Causes of Water Hammer
Like any other moving fluid, flowing water has momentum. When subjected to a sudden change in flow, shock waves propagate through the system. This occurrence is referred to as ‘water hammer’. Flow changes can occur due to the operation of valves, the starting and stopping of pumps, or directional changes caused by pipe fittings. Other causes may be the rapid exhaustion of all air from the system or the recombination of water after water column separation.
When sudden changes in flow occur, the energy associated with the flowing water is suddenly transformed into pressure at that location. This excess pressure is known as surge pressure and is greater with large changes in velocity.
Water hammer is usually caused in high pressure (e.g. mains pressure) water systems either when a tap is turned off quickly, or by fast-acting solenoid valves, which suddenly stop the water moving through the pipes and sets up a shock wave through the water, causing the pipes to vibrate and ‘shudder’.
Water hammer will be made worse by having worn valves and pipework inadequately supported.
There are five probable areas to look at for the cause of water hammer and a number of possible cures – the appropriate cure will depend upon the actual cause and the installation. If the water hammer only started after some work was done on the plumbing system, start by looking at those areas; however, any change to a plumbing system may start water hammer in another area.
Probable Sources of Water Hammer
- Inadequately secured pipework – more likely to cause water hammer after new work has been done; if water hammer first occurs without new work having been done, check the other possible cures first.
- Ball and float valves.
- Fast acting valves.
- Worn stop valves.
- Trapped air.
Ensure that pipework is clipped, secured and supported at regular intervals using pipe clips of the appropriate size.
Ball /Float Valves
Water hammer can result from ripples inside open water tanks where the water level is controlled by a ball/float valve – the ripples being caused by the inflow of water with the result that the valve float ‘bobs’ up and down thus repeatedly opening and closing the valve. This repeated opening and closing of the valve sets up shock waves which reverberate along the pipework causing the water hammer effect.
If the water tank is plastic, a metal reinforcing plate fitted on the side of the tank where the float valve is installed will reduce the flexing of the tank.
Other than the above, ripples in the water will usually only cause problems if the ball/float valve is worn or if the valve has a low pressure valve nozzle fitted when the water supply is high pressure. Ensure that the washers and diaphragms in ball/float valves are not worn and that the appropriate valve nozzle is fitted. The latter should not be the problem if the water hammer started some time after the valve was fitted.
On small tanks, fitting a Torbeck cistern valve is another possible cure as it will prevent ripples on the water surface, so the valve will close cleanly.
Some electrically operated solenoid valves stop the flow of water instantaneously, so possibly setting up the shock wave through the pipework to cause the water hammer. Flexible hose attaching the valve to the water supply may be enough to absorb any shock in the pipework.
Stop valves (stopcocks) and taps can cause water hammer if they have loose gland packing and/or worn washer jumpers. Stop valves will generally be open when the water hammer shock wave travels through the pipework and the shock wave could well ‘rattle’ the valve handle and a loose jumper.
While it is possible to tighten the gland packing and replace a loose jumper, the easiest way to cure the problem is to replace the stop valve.
Problems associated with air entrapment can be minimized by preventing air from accumulating in the system. This can be accomplished by using air-relief valves positioned at the high points of the piping system. In areas of relatively flat terrain these should also be used in the vicinity of the pump discharge, near the middle of the line, and at the downstream end of the line.
Empty pipelines should be filled as slowly as possible to allow entrapped air to escape.
All pipework should be sized so that the flow velocity is not more than 1.5m/s.
Surge arrestors (relatively simple devices such as small pressure tanks or buffer vessels, which can absorb shock waves) can be installed at strategic positions in the pipeline to minimize the effects of water hammer in the system. Surge arrestors should be installed as close to the source of the problem as possible.