§ 6.2 Module 6 — Pumps
A curve sheet is evidence, but only inside its stated test conditions.
By the end of this lesson
Choose a flow on the horizontal axis and draw one vertical construction line. Its intersection with the head curve gives head; efficiency contours give hydraulic efficiency; a power curve gives shaft input; the NPSHr curve gives the manufacturer-specified suction requirement, conventionally tied to tested NPSH3 or a value above it. All four values describe the same duty point.
Do not mix a head from one impeller trim with efficiency from another, or read NPSHr at the design flow while checking runout. Note the liquid, speed, impeller diameter, test tolerance, and whether power is shaft input or motor input. The curve sheet does not promise field performance after wear, changed viscosity, entrained gas, poor suction approach, or a different speed.
| Trace | Read | Selection failure it guards |
|---|---|---|
| H–Q | head at flow | no intersection with the required system duty |
| efficiency | hydraulic efficiency | heat, energy cost, and operation far from preferred region |
| power | shaft kW or bhp | motor overload toward runout |
| NPSHr | metres or feet of liquid | cavitation and lost performance |
The best efficiency point (BEP) is the tested point of maximum pump efficiency for that curve. It is also a useful geometric reference for hydraulic loading, but not a magic point at which every vibration or reliability problem vanishes. A preferred operating region and an allowable operating region should come from the manufacturer for the selected pump.
Why curves are test records
Early pump buyers received claims about capacity. Standardized factory testing turned those claims into paired measurements of flow and head, with tolerances and stated conditions. The modern curve sheet inherits that logic: it is a compact test record, not a nameplate rating.Interactive 3D instrument
Curve-sheet takeoff
A 3D instrument you drive yourself, one variable at a time. It needs JavaScript and WebGL, so it is not shown in this static copy of the page.
Curve data belongs to a stated impeller, speed, liquid, test tolerance and sometimes correction basis. A traced image with those conditions cropped off is not a specification.
Keep the complete certified submittal and identify the exact curve revision used in the selection.
A casing may accept several impeller diameters. Each trim has its own head curve; efficiency islands cross the family. Interpolate only where the manufacturer permits. Diameter affinity is an approximation because trimming changes vane exit geometry and clearances—it is not exact geometric similarity. A speed family follows the fixed-impeller affinity laws more closely, but the system intersection still has to be recomputed.
For 80 L/s at 30.0 m and 78% pump efficiency, using ρ = 999 kg/m³, the shaft requirement is 30.14 kW. A 30 kW motor is not therefore automatically adequate: check the whole operating range, motor service factor under the governing standard, drive losses, liquid density, and the manufacturer’s non-overloading power curve.
Check your understanding
Read the same duty
1 auto-graded question with an explanation for every wrong answer. Requires JavaScript. (m6-l2-q1)
Lab 6.2
Check motor input from a curve takeoff
Write shaftKW(qLps, headM, efficiencyPct) using ρ = 999 kg/m³ and g = 9.80665 m/s². Graded in the browser against 2 assertions; the editor and harness require JavaScript.
Examination score
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Head, Loss and Lift · Module 6, Lesson 2