A Q-H curve is a map of flow against head for one hydraulic assembly: a casing, an impeller diameter, a speed and a stated test standard. It is not a promise that every point on the printed family of curves is available from the factory quoting you this week. Before you argue about efficiency, find the duty you will actually run and make the bidder mark it on the curve that belongs to the impeller they intend to ship.

Write the duty before you open the PDF

Required flow is the process flow, not the largest number on a brochure. Total dynamic head must include static lift, friction and any control-valve drop you are not willing to ignore. If you only have a model code copied from a nameplate, you do not yet have a duty. Two Chinese plants can stamp the same series letters on pumps whose best efficiency points sit in different places.

Put units on the page. KanBeng normalizes catalog envelopes to m³/h and metres. Many export quotes still mix m³/h, L/s and m³/s on the same PDF. If the curve is in US gallons and feet, convert once, in writing, and keep the original beside it.

Park the duty near BEP, then look at the region

Best efficiency point is where the pump is happiest. Running far to the left (low flow) raises radial loads, seal chamber pressure and vibration. Running far to the right can exceed NPSHr and motor power. Ask for the preferred operating region, not only the BEP number. If the factory cannot mark your duty, they are not yet bidding the machine you think they are bidding.

Impeller diameter is part of the offer

Catalog pages often show a nest of curves for different impeller cuts. The index envelope on KanBeng is the rectangle around those trims. Your purchase order needs one diameter, one speed and one test acceptance. A quote that says “ISW 50-160” without impeller diameter is incomplete.

NPSH is a second curve, not a footnote

NPSHr rises as flow rises. A single NPSHr number at BEP does not protect a duty on the right-hand side of the curve. State the NPSH available in your system and the margin you require. If neither side can compute NPSHa, say so in the RFQ rather than pretending the catalog number is a site measurement.

Documents to request with the curve

  • Performance curve for the offered impeller diameter and speed, with your duty point marked.
  • Power and efficiency at that duty, plus motor rating and efficiency class.
  • NPSHr curve, not a single cell in a table.
  • Named test standard and grade, typically ISO 9906 or the GB equivalent you actually intend to enforce.
  • Whether the curve is a typical catalog plot or a test of a geometrically similar pump.

How this sits on a KanBeng model page

The flow and head fields on a model record are normalized catalog ranges. They help you discard impossible families. They do not replace the marked curve. If the record still has a source-pending caveat, treat the envelope as a hypothesis and force the documents above into the RFQ.