The water after working on the turbine, imparts its energy to the vanes and runner, thereb y reducing its pressure le ss than that of atmospher ic pressure (Vacuum). As the water flows from higher pressure to lower pressure, it cannot come out of the turbine aand nd hence henc e a divergent divergent tube is is connected to the end of the turbine. Dr D r a f t t u b e is a divergent tube one end of which is connected to the outlet of the turbine and other end is immersed well below the ta ilrace (W ater l e v el el ) . T h e m aj aj o r f u n ct c t i o n o f t h e d r af a f t t u b e i s t o i nc n c r e as as e t h e p r es es s u r e from the inlet to outlet of the draft tube as it fl ows through it and hence increase it more than atmos phe ric pressure. The oth er function is to safel y discharge the water water that has worked on the turbine to tailra tailracc e. Depending on the shape and alignment, draft tubes a re classified a s follows:
Vertical divergent draft tube Moody draft tube or hydraucone
Elbow draft tube
Elbow draft draft tube tube changing changing from circle to rectang rectangle le V e r t i c a l d i v e r g e n t d r a f t t u b e The draft tube has the shape of a frustum of
a cone. This is generall y provided for low specific speed. The cone angle
is not to exceed 8 . For greater value value of the cone cone angle a ngle it is is seen that the flow flowing ing bod y of water may not touch the sides of the draft tub e (Leaving the boundary). T his will lead to the e dd y formation bringing down the efficiency of the dra ft tube.
Mo M o o d y ’ s d r a f t t u b e o r h y d r a u c o n e : This is a bell mouthe d draft tube or a conical tube with a solid conical central core The whirl of discharged water is very much reduced in this arrangement. arrangement . El E l b o w
draft
tube:
This
draft
horizontally horizontall y to the tail rac e.
tube
affords
to
discharge
the
water
El E l b o w d r a f t t u b e w i t h c i r c u l a r i n l e t a n d r e c t a n g u l a r o u t l e t : This is a
further improvement of the simple elbow dra ft tube. In all the t ypes mentioned above, the outlet of the draft tube should be situated below the tail water level.
