Power Plant Engineering

(3) Hydracone or Moody Draft Tube. This type of draft tube is also known as bell-mouth draft
tube. The whirl component of the water is large when the turbine works at part load conditions. Turbine
runs at high speed under low head conditions.
The high speed runner under low head has high whirling component. As water enters the draft
tube from the high speed runner, it whirls in the direction in which the runner rotates. The axis of
rotation is the vertical axis of the draft tube. The vortex tends to remain in the same plane because of its
gyroscopic properties and therefore, will not follow the center line of the tube. This causes eddies and
whirls in the curved portion of the draft tube and only a portion of the discharged area at the exit may be
effective. Actually water flows back into the part of the draft tube and causes serious eddies and losses.
The central cone arrangement of hydracone draft tube reduces the whirl action of the discharged
water and increases the efficiency of this draft tube to 85%.
Two different hydracone type draft tubes are shown in Fig. 11.28(e) and Fig.11.28(f). The flow
coming out of the high speed runner is whirling flow and can be considered as free vortex flow. The
pressure is minimum at the center of the free vortex, therefore, the cavitation may start by the liberation
of air and vaporization of water at the center of the draft tube immediately underneath the runner if the
velocity of whirl is large. To prevent the cavitation of runner under above-mentioned condition, the high
speed runners are used in low head plants, and the central cone of draft tube has been extended right
upto the tube as far as possible as shown in Fig. 11.28 (g). The tube shown in Fig. 11.28 (e) is used for
low speed turbines where the whirl velocity is not high.

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HYDRO-ELECTRIC POWER PLANTS
377
D
Tail race
5°
(2
.5
to
3
)D
(0
.6
to
1
)D
( ) Conical draft tube
a
( ) Trumpet shaped draft tube
b
D
(3 to 3.7)D
(2
t
o
2.
5 D
1°
D
(3 to 3.7)D
(2
t
o
2.
5)D
( )
c
( )
d
1
2
D
1.
5
D
(2
to
2
.5
)D
D
D
3D
D
D
( ) Hydracone
draft tube
e
( ) Moody draft tube
with law cone
f
(g)