CAV I TAT I O N
W H AT I S CAV I TAT I O N
• The rapid vaporization and recondensation of liquid as
it briefly flows through a region of low absolute pressure
is called cavitation.
• Does not happen in gasses since gas does not change
state in low pressure.
• Cavitation can cause serious damage.
• The temporary low pressure results due to temporary high
velocities.
• At a given location in a liquid flow where no energy is added or
removed, if the velocity head increases, there must be a
corresponding decrease in the pressure head. As long as there is
some liquid present to evaporate, there is a minimum absolute
pressure possible as called vapor pressure.
• Vapor pressure depends on the liquid and the temperature and is
usually lower than the atmospheric pressure.
(P crit) abs = Pv
(P crit) abs =Patm+ (P crit) gage
(P crit) gage= -(Patm-Pv)
Where;
(P crit) gage= minimum possible gage pr
(P crit) abs= minimum possible abs pr
Patm= absolute atmospheric pr
Pv= absolute vapor pr
• The gage pressure head in a flowing liquid can be negative, but no more
negative than Patm-Pv
• Same equation can be expressed in terms of pressure head by dividing all the
terms by gamma
• If at any point in a liquid the local velocity is so high that the pressure falls to its
vapor pressure, the liquid will vaporize (or boil) and bubbels of vapor will form.
• As the fluid moves on into a region of higher pressure, the bubels of vapor will
suddenly condense, in other words collapse or implode.
• When this occurs adjacent to solid walls, the collapse begins as a jet of liquid
entering the bubble from the side opposite the wall.
• The implosion heats the liquid immediately surrounding the cavity to about 2100
C for less than a microsecond.
• Although the jets are very small, they occur continuously with a high frequency,
so combined with the high temperatures and the shock waves caused by bubble
collapse, they may damage the wall material.
• Such action often severely and quickly damages turbine runners, pump
impellers because it rapidly makes holes in the metal.
• The damaging effect is called pitting.
• Cavitation may cause drop in efficiency of the machine or propeller or other
devices. It can also cause undesired cavitation noise and vibrations.
• In order to avoid cavitation it is necessary to keep the absolute pressure above
vapor pressure at all points.
1. Place the device below the intake level so that the liquid flows to it by gravitational force
rather than suction.
2. Design the machine in a way that there are no local velocities high enough to cause such low
pressures.
3. Admit atmospheric air in low pressure zones.
