Centrifugal Pump Turn-Down & Selection
Slow a pump (VFD) or trim its impeller and see what it really does on your system. The affinity laws scale the curve; the system curve (static lift + friction) fixes the true operating point — so you also see the minimum speed before it dead-heads and how much a VFD saves over throttling. Feeding a positive-displacement pump? Switch Duty to Fixed flow to read the supply pressure at a set flow. Pick a pump from the library or paste your own curve.
Operating point — 100% speed (1761 rpm)
Where pump meets system
46.0 psi at SG 1.00
At the corresponding point
0.20 kW input @ 90% motor
Useful work into the fluid
1761 rpm of 1761
At the operating flow
992 rpm — below this, dead-head
Curves
To deliver 2.0 gpm
1201 rpm
On the system curve at target flow
Corresponding-point efficiency
VFD vs throttling to get there
Full speed, 121.1 ft made, surplus burned in valve
68% speed, only 54.4 ft made
0.09 kW · 731 kWh/yr
With 40.0 ftof static lift, the VFD can’t follow the affinity cube law all the way down — savings are real but smaller than the textbook N³ would suggest.
Motor sizing
Max shaft power anywhere on the full-speed curve
Smallest NEMA frame that won't overload
Shaft power at the current operating point
How the numbers are made & assumptions
Affinity laws
With ratio r = (N/N_ref)·(D/D_ref): flow scales with r, head with r², power with r³. Efficiency is held at the “corresponding” point. NPSHr scales with speed² at the operating flow, so slowing the pump lowers the NPSH it needs (the VFD anti-cavitation benefit); impeller trim barely changes it. The reference curve is least-squares fitted, then every point is moved to (Q·r, H·r²).
Operating point
System-curve mode: the pump runs where its (scaled) curve crosses the system curve H = static + k·Q², with k from your friction loss at the design flow — that crossing, not the affinity point, is the real duty. Fixed-flow mode (PD feed): a downstream positive-displacement pump sets the flow, so we read straight up the pump curve at that flow and report the head it makes as the supply pressure.
Power
Hydraulic HP = Q·H·SG / 3960. Shaft (brake) HP = hydraulic ÷ pump efficiency. Input kW = shaft HP × 0.746 ÷ motor efficiency.
Impeller-trim affinity is an approximation good for modest trims (within the casing range). Library curves are digitized by eye off published charts (datasheet) — verify against the manufacturer’s selection software before procurement.
Read before you rely on these numbers
- Speed (VFD) affinity is accurate; impeller-trim affinity drifts on big trims — confirm trimmed curves with the maker.
- Efficiency is assumed constant along a corresponding point; it actually sags a little at low speed and small trim.
- Library curves are read off printed charts and are approximate (±a few %). Your selection software is the source of truth.
- NPSH margin uses a scaled NPSHr; always keep a real margin (≥ 3 ft / 1 m) and check suction conditions.