Pump curves and affinity laws
Throttle the discharge valve, change the speed or trim the impeller, and watch where the pump curve meets the system curve. Switch on common mistakes to see why the affinity laws don't apply along a system curve with static head.
Open full screen · Exam preparation only: always use the pump maker's published curves and data.
What this calculator works out
Where a centrifugal pump runs on its system curve (the operating point), how far that is from the best efficiency point, the brake power, and what the affinity laws predict when you change the speed or trim the impeller.
The formula in words
For the same pump at the same point on its curve, flow changes in proportion to speed, head with speed squared and power with speed cubed. Trimming the impeller diameter follows the same pattern for small trims. The affinity laws move points on the pump curve; with static head in the system, the new operating point is where the new pump curve crosses the system curve, which the simulator shows.
Worked example
- Imperial: the teaching pump's best efficiency point at 1,750 rpm is 500 gpm, 120 ft and 18.94 hp. Slowed to 1,400 rpm (a ratio of 0.8): 500 × 0.8 = 400 gpm, 120 × 0.64 = 76.8 ft, 18.94 × 0.512 = 9.70 hp.
- Metric: the same point is 113.6 m³/h, 36.58 m and 14.15 kW. At 0.8 times the speed: 90.9 m³/h, 23.41 m and 7.24 kW.
Watch the simulators explained on the CrewForge YouTube channel, or work through the free sample lesson to see how a full CrewForge lesson is built.
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More free simulators
Rigging, hydraulics, shaft alignment and pumps. The same simulators used in the lessons and the videos.