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08438 Evaporator Failure Caused by Corrosion and Flow

Picture for 08438 Evaporator Failure Caused by Corrosion and Flow
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Product Number: 51300-08438-SG
ISBN: 08438 2008 CP
Author: Pavel Yoffe
Publication Date: 2008
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This paper deals with three cases of boiler tube failure. 1). Flow assisted chelant corrosion was concepted as reason for inadmissible thinning of economizer tubes. There were discontinuous orange peel and extremely thin layer of iron oxides on those scalloped waterside. Some pearlite grains with iron leached off were found in surface vicinity under oxides layer. 2) Caustic gauging was recognized as damage mechanism of evaporator arch tube. The failure was localized in vicinity of protruding weld seam. It had character of ellipsoidal funnel with solid dense piece of iron and copper oxides inside and thin sodium contained deposit between the deposit and tube surface. The last was significantly etched under the deposit. 3) Quasi corrosion cracking was real cause for bent tube rupture by an axial crack. Neither internal attack nor significant structure degradation was observed in crack vicinity. The crack sides were strongly oxided and ash constituents penetrated into the crack. Nothing like said was found in neighborhood, hence the fault tube was “precracked” before its service beginning, for instance due to inefficient heat treatment after bending.
This paper deals with three cases of boiler tube failure. 1). Flow assisted chelant corrosion was concepted as reason for inadmissible thinning of economizer tubes. There were discontinuous orange peel and extremely thin layer of iron oxides on those scalloped waterside. Some pearlite grains with iron leached off were found in surface vicinity under oxides layer. 2) Caustic gauging was recognized as damage mechanism of evaporator arch tube. The failure was localized in vicinity of protruding weld seam. It had character of ellipsoidal funnel with solid dense piece of iron and copper oxides inside and thin sodium contained deposit between the deposit and tube surface. The last was significantly etched under the deposit. 3) Quasi corrosion cracking was real cause for bent tube rupture by an axial crack. Neither internal attack nor significant structure degradation was observed in crack vicinity. The crack sides were strongly oxided and ash constituents penetrated into the crack. Nothing like said was found in neighborhood, hence the fault tube was “precracked” before its service beginning, for instance due to inefficient heat treatment after bending.
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Failure analysis methodology is applied to the principal mechanisms by which boiler tubes fail during service.  sThe failure analysis procedure, or methodology for evaluation, is provided in a step by step approach.  Among the case histories discussed are: fatigue, erosion, short-term overheating, and hydrogen damage.
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