The draft tube connects the outlet of the turbine runner to the tailrace. It gradually increases in cross-sectional area along its length. The draft tube allows reaction turbines to be placed above the tailrace without losing efficiency. It recovers some of the kinetic energy lost at the runner outlet by converting it into pressure potential energy, guiding the water smoothly into the tailrace and preventing backflow into the runner. Common draft tube types include conical, elbow, and Moody spreading designs.

2. Definition
 The draft-tube is a pipe of
gradually increasing area which
connects the outlet of the runner
to the tail race.
 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 the tail race.

3. Why it is needed?
 In an Impulse turbine the available head is considerably
high and there is no significant effect on the efficiency if
the turbine is placed a couple of meters above the
tailrace.
 But in the case of Reaction turbines, if the net head is
low and if the turbine is installed above the tailrace,
there can be appreciable loss in available pressure
head.

4. Why it is needed?
 If the pressure at the exit of the turbine is lower 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.
 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.

5. Function
 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 loss of kinetic energy at the outlet.

7. Function
 It permits negative head to be established at the outlet
of the runner and thereby 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.

8. Function
 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 turbine will go waste to the tail race.
 The draft tube prevents the splashing of water coming
out of the runner and guides the water to the tail race.

9. Theory

10. Theory

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 draft tube

12. Types of Draft tube
 Conical draft tube

13. Types of Draft tube
 Simple elbow tubes

14. Types of Draft tube
 Moody spreading tube

15. Types of Draft tube
 Elbow draft tube with
circular inlet and
rectangular outlet