Finite Element Analysis (FEA) is conducted on the skirt-to-head weld of an Acetylene Converter to evaluate the impact of a hot-box installation. Positioned on the interior side of the skirt in the skirt-to-head weld area, the hot-box aims to mitigate thermal bending stresses within the joint. The thermal analysis is based on the process fluid’s thermal properties inside the vessel to predict bending stresses accurately.
Assessment
The primary objective of the FEA study was to analyze the thermal bending stresses within the skirt-to-head joint and determine the permissible number of operational cycles. A significant portion of the study focused on modeling the hot-box’s effects.
Typically, hot-boxes are utilized at temperatures exceeding 400°C. They are created by removing insulation material near the skirt-to-head weld area on the skirt’s interior. Consequently, thermal radiation from the vessel head heats the top area of the skirt, raising its metal temperatures closer to those of the vessel wall. This increased temperature enhances the radial expansion of the skirt’s top section, thus reducing bending stresses in the weld.
The effectiveness of heat radiation is influenced by factors such as the emissivity of the metal surface and the shape factor of the hot-box geometry, which represents the projection of radiating surfaces. Thermal conductance and radiation coefficients were estimated using the VDI-Wärme Atlas but were also varied to achieve a robust design solution. The sensitivity study indicated that thermal bending stresses are highly dependent on the hot-box’s radiation effectiveness. For design purposes, a conservative radiation effectiveness was chosen, resulting in the maximum possible weld stress. A fatigue assessment was subsequently conducted to estimate the weld’s lifespan.
Conclusion
The application of the hot-box, evaluated through both transient and steady-state operating cycles, resulted in a significant reduction in weld stresses. This reduction extended the allowable number of operational cycles well beyond the initial design specifications. This case study underscores the importance of performing sensitivity analyses on hot-box effectiveness to ensure a safe and reliable fatigue design.
