Case
Case
In the pump room and piping system installation is completed, often found in the system operation, when in the stop pump, power outage of a moment, the piping system will have a large water impact, impact pumps, valves and piping, some may be water hammer is very light, but some are very serious, or worse, will produce serious quality accidents, sin the height ofuch as: valve valve flap, pump blades, piping system, etc. by the water hammer shattered, broken, and this damage is the Water hammer caused.
Water hammer is in a sudden power outage or when the valve is closed too quickly, due to the inertia of the pressure water flow, resulting in a water shock wave, like a hammer blow, so called water hammer. The force generated by the back and forth of the water shock wave is sometimes so great that it destroys valves and pumps.so-called
Water hammer effect refers to the inside of the pipe, the pipe wall is smooth, the water flows freely. When an open valve is suddenly closed, the water flow creates a pressure on the valve and the pipe wall, mainly the valve. Due to the smooth wall, the subsequent flow of water in the role of inertia, quickly reach the maximum, and produce damaging effects, 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 spreads along the pipeline, very easy to cause local overpressure and pipeline rupture, damage to equipment, etc., so the water hammer effect protection has become one of the key water supply engineering process technology.
① The valve is suddenly opened or closed;
② Water pump unit suddenly stops or opens;
③ Single-pipe water supply to the high place (the difference in height of water supply terrain is more than 20 metres);
④ The total head (or working pressure) of the water pump is large;
⑤ The speed of water flow in the water pipeline is too large;
⑥ The water pipeline is too long and the terrain changes greatly.
The pressure rise caused by water hammer can be up to several times or even dozens of times the normal working pressure of the pipeline. This large-scale pressure fluctuations, the piping system caused by the hazards are mainly:
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 Conversely, too low a pressure will lead to pipe deflation, but also damage to valves and fixed parts;
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.
There are many protective measures for water hammer, but according to the reasons why water hammer may arise, take different measures.
1, Reduce the flow rate of the water pipeline, can reduce the water hammer pressure to a certain extent, but will increase the diameter of the water pipeline, increase engineering investment. The arrangement of water pipeline should be considered to try to avoid humps or drastic changes in slope.
Reduce the length of the water pipeline, the longer the pipeline, the greater the value of water hammer pump stop. From a pumping station to two pumping stations, with suction wells to connect the two pumping stations.
Stop pumping the size of the water hammer is mainly related to the pump room geometry head, the higher the geometric head, stop pumping the value of water hammer is also greater. 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 the water hammer elimination device
① Adopt constant pressure control technology:
The use of PLC automatic control system, frequency control of the pump, the entire water supply pump room system operation automatic control. As the pressure of the water supply network changes with the change of working conditions, the system often appears in the process of low pressure or overpressure phenomenon, easy to produce water hammer, resulting in damage to the pipeline and equipment, the use of PLC automatic control system, through the detection of the network pressure, feedback control of the pump start, stop 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 computer to maintain a constant pressure water supply, avoiding the need to set up a constant pressure water supply device, to avoid the need to set up a constant pressure water supply device. Pressure can be set by controlling the microcomputer to maintain a constant pressure water supply, to 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 water hammer pump stops, generally installed near the pump outlet pipe, the use of 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 drainage port will automatically open the water pressure relief to balance the pressure of the local pipeline, to prevent the water hammer on the equipment and pipeline impacts, eliminator can be divided into mechanical and hydraulic two kinds of eliminators mechanical eliminators action by manual recovery, hydraulic eliminators action Mechanical eliminator action by manual recovery, hydraulic eliminator can be automatically reset.
③ Installation of slow-closing check valves on the outlet pipe of large-calibre water pumps
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 limited
④ Set up one-way pressure regulator tower.
In the vicinity of the pumping station or pipeline construction of the appropriate location, the height of the one-way pressure regulator tower is lower than the pipeline pressure. When the pressure in the pipeline is lower than the water level in the tower, the 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 large water hammer
⑤ Setting up a bypass pipe (valve) in the pumping station
During normal operation of the pump system, the check valve closes because the water pressure on the pressure side of the pump is higher than the water pressure on the suction side. When the pump is suddenly stopped by an accidental power failure, the pressure at the outlet of the pumping station decreases sharply, while the pressure on the suction side rises sharply.
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 Setting multi-stage check valves
In the longer water pipeline, add one or more check valves, divide the water pipeline into several sections, and set check valves on each section. 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 for geometric water supply in the case of large differences in height; but can not eliminate the possibility of separation of the water column. Its biggest disadvantage is: the normal operation of the pump power consumption increases, water supply costs.
