Damage assessment of topping furnaces radiant tubes and creep behaviour of ASTM A335 P5 steel

Radiant tubes of topping furnaces, utilized for crude oil heating, have to ensure safe operation during long time under critical conditions for material stability, due to the combined effects of environment and creep. In this paper the authors report their experience about investigations on radiant tubes after more than 200,000 hours of service. Such tubes, made of ASTM A 335 grade P5 steel, were inspected in order to assess any damaging conditions incompatible for safe operating. For this purpose visual inspection of the external surfaces and diameter ultrasonic measurements were performed during a scheduled shutdown of the furnace: the most critical sections were investigated “in situ” by means of replica method and Vickers hardness surveys. Tubes were put out of service when their external surface showed burns caused by flame impingement and thickness was reduced to unacceptable values. Specimens cut from two decommissioned tubes were characterized by means of tension test and accelerated creep test at various temperatures: their residual life was calculated numerically taking into account also the effects of wall thickness reduction.