Power Plant Engineering

370 POWER PLANT ENGINEERING

11.12 Draft Tubes

Reaction turbines must be completely enclosed because a pressure difference exists between the

working fluid (water) in the turbine and atmosphere. Therefore, it is necessary to connect the turbine

outlet by means of a pipe known as draft tube upto tailrace level.

1. Output of reaction turbine when the tailrace level is above the turbine (submerged turbine.)
   The position of the turbine is shown in Fig. 11.25 and energies at all points are measured taking x-y as
   reference line, considering the energies of unit mass of water at all points, we can write

Ea = Eb = Hc + a

p

ρ

Head racelevel pa A

H H 0

pa

D Tail race level

X B C

h

Y

Fig. 11.25

W 1 (Work done per kg of water passing through the turbine)

= Eb – Ec = Ho pa



+

ρ

2

2

pVcc

g



+

ρ

= o a

H +p

ρ



2

2

pVach

g



++

ρ

as c

p

ρ

= a

p

ρ

+ h for pressure equilibrium

∴ W 1 = Ho – h

2

2

Vc

g

= H –

2

2

Vc

g

...(a)

where H is the net head between headrace and tailrace level and V, is the velocity of water leaving the
turbine.

2. Output of reaction turbine with draft tube. The arrangement of the turbine with draft tube
   is shown in fig. 11.26 and energies at all points are measured taking x-y as reference line.