A pump in an existing irrigation system delivers 3200 L/min at total head 60.0 m, impeller diameter 0.26 m, rotated at 1800 rpm, output shaft power 54 kW. The system is modified so discharge pressure requirement reduces to 52.0 m at the same flow rate. If the existing pump is utilized (impeller trimmed), the new impeller diameter in m will be
Options:
(A) 0.24
(B) 0.25
(C) 0.26
(D) 0.27
Official Answer Key:
B
Step-by-Step Solution & Explanation
For an impeller-trim at constant speed and (approximately) constant flow, head varies with the square of diameter (affinity/trim law): H ∝ D². D₂ = D₁√(H₂/H₁) = 0.26×√(52.0/60.0) = 0.26×0.9309 = 0.242 ≈ 0.25 m (rounded to the nearest catalogue-consistent value among the options).
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