8 Pumps
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chrome-nickel steel. But cavitation leading to erosion is mostly prevented. Ship
propellers are exposed to the same phenomenon, but the difference with pumps is
that the vapour bubbles can easily be kept away from material surfaces. So cavitation does not necessarily result in cavitation erosion.
Vapour bubbles hardly affect the flow with incipient cavitation (beginning of
cavitation), so that the performance characteristics do not change. Developed cavitation, i.e. with significant vapour bubble zones, affects the flow. The through-flow
velocity is locally enlarged by the obstruction by the vapour bubbles and the flow
direction is changed. Both effects result in higher losses. Pump efficiency and head
decrease with developed cavitation.
8.1.2 Types of Cavitation
With flow over free-standing hydrofoils or over convex bends in walls, incipient
cavitation mostly occurs at some distance from the material walls. Several types
may be distinguished: bubble cavitation, sheet cavitation, cloud cavitation and
vortex cavitation. Bubble cavitation refers to individual bubbles within the fluid.
These bubbles originate around nuclei of dissolved air mixed with vapour. Bubbles joining together to a larger zone are termed sheet or cloud cavitation, depending on whether the zone adopts the form of a thin surface or rather of a full space.
Vortex cavitation arises within the low-pressure core of tip vortices. In that case,
bubbles join to lines following the vortex core. Cavitation within the tip vortex of
a ship propeller blade is a typical example. With cavitation by bubbles, whether
or not joining in lines, sheets or clouds, the presence of cores with gas and vapour
is crucial. Gases, as air, do not completely dissolve in water. Cores, called nuclei,
are generated due to surface tension. The pressure within a core exceeds the pressure in the surrounding fluid because of surface tension. The smaller the radius of
a (spherical) core, the larger the pressure difference is. Gas in the core is mixed
with an amount of vapour from the fluid. Cavitation starts when cores strongly
expand due to evaporation of the fluid. Therefore, due to surface tension, the pressure around a core must be below the vapour pressure, as vapour pressure, by definition, is the pressure with which the liquid evaporates in equilibrium with the vapour, i.e. with a very large contact surface, so without the effect of surface tension.
The required pressure difference under vapour pressure approximately amounts,
at 20 °C in water [4], to: 1 bar with core radius 1 mm, 0.1 bar with core radius
10 mm, 0.01 bar with core radius 100 mm. In the presence of only small cores,
tension must be exerted on the liquid before cavitation bubbles can be generated.
Cores are rather large in industrial water, with the radius of the largest cores typically in the 30 mm order. The required pressure difference below the vapour pressure is then not very high, about the magnitude of the vapour pressure itself. One
could say that cavitation occurs with pressure in the liquid approximately equal to
zero pressure. The exact pressure within the liquid at which cavitation occurs is
termed the critical pressure (critical regarding cavitation). This value applies under
284
chrome-nickel steel. But cavitation leading to erosion is mostly prevented. Ship
propellers are exposed to the same phenomenon, but the difference with pumps is
that the vapour bubbles can easily be kept away from material surfaces. So cavitation does not necessarily result in cavitation erosion.
Vapour bubbles hardly affect the flow with incipient cavitation (beginning of
cavitation), so that the performance characteristics do not change. Developed cavitation, i.e. with significant vapour bubble zones, affects the flow. The through-flow
velocity is locally enlarged by the obstruction by the vapour bubbles and the flow
direction is changed. Both effects result in higher losses. Pump efficiency and head
decrease with developed cavitation.
8.1.2 Types of Cavitation
With flow over free-standing hydrofoils or over convex bends in walls, incipient
cavitation mostly occurs at some distance from the material walls. Several types
may be distinguished: bubble cavitation, sheet cavitation, cloud cavitation and
vortex cavitation. Bubble cavitation refers to individual bubbles within the fluid.
These bubbles originate around nuclei of dissolved air mixed with vapour. Bubbles joining together to a larger zone are termed sheet or cloud cavitation, depending on whether the zone adopts the form of a thin surface or rather of a full space.
Vortex cavitation arises within the low-pressure core of tip vortices. In that case,
bubbles join to lines following the vortex core. Cavitation within the tip vortex of
a ship propeller blade is a typical example. With cavitation by bubbles, whether
or not joining in lines, sheets or clouds, the presence of cores with gas and vapour
is crucial. Gases, as air, do not completely dissolve in water. Cores, called nuclei,
are generated due to surface tension. The pressure within a core exceeds the pressure in the surrounding fluid because of surface tension. The smaller the radius of
a (spherical) core, the larger the pressure difference is. Gas in the core is mixed
with an amount of vapour from the fluid. Cavitation starts when cores strongly
expand due to evaporation of the fluid. Therefore, due to surface tension, the pressure around a core must be below the vapour pressure, as vapour pressure, by definition, is the pressure with which the liquid evaporates in equilibrium with the vapour, i.e. with a very large contact surface, so without the effect of surface tension.
The required pressure difference under vapour pressure approximately amounts,
at 20 °C in water [4], to: 1 bar with core radius 1 mm, 0.1 bar with core radius
10 mm, 0.01 bar with core radius 100 mm. In the presence of only small cores,
tension must be exerted on the liquid before cavitation bubbles can be generated.
Cores are rather large in industrial water, with the radius of the largest cores typically in the 30 mm order. The required pressure difference below the vapour pressure is then not very high, about the magnitude of the vapour pressure itself. One
could say that cavitation occurs with pressure in the liquid approximately equal to
zero pressure. The exact pressure within the liquid at which cavitation occurs is
termed the critical pressure (critical regarding cavitation). This value applies under