Power Plant Engineering

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,

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POWER PLANT ENGINEERING
the leaving loss is also very high. It is therefore essential to incorporate some improvements in the
simple impulse turbine for practical use and also to achieve high performance. This is possible by mak-
ing use of more than one set of nozzles, blades, rotors, in a series, keyed to a common shaft, so that either
the steam pressure or the jet velocity is absorbed by the turbine in stages. The leaving loss also will then
be less. This process is called compounding of steam turbines. There are three main types
(a) Pressure-compounded impulse turbine.
(b) Velocity-compounded impulse turbine.
(c) Pressure and velocity compounded impulse turbine.
6.5. PRESSURE COMPOUNDED IMPULSE TURBINE
In this type of turbine, the compounding is done for pressure of steam only i e. to reduce the high
rotational speed of turbine the whole expansion of steam is arranged in a number of steps by employing
a number of simple turbine in a series keyed on the same shaft as shown. Each of these simple impulse
turbine consisting of one set of nozzles and one row of moving blades is known as a stage of the turbine
and thus this turbine consists of several stages. The exhaust from each row of moving blades enters the
succeeding set of nozzles. Thus we can say that this arrangement is nothing but splitting up the whole
pressure drop (Fig. 6.7).
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