The vessel under consideration features three horizontal trays positioned at different elevations. These perforated trays are supported by and welded to a support ring, which is itself welded to the vessel’s inner shell. In some instances, the weld between the support ring and the shell has cracked along a substantial part of the circumference. The client has requested an assessment of the tray connection’s integrity to the vessel. Since the trays are not involved in pressure containment, the primary failure criterion is the stability of the trays.
Analysis
It was hypothesized that the observed weld cracks were due to high stress levels and potential cyclic stresses in the welds. Therefore, the first step in the fitness-for-purpose analysis was identifying the high-stress mechanism. A finite element analysis (FEA) was conducted, and the related stress assessment was performed in accordance with ASME VIII Div. 2 standards.
Finite Element Analysis
The FEA results indicated two stress peaks in the partial penetration weld and the fillet weld connecting the tray support ring to the vessel wall. To determine if these stresses were likely to cause fatigue-like weld cracks, the number of allowable stress cycles was calculated conforming to ASME B&PV Section VIII Div. 2, Paragraph 5.36. This code provides rules to determine the alternating peak stress, which are then used in conjunction with Annex 3F to determine the allowable number of cycles for cases where no pre-existing fatigue damage is present.
Results
Based on the available information and modeling techniques, it was challenging for DRG to predict with a reasonable degree of certainty how the fatigue cracks would develop in detail. However, by making certain assumptions, it was possible to determine the minimum weld length required to maintain the tray’s supporting function. It was assumed that the support ring is attached to the vessel wall at least at two opposite sides. For this configuration, the minimum required weld length was determined. If the intact weld sections are not exactly opposite each other, the minimum required intact weld length increases, leading to earlier failure.
Recommendations
It was found that thermal stresses caused by temperature differences between the tray and the vessel wall are significant enough to produce fatigue issues if thermal cycling occurs. The high stresses result from restraining thermal expansion, so the solution lies in eliminating this restraint. High thermal stresses can be eliminated by removing part of the weld that connects the tray to the support ring (note: not the weld between the ring and the vessel, but the weld between the tray and the support ring). By removing this weld, the tray plate’s thermal expansion will no longer be restrained, thereby eliminating the bending moment on the support ring and the high stresses in the ring-to-vessel weld.
For further optimization of the tray weld integrity and to ensure long-term stability, it is recommended to address these thermal expansion issues proactively.
Conclusion
In conclusion, assessing and addressing the weld integrity of tray supports in vessels is crucial for maintaining structural stability. Finite element analysis provides valuable insights into stress mechanisms and helps determine necessary modifications to prevent fatigue-related failures. Implementing the recommended changes can significantly enhance the durability and performance of the vessel trays.
