Design Principle and Selection of Pressure Reducing Valve

1、 Working principle of pressure reducing valve
Direct acting pressure reducing valve
Figure 14-1a shows the structural diagram of a direct acting pressure reducing valve with an overflow valve (referred to as the overflow pressure reducing valve).
Compressed air with a pressure of P1 is input from the left end and throttled through valve port 10, and the pressure drops to P2 output. The size of P2 can be adjusted by pressure regulating tower springs 2 and 3. Rotate knob 1 clockwise, compress springs 2, 3, and diaphragm 5 to move valve core 8 downward, and increase the opening of valve port 10 to increase P2. If knob 1 is turned counterclockwise, the opening of valve port 10 decreases, and P2 decreases accordingly.
If P1 momentarily increases, P2 will also increase, causing the pressure inside diaphragm chamber 6 to increase. The thrust generated on diaphragm 5 will correspondingly increase, disrupting the balance of the original force and causing diaphragm 5 to move upwards. A small amount of airflow will be discharged through overflow holes 12 and exhaust holes 11. At the same time as the diaphragm moves up, due to the action of the reset spring 9, the valve core 8 also moves up. By reducing the intake valve port 10 and increasing the throttling effect, the output pressure decreases until a new balance is reached, and the output pressure basically returns to its original value. If the input pressure momentarily decreases, the output pressure also decreases, and the diaphragm 5 moves down, causing the valve core 8 to move down accordingly. The intake valve port 10 opens wider, reducing the throttling effect and returning the output pressure to its original value. Rotate knob 1 counterclockwise. Relax the adjustment springs 2 and 3. The thrust of the gas acting on the diaphragm 5 is greater than the force of the pressure regulating spring. The diaphragm bends upwards and closes the intake valve port 10 with the action of the return spring resoures. Rotate knob 1 again, and the top of intake valve core 8 will detach from overflow valve seat 4. The compressed air in diaphragm chamber 6 will be discharged through overflow hole 12 and exhaust hole 11, leaving the valve in a state of no output.
In short, the overflow pressure reducing valve relies on the throttling effect of the air inlet to reduce pressure, the balance effect of the force on the diaphragm, and the overflow effect of the overflow hole to stabilize pressure; Adjusting the spring can change the output pressure within a certain range. To prevent the pollution of the surrounding environment caused by a small amount of gas emitted by the overflow type pressure reducing valve mentioned above, a pressure reducing valve without an overflow valve (i.e. a regular pressure reducing valve) can be used, with its symbol as follows
When the output pressure of the pressure reducing valve is high or the diameter is large, directly adjusting the pressure with a pressure regulating spring will inevitably result in excessive spring stiffness. When the flow rate changes, the output pressure fluctuates greatly, and the structural size of the valve will also increase. To overcome these shortcomings, pilot operated pressure reducing valves can be used. The working principle of a pilot operated pressure reducing valve is basically the same as that of a direct acting valve. The pressure regulating gas used in the pilot operated 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 operated pressure reducing valve. Figure 14-2 shows the structural diagram of the internal pilot pressure reducing valve. Compared with the direct acting pressure reducing valve, this valve adds a nozzle baffle amplification link composed of nozzle 4, baffle 3, fixed orifice 9, and air chamber B. When there is a slight change in the distance between the nozzle and the baffle, it will cause a significant change in the pressure in chamber B, causing a significant displacement of diaphragm 10 to control the up and down movement of valve core 6, causing inlet valve port 8 to open or close larger, improving the sensitivity of valve core control, and thus improving the stability accuracy.
Figure 14-3 shows the main valve of an external pilot operated pressure reducing valve, which operates on the same principle as the direct acting type. There is also a small direct acting pressure reducing valve (shown at the end of the figure) outside the main valve body, which controls the main valve. This type of valve is suitable for situations with a diameter of over 20mm, long-distance (within 30m), high altitude, dangerous locations, and difficult pressure regulation.
Fixer
Setpoint is a high-precision pressure reducing valve mainly used for pressure setting. There are currently two types of pressure regulators: their gas source pressures are 0.14MPa and 0.35MPa, and their output pressure ranges are 0-0.1MPa and 0-0.25MPa, respectively. Its output pressure fluctuation is not greater than 1% of the maximum output pressure, and is commonly used in situations where precise air source pressure and signal pressure need to be supplied, such as pneumatic experimental equipment, pneumatic automatic devices, etc.
Figure 14-4 shows the working principle diagram of the fixer. 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 certain differential pressure reducing valve. The main function is to ensure a stable air source flow rate for the nozzle; 3 is the nozzle baffle device and pressure regulating part, which plays a role in regulating and amplifying pressure, and uses the amplified air pressure to control the main  prototype parts.
Due to the functions of setting, comparing, and amplifying, the regulator has high voltage stabilization accuracy.
When the constant value device is in a non working state, the compressed air input from the air source enters room A and main room after being filtered by filter 1. The main valve core 19 is pressed onto the valve seat under the action of spring 20 and air supply pressure, causing chamber A to disconnect from chamber B. The airflow entering chamber A flows through valve port (also known as valve) 12 to chamber F, and then depressurizes through constant orifice 13 before entering chambers G and D. Due to the lack of force on diaphragm 8 at this time, the distance between baffle 5 and nozzle 4 is relatively large, resulting in a smaller airflow resistance when gas flows out of nozzle 4. The air pressure in chambers G and D is lower, and diaphragms 3 and 15 remain in their original positions. The trace gases entering the chamber are mainly discharged from the exhaust port through valve port 2 through chamber B; Another part is evacuated from the output port. At this point, there is no airflow output from the output port. Evacuating trace amounts of gas from the nozzle is necessary to maintain the operation of the nozzle baffle device. As it is a non power consumption gas, it is hoped that its consumption will be as low as possible.
When the constant value device is in operation, rotate the handle 7, press down the spring 6, and push the diaphragm 8 along with the baffle 5 to move down. The distance between the baffle 5 and the nozzle 4 decreases, and the airflow resistance increases, causing the air pressure in chambers G and D. Under the pressure of chamber D, diaphragm 16 moves downwards, closing valve port 2 and pushing main valve core 19 downwards to open the valve port. Compressed air is output from the output port through chambers B and H. At the same time, the pressure in the H chamber rises and feeds back to diaphragm 8. When the feedback force on diaphragm 8 is balanced with the spring force, the setter outputs a certain pressure of gas. When the input pressure fluctuates, such as an increase in pressure, the air pressure in chambers B and H instantly increases, causing diaphragm 8 to move up, resulting in an increase in the distance between baffle 5 and nozzle 4, and a decrease in air pressure in chambers G and D. Due to the increase in pressure in chamber B and the decrease in pressure in chamber D, diaphragm 15 moves upwards under the pressure difference, causing the main valve port to decrease and the output pressure to decrease until it stabilizes at the set pressure. In addition, as the input pressure increases, the pressure in chamber E and the instantaneous pressure in chamber F also increase. Under the action of the differential pressure between the upper and lower parts, diaphragm 3 moves up and closes the stabilizing valve port 12. Due to the strengthened throttling effect, the air pressure in chamber F decreases, and the pressure difference between the front and rear of orifice 13 is always kept constant. Therefore, the gas flow through orifice 13 remains unchanged, which improves the sensitivity of the nozzle baffle. When the input pressure decreases, the pressure in chambers B and H decreases instantaneously. Diaphragm 8 and baffle 5 move downwards due to force balance failure. The distance between nozzle 4 and baffle 5 decreases, and the pressure in chambers G and D increases. Diaphragms 3 and 15 move downwards. The downward movement of diaphragm 15 increases the opening of the main valve port, causing the air pressure in chambers B and H to rise until it balances with the set pressure. And the diaphragm 3 moves down, causing the pressure stabilizing port 12 to open wider, causing the air pressure in chamber F to rise, maintaining a constant pressure difference between the front and rear of constant orifice 13. Similarly, when the output pressure fluctuates, it will receive the same adjustment as when the input pressure fluctuates.
Due to the feedback effect of the output pressure and the amplification effect of the nozzle baffle, the constant value device controls the main valve, enabling it to respond to small pressure changes, so that the output pressure can be adjusted in a timely manner and the outlet pressure is basically stable, which means the constant value stabilization accuracy is high.
2、 Basic performance of pressure reducing valves
(1) Pressure regulation range: It refers to the adjustable range of the output pressure P2 of the pressure reducing valve, within which the specified accuracy is required. The range of pressure regulation is mainly related to the stiffness of the pressure regulating spring machining.
(2) Pressure characteristic: It refers to the characteristic of output pressure fluctuation caused by input pressure fluctuation when the flow rate g is a constant value. The smaller the fluctuation of output pressure, the better the characteristics of the pressure reducing valve. The output pressure must be lower than the input pressure – fixed value to basically remain unchanged with changes in input pressure.
(3) Flow characteristic: It refers to the persistence of the input pressure – timing, and the output pressure changing with the change of output flow g. When the flow rate g changes, the smaller the change in output pressure, the better. The lower the output pressure, the smaller the fluctuation with the change of output flow rate.
3、 Selection of pressure reducing valves
Select the type and pressure regulating accuracy of the pressure reducing valve according to the usage requirements, and then select its diameter according to the required maximum output flow rate. When determining the air supply pressure of the valve, it should be greater than the maximum output pressure of 0.1MPa. The pressure reducing valve is generally installed after the water and air filter, before the oil mist or constant value device, and attention should be paid not to connect its inlet and outlet in reverse; When the valve is not in use, the knob should be relaxed to prevent the diaphragm from being frequently compressed and deformed, which may affect its performance.