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Centrifugal Pump Affinity Laws |
.. Dave DeClerck, MP Pumps, fall 2002 |
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Selecting a pump to best suit a particular application can be challenging, especially when the speed is different from the published speed or the performance desired does not line up with the published curve. Using affinity laws to approximate the performance differences will help properly size the pump to the application.
Anyone can use affinity law formulas to approximate the performance of a centrifugal pump when the shaft speed or impeller diameter is changed. If you can remember three simple relationships, you will never have to look up affinity formulas.
The relationships are:
Simply Flow-linear, Head-square, Power-cube
Example: a data point off of a pump curve is 100 gpm, 150 ft, 3500 rpm and 8.0 diameter impeller that requires 7.5 horsepower.
What if the impeller is trimmed to 7.5"?
What if the speed is changed to 2850 rpm?
An entire pump curve can be generated by stepping several data points from a known curve.
The affinity laws are an approximation. If you consider the mathematical relationship, it assumes that the pump efficiency remains constant when the impeller is trimmed or the shaft speed is changed. Most pump performance curves vary where a trimmed impeller has lower efficiency and requires a higher horsepower than the calculated performance. Use the affinity relationship to solve small amounts of change to the performance from known data.
More on pump affinity laws:
One-hour
Centrifugal Pump University, then
test your
knowledge
McNally
Institute: Technical papers on pumps and seals
Estimations
require more complex analysis than the commonly applied affinity laws