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3-1.Introduction:
A special case of axial flow turbine is the so called impulse turbine. This type of turbine has no change in pressure across the rotor and thus has zero reaction. Impulse turbines are chosen when the head available exceeds about 300 m.
The impulse turbine is very easy to understand. A nozzle transforms water under a high head into powerful jet. The momentum of this jet is destroyed by striking the runner, which absorbs the resulting force. If the velocity of the water leaving the runner is nearly zero, all of the kinetic energy of the jet has been transformed into mechanical energy, so the efficiency high.
A practical impulse turbine was invented by Lester A. Pelton (1829-1908) in California around 1870. There were high-pressure jets there used in placer mining, and a primitive turbine called the hurdy-gurdy, a mere rotating platform with vanes, had been used since the 60 s, driven by such jets. Pelton also invented the split bucket, now universally used, in 1880. Pelton is a trade name for the products of the company he originated, but the term is now used generically for all similar impulse turbines.

3-2. Basic Equations:
If the water is not slowed down as it passes over the bucket surface, it follows that there is no change in the relative velocity across the rotor. Thus the rotor blades will be shaped such that W1=W2. In reality friction slows it down slightly and we define a blade friction coefficient as: