Root Cause Failure Analysis of Secondary Superheater Tube from Oil-Fired Power Boiler

The root cause failure investigation for a ruptured secondary superheater (SSH) tube in a heavy oil-fired boiler was undertaken. The technical investigation disclosed severe microstructural degradation in terms of intermetallic phase precipitation, creep cavities and carburization of tube’s external surface. The dimensional survey revealed significant thickness reduction along periphery of fractured surface. A semi-quantitative analysis of surface deposits was performed by scanning electron microscopy and x-ray diffraction to characterize and identify the sources of corrosive deposits. The detection of low melting eutectic vandates in external deposits induced the possibility of fireside corrosion. A comparative analysis was drawn between competing mechanisms of fireside corrosion and long-term overheating to reckon the root cause of the failure. Furthermore, a microstructural evolution model for fine-grained austenitic stainless steel (SA-213 TP347 HFG) suffering simultaneously from fireside corrosion and long-term overheating was developed. The investigation was concluded with notable recommendations and protective measures against fireside corrosion for superheater tubes for boiler life extension and maintaining overall equipment reliability.