In this article we will discuss about the draft tube and it’s importance. Later in this article we will discuss types of draft tube and it’s advantages .
Draft Tube :
Draft Tube is a pipe of gradually increasing area which connect outlet of the runner to the tail race. It is used for discharging the water and one end is connect with turbine exit and other end below the tail race level.
Why do we required draft tube in reaction turbine ?
In case of Kaplan and Francis Turbines (or reaction turbines) the head available at the turbine inlet is generally low, hence the turbine is placed much closer to the tail race to obtain maximum head.Since most of the pressure energy of the water is converted into the Mechanical Energy of the turbine, hence the pressure at the exit of the turbine is below the atmospheric pressure.
Note: Fluid flow from high pressure zone to low pressure zone
We have already discussed in the Kaplan and Francis turbines, the turbine exit placed near to tale race. So, if the pressure of water at the exit of turbine will be less than atmospheric pressure than it will cause back flow of water from the tail race (which is at atmospheric pressure) to the turbine.
To avoid this problem of back flow a draft tube is used between turbine exit and tale race. It increases the pressure of exit water at the expense of its kinetic energy.
Now, the increased pressure of water becomes some what above to the atmospheric pressure. It solves our problem of back flow of water from the tail race to the exit of turbine.
Note: Back flow of water can cause serious damage to the turbine and can stop turbine from functioning.
Do Pelton (or Impulse) turbines needs Draft Tube?
No, in case of impulse turbines the head available to the turbine is very large which results in an above atmospheric pressure at the turbine exit. Since we have a above atmospheric pressure of water at the turbine exit then there is no problem of back flow of water.
Types of Draft Tube :
There are four type of draft tube which are available in the field of fluid machinery.
- Conical Draft 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%
- Simple Elbow : 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%
- Moody Spreading Tube : This will reduce the whirling speed of the water. The efficiency around is 88 % which is good for the outlet of the water. It has two passes one inlet and two outlet. It has central solid core where it distributes two parts of an outlet.
- Elbow type with rectangular outlet : 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
Advantages of Draft Tubes :
- It permits negative head at the outlet of the runner so that water may be discharge easily even if turbine is placed above the tail race level.
- It convert large proportion of rejected kinetic energy at outlet in potential energy.
Application of Draft Tubes :
Draft tube is used to increase the pressure from low turbine exit to the pressure of surrounding .
Draft Tube Efficiency :
The Ratio of actual conversion of kinetic head into pressure head in draft tube to the kinetic head at inlet of draft tube.