Causes of water hammer phenomenon and solutions

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This post was last edited by GMFPJ on 2017-2-23 at 10:34. After the installation of pump rooms and piping systems, it is common to observe that during operation, when the pumps or power supply is turned off, a large force of water is generated in the piping system; this force impacts the pumps, valves, and pipes. In some cases, the effect of water hammer is mild, but in other cases it can be severe, even leading to serious accidents. For example, the valve discs, pump impellers, and piping components can be destroyed as a result of water hammer – such damage is caused by water hammer effects. What is 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. The force generated by the back-and-forth movement of water flow shock waves can sometimes be very large, thereby damaging valves and pumps. The \"water hammer effect\" refers to the situation inside a water pipe, where the inner wall is smooth and water flows freely. When an open valve suddenly closes, the water flow exerts pressure on the valve and the pipe walls, with the valve being the main target of this pressure. Due to the smooth wall of the pipe, the subsequent water flow, driven by inertia, quickly reaches its maximum velocity and causes destructive effects; this is what is known in fluid dynamics as the \"water hammer effect\", or positive water hammer. This factor must be taken into account in the construction of water supply pipelines. Conversely, when a closed valve is suddenly opened, water hammer occurs as well; this is known as negative water hammer, and it also has a certain degree of destructive power, but not as much as the former. A sudden power outage or startup of the electric water pump unit can also cause pressure surges and water hammer effects. The shock wave generated by this pressure propagates along the pipeline, easily leading to localized overpressure that can cause pipe rupture and equipment damage; therefore, protection against the water hammer effect is one of the key process technologies in water supply systems. Conditions for water hammer to occur: 1. Sudden opening or closing of a valve ； 2. Sudden shutdown or startup of the water pump unit ； 3. Single-pipe water transmission to higher elevations (when the elevation difference in the water supply area exceeds 20 meters) ； 4. High total head (or operating pressure) of the water pump ； 5. Excessively high flow velocity in the water delivery pipes ； 6. The water conveyance pipelines are too long, and the terrain varies greatly. 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 pressure fluctuations cause the following main damages to piping systems: 1. They induce intense vibrations in the pipes, leading to the disconnection of pipe joints ； 2. Damage to the valves; extremely high pressure can cause the pipes to burst, resulting in a decrease in pressure within the water supply network ； 3. Conversely, too low a pressure can cause the tube to collapse, as well as damage the valves and fixing elements ； 4. It can cause the water pump to run in reverse, damaging the equipment or pipes inside the pump room; in severe cases, it may lead to flooding of the pump room, resulting in serious accidents such as injuries and deaths, and affecting production and daily life. The calculation of water hammer pressure, specifically for the water hammer pressure that occurs when a pump is stopped, should be carried out using the following formula. When the value of water hammer pressure calculated in this way exceeds the test pressure of the pipeline, technical measures must be taken to eliminate the water hammer effect resulting from the stoppage of the pump. Δp = ρcv Where: △p —— maximum water hammer pressure (Pa) ； ρ —— density of water (kg/m3) ； c —— propagation speed of water hammer waves (m/s) ； v——flow velocity of water in the pipeline (m/s) ； c0 — the propagation speed of sound in water; it is advisable to use c0 = 1435 m/s (pressure range: 0.10 MPa to 2.50 MPa, water temperature: 10°C) ； K — the volumetric elastic modulus of water; it is advisable to take K = 2.1×109 Pa ； E — the elastic modulus of the pipe material; for steel pipes, E = 20.6×1010 Pa, for cast iron pipes, E = 9.8×1010 Pa, and for plastic (PE) composite pipes with wire mesh reinforcement, E = 6.5×1010 Pa ； di——Nominal diameter of the pipe (mm) ； δ —— Wall thickness of the pipe (mm). Taking a standard DN100 steel pipe (with an internal diameter of 105 mm and a wall thickness of 4.5 mm) and a flow rate of q = 20 L/s = 0.02 m3/s as an example, we have: Δp = ρcv, where v = q/A = 0.02/A = 2.31 m/s. The hydrodynamic pressure Δp = 1000 × 1295.48 × 2.31 = 2992558.8 Pa = 2.99 Mpa. Measures to eliminate or reduce hydrodynamic effects: There are many measures available for protecting against hydrodynamic effects, but different approaches need to be adopted depending on the possible causes of such effects. 1. Reducing the flow velocity in the water transmission pipelines can, to some extent, lower the water hammer pressure, but it requires a larger diameter for the pipelines, thereby increasing the project costs. When arranging water transmission pipelines, care should be taken to avoid bulges or sudden changes in slope, in order to reduce the length of the pipelines; the longer the pipelines, the greater the water hammer effect when the pumps are stopped. Changing from one pumping station to two, with suction wells connecting the two pumping stations. The magnitude of the water hammer when the pump stops is primarily 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 stops. Therefore, a suitable pump head should be selected based on the local 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) Use constant pressure control technology: Employ a PLC-based automatic control system to achieve variable frequency speed control of the pumps, thereby providing automatic control over the entire water supply pump room system. Since the pressure in the water supply network changes continuously as operating conditions vary, low pressure or overpressure conditions often occur during system operation, which can lead to water hammer and cause damage to pipes and equipment. By using a PLC-based automatic control system, it is possible to monitor the pressure in the network and use feedback control to regulate the start/stop of pumps as well as their speed, thereby controlling the flow rate and maintaining pressure at a constant level. The supply pressure can be set via a microcomputer, ensuring a constant pressure supply and reducing excessive pressure fluctuations, which in turn lowers 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, and 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 threshold, 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 intervention to be reset after they operate, while hydraulic suppressors can reset automatically. (3) Installing a slow-closing check valve on the outlet pipe of large-diameter water pumps can effectively eliminate water hammer when the pump stops operating; however, since a certain amount of water flows back during the operation of the valve, the suction well must be equipped with an overflow pipe. 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 according to the conditions of the water pump and pipelines, usually ranging from 10–30 seconds. It is worth noting that when a wave is present in the pipeline and cushioning water hammer occurs, the function of the 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. Furthermore, the check valves used in unidirectional voltage regulation towers must be absolutely reliable; should such valves fail, it could lead to significant water hammer effects. (5) A bypass pipe (valve) is installed in the pump station. When the pump system is operating normally, the check valve closes because the water pressure on the pressure side of the water pump is higher than that on the suction side. When a power failure causes the pump to stop suddenly, the pressure at the outlet of the pump station drops sharply, while the pressure on the suction side rises dramatically. Under this pressure difference, the transient high-pressure water in the water intake main pushes aside the valve disc of the check valve and allows the transient low-pressure water flowing in the water pressurization main to pass through, 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, the water hammer pressure rises and drops on both sides of the pump station are controlled, thereby effectively reducing and preventing the hazards caused by 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 pipes during the water hammer phenomenon, each check valve closes sequentially, dividing the backflow into several segments. Since the static head of water in each segment of the conveyance pipe (or backflow segment) is quite low, this reduces the pressure rise caused by 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 the power consumption of the water pump increases during normal operation, resulting in higher water supply costs.