Good practice is a minimum 0.3 of inlet diameter from the sump floor to the inlet of the pump, if more than this it is ok. Question, how do you know it is cavitating?It seems unlikely that pumps of this nature would be suffering cavitation unless the inlets are blocked.I would be checking flow rates and heads against the pump curve.    
 

He also said the operators said the pumps are "sometimes" cavitating.  He should correlate "when it cavitates" and relate it to water level in the wetwell.  It may not "cavitate" when the well is flooded due to storm conditionsHe did say that "...The owners are telling me cavitation is happening because of vortexing. They think this because of the fact that the inlet effluent 'waterfalls' into the wet well. It falls probably about a foot or so into the well. There is a steel grading to help disrupt the flow, but there are still reports of cavitation. I have visited the site a few times and watched the wet well for a Screw pumps while, never once saw any vortexing. So I am suspicious of vortexing..."  
 

"One other tid-bit to note: I was looking over the drawings for the wet well and the drawings indicate that the suction virtually rests on the ground in the wet well (hard to confirm if this is actually how it was built). This seems to me like a terrible design, or is this normal? "Having the suction at the bottom of the wet well is proper design.  This prevents vortexes forming underneath the suction pipe.  I ran across this very situation many years ago in a very very large raw sewage pump station in a very very large municipal wastewater treatment plant.  The suction was elevated and hydraulic modelling showed vortexing occuring underneath the raised suction.

2011-09-23

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