Introduction: Water hammer is usually recognized by the banging or thumping noise that is heard when valves are shut off. Although this is an easy way to recognize the problem, water hammer doesn’t always make these telltale noises. Water hammer occurs when the flow of moving water is suddenly stopped by a closing valve. This sudden stop causes the whole column of water behind the valve to slam into the valve, and itself, like a freight train crashing into a wall. The tremendous spike of pressure that is caused is called water hammer, and it not only acts like a tiny explosion inside pipes, it can be just as destructive.
Effects and mitigation: 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 plumbing this is experienced as a loud bang resembling a hammering noise. Water hammer can cause pipelines or equipment such as ﬁlter housing 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 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 .
Dynamic Equations: The water hammer effect can be simulated by solving the following partial differential equations where V is the ﬂuid velocity inside pipe, ρ is the ﬂuid density and Bm is the equivalent bulk modulus, f is the friction factor.
Possible causes: Sudden valve closure, Pump failure , 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).
Software: Most water hammer software packages use the method of characteristics to solve the deferential 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 noncommercial 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
Mitigating measures: Water hammer has caused accidents and fatalities, but is usually less threatening. In many cases damage is limited to breakage of pipes or appendages. An engineer should always assess (at least qualitatively) risk of a pipeline burst. Pipelines with hazardous goods should always receive special attention and should be thoroughly investigated.
The following characteristics may reduce or eliminate water hammer:
- Low ﬂuid velocities.
- Slowly closing valves. Toilet ﬂush valves are available in a quiet ﬂush type that closes quietly.
- High pipeline pressure rating (expensive).
- Good pipeline control (start-up and shut-down 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 water towers, but are pressurized. 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 meters on large pipelines. They come in many shapes, sizes and conﬁgurations. Such vessels often are called accumulators.