DRAFT TUBE IN TURBINES: TYPES, FUNCTION, EFFICIENCY AND PURPOSE OF A DRAFT TUBE

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machine, various types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine.

We have also seen the fundamental of Pelton wheel or Pelton hydraulic turbine, basics of radial flow reaction turbines, difference between inward radial flow reaction turbine and outward radial flow reaction turbine, Francis turbine and Axial flow reaction turbine in our recent posts.

Now we will focus here to understand the basics of draft tube and its classifications with the help of this post. We will also find out the efficiency of draft tube and its function in hydraulic turbine operation.

Draft - Tube

A draft - tube is basically a pipe of gradually increasing area that will connect the outlet of the turbine runner to the tail race where the water will be finally discharged from the turbine. Therefore, a draft tube will be used to discharge the water from the exit of the turbine to the tail race.

One end of the draft tube will be connected to the outlet of the runner while other end of draft tube will be submerged below the level of water in the tail race.

This pipe of gradually increasing area will be termed as draft tube.

Function of Draft - Tube in turbine

There are following important functions of a draft tube in the operation of a turbine as mentioned here.

Primary function of a draft tube is basically to provide a passage for water discharge from the turbine.

Draft tube will reduce the velocity of discharged water and hence it will help to minimize the loss of kinetic energy at the outlet.

Draft tube will allow establishing a negative head at the outlet of the runner and hence a draft tube will help to increase the net head on the turbine. With the presence of draft tube, turbine may be placed above the tail race without any loss of net head. In doing so, it will be quite easy to inspect the turbine properly.

A draft tube will convert a large portion of the kinetic energy, rejected at the outlet of the turbine, in to useful pressure energy. If a draft tube will not be used with the turbine, kinetic energy rejected at the outlet of the turbine will go waste to the tail race.

With the application of draft tube, the net head on the turbine and efficiency of the turbine will be increased and therefore turbine will develop more power.

A clear understanding of the function of the draft tube in any reaction turbine is very much important for the purpose of its design.

Types of Draft - Tubes

There are following types of draft tubes those are usually used in turbine operation and these are as mentioned here

Conical draft tubes
Simple elbow draft tubes
Moody spreading draft tubes
Elbow draft tubes with circular inlet and rectangular outlet

Conical draft tubes

Conical draft tube will have circular inlet and circular outlet. We can consider it as simple taper tube. Taper angle will vary from 4 degree to 7 degree as displayed in figure.

Conical draft tube will be made by mild steel plates. Conical draft tubes will have efficiency up to 90%.

Simple elbow draft tubes

Simple elbow draft tubes will have circular cross-section throughout i.e. from inlet to outlet of draft tube.

As name suggest, simple draft tube will be basically a tube with uniform cross-section turned into 90 degree.

Simple elbow draft tube will be made by concrete with steel lining at its inlet in order to reduce the effect of cavitation. Simple elbow draft tubes will have efficiency up to 60%.

Moody spreading draft tubes

Moody spreading draft tubes are very identical to conical draft tubes and such type of draft tubes are used with vertical shaft turbine.

Moody spreading draft tubes will have efficiency up to 85%.

A Moody spreading draft tube will have a solid central core at center, as displayed in figure, to reduce the whirling.

Elbow draft tubes with circular inlet and rectangular outlet

As name suggest, Elbow draft tubes will have circular inlet and rectangular outlet and it is displayed in following figure.

Elbow draft tubes with circular inlet and rectangular outlet will be made by concrete with steel lining at its inlet in order to reduce the effect of cavitation. Such draft tubes will have efficiency up to 60% to 80%.

Out of above types of draft tubes, Conical draft tubes and Moody spreading draft tubes are most efficient.

Efficiency of Draft – Tube

The efficiency of a draft tube will be defined as the ratio of actual conversion of kinetic head into pressure head in the draft tube to the kinetic head available at the inlet of draft tube.

Where,

V₁ = Velocity of water at the inlet of draft tube

V₂ = Velocity of water at the outlet of draft tube

h_f = Loss of head in the draft tube

Do you have any suggestions? Please write in comment box. Further we will find out, in our next post, Specific speed and its importance in turbine operation.