Thermal-Structural Coupled Analysis of Skirt-to-Shell Weld Overlay Structure

Literature Overview

Published in 2023 in the journal "Large Fertilizer" (Dafert), this paper by Li Yuping and Tao Jiang from Zhenhai Petrochemical Construction Engineering Co., Ltd. addresses a contemporary challenge in large-scale pressure vessel and storage tank fabrication: the thermal-structural coupled analysis of skirt-to-shell weld overlay structures. In petrochemical applications, particularly for large hydrogenation reactors, ammonia synthesis loops, and high-pressure storage vessels, the skirt-to-shell junction is a critical structural transition zone that often requires weld overlay to achieve corrosion resistance or hydrogen resistance in the joint region.

Core Technical Content

The skirt-to-shell connection in large pressure vessels represents a geometric discontinuity where the cylindrical shell meets the skirt support structure. When weld overlay is applied to this junction—typically to provide a corrosion-resistant or hydrogen-resistant lining—the interaction between thermal gradients from the welding process and the structural response of the assembly becomes a complex coupled problem.

Thermal-Structural Coupling Mechanism

The welding process introduces localized thermal cycles that cause differential expansion and contraction between the overlay layer, the base shell, and the skirt. This differential movement generates residual stresses that can be tensile in nature, potentially exceeding the yield strength of the overlay material in localized regions. The coupled analysis considers:

• Thermal phase: Heat generation from arc energy, heat conduction through the multi-layer assembly, convective and radiative heat losses, and the resulting temperature field evolution over time.
• Structural phase: Elastic-plastic deformation response to thermal gradients, constraint effects from the skirt-to-shell geometry, and the development of residual stress and distortion patterns.
• Coupling effects: The feedback between temperature-dependent material properties (yield strength, thermal conductivity, coefficient of thermal expansion) and the mechanical response.

Typical Material Systems and Parameters

Finite Element Model Configuration

The coupled analysis typically employs a sequential or fully coupled approach in finite element software. The thermal model incorporates the moving heat source from welding, with arc power typically ranging from 3–8 kW for submerged arc or electroslag overlay processes. The structural model accounts for the elastic-plastic behavior of all materials, with temperature-dependent constitutive laws. Boundary conditions represent the constraint imposed by the vessel's overall geometry and support system.

Key outputs from the coupled analysis include:

1. Peak temperature distribution at the weld zone
2. Residual stress field in the overlay layer and heat-affected zone
3. Distortion patterns of the skirt and adjacent shell
4. Stress concentration factors at geometric discontinuities

Engineering Practice Implications

For pressure vessel fabrication, the results of such coupled analysis directly inform several critical decisions:

• Welding sequence optimization: The order of overlay passes and the direction of travel can be adjusted to minimize peak residual stresses and distortion.
• Preheat and interpass temperature control: Analysis results provide quantitative justification for specified preheat levels, typically 100–150 °C for carbon steel substrates receiving stainless steel overlay.
• Post-weld heat treatment requirements: If the calculated residual stresses exceed acceptable limits (generally 0.3σ_y for the overlay material), PWHT becomes mandatory, typically at 600–650 °C for 2–4 hours depending on thickness.
• Structural integrity assessment: The analysis provides data for fitness-for-service evaluations of the skirt-to-shell junction under operating conditions.

Common Defects and Countermeasures

Key Questions and Reflections

This literature raises important questions about the adequacy of simplified design approaches for skirt-to-shell overlay structures. Traditional design codes such as GB/T 150 and ASME VIII Div.1 provide stress limits for the junction based on elastic analysis, but the actual residual stress state from overlay welding may significantly influence the fatigue and creep performance of the structure. The coupled analysis approach provides a more realistic assessment that can bridge the gap between code requirements and actual structural behavior.

The use of such analysis in a 2023 publication from a major petrochemical construction company reflects the industry's growing recognition that computational methods must complement traditional engineering practice, particularly for large-diameter vessels where the skirt-to-shell junction represents a significant fraction of the total structural weight and stress concentration potential.

Summary

This 2023 literature demonstrates the application of thermal-structural coupled finite element analysis to a practically important problem in petrochemical pressure vessel fabrication. The approach provides quantitative insight into residual stress development, distortion behavior, and structural integrity of skirt-to-shell weld overlay structures. For engineers involved in the design and fabrication of large-scale pressure vessels, this methodology offers a valuable tool for optimizing welding procedures, predicting potential failure modes, and ensuring long-term structural reliability under combined thermal and mechanical loading conditions.