Since the early 1980s, cracks have been observed in the off-gas cleaning section of the ethylbenzene hydroperoxide (EBHP) reaction unit. To address environmental concerns, EBHP must be removed from the off-gas before flaring. This process involves absorbing EBHP in norit beds within norit vessels, followed by cleaning the vessels with high-temperature, EB-free off-gas. The four norit vessels operate sequentially, causing cyclic heating and cooling of the interconnecting lines.
The Dynaflow Research Group was commissioned to conduct a fatigue analysis of these interconnecting lines. The primary objective was to assess the remaining lifespan of the lines, including the tee connections and the local connections to the various norit vessels.
Analysis
The fatigue analysis required a detailed understanding of the temperature distribution within the system over time. Finite element analysis was used to evaluate multiple temperature distributions during each cycle. This analysis provided the stress range at critical points in the system. These stress ranges were then utilized to estimate the fatigue life of the components in accordance with ASME B31.3 and ASME VIII-2 standards.
Additionally, the temperature gradient through the pipe wall was evaluated to ensure that it would not lead to fatigue cracks.
Results
The initial analysis revealed a significant fatigue issue at the tee connections between the vessel lines and the main line leading to the heat exchangers. Fatigue cracks were projected to develop within approximately 10 years, based on the analysis approach. This issue was addressed in 2012 by replacing the original pipe-to-pipe tees with welded tees. The revised configuration is expected to have a service life exceeding 100 years for the entire pipe section. An assessment of the nozzles showed that the loads on the nozzles are within the endurance limit, indicating no risk of fatigue.
