Discussion on the Application of Anti - water Hammer Pressure Tank in Abating Stop Water Hammer

A discussion on advantage of air chambers for water hammer for diminish water hammer when cut off pump.

Liu yi Chen min

(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

Stop water hammer is a phenomenon of water hammer, refers to the pump unit due to sudden power failure or other reasons caused by the sudden stop valve open state, in the pump and piping system, due to sudden changes in flow rate caused A series of rapid pressure alternating hydraulic lift impact. Under normal circumstances, the most serious pump stop water hammer, which has a great threat to the safety of pumping stations and pipelines, there are several domestic pumping station water pump had occurred due to stop pumping led to pump house submerged or pipeline rupture major accident .

Pump stop water hammer size and pump room in the water pipeline and the specific circumstances. In the pump room and water pipeline design should take into account the water hammer may occur, and take appropriate precautions to prevent the occurrence of water hammer, or the impact of water hammer control within the allowable range. Common preventive measures are the following:

(1) reduce the flow rate of the water pipeline can be reduced to some extent, water hammer pressure, but will increase the diameter of the water pipe to increase project investment.

(2) Water pipeline layout should be considered as far as possible to avoid hump or sharp gradient.

(3) Through the simulation calculation, the pump unit with larger inertia GD2 or the flywheel with enough inertia can be selected to reduce the water hammer value to a certain extent.

(4) set water hammer elimination device: such as pressure tank, two-way pressure regulating tower, one-way pressure regulating tower, water hammer eliminator, slow closing check valve and so on. Easy to operate, the effect is remarkable, and is not affected by the terrain. It is very common in Europe (Britain, France and other countries), but it is not widely used domestically. Our hospital through the practice of design in foreign water supply project, has accumulated some experience in the design and calculation and use of the device, in this introduction.

1 anti-water hammer pressure tank structure and working principle

Anti-water hammer pressure tank used to reduce the stop pump water hammer connected to the water pump after the pressure check valve on the water supply pipe from the pressure tank (containing gas and water separation diaphragm) and the ball tank outlet drilling check valve, Gate valve composition (see Figure 1). It uses a specific law of gas volume and pressure to work. As the pressure in the pipe changes, the pressure tank replenishes the pipe or absorbs too much pressure in the pipe, and the anti-water hammer is similar in principle to the two-way pressure regulating tower.

In normal operation, the volume of gas-water in the pressure tank is shown in Fig. 2a. When the pump stops suddenly and the pressure water supply pipe starts to form a negative pressure at the beginning, even when the water column is separated, the pressure tank replenishes the pipeline, effectively balances the negative pressure and prevents the water column from separating , Cut water hammer strength. When the water hammer wave returns, the starting pressure of the pressure water supply pipe rises, the pressure tank enters the water from the pipeline to absorb the excessively high pressure in the pipeline, and the pressure tank The increase in the amount of water, the gas is compressed (see Figure 2c), played a cushion role, which effectively resist the water hammer on the pipeline system adverse effects.

Anti-water hammer pressure tank in the internal structure and domestic pressure water supply equipment commonly used in regulating the pressure tank is basically the same, but both in the outlet valve settings are different. Anti-water hammer pressure tank in its outlet pipe section in addition to the installation of the inspection gate valve, but also set up a check valve, that is, in the valve flap to drill a certain aperture holes (aperture must be calculated to determine), which is The most distinctive part of the device. Its role is: When the pressure water supply pipe at the beginning of the formation of negative pressure or even when the separation of water, the check valve fully open, the pressure tank can quickly add the required amount of water pipe; when the water hammer wave back pressure pipeline pressure When it rises, the amount of water enters the pressure tank through the small hole in the check valve flap, which on the one hand absorbs the excessive pressure in the pipeline and on the other hand increases the head loss when the water flows into the pressure tank and consumes the energy of the water hammer wave , Reducing the impact on the pipeline system caused by the pressure swinging water hammer wave back and forth.

2 anti-water hammer pressure tank selection

Anti-water hammer pressure tank operation principle is very simple, but the selection calculation is not simple. Not only the mobility and continuity of the water flow need to be taken into account, but also the influence of the air temperature change on the air volume due to the drastic change of the air volume inside the tank. This makes the calculation complicated and not easy to be accurate. The current common calculation methods are the following:

(1) empirical formula calculation method

Its characteristic is the algorithm is simple, can quickly estimate the size of the required pressure tank, commonly used in feasibility studies and program design stage. The formula is as follows:

V = 10Q · L

Where: V is the total volume of the pressure tank, m3; Q is the pressure of the water supply pipe flow, m3 / s; L is the length of the pressure water supply pipe, m.

(2) graphic method

Calculate the required pressure tank volume based on the pamkin chart. Its characteristic is the algorithm is relatively simple, according to the pipe system can withstand water hammer pressure range (safety pressure range) to determine the most economical pressure tank volume, the results of relative empirical formula calculation method is more accurate, commonly used in the preliminary design stage . Due to the calculation of the chart more, this is not described in detail.

(3) electricity algorithm

The calculation principle is the application of elastic water column theory, the establishment of water hammer process equations of motion and continuous equations, combined with various factors and boundary conditions, calculate the required pressure tank volume. The algorithm is complicated, but the result is very accurate. It is often used in the construction design stage. Due to the different product specifications of manufacturers, the selection calculation process is also slightly different, the construction drawings design manufacturers may assist in the calculation and selection.

3 anti-water hammer pressure tank advantages and disadvantages and applicable conditions

Compared to other water hammer devices, water hammer pressure tank has the following advantages:

(1) Anti-water hammer effect is good. It not only possesses the function of balancing the negative pressure and reducing the positive pressure of the bidirectional pressure regulating tower, but also has the effect of reducing the pressure swing back and forth, and can minimize the various hazards of the water hammer to the piping system.

(2) Not subject to topographical conditions, flexible application. The volume and pressure of gas in the tank vary with the pipeline pressure, and the change of the water volume in the tank is limited to the volume of the pressure tank. This effectively avoids the restriction of the pressure of the pumping station and the surrounding topography of the bidirectional pressure regulating tower due to the free change of the water level with the pipeline pressure The weaknesses apply to all kinds of water pressure and different topography.

(3) safe and reliable operation. The utility model has the advantages of simple structure and automatic operation, and avoids the accident caused by device failure or man-made reason such as water hammer eliminator and the like. Even if the drilling check valve fails, the anti-water hammer pressure tank can still achieve a certain balance of negative pressure through the small holes on the valve flap and all the functions of reducing the positive pressure and reducing the pressure swinging back and forth without causing too much harm.

(4) easy to use, easy to maintain. After the installation of the pressure tank can be automatically run without debugging, eliminating the need for slow closure check valve as a resistance to water hammer device complicated debugging process. The whole device is simple in structure, and does not need to be invigorated frequently, and basically has no maintenance burden.

Anti-water hammer pressure tank is the disadvantage of higher prices, consumption of steel, and high sealing requirements of the device. When the pressure water supply pipe flow is too large or the water distance is too long, resulting in pressure tank volume is too large, resulting in the purchase cost is too high and processing difficulties and other issues. Therefore, anti-water hammer pressure tank is generally suitable for small and medium size, the length of the water pipe pumping station, in particular, should also be combined with the actual situation of the project by calculation and economic performance of a comprehensive comparison to determine.

4 Conclusion

Although the anti-water hammer pressure tank in the country's experience in the design and use of small, but it has a large number of foreign engineering practice and years of operational management experience has become a well-established anti-water hammer device. It has a comprehensive anti-water hammer performance, application flexibility, safety and reliability, easy operation and maintenance. In the case of small-scale projects and economic conditions permitting, it is worth to promote the use of domestic projects.

references:

1 water supply and drainage design manual. Beijing: China Building Industry Press

2 Jiang Naichang. Pumps and pump stations. Beijing: China Building Industry Press, 1993