Evolution from Francis turbine to Kaplan turbine

Francis turbine converts energy at high pressure heads which are not easily available and hence a turbine was required to convert the energy at low pressure heads, given that the quantity of water was large enough.

Nomenclature of a velocity triangle: A general velocity triangle consists of the following vectors:[1][2] Generally, the Kaplan turbine works on low pressure heads (H) and high flow rates (Q).

This implies that the specific speed (Ns) on which a Kaplan turbine functions is high, as specific speed (Nsp) is directly proportional to flow (Q) and inversely proportional to head (H).

On the other hand, the Francis turbine works on low specific speeds i.e., high pressure heads.

In the figure, it can be seen that the increase in specific speed (or decrease in head) have following consequences: Hence, these are the parameter changes that have to be incorporated in converting a Francis turbine to a Kaplan turbine.