Every pump needs a certain amount of pressure on the suction to prevent cavitation...which is bad. When liquid goes through the volute (in centrifigul pumps) it goes from a low volume space to a higher volume space while being flung around by the impeller. This can cause a condition where you have formation and subsequent collapse of tiny bubbles which is commonly know as cavitation. It sounds like marbles or something in your pump while in actuallity your pump is eating itself to a slow death...sometimes not so slow...SO...if you apply a head of water at the suction of the pump you can alleviate this condition but also due to evaporation you can have a make up area so your HX doesn't get air bound (air binding of HX is a lesson for another day)
So there you have it:NPSH, Cavitation, and YOU.
