Page 28 - Steam Turbines Design, Applications, and Rerating

Introduction 9

Note that these material classes do not define the situation in which
the operating pressure is 700 psig (48.3 bar) or less, with an operating
temperature exceeding 750°F (399°C). For this combination of operat-
ing limits, Class 3 construction, with the appropriate material, is uti-
lized. In other words, 700 psig (48.3 bar) construction is utilized with
the parts cast in the appropriate steel alloy (carbon-moly steel to
825°F, chrome-moly steel to 900°F (440 and 482°C, respectively).

1.2.4 Steam balance considerations
The steam balance of a process plant can be quite complicated because
of the multiple steam pressure levels often required.
Selecting a turbine to complement a particular steam balance is
made easier, however, by the wide variety of turbines available. Con-
densing, back-pressure or extraction/induction turbines can be used, as
required, in designing both new plants and additions to existing plants.
Steam for process use, for example, can be supplied from the exhaust
of a back-pressure turbine or from an extraction turbine. The choice
would depend on the number of pressure levels involved, the design of
the remainder of the plant, number of turbines required, etc. This ver-
satility simplifies the job of optimizing your steam balance.
The steam balance diagrams in Figs. 1.8 through 1.11 illustrate how
various types of turbines have been used to supply both shaft power
and steam for other uses.

1.3 Overview of Steam Turbine Types
and Controls
Figures 1.12a through 1.12h illustrate the types of turbines most fre-
quently used in industrial and cogeneration applications. Figures
1.12a through 1.12d show noncondensing designs that exhaust to a
header from which the steam is used for process or supply to a lower
pressure turbine. Figures 1.12e through 1.12h represent condensing
units that exhaust at the lowest pressure obtainable using water or
air as a heat sink.
Figures 1.12a and 1.12e illustrate straight noncondensing and
straight condensing turbines, simple types in which no flow is removed
from the turbine between its inlet and exhaust.
Figures 1.12b and 1.12f show the next simplest variations, in which
steam is made available for process from an uncontrolled, or nonauto-
matic, extraction. The extraction pressure is proportional to the flow
passing beyond the extraction through the unit to its exhaust and is
thus related to the inlet steam flow and the extraction itself. Variations
may include two or more such uncontrolled extractions.