Draft Tube: Definition, Types, Function, Efficiency [With PDF]


Draft Tube is an important component of a reaction turbine. The component is like a pipe in which the area increases gradually which connects the outlet of the runner to the tail race. There are two ends in which one end is connected to the runner outlet and the other end is submerged below the level of water in the tail-race also in the case of water turbine. It converts excess kinetic energy into static pressure. For making this component we use materials such as cast steel and cemented concrete.

What is a Draft Tube

A draft tube is a tube that establishes a connection between the water turbine’s outlet and the tailrace, which is the water channel responsible for carrying the water away from the turbine. Typically positioned at the turbine’s exit, the draft tube transforms the kinetic energy of the water into static pressure. Cast steel and cemented concrete are commonly employed materials in the construction of draft tubes.

What is the Purpose of the draft tube?

The primary function of the draft tube is to convert the kinetic energy of water into pressure energy by gradually reducing the water velocity and increasing its pressure as it approaches the tailrace. This is achieved by utilizing a pipe that gradually expands its cross-sectional area. By doing so, the draft tube elevates the water pressure to match atmospheric pressure. It is crucial for the tube to possess sufficient strength. It should withstand the high pressure and velocity of the water.

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The efficiency of a Draft Tube

The efficiency of the draft tube can be defined as the ratio of the actual conversion of kinetic energy into the pressure energy in the draft tube available at the inlet of the draft tube. So this means, Actual conversion of kinetic energy into pressure energy / kinetic energy available at the inlet of the draft tube.

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formula for draft tube

V2= Fluids velocity at the inlet of the draft tube or at the outlet of the turbine
V3= Fluids velocity at the outlet of the draft tube
g = gravitational acceleration
hd= head losses in the draft tube

Types of Draft Tube

Draft tubes are mainly classified in 4-types which are as follows:

  • Simple Elbow draft tube
  • Elbow with a varying cross-section area
  • Moody Spreading tube
  • Conical Diffuser or Straight divergent

1. Simple Elbow draft tube

This type is used for the low head. The Efficiency of this tube is about 60 percent which is moderate. The area of the inlet and outlet are the same. The outlet section is changed a little.

simple elbow draft tube
road draft tube
road draft tube conversion kit

2. Elbow with varying cross-section

It is generally used in the Kaplan Turbine. In this tube the upper section is circular and the outlet is a rectangular section.

draft tube with varying cross section

3. Moody Spreading Tube

This reduces the whirling speed of the water. The efficiency is around 88 percent which is good. It has two passages namely an inlet and outlets. It has a central solid core where it distributes two parts of an outlet.

Moody Spreading draft tube

4. Conical Diffuser or Straight Divergent

The cone angle is less than 10 degrees. If the angle is high, cavitation will occur.

Conical draft tube
Credits: By Adhish24

The function of a Draft Tube

It reduces the velocity of the fluid and hence increases the pressure of the fluids to the atmospheric level at the tailrace.

Advantages of a Draft Tube

  • It is an important component which is used to transform the water into energy.
  • It is generally found in the piping system.
  • It is also found in Dams where the turbine finds it difficult to deliver the mechanical work.
  • Draft tubes Prevent the splashing water coming out of the runner and its guide water to the tailrace.

Applications of a draft Tube

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

Video of Draft Tube in Francis Turbine

Credits: Lesics

More Resources for You

Kaplan Turbine
Francis Turbine
Turgo turbine

Reference (External Links)

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