The vapour filled bubbles are called cavity and presumed to form around a dust nuclei, which may be present in the liquid. These bubbles are carried along by the flow to high pressure .regions, where the vapour condenses and bubbles suddenly collapse. The surrounding liquid then rushed from all direction collides at the CG and a high pressure of around 1000 atm is generated. The entire phenomenon of formation, growth, travel and collapse of bubbles is referred as cavita tion.

Effect of cavltatlon

Considerable cracking or rattling may be produced leading to objectionable variations.

Pressure velocity distribution in this effected region. is greatly destroyed.

Due to the Vibration and pitting, the efficiency of hydraulic machinery will be reduced. .

The metallic and concrete walls containing the water stream may be

severely damaged.

The cavita tion is harmful as it affects the hydraulic machines in following ways; > Loss of material due to pitting.

Fitting and erosion of metal parts.

Reducing the actual volume of liquid due to formation of bubbles

Knocking vibration andhoise the machine parts.

The cavitation effects can be reduced by following ways;

Setting the turbine near the tail race.

The velocity’should be controlled so as not to exceed a safe limit.

Making the runner blade and interior. face of draft tube from especially

chosen resistant material.