Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change). A water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. It is also called hydraulic shock.
This pressure wave can cause major problems, from noise and vibration to pipe collapse. It is possible to reduce the effects of the water hammer pulses with accumulators, expansion tanks, surge tanks, and other features.
Rough calculations can be made either using the Zhukovsky (romanised as Joukovsky or Joukowsky) equation, or more accurate ones using the method of characteristics.
When a pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving, thereby building up high pressure and a resulting shock wave. In domestic plumbing this is experienced as a loud banging resembling a hammering noise. Water hammer can cause pipelines to break if the pressure is high enough. Air traps or stand pipes (open at the top) are sometimes added as dampers to water systems to absorb the potentially damaging forces caused by the moving water.
In hydroelectric generating stations, the water travelling along the tunnel or pipeline may be prevented from entering a turbine by closing a valve. For example, if there is 14 km of tunnel of 7.7 m diameter full of water travelling at 3.75 m/s, that represents approximately 8000 megajoules of kinetic energy that must be arrested. This arresting is frequently achieved by a surge shaft open at the top, into which the water flows. As the water rises up the shaft its kinetic energy is converted into potential energy, which causes the water in the tunnel to decelerate. At some HEP stations, what looks like a water tower is actually one of these devices, known in these cases as a surge drum.
In the home a water hammer may occur when a dishwasher, washing machine or toilet shuts off water flow. The result may be heard as a loud bang, repetitive banging (as the shock wave travels back and forth in the plumbing system), or as some shuddering.
On the other hand, when an upstream valve in a pipe closes, water downstream of the valve attempts to continue flowing creating a vacuum that may cause the pipe to collapse or implode. This problem can be particularly acute if the pipe is on a downhill slope. To prevent this, air and vacuum relief valves or air vents are installed just downstream of the valve to allow air to enter the line to prevent this vacuum from occurring.
Other causes of water hammer are pump failure and check valve slam (due to sudden deceleration, a check valve may slam shut rapidly, depending on the dynamic characteristic of the check valve and the mass of the water between a check valve and tank).
Steam distribution systems may also be vulnerable to a situation similar to water hammer, known as steam hammer. In a steam system, a water hammer most often occurs when some of the steam condenses into water in a horizontal section of the piping. Steam picks up the water forming a "slug" and hurls this at high velocity into a pipe fitting, creating a loud hammering noise and greatly stressing the pipe. This condition is usually caused by a poor condensate drainage strategy.
Where air filled traps are used, these eventually become depleted of their trapped air over a long period of time through absorption into the water. This can be cured by shutting off the supply, opening taps at the highest and lowest locations to drain the system (thereby restoring air to the traps), and then closing the taps and re-opening the supply.
If a stream of high pressure water impinges on a surface, water hammer can quickly erode and destroy it. In the 2009 Sayano–Shushenskaya hydroelectric power station accident, the lid to a 640 MW turbine was ejected upwards, hitting the ceiling above. During the accident, the rotor was seen flying through the air, still spinning, about 3 meters above the floor. Unrestrained, 256 cubic metres (67,600 US gal) per second of water began to spray all over the generator hall. The geyser caused the structural failure of steel ceiling joists, precipitating a roof collapse around the failed turbine.
When an explosion happens in an enclosed space, water hammer can cause the walls of the container to deform. However, it can also impart momentum to the enclosure if it is free to move. An underwater explosion in the SL-1 nuclear reactor vessel caused the water to accelerate upwards through 2.5 feet (0.76 m) of air before it struck the vessel head at 160 feet per second (49 m/s) with a pressure of 10,000 pounds per square inch (69,000 kPa). This pressure wave caused the 26,000 pounds (12,000 kg) steel vessel to jump 9 feet 1 inch (2.77 m) into the air before it dropped into its prior location.
Water hammer has caused accidents and fatalities, but usually damage is limited to breakage of pipes or appendages. An engineer should always assess the risk of a pipeline burst. Pipelines transporting hazardous liquids or gases warrant special care in design, construction, and operation. Hydroelectric power plants especially must be carefully designed and maintained because the water hammer can cause water pipes to fail catastrophically.
The following characteristics may reduce or eliminate water hammer:
One of the first to successfully investigate the water hammer problem was the Italian engineer Lorenzo Allievi.
Water hammer can be analyzed by two different approaches-rigid column theory, which ignores compressibility of the fluid and elasticity of the walls of the pipe, or by a full analysis that includes elasticity. When the time it takes a valve to close is long compared to the propagation time for a pressure wave to travel the length of the pipe, then rigid column theory is appropriate; otherwise considering elasticity may be necessary. Below are two approximations for the peak pressure, one that considers elasticity, but assumes the valve closes instantaneously, and a second that neglects elasticity but includes a finite time for the valve to close.
The pressure profile of the water hammer pulse can be calculated from the Joukowsky equation
So for a valve closing instantaneously, the maximum magnitude of the water hammer pulse is:
where ΔP is the magnitude of the pressure wave (Pa), ρ is the density of the fluid (kg m), a0 is the speed of sound in the fluid (ms), and Δv is the change in the fluid's velocity (ms). The pulse comes about due to Newton's laws of motion and the continuity equation applied to the deceleration of a fluid element.
As the speed of sound in a fluid is , the peak pressure depends on the fluid compressibility if the valve is closed abruptly.
When the valve is closed slowly compared to the transit time for a pressure wave to travel the length of the pipe, the elasticity can be neglected, and the phenomenon can be described in terms of inertance or rigid column theory:
Assuming constant deceleration of the water column (dv/dt = v/t), gives:
The above formula becomes, for water and with imperial unit: P = 0.0135 V L/t. For practical application, a safety factor of about 5 is recommended:
where P1 is the inlet pressure in psi, V is the flow velocity in ft/sec, t is the valve closing time in seconds and L is the upstream pipe length in feet.
When a valve with a volumetric flow rate Q is closed, an excess pressure ΔP is created upstream of the valve, whose value is given by the Joukowsky equation:
In this expression:
The hydraulic impedance Z of the pipeline determines the magnitude of the water hammer pulse. It is itself defined by:
The latter follows from a series of hydraulic concepts:
Thus, the equivalent elasticity is the sum of the original elasticities:
As a result, we see that we can reduce the water hammer by:
The water hammer effect can be simulated by solving the following partial differential equations.
where V is the fluid velocity inside pipe, is the fluid density and B is the equivalent bulk modulus, f is the friction factor.
Column separation is a phenomenon that can occur during a water-hammer event. If the pressure in a pipeline drops below the vapor pressure of the liquid, cavitation will occur (some of the liquid vaporizes, forming a bubble in the pipeline, keeping the pressure close to the vapor pressure). This is most likely to occur at specific locations such as closed ends, high points or knees (changes in pipe slope). When subcooled liquid flows into the space previously occupied by vapor the area of contact between the vapor and the liquid increases. This causes the vapor to condense into the liquid reducing the pressure in the vapor space. The liquid on either side of the vapor space is then accelerated into this space by the pressure difference. The collision of the two columns of liquid (or of one liquid column if at a closed end) causes a large and nearly instantaneous rise in pressure. This pressure rise can damage hydraulic machinery, individual pipes and supporting structures. Many repetitions of cavity formation and collapse may occur in a single water-hammer event.
Most water hammer software packages use the method of characteristics to solve the differential equations involved. This method works well if the wave speed does not vary in time due to either air or gas entrainment in a pipeline. The Wave Method (WM) is also used in various software packages. WM lets operators analyze large networks efficiently. Many commercial and non commercial packages are available.
Software packages vary in complexity, dependent on the processes modeled. The more sophisticated packages may have any of the following features:
Marcus Vitruvius Pollio described in the 1st century B.C. the effect of water hammer in lead pipes and stone tubes of the Roman public water supply. Water hammer was exploited before there was even a word for it: In 1772, Englishman John Whitehurst built a hydraulic ram for a home in Cheshire, England. In 1796, French inventor Joseph Michel Montgolfier (1740–1810) built a hydraulic ram for his paper mill in Voiron. In French and Italian, the terms for "water hammer" come from the hydraulic ram: coup de bélier (French) and colpo d'ariete (Italian) both mean "blow of the ram". As the 19th century witnessed the installation of municipal water supplies, water hammer became a concern to civil engineers. Water hammer also interested physiologists who were studying the circulatory system.
Although it was prefigured in work by Thomas Young, the theory of water hammer is generally considered to have begun in 1883 with the work of German physiologist Johannes von Kries (1853–1928), who was investigating the pulse in blood vessels. However, his findings went unnoticed by civil engineers. Kries's findings were subsequently derived independently in 1898 by the Russian fluid dynamicist Nikolay Yegorovich Zhukovsky (1847–1921), in 1898 by the American civil engineer Joseph Palmer Frizell (1832–1910), and in 1902 by the Italian engineer Lorenzo Allievi (1856–1941).
العربية طرق مائي ▪ Bosanski Prijelazno strujanje ▪ Català Cop d'ariet ▪ Deutsch Druckstoß ▪ Eesti Hüdrauliline löök ▪ Español Golpe de ariete ▪ Esperanto Premfrapo ▪ فارسی ضربه قوچ ▪ Français Coup de bélier ▪ 한국어 수격작용 ▪ Հայերեն Հիդրավլիկական հարված ▪ Hrvatski Vodni udar ▪ Ido Arieto hidraulikala ▪ Italiano Colpo d'ariete ▪ עברית הלם מים ▪ Қазақша Гидравликалық соққы ▪ Latviešu Hidrauliskais trieciens ▪ Magyar Vízütés ▪ Nederlands Waterslag (hydraulica) ▪ 日本語 水撃作用 ▪ Norsk bokmål Trykkstøt ▪ Polski Uderzenie hydrauliczne ▪ Português Golpe de aríete ▪ Română Curgere nestaționară ▪ Русский Гидравлический удар ▪ Українська Гідравлічний удар ▪ Tiếng Việt Búa nước ▪ 中文 水錘作用 ▪