Handbook of Turbomachinery Second Edition Revised - Ventech!

more susceptible to temper embrittlement during service, even at the lower
temperatures of operation.
Casings
Casing Design and Features
Turbine casings (or shells) must contain the steam pressure and maintain
support and alignment for the internal stationary components. Casings are
designed to withstand temperature and pressure up to the maximum steam
condition.
Early designs of casings (prior to about 1963) were classic pressure
vessel shapes with single-walled cylinders and hemispherical ends, together
with a horizontal, bolted, flanged joint for access. The casings were designed
to withstand steady-state pressure loads at high temperatures. Conservative
design led to thick sections. As technology advanced, steam temperatures
and pressures increased and more severe thermal transients were imposed on
casings during startup, load changes, and shutdown. This led to the
occurrence of fatigue, creep, and distortion.
Turbine casing design has evolved over the years and casings are now
multiple pressure vessels (for example, an inner and outer casing in the HP
and IP cylinder, or a triple casing), allowing smaller pressure differentials
and realizing thinner wall thickness and sections. These thinner sections
allow for lower temperature drop across the casing section and thus lower
thermal stresses. The exhaust steam flows back along the turbine axis
through the space between casings to allow for quicker warming of the
turbine during starts. LP casings may also be of multiple part design with
the inner casing containing the diaphragms supports and the outer casing
directing the exhaust to the condensers.
Steam chests and valves were often integral to older turbine casings.
Standard design practice now is to separate these components. However, the
materials of construction and degradation of these components is similar to
casings and, as a result, the methods for condition assessment of casings are
also appropriate to steam chests and valves.
Casing Materials of Construction
In the 1940s casings were manufactured from C-Mo steels; by about 1947,
CrMo and CrMoV steels were introduced to prevent graphitization. In the
1950s, the primary emphasis was on creep strength, and these early materials
had poor creep-rupture ductility (and high notch sensitivity). High carbon
levels were typical, as were relatively high contents of detrimental elements
such as P, S, Sn, As, and Sb. Modern materials have strict controls on these
Copyright © 2003 Marcel Dekker, Inc.