Most oversizing happens for an understandable reason: the duty point on the enquiry is the duty point somebody hopes for, and the pump chosen against it is the pump that meets it exactly. Then the system resistance turns out lower than calculated, the pump runs out to the right of its curve, and it spends its life cavitating at a flow nobody asked for.
Start with the system, not the pump
A pump curve tells you what the machine can do. A system curve tells you what your pipework demands. The operating point is where the two cross, and it moves whenever a valve throttles, a filter blinds or a tank level changes. Ask for both curves plotted together before you approve a selection.
Check the margin, not just the point
Three numbers decide whether a selection survives its first year: how far the duty sits from best efficiency, how much NPSH margin exists at the highest expected flow, and what the shut-off head does to the downstream fittings. A selection that looks efficient at the rated point and has no NPSH margin at run-out is not a selection, it is a warranty claim in waiting.
What to send us
Flow and head get you a quotation. Flow, head, the system curve, the medium with its temperature and solids content, and the suction arrangement get you the right pump. The second list is not much longer, and it is the difference between a machine that runs for a decade and one that is back on the bench in a season.
