Correct Answer - Option 2 : 5
The specific speed of a pump, Ns is given by
\({N_s} = \;\frac{{N\sqrt Q }}{{{H^{3/4}}}}\)
N is in rpm and Q is in lps and H is in m.
Or
\(H = {\left( {\frac{{N\sqrt Q }}{{{N_s}}}} \right)^{\frac{4}{3}}}\)
Now,
Putting N = 750 rpm, Q = 4000 lpm and Ns = 720 in above equation, we get H = 17.36 m
Putting N = 750 rpm, Q = 4000 lpm and Ns = 780 in above equation, we get H = 15.6 m
Average head for one impeller, Ha = (17.36 + 15.6) /2 = 16.48 m
No of impellers required will be = Total head/head per impeller = 80/16.48 = 4.85 i.e. 5 nos. of impeller required.