Investigations on the Failure of Economizer Tubes in a High-Pressure Boiler
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Investigations on the Failure of Economizer Tubes in a High-Pressure Boiler M. Mobin Æ A. U. Malik Æ M. Al-Hajri
Submitted: 20 May 2007 / in revised form: 18 November 2007 / Published online: 9 January 2008 ASM International 2008
Abstract This article describes the results of an investigation concerning the failure of economizer tubes of a high-pressure boiler in a dual-purpose power/water cogeneration plant. The failure was observed in the form of rupturing of one tube and a macrohole or pinhole in another tube. The boiler had an operating period of 116,123 h since its inception. For approximately the first 100,000 h, the fuel for the boiler was crude oil, which was replaced by Bunker C oil. The boiler tube is fabricated from carbon steel SA 210A1. The location of the failure was determined by on-site visual inspection of the boiler. Detailed macroand microexaminations of inner and outer scales on the tube were begun to determine the cause of the rupture. The composition of the fire- and waterside scale and ash deposited on the outer surface of the tubes was analyzed by energy-dispersive x-ray (EDX) technique. The reduction percentage of wall thickness of the tube facing inside and outside the furnace was calculated. The cause of the failure of the economizer tube appears to be H2SO4 dew-point corrosion. The relatively low temperature of feedwater lowered the tube metal temperature and promoted the condensation of H2SO4. The external deposits on the tubes, as a result of bunker oil firing, further helped to lower the tube metal temperature, thus promoting H2SO4 condensation over the deposit and subsequent corrosion of the tube
wall. Recommendations are given to prevent/minimize such failures. Keywords Economizer tube High-pressure boiler Dew-point corrosion Energy-dispersive x-ray
Background of the Failure During normal boiler operation a significant water leakage was observed at the flue gas outlet duct (upstream of economizer). Subsequently, the boiler was shut down and the manhole was opened for economizer inspection. The water leakage was from the bottom of the economizer coils at the 13th and 14th rows—second tubes from the top rows of the tube bank, approximately 300 mm away from the waterwall tube. The external boiler tubes were washed, followed by a detailed visual inspection. The important observations made were: • • • •
M. Mobin (&) Department of Applied Chemistry, Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh 202002, India e-mail: [email protected] A. U. Malik M. Al-Hajri Saline Water Desalination Research Institute (SWDRI), Saline Water Conversion Corporation, P.O. Box 8328, Al-Jubail 31951, Saudi Arabia
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One tube ruptured (fish-mouth opening) producing a 40 mm long, 3 mm diam hole. There were defects at the original welds in 17 other tubes. Dimensional checking at random locations revealed tube thinning from the external surface. Tube
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