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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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