Main Components of Francis Turbine

Following are the components of Francis turbine (Figure 9.9)

Figure 9.9 Francis Turbine

Penstock: Penstock is a waterway to carry water from the reservoir to the turbine casing. It is very similar to all types of turbines.

Spiral or Scroll Casing: In a spiral casing, the cross-sectional area decreases around the periphery of guide wheel from the entrance to tip. A spiral casing is shown in Figure 9.9. This type of casing is used to prevent the eddy formation that causes the loss in efficiency.

Guide Vane: Guide vanes have aerofoil cross section to reduce the friction loss and prevent the eddies formation. Each guide vane can turn about its pivot by means of regulating shaft and link connected to guide vane, so that it can be opened or closed to allow the passage for variable quantity of water according to the needs. The regulating shaft is operated by means of governor whose function is to keep the speed constant to the turbine at varying loads.

Runner: Runner is fixed to the turbine shaft. Water flows radially inward in to the runner and exits axially. For high specific speed runner, it is wider than low specific speed runner, since it has to handle large amount of water. It is made of corrosion resistant materials.

Draft Tube: If the water is discharged freely from the runner, the effective head of the turbine will be reduced and will be equal to height of the reservoir from runner outlet. So a taper draft tube is used at the outlet of turbine to increase the head of water by the height of the draft tube. Since, there is big loss of head due to high kinetic energy at the outlet of turbine. The loss is recovered by converting these kinetic head into pressure head at the exit of draft tube. It is mainly used in reaction turbine (Figure 9.10).

Figure 9.10 Draft Tube

Let H_d is the height of draft tube above the tail race and x is distance of bottom of draft tube from tail race level. From Bernoulli’s equation “Total energy at any section during flow remains constant.”