Size a Feed Pump From Flow, Head, and Efficiency in One Pass
Calculate hydraulic, shaft, and motor power for a boiler feed pump from flow, head, density, and efficiency, with worked examples at realistic duty points.
Sizing a boiler feed pump starts with the standard pump power equation, but feedwater complicates it: near-boiling water is about 4% less dense than cold, and drum pressure has to be converted into head at that density. The calculator handles the unit conversions and efficiency chain — hydraulic to shaft to motor input — so you can sanity-check a duty point in seconds.
Enter flow, head, density, and efficiencies to compute the power chain; the worked-example table covers common duty points.
A boiler feed pump has to push water into the boiler against full drum pressure plus system losses. The sizing math is the standard pump power equation: hydraulic power Ph = ρ·g·Q·H, and shaft power = Ph ÷ pump efficiency. Feedwater near 100 °C has a density of about 958 kg/m³ — noticeably below cold water, which is why the temperature field matters. Results are estimates (~) for first-pass sizing, not a substitute for a manufacturer's curve.
Shaft power: ~28.0 kW
Worked Examples at 70% Pump Efficiency, ρ = 958 kg/m³
| Flow (m³/h) | Head (m) | Hydraulic kW | Shaft kW |
|---|---|---|---|
| 10 | 100 | ~2.6 | ~3.7 |
| 25 | 100 | ~6.5 | ~9.3 |
| 50 | 150 | ~19.6 | ~28.0 |
| 75 | 200 | ~39.2 | ~55.9 |
| 100 | 250 | ~65.3 | ~93.2 |
| 150 | 300 | ~117.5 | ~167.8 |
Head from pressure: 1 bar of boiler pressure is roughly 10.6 m of head for hot feedwater (H = p ÷ ρg). Add static lift and friction losses, then a 10–20% margin for economizer fouling and valve wear before choosing a pump frame.