Cracking of radiant tube of heater coil carring heavy oil in a hydrocracking unit

Failure investigation was made on a cracked radiant tube made of 9Cr–1Mo (wt.%) alloy steel (ASTM A 213-T9). The failed tube was removed from a heater coil that carried crude vacuum residue and on external fuel gas firing in a hydrocracking unit operating at high pressure. The failed tube had been in service for about 15 years. The tube visually exhibited cracks on the external surface in the longitudinal axis direction. Cross-sectional examinations showed a straight and un-branched crack that reaches up to 70% of the tube’s wall thickness. Examination of the internal surface along the cracked area showed a surface of wavy topography. The cracking of the tube was attributed to long-term service at high temperature in sulphur bearing crude. High temperature sulphidic corrosion at the internal tube surface is believed to have aided the dissociation of carbides, resulting in the diffusion of carbon towards the hot external surface where fresh carbides precipitated and accumulated during shut-downs. Areas of carbide accumulation exhibited high hardness and were brittle, facilitating cracking at the external surface during start up operations.