What is water hammer?
Water hammer is in a sudden power failure or in the valve closed too fast, due to the inertia of the pressure water flow, the flow shock wave is generated, just like a hammer, so called water hammer. The back-and-forth force of the water shock wave, sometimes large, can break valves and pumps.
When an open valve is suddenly closed, the flow of water exerts a pressure on the valve and pipe wall. Due to the smooth wall of the pipe, the subsequent water flow under the action of inertia quickly reaches the maximum and produces destructive effect, which is the "water hammer effect" in fluid mechanics, that is, positive water hammer. This factor should be considered in the construction of water supply pipelines.
Conversely, a closed valve that suddenly opens will also produce a water hammer, called a negative water hammer, which also has some destructive power, but not as much as the former. The electric pump unit will also cause the impact of pressure and water hammer effect when the power is suddenly cut off or started. The shock wave of such pressure propagates along the pipeline, which easily leads to local overpressure of the pipeline and causes pipeline rupture and equipment damage. Therefore, the protection of water hammer effect becomes one of the key technology of water supply engineering.
Conditions of water hammer:
1. The valve opens or closes suddenly;
2. The pump unit stops or starts suddenly;
3. Single pipe to high water (water supply terrain height difference of more than 20 meters);
4. The pump total head (or working pressure) is large;
5. The water flow speed in the water pipeline is too large;
6. The water pipeline is too long, and the terrain changes greatly.
The harm of water hammer effect:
The pressure increase caused by the water hammer can reach several times or even dozens of times the normal working pressure of the pipeline.The main harm of this large pressure fluctuation to the pipeline system is:
1.Cause strong vibration of pipeline, pipe joint disconnected;
2. Damage to the valve, serious pressure is too high to cause pipeline burst, water supply network pressure is reduced;
3. On the contrary, too low pressure will lead to the collapse of the pipe, but also damage the valve and fixture ;
4. Cause the pump reversal, damage the pump room equipment or pipelines, seriously cause the pump room flooded, resulting in casualties and other major accidents, affecting production and life.
Protective measures to eliminate or mitigate water hammer:
There are many protective measures against water hammer, but different measures should be taken according to the possible causes of water hammer.
1. Reducing the flow rate of the water transmission pipe line can reduce the water hammer pressure to a certain extent, but it will increase the diameter of the water transmission pipe and increase the project investment. The distribution of water transmission lines should be considered to avoid the occurrence of humps or abrupt slope changes. The size of the water hammer is mainly related to the geometric head of the pump room. The higher the geometric head is, the larger the value of the water hammer is. Therefore, reasonable pump head should be selected according to the local actual situation. After the pump stops in an accident, the pump should be started when the pipeline behind the check valve is filled with water. Do not fully open the pump outlet valve when starting the pump, otherwise it will produce a great impact of water. Many major water hammer accidents in pumping stations occur under this condition.
2. Set up water hammer elimination device:
(1) Constant pressure control technology:
As the pressure of the water supply network constantly changes with the change of the working condition, the phenomenon of low pressure or overpressure often occurs in the system operation process, which is easy to produce water hammer, resulting in the destruction of the pipeline and equipment. The automatic control system is adopted, through the detection of pipe network pressure, feedback control of pump start, stop and speed regulation, control flow, and then make the pressure maintain a certain level.The water supply pressure of the pump 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 mainly prevents the water hammer from stopping the pump, which is generally installed near the pump outlet pipeline. It uses the pressure of the pipeline itself as the power to realize the low pressure automatic action, that is, when the pressure in the pipeline is lower than the set protection value, the drain port will automatically open the water release and pressure relief, so as to balance the pressure of the local pipeline and prevent the impact of the water hammer on the equipment and pipeline. Eliminator can be generally divided into mechanical and hydraulic two kinds, mechanical eliminator action by manual recovery, hydraulic eliminator can automatically reset.
(3)Install slow-closing check valve on the large diameter water pump outlet pipe.
It can effectively eliminate the pump-stopping water hammer, but because there is a certain amount of water backflow during the valve action, the suction well must have an overflow pipe. There are two types of slow closing check valves : heavy hammer type and energy storage type. This valve can adjust the valve closing time within a certain range as needed. Generally, the valve is closed by 70 % ~ 80 % within 3 ~ 7 s after the blackout, and the remaining 20 % ~ 30 % of the closing time is adjusted according to the conditions of the pump and pipeline, generally in the range of 10 ~ 30 s. It is worth noting that when there is a hump in the pipeline and a bridge water hammer occurs, the role of the slow closing check valve is very effective.
(4) Set the One-Way Pressure Regulating Tower
Constructed near the pumping station or at the appropriate location of the pipeline, the height of the one-way surge tower is lower than the pipeline pressure there. When the pressure in the pipeline is lower than the water level in the tower, the surge tower replenishes water to the pipeline to prevent the water column from breaking and avoid bridging the water hammer. However, its pressure-reducing effect on water hammers other than pump-stopping water hammers such as valve-closing water hammers is limited. In addition, the performance of the check valve used in the one-way pressure regulating tower is absolutely reliable. Once the valve fails, it may lead to a large occurrence.
(5) Set bypass pipe ( valve ) in the pump station.
During the normal operation of the pump system, the check valve is closed because the water pressure on the water side of the pump is higher than the water pressure on the suction side. When the pump is suddenly stopped after the accident, the pressure at the outlet of the pump station decreases sharply, while the pressure on the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the water absorption main pipe is the transient low-pressure water that pushes the check valve plate to the pressure water main pipe, and increases the low water pressure there. On the other hand, the water hammer pressure on the suction side of the pump is also reduced. In this way, the water hammer rise and fall on both sides of the pump station are controlled, thus effectively reducing and preventing the harm of water hammer.
(6) Set multi-stage check valve
In a long water pipeline, one or more check valves are added to divide the water pipeline into several sections, and check valves are set on each section. When the water in the water conveyance pipe flows backward during the water hammer process, each check valve is closed one after another to divide the backwash water flow into several sections. Because the hydrostatic head in each water conveyance pipe ( or backwash water flow section ) is quite small, the water hammer pressure rise is reduced. This protective measure can be effectively used in the case of large elevation difference of geometric water supply. But the possibility of water column separation cannot be eliminated. Its biggest disadvantage is that the power consumption of the pump increases and the cost of water supply increases during normal operation.
(7) The automatic exhaust and air supply device is set at the high point of the pipeline to reduce the influence of water hammer on the pipeline.
Post time: Jan-11-2023