What is the water hammer phenomenon?
Water hammer is called water hammer due to the inertia of the pressurised water flow that produces a water flow shock wave, like a hammer blow, when there is a sudden power failure or when the valve is closed too quickly. The force generated by the back and forth of the water shock wave is sometimes so great that it destroys valves and pumps.
When the open valve is suddenly closed, the water flow on the valve and pipe wall, will produce a pressure. As the pipe wall is smooth, the subsequent flow of water in the role of inertia, quickly reach the maximum, and produce damage, which is the fluid mechanics of the “water hammer effect”, that is, positive water hammer. In the construction of water supply pipelines should consider this factor.
On the contrary, the closed valve in the sudden opening, will also produce water hammer, called negative water hammer, also has a certain destructive force, but not as big as the former. Electric pumping units suddenly stop or start, the same will also cause pressure shock and water hammer effect. This pressure shock wave along the pipeline propagation, very easy to lead to pipeline local overpressure and pipeline rupture, damage to equipment, etc., so the water hammer effect protection has become one of the key water supply engineering technology.
Water hammer conditions:
1. Sudden opening or closing of valve
2. Sudden stopping or opening of water pump unit;
3. Single pipe to the high place of water supply (water supply terrain height difference of more than 20 metres);
4. The total head (or working pressure) of the water pump is large;
5. Excessive velocity of water flow in the water pipeline;
6. Long water pipelines and large changes in terrain.
Hazards of water hammer:
Water hammer caused by the pressure increase, up to several times the normal working pressure of the pipeline, or even dozens of times. This large pressure fluctuations, the piping system caused by the hazards are:
1. Cause strong vibration of the pipeline, pipeline joints disconnected;
2. Destruction of valves, serious pressure is too high to cause pipe burst, water supply network pressure reduction. 3;
3. Conversely, too low a pressure will lead to pipe deflation, but also damage to valves and fixtures;
4. Cause pump reversal, damage to equipment or piping in the pump room, serious flooding of the pump room, resulting in personal injury and other major accidents, affecting production and life.
Eliminate or reduce water hammer protection measures:
There are many protective measures for water hammer, but according to the possible causes of water hammer, to take different measures.
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 the project investment. The arrangement of the water pipeline should be considered to try to avoid humps or drastic changes in slope. The size of the pump stopping water hammer is mainly related to the geometric head of the pumping station, the higher the geometric head, the greater the value of the pump stopping water hammer.
Therefore, according to the local actual situation should be selected reasonable pump head. After the accident stops the pump, should wait until after the check valve pipeline full of water and then start the pump. Start the pump when the pump outlet valve should not be fully open, otherwise there will be a large water shock. A lot of pumping stations of major water hammer accidents in this case.
2. Set up water hammer elimination device:
(1) The use of constant pressure control technology
As the pressure of the water supply network with the changing conditions and constantly changing, the system often occurs during operation of low pressure or overpressure phenomenon, easy to produce water hammer, resulting in damage to pipelines and equipment, the use of automatic control systems, through the detection of the network pressure, feedback control of pumps on, off and speed adjustment, control flow, and then make the pressure to maintain a certain level, you can set the water supply pressure of machine pumps through the control of the microcomputer to Maintain constant pressure water supply, avoid excessive pressure fluctuations, so that the probability of water hammer is reduced.
(2) Installation of water hammer eliminator
The equipment is mainly to prevent stopping the pump water hammer, generally installed near the pump outlet pipe, using the pressure of the pipeline itself as a driving force to achieve low-pressure automatic action, that is, when the pressure in the pipeline is lower than the set value of the protection, the drain will automatically open the water pressure relief to balance the pressure of the local pipeline to prevent water hammer on the equipment and pipeline impacts, eliminator is generally divided into mechanical and hydraulic two kinds of eliminators, mechanical eliminators action by manual restoration, hydraulic eliminators action Mechanical eliminator action by manual recovery, hydraulic eliminator can be automatically reset.
(3) In the large calibre pump outlet pipe installed on the slow-closing check valve
Can effectively eliminate water hammer pump stop, but because the valve action has a certain amount of water backflow, suction wells must have an overflow pipe. Slow closing check valve has two kinds of heavy hammer type and accumulator type. This valve can be adjusted according to the need for a certain range of valve closing time. Generally in the power outage 3 ~ 7 s after the valve closed 70% to 80%, the remaining 20% to 30% of the closing time is adjusted according to the pump and pipeline conditions, generally in the range of 10 ~ 30 s. It is worth noting that when there is a hump in the pipeline and the occurrence of bridging water hammer, the role of slow-closing check valve is very effective.
(4) Set up a one-way pressure regulator tower
In the vicinity of the pumping station or the appropriate location of the pipeline construction, the height of the one-way pressure regulator tower is lower than the pressure of the pipeline at the place. When the pressure in the pipeline is lower than the water level in the tower, the pressure regulator tower to the pipeline to make up water to prevent the water column pulled off to avoid bridging water hammer. But its water hammer other than stopping the pump water hammer, such as shut-off valve water hammer pressure reduction effect is limited. In addition one-way pressure regulator tower using the performance of the check valve to be absolutely reliable, once the valve failure, may lead to the occurrence of larger
(5) Set up in the pumping station bypass pipe (valve)
In the normal operation of the pump system, due to the pump pressure side of the water pressure is higher than the suction side of the water pressure, check valve closed. When the accident power failure suddenly stop the pump, the pump station at the exit of the pressure is sharply reduced, while the suction side of the pressure is rising. Under this differential pressure, the suction pipe in the transient high-pressure water that is pushed away from the check valve valve plate flow to the transient low-pressure water in the pressure pipe, and make the low water pressure rise; on the other hand, so that the pump suction side of the water hammer pressure is also reduced. In this way, both sides of the pumping station water hammer rise and fall pressure are controlled, thus effectively reducing and preventing water hammer hazards
(6) Set up multi-stage check valve
In the longer water pipeline, the addition of one or more check valves, the water pipeline is divided into several sections, each section is set on the check valve. When the water hammer process in the water pipeline backflow, the check valve successive closure of the backwash water flow is divided into several segments, due to each section of the water pipeline (or backwash water flow section) within the hydrostatic pressure head is quite small, thus reducing the water hammer pressure.
This protective measure can be effectively used in the case of a large difference in geometric water supply heights; however, it cannot eliminate the possibility of water column separation. Its biggest disadvantage is: the normal operation of the pump power consumption increases, water supply costs.