Power Plant Engineering

202 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).

Steam chest pressure

Initial steam velocity

Boiler

steam

Clearance

Cylinder (casing)

Exhaust

diaphragm

Wheel

Shaft

Lost velocity

Condenser pressure

NMNMNM NNNN

Fig. 6.7. Pressure Compounded Impulse Turbine.

from the steam chest pressure to the condenser pressure into a series of smaller pressure drop across

several stages of impulse turbine and hence this turbine is culled, pressure-compound impulse turbine.

The pressure and velocity variation are also shown. The nozzles are fitted into a diaphragm

which is locked in the casing. This diaphragm separates one wheel chamber from another. All rotors are