Consider a capital draft-tube as shown,

Applying Bernoulli's equation to inlet [(1)-(1)]

and outlet [(2){2)] take section [(2)(2)] as datum line,

$\frac{P_1}{pg} + \frac{v1^2}{2g} + (Hs + y) = \frac{P_2}{P_g} + \frac{v_2^2}{2g} + 0 + hf$ -----(1)

$h_f$ = losses of energy between (1) - (1) and (2) - (2) …