Compounding in Impulse Turbine
If high velocity of steam is allowed to flow through one row of moving blades, it produces a rotor speed of about 30000 rpm which is too high for practical use.
It is therefore essential to incorporate some improvements for practical use and also to achieve high performance. This is possible by making use of more than one set of nozzles, and rotors, in a series, keyed to the shaft so that either the steam pressure or the jet velocity is absorbed by the turbine in stages. This is called compounding. Two types of compounding can be accomplished: (a) velocity compounding and (b) pressure compounding
Either of the above methods or both in combination are used to reduce the high rotational speed of the single stage turbine.
The Velocity - Compounding of the Impulse
Turbine
The velocity-compounded impulse
turbine was first proposed by C.G. Curtis to solve
the problems of a single-stage impulse turbine for
use with high pressure and temperature steam. The Curtis
stage turbine,
as it came to be called, is composed of one stage
of nozzles as the single-stage turbine, followed
by two rows of moving blades instead of one. These
two rows are separated by one row of fixed blades
attached to the turbine stator, which has the function
of redirecting the steam leaving the first row of
moving blades to the second row of moving blades.
A Curtis stage impulse turbine is shown in Fig. 23.1
with schematic pressure and absolute steam-velocity
changes through the stage. In the Curtis stage, the
total enthalpy drop and hence pressure drop occur
in the nozzles so that the pressure remains
constant in all three rows of blades.
|
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| Figure 23.1 Velocity Compounding arrangement |
Velocity is absorbed in two stages.
In fixed (static) blade passage both pressure and
velocity remain constant. Fixed blades are also called
guide vanes. Velocity compounded stage is also called Curtis stage. The
velocity diagram of the velocity-compound Impulse
turbine is shown in Figure 23.2.
Figure 23.2 Velocity diagrams for the Velocity-Compounded
Impulse turbine |
The fixed blades are used to guide the outlet steam/gas from the previous stage in such a manner so as to smooth entry at the next stage is ensured.
K, the blade velocity coefficient may be different in each row of blades
Work
done =  |
(23.10) |
End
thrust =  |
(23.11) |
The optimum velocity ratio will depend on number of stages and is given by 
• Work is not uniformly distributed (1st >2nd )
• The fist stage in a large (power plant) turbine is velocity or pressure compounded impulse stage.
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