On these straight line pump curves it's sometimes hard to tell. Estimate it at about where 75% to 85% of motor power rating is being consumed, estimating the efficiency of the pump at that point of around 75%. It might be a long shot, but it's the only shot you've got without the real curve.
"The kW in the PDF are much higher" I forget exactly, but I think starting currents can be maybe 2-4 times higher than run currents. EE is my weak suit.
Off-BEP ops with a VFD isn't the same problem for a pump varying flow while running at constant speed at a fixed BEP, as a lot of the forces and pressures retain balance as the BEP moves more closely with the change in flowrate. The problems encountered with VFDs manifest themselves on the power side.
In engineering, if you've really got a handle on what you are doing, you will always realize that trade-offs are being made and that the trade-offs come at a definate cost... somewhere. You've got to figure out where the penalties are being levied and how big they are. "No free lunches" and "You can't fool Mother Nature" are the reoccuring themes.
At 100% BEP, a control valve is probably fully open and no efficiency is lost there, but at 100% flow with the VFD on, you're paying maybe 5% eff loss just to have it there. That's why I say 85-100% BEP flow is better done with a control valve, as actual power losses are less than, or very close to, the VFD's 5% loss, - possible problems with power line harmonics and bearing corrosion. From 50% to 85% is VFD territory, provided you have available head when speed is lower (not for example as for Boiler Feed Water systems, etc. where head requirements remain high even for small flows, or for longer pipelines with + elevation changes). For a VFD to be economical, you have to spend some major percentage of pump time at 50-85% flowrates, which in that case may mean that you've set your system flowrate target too high and it probably should be lowered, making the control valve the more optimal solution once again. Lower than 50% BEP many system start to fall apart due to multiple efficiency reductions and not many systems work well at those flowrates at all, aside from cooling water closed circuits, etc. See my drift? A VFD on a submersible well pump is a real waste of money, as the high constant head required to reach the surface, even at low flows, will blow that idea. But I'm sure that somewhere they can be useful, I just don't often see too many of those cases in my pipeline work. If I need low to no flow, I'll pump to a tank at BEP flowrate, then shut the pipeline down until the tank starts getting low enough to pump full BEP flowrate into it again. That's he best way to save money. In any case, my usual objective is to pump as much as possible from point A to point B and actually neither a control valve or a VFD helps me out much there. I've generally lived a pretty long life without much use for them.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
