Recovery Boiler Waterwall Tube Failure Analysis with Implications for Furnace Replacement and New Construction, 1998 Engineering Conference Proceedings

An investigation of a water wall tube rupture in a low pressure, recovery boiler was undertaken to determine why the tube thinned to failure in less than five years of service. The failure mechanism, sulfidation corrosion. is not unusual near the furnace bed but the analysis yielded a number of interesting issues relating to boiler design and fabrication practices. The failure was located just above the transition from the stainless steel composite tubing to the carbon steel tubing. While sulfidation was found throughout this elevation of the furnace, the ruptured tube exhibited significantly higher corrosion rates than the adjacent tubes. The relatively rapid corrosion thinning exhibited by the failed tube was attributed to the combined effects of a number of factors; the installation of a previously used tube, a misunderstanding of the operating environment inside the furnace and starting the boiler up without an initial acid cleaning.

Modified burning strategies will alter the environment at the tube surface and may affect the location of corrosion. The height of corrosion resistant tubes should be considered with respect to where sulphidation may occur, not where thinning has occurred in the past. In addition, metallographic examination of the welds at the transition between the composite tubes and carbon steel tubes revealed a secondary problem relating to welding practices which has implications for both new and retrofit furnaces at composite to carbon steel cut lines.