Page 128 - Steam Turbines Design, Applications, and Rerating

Chapter
6

Turbine Blade Design Overview

The most critical aspect of steam turbine reliability centers on the
bucket design. Since buckets, or rotating blades, are subjected to
unsteady steam forces during operation, the phenomenon of vibration
resonance must be considered. Resonance occurs when a stimulating
frequency coincides with a natural frequency of the system. At reso-
nance conditions, the amplitude of vibration is related primarily to the
amount of stimulus and damping present in the system.
High bucket reliability requires designs with minimum resonant
vibration. As will be seen later, the design process starts with accurate
calculation of bucket natural frequencies in the tangential, axial, tor-
sional, and complex modes, which are verified by test data. In addition,
improved aerodynamic nozzle shapes and generous stage axial clear-
ances are used to reduce bucket stimulus. Bucket covers are used on
some or all stages to attenuate induced vibration. These design prac-
tices, together with advanced precision manufacturing techniques,
ensure the necessary bucket reliability.
Almost all of the blading used in modern mechanical drive steam tur-
bines is either of drawn or milled type construction. Drawn blades are
machined from extruded airfoil shaped pieces of material stock. Refer to
Fig. 6.1 for the steps in machining a drawn blade. Milled blades are
machined from a rectangular piece of bar stock. Machining steps in the
manufacture of milled blades are shown in Figs. 6.2 and 6.3. The cost of
a drawn blade is much less than the cost of a milled blade, the reasons
being obvious from Figs. 6.1 to 6.3 showing the number of steps needed
to produce each blade.
As will be seen later, a certain percentage of steam turbine blades are
neither drawn nor milled type construction. These blades are usually
large, last-stage blades of steam turbines or jet gas expanders. They are
either made by forging or a precision cast process.
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