In this study, the details and results of the stress analysis performed for the design of two platforms attached to the reinforcement rings of a vacuum tower were presented. The platforms were positioned at two different elevations. The highest platform was designed to encircle the vacuum tower almost entirely, while the lower platform comprised two sections located on each side of the tower.
Analysis
The connection between the platforms and the vacuum tower was modeled in detail using Finite Element Methods (FEM). Various stresses in the vacuum tower and the platforms were assessed under the design and operating conditions provided by the client. Detailed assessment of the vacuum rings and supporting bolts was also conducted. Given the low number of thermal cycles, the most critical loads identified were the sustained loads, including weight and external forces. Therefore, the primary (sustained) stresses on the vacuum rings and the attached bolts were specifically evaluated.
Results
The stress analysis using Finite Element techniques provided the following key findings:
Sustained Stresses: Under the most conservative conditions (design temperature and maximum corrosion), the sustained stresses due to weight, wind, pressure, and platform loads at the location just above the upper vacuum ring were found to be close to the allowable value. Under operating conditions, which result in a lower temperature and a correspondingly higher allowable stress, the sustained stresses were well below the allowable limits.
Temperature Gradients: The self-limiting stresses caused by temperature gradients were insignificant. Additionally, with only about six full thermal cycles over 30 years, the fatigue damage due to thermal loads was deemed negligible.
Bolt Connections: Even under the most conservative conditions, and including a safety factor of 2 on loads, the bolt connections were adequately designed to withstand the sustained loads from wind, weight, pressure, and the applied platform loads.
To further optimize the design and ensure safety, it was advised to repair the corroded parts. This repair would help decrease both primary and secondary stresses, which becomes particularly important if the number of thermal cycles increases in the future.
