Power Plant Engineering

6.3. THE SIMPLE IMPULSE TURBINE
This type of turbine works on the principle of impulse and is shown diagrammatically. It mainly
consists of a nozzle or a set of nozzles, a rotor mounted on a shaft, one set of moving blades attached to
the rotor and a casing. The uppermost portion of the diagram shows a longitudinal section through the
upper half of the turbine, the middle portion shows the development of the nozzles and blading i.e. the
actual shape of the nozzle and blading, and the bottom portion shows the variation of absolute velocity
and absolute pressure during flow of steam through passage of nozzles and blades. The example of this
type of turbine is the de-Laval Turbine.
It is obvious from the figure that the complete expansion of steam from the steam chest pressure
to the exhaust pressure or condenser pressure takes place only in one set of nozzles i.e. the pressure drop
takes place only in nozzles. It is assumed that the pressure in the recess between nozzles and blades

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STEAM TURBINE
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remains the same. The steam at condenser pressure or exhaust pressure enters the blade and comes out
at the same pressure i.e. the pressure of steam in the blade passages remains approximately constant and
equal to the condenser pressure. Generally, converging-diverging nozzles are used. Due to the relatively
large ratio of expansion of steam in the nozzles, the steam leaves the nozzles at a very high velocity
(supersonic), of about 1100 m/s. It is assumed that the velocity remains constant in the recess between
the nozzles and the blades. The steam at such a high velocity enters the blades and reduces along the
passage of blades and comes out with an appreciable amount of velocity (Fig. 6.6).
As it has been already shown, that for the good economy or maximum work, the blade speeded
should be one half of the steam speed so blade velocity is of about 500 m/s which is very en high. This
results in a very high rotational speed, reaching 30,000 r.p.m. Such high rotational speeds can only be
utilised to drive generators or machines with large reduction gearing arrangements.
Steam
Entering
Exhaust
Steam
Shroud
Casing
Nozzle
Blade
Roter
Labyrinth
packing
Bearing
B
lad
e mo
ti
o
n
di
rec
ti
on
St
e
a
m
pre
s
s
u
re
Velocity
Pressure
Lost velocity
Condenser
pressure
Entering velocity of steam
Fig. 6.6. Impulse Turbine.
In this turbine, the leaving velocity of steam is also quite appreciable resulting in an energy loss,
called “carry over loss” or “leaving velocity loss”. This leaving loss is so high that it may amount to
about 11 percent of the initial kinetic energy. This type of turbine is generally employed where relatively
small power is needed and where the rotor diameter is kept fairly small.