Health assessment of 22 years service-exposed radiant tube from an oil refinery

Radiant tube conforming to ASTM A-200 T5 grade of steel and service exposed for 20 years from an oil refinery was investigated for health check and safety aspect. Microstructural study of the service exposed tube revealed a ferritic–martensitic structure with significant spheroidization of second phase particles. It appears that there is total disintegration of martensitic lath boundaries in some areas. The martensitic lath grains seemed to have broadened and lath boundaries are decorated with coarse and rounded carbides. As a result, with carbides the material has softened. Analysis of hot tensile and accelerated stress rupture data revealed that the residual life at 575 °C and an operating hoop stress level of (28.11 MPa) gave a minimum balance of life of approximately four and a half years for the service-exposed radiant tube. This is provided there is no evidence of localised damage in the form of surface cracks, cavitations or dents. A further life assessment of the service-exposed radiant tube is recommended after an additional 2–3 years of service life an economical and safety reasons viewpoint. During the next shut down of the plant, NDT (non-destructive) tests viz. dimensional (thickness and diameter)measurement, hardness measurement and in situ metallography may be carried out to assess the condition of the materials for their future serviceability.