In the pump and pipeline system installation is completed, often found in the system is running, when the pump, power outages of the moment, the pipeline system will have a large impact of water, the impact of the pump, valves and pipelines, and some May be very light attack, but some are very serious, and even those who will produce serious quality accidents, such as: valve discs, pump blades, piping systems were broken by water, broken, this damage is caused by water hammer , In foreign engineering, we have encountered such a phenomenon.
What is water hammer phenomenon?
Water hammer is in the sudden power failure or when the valve is closed too fast, due to the inertia of the pressure water flow, resulting in water shock wave, like a hammer beat the same, so called water hammer. The flow of shock waves back and forth the force generated, sometimes large, thus destroying the valves and pumps. Water hammer effect refers to the inside of the pipe, the tube wall is smooth, the water flow freely. When the open valve suddenly closes, the water flow to the valve and the pipe wall, mainly the valve will produce a pressure. As the pipe wall is smooth, the subsequent flow of water under the action of inertia, quickly reach the maximum, and produce damage, which is the fluid mechanics of the "water hammer effect", that is, water hammer. This factor should be taken into account in the construction of water supply pipelines. On the contrary, the closure of the valve in the sudden opening, will produce water hammer, called the water hammer, there is a certain destructive, but not the former big. Electric pump unit sudden power outage or start, the same will also cause the impact of pressure and water hammer effect. This kind of pressure shock wave along the pipeline, can easily lead to pipeline overpressure caused by pipeline rupture, damage to equipment, so the water hammer effect of water supply project to become one of the key technology.
Second, the conditions of water hammer
1, the valve suddenly opened or closed;
2, the pump unit suddenly stop or open;
3, single pipe to the high water (water supply terrain height difference of more than 20 meters);
4, the pump total head (or work pressure) large;
5, the water flow in the pipeline is too large;
6, the water pipeline is too long, and the terrain changes.
Third, the water hammer effect of what is the harm?
Water hammer caused by the pressure rise, up to the normal pressure of the pipeline several times, or even several times. This substantial pressure fluctuations, the damage caused by the pipeline system are:
1, causing a strong pipe vibration, pipe fittings disconnected;
2, damage to the valve, a serious pressure caused by high pipe burst pipe, water supply pipe network pressure to reduce;
3, on the contrary, the pressure is too low will lead to collapse of the tube collapse, but also damage the valve and fixed pieces;
4, causing the pump reversal, destruction of the pump room equipment or pipes, causing serious pump house submerged, causing personal injury and other major accidents, affecting production and life.
Fourth, the elimination or mitigation of water hammer protection measures
There are many protective measures for water hammer, but depending on the possible causes of water hammer, take different measures.
1, reduce the flow rate of the pipeline, to a certain extent, reduce the water hammer pressure, but will increase the diameter of the water pipe, increase investment in the project. Pipeline layout should be considered to avoid the emergence of hump or slope drastic changes.
Reduce the length of the pipeline, the longer the pipeline, the greater the value of the pump water hammer. By a pump station to change the two pumping stations, with the suction wells to connect the two pumping stations.
The size of the pump water hammer is mainly related to the geometric head of the pump house, the higher the geometric head, the greater the value of the pump water hammer. Therefore, should be based on the actual situation of the local use of a reasonable pump head.
After the accident stopped the pump, should be to check the valve after the pipeline is full of water and then start the pump.
When the pump pump outlet valve do not fully open, otherwise it will produce a lot of water impact. Many of the major water hammer accidents in the pumping station are more in this case.
2, set the water hammer to eliminate the device
1) using constant pressure control technology:
Using PLC automatic control system, the pump frequency control, the entire water supply pump system operation to implement automatic control. As the pressure of the water supply pipe network changes with the working conditions, the system often occurs during the operation of low pressure or overpressure phenomenon, prone to water hammer, resulting in the destruction of pipes and equipment, the use of PLC automatic control system, through the pipe network Pressure control, feedback control pump open, stop and speed regulation, control flow, and then maintain a certain level of pressure, you can control the computer to set the pump water supply pressure to maintain constant pressure water supply, to avoid excessive pressure fluctuations, so that The probability of generating water hammer is reduced.
2) Install the water hammer eliminator
The device is mainly to prevent the pump water hammer, generally installed in the vicinity of the pump outlet pipe, the use of the pressure of the pipeline itself to achieve low-pressure automatic action, that is, when the pressure in the pipeline below the set protection value, the drain will automatically open the water Pressure relief, in order to balance the pressure of local pipelines to prevent the impact of water hammer on equipment and pipelines, eliminator can be divided into mechanical and hydraulic two, mechanical eliminator after the manual recovery by the hydraulic eliminator can automatically Reset.
3) Install the slow closing check valve on the large diameter pump outlet pipe
Can effectively eliminate the pump water hammer, but because the valve when there is a certain amount of water flow back, suction wells must have overflow pipe. Slow closing check valve with heavy hammer and storage type two. This valve can be adjusted according to the need for a certain range of valve closure time. Usually within 3 to 7 s after the power failure valve closed 70% to 80%, the remaining 20% to 30% of the closing time is based on the pump and pipeline regulation, generally in the range of 10 ~ 30 s. It is worth noting that when the hump in the pipeline and the emergence of a water hammer, the role of slow closing check valve is very limited.
4) Set the one-way surge tower
In the vicinity of the pump station or in the proper place of the pipeline, the height of the one-way surge tower is lower than the pipe pressure there. When the pressure inside the pipeline is lower than the water level inside the tower, the regulator tower to the pipeline water, to prevent the water column pull off, to avoid bridging the water hammer. But its role in stopping the water hammer, such as water hammer, such as closing the water hammer, is limited. In addition, the performance of the one-way valve used in a one-way regulator tower is absolutely reliable, and once the valve fails, it may cause a large water hammer to occur.
5) Set the bypass pipe (valve) in the pump station
When the pump system is in normal operation, the check valve is closed due to the water pressure on the suction side of the pump. When the accident power suddenly stopped the pump, the pump station exit pressure dropped sharply, while the suction side of the pressure is soaring. Under this differential pressure, the transient high pressure water in the main pipe of the water inlet is pushed by the check valve plate to the hydraulic low pressure water of the water main pipe and the lower water pressure is increased; on the other hand, The water hammer boost on the suction side is also reduced. In this way, the water pump on both sides of the water hammer l, buck are controlled, so as to effectively reduce and prevent the water hammer hazards.
6) Set the multi-level check valve
In the longer water supply pipeline, add one or more check valves, the water pipe is divided into several sections, each section are set on the check valve. When the water hammer in the process of water back to the water, the check valves have been closed to return to the flow of water into several sections, due to each section of the water pipe (or return flow section) within the hydrostatic head is relatively small, thus reducing the water Hammer boost. This protective measure can be effectively used for large differences in geometric water supply; but it can not eliminate the possibility of separation of water column. The biggest drawback is that the normal operation of the pump power consumption increases, the water supply costs.