What is a pressure relief valve?

What is a pressure relief valve?

What is a pressure relief valve?

The pressure reducing valve is a valve that can reduce the inlet pressure to a certain required outlet pressure through regulation and make the outlet pressure automatically maintain stable by relying on the energy of the medium itself. From the point of view of hydrodynamics, the pressure reducing valve is a throttling element whose local resistance can be changed. That is to say, by changing the throttling area, the velocity and kinetic energy of the fluid can be changed, resulting in different pressure losses, so as to achieve the purpose of pressure reduction. Then, by the adjustment of the control and regulation system, the fluctuation of the pressure behind the valve is balanced with the spring force, and the pressure behind the valve is kept constant within a certain error range.

Experts said that the pressure relief valve is a special device to automatically reduce the working pressure of the pipeline, which can reduce the high liquid pressure of the pipeline in front of the valve to the level required by the pipeline behind the valve. The transmission medium here is mainly water. The pressure relief valve is widely used in high-rise buildings, areas with high water pressure in urban water supply network, mines and other occasions to ensure that all water points in the water supply system obtain appropriate service water pressure and flow. Since the leakage rate and waste degree of water are almost proportional to the water pressure of the water supply system, the pressure reducing valve can improve the operation condition of the system and potentially save water. According to statistics, its water saving effect is about 30%. There are many types of pressure reducing valve, such as diaphragm type, internal spring piston type and so on. The basic principle of the pressure reducing valve is to reduce the water pressure by the local resistance of the flow passage in the valve to the water flow. The range of the water pressure drop is automatically adjusted by the water pressure difference between the inlet and outlet of the diaphragm connecting the valve disc or the two sides of the piston. In 2007, Shanghai Minggao Valve Co., Ltd. introduced foreign technology and produced some new pressure reducing valves, such as constant ratio pressure reducing valve. The principle of constant ratio pressure reduction is to control the water pressure ratio of floating piston in the valve body. The pressure reduction ratio at the inlet and outlet is inversely proportional to the area ratio of piston at the inlet and outlet. This pressure relief valve works stably and without vibration, which further improves the production technology of domestic pressure relief valve. There is no spring in the valve body, so there is no consideration of spring corrosion and metal fatigue failure. The sealing performance is good and no leakage, so the pressure (water flow) is reduced It also reduces static pressure (when the flow is 0); especially when the pressure is reduced, it does not affect the water flow. The pressure reducing valve usually has various specifications such as dn15-dn100. The working pressure before and after the valve is < 1MPa and 0.1-0.5mpa respectively, and the error of pressure regulating range is ± 5% – 10%.
Principle and construction type of pressure reducing valve
The pressure reducing valve is a special device which can automatically reduce the working pressure of the pipeline. It can reduce the high water pressure of the pipeline in front of the valve to the level required by the pipeline behind the valve. The pressure relief valve is widely used in high-rise buildings, areas with high water pressure in urban water supply network, mines and other occasions to ensure that all water points in the water supply system obtain appropriate service water pressure and flow. Since the leakage rate and waste degree of water are almost proportional to the water pressure of the water supply system, the pressure reducing valve can improve the operation condition of the system and potentially save water. According to statistics, its water saving effect is about 30%.
There are many types of pressure reducing valve, such as diaphragm type, internal spring piston type and so on. The basic principle of the pressure reducing valve is to reduce the water pressure by the local resistance of the flow passage in the valve to the water flow. The range of the water pressure drop is automatically adjusted by the water pressure difference between the inlet and outlet of the diaphragm connecting the valve disc or the two sides of the piston. In recent years, some new types of pressure reducing valves have appeared, such as constant ratio pressure reducing valves. The principle of constant pressure reduction is to control the water pressure ratio of floating piston in the valve body. The pressure reduction ratio at the inlet and outlet is inversely proportional to the piston area ratio at the inlet and outlet. This pressure reducing valve works stably without vibration; there is no spring in the valve body, so there is no consideration of spring corrosion and metal fatigue failure; the sealing performance is good without leakage, so it not only reduces the pressure (when the flow is moving) but also reduces the static pressure (when the flow is 0); especially when the pressure is reduced, it does not affect the water flow. The pressure reducing valve usually has dn50-dn100 and other specifications. The working pressure before and after the valve is less than 1MPa and 0.1-0.5mpa respectively, and the error of pressure regulating range is ± 5% – 10%.

It should be noted that although there is a large head loss of water flow through the pressure reducing valve, the overall energy saving is still achieved due to the reduction of water waste, the reasonable distribution of system flow and the improvement of system layout and working conditions.

Principle of pressure reducing valve

The pressure reducing valve reduces the water pressure by the local resistance of the flow passage in the valve to the water flow. The range of the water pressure drop is automatically adjusted by the water pressure difference between the inlet and outlet on both sides of the diaphragm connecting the valve disc or the piston. The principle of constant pressure reduction is to control the water pressure ratio of floating piston in the valve body. The pressure reduction ratio at the inlet and outlet is inversely proportional to the piston area ratio at the inlet and outlet. This pressure reducing valve works stably without vibration; there is no spring in the valve body, so there is no consideration of spring corrosion and metal fatigue failure; the sealing performance is good without leakage, so it not only reduces the pressure (when the flow is moving) but also reduces the static pressure (when the flow is 0); especially when the pressure is reduced, it does not affect the water flow.

Basic performance of pressure reducing valve

• (1) Pressure regulating 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 pressure regulating range is mainly related to the rigidity of the pressure regulating spring.
• (2) Pressure characteristic: it refers to the characteristic of output pressure fluctuation caused by input pressure fluctuation when flow 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 – set value to basically not change with the input pressure.
• (3) Flow characteristics: it refers to the input pressure – timing, the output pressure changes with the change of output flow G. When the flow rate G changes, the smaller the change of the output pressure is, the better. Generally, the lower the output pressure is, the smaller its fluctuation with the change of output flow.

How to adjust the pressure reducing valve

• 1. Close the gate valve in front of the pressure reducing valve, open the gate valve behind the pressure reducing valve, and manufacture the downstream low-pressure environment; secondly, turn the adjusting screw to the highest position (relative to the lowest outlet pressure), and then close the gate valve behind the pressure reducing valve; then slowly open the gate valve in front of the pressure reducing valve to the full open position; then slowly turn the adjusting screw clockwise to adjust the outlet pressure to the required position Pressure (based on the gauge pressure behind the valve); after adjustment, lock the lock nut and open the gate valve behind the pressure reducing valve; finally, if the outlet pressure is higher than the set pressure during adjustment, readjustment must be started from the first step, i.e. it can only be adjusted from low pressure to high pressure. After a few steps, the pressure relief valve can be adjusted.
• 2. The pressure reducing valve can set the outlet pressure 200X type pressure reducing valve (adjustable pressure reducing valve) working principle through the valve spring: when the valve supplies water from the inlet end, the water flows through the needle valve into the main valve control room, and the outlet pressure acts on the guide valve through the conduit. When the outlet pressure is higher than the pilot spring setting, the pilot closes. When the control room stops draining, the pressure in the main valve control room will rise and the main valve will be closed, and the outlet pressure will not rise any more.
• 3. When the outlet pressure of the valve drops to the set pressure of the guide valve spring, the guide valve opens and the control room drains to the downstream. Because the water discharge of the pilot valve system is greater than that of the needle valve, the pressure of the main valve control room drops. The inlet pressure opens the main valve. In the stable state, the water inlet and drainage of the control room are the same, the opening of the main valve is unchanged, and the outlet pressure is stable. The outlet pressure can be set by adjusting the guide valve spring.
• 4. The pressure reducing valve is a special device which can automatically reduce the working pressure of the pipeline. Its function is to reduce the medium with higher pressure to the given pressure after the given pressure reducing range. It can reduce the high liquid pressure in the front line of the valve to the level required by the rear line of the valve. The transmission medium here is mainly water. The pressure relief valve is widely used in high-rise buildings, areas with high water pressure in urban water supply network, mines and other occasions to ensure that all water points in the water supply system obtain appropriate service water pressure and flow. Since the leakage rate and waste degree of water are almost proportional to the water pressure of the water supply system, the pressure reducing valve can improve the operation condition of the system and potentially save water. According to statistics, its water saving effect is about 30%.

Selection of pressure reducing valve

Select the type and regulation accuracy of pressure reducing valve according to the use requirements, and then select its diameter according to the required maximum output flow. When determining the air supply pressure of the valve, make it greater than the maximum output pressure of 0.1MPa. The pressure reducing valve is generally installed after the water separator and air filter, before the oil atomizer or the setter, and pay attention not to connect the inlet and outlet reversely; when the valve is not in use, the knob should be loosened to avoid that the diaphragm is often compressed and deformed, which will affect its performance.

Selection principle of pressure reducing valve

• 1. The fluctuation of the inlet pressure of the pressure reducing valve shall be controlled within 80% – 105% of the given value of the inlet pressure. If it exceeds this range, the performance of the pressure reducing valve will be affected.
• 2. Generally, the pressure PC behind the pressure reducing valve should be less than 0.5 times of the pressure in front of the valve, that is, PC < 0.5p1. Each spring of the pressure reducing valve is only applicable within a certain range of outlet pressure, and the spring shall be replaced if it is out of the range.
• 3. In the case of high working temperature of medium, pilot piston type pressure reducing valve or pilot bellows pressure reducing valve is generally selected.
• 4. When the medium is air or water (liquid), the direct acting membrane type pressure reducing valve or pilot membrane type pressure reducing valve should be selected generally.
• 5. When the medium is steam, pilot piston type pressure reducing valve or pilot bellows pressure reducing valve should be selected.
• 6. For the convenience of operation, adjustment and maintenance, the pressure reducing valve shall be generally installed on the horizontal pipeline.

Classification of pressure reducing valves

Action reducing valve

Simplest pressure reducing valve, direct acting type, with flat diaphragm or bellows. Because the pressure reducing valve is a separate structure, there is no need to install an external sensing line downstream. The pressure reducing valve is the smallest and the most economical one of the three pressure reducing valves, which is specially designed for low and medium flow. The accuracy of the direct acting pressure reducing valve is usually + / – 10% of the downstream set point.

Membrane pressure reducing valve

In this type of pressure reducing valve, the double diaphragm replaces the piston in the internal guide pressure reducing valve. The increased diaphragm area of the pressure reducing valve can open a larger main valve, and its capacity is larger than the internal guided piston pressure reducing valve under the same pipe size. In addition, the pressure relief valve diaphragm is more sensitive to pressure changes with an accuracy of + / – 1%. The higher accuracy is due to the positioning of the downstream sensing line (outside of the valve), where there is less gas or liquid turbulence. The pressure reducing valve is very flexible and can adopt different types of pressure reducing valve guides (for example, pressure valve, temperature valve, air loading valve, solenoid valve or several valves are suitable for matching at the same time).

Piston type pressure reducing valve

This type of pressure reducing valve integrates two kinds of valves: pilot valve and main valve. The design of the pilot valve is similar to the direct acting pressure reducing valve. The exhaust pressure from the pilot valve acts on the piston to open the main valve. If the main valve is too large to open directly, this design will use the inlet pressure to open the main valve. Therefore, this type of pressure reducing valve has higher capacity and accuracy (+ / – 5%) under the same pipe size compared with the direct acting pressure reducing valve. The same as the direct acting pressure reducing valve is that the pressure inside the pressure reducing valve senses the pressure, and the sensor line does not need to be installed outside.

Installation requirements of pressure reducing valve

1. The pressure relief valve shall be installed after the pressure test and flushing of the water supply network are qualified.
Inspection quantity: full inspection.
Inspection method: check the pipeline pressure test and flushing records.
2. Before the installation of the pressure reducing valve, it shall be checked that the specification and model shall be consistent with the design; the control pipeline outside the valve and the connecting parts of the guide valve shall not be loose; the appearance shall be free of mechanical damage, and the foreign matters inside the valve shall be removed.
Inspection quantity: full inspection.
Inspection method: compare with the drawing, observe and check with the wrench
3. The flow direction of pressure relief valve shall be consistent with that of water supply network.
Inspection quantity: full inspection.
Inspection method: observation and inspection.
4. The filter shall be installed on the water inlet side, and the control valve shall be installed before and after it.
Inspection quantity: full inspection.
Inspection method: observation and inspection.
5. The adjustable pressure reducing valve should be installed horizontally and the valve cover should be upward.
Inspection quantity: full inspection.
Inspection method: observation and inspection.
6. The proportional pressure reducing valve should be installed vertically; when it is installed horizontally, the orifice of the single breathing hole pressure reducing valve should be downward, and the orifice of the double breathing hole pressure reducing valve should be horizontal.
Inspection quantity: full inspection.
Inspection method: observation and inspection.
7. When installing the pressure reducing valve without pressure gauge, the pressure gauge shall be installed at the front and rear adjacent parts.
Inspection quantity: full inspection.
Inspection method: observation and inspection.

Precautions for pressure reducing valve

• 1. For the convenience of operation and maintenance, the valve is generally installed vertically on the horizontal pipeline.
• 2. The pressure reducing valve must be installed in strict accordance with the direction of the arrow on the valve body to keep consistent with the flow direction of the fluid. If the water quality is not clean and contains some impurities, a filter must be installed at the upstream water inlet of the pressure reducing valve.
• 3. In order to prevent overpressure behind the valve, a pressure reducing valve shall be installed at least 4m away from the valve outlet.
• 4. The pressure reducing valve plays a certain check role in the pipeline. In order to prevent the harm of water hammer, small expansion water tank can also be installed to prevent damage to the pipeline and valve. The filter must be installed in front of the inlet pipe of the pressure reducing valve, and the expansion water tank must be installed behind the outlet pipe of the pressure reducing valve.
• 5. If you need to install a pressure relief valve in a hot water system, you must install a check valve between the pressure relief valve and the expansion tank. In this way, the expansion tank can not only absorb the volume of water increased by thermal expansion, but also prevent the influence of hot water return or pressure fluctuation on the pressure relief valve, so as to ensure the long-term normal operation of the pressure relief valve.

There are many types of pressure reducing valves, which can meet the requirements of different environments for different pressure reducing valves. Generally, the pressure reducing valve is installed after the water separator, before the oil atomizer or the constant value device, and attention should be paid to the corresponding inlet and outlet; when not in use, the knob should be loosened to avoid the long-term compression deformation of the diaphragm, which will affect the performance.

Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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