FRANCIS TURBINE

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.

GUIDE MECHANISM

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

i) Slow  

ii) Medium  

iii) Fast

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.

DRAFT TUBE

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.