Failure analysis of a boiler stack in a refinery industry

Severe thinning and failure were observed in a boiler stack made of carbon steel plates. The steam boiler was water-tube type with natural circulation and forced draft. It operates with natural gas at a load of 45 TPH. The upper part of the boiler stack fell down after 12 years of service. Visual examination, chemical analysis and microstructure investigation were carried out. The constituents and nature of the corrosion products were identified via XRD and EDS analysis. Hardness measurements were also performed to assess the suitability of the material to withstand the working conditions. The root cause of the stack failure was determined as a three-fold effect where, the stack plates were primarily heat-damaged, in addition to the erosive and corrosive actions of the flue gasses. The recommended action to avoid similar future failures is to improve the resistance to erosion, by either increasing the stack diameter to decrease velocity or using a harder material to reduce damage. It was highly advised as well to maintain the stack metal surface above 138°C to minimise the dew-point corrosion.