Page 240 - Steam Turbines Design, Applications, and Rerating

Chapter
12

Reaction vs. Impulse Type

Steam Turbines

12.1 Introduction
Historically, steam turbines have been divided into impulse and reac-
tion turbines, from the thermodynamic aspect (Fig. 12.1), and into
axial and radial flow turbines to denote the principal direction of the
steam flow. Axial machines, and only this kind will be considered here,
are also identified according to their rotor construction as disk type or
drum type turbines. These are shown in Fig. 12.2. Because the terms
are normally understood, the impulse turbine with little or no reaction
corresponds to the disk type of construction, and the reaction turbine
to the drum type. For simplicity, the terms impulse turbine and reac-
tion turbine will be used here.
The impulse turbine, first built in 1883 by the Swedish engineer De
Laval, was inevitably at an advantage in the early days of steam tur-
bines because it is well suited to small flow volumes. The subsequent
gradual but ever accelerating increase in electricity consumption led to
a rapid rise in the unit capacities of power station turbines. The out-
puts required for mechanical drives at first remained more modest,
most of these turbines being of the impulse type. This situation evi-
dently gave rise to the view that this kind of machine is more suitable
for mechanical drives.
However, the way in which a steam turbine functions has only an
indirect and, as will be shown, slight influence on its reliability; when
properly designed, any steam turbine can be made dependable.
In the years that followed, reaction turbines (first introduced by C. A.
Parsons in 1884) also came to be used for mechanical drives. The
largest units in service in the 1980s, driving pumps and compressors,
were reported to be reaction machines.
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