Failure Analysis of Burst Tube Caused by Corrosion Thinning of the Tube Wall of Boiler Economizer

Abstract A boiler economizer is in continuous operation during several burst pipes. Burst tubes are located near the elbow below the weld, and the weld on both sides of the tube wall is thinning seriously. There is a flue gas formation vortex in the furnace chamber; the outer wall of the tubes is thicker ash, and fins and tubes are corroded to varying degrees. This study employed meticulous macroscopic observation, precise chemical composition analysis, detailed metallography, advanced scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to determine the causes of the coal economizer tube burst and tube wall thinning, ensuring the accuracy of the findings. The chemical composition, microstructure, and grain size of the base material met the requirements of the relevant standards. The heat-affected zone and weld seam show the Widmannstatten structure, a superheated tissue caused by excessive heat input during welding. The outer wall of the pipe corrosion is more serious, resulting in serious wall thinning, and ultimately, the pipe bursts, unable to withstand the pressure inside the pipe. It was eventually determined that the cause of the pipe burst was that the operating temperature of the economizer was close to the dew point, and the flue gas contained sulfur dioxide, sulfur trioxide, and hydrogen chloride that met water vapor at that temperature to form acid. Acid on the outer wall of the pipe was caused by flue gas dew point corrosion, seriously causing the pipe wall to thin. When the pipe wall is thinned to a certain extent due to corrosion, it cannot withstand the pressure inside the pipe, and rupture will occur.