DRAFT TUBE

Draft tube is used to increase the pressure from the low turbine exit

pressure to the pressure of the surrounding to which the fluid is

rejected.

The draft tube is pipe of gradually increasing area which connects the outlet of the runner to the tail race. It is used to discharging water from the exit of the turbine to the tail race. This pipe of gradually increasing area is called Draft tube. One end of the draft tube is connected to the outlet of the runner while the other end is sub-merged below the level of water in the tail race.

PURPOSES OF DRAFT TUBE1. It permits a 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 with out any loss of net head and hence turbine may be inspected properly

2. It converts a large proportion of the kinetic energy(V2

2/2g) 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.

Hence by using draft tube, the net head on the turbine increases. The turbine developed more power and also the efficiency of the turbine increases.

If a reaction turbine is not fitted with a draft tube, the pressure at the outlet of the runner will be equal to atmospheric pressure. The water from the outlet of the runner will discharge freely into the tail race. The net head on the turbine will be less than that of reaction turbine fitted with a draft tube.

Also Without the draft tube, the kinetic energy rejected at the outlet of the turbine will go waste to the tail race.

Types of Draft Tube

1. Conical diffuser or straight divergent tube-

This type of draft tube consists of a conical diffuser with half angle generally less than equal to 10° to prevent flow separation. It is usually employed for low specific speed, vertical shaft Francis turbine. Efficiency of this type of draft tube is 90%

2. Simple elbow type draft Tube-It consists of an extended elbow type tube. Generally, used when turbine has to be placed close to the tail-race. It helps to cut down the cost of excavation and the exit diameter should be as large as possible to recover kinetic energy at the outlet of runner. Efficiency of this kind of draft tube is less almost 60%

3. Elbow with varying cross section-It is similar to the Bent Draft tube except the bent part is of varying cross section with rectangular outlet. The horizontal portion of draft tube is generally inclined upwards to prevent entry of air from the exit end

Use of draft tube in reaction turbine

To reduce velocity/ kinetic energy of water making exit thereby converting kinetic energy to pressure head allowing turbine to be installed above tail pool level.

What is the function of draft tubeDue to high KE at the exit of reaction turbine, there would be a loss of overall reduction in KE. So to prevent this loss, the draft tube is provided which produces net efficiency

purpose of draft tube in hydraulic turbines

It makes possible the installation of the turbine above the tail race level without the loss of head

the velocity of water at the runner outlet is very high. By employing a draft tube of increasing cross sectional area, the discharge takes place at a much lower velocity and thus, a part of the kinetic energy that was going as a waste is recovered as a gain in the pressure head, and this increases the efficiency of the turbine.

The draft tube prevents the splashing of water coming out of the runner and guides the water to the tail race.

What turbine does not require draft tube?Draft tube is used to increase the pressure

from the low turbine exit pressure to the pressure of the surrounding to which the fluid is rejected.

Only reaction turbines require a draft tube as there is low pressure at the exit of the turbine, where as in impulse turbine the pressure at the inlet and the exit is essentially the same, which is above the pressure to which the fluid rejected and thus it require no draft tube.

Consider a capital draft tube

Hs = vertical height of draft tube above the tail race

Y = distance of bottom of draft tube from tail race

Applying Bernoulli’s equation to inlet (1 – 1) and outlet (2 – 2) of the draft tube and taking section 2 – 2 as the datum line

We get (P1/ ρg) + (V1

2/2g) + (Hs + y) = (P2/ ρg) + (V22/2g) + 0 + hf

----------------(1)

Where hf = loss of energy between section 1-1 and 2-2

But (P2/ ρg) = atmospheric pressure head + y = (Pa/ ρg) + y

 

Substituting this value of (P2/ ρg) in equation (1), we get

This equation (P1/ρg) is less than atmospheric pressure

EFFICIENCY

Actual conversion of kinetic head into pressure head Kinetic head at the inlet of draft tube

Efficiency =

Actual conversion of kinetic head into pressure head =

Theoretical conversion of kinetic head into pressure head in draft tube =

EFFICIENCY =

V1= Velocity of water at inlet of Draft tubeV2= Velocity of water at outlet of Draft tube andhf= Loss of head in the Draft tube

THANKS TO ALL . . . . .

AJESH BY

R A RAYANKARA