Cavitation is where the pressure in a fluid falls to vapour pressure, therby instantaneously turning the liquid into its vapour form and forming bubbles. Typically the noise you hear isn't the bubbles forming, its when the pressure comes back to VP and these same bubbles collapse in on themselves with a bang. If you get bubbles forming near the leading edge of a lifting surface then the liquid does not flow over the aerofoil as it is supposed too, it just makes streams of bubbles in the liquid and makes minimal lift. Aerofoil doesn't work and so the propellor doesn't work. Often occurs with outboard motors when the leading edges of the propellor gets damaged, by dings, gouges, dents etc.
Same theory will apply to impellers in a centrifugal pump. Too much restriction on the inlet side, as Butcher said, and the pressure at the inlet side of the impeller drops as the pump is having to 'suck' liquid in.
Having a res on the inlet of the pump reduces as much as possible the restriction on the inlet, but may slightly increase total loop restriction depending on its design.
Not sure about Net Postive Suction Head. Thought it was a good thing, as my understanding of it was that the Suction pressure was Net Positive, ie above VP, so reducing the likelyhood of cavitation occurring. Thought zero positive suction head was where bubbles start forming and as pressure on the inlet side falls below NPSH you get more and more bubbles forming.
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