In simple terms it is the case that many materials stay as liquids while at atmospheric pressure. At some pressure less than this some/many turn into a vapour. This is essentially the vapour pressure. So if a pump sucks to hard or the liquid in it is unable to fall in behind the moving pump "blade" then the local pressure falls to less than the vapour pressure and fluid at that point vapourises. This means that the pump is trying to pump low density vapour. For one reason or another a pump designed to pump liquid will not properly pump what is effectively "gas". A positive displacement pump will pump, but the output liquid volume will be very low once the displaced vapour condenses back to liquid. A centrifugal pump will not generate enough head to work either because the density of material thrown outwards is low giving low output pressure and/or because cavitation will prevent new fluid from being drawn into the pump - rather like trying to tow a car on a very stretchy tow rope.
PUMP
Re: PUMP
