Zhuojin Pipe Fitting Co., LtdThermal-Structural Coupled Analysis of Overlay-Welded Skirt-to-Shell Junction in Vertical Vessels
Literature Overview
The paper by Li Yuping and Tao Jiang, published in Large Fertilizer (2023, Vol. 46, No. 4, pp. 229-231), addresses a critical engineering challenge in the fabrication of large vertical pressure vessels — the connection between the cylindrical shell and the supporting skirt. When overlay welding is adopted for this junction, spatial constraints during actual manufacturing can lead to multiple forming configurations, each producing distinct stress distributions. The authors performed thermal-structural coupled finite element analyses on four representative configurations and derived practical recommendations for field implementation.
Core Technical Problem
Vertical vessels in petrochemical and fertilizer industries often require a robust skirt-to-shell joint that can withstand operational loads, thermal cycling, and wind/seismic forces. The overlay welding approach allows the use of a thinner skirt plate with a corrosion-resistant or wear-resistant weld overlay at the transition zone, but the geometry of the weld buildup is not always straightforward in confined spaces. The four configurations examined can be summarized as follows:
| Configuration | Description | Key Geometric Feature |
|---|---|---|
| Structure 1 | Ideal overlay weld with smooth fillet transition | Continuous concave fillet, no undercut, uniform reinforcement |
| Structure 2 | Partially obstructed overlay, incomplete fillet | Flat top surface, abrupt shoulder |
| Structure 3 | Multi-pass overlay with step-like profile | Staircase geometry due to pass-by-pass buildup |
| Structure 4 | Overlay with significant undercut and reinforcement | Irregular profile, stress concentration at root |
Thermal-Structural Coupled Analysis Methodology
The analysis employed a sequential coupled approach: first, the thermal field during the overlay welding process was simulated to capture the transient temperature distribution, and then the resulting temperature field was mapped onto a structural model to compute residual stresses and deformation. The weld heat input, cooling rate, and solidification sequence were all modeled to reflect realistic field conditions.
Key findings from the coupled analysis include:
- Structure 1 exhibited the lowest equivalent stress at the weld root, with stress levels well below the yield strength of the base material under all loading conditions.
- Structures 2 and 3 showed elevated stress concentrations at the geometric discontinuities, particularly at the shoulder where the overlay meets the shell plate.
- Structure 4 produced the highest peak stresses, driven by the combination of undercut-induced stress concentration and residual tensile stress from the welding process.
Engineering Practice Recommendations
The authors conclude that Structure 1 should be the preferred configuration in all cases where spatial accessibility permits. When the overlay position is constrained and Structure 1 cannot be achieved, the paper recommends using a fully forged transition section that machinarily forms a smooth, continuous curvature between the shell and the skirt. This approach eliminates the need for a multi-pass overlay in a confined geometry and provides a metallurgically homogeneous transition.
From a quality assurance standpoint, this recommendation aligns with common industry practice:
- Forged transition pieces avoid the heterogeneity of weld metal and HAZ that is inherent in overlay joints.
- Machining a smooth curvature eliminates stress concentration factors that would otherwise be introduced by a step-like or undercut weld profile.
- The forged material can be heat-treated as a single piece, ensuring uniform mechanical properties throughout the transition zone.
Key Reflections
This study is particularly valuable for engineers involved in the design and fabrication of large vertical vessels in the ammonia, urea, and ethylene oxide industries. The four-configuration comparison provides a clear decision framework: attempt Structure 1 first, and fall back to a forged transition if spatial constraints make it impractical. The paper also implicitly highlights the importance of pre-fabrication planning — the geometry of the overlay weld should be determined during the design phase, not improvised during fabrication.
One area where the analysis could be extended is the consideration of fatigue life under cyclic loading. The residual stress distributions identified in Structures 2 through 4 would likely accelerate crack initiation at the weld root under repeated pressure cycling. Incorporating fatigue analysis using the S-N curve approach for the specific steel grade would provide a more complete engineering basis for rejecting non-ideal configurations. Additionally, the effect of post-weld heat treatment on residual stress relief should be evaluated for each configuration, as PWHT can significantly reduce the tensile residual stress component that drives fatigue crack growth.
In summary, this paper offers a practical, analysis-driven guideline for skirt-to-shell overlay welding in vertical vessels, reinforcing the principle that geometric continuity and metallurgical homogeneity are paramount in high-integrity pressure vessel construction.