What is: Water Hammer.

Hydraulic shock in pipelines is commonly referred to as water hammer.
However, water hammer is only one result of the harmful effects created by
hydraulic shock. Hydraulic shock occurs when fluid flowing through a pipe
is subjected to a sudden, rapid change in velocity. The pressure wave
generated travels back and forth within the piping until the energy is
dissipated.



When the tubes are not adequately secured or supported, or the tube runs
are particularly long, these rebounding pressure waves cause the tubes to
vibrate and hit against the supporting structure causing the noise referred
to as water hammer. The noise is objectionable but not, in itself, inherently
dangerous. Noise may not be as noticeable in plastic pipes but damaging
shock stresses are still imposed on pipes and fittings. Hydraulic shock can
cause damage to joints, taps, valves, meters and even to the pipeline itself.
Water hammer effects can be generated by foot action taps, solenoid valves
in clothes and dishwashing machines, quick acting quarter-turn taps and
pumps. Tube should be fixed in position securely at the spacings shown in
the table on page 44 to minimise noise associated with hydraulic shock.
Water hammer effects may be minimised by reducing the velocity of the
water flow in the tubes, reducing the inlet pressure of the water in the system,
closing manually operated taps slowly and by fitting slow acting solenoid
valves.
In certain cases it may be necessary to fit a water hammer arrestor as close
as possible to the source of the problem. These devices are available from
plumbing merchants.