintain the pressure of the medium within a set range. .
This post was last edited by zhoudingshengs on 2023-11-13 at 16:51. Additional information: I. Classification of self-acting pressure control valves: 1. Classified by control location downstream or upstream of the valve: self-acting downstream (pressure-reducing) control valves, and self-acting upstream (pressure-relieving) control valves. 2. Classified by whether they are equipped with a controller: (1) Direct-acting self-acting control valves: Direct-acting pressure control valves regulate pressure by using the medium itself to directly control the valve. Direct-acting pressure regulating valves come in two types: those with pressure measurement after the valve and those with pressure measurement before the valve. Pressure is taken after the valve to keep the pressure there within a set range, thereby achieving the effect of pressure reduction after the valve. Pressure is taken before the valve to keep the pressure there within a set range, thereby achieving the effect of pressure relief before the valve. (2) Operator-operated self-acting control valve: An operator-operated control valve achieves automatic pressure regulation through mutual control between two valves. II. The difference between self-acting pressure control valves and control valves lies in the fact that control valves require external energy sources (such as power or air supply) for operation, and they also need to receive signals from external control instruments in order to change the relative position of the flow-blocking elements inside the valve, thereby adjusting the fluid flow rate. On the other hand, a self-acting pressure control valve requires no external power source nor any control signal from external instruments; it can regulate pressure based solely on the pressure signal of the medium being controlled.