A discussion on advantage of air chambers for water hammer for diminish water hammer when cut off pump.
(Nanchang Engineering &Research Institule of Nonferrous Metals Nanchang 330002)
This paper introduces equipment used for combating water hammer, conformation; working elements and merit fault. And make it using abroad for the better work in the future.
Keyworda water hammer when cut off pump, air chambers for water hammer, elements,application
Pump stop water hammer is one of the water hammer phenomena. It refers to the sudden change of flow rate in the water pump and pipeline system when the water pump unit suddenly stops with the valve open due to a sudden power outage or other reasons. A series of hydraulic shock phenomena in which the pressure alternately rises and falls sharply. Under normal circumstances, water hammer when the pump is stopped is the most serious, and it poses a great threat to the safety of the pump room and pipelines. Several pump rooms in China have experienced major accidents such as flooding of the pump room or rupture of pipelines due to water hammer when the pump was stopped. .
The size of the water hammer value when the pump is stopped is related to the specific conditions of the water pump and water pipeline in the pump room. Possible water hammer should be considered when designing pump rooms and water pipelines, and corresponding preventive measures should be taken to avoid the occurrence of water hammer, or to control the impact of water hammer within the allowable range. Common preventive measures include the following:
(1) Reducing the flow rate of the water pipeline can reduce the water hammer pressure to a certain extent, but it will increase the diameter of the water pipeline and increase project investment.
(2) When laying out water pipelines, consideration should be given to avoiding humps or drastic changes in slope.
(3) Through simulation calculations, choosing a water pump unit with a larger moment of inertia GD2 or adding a flywheel with sufficient inertia can reduce the water hammer value to a certain extent.
(4) Set up water hammer elimination devices: such as air pressure tanks, two-way pressure regulating towers, one-way pressure regulating towers, water hammer eliminators, slow-closing check valves, etc. Among them, it is simple to use anti-water hammer pressure tanks to reduce water hammer when the pump is stopped. It is easy to operate, has remarkable effects, and is not affected by terrain. It is very commonly used in Europe (Britain, France and other countries), but it is not widely used in China. Through the design practice in foreign water supply projects, our institute has accumulated some experience in the design calculation and use of this device, which is briefly introduced here.
1. Structure and working principle of anti-water hammer pressure tank
The anti-water hammer pressure tank is used to reduce water hammer when the pump is stopped. It is connected to the pressure water supply pipeline behind the water pump outlet check valve. It consists of a pressure spherical tank (containing a gas-water separation diaphragm) and a drilled check valve at the outlet of the spherical tank. Gate valve composition (see Figure 1 for details). It works using specific laws of gas volume and pressure. As the pressure in the pipeline changes, the air pressure tank replenishes water to the pipeline or absorbs the excessive pressure in the pipeline. The anti-water hammer principle is similar to the two-way pressure regulating tower.
During normal operation, the volume of air and water in the air pressure tank is shown in Figure 2a. When the water pump suddenly stops running, negative pressure forms at the beginning of the pressure supply pipe, and even when the water column separates, the air pressure tank replenishes water to the pipeline, which can effectively balance the negative pressure and prevent the water column from separating. , reduce water hammer intensity. At this time, the amount of water in the pressure tank decreases and the gas expands (see Figure 2b for details); when the water hammer wave returns, the pressure at the beginning of the pressure water supply pipe increases, and the pressure tank enters water from the pipe to absorb the excessive pressure in the pipe. As the amount of medium water increases, the gas is compressed (see Figure 2c for details), which acts as an air cushion, thus effectively resisting the adverse effects of water hammer on the pipeline system.
The internal structure of the anti-water hammer pressure tank is basically the same as the regulating pressure tank commonly used in domestic pressure water supply equipment, but the two are different in the setting of the outlet valve. In addition to the maintenance gate valve installed on the outlet pipe section of the anti-water hammer pressure tank, it is also equipped with a drilled check valve, that is, a small hole of a certain diameter is drilled on the valve disc (the hole diameter must be calculated and determined). This is also The most distinctive part of the installation. Its function is: when negative pressure or even water column separation is formed at the beginning of the pressure water supply pipe, the check valve is fully opened, and the air pressure tank can quickly replenish the required amount of water into the pipeline; when the water hammer wave returns, the pressure at the beginning of the pressure water supply pipe is When it rises, the water enters the pressure tank through the small hole on the check valve disc. On the one hand, it absorbs the excessive pressure in the pipeline. On the other hand, it increases the head loss when the water flows into the pressure tank, consuming the energy of the water hammer wave. , reducing the impact of the back and forth pressure swing caused by water hammer wave motion on the pipeline system.
2. Selection of anti-water hammer pressure tanks
The operating principle of the anti-water hammer pressure tank is very simple, but its selection calculation is not simple. Not only the movement and continuity of the water flow must be considered, but also the impact of air temperature changes on the air volume caused by rapid changes in the air volume in the tank. This makes calculations complex and less likely to be accurate. Common calculation methods currently include the following:
(1) Empirical formula calculation method
Its characteristic is that the algorithm is simple and can quickly estimate the volume of the required pressure tank. It is often used in feasibility studies and program design stages. The formula is as follows:
In the formula: V is the total volume of the pressure tank, m3; Q is the flow rate of the pressure water supply pipe, m3/s; L is the length of the pressure water supply pipe, m.
(2) Graphical method
Calculate the required pressure tank volume based on the Parmakine chart. Its characteristic is that the algorithm is relatively simple and can determine the most economical pressure tank volume based on the water hammer pressure change range (safe pressure change range) that the pipeline system can withstand. The results are more accurate than the empirical formula calculation method and are often used in the preliminary design stage. . Since there are many calculation charts, they will not be described in detail here.
(3) Electrical algorithm
The calculation principle is to apply elastic water column theory to establish the motion equation and continuity equation of the water hammer process, and then combine various influencing factors and boundary conditions to calculate the required volume of the pressure tank. The algorithm process is complex, but the results are very accurate and are often used in the construction drawing design stage. Because the product specifications of each manufacturer are different, their selection and calculation processes are also slightly different. During the construction drawing design, you can ask the manufacturer to assist in calculation and selection.
3. Advantages, disadvantages and applicable conditions of anti-water hammer pressure tanks
Compared with other anti-water hammer devices, anti-water hammer pressure tanks have the following advantages:
(1) Good resistance to water hammer. It not only has the function of balancing the negative pressure and reducing the positive pressure of the two-way pressure regulating tower, but also has the effect of reducing the pressure swing back and forth, which can minimize the various harms caused by water hammer to the pipeline system.
(2) Not restricted by terrain conditions, flexible application. The volume and pressure of the gas in the tank changes with the pipeline pressure, limiting the changes in the water volume in the tank within the volume range of the pressure tank, effectively avoiding the two-way pressure regulating tower being restricted by the pump station pressure and surrounding terrain due to the free change of water level with pipeline pressure. Weak points, suitable for various water supply pressures and different terrains.
(3) Safe and reliable operation. The device has a simple structure and operates automatically, avoiding accidents caused by equipment failure or human factors such as water hammer eliminators. Even if the drilled check valve fails, the anti-water hammer pressure tank can still achieve a certain balanced negative pressure and all the functions of reducing positive pressure and reducing pressure swing through the small hole on the valve disc, without causing too much harm.
(4) Easy to use and maintain. The air pressure tank can operate automatically without debugging after installation, eliminating the complicated debugging process when using a slow-closing check valve as an anti-water hammer device. The whole device has a simple structure and does not require frequent air replenishment, so there is basically no maintenance burden.
The disadvantages of anti-water hammer pressure tanks are that they are more expensive, consume steel, and require high device sealing. When the flow rate of the pressure water supply pipeline is too large or the water transmission distance is too long, the volume of the pressure tank will be too large, resulting in problems such as high purchase costs and difficult processing. Therefore, anti-water hammer pressure tanks are generally suitable for small and medium-sized pumping stations with small water pipe lengths. The specific conditions should be determined based on calculations and comprehensive comparison of economic performance based on the actual conditions of the project.
Although there is not much experience in the design and use of anti-water hammer pressure tanks in China, it has become a technologically mature anti-water hammer device after a large number of engineering design practices and many years of operation and management experience abroad. It has the advantages of comprehensive water hammer resistance, flexible application, safety and reliability, and convenient operation and maintenance. When the project scale is small and economic conditions permit, it is worthy of promotion and adoption in domestic projects.
1Water supply and drainage design manual. Beijing: China Construction Industry Press
2Jiang Naichang. Water pumps and water pumping stations. Beijing: China Construction Industry Press, 1993
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