Whow! Thank you all so much. Everything you say makes a lot of sense. First of all, it is effluent water in a wastewater treatment plant. Secondly the moment I was asked to help on this project, I requested to see the failed impelers (two out of three were replaced last year), the answer was no impellers, no pictures. So no help here either. I requested some flow/pressure monitoring to determine exactly what the pumps are pumping at what pressure, but client has been kind of slow.
I think I made a mistake saying that it was 48" suction when in fact it is a 36" common suction. There are also three "low pressure effluentpumps" drawing water off the same suction but they have not been a target of this project. There are four pumps in question, three of which run coninuously in parallel. Only one of four pumps has somewhat better suction piping arrangement (several feet of straight line prior to suction flange). Three pumps provide anywhere from 4,000 to 6,000 gpm at measured system pressure around 300 feet TDH. When I took instantenuous measurements of system's flow and pressure it appears that pumps are pumping 30% less flow (for the recorded system pressure) than they should, based on the existing pumps' design curve.