The first valve is based on the water hammer phenomenon that occurs in the hot water heating system, so the following information is given. In order to ensure the safety of the system, active preventive measures must be taken; the occurrence of water hammer is minimized, and the check valve is a very important component in the heating system piping. This paper studies the check valve in the heating system. The waterproof hammer acts to establish the boundary conditions when the upper valve is opened and closed. The defects of the ordinary check valve and the characteristics of the waterproof hammer check valve are analyzed. The mechanism of optimizing the closing characteristics to eliminate the water hammer and prevent the pump from reversing is studied. The conclusions with practical significance are obtained, which is helpful for the correct selection and use of the check valve. significance.
I. Introduction
With the development of China's national economy and the continuous improvement of people's living standards. The urban central heating system has gradually replaced the household-type distributed heating furnace. At present, cities such as Beijing and Shenyang have already built or are building large-scale centralized heating systems. The future centralized heating system will develop in the direction of large-scale and complicated. . This large-scale centralized heating system has a large heating scale. Once a water hammer accident has a strong destructive power and serious consequences, it is necessary to take active preventive measures to minimize and prevent the occurrence of water hammer.
The check valve is a very important component in the heating system piping. The installation position, structural parameters and closing curve of the check valve have a great influence on the safety and reliability of the heating system. If the check valve is used properly; it can play the role of ice hammer protection; but if it is not used properly, it will not only protect the heating equipment, but also cause a lot of water in the heating system pipeline. Therefore, it is of great practical and academic significance to study the role of the check valve in the waterproof hammer in the heating system.
From the perspective of fluid transient motion; for the check valve requirements, it can only be turned on or off when the power is interrupted, and the opening and closing speed should be required according to the flow condition in the heating pipeline. An improper opening and closing speed will cause a sudden increase or decrease in the pressure in the system piping, which will cause the tube to rupture when the pressure rise is large enough. To ensure the safety of the system, the structural form and closing curve of the check valve must be properly selected to make it function as a waterproof hammer. In this paper, the waterproof hammer function of the check valve in the heating system is studied. The boundary conditions of the check valve when opening and closing are established. The defects of the ordinary check valve and the characteristics of the waterproof hammer check valve are analyzed. The optimized closing characteristics are studied. Eliminating the water hammer and preventing the pump from reversing the mechanism, and drawing practical conclusions, it has guiding significance for the correct selection and use of the check valve in the heating system.
Second, check valve and its application
Check valves are a type of valve factory used to prevent backflow of media in pipes or equipment. It uses the kinetic energy of the fluid to turn it on. In heating systems, check valves are often installed at the pump outlet and where other fluids are not allowed to flow in the opposite direction. Choosing the right type of check valve has a great impact on water hammer protection. For example, a large number of water hammer accidents in the pump station are related to the checkout of the pump outlet installation. After the accident stops the pump, the water flows through the Beijing to start the reverse flow. Under the action of the drain water flow, the check valve is quickly closed. If the check valve is affected by the fault or the friction of the bearing and its inertia, the check valve may lag behind the start of the reverse flow. The time, that is, when the reverse flow rate is increased to a certain value, even when the maximum reverse flow rate is quickly closed, a very large pressure can be generated at the check valve. The closing of the check valve flap at the flow rate of zero is the best condition for closing. It is not possible to complete the flap closing when the flow rate is zero. It is generally believed that it is normal to close it near zero flow.
In the hot water heating system, in order to reduce the accidental pump water hammer, a pressure relief bypass pipe with a check valve may be disposed between the pressure water pipe of the circulating water pump and the suction pipe. When the circulating water pump is running, the check valve is closed because the water pressure on the water outlet side of the pump is higher than the water pressure on the water suction side. When the pump is suddenly stopped, the pressure on the pump outlet side is drastically reduced, and the pressure on the water suction side is greatly increased. Under the pressure difference, the water in the suction side of the circulating water pump pushes the check valve to the pump outlet side. The pipe network system reduces the pressure increase in the water absorption side pipe network; reduces and prevents the water hammer damage. The check valve on the pressure relief bypass pipe should be a product with less resistance and flexible opening. The larger the diameter of the pressure relief bypass pipe is, the more powerful it is to reduce the water hammer. The economical pipe diameter can be determined by transient calculation according to the specified pressure limit.
When there is no bypass pipe between the pressure water pipe and the suction pipe of a circulating water pump of a heating system, the calculation curve of the outlet pressure of the water pump changes with time, and the pressure relief bypass pipe between the system and the non-return The calculation curve of the pump outlet pressure change. It can be seen from the figure that adding a bypass design with a check valve is obviously effective for controlling the water hammer pressure; the pressure oscillation in the pipeline is quickly attenuated to a new stable state. The amplitude of the fluctuations of the Zhuangtou is also much smaller than when the design is not bypassed.
Third, the boundary conditions when the check valve is opened and closed
If the baseline of the hydraulic gradient coincides with the axis of the valve; then in the steady flow case F, the pressure drop ΔH0 through the orifice of the check valve is related to Q0 as follows:
This is the boundary condition when the check valve is opened and closed. The dimensionless opening in the formula is also called the valve closing curve. For a constant flow, the flow rate is zero when the Ï„=1 check valve is closed, and Ï„=0.
When the boundary condition equation of the upper valve opening and closing is combined with other boundary condition equations in the heating system, the one-dimensional unsteady flow equation of motion and the continuous equation, it can be found that when the non-dimensional opening of the check valve changes, The heating system calculates the change in section pressure. The purpose of the calculation is to find the optimal check valve closing process in order to determine reasonable stop valve structural parameters (eg valve diameter, counterweight, etc.).
Fourth, ordinary check valve and waterproof hammer check valve
Choosing the right type of check valve has a great influence on the water hammer protection of the heating system, especially the check valve installed at the pump outlet, especially the proper type. When the accident stops the pump, it is hoped that it will automatically close quickly, preventing the water from flowing back and impacting the pump and causing the flyback reversal. However, if it is improperly closed, it will cause serious pipe network water hammer, posing a threat to safe heating, severe water hammer. It will cause damaging damage to the heating system, affecting and losing production and life.
1. Defects of ordinary check valves
The conventional check valves commonly used in heating systems are referred to herein as ordinary check valves and are mainly composed of a valve body, a valve flap and a valve cover, which have the following disadvantages:
(1) When the accident stops the pump, the valve closes rapidly, causing the cavity and the cavity behind the valve to repeatedly generate and self-extinguish under the action of the positive and negative water hammer wave, causing the cavitation damage of the valve and shortening its service life.
(2) When the valve action is not flexible, the closing is slow, the water is reversed, and the impact impeller reverses and the pump equipment is destroyed.
(3) During normal operation, the valve flap floats in the water and oscillates and vibrates continuously, so the flow resistance is large, the friction is large, the energy consumption is high, and the service life is short.
2. Water hammer check valve features
The hot water heating system has the practical problems in the development of water hammer central heating. The influence of the check valve type on the water hammer pressure of the heating system has attracted more and more attention from the domestic and international heating circles.
Water hammer check valves should be used as much as possible in hot water heating systems. The waterproof hammer check valve overcomes the shortcomings of the ordinary check valve and has the following features:
(1) The valve can be opened quickly and promptly after the pump is started.
(2) In normal operation, the valve flap is required to have the largest possible opening angle and can be stabilized in the fully open position.
(3) The valve has an optimized closing characteristic when the pump is stopped. When the pump is suddenly stopped, the water can be prevented from flowing back to protect the water pump from flying reverse. The purpose of protecting the water pump is achieved, and the pump can be slowly closed in the final stage of closing. Reduce the water hammer in the pipeline caused by the sudden closing to achieve the purpose of protecting the pipeline.
Install a normal check valve at the pump outlet; when the pump is stopped, the pressure on the downstream side of the check valve changes with time (Fig. 2(a)); and install the Danfoss 402M waterproof hammer check valve at the pump outlet. The measured curve of the pressure on the downstream side of the check valve with time when the pump is stopped (Fig. 2(b)). The test tube has a diameter of 150 mm, and the pipe flow rate is 155 m3/h in the normal state; the pressure on the downstream side of the check valve is 5 bar. It can be seen from Fig. 2 that when the ordinary check valve is installed, the maximum pressure at the check valve is 20 bar when the pump is stopped, and the maximum pressure at the check valve is only 10 bar when the waterproof hammer check valve is installed under the same working condition. , reducing the water hammer caused by stopping the pump.
V. Optimizing the closing characteristics to eliminate the water hammer and prevent the pump from reversing
According to the hydraulic transition process theory of stopping the pump, when the check valve is not installed at the pump outlet, when the pump is suddenly stopped, the water hammer process can be divided into three stages.
The first valve is based on the water hammer phenomenon that occurs in the hot water heating system, so the following information is given. In order to ensure the safety of the system, active preventive measures must be taken; the occurrence of water hammer is minimized, and the check valve is a very important component in the heating system piping. This paper studies the check valve in the heating system. The waterproof hammer acts to establish the boundary conditions when the upper valve is opened and closed. The defects of the ordinary check valve and the characteristics of the waterproof hammer check valve are analyzed. The mechanism of optimizing the closing characteristics to eliminate the water hammer and prevent the pump from reversing is studied. The conclusions with practical significance are obtained, which is helpful for the correct selection and use of the check valve. significance.
I. Introduction
With the development of China's national economy and the continuous improvement of people's living standards. The urban central heating system has gradually replaced the household-type distributed heating furnace. At present, cities such as Beijing and Shenyang have already built or are building large-scale centralized heating systems. The future centralized heating system will develop in the direction of large-scale and complicated. . This large-scale centralized heating system has a large heating scale. Once a water hammer accident has a strong destructive power and serious consequences, it is necessary to take active preventive measures to minimize and prevent the occurrence of water hammer.
The check valve is a very important component in the heating system piping. The installation position, structural parameters and closing curve of the check valve have a great influence on the safety and reliability of the heating system. If the check valve is used properly; it can play the role of ice hammer protection; but if it is not used properly, it will not only protect the heating equipment, but also cause a lot of water in the heating system pipeline. Therefore, it is of great practical and academic significance to study the role of the check valve in the waterproof hammer in the heating system.
From the perspective of fluid transient motion; for the check valve requirements, it can only be turned on or off when the power is interrupted, and the opening and closing speed should be required according to the flow condition in the heating pipeline. An improper opening and closing speed will cause a sudden increase or decrease in the pressure in the system piping, which will cause the tube to rupture when the pressure rise is large enough. To ensure the safety of the system, the structural form and closing curve of the check valve must be properly selected to make it function as a waterproof hammer. In this paper, the waterproof hammer function of the check valve in the heating system is studied. The boundary conditions of the check valve when opening and closing are established. The defects of the ordinary check valve and the characteristics of the waterproof hammer check valve are analyzed. The optimized closing characteristics are studied. Eliminating the water hammer and preventing the pump from reversing the mechanism, and drawing practical conclusions, it has guiding significance for the correct selection and use of the check valve in the heating system.
Second, check valve and its application
Check valves are a type of valve factory used to prevent backflow of media in pipes or equipment. It uses the kinetic energy of the fluid to turn it on. In heating systems, check valves are often installed at the pump outlet and where other fluids are not allowed to flow in the opposite direction. Choosing the right type of check valve has a great impact on water hammer protection. For example, a large number of water hammer accidents in the pump station are related to the checkout of the pump outlet installation. After the accident stops the pump, the water flows through the Beijing to start the reverse flow. Under the action of the drain water flow, the check valve is quickly closed. If the check valve is affected by the fault or the friction of the bearing and its inertia, the check valve may lag behind the start of the reverse flow. The time, that is, when the reverse flow rate is increased to a certain value, even when the maximum reverse flow rate is quickly closed, a very large pressure can be generated at the check valve. The closing of the check valve flap at the flow rate of zero is the best condition for closing. It is not possible to complete the flap closing when the flow rate is zero. It is generally believed that it is normal to close it near zero flow.
In the hot water heating system, in order to reduce the accidental pump water hammer, a pressure relief bypass pipe with a check valve may be disposed between the pressure water pipe of the circulating water pump and the suction pipe. When the circulating water pump is running, the check valve is closed because the water pressure on the water outlet side of the pump is higher than the water pressure on the water suction side. When the pump is suddenly stopped, the pressure on the pump outlet side is drastically reduced, and the pressure on the water suction side is greatly increased. Under the pressure difference, the water in the suction side of the circulating water pump pushes the check valve to the pump outlet side. The pipe network system reduces the pressure increase in the water absorption side pipe network; reduces and prevents the water hammer damage. The check valve on the pressure relief bypass pipe should be a product with less resistance and flexible opening. The larger the diameter of the pressure relief bypass pipe is, the more powerful it is to reduce the water hammer. The economical pipe diameter can be determined by transient calculation according to the specified pressure limit.
When there is no bypass pipe between the pressure water pipe and the suction pipe of a circulating water pump of a heating system, the calculation curve of the outlet pressure of the water pump changes with time, and the pressure relief bypass pipe between the system and the non-return The calculation curve of the pump outlet pressure change. It can be seen from the figure that adding a bypass design with a check valve is obviously effective for controlling the water hammer pressure; the pressure oscillation in the pipeline is quickly attenuated to a new stable state. The amplitude of the fluctuations of the Zhuangtou is also much smaller than when the design is not bypassed.
Third, the boundary conditions when the check valve is opened and closed
If the baseline of the hydraulic gradient coincides with the axis of the valve; then in the steady flow case F, the pressure drop ΔH0 through the orifice of the check valve is related to Q0 as follows:
This is the boundary condition when the check valve is opened and closed. The dimensionless opening in the formula is also called the valve closing curve. For a constant flow, the flow rate is zero when the Ï„=1 check valve is closed, and Ï„=0.
When the boundary condition equation of the upper valve opening and closing is combined with other boundary condition equations in the heating system, the one-dimensional unsteady flow equation of motion and the continuous equation, it can be found that when the non-dimensional opening of the check valve changes, The heating system calculates the change in section pressure. The purpose of the calculation is to find the optimal check valve closing process in order to determine reasonable stop valve structural parameters (eg valve diameter, counterweight, etc.).
Fourth, ordinary check valve and waterproof hammer check valve
Choosing the right type of check valve has a great influence on the water hammer protection of the heating system, especially the check valve installed at the pump outlet, especially the proper type. When the accident stops the pump, it is hoped that it will automatically close quickly, preventing the water from flowing back and impacting the pump and causing the flyback reversal. However, if it is improperly closed, it will cause serious pipe network water hammer, posing a threat to safe heating, severe water hammer. It will cause damaging damage to the heating system, affecting and losing production and life.
1. Defects of ordinary check valves
The conventional check valves commonly used in heating systems are referred to herein as ordinary check valves and are mainly composed of a valve body, a valve flap and a valve cover, which have the following disadvantages:
(1) When the accident stops the pump, the valve closes rapidly, causing the cavity and the cavity behind the valve to repeatedly generate and self-extinguish under the action of the positive and negative water hammer wave, causing the cavitation damage of the valve and shortening its service life.
(2) When the valve action is not flexible, the closing is slow, the water is reversed, and the impact impeller reverses and the pump equipment is destroyed.
(3) During normal operation, the valve flap floats in the water and oscillates and vibrates continuously, so the flow resistance is large, the friction is large, the energy consumption is high, and the service life is short.
2. Water hammer check valve features
The hot water heating system has the practical problems in the development of water hammer central heating. The influence of the check valve type on the water hammer pressure of the heating system has attracted more and more attention from the domestic and international heating circles.
Water hammer check valves should be used as much as possible in hot water heating systems. The waterproof hammer check valve overcomes the shortcomings of the ordinary check valve and has the following features:
(1) The valve can be opened quickly and promptly after the pump is started.
(2) In normal operation, the valve flap is required to have the largest possible opening angle and can be stabilized in the fully open position.
(3) The valve has an optimized closing characteristic when the pump is stopped. When the pump is suddenly stopped, the water can be prevented from flowing back to protect the water pump from flying reverse. The purpose of protecting the water pump is achieved, and the pump can be slowly closed in the final stage of closing. Reduce the water hammer in the pipeline caused by the sudden closing to achieve the purpose of protecting the pipeline.
Install a normal check valve at the pump outlet; when the pump is stopped, the pressure on the downstream side of the check valve changes with time (Fig. 2(a)); and install the Danfoss 402M waterproof hammer check valve at the pump outlet. The measured curve of the pressure on the downstream side of the check valve with time when the pump is stopped (Fig. 2(b)). The test tube has a diameter of 150 mm, and the pipe flow rate is 155 m3/h in the normal state; the pressure on the downstream side of the check valve is 5 bar. It can be seen from Fig. 2 that when the ordinary check valve is installed, the maximum pressure at the check valve is 20 bar when the pump is stopped, and the maximum pressure at the check valve is only 10 bar when the waterproof hammer check valve is installed under the same working condition. , reducing the water hammer caused by stopping the pump.
V. Optimizing the closing characteristics to eliminate the water hammer and prevent the pump from reversing
According to the hydraulic transition process theory of stopping the pump, when the check valve is not installed at the pump outlet, when the pump is suddenly stopped, the water hammer process can be divided into three stages.
l Pump working condition
After the power failure, the pump is evaluated by inertia rotation. The customer should choose the check valve suitable for the working environment when purchasing, which has a great influence on the water hammer protection of the heating system, especially at the pump outlet. Pay particular attention to the selection of the appropriate type of return valve.
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