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
hf
= 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.
Reference:
Fluid
mechanics, By R. K. Bansal
Image
courtesy: Google
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