Abstract: In the hot water heating system. The phenomenon of water hammer exists objectively. In order to ensure the safety of the system, active preventive measures must be taken; to minimize the occurrence of water hammer, the check valve is a very important component in the pipeline of the heating system. The water hammer effect of the return valve in the heating system establishes the boundary conditions for the opening and closing of the upper return valve. Analyzed the defects of ordinary check valves and the characteristics of waterproof hammer check valves, studied the mechanism of optimizing the closing characteristics to eliminate water hammer and prevent the pump from reversing, and draw practical conclusions, which have guidance for the correct selection and use of check valves significance.
With the development of our national economy and the continuous improvement of people’s living standards. Urban central heating systems have gradually replaced household-style decentralized heating furnaces. At present, large-scale central heating systems have been or are being built in cities such as Beijing and Shenyang. In the future, central heating systems will develop in the direction of large-scale and complex . This large-scale central heating system has a large heating scale. Once a water hammer accident occurs, the damage is strong and the consequences are serious. Therefore, active preventive measures must be taken to minimize and prevent the occurrence of water hammer.
The check valve is a very important part of the heating system pipeline. 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 a role in ice hammer protection; but if it is not used properly, it will not only fail to protect the heating equipment, but it will also cause a lot of water scalding in the heating system pipeline. Therefore, the research on the water hammer effect of the check valve in the heating system has very important practical and academic significance.
From the perspective of fluid transient movement; the check valve can only be opened or closed when the power is interrupted under the requirements of the check valve, and there should be certain requirements for the opening and closing speed according to the flow conditions in the heating pipeline. Inappropriate opening and closing speed will cause the pressure in the pipeline of the system to rise or drop suddenly, and when the pressure rise is large enough, it will cause the pipe to rupture. In order to ensure the safety of the system, the structure and closing curve of the check valve must be reasonably selected to make it play the role of a waterproof hammer. This paper studies the water hammer effect of the check valve in the heating system, establishes the boundary conditions when the check valve opens and closes, analyzes the defects of the ordinary check valve and the characteristics of the water hammer check valve, and studies the optimal closing characteristics The mechanism of eliminating water hammer and preventing the reverse of the water pump, and drawing practical conclusions, is of guiding significance for the correct selection and use of check valves in the heating system.
2. Check valve and its application
Check valve is a kind of valve factory used to prevent the back flow of the medium in the pipeline or equipment. It uses the kinetic energy of the fluid to open. In heating systems, check valves are often installed at the outlet of the pump and other places where the reverse flow of fluid is not allowed. Choosing a suitable check valve type has a great influence on water hammer protection. For example, a large number of water hammer accidents in pumping stations are related to the check valve installation at the pump outlet. After the pump was stopped in an accident, the water flow began to flow backwards through Beijing. Under the action of the reversed water flow, the check valve was quickly closed. If the check valve is affected by a malfunction or the friction of the support and its inertia, the check valve may lag behind the start of the reverse flow. When the reverse flow increases to a certain value, or even closes quickly at the moment of the maximum reverse flow, a very large pressure can be generated at the check valve. It is the best state for the check valve to close when the flow rate is zero. It is impossible to complete the valve closure when the flow rate is zero. It is generally believed that it is normal as long as the shutdown is completed near zero flow.
In the hot water heating system, in order to reduce the water hammer of the accidental pump stop, a pressure relief bypass pipe with a check valve can be installed between the pressure pipe and the suction pipe of the circulating water pump. When the circulating water pump is running, because the water pressure on the outlet side of the pump is higher than the water pressure on the suction side, the check valve is closed. When the pump is suddenly stopped, the pressure on the outlet side of the pump drops sharply, while the pressure on the suction side increases greatly. Under the action of this pressure difference, the water in the suction side pipeline of the circulating water pump pushes open the check valve to the outlet side of the pump. The pipe network system reduces the pressure increase in the suction side pipe network; reduces and prevents the harm of water hammer. The check valve installed 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, the more effective it is to reduce the water hammer. According to the specified pressure limit, the economical pipe diameter can be determined through transient calculations.
- (A) No bypass line
- (B) There is a bypass pipeline
The calculation curve of the pressure at the outlet of the water pump over time is given when there is no bypass pipe between the pressurized water pipe and the suction pipe of the circulating water pump of a certain heating system. The calculation curve of the pressure change at the outlet of the water pump after the pipe is piped (Figure 1(b)). 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 decays quickly to reach a new stable state. The amplitude of the Zhuangtou fluctuation is also much smaller than that without the bypass design.
3. Boundary conditions when the check valve is opened and closed
If the reference line of the hydraulic gradient is consistent with the axis of the valve; in the steady flow condition F, the head drop △H0 through the orifice of the check valve is related to Q0 as follows:
- (1) Among them, CD is the flow coefficient, and AG is the open flow area of the valve port. During the opening and closing process of the check valve, the instantaneous flow rate and pressure head drop still maintain the above relationship:
- (2) If you define the valve’s infinite steel opening as
- (3) It is a function of the time valve t during the opening and closing process of the check valve, then according to formula (1) and formula (2):
- (4) 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 steady flow, when τ=1, the flow is zero when the check valve is closed, and τ=0.
The boundary condition equations when the upper return valve is opened and closed are combined with other boundary condition equations in the heating system, the motion equation of one-dimensional unsteady flow, and the continuous equation. It can be found that when the non-return valve opening changes, Changes in the pressure of each calculated section of the heating system. The purpose of the calculation is to find the best check valve closing process in order to determine the reasonable structural parameters of the check valve (such as valve diameter, counterweight, etc.).
Ordinary check valve and waterproof hammer check valve
Choosing a suitable check valve type has a great impact on the water hammer protection of the heating system. Especially the check valve installed at the pump outlet must pay special attention to selecting the appropriate type. When the pump is stopped in an accident, it is hoped that it can be automatically shut down quickly to prevent the backflow of water from impacting the pump and causing a runaway reversal. However, if it is closed improperly, it will cause serious water hammer in the pipe network, posing a threat to safety heating, and serious water hammer. It will cause paralytic damage to the heating system, and cause impact and loss on production and life.
1. Defects of ordinary check valve
The check valve commonly used in current heating systems is called ordinary check valve here. It is mainly composed of valve body, valve clack and valve cover. It has the following shortcomings:
- (1) When the pump is stopped in an accident, the valve is closed quickly, causing cavities behind the valve. The cavities are repeatedly generated and self-extinguished under the action of positive and negative water hammer waves, causing cavitation damage to the valve and shortening its life.
- (2) When the valve action is not flexible, it will close slowly, the water will flow backwards, and the impeller will hit the impeller to cause runaway and reverse rotation, which will damage the pump equipment.
- (3) During normal operation, the valve flap floats in the water, constantly swinging and vibrating, resulting in high flow resistance, high friction, high energy consumption, and short life.
2. Features of waterproof hammer check valve
The hot water heating system has practical problems in the development process of water hammer central heating. The influence of check valve type on the water hammer pressure of heating system has attracted more and more attention from domestic and foreign heating circles.
In the hot water heating system, the waterproof hammer check valve should be used as much as possible. The waterproof hammer check valve overcomes the shortcomings of ordinary check valves and has the following characteristics:
- (1) The valve can be opened promptly and quickly after starting the pump.
- (2) During normal operation, the valve disc is required to have the largest possible opening angle and be able to stabilize in the fully open position.
- (3) The valve has an optimized closing characteristic when the pump is stopped. When the pump is suddenly stopped, it can prevent the water from flowing backward to protect the pump from runaway and reverse, so as to protect the pump and enable it to achieve slow closing in the final stage of closing. , Reduce the water hammer in the pipeline caused by sudden closing, and achieve the purpose of protecting the pipeline.
Figure 2 shows the installation of an ordinary check valve at the outlet of the pump; the measured curve of the pressure on the downstream side of the check valve with time when the pump is stopped (Figure 2(a)); and the installation of the Danfoss 402M waterproof hammer at the outlet of the pump Check valve, the measured curve of the pressure on the downstream side of the check valve with time when the pump is stopped (Figure 2(b)). The diameter of the test pipe is 150 mm, and the pipe flow rate is 155 cubic meters per hour in a steady state; the pressure on the downstream side of the check valve is 5 bar. It can be seen from Figure 2 that when the ordinary check valve is installed, the maximum pressure at the check valve reaches 20 bar when the pump is stopped, while the maximum pressure at the check valve is only 10 bar when the waterproof hammer check valve is installed under the same working conditions. , Reduce the water hammer caused by stopping the pump.
- (A) Ordinary check valve
- (B) Waterproof hammer check valve
5. Optimize the mechanism of closing characteristics to eliminate water hammer and prevent the pump from reversing
According to the theory of the hydraulic transition process of stopping the pump, the water hammer process can be divided into three stages when there is no check valve at the outlet of the pump, and when the pump is suddenly stopped.
l Water pump working condition
After the power failure, the water pump rotates due to inertia, and the water flow in the pipe continues to flow in the positive direction, but its speed gradually decreases until the water flow speed becomes zero. This stage is also called “positive flow before turning”.
2 Brake condition
The transiently stationary water begins to flow backwards due to the effect of the static water head, and the backwashing water flow acts as a restraint on the pump impeller that is still rotating, so the speed continues to decrease until the speed reaches zero. This stage is also called “countercurrent forward rotation”.
3 Turbine operating conditions
With the increase of the inverted water flow, the water pump starts to reverse and gradually accelerates, and finally rotates in reverse at the runaway speed. This stage is also called “countercurrent reversal”.
Through analysis, it can be seen that after the pump is stopped, the water flow flows from the forward direction to the reverse direction. In theory, there is a zero flow point, that is, the instantaneous water flow is static. If the valve is closed quickly at this time, the inertial impact of the water flow is the smallest, that is, the water hammer is generated. The smallest. But in fact, due to the existence of many factors, the time when the zero flow point appears may be determined accurately. In addition, it takes a while for the valve to close, and it is impossible to complete the valve flap closing at the zero flow point. Therefore, the valve closing parameter is set to close a larger angle at a faster speed when the opening is large, and cut off a large amount of water flow. At this time, the dimensionless opening of the valve does not change much. However, close to the closing time, closing the valve with a smaller amplitude will also cause a significant change in the dimensionless opening, which will cause a significant change in the flow rate and pressure. Therefore, when it is about to close, use a slower speed to close a smaller angle and extend the closing time of the valve. At this time, although the closing time is longer, because the opening of the check valve is already very small, the valve resistance to the reverse flow is very high. Large, the reverse flow is small, so it will not cause the pump to reverse overspeed, which reduces the water hammer and solves the problem of pump overspeed reverse.
The check valve is a component in the entire heating system, and its action requirements are not static, but depend on the system. A certain closing speed may be appropriate for one system, but may not be appropriate for another system. Correct analysis and determination of the check valve closing procedure is the key to eliminating water hammer and preventing reversal; analysis of hammer burn And the closing parameters should be based on the specified closing time and the specified maximum (minimum) pressure limit. Perform adjustment calculations reasonably to effectively control the valve-closing process.
- 1 The method of connecting a pressure relief bypass pipeline with a check valve between the outlet pipeline and the pressure pipe of the water pump has a good effect on controlling the water hammer pressure when the water pump fails.
- 2 The waterproof hammer check valve overcomes the shortcomings of ordinary check valves, and the waterproof hammer check valve should be used as much as possible in the hot water heating system.
- 3 A waterproof hammer check valve should be installed at the outlet of the water pump. That is, it can prevent the overspeed reversal when the pump suddenly stops. It can also reduce the water hammer caused by the sudden stop of the pump.
- 4 The action requirements of the non-return widening are not set in stone. It should be based on the system. This article gives the boundary condition equation describing the non-reverberation and closing time.
Link to this article：Waterproof hammer function of check valve in heating system
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