Water hammer is called water hammer because of the inertia of pressure water flow when the power is suddenly cut off or the valve is closed too fast, which produces water flow shock wave, just like a hammer hitting. The back and forth force of water shock wave can sometimes be very large, thus damaging the valve and water pump.
When the open valve is suddenly closed, the water flow will produce a pressure on the valve and pipe wall. Due to the smooth pipe wall, the subsequent water flow quickly reaches the maximum under the action of inertia and produces destructive effect, which is the “water hammer effect” in hydrodynamics, that is, positive water hammer. This factor should be considered in the construction of water supply pipeline.
On the contrary, after the closed valve is suddenly opened, it will also produce a water hammer, called negative water hammer, which also has a certain destructive force, but it is not as big as the former. When the electric water pump unit is suddenly powered off or started, it will also cause pressure impact and water hammer effect. This pressure shock wave propagates along the pipeline, which can easily lead to local overpressure of the pipeline, resulting in pipeline rupture and equipment damage. Therefore, water hammer effect protection has become one of the key technologies of water supply engineering.
Conditions of water hammer
1. Sudden opening or closing of the valve;
2. Sudden stop or start of water pump unit;
3. Water delivery from single pipe to high place (the topographic elevation difference of water supply exceeds 20m);
4. The total head (or working pressure) of the water pump is large;
5. Excessive flow velocity in water transmission pipeline;
6. The water pipeline is too long and the terrain changes greatly.
Harm of water hammer effect
The pressure rise caused by water hammer can reach several times or even dozens of times of the normal working pressure of the pipeline. The hazards of this large pressure fluctuation to the pipeline system mainly include:
1. Cause strong vibration of the pipeline and disconnect the pipeline joint;
2. The valve is damaged, and the serious pressure is too high, resulting in pipe explosion and pressure reduction of water supply network;
3. On the contrary, too low pressure will lead to the collapse of the pipe and damage the valve and fixing parts;
4. Cause the pump to reverse, damage the equipment or pipeline in the pump room, seriously submerge the pump room, cause casualties and other major accidents, and affect production and life.
Protective measures to eliminate or reduce water hammer
There are many protective measures for water hammer, but different measures should be taken according to the possible causes of water hammer.
1. Reducing the flow velocity of water transmission pipeline can reduce the water hammer pressure to a certain extent, but it will increase the diameter of water transmission pipeline and increase the project investment. During the layout of water transmission pipeline, hump or sharp change of slope shall be avoided as far as possible. The size of pump stop water hammer is mainly related to the geometric head of pump house. The higher the geometric head is, the greater the value of pump stop water hammer is. Therefore, the reasonable pump head shall be selected according to the local actual situation. After emergency pump shutdown, the pump shall be started after the pipeline behind the check valve is filled with water. When starting the pump, do not fully open the outlet valve of the pump, otherwise it will produce great water impact. The major water hammer accidents of many pump stations are mostly caused in this case.
2. Set water hammer elimination device
(1) Constant pressure control technology is adopted:
As the pressure of the water supply pipe network changes constantly with the change of working conditions, low pressure or overpressure often occurs during the operation of the system, which is easy to produce water hammer, resulting in damage to pipelines and equipment, the automatic control system is adopted to feed back the start, stop and speed regulation of the water pump through the detection of the pipe network pressure, control the flow, and then maintain a certain level of pressure, The pump water supply pressure can be set by controlling the microcomputer to maintain constant pressure water supply, avoid excessive pressure fluctuation and reduce the probability of water hammer.
(2) Install the water hammer eliminator
The equipment is mainly used to prevent water hammer from stopping the pump. It is generally installed near the outlet pipeline of the water pump. The pressure of the pipeline itself is used as the power to realize low-pressure automatic action, that is, when the pressure in the pipeline is lower than the set protection value, the drainage outlet will automatically open to release water and pressure, so as to balance the pressure of local pipeline and prevent the impact of water hammer on equipment and pipeline, Generally, the eliminator can be divided into mechanical type and hydraulic type. After the mechanical eliminator acts, it can be restored manually, and the hydraulic eliminator can be reset automatically.
(3) Install slow closing check valve on the outlet pipe of large diameter water pump
It can effectively eliminate the water hammer of pump shutdown, but because there is a certain amount of water backflow when the valve acts, the suction well must have an overflow pipe. Slow closing check valve has two types: heavy hammer type and energy storage type. The closing time of the valve can be adjusted within a certain range as required. Generally, the valve is closed 70% ~ 80% within 3 ~ 7 s after power failure, and the remaining 20% ~ 30% closing time is adjusted according to the conditions of water pump and pipeline, generally in the range of 10 ~ 30 s. It is worth noting that when there is a hump in the pipeline to bridge the water hammer, the slow closing check valve is very effective.
(4) One way Surge Tower
It shall be built near the pump station or at the appropriate position of the pipeline, and the height of the one-way surge tank shall be lower than the pipeline pressure there. When the pressure in the pipeline is lower than the water level in the tower, the surge tank makes up water to the pipeline to prevent the water column from breaking and to avoid bridging the water hammer. However, its depressurization effect on water hammer other than pump stop water hammer, such as valve closing water hammer, is limited. In addition, the performance of the one-way valve used in the one-way pressure regulating tower shall be absolutely reliable. Once the valve fails, it may lead to major accidents
(5) A bypass pipe (valve) is set in the pump station
During the normal operation of the pump system, the check valve is closed because the water pressure at the water pressure side of the pump is higher than that at the water suction side. When the pump is suddenly stopped due to power failure, the pressure at the outlet of the water pump station decreases sharply, while the pressure at the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the suction main pipe is the transient low-pressure water that pushes the check valve plate to flow to the pressurized water main pipe, and increases the low water pressure there; On the other hand, the pressure rise of water hammer on the suction side of the water pump is also reduced. In this way, the water hammer rise and pressure drop on both sides of the water pump station are controlled, so as to effectively reduce and prevent the harm of water hammer
(6) Set multi-stage check valve
In the longer water transmission pipeline, one or more check valves are added to divide the water transmission pipeline into several sections, and check valves are set on each section. When the water in the water delivery pipe flows back in the process of water hammer, each check valve is closed successively to divide the backwash flow into several sections. Because the hydrostatic head in each section of water delivery pipe (or backwash flow section) is quite small, the water hammer pressure rise is reduced. This protective measure can be effectively used in the case of large geometric water supply height difference; However, the possibility of water column separation cannot be eliminated. Its biggest disadvantage is that the power consumption of water pump increases and the cost of water supply increases during normal operation.
(7) Automatic exhaust and air replenishment devices are set at the high points of the pipeline to reduce the impact of water hammer on the pipeline.
Post time: Oct-26-2021
