off the curve

[soupsamich]

We have done some testing on a newly installed pump onsite and found that the balancing firm appeared to leave the pump off the end of the curve. I have seen pump curves that show that when a pump goes off the end of the curve it comes back on itself as far as flow goes, but at a lower head. It ends up making the curve look like a bullet. The published curves show only the end of the curve, but not past it. Does anyone have any sources that show this, I cannot remember where I have seen these curves. Thanks

 

[tstead]

Manufacturers limit the end of curve (I am assuming you are talking about centrifugal pumps) based upon the whim of the pump designer. Maybe at those flows, the NPSHr skyrockets. Maybe the radial thrust is too great and will lead to unaceptable operation and may have a good chance of failing during the warranty period.

If you are looking at submersible pumps, maybe the end of curve has been cut off to make the pump look like it is non-overloading. For these pumps, an arbitraty NPSHr is assumed (34 feet or so) and the pump is limited to that. After all, if you are pumping water, you can't pump down too far at those flows before you are guaranteed to cavitate.

Maybe at those flows, the torque on the pump shaft is too high and it might break.

Sure, the pump can theoretically run past the end of curve but if you need to run in that portion of the curve (say, to meet occasional higher flow requriements) then you probably need a larger pump. If the proper size pump is too large for normal operation, then consider using a VFD to meet the normal duty point.

You really don't want to be replacing bearings and seals and an occasional shaft every time you turn abound. Pumps should last a long time and there is no reason why people need to be constantly working on pumps.