Page 27 - Steam Turbines Design, Applications, and Rerating

8 Chapter One

flows will be high. The multivalve arrangement usually improves effi-
ciency over the full operating range of a steam turbine.
Single-stage turbines are available in six classes of construction.
Class 1 (cast iron) is suitable for pressures not exceeding 250 psig (17.2
bar) and for temperatures not exceeding 500°F (260°C). If either one of
these limits is exceeded, steel construction is required.
Classes 2 and 3 (carbon steel) incorporate construction features suit-
able for a maximum pressure of 700 psig (48.3 bar). Temperature limit
for Class 2 is 650°F, 750°F for Class 3 (343 and 399°C, respectively).
For pressures exceeding 700 psig (48.3 bar), the casting is formed
from a different pattern and otherwise utilizes construction features
suitable up to a maximum pressure of 900 psig (62 bar). Class 4, 5, or 6
is required, depending on temperature. Class 4 (carbon steel) is suit-
able to a maximum temperature of 750°F (399°C). Alloy steels are
required for temperatures exceeding 750°F, or 399°C. Class 5 (carbon-
moly steel) can be used to 825°F (440°C), Class 6 (chrome-moly steel) to
900°F (482°C).

Figure 1.7 Multivalve steam turbine. (Siemens Power Corporation, Milwaukee, Wis.
and Erlangen, Germany)