Kelvin

Select a pump against a circuit's duty

Take the duty from Circuits and Pipe Sizing, enter the maker's curve, and read the operating point, powers, motor size and NPSH margin.

Hydronic design · For owners, admins and engineers · Updated 7 Oct 2026

Pumps sets the duty a circuit asks for and selects a pump against it. The duty isn't typed twice: the flow is the circuit's total from Circuits and the head is the index circuit from Pipe Sizing, plus whatever else the pump pushes through.

Select a pump

  1. 1

    Under Circuit, choose the circuit, then give the pump a Pump name.

  2. 2

    Under Duty, choose the Pipe network the head comes from. Design flow and Network ΔP then come from Circuits and Pipe Sizing: leave the cells blank to use them, or type a figure to select against another.

  3. 3

    Under Pump curve, enter points off the manufacturer's curve, or copy one From the curve library, and name the Datasheet it came from.

  4. 4

    Under Component pressure drops, add the chiller, strainer, control valve and anything else in series with the index circuit, each with its drop at design flow and its source.

  5. 5

    Choose the Arrangement, and a Motor efficiency class if you want the motor sized.

  6. 6

    To check NPSH, tick Check NPSH at the suction and enter the Suction losses. Left blank, Static at the suction takes the circuit's standing pressure and Fluid temperature its return temperature, the warmest the pump sees. Type either to use another figure.

  7. 7

    Choose Save new pump, or Save pump for one already saved.

The curve

  • Each point needs a flow and a head, one point per flow, and the curve needs at least two points. Efficiency, absorbed power and NPSHr are optional, but each goes on every point or on none.
  • Kelvin fits the curve to your points and never extends it: an operating point beyond the highest or below the lowest flow entered is off the curve, so there is none. Enter the whole published curve.
  • From the curve library copies a pump curve's points, the speed it's published at and its citation into the table. The points stay editable, and the pump never reads the library row again. If points are already typed, Kelvin asks before replacing them.
  • A curve isn't saved without the datasheet it came from.

The system curve and operating point

The system curve runs through the duty point. On a closed circuit it passes through the origin: the pump sees friction alone, and the circuit's standing pressure adds nothing to its head. Only an open system, such as a cooling tower or an open tank, has a lift between its free surfaces to enter under Static lift (open systems only). Leave it blank on a closed circuit.

Curve and operating point draws both curves, with the operating point where they meet. If they cross more than once, the highest-flow crossing is used and a warning asks you to check the pump isn't selected in its unstable region.

Duty / standby runs one pump, so the selection is against the single curve. Parallel, n running adds the flows of the number of identical pumps under Pumps running, at equal head, and the system meets that combined curve.

Power and motor

  • Hydraulic power is the flow times the pressure rise at the operating point, or at the duty point when there's no operating point.
  • Shaft power comes from the curve's efficiency at the operating flow, or from its absorbed-power series. Kelvin never assumes an efficiency: with neither on the curve, there's no shaft power and no motor.
  • With a Motor efficiency class chosen, the motor is the smallest rated size at or above the shaft power, at that size's efficiency, with its input power.
  • A motor loaded under half its rating carries a note that its input power is understated: the efficiency is a full-load figure, and Kelvin holds no part-load model.

NPSH

NPSH available is the gauge pressure at the suction plus atmospheric pressure, less the fluid's vapour pressure and the suction losses, in metres of the fluid. It's set against the curve's NPSHr at the operating flow.

  • Adequate: available clears required by a metre or more.
  • Tight margin: under about a metre to spare. That metre is Kelvin's prompt to look again, not a standard's figure: the manufacturer states the margin their pump needs.
  • Would cavitate: available is below required.
  • Not checked: the curve carries no NPSHr at the operating flow.

The vapour pressure is water's. A glycol's is lower, so for a glycol the figure reads low, on the safe side, and a warning says so. Atmospheric pressure comes from the elevation on an HVAC project's Design Conditions, or is taken as standard sea-level pressure.

When there's no operating point

  • The curves don't meet inside the range entered. Enter more of the published curve, or choose another pump.
  • The duty head is below the static lift entered, so the system curve can't be drawn. A closed circuit has no static lift: clear the cell.
  • If the static lift equals the circuit's static pressure, the page points it out: an earlier version copied the standing pressure into the lift. Clear it unless this is an open system with exactly that lift.
  • With no curve at all, the duty is still reported, but there's no operating point, shaft power or NPSHr.
  • A component drop isn't saved without its source.

The motor efficiency classes and library curves are Kelvin's own reference data: where the library holds none, the cell says so. See missing reference tables.