Question Bank • QB_EXP_IDE_PMP_004 NAT • 1 Mark

Section 5: Irrigation and Drainage Engineering

Centrifugal Pumps: Performance Curves & Affinity Laws

Subtopic: Affinity law discharge scaling with speed

Question Statement:

A centrifugal irrigation pump running at a rotational speed of $N_1 = 1450\text{ rpm}$ delivers a discharge of $Q_1 = 40.0\text{ L/s}$. If the impeller diameter remains unchanged and the pump speed is increased to $N_2 = 1750\text{ rpm}$, the new discharge $Q_2$ will be ________ $\text{L/s}$ (round off to two decimal places).

Answer Key:

48.28

Step-by-Step Solution & Explanation

From the pump affinity laws for identical impeller diameter ($D_1 = D_2$): $\frac{Q_2}{Q_1} = \frac{N_2}{N_1}$ $Q_2 = Q_1 \left( \frac{N_2}{N_1} \right) = 40.0 \times \left( \frac{1750}{1450} \right) = 40.0 \times 1.2069 = 48.28\text{ L/s}$
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