Abstract: In hot water heating system. Water hammer phenomenon is an objective reality, in order to ensure the safety of the system, we must take positive precautions; to minimize the occurrence of water hammer phenomenon, check valve is a very important component of the heating system piping, this article studies only Back to the valve in the heating system in the role of water hammer, the valve set up on the opening and closing of the boundary conditions. The defects of ordinary check valve and the characteristics of water hammer check valve are analyzed. The mechanism of eliminating water hammer and preventing water pump reversal by optimizing the closing characteristics is studied. Practical conclusions are drawn, which is helpful for the correct selection and use of check valve significance. I. Introduction As the development of our national economy and the continuous improvement of people's living standards, Urban central heating system has gradually replaced household-based decentralized heating stoves. At present, large-scale central heating systems have been or are being constructed in Beijing, Shenyang and other cities, and the future centralized heating systems will move toward large-scale and complicated ones . This large-scale central heating system has a large heating capacity. Once a water hammer accident is destructive and the consequences are serious, proactive 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 piping. The installation position of the check valve, the structural parameters and the closing curve have a great influence on the safety and security of the heating system. If the check valve is properly used; can play the role of ice hammer protection; but if not used properly, not only can not play a protective role in heating equipment, but also in the heating system piping caused a lot of hot water. Therefore, it is of great practical significance and academic significance to study the function of check valve in water heating system. From the point of view of fluid transients; for check valves only open or close when the power is interrupted, and there are certain requirements for opening and closing speed depending on the flow conditions in the heating circuit. Improper opening and closing speed will cause a sudden increase or decrease in the pressure in the system line, when the pressure rise is large enough will cause the tube to rupture. To ensure the safety of the system for the plant, the structure of the check valve and the closing curve must be selected reasonably to make it function as a water hammer. In this paper, we study the water hammer effect of the check valve in the heating system, establish the boundary conditions when the check valve is opened and closed, analyze the defects of the check valve and the characteristics of the water hammer check valve, The elimination of water hammer and the mechanism that prevents the pump from reversing leads to practical conclusions that are instructive for the correct selection and use of check valves in heating systems. Second, the check valve and its application Check valve is used to prevent the pipeline or equipment backflow of a valve plant. 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 in other places where the fluid is not allowed to flow in the opposite direction. Choosing a suitable check valve type has a significant impact on water hammer protection. For example, a large number of water hammer incidents in a pumping station are related to check-ups installed at the pump outlet. After the accident, the flow of water through the pump stopped flowing back to Beijing and the check valve quickly closed under the action of pouring water flow. If the check valve could not start flowing backward due to the failure or the friction and inertia of the support, Of the time, that is, when the amount of backflow increases to a certain value, even at the moment of maximum backflow, it can cause a very large pressure at the check valve. Check valve flap at zero flow when closed is closed the best state, to complete the valve at zero flow is not possible to close. It is generally accepted that it is normal to close at zero flow. In the hot water heating system, in order to reduce the accident to stop the pump water hammer, you can set a pressure relief pipe with a check valve between the pressure pipe and the suction pipe of the circulating pump. When the circulating pump is running, the check valve is closed due to the water pressure higher than the suction side of the water pump. When the sudden stop pump moment, the pressure on the water side of the pump drastically decreased, while the pressure on the water side increased significantly. Under this pressure difference, the water in the water circuit on the circulating pump pushed the check valve to the side of the pump Pipe network system, thereby reducing the water pressure on the suction side of the pipe network increase; reduce and prevent the hazards of water hammer. Pressure relief valve installed on the check valve should be used less resistance, open the flexible products. The larger the diameter of the pressure relief bypass pipe, the more effective it is to reduce the water hammer and the economical diameter can be determined by transient calculation according to the specified pressure limit. (A) without bypass pipe (b) with bypass pipe Figure 1 pump outlet pressure a time curve Figure 1 shows a heating system circulating pump without a bypass pipe between the pressure pipe and suction pipe, circulation (Fig. 1 (a)), and the calculation curve of the outlet pressure change of the pump with the pressure-relief bypass pipe between the system and the non-return valve after the pump is de-energized (Fig.1 (b) ). It can be seen from the figure that adding a by-pass design with a check valve is obviously effective in controlling the water hammer pressure; the pressure oscillation in the pipe quickly decays to reach a new steady state. The amplitude of the ripple is also much smaller than when there is no by-pass design. Third, the check valve opening and closing of the boundary conditions If the hydraulic gradient of the baseline and the valve axis line; then in the steady flow F, through the check valve orifice pressure drop â–³ H0 and Q0 are as follows: (1 ) Where CD is the flow coefficient, AG is the valve opening flow area. In the opening and closing of the check valve, the instantaneous flow and pressure drop still maintain the above relationship: (2) If the valve is defined as the infinity of steel opening (3) It is in the check valve opening and closing process when the valve t According to Eq. (1) and Eq. (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, Ï„ = 1 flow is zero when check valve is closed, Ï„ = 0. When the last valve opening and closing of the boundary condition equation and heating system in the other boundary conditions equation, one-dimensional unsteady flow equation of motion and the continuous equation of the solution, you can find when the non-dimensional valve opening changes, Heating system of the calculated section of the pressure changes. The purpose of calculation is to find the best check valve closing process in order to determine a reasonable check valve structure parameters (such as valve diameter, counterweight, etc.). Fourth, the ordinary check valve and water hammer check valve Select the appropriate check valve type, the heating system water hammer protection has a great impact, especially in the pump outlet check valve with particular attention to choose the right The type. It is hoped that the pump will automatically shut down quickly and stop the water from flowing back and forth when the pump stops. However, if it is shut down improperly, it will cause serious water hammer in the pipe network and threaten the safety of heating. Severe water hammer It will cause paralyzing damage to the heating system and will have an impact and loss on production and life. 1. General check valve defects Currently used in the heating system check valve is here called the ordinary check valve, mainly by the valve body, valve flap and valve cover, it has the following disadvantages: (1) accident stop When the pump valve is closed quickly, resulting in the valve after the hole, hole in the positive and negative water hammer wave under the action of repeated generation and self-extinguishing, resulting in cavitation erosion valve to shorten its life expectancy. (2) When the valve action is not flexible, the closure is slow, the water backflow, the impact of the impeller to produce runaway reversal and damage to the pump equipment. (3) During normal operation, the valve flap floats in the water, swinging and vibrating constantly, so that 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 Hot water heating system occurred in the process of development of water hammer central heating problems, the type of check valve on the heating system water hammer pressure is more and more caused by the heating industry at home and abroad attention . Water heating system should be used as much as possible waterproof check valve. Water hammer check valve to overcome the shortcomings of the ordinary check valve, has the following characteristics: (1) after the pump can open the valve promptly and quickly. (2) During normal operation, it is required that the valve flap has the largest opening angle and be stable in the fully open position. (3) When the pump is shut down, the valve has an optimized shut-off feature, which can prevent the water from flowing backward when the pump is suddenly stopped to prevent the pump from runaway and reverse, to achieve the purpose of protecting the pump, and to enable it to be closed slowly in the final stage of closing , To reduce the sudden closure caused by water hammer in the pipeline, to protect the purpose of the pipeline. Figure 2 shows the actual check valve installed at the pump outlet; when the stop pump when the downstream side of the check valve pressure changes with time curve (Figure 2 (a)); and installed at the pump Danfoss Danfoss 402M waterproof hammer Check valve, stop the pump when the downstream side of the check valve pressure changes over time measured curve (Figure 2 (b)). Test diameter 150 mm, steady flow of 155 m3 / h; check valve downstream side pressure 5 bar. It can be seen from Figure 2 that when the normal check valve is installed, the maximum pressure at the check valve caused by the stop pump reaches 20 bar, and the installation of the water hammer check valve under the same conditions, the maximum pressure at the check valve is only 10 bar , Reducing the water hammer caused by stopping the pump. (A) ordinary check valve (b) water hammer check valve Figure 2 downstream of the check valve pressure - time curve Fifth, the optimization of the closing characteristics of the mechanism to eliminate the water hammer and prevent pump reversal According to stop pumping hydraulic transition process theory, at Pump outlet is not set check valve, when a sudden stop pump, the water hammer process can be divided into three stages. l Pump conditions After power failure, the pump due to inertial rotation, the water flow in the pipe continued to flow, but its speed gradually reduced until the flow rate becomes zero. This phase is also known as "positive flow only." 2 braking conditions Transient stationary water, due to the role of hydrostatic head began to reverse flow back to the water flow is still positive pump impeller play a braking effect, so the speed continues to decrease until the speed is zero. This phase is also known as "counter-current positive." 3 Turbine conditions With the pouring water flow increases, the pump began to reverse and gradually accelerated, and finally in the case of runaway speed reverse rotation. This stage is also known as "counter-current reversal." It can be seen from the analysis that after stopping the pump, the water flow from the forward flow to the reverse flow has a zero flow point in theory, that is, the instantaneous flow is still. If the valve closes rapidly at this moment, the inertial impact of the water flow is the smallest, Min. But in fact due to many factors, it is possible to determine the timing of zero flow point is very accurate. In addition, it takes time for the valve to close, and it is not possible to close the valve at zero flow. Therefore, the valve closing parameter is set to a larger opening, with a faster closure of a larger angle, cut off a large number of water flow, then the valve dimensionless opening change is not large. However, close to the closure, the smaller closure of the valve also causes a significant change in the non-dimensional opening, causing significant changes in flow rate and pressure. So close to close with a slower speed to close a smaller angle to extend the closing time of the valve, although the closing time longer, but the check valve opening has been very small, the flow of water by valve resistance is very Large, counter-flow is small, so not to reverse the pump over speed, so that reduces the water hammer and solve the pump speed reversal problem. Check valve is a part of the heating system, its action requirements are not static, but according to the system. Some shut-off speed may be appropriate for a system, but may not be appropriate for the other system. Proper analysis and verification of the shut-off valve closure are key to eliminating water hammer and preventing reversal; And the closing parameter shall be based on the specified closing time, the maximum (minimum) pressure limit. Reasonably adjust the calculation, effectively control the process of closing the valve. VI Conclusion 1 in the pump outlet pipe and pressure pipe connected with the check valve of the pressure relief bypass pipe approach to control the pump pressure water hammer failure has a very good effect. 2 Water hammer check valve to overcome the shortcomings of the ordinary check valve, hot water heating system should be used as much as possible waterproof check valve. 3 Water hammer check valves should be provided at the pump outlet. That can prevent the pump suddenly stop pump speeding reversal. But also can reduce the sudden stop pump caused by water hammer. 4 stop wide motion requirements are not static. It should be based on the system, this article gives the description of the boundary conditions after the closure of the echo equation.
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