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Water Hammer; Noisy Plumbing and banging pipes. 


Every time you flush the toilet, use the washing machine, dishwasher or open certain taps, you hear a loud rapid banging noise? You may have water hammer. 


Water hammer often only occurs when taps (or flow rates) are open at a certain flow speeds. Obscure and often hard to diagnose but a very common problem that is unusually easily fixed, yet it can be a massive and loud problem. Pipes especially copper or other steel pipes can be very good sound conductors.

Water hammer usually occurs in the following situations.


Loose pipes in close proximity to fixed wooden materials*

  1. Typically when the taps are closed especially if they are closed quickly the change of pressure causes the pipe to move, wobble or shake. If the pipe is not fixed, clipped in often enough, or is too close to a the wooden material a loud bang is heard, the sound conduction of the pipes and wood exacerbate the problem.

  2. When the toilet is flushed or when certain taps are open at certain flow rates the loose pipe are vibrating just enough from the flow to cause a contact, and usually very loud and annoying rapid, machine gun type noise. This scenario is rarer but still a common problem, and many plumbers can miss the cause. Usually locating the offending wooden material, cutting out a small piece by the pipe, or making the hole which the pipe passes though bigger will solve the problem. This is not necessarily straight forward if the pipe is between the floor and ceiling though, and if extra fixings can be attached to the pipe work this should be considered. *(such as floor boards, joists, ply wood under tiling in bathrooms and kitchens, kitchen units and bathroom vanity units)

Fast Changes of pressure.

  1. Fast changes of pressure such as shutting of taps or fast close cistern valves (toilet or storage tank float valves). Can result in surges of pressure producing noise usually a thump noise from the plumbing system. Changing the valves can solve the problem when that is the source  Half turn taps and lever taps can be changed to more traditional taps if necessary though simply using the taps without fast closing them solves the problem.

  2. Combination boilers, tank-less water heaters, or instantaneous water heaters can experience noise with fast pressure changes from, rapidly opening and closing taps. especially as the appliances contain more components which can add to the noise. Diverter valves in combination boilers can also be a problem, producing noise when closing or opening, this can be a sign the part needs to be replaced.

  3. Faulty values, often cistern valves, opening and closing rapidly can produce extremely loud machine gun type noises.

Thermal expansion.

Thermal expansion on hot water and heating pipes, as well as radiators can be another source of annoying noises, though this is not directly related to water hammer. Lagging of pipes can reduce expansion and contraction rates and therefore noise, and this saves energy so you will see a return on any investment.


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See below this useful though technical extract from an article in wikipedia

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Water hammer

From Wikipedia, the free encyclopedia


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For a hammer powered by water, see Trip hammer.

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). 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's 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 and other features.

Rough calculations can be made either using the Joukowsky equation,[1]or more accurate ones using the method of characteristics.


Cause and effect

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 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 provide a cushion to absorb the force of moving water to prevent damage to the system.

In hydroelectric generating stations, the water travelling along the tunnel or pipeline may be prevented from entering a turbine by closing a valve. But if there is, say, 14 km of tunnel of say 7.7m diameter, full of water travelling at say 3.75 m/sec,[2] that represents a very large amount of kinetic energy that must be arrested. This is frequently achieved by a surge shaft[3] open at the top, into which the water flows. As the water rises up the shaft, converting kinetic energy into potential energy, it decelerates the water in the tunnel. 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, water hammer may occur when a dishwasherwashing 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 and 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).

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