There may be, on the low-pressure side of the runner, regions in which the pressure falls to values considerably below atmospheric. In a liquid, however, the pressure cannot fall below the vapour pressure at the temperature concerned. If at any point the vapour pressure is reached, the liquid boils and small bubbles of vapour form in large numbers. These bubbles are carried along by the flow, and on reaching a point where the pressure is higher they suddenly collapse as the vapour condenses to liquid again. A cavity results and the surrounding liquid rushes in to fill it. The liquid moving from all directions collides at the centre of the cavity, thus giving rise to very high local pressures (up to 1 GPa). Any solid surface in the vicinity is also subjected to these intense pressures, because, even if the cavities are not actually at the solid surface, the pressures are propagated from the cavities by pressure waves similar to those encountered in water hammer. This alternate formation and collapse of vapour bubbles may be repeated with a frequency of many thousand times a second. The intense pressures, even though acting for only a very brief time over a tiny area, can cause severe damage to the surface. The material ultimately fails by fatigue, aided perhaps by corrosion, and so the surface becomes badly scored and pitted. Parts of the surface may even be torn completely away. Associated with cavitating flow there may be considerable vibration and noise; when cavitation occurs in a turbine or pump it may sound as though gravel were passing through the machine.
Not only is cavitation destructive: the larger pockets of vapour may so disturb the flow that the efficiency of a machine is impaired. Everything possible should therefore be done to eliminate cavitation in fluid machinery, that is, to ensure that at every point the pressure of the liquid is above the vapour pressure. When the liquid has air in solution this is released as the pressure falls and so air cavitation also occurs. Although air cavitation is less damaging than vapour cavitation to surfaces, it has a similar effect on the efficiency of the machine.
Since cavitation begins when the pressure reaches too low a value, it is likely to occur at points where the velocity or the elevation is high, and particularly at those where high velocity and high elevation are combined.
Cavitation is likely to occur on the inlet side of a pump particularly if the pump is situated at a level well above the surface of the liquid in the supply reservoir. For the sake of good efficiency and the prevention of damage to the impeller, cavitation should be avoided.
Applying the energy equation between the surface of liquid in the supply reservoir and the entry to the impeller (where the pressure is a minimum) we have, for steady conditions
p0 /ρg + z1 - hf = pmin /ρg + v12 /2g
Re-arranging the above equation gives
pmin /ρg = p0 /ρg - hf - v12 /2g + z1
For cavitation not to occur
pmin > pv
p0 /ρg - pv /ρg - hf - v12 /2g + z1 > 0
A parameter called Nett Positive Suction Head (NPSH) is defined as
NSPHa = p0 /ρg - pv /ρg - hf + z1
The NPSH available at the inlet flange of the pump can be calculated from the above equation. The pump curves in the pump catalog generally give the NPSH required at each volume flow the pump is required to do. For good pump operation
NPSHavailable > NPSHrequired