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Manuscript Title: Impact of Boiler Water Chemistry on Waterside Tube Failures
Abstract: This paper emphasis on the study of typical premature
failure of water wall tubes of two thermal power plant boiler of same capacity
(250 MW) and same operational parameter but with different boiler water
chemistry. The investigation concludes
on the waterside corrosion in both of the case. One boiler is running with coordinated
phosphate treatment (CPT) and another with all volatile treatment (AVT). The causes of corrosion were discovered and
proposed measures for their elimination were given. Visual examination,
chemical analysis of deposits, oxide scale thickness measurement and micro
structural examination were carried to ascertain the probable cause/causes of
failure. From the investigation, it was finally concluded that the combination
of localized high tube metal temperature and wall thinning due to under
deposit corrosion led to the premature tube failure in boiler running with
coordinated phosphate treatment and localized pitting corrosion in boiler
running with AVT. Based on the results and discussions, a possible way to
combat the corrosion was proposed.
Keywords:
Boiler Water Chemistry, Boiler Tube Failure, Caustic Gauging Corrosion,
Pitting Corrosion, Boiler Deposits
I.
Introduction
Thermal power plants contribute about 75% to all India
installed capacity of electric power generating stations. In worldwide energy
sector, about 37% of electricity is produced by combusting coal [1-2]. Most of
the Indian industrial boilers has been a prominent problem of boiler tube
failure (BTF). The tube failure causes
loss in generation and which in turn responsible for massive economic loss. All
type of boiler tubes have their defined life period and can fail due to various
failure mechanisms. So, Successful and reliable operation of steam generating
equipment needs the use of the best available methods to prevent scale and
corrosion. In the boiler feed water cycle the ingress of contaminants,
deposition of contaminants, and corrosion were found as the major carriers of
potential problems who may have major role for the analysis of boiler tube
failures. Failure can occur in all boiler areas: economizers, waterwalls,
super-heaters (SH) and re-heaters (RH). Figure 1 shows simplified schematic of a
coal fired sub critical boiler. The boiler tubes are of various sizes
and thickness depending upon the pressure and mid wall metal temperature. According
to the failures by location, water wall tubes are the second highest failure
location after superheater tubes. However, according to the failures by
material, carbon steel tubes statistically lead as the most frequent material
causing failures. Correct tube material selection to resist the surrounding
temperature is also one other decisive factor to stop the chances of BTF.
Normally the water touched areas like economizer and waterwalls are made of
boiler grade carbon steel. Superheater and reheater will have combination of
low alloy tubes of stainless steels tubes. Figure 2[a] and [b] show the schematics
of heat transfer modes in the radiant
and convective section of coal fired boiler. When the tube metal is in contact
with the steam over period of time, the oxidation process may begin to form a
layer of protective magnetite (Fe3O4) scale. Ferrous
hydroxide [Fe (OH) 2] is believed to be an intermediate in this
process, converting to magnetite above 100°C according to the Schikorr
reaction:
Fe
+ 2 H2O → Fe (OH) 2 + H2
Followed by reaction:
3Fe (OH) 2 → Fe3O4 +
2 H2O + H2
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