Handbook of Turbomachinery Second Edition Revised - Ventech!

Geothermal Unit Turbines
Because of the low-enthalpy steam conditions in most geothermal units,
very large flow rates are required; as a result, such units may typically
contain setups such as two double-flow, low-pressure cylinders.
Rotors
Rotors are coupled together, except for cross-compound designs where there
may be two shafts driving two generators that are coupled electrically. The
rotors rest in one or, more typically, two journal bearings. The journal
bearings help hold the rotor in the proper radial position and provide
support that will withstand the reaction from shaft rotation. Thrust bearings
are typically located between the HP and IP and first LP sections in fossil
units. They take axial thrust and hold the position of the turbine rotor
axially relative to the stationary stages. Rotors are equipped with a turning
gear to allow them to be turned during shutdown and startup to prevent
thermal bending or warping caused by uneven cooling or warming.
This section briefly reviews the design features, materials of construction,
and damage mechanisms pertinent to steam turbine rotors.
Overview of Rotor Design and Features
ThreegeneraltypesofrotorconstructionareshownschematicallyinFig.30.
In HP and IP turbines of fossil plants, rotors are ordinarily of the
monoblock or welded construction. In nuclear units, the earliest HP rotors
tended to be of monoblock construction, followed for a period of time by
the use of shrunk-on construction. Most recently the trend has been back to
monoblock construction in these machines [24].
Most commonly, because of their large size, LP turbines have tended
to be the built-up (shrunk-on) design with an integrally forged shaft onto
which disks are shrunk and keyed. However, all three types of rotor
construction are in use in the LP rotors; Figure 31 shows, for example, a
welded LP rotor.
Rotor Materials of Construction
Fossil HP and IP rotors. Rotors in the 1950s were made of 1 CrMoV
(typical composition is 0.9 to 1.5% Cr, 0.7 to 1.5% Mo, and 0.2 to 0.35% V),
prepared in open hearth furnaces (‘‘C’’ grade). In the late 1950s, electric
furnace steel making and vacuum deoxidation and degasing techniques led
to improved quality and cleanliness (‘‘D’’ grade). Some manufacturers use a
Copyright © 2003 Marcel Dekker, Inc.