Impulse Turbine is the first stage in the steam turbine. Its is the control stage. Here the blades will like cup shaped.
The steam enters through the inner casing nozzle at constant pressure and temperature and enters the stationary blades. The stationary blades acts as a nozzle, it increases the kinetic energy and decreases the pressure of the fluid.
Fluid with high velocity impacts the rotor and moves it. As the fluid strikes the moving blades in the rotor, the kinetic energy decreases and pressure remains constant because here the kinetic energy is used to convert into mechanical energy
The remaining stages in the turbine are reaction turbine. The steam after leaving the impulse stage enters the reaction stationary blades. Here both stationary and rotary blades acts as nozzle. when steam enters stationary blades velocity increases and pressure decreases.
The fluid with increased velocity enters the rotary blades , here the pressure decreases ad the velocity also decreases as both pressure and kinetic Energy are required to convert into Mechanical Energy.The entalpy and kinetic energy decreases through the stages