To study constructional features of Francis Turbine | BME Lab Manual

To study constructional features of Francis Turbine.

OBJECT: To study constructional features of Francis Turbine.

APPARATUS USED: Model of Francis Turbine

THEORY:

FRANCIS TURBINE :

A Francis Turbine ahs the following main components:

1) Penstock: It is the water ways used to carry the water from the reservoir to the turbine.

2) Scroll casing: It is the casing around the turbine wheel and it evenly distributes the water around the circumference of the wheel. Area of cross-section of a scroll casing reduces uniformly from maximum at the entrance to zero at the exist. Since the quantity of water reduces from maximum to zero corresponding otherwise, velocity will vary all around the circumference of the wheel and water will not enter the runner with constant velocity. Material sued for the casing is generally concrete for loco head rolled steel for medium heads and cast steels for high heads.

3. Guide Mechanism: It has a guide wheel consisting of guide vanes. Each guide vane can be moved on its pivot centre, thus changing the area of flow into the runner. This regulates the flow to meet the varying requirement of demand.

4. Draft tube: The pressure at the exist of the runner of a reaction turbine is generally less than atmospheric pressure. The water at exit cannot be directly discharged to the tail race. A tube or pipe of gradually increasing area is used for discharging water from the exist of the turbine to the tail race. This tube of increasing area is called draft tube. The free end of the draft tube is submerged deep into the tail race.

5. Runner: The runner of Francis turbine is a circular wheel on which a series of radical curved vanes are fixed. The radial vanes are so shaped that the water enters and leaves the runner without shock. The number of runners varies between16 to 24. The runner is keyed to the shaft which is coupled to the generator shaft.

Figure of Francis Turbine

Working of a Francis Turbine:

The working of a Francis turbine is shown in fig. A Francis turbine is a mixed flow turbine in which the water enters at the circumference of the runner and travels towards the axis of the runner. Finally it comes out equally along the shaft.

It operates under medium heads and requires medium quantity of flow. The water first passes through the guide vanes and then through runner vanes. Finally the water is discharged into the tail race through a draft tube. There is a difference of pressure between guide vanes and runner which is called as reaction pressure. This reaction pressure is responsible for the motion of the runner.