The steam turbine in which the steam expands while passing through nozzle and remains at the constant pressure over the blades is called as impulse turbine. In the given figure single stage impulse turbine is shown. In this kind of turbine there is one set of fixed nozzles which is followed by the one ring of moving blades. The blades are attached to the rim of wheel, which is keyed to the shaft. The steam expands from its initial pressure to the final pressure in only one set of nozzle. The jet of steam with a very high velocity enters the moving blades. The jet of steam gets deflected when passing over these blades, exerts force on them and in this way the rotor starts rotating.
De-laval, Curtis, Zoelly, Rateau are the examples of this type of turbine. Delaval turbine is impulse turbine which is suitable for low pressure steam supply. But the only disadvantage of this type of turbine is its very high speed (usually 30000rpm) so its use is limited. It is as shown in figure. The steam is expanded from boiler pressure to condenser pressure in a single stage only; its velocity will be very high. But this speed is too high for practical applications.