Water Hammer
Each time the rate of flow in a pipe system
changes a pressure wave is created. This can result in such
large variations in pressure as to cause a water hammer which
may exceed the permissible load on the pipes (force acting
upon it). (see Fig 9)
Inside the pump systems, changes in the rate of flow
can occur in the case of e.g. power cut, sudden blockages,
rapid closing of valves etc. If this takes place at one end
of a long pipe line, the pressure waves will rebound from
the other end and may cause damage when they return to their
point of origin – particularly if this end is completely
shut off and the increased pressure cannot escape. The risk
of water hammer may necessitate the installation of devices
to minimize the effect of the pressure waves and will often
require special operating instructions.
Rapid filling of a pressure pipeline and variations
between trapped air masses may also cause sharp rises in pressure.
Pipelines should therefore be designed to allow bleeding wherever
required and filling speed should be kept low.
The speed of the pressure wave depends on
the pipe material, wall thickness and the substance carried
by the pipe.
All known materials show to a varying degree
a tendency to suffer fatigue when subjected to dynamic forces.
Incidences of water hammer will therefore reduce the service
life of the pipes the extent of such reduction depending upon
the composition of the dynamic forces i.e.:
- The duration of the pressure rise
- The maximum value of the above compared
with the level of the static mean stress.
- The time interval between rises in pressure
(frequency) etc.
The following permissible pressure rises
apply for pressure pipes used in water supply systems:
Where pressure rises occur infrequently,
e.g. pressure testing, power failure etc., the permissible
maximum pressure may exceed the nominal pressure by 50%.
For PVC pipes it furthermore applies that
where pressure rises occur frequently the permissible maximum
pressure may exceed the nominal pressure by 25%, but such
pressure rises must not result in pressure amplitude higher
than 30%.
In case of doubt please .
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