Selection of an appropriate turbine to a large extent is dependent upon the available water head and to a lesser extent on the available flowrate. In general, impulse turbines are used for high head sites, and reaction turbines are used for low head sites. Kaplan turbines with adjustable blade pitch are suitable for wide ranges of flow or head conditions, since their peak efficiency can be achieved over a wide range of flow conditions. Small turbines (less than 10 MW) may have horizontal shafts, and even fairly large bulb-type turbines up to 100MW or so may be horizontal. Very large Francis and Kaplan machines usually have vertical shafts because this makes best use of the available head, and makes installation of a generator more economical. Pelton turbines may be installed either vertically or horizontally.Some impulse turbines use multiple water jets per runner to increase specific speed and balance shaft thrust. Turbine type, dimensions and design are basically governed by the following criteria:

  • Net head.
  • Variation of flow discharge through the turbine.
  • Rotational speed.
  • Cavitation problems (quality of water available from penstock).
  • Cost.

The main criterion considered in turbine selection is the net head. The figure given below (Turbine Application Chart) specifies the range of operating heads for each turbine type.The figure and the table below show some overlapping, so that for a given head several types of turbines can be used. The selection is particularly critical in low-head schemes, where large discharges need to be handled to be economically viable.


Figure 28: Turbine Application Chart based on Head and Discharge.

taken by: http://cdn.intechopen.com/pdfs-wm/40550.pdf


A significant factor in the comparison of different turbine types is their relative efficiencies both at their design point and at reduced flows. Typical efficiency curves are shown in the figure below. An important point to note is that the Pelton and Kaplan turbines retain very high efficiencies when running below design flow; in contrast the efficiency of the Crossflow and Francis turbines falls away more sharply if run at below half their normal flow. Most fixed-pitch propeller turbines perform poorly except above 80% of full flow.


Figure 29: Efficiency of Various Turbines based on Discharge rate

taken by: http://cdn.intechopen.com/pdfs-wm/40550.pdf