Water hammer can be very harmful; it’s essential to learn how to protect against it!

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Water hammer phenomenon: Water hammer occurs when there is a sudden power outage or when a valve is closed too quickly; due to the inertia of the pressurized water flow, shock waves are generated, similar to the impact of a hammer, which is why it is called water hammer. Water hammer in pump stations includes starting water hammer, valve-closing water hammer, and pump-stopping water hammer (which occurs due to sudden power outages, etc.). The first two types of water hammer do not cause any problems that could threaten the safety of the unit under normal operating conditions. The water hammer pressure generated by the latter is often very high, leading to accidents. Pump shutdown water hammer: Pump shutdown water hammer refers to the hydraulic shock phenomenon in which, when a pump is stopped due to a sudden power failure or other reasons with valves left open, sudden changes in flow velocity cause fluctuations in pressure within the pump and pressure pipelines. For example, failures in power systems or electrical equipment, as well as occasional malfunctions in water pump units, can all lead to the centrifugal pump stopping due to valve opening, thereby causing water hammer resulting from the pump shutdown. The maximum pressure caused by pump surge can reach 200% of the normal operating pressure; in some cases, it can even be higher, leading to damage of pipes and equipment. Typically, such incidents result in water leaks or water supply interruptions ； Serious accidents can result in flooded pump rooms, damaged equipment, destroyed facilities, and even casualties. The hazards of the water hammer effect: The pressure increase caused by water hammer can reach several times, or even dozens of times, the normal operating pressure of the pipeline. Such significant fluctuations in pressure can cause the following damages to piping systems: 1. They induce intense vibrations in the pipes, leading to the disconnection of pipe joints ； 2. Valves are damaged; excessively high pressure leads to pipe bursts, resulting in a decrease in pressure within the water supply network ； 3. Conversely, too low a pressure can cause the pipe to collapse, as well as damage the valves and fixing elements ； 4. It causes the water pump to rotate in reverse, damaging equipment or pipelines within the pump house; in severe cases, it leads to flooding of the pump house, resulting in major accidents such as casualties, thus affecting production and daily life. Protective measures to eliminate or mitigate water hammer. There are many protective measures against water hammer; however, different measures must be taken depending on the possible causes of water hammer. 1. Reducing the flow velocity in water transmission pipelines can, to some extent, decrease water hammer pressure; however, it will also increase the diameter of the pipelines, thereby raising construction costs. When laying out water transmission pipelines, every effort should be made to avoid the occurrence of humps or abrupt changes in slope. Shorten the length of the water conveyance pipeline; the longer the pipeline, the greater the value of pump-stop water hammer. One pumping station is changed to two pumping stations, which are connected by a suction well. The magnitude of pump-stop water hammer is mainly related to the geometric head of the pump house; the higher the geometric head, the greater the value of the pump-stop water hammer. Therefore, a reasonable pump head should be selected based on local actual conditions. After shutting down the pump due to an accident, wait until the pipeline behind the check valve is filled with water before restarting the pump. When starting the pump, do not fully open the outlet valve of the water pump, otherwise it will cause significant water shock. Many major water hammer incidents at pumping stations occur under such circumstances. 2. Install a water hammer suppression device: (1) Adopt constant pressure control technology: Use a PLC-based automatic control system to carry out variable frequency speed control of the pumps, thereby enabling automatic operation of the entire water supply pump room system. Since the pressure in the water supply network constantly changes with operating conditions, low or excessive pressure often occurs during system operation, leading to water hammer and subsequent damage to pipes and equipment. By employing a PLC-based automatic control system, the pressure within the network is monitored; this information is used to regulate the start/stop operations and rotational speed of the pumps, thereby controlling the flow rate and maintaining pressure at a desired level. The desired water supply pressure can be set via a control computer, ensuring constant-pressure water supply. This approach prevents excessive pressure fluctuations and reduces the likelihood of water hammer occurring. (2) Installation of a water hammer suppressor. This device is primarily used to prevent water hammer effects that occur when the pump stops operating; it is usually installed near the outlet pipe of the pump. It utilizes the pressure in the pipe itself as power to trigger automatic operation at low pressures. That is, when the pressure in the pipe falls below a set safety level, the discharge valve opens automatically to release water and balance the pressure in that section of the pipe, thereby preventing water hammer from causing damage to the equipment and pipes. Water hammer suppressors can generally be divided into mechanical and hydraulic types – mechanical suppressors require manual restoration after they operate, while hydraulic suppressors can reset automatically. (3) Installing a slow-closing check valve on the discharge pipe of large-diameter water pumps can effectively eliminate pump-stop water hammer. However, since a certain amount of water flows backward when the valve operates, the suction well must be equipped with an overflow pipe. There are two types of slow-closing check valves: weight-type and accumulator-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 according to the conditions of the water pump and pipelines, usually ranging from 10–30 seconds. It is worth noting that when a surge occurs due to a kink in the pipeline, the function of a slow-closing check valve is very limited. (4) Install a one-way pressure regulating tower, which should be built near the pump station or at an appropriate location along the pipeline; the height of this tower should be 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. However, its effect on reducing pressure for water hammers other than pump-stop water hammers, such as valve-closing water hammers, is limited. In addition, the performance of the check valve used in a one-way pressure regulating tower must be absolutely reliable; should this valve fail, it could lead to significant water hammer. (5) Install a bypass pipe (valve) within the pump station. When the pumping system is operating normally, since the water pressure on the discharge side of the pump is higher than that on the suction side, the check valve remains closed. After the sudden pump shutdown due to a power failure in an accident, the pressure at the outlet of the pumping station dropped sharply, while the pressure on the suction side rose sharply. Under this pressure difference, the transiently high-pressure water in the water suction main pushes open the check valve disc and flows toward the transiently low-pressure water in the pressurized water main, thereby increasing the low water pressure at that location ； On the other hand, the water hammer pressure rise on the water intake side of the water pump is also reduced. In this way, both the pressure rise and drop caused by water hammer on both sides of the pump station are controlled, thereby effectively reducing and preventing the hazards of water hammer. (6) Install multi-stage check valves: In longer water transmission pipelines, one or more check valves are added to divide the pipeline into several sections, with a check valve installed in each section. When water flows backward in the water conveyance pipe during a water hammer event, the check valves close sequentially, dividing the backflow into several segments. Since the hydrostatic head within each segment of the pipe (or each segment of the backflow) is quite small, this reduces the pressure rise caused by the water hammer. This protective measure can be effectively used in situations where there is a large geometric difference in water supply levels ； But it cannot eliminate the possibility of water column separation. Its biggest drawback is that during normal operation, the power consumption of the water pump increases, leading to higher water supply costs.