Zhuojin Pipe Fitting Co., LtdFinite Element Study of Shoulder Reinforcement for Extruded Tees
Literature Overview
This paper by Guo Hong and colleagues from Taiyuan University of Technology was published in the Journal of Taiyuan University of Technology (2016, Vol. 47, No. 4, pp. 532-535). Funded by the Ministry of Housing and Urban-Rural Development science and technology program, the study investigates shoulder reinforcement (gusset plate reinforcement) for extruded tees with stepped branch diameters used in direct-buried district heating pipelines. The work fills a gap in design methodology for large-diameter crossing branch tees.
Core Technical Points
The study systematically analyzed multiple configurations of stepped branch extruded tees with shoulder reinforcement using ANSYS linear elastic analysis. The key findings regarding stress distribution and influencing factors are summarized below:
| Configuration | Branch-to-Main Ratio | Shoulder Plate Thickness | Maximum Equivalent Stress | Stress Reduction vs. Unreinforced |
|---|---|---|---|---|
| Base case | 0.5 | 0 | Reference | - |
| Configuration A | 0.5 | 0.2t_main | Moderate reduction | 15-25% |
| Configuration B | 0.7 | 0.3t_main | Significant reduction | 30-40% |
| Configuration C | 0.7 | 0.4t_main | Substantial reduction | 35-45% |
The study revealed that the shoulder plate thickness, the branch-to-main diameter ratio, and the relative position of the shoulder plate along the branch pipe axis are the three primary factors governing stress reduction effectiveness. The optimal shoulder plate thickness was found to be approximately 0.3 times the main pipe wall thickness for typical district heating pipeline configurations.
Process and Standards Analysis
Extruded tees are manufactured by extruding a pipe section through a die to create the branch opening, a process that is more economical than forging for large diameters. However, the extrusion process introduces significant plastic strain at the intersection, which can lead to:
- Localized thinning of the main pipe wall near the branch opening
- Residual stress patterns that may promote stress corrosion cracking in aggressive environments
- Reduced fatigue resistance at the intersection line due to work hardening and microstructural changes
The Chinese standard CJJ 34 (Technical Code for District Heating Engineering) does not provide specific guidance for shoulder reinforcement design of crossing branch tees. The authors' findings directly address this gap by providing quantitative stress reduction data that can be incorporated into design calculations.
Integration with Engineering Practice
In district heating applications, tees are subjected to repeated thermal cycling (typically 60-90 degrees C supply temperature with ambient return), which creates cyclic thermal stresses superimposed on internal pressure stresses. A case study from a Beijing district heating network involved DN800 extruded tees with DN400 branch connections that experienced cracking at the intersection line after approximately 8 years of service. Post-failure analysis revealed that the intersection line stress exceeded the material's fatigue limit due to the absence of shoulder reinforcement.
The implementation of shoulder reinforcement according to the methodology in this paper requires careful welding design:
- The shoulder plate is typically attached using fillet welds around the branch pipe circumference
- Weld qualification per NB/T 47014 or ISO 9606-1 is mandatory for the specific plate thickness
- The weld toe geometry must be ground smooth to minimize secondary stress concentrations
- Post-weld stress relief may be required for thick sections to reduce residual stresses
Key Questions and Reflections
The linear elastic approach is appropriate for initial design but may not capture the full benefit of shoulder reinforcement under cyclic loading, where plastic deformation at the weld toe can redistribute stresses. Additionally, the interaction between the shoulder plate weld and the extrusion-induced residual stress pattern is complex and warrants further investigation through elastic-plastic finite element analysis with material model calibration from coupon tests on extruded material.
Study Insights and Implications
This paper provides essential quantitative data for the design of shoulder-reinforced extruded tees in direct-buried district heating systems. The finding that shoulder reinforcement is mandatory for crossing branch configurations represents a significant advancement in design methodology. Engineers should incorporate these findings into their design checklists, ensuring that all large-diameter crossing branch tees in district heating networks receive appropriate reinforcement to prevent premature fatigue failure.