Design of Large Capacity Unit Hot Extrusion Oblique Tee

Literature Overview

The paper by Liu Tong and Zhao Xiantai, published in the Journal of Liaoning Technical University (Natural Science Edition) in 2004, addresses the design of hot-extruded 45° oblique tees for 300 MW and above ultra-supercritical and supercritical power generation units. At the time of writing, forged oblique tees for main steam piping in such high-capacity units were entirely imported, resulting in high costs, excessive weight, and complications for the piping system's stress analysis and support design. The authors proposed a "stress-area compensation calculation method" by analyzing the design and forming mechanisms of hot-extruded straight tees and forged oblique tees. A 45° hot-extruded oblique tee for a 300 MW unit was designed and verified through finite element stress analysis and stress state simulation experiments, demonstrating that it can fully replace imported forged 45° tees while reducing weight and cost.

Core Technical Points

The hot extrusion process for tees involves forcing heated billet material through a die to form the desired tee geometry. Unlike forged tees, which are produced through multi-step forging operations, hot-extruded tees offer a more continuous forming process that can produce complex geometries in a single or fewer operations. The key challenge in designing oblique tees (as opposed to straight tees) lies in managing the asymmetric material flow and stress distribution that results from the angled branch connection.

The proposed "stress-area compensation calculation method" is the central contribution of this paper. This method accounts for the fact that in an oblique tee, the cross-sectional areas at different locations of the branch and run are not aligned, leading to non-uniform stress distributions. The method compensates for these geometric asymmetries by adjusting the calculated stress values based on the actual area distributions at critical sections.

Process and Standards Analysis

The finite element analysis conducted in this study verified that the hot-extruded tee meets the strength requirements specified in applicable codes. The stress state simulation experiments further confirmed that the material flow during extrusion produces a favorable grain structure and internal quality. The design methodology follows a systematic approach: first establishing the geometric requirements from the piping layout, then applying the stress-area compensation method to determine wall thicknesses, and finally verifying through finite element analysis that the design satisfies code requirements.

Study Insights and Implications

This paper is particularly significant for the domestic pipe fitting industry, as it demonstrates that hot extrusion technology can produce high-quality oblique tees that replace expensive imported forgings. The stress-area compensation calculation method represents a practical engineering solution to the challenge of designing asymmetric fittings, and its validation through both finite element analysis and physical simulation experiments provides strong confidence in its applicability.

From a materials engineering perspective, the hot extrusion process produces a fibrous grain structure that follows the material flow direction, which can be advantageous for fatigue resistance in the critical shoulder regions of the tee. However, it also requires careful control of the extrusion temperature and strain rate to avoid defects such as internal cracking, voids, or unfavorable grain boundary segregation. The reduction in weight achieved through hot extrusion not only reduces material costs but also decreases the loads on piping supports, which is particularly important in high-temperature, high-pressure main steam systems where support design is a critical consideration.

This work exemplifies the importance of integrating process engineering with structural design, where the manufacturing method directly influences the structural performance and economic viability of the final product.