Basically the hydraulics turbines are classified as impulse and reaction turbines

1) Impulse turbine:

an impulse turbine as the name suggests works on the principle of impulse. In theses turbines, the head or pressure energy of water is first converted into kinetic energy by means of a nozzle or set of nozzles kept close to the runner.

This high velocity jet produced by nozzle is alilomel to impinge on the set of buckets fixed on the other periphery of the wheel or runner. The direction of jet is changed by buckets. The change of momentum of water causes the wheel to rotate, thus produces mechanical energy. It should be noted that the pressure of water is atmospheric and remains constant while passing over the runner Examples of important impulse turbines are pelton wheel girard turbine, Turgo turbine etc.

2) Reaction turbines:

In these turbines, a part of pressure energy is first converted into kinetic energy before supplied to runner.

Therefore, the water enters the runner having partly the pressure energy and partial the kinetic energy and both these energies are reduced simultaneously while passing over the runner and produce mechanical energy. Hence these turbines work on the principle of impulse reaction.

The runner of these turbines being under pressure above atmospheric if requires the blade of turbine to run in closed passages which are completely filled with water in all conditions.

Examples of reaction turbine are Francis, Kaplan and propeller turbines.