Page 114 - Steam Turbines Design, Applications, and Rerating

Chapter
5

Rotors for Reaction Turbines

During the first two decades of this century, experienced manufactur-
ers used solid forged rotors for small turbines. For larger units they
preferred built-up rotors, consisting of a number of disks shrunk on to
a central shaft. For small machines the solid forged rotor is still stan-
dard, but for the large machines the shrunk disk design was subse-
quently discarded because of its higher stress levels. Articles from a
number of independent sources deal with the stress levels and quality
of this type of rotor.

5.1 Solid Rotors
Solid rotors can be defined as those forged from a single piece
(monoblock). Figure 5.1 shows a solid rotor forging being produced in a
steel plant. Only after extensive testing, which is described later, is the
rotor accepted for further machining. Figure 5.2 shows a solid rotor
being machined in a lathe.
The bladed rotor of an 11.5-MW turbine is shown in Fig. 5.3, while
Fig. 5.4 shows a section through a 15-MW mechanical drive turbine
rotor.
The choice of whether a solid rotor can be used is dependent on the
start-up procedure. The temperature and stress conditions arising
because of the admission of hot steam onto the cold rotor must be
exactly known and controlled. This is because the thermal stresses due
to the temperature difference between the surface and the center line
of the rotor determine whether it can be manufactured as a solid rotor.
The factors responsible for the stress levels in the rotor are basically
the steam temperature, the geometry (diameter) of the rotor and, in
particular, the time available to reach the full-load condition. The exact
relationship between these criteria has been established following
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