Answer Posted / shaktisinh zala
When the absolute pressure at some point in a liquid
reaches the vapour pressure corresponding to the liquid
temperature there, bubbles, cavities, form within the
liquid at this point. These cavities contain vapour and
possibly some gases coming out of solution. When the liquid
is in motion and a pressure increase occurs without a
substantial change in temperature, as in the case of flow
through a running impeller, the vapour in the bubbles
cannot be sustained, it condenses very rapidly and
therefore the bubbles collapse quite suddenly, giving rise
to shocks in the nature of implosions. This phenomenon is
called cavitation.
The formation of bubbles gives rise to hydraulic
performance reduction possibly to an unacceptably large
scale,to rough running and possibly to noise. As the vapour
bubbles collapse in an extremely short period of time, in
microseconds, pressure shocks traveling at the local speed
of sound, meet the metallic boundaries at very high local
surface pressure that can be higher than the ultimate
strength or fatigue limit of the material, causing material
break up, pitting and erosion followed by corrosion. The
damage is more extensive if the bubbles implode on the
metallic boundaries.
The pressure of the liquid going into a centrifugal pump
drops as it flows from the suction pipe through the suction
nozzle and into the impeller. The pressure drop is a
function of many factors, including pump geometry,
rotational speed, frictional and hydraulic shock losses,
and rate of flow.
If the pressure at any point within the pump falls below
the vapor pressure of the liquid being pumped, vaporization
and so bubble formation will occur. As the pressure
increases the local pressure is above the vapour pressure
and so the bubbles collapse spontaneously and cavitation
occurs
When cavitation occurs due to insufficient head at the pump
suction
the type of cavitation taking place is termed Normal
Cavitation.
Normal Cavitation results in:-
Pump performance deterioration - reduction in total head
developed,
flowrate reduction, pump efficiency reduction.
Increase in the level of pump vibrations and of noise
level, flowrate
pressure and power input fluctuations if the cavitation
is severe
enough.
Pump impeller and possibly casing pitting and erosion.
In carrying out cavitation tests (constant speed and
flowrate but
decreasing suction head) the point at which the total head
developed falls by a predetermined amount (usually 3%) is
regarded
as the point at which the pump becomes critical
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