Cavitation
Cavitation is caused by the vaporization of liquid prior to the low pressure area of the pump. It is due to insufficient NPSHa, and the consequent implosion (or collapsing) of vapor bubbles as they pass to regions of higher pressure. If the vapor bubbles collapse near an adjacent surface-such as the impeller vane in a centrifugal pump-it is subject to tremendous shock from the inrush of liquid into the cavity left by the bubble. This shock wave removes molecules of metal and the
pump parts look badly corroded and sponge-like.
The energy expended in accelerating the liquid to high velocity in filling the void left by the bubble is a loss and causes a drop in capacity associated with cavitation. The loss in capacity is the result of pumping a mixture of vapor and liquid instead of just liquid.
All conventional centrifugal pumps are subject to the negative effects of cavitation if there is insufficient NPSHa. Water at 70°F [21°C] for example increases in volume about 54,000 times when vaporized and thus even a small amount of cavitation will significantly reduce capacity.
It should be noted that the Discflo pump, which operates on the boundary layer/viscous drag principle, produces a very smooth laminar flow pattern within the pump. Testing has shown this gives minimal pressure drop between the pump inlet and the eye of the pump, so that the resulting NPSH required by a Discflo pump is approximately half to a third of that required by a standard centrifugal pump for the same service conditions. Also, with a Discflo pump, the NPSHr curve is stable all the way to shut off.
By observing the operation of transparent disc pumps, we can demonstrate that there is little to no pre-rotation in the suction end of the pump. The result is extremely low NPSH values. In addition, the layers of fluid near the discs in the disc pump act as a buffer or "shock absorber" to protect the discs against the effects of cavitation and implosions. Even under low NPSH conditions, the Discflo pump suffers little to no wear.
