Environmental Engineering Reference
In-Depth Information
Denting of the tubes, fretting wear and erosion-corrosion can be detected
through a normal in-service inspection before leakage occurs, whereas it is
diffi cult to detect regional pitting corrosion and cracking (fretting-fatigue,
stress corrosion and formation of intergranular) before leakage occurs. The
fundamental cause of fretting is related to the design of the stream gen-
erator. As a result, the most effective management option is dependent
upon the design. In most cases plugging of the affected tubes is an effec-
tive solution when damage is found in a particular part of a certain design.
The occurrence of erosion-corrosion and corrosion fatigue is limited to
once through steam generators (OTSGs), and management options vary
depending on the characteristics of particular power plants (Morgan and
Livingston, 1995 ).
To avoid maintenance cost increases, suspension of operation or reduc-
tion of output, it has become possible to replace the existing steam gener-
ator with one using corrosion resistant alloys (Alloy 690). As of 2011, over
100 steam generators have been replaced around the world. For most of
the replaced stream generators, thermally treated Alloy 690TT has been
used. The power plant Cook-2 used this alloy for the fi rst time in 1989. With
advanced methods and greater experience, it no longer takes much time
to replace a steam generator. Developments in design and material allows
newer steam generators to have a long service life. Crevices can be removed,
allowing a steam generator to have low residual stress. New generator
designs also have improved accessibility for secondary lancing and chem-
ical cleaning (Morgan and Livingston, 1995). Improved corrosion-resistant
materials for SG tubes include high temperature mill annealed Alloy 600
(Alloy 600 HTMA), mill annealed Alloy 690 (Alloy 690 MA) and Alloy
690TT. Alloy 690TT has only recently been used in new steam generators.
Ferritic stainless steel is used for tube support structure.
7.3.4 Management techniques for pressurizer
nozzles and the CRDM
To minimize occurrence of damage to pressurizer nozzles and the CRDM,
basic management strategies consist of: operation within operating guide-
lines; inspection and monitoring; assessment of any degradation that is
detected; and maintenance. The main degradation mechanisms which can
occur in pressurizer nozzles and the CRDM are thermal fatigue, vibratory
fatigue, SCC and boric acid corrosion. Because coolant leakage through the
heater sheath, instrument penetrations or manway cover gasket can cause
corrosion and SCC of other equipment of the pressurizer system, this must
be controlled. Molybdenum di-sulphide lubricant should also not be used in
a steam exposed environment, because experience suggests that MoS
2
has
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