Why it is needed?
 In an impulse turbine the
available head is high so there is
no effect in the efficiency when
the turbine is placed above the
tail race
 But in the reaction turbine if the
net head is low and if the turbine
is placed above the tail race there
can be an appreciable loss in
available pressure head

Why it is needed?
 If the exit pressure is lower than the presure of fluid in the
tailrace,a backflow of fluid into the turbine can result in
significant damage.
 This can be reduced by placing the diffusing pipe at the exit
of runner, thus the pressure head is increased by
decreasing the exit velocity ,and the overall efficiency is
improved.
 Turbines need to have a minimum amount of water to
propel them inorder to produce enough energy.without
these tubes ,the pressure could drop because of lack of
water ,and inturn the entire turbine could fail to work

Function
 The primary purpose of draft is
to monitor the influx of water.
 The lowest portion of the
turbine is the tail race where
water pools.The draft tube
connects the turbine to the tail
race ,permitting the turbine to
be outside of water but still has
access to water.
 It also reduces the velocity of
discharged water to minimise
the loss of kinetic energy at the
outlet.

Efficiency
 The efficiency of the draft tube is defined as the ratio
of actual conversation kinetic head into pressure head
in the draft tube to the kinetic head to the inlet of the
draft tube.

Types of draft tube
 Conical or divergent draft tube
 Simple Elbow type draft tube
 Elbow draft tube with circular inlet and rectangular
outlet
 Hydracone or Moody spreading draft tube

Conical or divergent draft tube:
 The shape of the tube resembles that of a frustrum of a
cone. It is commonly used in the Francis turbine. The
cone angle varies from 4° to 8°.The efficiency of the
conical tube is about 8.5% to 90%.

Simple Elbow type draft tube:
 It may be in the form of a simple elbow type or elbow
tube with a circular inlet and a rectangular outlet
section. The latter type is used in the Kaplan turbine
with an efficiency of about 70%.

Elbow draft tube with circular inlet
and rectangular outlet

Hydracone or Moody spreading
draft tube
this is a modification of conical tube and a
solid conical cone is provided in the centre of
the tube with a flare at the bottom end. Such
an arrangem Hydracone or Moody spreading
draft tube ent allows a large exit area without
excessive length. The solid coreat the centre
enables full flow and Hydracone or Moody
spreading draft tube reduces the eddy losses.
The efficiency of the tube is about 85%.

Phenomenon in the draft tube
flow
 swirl flow
 Flow in bend
 Positive pressure gradient in diffuser seperation

Swirl flow
Vortex filament at part load Vortex filament at full load

MERITS
 It permits negative head to be established at the outlet of
the runner there by to increase the net head on the turbine
 The turbine may be placed above the tail race without any
loss of net head and hence turbine may be inspected
properly
 The draft tube prevents the splashing water coming out of
the runner and guide the water to the tail race.
 It also converts a large portion of kinetic energy rejected at
theoutlet of the turbine into usefull pressure
energy.without thedraft tube ,the kinetic energy rejected at
the outline of the turbine will go waste to the tail race

 A draft tube is one important part of a turbine, which is
used to transform water into energy.
 It's found within the piping system in jets, dams, or
anywhere else where turbines help do difficult mechanical
work.
 Turbines need to have a minimum amount of water to
propel them in order to produce enough energy. Without
these tubes, the pressure could drop because of lack of
water, and in turn, the entire turbine could fail to work and
power could be lost.
 . To reduce velocity/ kinetic energy of water making exit
thereby converting kinetic energy to pressure head
allowing turbine to be installed above tail pool

Draft tubes merits and demerits

In this document