Shot
Peening Success Stories for Turbomachinery / Power Generation
Applications
Shot
peening has proved its effectiveness in extending the
service life and enhancing the performance of metal
components by protecting them against fatigue, fretting
fatigue, stress corrosion cracking and a variety of
other failure mechanisms. Following are some of these
success stories for turbomachinery / power generation
applications:
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Feedwater
Heaters
Large
thermal fatigue cracks were discovered in eight high
pressure feedwater heaters used in a power generation
application. These units operated in both an elevated
temperature and thermal fatigue environment. Startups
and shut downs caused thermal fatigue. Steady state
operation was at 480-660 °F (250-350 °C). The
cracks were circumferential in the weld heat affected
zone between the water chamber and tube sheet. Fatigue
cracking was attributed to years of service and 747
startups and shutdowns of the unit. This caused concern
about the remaining life of the units. The cracked locations
were machined and shot peened. Subsequent inspections
showed that no additional fatigue cracks developed after
five years of service and 150 startup and shutdown cycles.
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Diaphragm Couplings
Metal
diaphragm couplings are often used in turbomachinery
applications. These couplings accommodate system misalignment
through flexing. This flexing, or cyclic loading, poses
concerns for fatigue failures. Researchers concluded
that the Electro-Chemical Machining (ECM) process produces
parts that are geometrically near-perfect. However,
they found under scanning electron microscope observation
that small cavities sometimes developed on the surface
of a part as a result of ECM. These cavities apparently
generate stress concentrations that lead to premature
failures. Shot peening after ECM was able to overcome
this deficiency and has significantly improved the endurance
limit of the diaphragm couplings.
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 Turbomachinery
Blades And Buckets
A
very common fretting environment is the dovetail root
of turbomachinery blades. Shot peening is commonly used
to prevent fretting failures of these roots. Blade roots
have a characteristic fir tree shape. The tight mating
fit coupled with demanding loading conditions require
that the surfaces be shot peened to prevent failure
associated with fretting. Many turbine and compressor
blade roots are shot peened as OEM parts and re-shot
peened upon overhaul to restore fatigue debits otherwise
lost to fretting. The discs or wheels that support the
blades should also be peened.
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