Analysis of Large Tees Used in Subcritical Boilers

Literature Overview

This paper by Lang Liping from Harbin Boiler Works Co., Ltd., published in Power Station System Engineering in 2003, provides a comparative analysis of three structural types of large tees used in subcritical boilers: welded, extruded, and punched-welded configurations. The study examines the performance, structural characteristics, and manufacturing processes of each type, offering guidance for selection in high-temperature, high-pressure boiler applications.

Core Technical Content and Analysis

Subcritical boilers operate at pressures typically between 12.5 MPa and 17 MPa, with steam temperatures ranging from 535°C to 570°C. The large tees used in these applications must withstand severe thermal cycling, high internal pressure, and erosive flow conditions. The three structural types each offer distinct advantages and challenges.

Comparison of Three Tee Structural Types

Feature	Welded Tee	Extruded Tee	Punched-Welded Tee
Manufacturing Process	Pipe cutting, forming, welding	Hot or cold extrusion from solid billet	Pipe punching followed by welding
Material Utilization	High	Lower	Moderate
Weld Quality	Critical control required	No welds (homogeneous)	Critical control required
Dimensional Accuracy	Good	Excellent	Good
Cost	Moderate	High	Moderate
Applicability	Large diameters	Medium to large diameters	Medium diameters
Heat Treatment	Required for weld HAZ	Uniform throughout	Required for weld HAZ

The welded tee involves cutting and forming pipe sections followed by butt welding to create the three-way junction. This method offers high material utilization but introduces weld HAZ concerns that require careful heat treatment and non-destructive testing. The extruded tee is manufactured through hot or cold extrusion from a solid billet, producing a homogeneous material structure without welds, which eliminates welding-related defects but increases material cost and limits maximum diameter. The punched-welded tee combines pipe punching with subsequent welding, offering a compromise between material efficiency and structural integrity.

Engineering Practice Implications

In subcritical boiler applications, tee selection involves a careful balance between manufacturing cost, structural reliability, and service life. The welded tee is most commonly used for large-diameter applications where material cost is a significant concern. However, weld quality becomes the critical failure mode, requiring rigorous qualification of welding procedures, welder certification, and comprehensive NDT coverage including radiographic testing and ultrasonic testing.

For the welded tee, the heat-affected zone (HAZ) microstructure and mechanical properties are paramount. The base material is typically a chromium-molybdenum alloy steel such as 12Cr1MoV or 15CrMoG, which requires careful preheating and post-weld heat treatment to ensure adequate toughness and resistance to creep rupture. The extruded tee eliminates welding concerns but requires precise control of extrusion temperature and speed to avoid grain coarsening and ensure uniform mechanical properties throughout the component.

Study Insights and Reflections

This paper provides valuable comparative data for engineering decision-making in boiler component selection. The analysis of manufacturing processes, structural performance, and material behavior under service conditions offers a comprehensive framework for evaluating tee alternatives. The emphasis on welding quality in welded and punched-welded tees underscores the importance of welding procedure qualification and in-process monitoring in high-pressure applications.

From a quality management perspective, the selection of tee type should be based on a systematic risk assessment considering manufacturing capability, inspection requirements, service conditions, and cost constraints. The extruded tee, despite higher manufacturing cost, offers superior structural reliability due to the absence of welds, which may be justified in critical applications where failure consequences are severe.