1. Stage efficiency :

A stage is defined as the combination of a ring of nozzles (fixed blades) and a ring of moving blades. The energy supplied corresponds to the isentropic heat drop, ΔH in the nozzles. The stage efficiency, ηs is given by, $$η_s=\dfrac{\text{work done}}{\text{energy supplied per stage}}$$ The stage efficiency becomes equal to the blade efficiency if there are no friction losses in the nozzles. Thus, $$η_s=η_n×η_b$$

2. Blade efficiency

The energy supplied per stage of an impulse turbine is equal to kinetic energy given by $η_b=\dfrac{ṁC_i^2}{2}$ and assuming that the kinetic energy leaving the stage is wasted.

Blade efficiency can be defined as,

$$η_b=\dfrac{\text{workdone on the blades}}{\text{K.E supplied to the blade}}$$ Blade efficiency can also be defined as, $$η_b=\dfrac{\text{change in K.E.of steam}}{\text{K.E supplied}}$$ Blade efficiency will be maximum when $C_0$ (Absolute velocity of steam at exit to moving blades) is minimum, that is, when $β=90^o$, or the discharge is axial.