After a disassembly of a single stage centrifugal pump, one finds the outer circumference of the impeller badly pitted and worn. Not having a spare impeller on board, it is decided to turn down the impeller to eliminate the wear/pitting. What will be the pump capacity, in GPM, after the reduction of impeller diameter by 0.5 inches? GIVEN: Original pump capacity = 30 GPM Original impeller diameter = 8 in.
Official USCG Answer & Rule Citation
Correct Answer: Option C — 28.12 GPM
Centrifugal pump affinity laws state that capacity is directly proportional to impeller diameter. Using the ratio (D2/D1), where D2 is 7.5 inches and D1 is 8 inches, the new capacity is 30 GPM multiplied by (7.5/8.0). This calculation yields 28.125 GPM, confirming option C as the correct performance result.
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