What is cavitation?
Cavitation is the localized formation and subsequent collapse of cavities, or
bubbles, in a liquid. The formation of cavities (or bubbles) is induced by flow
separation, or non-uniform flow velocities, inside a pump casing that result in
localized low pressure points.
(This is analogous to the effect of air flowing against the leading edge of an
airplane wing where flow separation and difference in air velocities reduce the
pressure at the top of the wing causing the plane to be air-lifted.)
As the liquid moves from a low pressure point to a high pressure point inside the
casing, the cavities collapse resulting in increased vibration and noise. The pump
is said to be cavitating. Depending on its severity and duration, cavitation can
cause pump damage, failure, or impairment of performance..
Cavitation is usually caused by insufficient NPSHA. When there is insufficient
positive pressure to suppress them, cavities form quickly when the liquid passes
through a localized low pressure point in the casing. Cavitation is also likely to
occur when the liquid has a high amount of entrained, or dissolved, gas. In such
service a higher NPSH margin must be maintained for vapor suppression..
Cavitation can occur at both the suction and discharge sides of an impeller. Most
suction side cavitation is caused by insufficient NPSHA, whereas most discharge
side cavitation is caused by discharge flow recirculation when the pump operates
at low flow whereby a higher amount of liquid recirculates back to suction. The
flow recirculation creates localized low pressure points causing the formation of
cavities at the discharge side of the impeller. This can cause what is referred to
as discharge cavitation.
C: basics, operation
F: cavitation definition