STEAM TURBINE 201
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
EnteringExhaust
SteamShroud CasingNozzle BladeRoter Labyrinth
packingBearingBlade motiondirectionSteampressureVelocityPressureLost velocityCondenser
pressureEntering velocity of steamFig. 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.
6.4 Compounding of Impulse Turbine
Compounding is a method for reducing the rotational speed of the impulse turbine to practical
limits. As we have seen, if the high velocity of steam is allowed to flow through one row of moving
blades, it produces a rotor speed of about 30,000 r.p.m. which is too high for practical use. Not only this,