Primming assisted failure of superheater tubes in a sugarcane industry

The superheater tubes in steam generating boilers are commonly made from low alloy steels T11 and T22 with Cr and Mo as the main alloying elements. These tubes are designed for long-term service under normal operating conditions. These tube materials are exposed to high temperature steam internally, as well as fire side combustion products externally. High temperature and environment on either side of the tubes can cause failures which are commonly known as fire side and steam side corrosion or failures. In the present work, failure of the superheater tubes in a sugarcane industry was investigated. Different failure mechanisms were identified on steam side and fire side of the ASTM A213-T11 and ASTM A213-T22 superheater tubes operating at the service temperature of 240 and 380 degrees C, respectively. To identify the failure mechanisms, microstructural examinations of the damaged areas and characterization of corrosion deposits were con ducted on both sides of the tubes. A large amount of sodium chloride and sodium-ironoxygen compounds deposits were found inside the tubes, which could be attributed to carryover or primming. The deposits caused dissolution of the iron oxide layer and later the tube surface, causing a progressive caustic corrosion process which eventually led to thinning of the tube, and fish mouth rupture. A spherical iron particle was also formed by local melting of the tube's internal surface, which was cooled by the flowing steam. Furthermore, the external surface of tube bulged due to overheating, caused by excessive deposits accu mulation inside the tube surface. According to the study, most of the problems were caused by the Estamine, which had not been maintained properly for treating feed water.