Question

A centrifugal pump with a 12- in diameter impeller requires a power input of 60 hp when the flowrate is 3200 gpm against a 60 ft head. The impeller is changed to one with a 10 in. diameter. Determine the expected flowrate, head, and input power if the pump speed remains the same

Answer 1

a) expected flow rate is 1852 gpm

b) head is 41.7 ft

c) input power is 21.1 hP

Step-by-step explanation:

from flow coefficient equation C_Q = Q/(W×D³)

Q ∝ d³

Qd₁/Qd₂ = d₁³/d₂³

where Qd₁ and Qd₂ are different discharges with respect to different diameter, (Qd₁ = 3200 gpm) and d₁ is diameter one ( 12 in) while d₂ is diameter 2 ( 10 in),

now we substitute

Qd₂ = Qd₁(d₂³/d₁³)

Qd₂ = 3200( 10/12)³

Qd₂ = 1852 gpm

∴ expected flow rate is 1852 gpm

next we calculate the head

h ∝ d²

hd₁/hd₂ = d₁²/d₂²

where hd₁ and hd₂ are different heads with respect to different diameter, (hd₁ = 60 ft) and d₁ is diameter one ( 12 in) while d₂ is diameter 2 ( 10 in),

now we substitute

hd₂ = hd₁(d₂²/d₁²)

hd₂ = 60 ( 10/12 )²

hd₂ = 41.7 ft

∴ head is 41.7 ft

Now calculate the input power

W ∝ d⁵

Wd₁/Wd₂ = d₁⁵/d₂⁵

where Wd₁ and Wd₂ are different power with respect to different diameter, (Wd₁ = 60 hp) and d₁ is diameter one ( 12 in) while d₂ is diameter 2 ( 10 in),

now we substitute

Wd₂ = Wd₁(d₂⁵/d₁⁵)

Wd₂ = 60 ( 10/12 )⁵

Wd₂ = 21.1 hP

∴ input power is 21.1 hP