The draft tube is a pipe connected at one end to the outlet of the turbine runner and the other end submerged in the tailrace. It functions to reduce the velocity of the water discharged from the turbine, minimizing kinetic energy losses and allowing the turbine to be placed above the tailrace without reducing efficiency. The draft tube converts kinetic energy to pressure energy, guiding water from the runner outlet without splashing and permitting negative pressure head to be established, improving the net head on the turbine.

1. SARDAR PATEL COLLEGE
OF ENGINEERING
Topic: Draft Tube

2. Draft tube

3.  The draft-tube is a pipe of gradually
increasing area which connects the outlet of
the runner to the tailrace.
 One end of the draft tube is connected to
the outlet of the runner while the other end
is submerged below the level of water in
thetail race.

4. The draft tube is a conduit which connects the runner exit to the tail
race where the water is being finally discharged from the turbine. The
primary function of the draft tube is to reduce the velocity of the
discharged water to minimize the loss of kinetic energy at the outlet

5.  Ifthe pressure at the exit of the turbine islower than the pressure of fluid
in the tailrace, a back flow of liquid into the turbine can result in
significant damage. By placing a diffusing pipe at the exit of the
runner, the pressure head is increased by decreasing the exit velocity,
and both the overall efficiency and the output of the turbine can be
improved.
 Turbinesneed to have a minimumamount 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.

6.  In an Impulseturbine the available head isconsiderably high and there
is no significant effect on the efficiency if the turbine is placed a
couple of meters above the tailrace.
 Butin the case of Reaction turbines, if the net head is low and ifthe
turbine is installed above the tailrace, there can be appreciable
lossin available pressure head.

7.  The primary purpose of the draft tube is to monitor the influx
of water.
 The turbine has something called a "tail race," which is the
lowest portion of a dam or construction project where water
pools. The tube connects the turbine to this tail race,
permitting the turbine to be outside of the water but still have
access to the water.
 Reduce the velocity of the discharged water to minimize
the lossof kineticenergy at the outlet.

8.  Itpermitsnegative head to be established at the outlet of the runnerand
therebyincrease the net head on the turbine.
 The turbine may be placed above the tail race without any lossof net
head and hence turbine may be inspected properly.

9.  It converts a large portion of the kinetic energy rejected at the outlet
of the turbine into useful pressure energy.
 Without the draft tube ,the kinetic energy rejected at the outlet of the
turbinewillgo waste to the tailrace.
 The draft tube prevents the splashing of water coming out of the
runnerand guides the water to the tailrace.

11. Efficiency
 The efficiency of a draft tube is defined as the ratio of actual
conversion of kinetic head into pressure head in the draft tube to the
kinetic head at the inlet of the drafttube

12. Typesof Drafttube
 Conical draft tube

13. Typesof Draft tube
 Simple elbow tubes

14. Typesof Drafttube
 Elbow draft tube with circular
inlet and rectangularoutlet

15. Typesof Drafttube
 Moody spreading tube