After the installation of the pump room and the piping system, it is often found that when the system is running, when the pump is stopped or the power is cut off, the piping system will have a great impact of water, impacting the pump, valve and pipeline, and some Water hammer may be very light, but some are serious, and even worse, serious quality accidents may occur. For example, valve discs, water pump blades, piping systems, etc. are crushed or broken by water. Such damage is caused by water hammer. Yes, we have also encountered this phenomenon in foreign projects.
What is water hammer?
Water hammer is called water hammer because of the inertia of the pressure water flow when the power is suddenly cut off or when the valve is closed too fast. The force generated by the shock wave of the water flow can sometimes be very large, which can damage the valve and the pump. The water hammer effect means that inside the water pipe, the inner wall of the pipe is smooth and the water flows freely. When the opened valve is suddenly closed, the water flow on the valve and pipe wall, mainly the valve will generate a pressure. Because the pipe wall is smooth, the subsequent water flow quickly reaches the maximum under the action of inertia, and produces a destructive effect. This is the “water hammer effect” in fluid mechanics, that is, positive water hammer. This factor must be considered in the construction of water supply pipelines. On the contrary, after a closed valve is suddenly opened, water hammer will also occur, called negative water hammer, which also has a certain destructive power, but it is not as great as the former. When the electric water pump unit is suddenly powered off or started, it will also cause pressure shock and water hammer effect. The shock wave of this pressure propagates along the pipeline, which can easily lead to local over pressure in the pipeline, causing pipeline rupture and equipment damage. Therefore, water hammer effect protection has become one of the key process technologies for water supply projects.
The conditions for water hammer
1. The valve opens or closes suddenly;
2. The water pump unit suddenly stops or turns on;
3. A single pipe delivers water to high places (the height difference of the water supply terrain exceeds 20 meters);
4. The total head (or working pressure) of the pump is large;
5. The water velocity in the water pipeline is too high;
6. The water pipeline is too long and the terrain changes greatly.
What is the hazard of the water hammer effect?
The pressure rise caused by water hammer can reach several times, or even dozens of times, the normal working pressure of the pipeline. The main hazards caused by this large pressure fluctuation to the pipeline system are:
1. Cause the pipeline to vibrate strongly and disconnect the pipeline joint;
2. Destroy the valve, severely high pressure will cause the pipeline to burst, and the pressure of the water supply network will decrease;
3. On the contrary, if the pressure is too low, it will cause the collapse of the pipe and damage the valve and fixing parts;
4. Cause the water pump to reverse, damage the equipment or pipelines in the pump room, seriously cause the pump room to be submerged, cause personal injury and other major accidents, and affect production and life.
Protective measures to eliminate or reduce water hammer
There are many protection measures for water hammer, but different measures need to be taken according to the possible causes of water hammer.
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. When arranging water pipelines, consideration should be given to avoid humping or sudden changes in slope as much as possible.
Reduce the length of the water pipeline, the longer the pipeline, the greater the water hammer value when the pump is stopped. From one pumping station to two pumping stations, the two pumping stations are connected by suction wells.
After stopping the pump in an accident, wait until the pipeline is full of water after the check valve before starting the pump.
When starting the pump, the outlet valve of the water pump should not be fully opened, otherwise a great water shock will occur. Many major water hammer accidents at pumping stations occur under these circumstances.
Set up water hammer elimination device
1）Using constant pressure control technology:
The PLC automatic control system is adopted to control the pumps by frequency conversion, and the operation of the entire water supply pump room system is automatically controlled. As the pressure of the water supply pipe network changes continuously with changes in working conditions, low pressure or over pressure often occurs during the operation of the system, which is prone to water hammer, resulting in damage to pipelines and equipment. The PLC automatic control system is adopted to control the pipe network. Pressure detection, feedback control the start and stop of the water pump and speed adjustment, control the flow rate, and then maintain the pressure at a certain level. The water supply pressure of the pump can be set by controlling the microcomputer to maintain the constant pressure water supply, avoiding excessive pressure fluctuations. The probability of water hammer is reduced
2）Install water hammer eliminator
This equipment mainly prevents water hammer when the pump is stopped. It is generally installed near the outlet pipe of the pump, and uses the pressure of the pipe itself as the power to achieve low-pressure automatic action, that is, when the pressure in the pipe is lower than the set protection value, the drain will automatically open to discharge water Relief pressure to balance the pressure of local pipelines and prevent the impact of water hammer on equipment and pipelines. Eliminators can generally be divided into mechanical and hydraulic types. The mechanical eliminator can be restored manually after the action, and the hydraulic eliminator can automatically Reset.
3）Install a slow-closing check valve on the outlet pipe of the large-diameter water pump
It can effectively eliminate the water hammer when the pump is stopped, but because a certain amount of water flows back when the valve is operating, 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 kind of valve can adjust the valve closing time within a certain range as required. Generally, the valve closes 70% to 80% within 3 to 7 s after a power failure, and the remaining 20% to 30% of the closing time is adjusted according to the conditions of the pump and pipeline, generally in the range of 10 to 30 s. It is worth noting that when there is a hump in the pipeline to bridge the water hammer, the effect of the slow closing check valve is very limited.
4）Set up a one-way surge tank
Build near the pumping station or at the appropriate location of the pipeline, and the height of the one-way surge tank is lower than the pipeline pressure at that location. When the pressure in the pipeline is lower than the water level in the tower, the surge tank replenishes water to the pipeline to prevent the water column from breaking and avoid bridging the water hammer. However, it has limited depressurization effect on water hammer other than the water hammer when the pump is stopped, such as the valve water hammer. In addition, the performance of the one-way valve used in the one-way pressure regulating tower must be absolutely reliable. Once the valve fails, it may cause a large water hammer.
- 5）Install a bypass pipe (valve) in the pump station
When the pump system is operating normally, the check valve is closed because the water pressure on the pressure side of the pump is higher than the water pressure on the suction side. When the pump was suddenly stopped after an accidental power failure, the pressure at the outlet of the water pump station dropped sharply, while the pressure on the suction side rose sharply. Under this differential pressure, the transient high-pressure water in the suction manifold pushes open the check valve valve plate and flows to the transient low-pressure water in the pressurized water main, and increases the low water pressure there; on the other hand, the water pump The water hammer boost on the suction side is also reduced. In this way, the rise and fall of water hammer on both sides of the water pumping station are controlled, thereby effectively reducing and preventing water hammer hazards.
6）Set up multi-stage check valve - In the long water pipeline, one or more check valves are added to divide the water pipeline into several sections, and each section is equipped with a check valve. When the water in the water delivery pipe flows back during the water hammer process, the check valves are closed one after another to divide the back flushing water flow into several sections. Since the static water pressure head in each section of the water delivery pipe (or the back flushing water flow section) is quite small, the water is reduced. Hammer boost. This protective measure can be effectively used in situations where the geometric water supply height difference is large; but it cannot eliminate the possibility of water column separation. Its biggest disadvantage is that the pump power consumption increases during normal operation and the water supply cost increases.
