Abstract
The duct line in FCC unit is generally designed using carbon steel with inside refractory lining for a high temperature of service fluid. The refractory could be degraded during operation due to various causes. When the refractory is damaged and a high temperature of service fluid is affected to the duct directly, the surface of the duct is exposed to the elevated temperature, and it is called a hot spot. The temperature increase of hot spot does not lead to immediate failure, but it causes structural rupture or bulging of the duct due to creep in the long term. When hot spot occurs, the steam cooling method is generally used to mitigate the creep damage by spraying the steam onto the hot spot area. In case of extensive hot spot, it is difficult to apply the steam cooling method properly. If hot spot is not controlled by mitigation method, it is necessary to check the fitness-for-service of the duct line and determine the remaining life to prepare the maintenance plan to replace or repair the steel part and refractory lining of the duct line.
In this paper, a case study of fitness-for-service assessment is performed for the refractory lined duct with extensive hot spot in FCC unit. Piping stress analysis is performed to calculate stresses by weight and pressure. And remaining life against creep damage is evaluated with ASME B31.3 Appendix V. And detailed finite element analysis is performed under same condition, and creep life is calculated according to API 579 Part 10 Level 2 Assessment.
Through this process, fitness-for-service assessments for the refractory lined duct with hot spot are compared and its effectiveness is discussed.
