Zhuojin Pipe Fitting Co., LtdDesign of Hot-Extruded Oblique Tee for Large-Capacity Power Units
Literature Overview
This paper, published in the Journal of Liaoning Technical University (2004, Vol. 23, Z1), addresses the design of hot-extruded 45° oblique tees for main steam piping in 300 MW and above subcritical and supercritical power units. The authors from Fuxin Pressure Vessel and Piping Company propose a "stress-area compensation calculation method" to design hot-extruded oblique tees that can replace imported forged oblique tees. Finite element analysis and stress state simulation experiments were used to verify that the design meets strength requirements under operating conditions.
Core Technical Content
The problem addressed is significant: at the time of publication, all forged oblique tees for main steam piping in large-capacity units in China were imported, resulting in high costs, long lead times, and complications for pipe system stress analysis and support design. The heavy weight of imported forged tees imposed additional challenges on pipe routing, support design, and thermal expansion management.
The proposed solution is to use hot extrusion instead of forging to produce oblique tees. Hot extrusion offers advantages in material utilization, dimensional accuracy, and cost reduction. However, the transition from straight tees (where hot extrusion was already established) to oblique tees required new design methodologies, as the stress distribution at the branch junction is fundamentally different for angled branches.
Design Methodology: Stress-Area Compensation Calculation
The "stress-area compensation calculation method" proposed in this paper is the central contribution. This method accounts for the stress redistribution that occurs at the junction of the main pipe and the oblique branch by compensating the effective load-bearing area based on the actual stress distribution rather than assuming uniform stress across the cross-section.
| Design Parameter | Forged Tee (Imported) | Hot-Extruded Tee (Proposed) |
|---|---|---|
| Manufacturing process | Forging | Hot extrusion |
| Material utilization | Lower | Higher |
| Weight | Heavier | Lighter |
| Cost | High (imported) | Reduced (domestic) |
| Dimensional accuracy | Good | Excellent |
| Surface quality | Requires machining | Good as-extruded |
| Stress distribution | Uniform assumption | Compensated calculation |
Finite Element Verification
The paper describes finite element stress analysis and stress state simulation experiments to validate the design. The FEA model would have included:
- Geometry: Accurate representation of the 45° oblique tee with appropriate wall thickness distribution
- Material properties: Temperature-dependent stress-strain curves for the main steam piping material (typically 12Cr1MoV or similar)
- Loading conditions: Internal pressure, thermal gradients, and weight loads
- Boundary conditions: Fixed supports representing pipe connection constraints
- Mesh refinement: Concentrated elements at the branch junction where stress concentrations are expected
The stress analysis would have identified critical locations, particularly at the root of the branch and at the saddle of the main pipe, where the geometric discontinuity creates stress concentration factors significantly above the nominal stress level.
Engineering Practice Integration
The design of hot-extruded oblique tees for power plant main steam piping involves several critical engineering considerations:
- Material selection: The main steam piping material must withstand high temperatures (typically 540-566°C for subcritical units) and pressures (16-17 MPa or higher), requiring careful selection of alloy steel grades with appropriate creep resistance.
- Wall thickness design: The wall thickness must account for internal pressure, bending moments from pipe weight and thermal expansion, and stress concentration at the branch junction. The stress-area compensation method provides a more accurate basis for wall thickness determination than simple pressure vessel formulas.
- Manufacturing process control: Hot extrusion parameters—extrusion temperature, die design, reduction ratio, and cooling rate—must be carefully controlled to achieve the required mechanical properties and dimensional accuracy. The grain flow pattern in the extruded product must be aligned with the principal stress directions to maximize structural integrity.
- Quality assurance: Non-destructive testing (UT, MT, PT) must be applied to the extruded tee, with particular attention to the branch junction area where internal defects are most likely to be detrimental. Mechanical property testing, including tensile, impact, and hardness tests, must confirm that the material meets the specified requirements.
Study Insights and Implications
This paper represents an important milestone in the domestication of critical piping components for large-capacity power units in China. The development of a validated design methodology for hot-extruded oblique tees eliminates dependence on imports and reduces costs while maintaining structural integrity. The stress-area compensation calculation method provides a practical engineering tool that bridges the gap between simplified pressure vessel formulas and full finite element analysis, making it accessible for routine design work. The successful validation through FEA and stress state simulation experiments demonstrates the importance of computational analysis in modern fitting design, enabling confident adoption of new manufacturing processes for critical components. This work also highlights the broader principle that process innovation—replacing forging with hot extrusion—can yield significant benefits in cost, weight, and material efficiency, provided that rigorous design and validation methodologies are established to ensure structural reliability.