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Pressure reducing valve

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The pressure reducing valve is a valve that adjusts the inlet pressure to a certain required outlet pressure and relies on the energy of the medium to automatically maintain a stable outlet pressure. From the point of view of fluid mechanics, the pressure reducing valve is a throttling element whose local resistance can be changed, that is, by changing the throttling area, the flow velocity and the kinetic energy of the fluid are changed, resulting in different pressure losses, thereby achieving the purpose of reducing pressure. Then rely on the adjustment of the control and adjustment system to balance the fluctuation of the pressure behind the valve with the spring force, so that the pressure behind the valve remains constant within a certain error range.


1 basic information

The pressure reducing valve is a valve that adjusts the inlet pressure to a certain required outlet pressure and relies on the energy of the medium to automatically maintain a stable outlet pressure.

The pressure reducing valve is a special device that automatically reduces the working pressure of the pipeline. It can reduce the higher liquid pressure of the pipeline before the valve to the level required by the pipeline behind the valve. The transmission medium here is mainly water. Pressure reducing valves are widely used in high-rise buildings, areas with high water pressure in urban water supply networks, mines, and other occasions to ensure that the water points in the water supply system get the proper service water pressure and flow.

In view of the fact that the water leakage rate and the degree of waste are almost proportional to the water pressure of the water supply system, the pressure reducing valve can improve the system's operating conditions and potential water saving effects. According to statistics, its water saving effect is about 30%. There are many types of pressure reducing valve constructions, such as membrane type and inner spring piston type.

The basic function principle of the pressure reducing valve is to reduce the water pressure by the local resistance of the flow channel in the valve to the water flow. The range of the water pressure drop is automatically adjusted by the membrane connected to the valve disc or the water pressure difference between the inlet and outlet of the piston.

The principle of constant pressure reduction is to use the water pressure ratio control of the floating piston in the valve body. The pressure reduction ratio at the inlet and outlet sides is inversely proportional to the piston area ratio at the inlet and outlet sides. This pressure reducing valve works smoothly without vibration, making domestic pressure reducing valve production. The technology is further improved; there is no spring in the valve body, so there is no concern about spring corrosion and metal fatigue failure; good sealing performance and no leakage, so both dynamic pressure (when water flows) and static pressure (when the flow is 0); especially Does not affect water flow while reducing pressure. Pressure reducing valves usually have various specifications such as DN15 to DN100. The working pressure before and after the valve are <1MPa and 0.1 to 0.5MPa, respectively, and the pressure range error is ± 5% to 10%.

JB / T2203-1999 "Length of pressure reducing valve structure" is a general class. At present, most domestic pressure reducing valve manufacturers are designed and produced according to this standard, but this standard is not exhaustive and the specifications are incomplete. For example, the large nominal diameter of gas pressure reducing valve is DN500, and the large nominal diameter of water pressure reducing valve is DN1000. According to the specifications of the pressure reducing valve produced by the manufacturer and the information available, the connection dimensions of the manufacturers are not uniform. For example, the DN500 gas pressure reducing valve, it is recommended that the General Machinery Research Institute review the structure length of the pressure reducing valve JB / T2203-1999. Make revisions. It is recommended that design institutes and users choose according to standards, and manufacturers of pressure reducing valves design and manufacture according to standards.

2 working principle

Pressure reducing valve is a necessary accessory for pneumatic regulating valve. Its main function is to reduce the pressure of the air source.

Pressure and stabilize to a fixed value, so that the regulating valve can obtain a stable air source power for regulating control. According to the structure, it can be divided into film type, spring film type, piston type, lever type and bellows type. According to the number of valve seats, it can be artificial single seat type and double seat type. It can be divided into positive action type and Reaction type.

Direct acting pressure reducing valve

The direct-acting pressure reducing valve is a structural diagram of a direct-acting pressure reducing valve with an overflow valve (referred to as an overflow pressure reducing valve).

Compressed air with pressure P1 is input from the left end and throttled through valve port 10, and the pressure drop is output to P2. The size of P2 can be adjusted by the pressure regulating springs 2 and 3. Turn the knob 1 clockwise to compress the springs 2, 3 and the diaphragm 5 to move the valve core 8 downward, and increase the opening of the valve port 10 to increase P2. If the knob 1 is turned counterclockwise, the opening degree of the valve port 10 decreases, and P2 decreases accordingly.

If P1 rises momentarily, P2 will rise accordingly, causing the pressure in the diaphragm air chamber 6 to increase, and the thrust generated on the diaphragm 5 will increase accordingly. This thrust destroys the original force balance and makes the diaphragm 5 upward. When moving, a small part of the air flow is discharged through the overflow hole 12 and the exhaust hole 11. While the diaphragm is moving up, the valve core 8 is also moved upward due to the action of the return spring 9 and the small intake valve port 10 is closed, and the throttling effect is increased to reduce the output pressure until a new balance is reached. The output pressure is basically Back to the original value. If the input pressure drops instantaneously, the output pressure also drops, the diaphragm 5 moves down, the valve core 8 moves down, the intake valve port 10 opens, and the throttling effect is reduced, so that the output pressure also basically returns to the original value. Turn Knob 1 counterclockwise. The adjustment springs 2 and 3 are relaxed, the thrust force of the gas on the diaphragm 5 is greater than that of the pressure regulating spring, the diaphragm is curved upward, and the intake valve port 10 is closed by the action of the reset spring. Turn the knob 1 again, the top of the inlet valve core 8 and the relief valve seat 4 will be disengaged, 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 non-output state. .

In short, the relief pressure reducing valve is pressure-reduced by the throttling effect of the air inlet, and is balanced by the force of the diaphragm.

Stabilize with the overflow effect of the overflow hole; adjust the spring to make the output pressure change within a certain range. In order to prevent the above-mentioned overflow pressure reducing valve from polluting the surrounding environment with a small amount of gas, a pressure reducing valve without a relief valve (that is, a common pressure reducing valve) can be used.

Pilot operated pressure reducing valve

Internal pilot-operated pressure reducing valve When the output pressure of the pressure reducing valve is high or the diameter is large, the pressure is directly adjusted by a pressure regulating spring, the spring stiffness must be too large, and the output pressure fluctuates greatly when the flow rate changes, and the structure of the valve The size will also increase. To overcome these shortcomings, pilot operated pressure reducing valves can be used. The pilot operated pressure reducing valve works basically the same as the direct acting type. 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 pressure reducing valve . Compared with the direct-acting pressure reducing valve, the enlargement of the nozzle baffle composed of the nozzle 4, the baffle 3, the fixed orifice 9 and the air chamber B is added. When the distance between the nozzle and the baffle changes slightly, the pressure in the B chamber will change significantly, which will cause a large displacement of the diaphragm 10 to control the vertical movement of the valve core 6 to make the inlet The air valve port 8 is opened large or closed small, which improves the sensitivity of valve core control, that is, the accuracy of voltage stabilization is improved.

There is also a small direct acting pressure reducing valve outside the main valve body to control the main valve. This type of valve is suitable for occasions with a diameter of more than 20mm, long distance (within 30m), high places, dangerous places, and difficult to adjust the pressure.

3 categories

(1) Direct-acting pressure reducing valve

Simple pressure reducing valve, direct acting pressure reducing valve with flat diaphragm or bellows. Because it is independent

Structure, so there is no need to install external sensing lines downstream. It is one of the three types of pressure reducing valves that is small in size and economical in use, and is designed for low and medium flow rates. Direct acting pressure reducing valves are typically +/- 10% of the downstream setpoint.

(二) Piston pressure reducing valve

This type of pressure reducing valve combines two types of valves-pilot valve and main valve-in one. The design of the pilot valve is similar to that of a direct acting pressure reducing valve. The exhaust pressure from the pilot valve acts on the piston, causing the piston to open the main valve. If the main valve is large and cannot be opened directly, this design will use the inlet pressure to open the main valve. Therefore, this type of pressure reducing valve has a higher capacity and degree (+/- 5%) at the same pipe size compared to a direct acting pressure reducing valve. Similar to the direct-acting pressure reducing valve, the pressure sensing valve internally senses the pressure without externally installing a sensing line.

(Three) membrane pressure reducing valve

In this type of pressure reducing valve, a double diaphragm replaces the piston in an internally guided pressure reducing valve. This increased diaphragm area opens up a larger main valve and has a larger capacity than an internally piloted piston pressure reducing valve for the same pipe size. In addition, the diaphragm is more sensitive to pressure changes, up to +/- 1%. Higher performance 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 use different types of pilot valves (such as pressure valves, temperature valves, air loading valves, solenoid valves or several types of valves at the same time).

References

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