What is the water hammer effect?
Water hammer effect refers to the phenomenon that the flow rate changes suddenly and the pressure fluctuates greatly due to the sudden opening and closing of the valve and the sudden start and stop of the water pump during the transportation of water or other liquids. To put it in layman terms: Sudden power failure or valve closing too fast, due to the inertia of the pressure water flow, a water shock wave is generated, just like a hammer hit, we call it the water hammer effect.
How does the water hammer effect damage the pipeline?
The wall of the water supply pipeline is smooth, and the subsequent water flow is accompanied by inertia, and the hydraulic power quickly reaches the maximum, so it is easy to cause destructive effects (such as damaging valves and pumps, etc.). This is the water hammer effect in hydraulics, also called positive water hammer; on the contrary, The sudden opening of a valve or a water pump will also produce a water hammer effect, called negative water hammer. This kind of pressure shock wave with large fluctuations can easily cause the pipeline to rupture due to local overpressure and damage the equipment. Therefore, the water hammer effect protection is the water supply pipeline. Key factors that must be considered in engineering design and construction.
How is water hammer produced?
- Valve opens or closes suddenly;
- Water pump unit suddenly stops or turns on;
- Single pipe delivers water to high places (the height difference of the water supply terrain exceeds 20 meters);
- The total head (or working pressure) of the pump is large;
- Water flow rate in the water pipeline is too large;
- Water pipeline is too long and the terrain changes greatly.
- Irregular construction is a hidden danger in water supply pipeline engineering: the production of cement thrust piers such as tees, elbows, reducers, and other nodes does not meet the requirements.
What are the hazards of the water hammer effect to the pipeline?
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:
- Cause the pipeline to vibrate strongly, and the pipeline joint is disconnected;
- Destroy the valve, severely high pressure causes the pipeline to burst, and the pressure of the water supply network is reduced;
- On the contrary, if the pressure is too low, it will cause the tube to collapse, and it will also damage the valve and fixing parts;
- Water hammer will cause the 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 have an impact on production and life.
What are the protective measures to eliminate or reduce water hammers?
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 delivery 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.
- The size of the water hammer when the pump is stopped is mainly related to the geometric head of the pump room. The higher the geometric head, the greater the value of the water hammer when the pump is stopped. Therefore, a reasonable pump head should be selected according to local actual conditions.
- After stopping the pump due to an accident, wait for the pipeline after the check valve to be filled with water before starting the pump.
- Do not open the pump outlet valve fully when starting the pump, otherwise, it will cause a big water shock. Many major water hammer accidents at pumping stations occur under these circumstances.
What is the water hammer when the pump stops?
The so-called stop pump water hammer refers to the pipeline without PLC hydraulic check butterfly valve installed at the water pump port. Sudden power failure or other reasons cause the water pump to stop running suddenly, and the pressure rises and falls due to the sudden change in the flow rate in the water pump and pressure pipeline. The phenomenon of hydraulic shock. For example, the power system or electrical equipment failure, the accidental failure of the water pump unit, etc., may cause the centrifugal pump to stop running suddenly. Since the pipeline is not installed with a PLC hydraulic control check butterfly valve group that can be automatically closed, the valve cannot be closed in time manually, which causes the shutdown Pump water hammer.
The maximum pressure of the water hammer when the pump is stopped can reach 200% of the normal working pressure, and even higher can cause damage to the pipeline and equipment, and lighter accidents can cause water overflow and water stoppage. Serious accidents can cause the pump room to be flooded, equipment damage, facilities destroyed, and even personal injury accidents.
How to set up a water hammer elimination device
- Adopt 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. Since the pressure of the water supply pipe network changes continuously with changes in working conditions, low pressure or overpressure 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 pass the control of the pipe network. Pressure detection, feedback control the start, stop and speed adjustment of the water pump, control the flow, and then maintain the pressure at a certain level, you can set the pump water supply pressure by controlling the microcomputer to maintain the constant pressure water supply, avoid excessive pressure fluctuations, and make The probability of water hammer is reduced.
- Install PLC hydraulic control check butterfly valve on the large diameter pump outlet pipe
This valve is an important equipment behind medium and large pipeline pumps. It is widely used in medium and large pipelines such as water conservancy and electric power. It can effectively avoid the impact of water hammers on pumps and pipelines.
This butterfly valve product is a high-tech high-end butterfly valve product developed and applied at the end of the last century. It can precisely control and adjust the valve opening and closing time, and can accurately realize the fast and slow closing time.
The valve can be divided into heavy hammer type and fully hydraulic control type, as well as pressure holding type and locking type. It is an irreplaceable product for the backstop function of the pump.
- Install water hammer eliminator
This equipment mainly prevents water hammer from stopping the pump. It is generally installed near the outlet pipe of the pump. It 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. Mechanical eliminators are manually restored after action, and hydraulic eliminators can be automatically restored. Reset.
- 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 surge tank must be absolutely reliable. Once the valve fails, it may cause a large water hammer.
- 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 water pump side is higher than the water pressure on the suction side. When the pump was suddenly shut down after an accidental power failure, the pressure at the outlet of the water pumping station dropped sharply, while the pressure on the suction side rose sharply. Under this differential pressure, the transient high-pressure water in the suction header is the transient low-pressure water that pushes open the check valve plate and flows to the water pressure header, 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.
- 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 provided 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 backflushing water flow into several sections. Because the static water pressure head in each section of the water delivery pipe (or backflushing 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. The biggest disadvantage is that the pump power consumption increases during normal operation and the cost of water supply increases.