Dynaflow Research Group (DRG) conducted a static analysis of a rectifier or afterwash column, which consisted of a carbon steel shell with a 15 mm thick dividing plate welded from wall to wall. The column stood approximately 70 meters tall, with the dividing wall spanning 70% of its height. Both the vessel and the dividing wall were supported by a set of stiffeners and support rings. The primary load acting on the dividing plate was differential pressure. DRG performed a detailed stress assessment of the design, with a focus on fatigue analysis.
Assessment
The stress assessment included a thorough verification to ensure that the stresses and deflections in the dividing wall and shell remained within acceptable limits, as specified by the ASME Boiler and Pressure Vessel (B&PV) Section VIII Division 2 Code. The analysis was performed using Finite Element software packages Pro/Mechanica™ and FEMap™ to evaluate the connection between the column and the dividing wall, which was considered the most critical area.
Due to the reduced thickness resulting from the high corrosion allowance, the maximal deflection of the dividing wall (between two lateral stiffeners) was found to be significant.
Results
To mitigate this deflection, DRG investigated several potential solutions involving the addition of stiffeners. However, these solutions were either infeasible for technical reasons or ineffective. Detailed stress analysis revealed that reducing the corrosion allowance led to a decreased deflection of the dividing plate, which the client accepted.
Overall, the stress values were within the acceptable limits defined by the ASME B&PV Section VIII Division 2 Code. However, the local membrane stress at the junction between the lateral stiffener and the circumferential support ring exceeded the secondary allowable stress relevant for fatigue. In accordance with the ASME Section VIII Code, a detailed elastic-plastic assessment was conducted, showing sufficient redistribution of the stresses after reaching the yield point. Consequently, the membrane stresses were deemed acceptable, providing adequate protection against burst and collapse. The reduced number of cycles was found to be 1750, provided that the junction between stiffeners and rings was sufficiently rounded.
Conclusions
The detailed stress calculations performed by DRG were crucial to ensure compliance with the ASME B&PV Code Section VIII Division 2, particularly concerning sustained and fatigue stresses. This analysis demonstrated that while some local stresses exceeded secondary allowable limits, the overall design remained safe and effective through appropriate stress redistribution.
