If the opening degree of the valve in the outlet pipeline of the pressure relief valve changes, the pressure will also change. Am I correct in thinking this?

Personally, I think the original poster’s idea is incorrect. If the valve on the outlet pipeline of the pressure regulator is closed, then the pressures in the pipelines before and after the pressure regulator will be the same, meaning the pressure regulator would become useless. I believe it’s necessary to maintain a specified pressure difference in the pipelines before and after the pressure regulator.

It depends on whether the pressure regulator referred to by the poster is a manual one, or an automatic pressure regulator or an electric pressure regulator. If a gate valve or similar device is used to regulate the outlet pressure and flow rate by adjusting its opening degree, then the pressures before and after closing the outlet valve will be equal. In contrast, with automatic and electric pressure regulators, the pressure remains essentially unchanged after the outlet valve is closed, staying close to the set regulatory pressure.

Working Principle and Selection of Pressure Reducers I. Working Principle of Pressure Reducers Direct-acting Pressure Reducer Figure 14–1a shows the structural diagram of a direct-acting pressure reducer with an overflow valve (also referred to as an overflow-pressure reducing valve). Compressed air at pressure P1 is fed in from the left end and, after being throttled through valve port 10, its pressure drops to P2 before being discharged. The size of P2 can be adjusted by the pressure-regulating springs 2 and 3. Turning knob 1 clockwise compresses springs 2, 3 and diaphragm 5, causing valve core 8 to move downward and increasing the opening of valve port 10, thereby raising P2. If knob 1 is rotated counterclockwise, the opening of valve port 10 decreases, and P2 decreases accordingly. If P1 increases instantaneously, P2 will rise as well, causing the pressure in diaphragm chamber 6 to increase. The thrust generated on diaphragm 5 increases accordingly, and this thrust disrupts the original force balance, causing diaphragm 5 to move upward. A small amount of air flow is then discharged through overflow hole 12 and exhaust hole 11. As the diaphragm moves upward, the return spring 9 causes the valve core 8 to move upward as well, thereby reducing the size of the intake valve opening 10. This increases the throttling effect and leads to a decrease in the output pressure, until a new equilibrium is reached; at that point, the output pressure returns to its original value. If the input pressure drops instantaneously, the output pressure also drops; the diaphragm 5 moves downward, and the valve core 8 moves downward as well. This causes the intake valve opening 10 to widen, reducing the throttling effect and allowing the output pressure to return to its original value. Rotate knob 1 counterclockwise. This relaxes the adjusting springs 2 and 3; the thrust exerted by the gas on diaphragm 5 becomes greater than the force exerted by the pressure-regulating springs, causing the diaphragm to bend upward and closing the intake valve opening 10 due to the action of the return spring. When the knob 1 is rotated further, the top of the intake valve core 8 will disengage from the relief valve seat 4, and the compressed air in the diaphragm chamber 6 will be discharged through the relief hole 12 and the exhaust hole 11, putting the valve in a state with no output. In short, the relief valve reduces pressure through throttling at the air inlet, and maintains a constant pressure thanks to the force balance on the diaphragm and the overflow action of the overflow hole ； The adjustment spring allows the output pressure to be changed within a certain range. To prevent contamination of the surrounding environment by the small amount of gas discharged from the above-mentioned overflow-type pressure regulator, a pressure regulator without an overflow valve (i.e., a regular pressure regulator) can be used, whose symbol is shown in Figure 14–1c. Pilot-operated pressure reducing valve: When the output pressure of a pressure reducing valve is high or its diameter is large, using a regulating spring to control the pressure means that the spring stiffness must be very high; as the flow rate changes, the output pressure fluctuates significantly, and the structural dimensions of the valve also increase. To overcome these drawbacks, a pilot-operated pressure relief valve can be used. The working principle of a pilot-operated pressure reducing valve is basically the same as that of a direct-acting one. The pressure-regulating gas used in the pilot-operated pressure regulator is supplied by a small direct-acting pressure regulator. If a small direct-acting pressure reducing valve is installed inside the valve body, it is called an internal pilot-type 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 an internal pilot-operated pressure reducing valve. Compared with a direct-acting pressure reducing valve, this valve features a nozzle-throttle amplification element composed of nozzle 4, baffle 3, fixed throttle orifice 9, and air chamber B. When there is a slight change in the distance between the nozzle and the baffle, it causes a significant change in the pressure in chamber B, which in turn leads to a large displacement of diaphragm 10. This displacement controls the upward and downward movement of valve spool 6, thereby adjusting the size of the intake valve opening – improving the sensitivity of control over the valve spool and thus enhancing the accuracy of pressure stabilization. Figure 14–3 shows the main valve of an external pilot-operated pressure reducing valve, whose operating principle is the same as that of a direct-acting type. Outside the main valve body, there is also a small direct-acting pressure reducing valve (not shown in the diagram), which is used to control the main valve. Such valves are suitable for diameters of 20 mm and above, in long-distance locations (within 30 m), at high altitudes, in hazardous areas, and in situations where pressure regulation is difficult. Settler: A settler is a high-precision pressure reducing valve primarily used for setting pressure values. Currently, there are two types of regulators with different pressure specifications: their air supply pressures are 0.14 MPa and 0.35 MPa respectively, while their output pressure ranges are 0–0.1 MPa and 0–0.25 MPa respectively. Its output pressure fluctuation is no more than 1% of the maximum output pressure, and it is commonly used in applications that require a precise supply of air pressure and signal pressure, such as pneumatic testing equipment and pneumatic automatic devices. Figure 14–4 shows the working principle diagram of the constant value device. It consists of three parts: 1 is the main closing part of the direct-acting pressure relief valve ； 2 is a constant pressure drop device, equivalent to a differential pressure valve. Its main function is to provide a stable gas flow to the nozzle ； 3 is the nozzle baffle device and pressure regulation section, which serves to regulate pressure and amplify it; the amplified air pressure is then used to control the main valve section. Since the regulator has functions of setting, comparison, and amplification, it achieves high voltage stabilization accuracy. When the regulator is not in operation, the compressed air supplied from the air source passes through filter 1 before entering chamber A and the main chamber. The main spool 19 is pressed against the valve seat by the spring 20 and the air source pressure, thereby separating chamber A from chamber B. The airflow entering Chamber A passes through the valve opening (also known as a flap) 12 to Chamber F, and after pressure reduction via the constant orifice 13, it enters Chambers G and D respectively. Since no force has yet been applied to diaphragm 8, the distance between baffle 5 and nozzle 4 is large; as a result, the airflow resistance when gas flows out of nozzle 4 is low, the air pressure in chambers G and D is low, and diaphragms 3 and 15 remain in their original positions. The trace gases entering the single-chamber are mainly discharged from the exhaust port through valve 2 in chamber B ； Another portion is discharged from the outlet. At this time, no air flow is emitted from the outlet; it is necessary for a small amount of gas to be discharged through the nozzle in order to keep the nozzle baffle device functioning. Since this involves no power consumption for gas flow, it is desirable that its consumption be as low as possible. When the regulator is in operation, turning the handle 7 compresses the spring 6 and pushes the diaphragm 8 along with the baffle 5 downward; as a result, the distance between the baffle 5 and the nozzle 4 decreases, the air flow resistance increases, and the air pressure in chambers G and D rises. Under the pressure of the air in chamber D, diaphragm 16 moves downward, closing valve port 2 and pushing main spool 19 downward to open the valve port, allowing compressed air to be discharged through outlet ports via chambers B and H. At the same time, the pressure in chamber H rises and is transmitted to diaphragm 8; when the feedback force acting on diaphragm 8 balances the spring force, the regulator outputs gas at a constant pressure. When the input pressure fluctuates, such as when it rises, the air pressure in chambers B and H increases instantaneously, causing diaphragm 8 to move upward. This increases the distance between baffle 5 and nozzle 4, resulting in a decrease in the air pressure in chambers G and D. Due to the increased pressure in chamber B and the decreased pressure in chamber D, diaphragm 15 moves upward under the effect of this pressure difference, which reduces the size of the main valve opening and lowers the output pressure until it stabilizes at the set pressure. Furthermore, as the input pressure increases, the pressure in chamber E and the instantaneous pressure in chamber F also rise; under the effect of this pressure difference, diaphragm 3 moves upward, thereby reducing the opening of pressure-regulating valve 12. Due to the enhanced throttling effect, the air pressure in chamber F decreases, while the pressure difference before and after the throttle orifice 13 remains constant. As a result, the gas flow rate through the throttle orifice 13 stays unchanged, thereby improving the sensitivity of the nozzle baffle. When the input pressure decreases, the pressures in chambers B and H drop instantaneously; diaphragms 8 together with baffle 5 move downward due to the disruption of force balance, the distance between nozzle 4 and baffle 5 decreases, the pressures in chambers G and D rise, and diaphragms 3 and 15 move downward. The downward movement of diaphragm 15 increases the opening degree of the main valve, allowing the air pressure in chambers B and H to rise until it reaches equilibrium with the set pressure. As the diaphragm 3 moves downward, the pressure relief port 12 opens wider, causing the air pressure in chamber F to rise, while always maintaining a constant pressure difference across the constant-throttling orifice 13. Similarly, when the output pressure fluctuates, the same adjustment will be made as when the input pressure fluctuates. Since the regulator uses the feedback from the output pressure and the amplifying effect of the nozzle baffle to control the main valve, it is able to respond to minor pressure changes, thereby enabling timely adjustment of the output pressure and maintaining a relatively stable outlet pressure – that is, it offers high precision in pressure stabilization. II. Basic performance of pressure reducing valves (1) Pressure regulation range: This refers to the adjustable range of the output pressure P2 of the pressure reducing valve, within which a specified accuracy must be achieved. The voltage regulation range is mainly related to the stiffness of the voltage regulation spring. (2) Pressure characteristic: It refers to the characteristic in which, when the flow rate g is constant, fluctuations in the input pressure cause fluctuations in the output pressure. The smaller the fluctuation in output pressure, the better the performance of the pressure reducing valve. The output pressure must be lower than the input pressure by a fixed value in order to remain essentially unchanged regardless of variations in the input pressure. (3) Flow characteristics: It refers to the property in which, at a given input pressure and timing, the output pressure changes as the output flow rate g varies. When the flow rate g changes, it is desirable for the change in output pressure to be as small as possible. Generally, the lower the output pressure, the smaller the fluctuation of it with the output flow rate. III. Selection of pressure relief valves: Determine the type and pressure regulation accuracy of the pressure relief valve based on the application requirements, and then select its diameter according to the required maximum output flow rate. When determining the air supply pressure for the valve, it should be set to 0.1 MPa higher than the maximum output pressure. The pressure relief valve is generally installed after the water and air separator, and before the oil mister or regulator; make sure not to connect its inlet and outlet in reverse ； When the valve is not in use, the knob should be loosened to prevent the diaphragm from being constantly under pressure and deformed, which could affect its performance.

Just find the flow curve of a pressure relief valve and take a look, and you’ll understand. As the opening changes, the flow rate increases while the outlet pressure decreases. Also, if you close the ball valve at the back, the pressure relief valve will close on its own as well. Finally, a pressure reducing valve is designed to reduce pressure – so how can the inlet and outlet pressures be equal? Unless it fails.