Water hammer caused by pump shutdown is very harmful; how to protect against it?

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Water hammer, also known as shock wave, is a hydraulic transient phenomenon in pressure pipelines caused by sudden changes in water flow velocity, which lead to pressure variations; it is also referred to as transient flow. That is, in pressure pipelines, when the flow velocity of the water changes abruptly for some reason, the inertia of the fluid causes the pressure of the liquid inside the pipeline to increase or decrease. It poses a great threat to water pump units and piping systems. Under normal circumstances, water hammer does not occur in water pumps during normal operation or normal shutdown. Classification of water hammer phenomena: From different perspectives, water hammer phenomena can be divided into 4 categories: (1) Based on the relationship between the valve closing time and the water hammer effect, they are classified as direct water hammer phenomena and indirect water hammer phenomena ； (2) Based on the characteristics of the water hammer wave, it can be divided into the continuous water column water hammer phenomenon and the separated water column water hammer phenomenon ； (3) Based on the hydraulic characteristics of water hammer, it can be divided into rigid water hammer phenomena and elastic water hammer phenomena ； (4) Based on the external conditions causing water hammer, it can be divided into start-up water hammer, valve-shutting water hammer, and pump-stop water hammer. Methods for calculating water hammer: Water hammer analysis and calculation can be carried out using analytical methods, graphical methods, characteristic line methods, and finite element methods. The analytical and graphical methods ignore the effect of frictional resistance, handle complex boundary conditions in a rather rough manner, and result in poor computational accuracy. Using the finite element method does not reduce computing time, and dealing with boundary conditions is less convenient than with the characteristic line method. By using the characteristic line method, the effects of frictional losses along the flow path and local frictional losses on water hammer calculations can be taken into account with relatively high accuracy. It also enables easy handling of various complex boundary conditions, such as those at connected pipelines, branching pipelines, valves, pressure regulation chambers, and machinery units, thereby improving the calculation accuracy. What are the hazards of pump shutdown water hammer? Pump shutdown water hammer is a type of water hammer phenomenon that occurs when the water pump unit loses power suddenly due to errors in the operation by pump station staff, outages caused by faults in the external power grid, or natural disasters such as strong winds, lightning strikes, and earthquakes. This sudden stoppage leads to the occurrence of water hammer effects within the pump station and pipeline systems. According to investigations, many water hammer incidents are of the type that occur when pumps are stopped, and these pose a serious threat to the safety of pump houses and pipelines. In China, several pump houses have experienced flooding or pipeline ruptures as a result of water hammer occurring when pumps were stopped. Although the duration of the water hammer phenomenon is short, the engineering accidents it causes cannot be ignored; in mild cases, this leads to vibration in the pump units and hydraulic impact noise ； In severe cases, the water pump unit is damaged by vibration, and the pipes crack, resulting in water supply disruptions. If the pump is stopped due to an accident, causing water column separation in the pipeline and the closure of flow, the resulting water hammer has an even more severe destructive effect. How to protect against pump shutdown water hammer: Since pump shutdown water hammer can lead to serious accidents in pumping stations and water transmission systems, it is necessary to take appropriate measures based on the specific circumstances in order to eliminate this phenomenon or reduce the pressure resulting from it. To date, there are various measures for protecting against pump shutdown water hammer, which can be roughly classified as follows: 1. Pressure stabilization through water replenishment (air injection). Pressure stabilization by water replenishment (air injection) can prevent water column separation or the water hammer caused by excessive pressure rise during flow interruption. Examples of this type include two-way surge chambers, one-way surge chambers, and air tanks. (1) Two-way pressure regulation tower: Built near the pump station or at an appropriate location along the pipeline. The water level of the two-way pressure regulation tower should be higher than that of the receiving tank at the end of the water transmission pipeline, taking into account the head loss along the pipeline. The pressure regulating tower adds water to the pipeline or releases excess pressure from it in response to changes in pressure within the pipeline, thereby effectively preventing or reducing water hammer pressure. (2) Unidirectional pressure regulating tower: Constructed near the pump station or at an appropriate location along the pipeline; the height of such a 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 supplies water to the pipeline to prevent the water column from breaking, thus avoiding water hammer. (3) Pressure tank: There is limited experience in its use in China, but it is widely used abroad. It operates based on a specific law relating gas volume and pressure. As the pressure in the pipeline changes, the air tank supplies water to the pipeline or absorbs excess pressure from it, functioning in a similar manner to a two-way pressure regulating tower. 2. Pressure relief and reduction: Pressure relief and reduction helps prevent sudden spikes in pressure. Examples of devices used for this purpose include pump-stop water hammer arresters, slow-closing check valves, and rupture discs. (1) There are mainly three types of pump-stop surge arresters: downward-opening pump-stop surge arresters, self-closing pump-stop surge arresters, and automatically resetting pump-stop surge arresters. They operate on a similar principle: during the shutdown of the pump, when the outlet pressure drops to a certain level, the suppressor opens; when the water hammer pressure wave moves back toward the pump, the suppressor discharges water outward, thereby eliminating the water hammer effect. The length of main pipe protected by surge arresters generally does not exceed 800 meters. (2) The slow-closing check valve is a type of check valve that eliminates water hammer through a slow-closing mechanism. There are various designs of such valves, which are simple and practical, and they have been widely used. Slow-closing check valves come in two types: weight-type and energy-storing type. This type of valve allows the closing time to be adjusted within a certain range as needed. Generally, the valve closes 70%–80% within 3–7 seconds after a power outage, while the remaining 20%–30% of the closing time is adjusted depending on the conditions of the water pump and pipelines, usually ranging from 10–30 seconds. (3) The burst disc functions similarly to a fuse in an electrical circuit; when the pressure in the pipeline rises due to water hammer and exceeds a predetermined value, the disc ruptures automatically, allowing the water to flow out and thereby reducing the pressure and eliminating the effect of water hammer. 3. Other types: (1) Appropriately increasing the pipe diameter and wall thickness, as well as reducing the flow velocity in the water delivery pipelines, can help to reduce water hammer pressure to a certain extent. (2) Reduce the pipeline length by changing from one pump station to two, and connect the two pump stations using a suction well. (3) Using a water pump unit with a large moment of inertia or equipping it with a flywheel having sufficient inertia can reduce the water hammer value to a certain extent. (4) Change the longitudinal profile layout of the pipeline. When arranging water supply pipelines, sharp changes in slope should be avoided as much as possible. Precautions for protecting against water hammer: Additionally, when selecting measures to protect against water hammer during pump shutdown, the following points should be taken into account: (1) The protective measures chosen should be appropriate to the scale and function of the pumping station and pipeline system, as well as the requirements regarding safety and the technical (management) level; it is advisable to opt for measures that are technically safe and reliable, economically reasonable, and easy to maintain. (2) In situations where water hammer hazards may occur, prevention and control should be carried out at an early stage. In aspects such as the design of pump stations and piping systems, the selection of pipe routes, the choice of pump units and pipes, and the determination of flow velocities within the pipes, measures should be considered to eliminate or mitigate the effects of water hammer. (3) Depending on the specific circumstances, comprehensive preventive measures (the application of several measures simultaneously) should be adopted as much as possible to enhance the safety and reliability of the protective function. (4) Sufficient attention should be paid to the requirements regarding the management, maintenance, and operation of protective measures; many serious water hammer incidents at pump stations in China have been caused by poor maintenance of protective equipment or improper operation. (5) The selection of protective measures must be carried out simultaneously with the precise calculation of pump shutdown water hammer and their mutual analysis. In short, the water hammer phenomenon that occurs when a pump is stopped, along with its hazards, arise under specific conditions, just like any other physical phenomenon. Therefore, most preventive measures are based on early protection against water hammer caused by pump shutdown. In practical engineering applications, it is necessary to understand their essence, consider various factors comprehensively, and apply them flexibly.