Questions about pressure relief valves

Reducing Valve A reducing valve is a device that regulates the flow rate of a fluid by controlling the degree of opening of the valve’s closing element, thereby reducing the pressure of the fluid. It also uses the pressure behind the valve to adjust the degree of opening of the closing element, so as to keep the pressure behind the valve within a certain range. Even when the inlet pressure changes, it ensures that the outlet pressure remains within the set range, thus protecting the equipment used in daily life and industrial processes. 　　A gas pressure reducing valve is an essential component of pneumatic control valves. Its main function is to reduce the pressure of the gas supply and stabilize it at a constant level, so that the control valve can receive a stable supply of gas power for regulation and control purposes. 　　Based on their structural design, they can be classified into diaphragm type, spring-diaphragm type, piston type, lever type, and bellows type ； Based on the number of valve seats, they can be manually classified as single-seat or double-seat types ； Based on the position of the valve disc, they can be divided into direct-acting and reverse-acting types. 　　I. Working Principle of Pressure Reducers 　　Direct-acting Pressure Reducer 　　 Figure 14–1 shows a direct-acting pressure reducer. Figure 14–1a depicts the structure of a direct-acting pressure reducer equipped with a relief valve (also known as a relief-pressure reducer). 　　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 degree of valve port 10 decreases, and P2 decreases accordingly. 　　If P1 increases instantaneously, P2 will increase as well, thereby raising the pressure in diaphragm chamber 6. 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 flows out 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 this 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 spool 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. Upon rotating the knob 1 further, the top end of the intake valve plug 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 output pressure can be adjusted within a certain range by adjusting the spring. 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 Figure 14–2 Internal structure of the pilot-operated pressure reducing valve. When the output pressure of the 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-operated 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 gas 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 results in 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 and improving the sensitivity of control over the valve spool – that is, enhancing the precision of pressure stabilization. 　　 Figure 14–3 shows the main valve of an external pilot-operated pressure reducing valve. Its 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 applications involving long distances (within 30 m), high locations, hazardous environments, and situations where pressure regulation is difficult. This post was last edited by yhlio on 2009-4-27 19:07]

What was said upstairs made it sound complicated. The function of a pressure relief valve is to reduce pressure and keep the outlet pressure stable. The principle is to reduce the flow rate within the valve body based on the equation of state in order to control the flow, and then to increase the volume instantly, thereby reducing pressure with minimal changes in temperature.

Is it better to install a pressure relief valve at the boiler’s exhaust port on the steam main, or at the point of use?

A pressure reducing valve regulates the flow rate of the medium by controlling the opening degree of the closing element inside the valve body, thereby reducing the pressure of the medium. It also adjusts the opening degree of the closing element based on the pressure behind the valve, so as to keep that pressure within a certain range. Even when the inlet pressure changes, it ensures that the outlet pressure remains within the set range, thus protecting the equipment used in daily life and production downstream. When the system pressure increases and exceeds the required ambient pressure, it is necessary to use a pressure relief valve to relieve the pressure of the gas. Most pressure relief valves are needle valves.

The function of a pressure relief valve is to reduce pressure and keep the outlet pressure stable

The function of a pressure relief valve is to reduce pressure and keep the outlet pressure stable

A pressure reducing valve is a specialized device designed to automatically lower the operating pressure in a pipeline; it reduces the high water pressure in the pipeline before the valve to the level required in the pipeline after the valve. Pressure relief valves are widely used in high-rise buildings, areas with excessively high water pressure in urban water supply networks, mines, and other settings, to ensure that all water usage points in the supply system receive an appropriate water pressure and flow rate. Given that the rate of water leakage and waste is almost directly proportional to the water pressure in the supply system, pressure reducing valves help improve the operational conditions of the system and have the potential to save water; statistics show that their water-saving effect is approximately 30%. There are many types of construction for pressure relief valves; common ones in the past included diaphragm type and internal spring piston type. The basic working principle of a pressure reducing valve is to reduce water pressure by creating local resistance to the flow of water within the valve’s flow channels; the range of pressure reduction is automatically adjusted by the pressure difference between the inlet and outlet waters on either side of the diaphragm or piston that connects the valve disc. In recent years, some new types of pressure relief valves have emerged, such as the constant ratio pressure relief valve, whose structural principle is shown in Figure 14.2-2. The principle of proportional pressure reduction relies on the control of the water pressure ratio by a floating piston within the valve body; the pressure reduction ratio at the inlet and outlet is inversely proportional to the ratio of the piston areas on those sides. This pressure relief valve operates smoothly without vibration ； There are no springs inside the valve body, so there is no risk of spring corrosion or metal fatigue failure ； It has excellent sealing properties and does not leak, thereby reducing both dynamic pressure (when water is flowing) and static pressure (when the flow rate is 0) ； In particular, it does not affect the water flow rate while reducing pressure. Pressure relief valves are typically available in various sizes ranging from DN50 to DN100. The operating pressures before and after the valve are respectively <1 MPa and 0.1–0.5 MPa, with a pressure regulation accuracy of ±5%–10%. It should be noted that although there is significant head loss as water flows through the pressure reducing valve, it is still energy-saving overall, as it reduces water waste, ensures a proper distribution of flow within the system, and improves the system’s layout and operating conditions.

Pressure relief valves are available in various sizes; the largest ones I’ve seen are of DN250, with a working pressure before the valve that can exceed 70 MPa (for the smaller DN15 models). The pressure regulation range has an error of ±5% to 10%.

During throttling expansion, the temperature drops. Typically, throttle valves are narrower in the middle and wider at the ends, with the outlet being even wider

One thing to keep in mind is that the tighter you turn the adjustment knob of the pressure relief valve, the larger the opening will be; conversely, the looser it is, the smaller the opening. Be careful when using it – do not stand directly facing the outlet of the valve