Scales a centrifugal pump's flow, head, and power from a rated condition to a new speed or impeller trim. Runs entirely offline; inputs persist in this browser via localStorage.
Read this before arguing with the pump: The affinity laws shift the pump curve, not the operating point. The pump actually runs where the shifted curve crosses the system curve. If the system has significant static head, the operating point does not slide along an affinity line — flow falls off faster than Q ∝ N as speed drops, and below some speed the pump develops too little head to move any water at all. Use this tool to answer "what is this pump capable of at this speed," then check it against the system.
Scaled Condition
Speed ratio
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Flow (Q ∝ N)
—
Head (H ∝ N²)
—
Power (P ∝ N³)
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Speed Table
The rated point scaled across the operating range. Compare the head column against the system's static head — below the speed where pump head drops under static, the pump moves nothing.
Hz
%
Flow (GPM)
Head (ft)
BHP
How the Numbers Are Derived
The affinity laws
For a centrifugal pump at constant impeller diameter, with r = N₂/N₁:
Q₂ = Q₁ · rH₂ = H₁ · r²P₂ = P₁ · r³
The same ratios apply approximately to impeller trim at constant speed, with r = D₂/D₁. Trim predictions degrade for cuts beyond roughly 10–15% of the full diameter and for very low specific-speed pumps — check the manufacturer's trim curves.
Since VFD frequency is proportional to synchronous speed, Hz ratios and RPM ratios are interchangeable for a fixed pole count (ignoring the small change in induction motor slip).
Field troubleshooting pattern
Pump underperforming at 60 Hz against the curve → possible wrong rotation (3-phase pumps run backward at roughly half flow and head — swap two leads), plugged impeller, excessive wear ring clearance, air entrainment, or a closed/failed valve.
Pump underperforming at reduced speed but the affinity-scaled numbers match what you measured → the pump is fine; the operating point moved because the system curve, not the pump, controls.
Amp draw noticeably above the r³-scaled expectation → check for mechanical drag, binding, or pumping denser/thicker fluid than water.
Verify actual speed at the drive keypad, not the setpoint — some drives display commanded rather than actual frequency depending on the parameter shown.
Low-speed cautions
Head falls with the square of speed. If system static head is a large share of rated head, useful flow disappears well before the drive reaches minimum speed.
Extended running at low speed reduces motor cooling (shaft-mounted fans slow down too) and can drop velocity below solids-carrying minimums in wastewater service.
Many drives and pump manufacturers set a minimum speed (commonly 40–60% for wastewater) for exactly these reasons — check the approved O&M data before running below it.
References
Hydraulic Institute — ANSI/HI standards on pump performance and variable speed operation. https://www.pumps.org/
Karassik et al., Pump Handbook — affinity law derivation and trim limitations.
Manufacturer performance curves for the specific pump — affinity scaling of a single point is no substitute for the published family of speed curves when they exist.