Finite Element Strength Analysis of Large-Diameter Equal-Thickness Eccentric Welded Oblique Tee under Combined Loading

Literature Overview and Research Context

This paper by Li Mengli, Ma Xiaohui, Shang Xiandong, Bi Jinpeng, Wang Junli, Hou Yihang, and Zhang Ming from Qilu University of Technology and related institutions presents a comprehensive finite element analysis of a large-diameter equal-thickness eccentric welded oblique tee subjected to combined internal pressure and external mechanical loading. Published in 2021 in the "Journal of Qilu University of Technology," Volume 35, Issue 1, the study was supported by the National Natural Science Foundation of China (Grant No. 51705265) and the National Key R&D Program of China (Grant No. 2016YFC0802303). The analysis follows the ASME code stress analysis methodology and evaluates the structural integrity of the tee component under both design and operating loading conditions.

Component Description and Loading Conditions

The eccentric welded oblique tee is a specialized pipe fitting used in large-diameter piping systems where the branch pipe intersects the main pipe at an oblique angle, and the wall thickness remains uniform throughout the component. This geometry is common in industrial applications such as cryogenic liquefied natural gas (LNG) facilities, petrochemical processing plants, and large-scale process piping systems where space constraints or process requirements dictate oblique branch connections.

The following table summarizes the key loading conditions and analysis parameters:

Loading Condition	Description	Maximum Stress	Stress Location
Design load	Combined internal pressure and external mechanical load	234.89 MPa	Outer belly (pipe end to the left)
Operating load	Sustained operating conditions	228.56 MPa	Acute angle at the intersection shoulder
Material design stress	Allowable stress per ASME code	Below maximum stress values	High stress regions identified

The combined loading scenario involves internal pressure acting on the pipe walls and external mechanical loads applied at the pipe ends, simulating the effects of thermal expansion, weight of connected equipment, and wind or seismic loads. The analysis was conducted using ANSYS Workbench, which provides an integrated environment for finite element modeling, mesh generation, and post-processing.

ASME Code Stress Analysis Methodology

The stress analysis followed the ASME Section VIII Division 2 methodology, which classifies stresses into primary general, primary local, and secondary categories. Each stress category has a corresponding allowable stress limit, and the superposition of stresses from different load cases is evaluated according to specific rules. The analysis procedure involves identifying the stress linearization paths (SLPs) at critical locations, extracting the membrane, bending, and peak stress components, and comparing the combined stresses against the applicable code limits.

The identification of high stress regions is a critical step in the analysis. In the design load case, the maximum von Mises equivalent stress of 234.89 MPa was identified at the outer belly of the tee, specifically at the pipe end oriented to the left. In the operating load case, the maximum stress of 228.56 MPa occurred at the acute angle shoulder of the intersection zone. Both locations exhibit stress concentrations that exceed the material design stress, necessitating detailed stress evaluation according to the code provisions.

Stress Linearization and Evaluation

Stress linearization is the process of separating the total stress field at a critical location into membrane (through-thickness average), bending (linear through-thickness variation), and peak (remaining nonlinear component) stress components. This separation is essential for code-based stress evaluation because each stress component is compared against different allowable limits. The following table presents the evaluation framework:

Stress Category	Allowable Limit	Evaluation Basis
Primary general stress	1.5 S	S is the allowable stress at design temperature
Primary local stress	3 S	S is the allowable stress at design temperature
Secondary stress	3 S	S is the allowable stress at design temperature
Primary plus secondary	1.5 S + 3 S	Combined limit for primary and secondary stresses

The analysis results, after stress linearization, demonstrated that the combined stresses at the evaluated locations satisfied the ASME code requirements for both design and operating conditions. This outcome confirms that the eccentric welded oblique tee geometry is structurally adequate for the specified loading conditions, provided that the material properties and fabrication quality meet the code requirements.

Engineering Practice and Fabrication Considerations

The identification of high stress regions at the intersection shoulder and the outer belly has direct implications for the fabrication and quality assurance of the eccentric welded oblique tee. The intersection shoulder, where the branch pipe meets the main pipe at an oblique angle, is inherently a stress concentration zone due to the geometric discontinuity. During fabrication, this region is subject to welding deformation and residual stresses that can further exacerbate the stress concentration.

The following table outlines the key fabrication and quality control considerations for the eccentric welded oblique tee:

Fabrication Aspect	Requirement	Quality Control Method
Weld joint geometry	Code-compliant preparation and fit-up	Visual inspection and radiographic testing
Weld residual stress	Controlled through post-weld heat treatment or stress relief	Strain measurement or magnetic force method
Surface quality at high stress zones	Smooth finish to avoid additional stress concentrations	Surface roughness measurement
Dimensional accuracy	Conformance to design drawings within tolerance	Coordinate measurement or laser scanning
Material certification	Mill test reports meeting code requirements	Chemical analysis and mechanical testing

The finite element analysis results should be integrated with the fabrication quality assurance program to ensure that the as-built component performs as predicted by the analysis. Any deviations from the assumed geometry, material properties, or loading conditions must be evaluated for their impact on the stress analysis results and the overall structural adequacy.

Key Reflections and Study Insights

This study demonstrates the essential role of finite element stress analysis in the design verification of complex pipe fittings subject to combined loading conditions. The eccentric welded oblique tee represents a challenging geometry due to the oblique intersection angle and the eccentricity, which create non-uniform stress distributions that are difficult to predict using simplified analytical methods. The application of ASME code stress analysis methodology provides a rigorous and internationally recognized framework for evaluating the structural adequacy of such components. Engineers should note that the identification of high stress regions does not necessarily indicate a design failure but rather highlights areas that require careful stress evaluation and potentially enhanced quality control during fabrication. The parametric nature of finite element analysis also enables the exploration of design modifications, such as local wall thickness reinforcement or geometric modifications at the intersection zone, to reduce stress concentrations and improve the structural performance of the component. The research contributes to the technical knowledge base for the design and qualification of large-diameter pipe fittings in critical industrial applications.