Pump Affinity Law — Flow vs Impeller Diameter
Enter your known values, leave one input blank, and solves for the missing one. Try different units for next level excitement!
Learning zone
When a pump is chronically oversized and the speed is fixed by a across-the-line motor, the shop answer is to put the impeller on a lathe. Take a 10 in impeller passing 400 gpm down to 9.5 in and you get 400 × 0.95 = 380 gpm at the same rpm. Machinists have been doing this since the 1920s, and manufacturer curves still print the trim range as a family of nested lines on one chart.
The trap: the diameter laws are an approximation, not physics. They hold well for trims within about 10–15% of full diameter; cut deeper and the vane tips no longer match the volute, efficiency falls several points, and the real flow lands short of prediction. Most manufacturers void the efficiency guarantee below roughly 80% of maximum diameter, and a trim is irreversible — you cannot weld the metal back on.
- = Flow at diameter 1
- = Impeller diameter 1
- = Flow at diameter 2
- = Impeller diameter 2
- Flow at diameter 1 — Pump Water Horsepower, Pump Brake Horsepower
- Impeller diameter 1 — Pump Affinity Law — Head vs Impeller Diameter, Round Duct Air Velocity
- Flow at diameter 2 — Pump Water Horsepower, Pump Brake Horsepower
- Impeller diameter 2 — Pump Affinity Law — Head vs Impeller Diameter, Round Duct Air Velocity