Handbook of Turbomachinery Second Edition Revised - Ventech!

ductility and poor toughness, since as the creep-rupture strength increases,
so does the fracture appearance transition temperature (FATT). More
recently, toughness has been improved through changes in steel-making
practices, alloy content, heat treatment, and increased steel purity.
Maximizing creep-rupture strength has been relaxed in favor of improving
creep ductility. However, creep cracking at the blade attachment areas
continues to be a problem for many operators.
The range of service conditions of rotors has changed over the years.
Under continuous service at temperatures up to 565 8C (1050 8F), creep
damage was the primary threat to integrity, and considerable long-term
(100,000 hr) creep data have been generated. However, as rotors have
become increasingly subject to cyclic service conditions (load cycling and
full startup/shutdown cycles), other potential damage mechanisms such as
low cycle fatigue and creep-fatigue have been introduced. Startup and
shutdown induce thermal stresses that become a maximum at temperatures
substantially lower than those that occur at steady state. Fracture toughness
becomes the dominant property in the resistance of the steel to brittle
fracture.
Fossil and nuclear Lp rotors. Typical damage mechanisms for LP rotors are
(1) low cycle fatigue cracking in the rotor bore, (2) high cycle fatigue
cracking of the shaft, (3) stress corrosion cracking of discs in areas of
condensation, especially in shrunk-on disks, and (4) stress corrosion
cracking of the blade attachment areas.
Most of the potential problem areas on the LP rotor are in regions
where stress concentrations are present. These stress concentrations may be
further aggravated by erosion and/or pitting. Corrosion caused by chemical
pitting at the surface of the rotor can lead to crack initiation and subsequent
propagation by rotating bending fatigue (transverse cracking). High levels
of concentrated bending stress can exist in the shaft beneath certain disks,
during conditions of journal bearing misalignment.
The disk and shaft assembly of large LP rotors includes either keys or
locking pins at each shrink-fit interface to prevent rotation of the disk
relative to the shaft. Keyways are stress concentrators and provide
interstices for deposition of chemical contaminants from the steam. Both
of these factors are important in assessing the problems associated with disk
cracking.
As with HP and IP rotors, the LP bores are highly stressed largely by
thermal transients that can lead to low cycle fatigue damage. Unlike the
high-temperature rotors, creep is not a factor because the maximum
temperature of operation is below the creep regime, which simplifies
evaluation. However, the older NiMoV and newer NiCrMoV steels are
Copyright © 2003 Marcel Dekker, Inc.