Scale flow, head and power for a centrifugal pump speed change or impeller trim — Q∝r, H∝r², P∝r³
Understand the pump affinity laws — how flow, head, and power scale with speed and impeller diameter, and the cube-law energy impact
For a geometrically similar centrifugal pump at constant efficiency, with ratio r:
The affinity laws are an idealization. Keep these limits in mind:
The affinity laws scale a centrifugal pump's performance when speed or impeller diameter changes. Flow varies directly with the ratio (Q2/Q1 = r), head varies with the square (H2/H1 = r²), and power varies with the cube (P2/P1 = r³). For a speed change r = N2/N1 with the impeller fixed; for an impeller trim r = D2/D1 with the speed fixed. Efficiency is assumed constant between the two points.
As a practical rule, keep an impeller trim within about 10–15% of the original diameter (r between roughly 0.85 and 1.0). Within that band the affinity laws predict head and flow well. Beyond ~15% the flow leaving the trimmed impeller no longer fully fills the casing, efficiency drops, and the actual head tends to fall below the affinity prediction — confirm any large trim against the manufacturer's published trim curve.
A variable-frequency drive (VFD) changes speed without machining, makes the speed-affinity laws essentially exact, is reversible, and saves the most energy because power drops with the cube of speed. Trimming the impeller is a permanent, lower-cost fix when no VFD is available, but it is limited to ~10–15% and slightly reduces peak efficiency. Use a VFD for variable duty or when future re-rates are likely; trim for a fixed, modest over-performance correction.