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| | Fluid transients (Water hammer phenomena) |
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If sudden velocity variations are caused in a pipe system, for example when pumps are shut off or valves are closed, a pressure wave develops which is transmitted in the pipe at a certain velocity that is determined by the type of material and the material thickness. This pressure wave can be detrimental and cause pipe and fittings to rupture. The maximum pressure increase over the existing pressure may be determined by Joukowsky´s formula if a valve is closed very rapidly.
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= pressure in meter water column compared with original pressure.
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= velocity of pressure wave in m/sec
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= change of flow in m3/s
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= acceleration due to gravity
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= pipe area in m2
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However, the maximum over-pressure can be critical. In most pumping situations it is the sub-pressures that are the major concern, since pipes are more sensitive to buckling than bursting due to over-pressure.
Fatigue: in an on-off regulated system, with frequent starts and stops, the risk for fatigue must be taken into account. Most pipe line materials are more sensitive to fatigue through sub-pressure than over-pressure.
If the pressure in some part of the pipe system drops below vapour pressure the pumping liquid is vaporised and vacuum bubbles are formed between liquid columns in the pipeline. Damaging pressure can occur when these cavities collapse.
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Engineering
