Working principle and selection of pressure reducing valve

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Working principle of pressure reducing valve
Direct acting pressure reducing valve
Fig. 14-1a is a structural diagram of a direct acting pressure reducing valve with an overflow valve (hereinafter referred to as an overflow pressure reducing valve).
The compressed air with the pressure of P1 is input from the left end and throttled through the valve port 10, and the pressure drops to P2 for output. The magnitude of P2 can be adjusted by the pressure adjusting springs 2 and 3. Turn the knob 1 clockwise, compress the springs 2, 3 and the diaphragm 5 to move the valve element 8 downward, and increase the opening of the valve port 10 to increase P2. If the knob 1 is rotated counterclockwise, the opening of the valve port 10 decreases, and P2 decreases accordingly.
If P1 rises instantaneously, P2 will rise accordingly, so that the pressure in the diaphragm air chamber 6 will rise, and the thrust generated on the diaphragm 5 will correspondingly increase. This thrust will destroy the balance of the original force, move the diaphragm 5 upward, and a small part of the air flow will be discharged through the overflow hole 12 and the exhaust hole 11. At the same time as the diaphragm moves up, the valve core 8 also moves up due to the action of the return spring 9, and the intake valve port 10 is closed. The throttling effect is increased, so that the output pressure drops until a new balance is reached, and the output pressure basically returns to the original value. If the input pressure drops instantaneously, the output pressure also drops, the diaphragm 5 moves down, the valve element 8 moves down, the inlet valve port 10 opens wider, and the throttling effect decreases, so that the output pressure basically returns to the original value. Turn knob 1 counterclockwise. The regulating springs 2 and 3 are released, and the thrust of the gas on the diaphragm 5 is greater than the force of the pressure regulating spring. The diaphragm bends upward, and the inlet valve port 10 is closed by the action of the return spring. When the knob 1 is rotated again, the top end of the intake valve core 8 will be disengaged from the overflow valve seat 4, and the compressed air in the diaphragm air chamber 6 will be discharged through the overflow hole 12 and the exhaust hole 11, so that the valve is in a no output state.
In short, the overflow pressure reducing valve is reduced by the throttling action of the air inlet, stabilized by the balance of the force on the diaphragm and the overflow action of the overflow hole; The output pressure can be changed within a certain range by adjusting the spring. In order to prevent the pollution of the surrounding environment caused by a small amount of gas released from the above overflow type pressure reducing valve, the pressure reducing valve without overflow valve (i.e. ordinary pressure reducing valve) can be used. Its symbol is shown in Fig. 14-1c.
Pilot-operated reducing valves 
When the output pressure of the pressure reducing valve is high or the diameter is large, if the pressure is directly regulated by the pressure regulating spring, the spring stiffness must be too large. When the flow rate changes, the output pressure fluctuates greatly, and the structural size of the valve will also increase. In order to overcome these disadvantages, a pilot type pressure reducing valve can be used. The working principle of the pilot type pressure reducing valve is basically the same as that of the direct acting type. The pressure regulating gas used in the pilot type pressure reducing valve is supplied by a small direct acting pressure reducing valve. If a small direct acting pressure reducing valve is installed inside the valve body, it is called an internal pilot pressure reducing valve; If a small direct acting pressure reducing valve is installed outside the main valve body, it is called an external pilot pressure reducing valve. Fig. 14-2 shows the structure of the internal pilot type pressure reducing valve. Compared with the direct acting pressure reducing valve, the valve adds an enlarged nozzle baffle composed of nozzle 4, baffle 3, fixed orifice 9 and air chamber B. When the distance between the nozzle and the baffle changes slightly, the pressure in chamber B will change significantly, which will cause a large displacement of the diaphragm 10 to control the up and down movement of the valve core 6, so that the inlet valve port 8 can be opened or closed, and the sensitivity of the valve core control can be improved, that is, the stability accuracy can be improved.
Fig. 14-3 shows the main valve of the external pilot type pressure reducing valve, and its working principle is the same as that of the direct acting type. There is also a small direct acting pressure reducing valve (not shown) outside the main valve body, which controls the main valve. This type of valve is suitable for occasions with a diameter of more than 20mm, a long distance (within 30m), a high place, a dangerous place and a difficult pressure regulation.
Setter
The setter is a high-precision pressure reducing valve, mainly used for pressure setting. At present, there are two pressure specifications of the setter: the air source pressure is 0.14MPa and 0.35Mpa respectively, and the output pressure range is 0-0.1mpa and 0-0.25mpa respectively. Its output pressure fluctuation is not more than 1% of the maximum output pressure, which is often used in occasions where accurate air source pressure and signal pressure need to be supplied, such as pneumatic experimental equipment and pneumatic automatic devices.
Fig. 14-4 shows the working principle of the setter. It consists of three parts: 1. The main closing part of the direct acting pressure reducing valve; 2 is a constant pressure drop device, equivalent to a differential pressure reducing valve. The main function is to make the nozzle obtain stable gas flow; 3 is the nozzle baffle device and the pressure regulating part, which plays the role of pressure regulation and pressure amplification, and uses the amplified air pressure to control the main valve part.
Since the setting device has the functions of setting, comparison and amplification, the voltage stabilization accuracy is high.
When the setter is in the non operating state, the compressed air input from the air source enters chamber a and the main chamber after being filtered by filter 1. The main valve element 19 is pressed against the valve seat under the action of the spring 20 and the air source pressure to disconnect the chamber a from the chamber B. The air flow entering chamber a passes through the valve port (also referred to as the valve) 12 to chamber F, and then enters chamber g and chamber d after being depressurized through the constant orifice 13. Since no force is applied to the diaphragm 8 at this time, the distance between the baffle 5 and the nozzle 4 is large, the air flow resistance when the gas flows out of the nozzle 4 is small, the air pressure in chambers g and D is low, and the diaphragms 3 and 15 remain in their original positions. The trace gas entering the chamber is mainly discharged from the exhaust port through the valve port 2 through the chamber B; Another part is evacuated from the outlet. At this time, there is no air flow output from the output port, and the discharge of trace gas from the nozzle is necessary to maintain the operation of the nozzle baffle device. Since it is a non power consumption gas volume, it is hoped that the smaller the consumption, the better.
When the setter is in the working state, turn the handle 7, press down the spring 6 and push the diaphragm 8 to move down together with the baffle 5. The distance between the baffle 5 and the nozzle 4 is reduced, and the air flow resistance is increased, so that the air pressure in chamber g and chamber D is increased. The diaphragm 16 moves down under the pressure of chamber D, closes the valve port 2, and pushes the main valve element 19 downward to open the valve port. The compressed air is output from the output port through chamber B and chamber H. At the same time, the pressure of the H chamber rises and is fed back to the diaphragm 8. When the feedback force received by the diaphragm 8 is balanced with the spring force, the constant value device outputs a certain pressure of gas. When the input pressure fluctuates, if the pressure rises, the air pressure in chambers B and H increases instantaneously, causing the diaphragm 8 to move upward, resulting in an increase in the distance between the baffle plate 5 and the nozzle 4, and a decrease in the air pressure in chambers g and D. As the pressure in chamber B increases and the pressure in chamber D decreases, the diaphragm 15 moves upward under the action of the pressure difference, reducing the main valve port and reducing the output pressure until it stabilizes to the set pressure. In addition, when the input pressure rises, the pressure in chamber E and the instantaneous pressure in chamber f also rise, and the diaphragm 3 moves up under the action of the up-down differential pressure to close the pressure stabilizing valve port 12. Since the throttling effect is strengthened, the air pressure in chamber f decreases, and the pressure difference between the front and rear of the throttle hole 13 is always kept constant, the gas flow through the throttle hole 13 is constant, and the sensitivity of the nozzle baffle is improved. When the input pressure decreases, the pressures of chambers B and H drop instantaneously, the diaphragm 8 and the baffle 5 move down due to the destruction of the force balance, the spacing between the nozzle 4 and the baffle 5 decreases, the pressures of chambers g and D rise, and the diaphragms 3 and 15 move down. The diaphragm 15 moves down to increase the opening of the main valve port and increase the air pressure of chamber B and chamber h until it is balanced with the set pressure. While the diaphragm 3 moves down, so that the pressure stabilizing port 12 is opened and the air pressure in chamber f is increased, and the pressure difference before and after the constant orifice 13 is always kept constant. Similarly, when the output pressure fluctuates, the same adjustment will be obtained as when the input pressure fluctuates.
Since the setter controls the main valve by using the feedback effect of the output pressure and the amplification effect of the nozzle baffle, it can respond to small pressure changes, so that the output pressure can be adjusted in time and the outlet pressure can be kept basically stable, that is, the accuracy of the constant pressure stabilization is high.
2、 Basic performance of pressure reducing valve
(1) Pressure regulating range of pressure reducing valve: it refers to the adjustable range of the output pressure P2 of the pressure reducing valve, within which the specified accuracy is required. The pressure regulating range is mainly related to the stiffness of the pressure regulating spring.
(2) Pressure characteristic of pressure reducing valve: it refers to the characteristic of output pressure fluctuation caused by input pressure fluctuation when the flow rate G is constant. The smaller the output pressure fluctuation, the better the characteristics of the pressure reducing valve. The output pressure must be lower than the input pressure - the fixed value will not change with the input pressure.
(3) Flow characteristics of pressure reducing valve: it refers to the persistence of input pressure timing and output pressure changing with the change of output flow G. When the flow rate G changes, the smaller the change in the output pressure, the better. Generally, the lower the output pressure, the smaller the fluctuation with the change of output flow.
3、 Selection of pressure reducing valve
Select the type and pressure regulation accuracy of the pressure reducing valve according to the use requirements, and then select its diameter according to the required maximum output flow. When determining the air source pressure of the valve, make it greater than the maximum output pressure by 0.1MPa. The pressure reducing valve is generally installed after the water separator air filter and before the oil mist or setting device, and attention shall be paid not to connect its inlet and outlet reversely; When the valve is not in use, the knob should be loosened to prevent the diaphragm from being deformed under pressure and affecting its performance.