Criteria for turbine selection

The turbine type is selected mainly from the head and discharge of the site, then refined by speed, efficiency and site conditions.

Major governing factors

1. Net head: the most important factor. High head (above ~300 m) suits Pelton; medium head (30–300 m) Francis; low head (below ~40 m) Kaplan/propeller.
2. Discharge and its variation: large discharge at low head favours Kaplan; small discharge at high head favours Pelton. A river with widely varying flow needs a turbine with a flat part-load efficiency curve.
3. Specific speed: $N_s = N\sqrt{P}/H^{5/4}$ combines head, power and speed into one number that fixes the runner type.
4. Rotational speed: higher speed means a smaller, cheaper generator, but is limited by cavitation and runaway speed.
5. Part-load efficiency: Pelton and Kaplan keep high efficiency at part load; Francis efficiency falls quickly below about 40–50% load.
6. Cavitation and setting level: reaction turbines must be set low enough (Thoma's $\sigma$), which adds excavation cost.
7. Sediment in water: silty Himalayan rivers wear Francis runners quickly; Pelton is easier to repair.
8. Cost, size and transport: runner size, powerhouse size and access road limits.
9. Overall efficiency and maintenance needs, and availability of spares and local expertise.