Discussion on the influence of water hammer wave i

2022-08-05
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Discussion on the impact of water hammer wave in fire water system pipeline

Abstract: Discussion on the impact of water hammer wave in fire water system pipeline, When the water flow stops rapidly, the kinetic energy of the water will be converted into pressure energy, and the water pressure in the pipe will rise sharply; On the contrary, if the water in the original steward road is static, the pump or valve suddenly starts or the valve suddenly opens, the water will obtain a large speed, that is, the pressure energy of the water will be converted into kinetic energy, and its pressure will drop sharply. The phenomenon that the pressure of water fluctuates greatly due to the rapid change of flow velocity is called water hammer. The pressure change caused by water hammer propagates along the pipeline in the form of pressure wave, and its propagation speed is about equal to the propagation speed of sound wave in the water. The propagation form is similar to the propagation form of shock wave generated by bomb explosion in the air. Where the pressure wave is transmitted, the water pressure rapidly increases by the same value on the original basis. We call this phenomenon pressure superposition. During the propagation of water hammer wave along the pipeline to the upstream with high pressure, the superimposed pressure becomes higher and higher, which may cause the allowable range of upstream pressure pipeline strength and cause pipeline rupture or equipment damage. Therefore, this phenomenon of water hammer with sharp pressure change must be strictly prevented in the production process

II. Calculation of instantaneous pressure change caused by water hammer

1. The calculation of direct water hammer (also known as complete water hammer) also shows that it attaches importance to the Chinese market.

if the valve closes very quickly, when the closing time t ≤ 2l/c, the instantaneous pressure rise caused by water hammer can be calculated by formula a

p=cv/g --------- a

2. Calculation of indirect water hammer (also known as incomplete water hammer)

if the closing time of the valve is not very fast, when the closing time t> 2l/c, the instantaneous pressure rise caused by water hammer can be calculated by formula B

At the same time, during the installation, 1 pay attention to the installation of the locking device

p=cv/g-l/(ct-l) ----- b

where: P - the pressure when the water hammer rises, m water column

v - average flow velocity in the pipe before water hammer, M/S

l - length of water pipe or hose, m

g - gravitational acceleration, 9, 8 m/S

c - propagation speed of water hammer wave, M/S

for example, the length of the water hose is 180m (new water hose), the diameter is 65mm, and the water flow speed is 2m/s. when a switch type water gun is connected to extinguish the fire, the position needs to be transferred. The time to close the water gun is 1 second, and try to determine the instantaneous rising pressure

problem solution:

1. The nature of water hammer

t=2l/c=2*180/80=4, 5 seconds

the closing time of the water gun is one second. If it is less than 4, 5 seconds, it is a direct (complete) water hammer

2. Instantaneous rising pressure p=cv/g=80*2/9, 8=16, 1m water column

another example: the length of cast iron pipe below dg200 is 600m. In a water spray test, the water flow velocity in the pipe is 4m/s, and the time to close the valve is 2S. Try to determine the instantaneous rising pressure

1. The nature of water hammer

t=2l/c=4*600/1200=2 seconds

the time to close the valve is 2 seconds, which is equal to 2l/c, which is also a complete water hammer

2. Instantaneous rising pressure

p=cv/g-l/(ct-l) =1200*4/9, 8=489, 7 (m water column)

from the above analysis, it can be seen that the water hammer effect is related to the material, flow rate and length of the pipeline, especially the speed of closing the valve. Fire water supply pipeline due to water hammer It often causes pipeline rupture and affects the normal production and power generation operation. Improper operation in the fire fighting process at the fire site will often lead to the explosion of water belt, resulting in the interruption of water supply at the fire site, affecting the fire fighting and causing the expansion of the fire

III. methods to reduce water hammer pressure

in order to reduce the impact of water hammer wave, the following methods are usually adopted:

1. Take the grid to the right to delay the opening and closing time of pipeline valves, so that the speed of water flow changes slowly. If the pipe length is l; The propagation speed of water hammer wave is C, and the propagation time of water hammer wave from one end to the other end is l/c, so the opening and closing time of the valve shall be greater than 2l/c

2. Shorten the pipeline length to shorten the propagation period, and the shorter the pipeline, the greater the rigidity

3. A buffer tank is installed on the pipeline. The water hammer wave propagates to the buffer tank, and the pressure decreases sharply

4. Install a pressure safety valve on the pipeline, so that when the pressure reaches a certain level, it will automatically open to release water and pressure

References:

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