The Francis turbine is mixed flow reaction turbine. This turbine is used for medium heads with medium discharge. Water enters the runner and flows towards the centre of wheel in the radial direction and leaves parallel to the axis of the turbine.
Description of main parts
Francis turbine consists mainly of the following parts
a) Spiral or scroll casing
b) Guide mechanism
c) Runner and turbine main
d) Draft tube
Spiral casing of scroll
The casing of the francis turbine is designed in a spiral form with a gradually increasing area. The advantages of this design are
i) Smooth and even distribution of water around the runner.
ii) Loss of head due to the formation of eddies is avoided.
iii) Efficiency of flow of water to the turbine is increased.
In big units stay vanes are provided which direct the water to the guide vanes. The casing is also provided with inspection holes and pressure gauge connection.
The selection of material for the casing depends upon the head of water to be supplied
For a head — upto 30 metres —concrete is used.
For a head — from 30 to 60 metres — welded rolled steel plates are used.
For a head of above 90 metres . — cast steel is used.
The guide vanes or wicket gates are fixed between two rings. This arrangement is in the form of a wheel and called guide wheel. Each vane can be rotated about its pivot centre. The opening between the vanes can be increased or decreased by adjusting the guide wheel. The guide wheel is adjusted by the regulating shaft which is operated by a governor. The guide mechanism provides the required quantity of
water to the runner depending upon the load conditions. The guide vanes are in general made of cast steel.
RUNNER AND TURBINE MAIN SHAFT
The flow in the runner of a modern Francis turbine is partly radial and partly axial. The runners may be classified as
The runner may be cast in one piece or made of separate steel plates welded together. The runner made of CI for small output, cast steel or stainless steel or bronze for large output. The runner blades should be carefully finished with high degree of accuracy. The runner may be keyed to the shaft which may be vertical or horizontal. The shaft is made of steel and is forged it is provided with a collar for transmitting the axial thrust.
The water after doing work on the runner passes on to the tall race through a tube called draft tube. It is made of riveted steel plate or pipe or a concrete tunnel. The cross —section of the tube increases gradually towards the outlet. The draft tube connects the runner exit to the tail race. This tube should be drowned approximately 1 metre below the tail race water level.
Working principles of Francis turbine
The water is admitted to the runner through guide vanes or wicket gates. The opening between the vanes can be adjusted to vary the quantity of water admitted to the turbine. This is done to suit the load conditions.
The water enters the runner with a low velocity but with a considerable pressure. As the water flows over the vanes the pressure head is gradually converted into velocity head. This kinetic energy is utilised in rotating the wheel Thus the hydraulic energy is converted into mechanical energy. The out going water enters the tail race after passing through the draft tube. The draft tube enlarges gradually and the enlarged end is submerged deeply in the tail race water. Due to this arrangement a suction head is created at the exit of the runner.