LittleInch,
Indeed I didn't think my question would generate this much response.
To give some background, we corporate engineers were visiting a plant when I asked this question. We'd audited many of the water pumping systems at another plant, and wanted to repeat the process. The systems involved are chilled water, CIP water, cooling tower water, and perhaps a couple others. Fairly constant pressure, except for variable friction losses, but great flow variability. Since these pumps were put in, we've reduced our utilities loads greatly, so throughout the company they tend to be oversized. In the original plant audited, the pumps were almost all single speed, running at 100% flowrate, with the unneeded flow recirculating back to the tank (Often 60% of the pump's flow). It was decided to throw some VFDs on, since the flowrates were always going to be variable, as they're driven by batch manufacturing processes.
When I got to the new plant, I found that the plant maintenance manager had already put VFDs on all such pumps. Whether this was part of the original design or a retrofit, I'm not sure. This pump audit was one of many things we were scheduled to do on the trip, so we didn't call ahead to get a handle on the existing situation. Once there, I wondered if there were any other adjustments that could be made to the few pumps in the system that never reached 100% design flowrate, given their variability. So I asked my original question, about impeller size.
Ever since that week, I've been back at the corporate office, a thousand miles away. I'm trying to get access to more pressure and flow data for the systems out there, but it's slow going. From the debate back and forth, I'm getting the feel that there's not dramatic electricity savings available for a pump with varying flow. Once I get access to real time flow and pressure data, I'll go back to the manufacturer to get pump curves and double check to see if a new impeller could pay for itself, or if a control valve would beat a VFD, but our ROI requirements are pretty strict, so I don't think this is going to go anywhere.
Thanks again for all your advice.