What is water hammer?

Water hammer is when the power is suddenly cut off or the valve is closed too fast, due to the inertia of the pressure water flow, a water shock wave is generated, just like a hammer hit, so it is called water hammer. The force generated by the shock wave of the water flow can sometimes be very large, which can damage the valve and the pump.

When the opened valve is suddenly closed, the water flow will generate a pressure on the valve and the pipe wall. 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 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. This kind of pressure shock wave propagates along the pipeline, which can easily lead to local overpressure in the pipeline and cause pipeline rupture and equipment damage. Therefore, water hammer effect protection has become one of the key technology of water supply engineering.

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.

Harm 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. Such large pressure fluctuations,the main hazards to the pipeline system are:

1. Cause the pipeline to vibrate strongly, and the pipeline joint is disconnected;

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.

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 water pipelines, consideration should be given to avoid humping or drastic changes in slope as much as possible.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 the actual local conditions. After stopping the pump in an accident, wait until the pipeline is full of water after the check valve before starting the pump. Do not open the pump outlet valve fully when starting the pump, otherwise a great water shock will occur. Many major water hammer accidents at pumping stations occur under these circumstances.

2. Set up water hammer elimination device

(1) Adopt constant pressure control technology:
     As 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. An automatic control system is adopted to control the pressure on the pipe network. The detection, feedback control the start, stop and speed adjustment of the water pump, 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 to avoid excessive pressure fluctuations and produce water The probability of hammering decreases.

(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-caliber 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 seconds after a power failure, and the remaining 20% to 30% closing time is adjusted according to the conditions of the pump and pipeline, generally in the range of 10 to 30 seconds. 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 effective.

(4) Set up a one-way surge tank

       Build near the pumping station or at the appropriate location of the pipeline. 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 pump-stop water hammer, such as valve-closing water hammer. In addition, the performance of the one-way valve used in the one-way pressure regulating tower must be absolutely reliable.

(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 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 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 backflushing water flow into several sections. Since the static water pressure head in each section of the water delivery pipe (or the 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. Its biggest shortcoming is that the pump power consumption increases during normal operation, and the water supply cost increases.

(7) The high point of the pipeline is equipped with automatic exhaust and air supplement devices to reduce the impact of water hammer on the pipeline

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