Follow that link and the pages before and after to learn about high suction energy pumps.
One basic concept:
High suction energy impeller designs are so efficient at passing large volumes of fluid, that the NPSHr 3% values for those pumps do not mean the same thing as NPSHr 3% values for other pumps. By the time the NPSHr 3% value is reached in testing, large amounts of incipient cavitation are occurring inside the pump, sufficient to quickly damage the pump. Therefore, large margins of NPSHa over NPSHr are required to use these pumps successfully, you will see margins referenced from 5-20 times or more, NPSHa over NPSHr.
As noted by PUMPDESIGNER, the higher suction energy pumps have additional problems with cavitation. These problems can require high NPSH margins to avoid incipient cavitation. But even with high NPSH margins, there can be cavitation problems at low flow. Pumps in this category should be carefully evaluated for minimum flow requirements. The pump manufacturer will provide a recommended minimum flow that may not be conservative enough. We have a set of high suction energy pumps operating with more than 30 feet of excess NPSH and they still have cavitation damage to the impellers every time we pull one out. They are operating at about 80 percent of BEP flow but this still doesn't seem to be enough.
JJPellin is exactly correct, although the pumps pass fluid efficiently, they hate to run much below BEP, they can vibrate and have serious recirculation cavitation.
