Correct Answer - Option 2 : 5

The specific speed of a pump, N_{s} 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 N_{s} = 720 in above equation, we get H = 17.36 m

Putting N = 750 rpm, Q = 4000 lpm and N_{s} = 780 in above equation, we get H = 15.6 m

Average head for one impeller, H_{a} = (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.**