Water hammer

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Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused by the kinetic energy of a fluid in motion when it is forced to stop or change direction suddenly. It depends on the fluid compressibility where there are sudden changes in pressure. For example, if a valve is closed suddenly at an end of a pipeline system a water hammer wave propagates in the pipe. Moving water in a pipe has kinetic energy proportional to the mass of the water in a given volume times the square of the velocity of the water.

\mathrm{kinetic\ energy} \propto \mathrm{mass} \times \mathrm{velocity}^2

or

\mathrm{kinetic\ energy} \propto \mathrm{volume} \times \mathrm{velocity}^2

For this reason, most pipe-sizing charts recommend keeping the flow velocity at or below 5 ft/s (1.5 m/s). If the pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving forward with some velocity, building up a high pressure and shock waves. In domestic plumbing this is experienced as a loud bang resembling a hammering noise. Water hammer can cause pipelines to break or even explode if the pressure is high enough. Air traps or stand pipes (open at the top) are sometimes added as dampers to water systems to provide a cushion to absorb the force of moving water in order to prevent damage to the system. (At some hydroelectric generating stations what appears to be a water tower is actually one of these devices.)

On the other hand, when a valve in a pipe is closed, the water downstream of the valve will attempt 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 and prevent this vacuum from occurring.

In the home water hammer often occurs when a dishwasher, washing machine, or toilet shuts off water flow, resulting in a loud bang or banging sound. A hydropneumatic device similar in principle to a shock absorber called a 'Water Hammer Arrestor' can be installed between the water pipe and the machine which will absorb the shock and stop the banging.

Steam heating systems for buildings may also be vulnerable to water hammer. In a steam system, water hammer most often occurs when some of the steam condenses into water in a horizontal section of the steam piping. Subsequently, steam picks up the water, forms a "slug" and hurls it at high velocity into a pipe elbow, 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 sometimes 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 and draining the system by opening taps at the highest and lowest locations, which restores the air to the traps and then closing the taps and opening the supply.

Contents

[edit] Possible causes

  • Sudden release of air from the system (fire hydrant, air release valve) causes the fluid to suddenly stop moving at the location of the device (as the flow rate is suddenly set to zero (in case of non controlled or oversized air valve) or because there is a much larger head drop for the fluid than for the air (fire hydrant)).
  • Pump trip or start
  • Valve closure or opening
  • 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 F=m*a). This may occur after full pumping station trip or during pump trip of a single pump (last one is often more critical as downstream pressure is maintained, but first one may be as critical if a large air vessel is present).
  • Resonance of appendages (mainly pumps) is a frequent cause of vibrations in the pipeline. Other fittings (bends etc) may sometimes induce vibrations due to shedding of vortices). These fluctuations are often of limited amplitude and will only cause problems if their frequency is close to natural frequencies in the pipeline.

[edit] Software

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. Many commercial and non commercial packages exist today. Software packages vary in complexity, dependent on the processes modeled. The more sophisticated packages may have any of the following features:

  • Multiphase flow capabilities
  • An algorithm for cavitation growth and collapse
  • Unsteady friction (The pressure waves will dampen as turbulence is generated and due to variations in the flow velocity distribution)
  • Varying bulk modulus for higher pressures (water will become less compressible)
  • Fluid structure interaction (The pipeline will react on the varying pressures and will cause pressure waves itself)

[edit] Mitigating measures

Accidents due to water hammer do not often result in casualties, though fatalities may occur. In many cases damage is limited to breakage of pipes or appendages. An engineer should allways asses at least gualitatively the risk of a pipeline burst. Pipelines with hazardous goods should allways receive special attention and should be thorougly investigated. Often accidents will occur that could hardly be foreseen (sudden release of air pockets) or are

  • Low velocities.
  • High pipeline pressure rating (expensive).
  • Good pipeline control (Start-up and ShutDown procedures).
  • Water towers (Used in many drinking water systems) help maintain steady flow rates and trap large pressure fluctuations.
  • Air vessels work in much the same way as the water tower, but are pressurised. They typically have an air cushion above the fluid level in the vessel, which may be regulated or separated by a bladder). Sizes of air vessels may be up to hundreds of cubic metres on large pipelines. They come in many shapes, sizes and configurations.
  • Air valves are often used to remediate low pressures at high points in the pipeline. Though effective, sometimes large number of air valves need be installed. These valves also let air into the system, which is often unwanted.
  • Increase pressure rating of the pipe.
  • UPS or uninterupted power supply is sometimes installed to dampen the initial pressure wave by keeping the system running for some time after a power trip.
  • Pumping station bypass.

[edit] Applications

  • The water hammer principle can be used to create a simple water pump called a hydraulic ram.
  • Leak detection is sometimes done using water hammer.
  • Detection of enclosed air pockets in a pipeline.
  • The US Navy is conducting field trials for mine clearing using water hammer.

[edit] See also

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