Draft Tube Theory

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Draft TubesThe water after working on the turbine, imparts its energy to the vanesand runner, thereb y reducing its pressure less than that of atmosphericpressure (Vacuum). As the water flows from higher pressure to lowerpressure, it cannot come out of the turbine and hence a divergent tube isconnected to the end of the turbine.Draft tube is a divergent tube one end of which is connected to the outletof the turbine and other end is immersed well below the tai lrace (Waterlevel). The major function of the draft tube is to increase the pressurefrom the inlet to outlet of the draft tube as it flows through i t and henceincrease it more than atmospheric pressure. The other function is to safel y

discharge the water that has worked on the turbine to tailrac e.Depending on the shape and al ignment, draft tubes are classified asfollows:

Moody draft tube or hydrauconeVertical divergent draft tube

2Vertical divergent draft tube The draft tube has the shape of a frustum ofa cone. This is generall y provided for low specific speed. The cone angle

is not to exceed 8 . For greater value of the cone angle i t is seen that the

flowing bod y of water may not touch the sides of the draft tube (Leavingthe boundary). This will lead to the edd y formation bringing down theefficiency of the draft tube.Moodys draft tube or hydrauc one: This is a bell mouthed draft tube or aconical tube with a solid conical central core The whirl of dischargedwater is very much reduced in this arrangement.Elbow draft tube: This draft tube affords to discharge the waterhorizontall y to the tail rac e.

Elbow draft tube

Elbow draft tube changingfrom circle to rectangle

Dr. M .N. S he sha P ra ka sh , P ro f e sso r , J . N.N . C o l leg e o f E ng i nee ri ng , Sh i mo g a 3

Elbow draft tube with circular inlet and rectangular outlet: This is afurther improvement of the simple elbow draft tube. In all the t ypesmentioned above, the outlet of the draft tube should be situated below thetail water level.