Introduction to Water Hammer
Water hammer, also known as hydraulic shock, refers to the phenomenon of sudden changes in flow velocity and large pressure fluctuations during the transportation of water or other liquids, due to reasons such as sudden opening or closing of valves or sudden starting or stopping of pumps. Simply put: when there is a sudden power outage or the valve closes too quickly, the inertia of the pressurized water flow creates a water shock wave, like a hammer striking, which we call water hammer.
In smooth-walled water supply pipes, the subsequent water flow, aided by inertia, quickly reaches its maximum hydraulic pressure, thus easily causing damage (such as damaging valves and pumps). This is the "water hammer effect" in hydraulics, also called positive water hammer; conversely, the sudden opening of valves or pumps also produces a water hammer effect, called negative water hammer. This large pressure shock wave can easily lead to pipe rupture due to local overpressure, equipment damage, etc. Therefore, protection against the water hammer effect is a crucial factor that must be considered in the design and construction of water supply pipeline projects.
Conditions for Water Hammer Generation
1. Sudden opening or closing of valves;
2. Sudden stopping or starting of pump units;
3. Single-pipe water delivery to a high elevation (water supply elevation difference exceeding 20 meters);
4. High total head (or working pressure) of the pump;
5. Excessive water flow velocity in the pipeline;
6. Long pipelines with significant terrain changes.
7. Non-standard construction is a hidden danger in water supply pipeline projects:
7.1 For example, the cement thrust blocks at nodes such as tees, elbows, and reducers do not meet the requirements.
Hazards of the Water Hammer Effect
The pressure increase caused by water hammer can reach several times, or even tens of times, the normal working pressure of the pipeline. This large pressure fluctuation causes the following main hazards to the pipeline system:
1. Causes strong vibration of the pipeline and disconnection of pipe joints;
2. Damages valves; severe overpressure can cause pipe bursts and reduce the pressure in the water supply network;
3. Conversely, excessively low pressure can lead to pipe collapse and damage to valves and fasteners;
4. Causes pump reversal, damaging equipment or pipelines in the pump room, and in severe cases, causing flooding of the pump room, resulting in major accidents such as personal injury and death, affecting production and daily life. Protective Measures to Eliminate or Reduce Water Hammer
There are many protective measures against water hammer, but different measures should be taken depending on the possible causes of water hammer.
1. Reducing the flow velocity in the water pipeline can reduce water hammer pressure to a certain extent, but it will increase the pipe diameter and increase engineering investment. When laying the water pipeline, efforts should be made to avoid humps or sudden changes in slope.
Reducing the length of the water pipeline is also important, as the longer the pipeline, the greater the water hammer value when the pump stops. Consider using two pumping stations instead of one, connecting them with a suction well.
The magnitude of water hammer when the pump stops is mainly related to the geometric head of the pumping station; the higher the geometric head, the greater the water hammer value. Therefore, a reasonable pump head should be selected based on local conditions.
After an accidental pump shutdown, the pump should not be restarted until the pipeline after the check valve is full of water.
When starting the pump, the pump outlet valve should not be fully open, otherwise a large water surge will occur. Many major water hammer accidents in pumping stations occur under these circumstances.
Pump Shutdown Water Hammer
Pump shutdown water hammer refers to the hydraulic shock phenomenon caused by sudden changes in flow velocity in the pump and pressure pipeline when the valve is opened and the pump stops due to sudden power failure or other reasons. For example, faults in the power system or electrical equipment, or accidental failures of the pump unit, can all cause centrifugal pumps to stop with the valve open, thus triggering pump shutdown water hammer.
The maximum pressure of pump shutdown water hammer can reach 200% of the normal working pressure, or even higher, which can damage pipelines and equipment. General accidents cause water leakage and water supply interruptions; serious accidents can cause flooding of the pump room, equipment damage, destruction of facilities, and even personal injury or death.