Pump school: predicting performance with speed variations

Question:

How do I predict changes in pump performance with variations in speed?

Answer:

There are fundamental laws which can be used to predict changes in pump performance with variations in speed. It is important in pump applications to be able to develop performance curves corresponding to various speeds from standard performance curves. The mathematical relationships between flow rate, head, power and speed which enable this are known as the Affinity Laws.

For variation in speed with constant impeller diameter, the following laws apply:

 

In using the above formulae, it is assumed that efficiency remains constant. In practice, the efficiency is slightly less at lower speeds since friction and drag constitute a larger proportion of hydraulic power. It is important to note that these laws do not apply to NPSH.

Speed change example

If the pump speed is 1,450rpm and duty is 200L/sec at 30m head; power = 120kW. What would be the pump duty and duty power if the pump speed is increased to 1,600rpm?

Q1 = 200l/s, H1= 30m, N1=1,450RPM, P1=120kW

N2=1600RPM

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Duty at 1,600rpm is 220.69l/s at 36.53m head. Duty power equals 161.23kW

Article courtesy of Kelair Pumps Australia “When Pump Knowledge Matters”

Phone: 1300 789 466 or visit www.kelairpumps.com.au

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