Water hammer is very harmful, how to prevent it?
Water hammer phenomenon
Water hammer is a shock wave caused by the inertia of the pressure water flow when there is a sudden power outage or when the valve is closed too quickly, just like a hammer hitting, so it is called water hammer.
Pump station water hammer includes start-up water hammer, valve closing water hammer and pump stop water hammer (caused by sudden power outage, etc.). The first two types of water hammer will not cause problems that endanger the safety of the unit under normal operating procedures. The water hammer pressure value formed by the latter is often very large, thus causing accidents.
Pump stop water hammer
The so-called pump stop water hammer refers to the hydraulic shock phenomenon of pressure rise and fall caused by sudden changes in flow rate in the water pump and pressure pipeline when the valve is opened and stopped due to sudden power outage or other reasons. For example, failure of the power system or electrical equipment, occasional failure of the water pump unit, etc., may cause the centrifugal pump to open the valve and stop, thus causing pump stop water hammer.
The maximum pressure of the pump-stop water hammer can reach 200% of the normal working pressure, or even higher, which can destroy pipelines and equipment. General accidents cause "water leakage" and water outages; serious accidents cause pump room flooding, equipment damage, facilities destruction, and even personal injury accidents.
Hazards of water hammer effect
The pressure increase caused by water hammer can reach several times or even dozens of times the normal working pressure of the pipeline. This large pressure fluctuation mainly causes the following hazards to the pipeline system:
1. Cause strong vibration of the pipeline and disconnection of the pipeline joint;
2. Damage to the valve. Severe excessive pressure causes pipeline burst and reduced pressure in the water supply network;
3. On the contrary, too low pressure will cause the collapse of the pipe and damage the valve and fixings;
4. Cause the water pump to reverse, damage the equipment or pipeline in the pump room, and seriously cause the pump room to flood, causing major accidents such as personal injury and death, affecting production and life.
Protective measures to eliminate or reduce water hammer
There are many protective measures for water hammer, but different measures need to be taken according to the possible causes of water hammer.
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. When arranging the water pipeline, it should be considered to avoid humps 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. Change from one pump station to two pump stations, and connect the two pump stations with a suction well.
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 water hammer value when the pump is stopped. Therefore, a reasonable water pump head should be selected according to the actual local conditions.
After the pump is stopped due to an accident, the water pump should be started after the pipeline behind the check valve is filled with water.
When starting the pump, do not fully open the outlet valve of the pump, otherwise it will cause a lot of water shock. Many major water hammer accidents in pump stations occur in this situation.
2. Set up water hammer elimination device
(1) Use constant pressure control technology:
Use PLC automatic control system to perform frequency conversion speed control on the machine pump and implement automatic control on the operation of the entire water supply pump room system. Since the pressure of the water supply network changes with the change of working conditions, low pressure or overpressure often occurs during the operation of the system, which is easy to cause water hammer, resulting in damage to pipelines and equipment. The PLC automatic control system is used to detect the pressure of the network, feedback control the start, stop and speed adjustment of the water pump, control the flow rate, and then maintain a certain level of pressure. The water supply pressure of the machine pump can be set by controlling the microcomputer to maintain constant pressure water supply, avoid excessive pressure fluctuations, and reduce the probability of water hammer.
(2) Install water hammer eliminator
This device is mainly used to prevent water hammer when the pump stops. It is usually installed near the outlet pipe of the water pump and uses the pressure of the pipe itself as power to realize low-pressure automatic action. That is, when the pressure in the pipe is lower than the set protection value, the drain outlet will automatically open to release water and pressure to balance the pressure of the local pipe and prevent water hammer from impacting the equipment and pipes. Eliminators can generally be divided into mechanical and hydraulic types. Mechanical eliminators are manually restored after actuation, while hydraulic eliminators can be automatically reset.
(3) Install slow-closing check valves on the outlet pipes of large-diameter water pumps
This can effectively eliminate water hammer when the pump stops. However, since a certain amount of water will flow back when the valve is actuated, the suction well must have an overflow pipe. Slow-closing check valves are available in two types: heavy hammer type and energy storage type. This type of valve can adjust the valve closing time within a certain range as needed. Generally, the valve is closed 70% to 80% within 3 to 7 seconds after a power outage. The remaining 20% to 30% of the closing time is adjusted according to the conditions of the water pump and the pipe, generally in the range of 10 to 30 seconds. It is worth noting that when there is a hump in the pipeline and a bridge water hammer occurs, the effect of the slow-closing check valve is very limited.
(4) Install a one-way pressure regulating tower
It is built near the pump station or at an appropriate location in the pipeline. The height of the one-way pressure regulating tower 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 pressure regulating tower replenishes water to the pipeline to prevent the water column from breaking and avoid bridge water hammer. However, its pressure-reducing effect on water hammers other than pump stop water hammer, such as valve closing water hammer, is limited. 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 larger 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 pump pressure side is higher than the water pressure on the suction side. When the pump is suddenly stopped due to a power outage, the pressure at the outlet of the pump station drops sharply, while the pressure on the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the suction main pipe pushes the check valve plate to flow to the transient low-pressure water in the pressure main pipe, and the low water pressure there is increased; on the other hand, the water hammer pressure rise on the suction side of the pump is also reduced. In this way, the water hammer pressure rise and pressure drop on both sides of the pump station are controlled, thereby effectively reducing and preventing water hammer hazards.
(6) Install multi-stage check valves
In a long water pipeline, one or more check valves are added to divide the water pipeline into several sections, and a check valve is installed on each section. When the water in the water pipeline flows back during the water hammer process, the check valves are closed successively to divide the backwash water flow into several sections. Since the hydrostatic head in each section of the water pipeline (or backwash water flow section) is quite small, the water hammer pressure rise is reduced. 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 power consumption of the water pump increases during normal operation and the water supply cost increases.
