Analysis of Large Tees Used in Subcritical Boilers

Literature Overview

This technical paper by Lang Liping from Harbin Boiler Works Co., Ltd., published in Power Station System Engineering (2003), provides a comprehensive analysis of large tee fittings used in subcritical boilers. The paper examines three main structural types: welded tees, extruded tees, and punch-welded (formed and welded) tees, analyzing their performance, structural characteristics, and manufacturing processes.

Technical Analysis of Three Tee Types

Welded Tees

Welded tees are fabricated by joining pipe sections through welding processes. This method offers flexibility in size and configuration but introduces weld quality concerns that are critical in high-temperature, high-pressure boiler applications.

Key characteristics:

• Fabrication method: Pipe cutting and welding
• Applicable standards: ASME B16.9, GB/T 12459
• Welding processes: SMAW, GTAW, SAW (depending on wall thickness)
• Quality concerns: HAZ properties, residual stress, weld defects

Extruded Tees

Extruded tees are formed by hot or cold extrusion of a billet or pipe blank through a die. This method produces homogeneous material without weld joints, offering superior structural integrity.

Key characteristics:

• Fabrication method: Hot extrusion or cold extrusion
• Material homogeneity: Excellent
• Wall thickness uniformity: Depends on die design and process control
• Applicable materials: Carbon steel, alloy steel, stainless steel

Punch-Welded (Formed and Welded) Tees

These tees are manufactured by forming a tee shape from a plate or pipe section and then welding the seams. This hybrid approach combines forming and welding technologies.

Key characteristics:

• Fabrication method: Mechanical forming followed by seam welding
• Economical for large sizes
• Quality depends on both forming and welding operations
• Potential for dimensional accuracy issues at intersections

Comparative Analysis

Welding Quality Considerations for Boiler Tees

In subcritical boiler applications, the operating conditions are severe: high temperatures (up to 540°C for steam), high pressures (up to 25 MPa), and cyclic thermal loading. The welding quality of tee fittings is therefore paramount.

Critical welding considerations include:

• Preheating temperature control to prevent cold cracking in low-alloy steels
• Interpass temperature control to limit grain growth in the HAZ
• Post-weld heat treatment (PWHT) to relieve residual stresses and temper the HAZ
• Non-destructive examination (NDE) including RT and UT for volumetric and surface defect detection

Fluid Dynamics Considerations

The paper mentions fluid effects as a keyword, which is relevant because tee fittings create flow disturbances that can lead to erosion-corrosion at the intersection area. In subcritical boilers, the high-velocity steam or water flow through tees can cause localized thinning at the inner radius of the bend, particularly at the run side.

Engineering Practice Insights

From decades of experience in boiler piping and fitting design, I can confirm that the choice of tee type significantly impacts the long-term reliability of subcritical boiler systems. Extruded tees, despite their higher cost, offer the best structural integrity because they eliminate weld joints entirely. However, for very large diameters (above DN400), extrusion becomes impractical, and welded or formed-welded tees become necessary.

The punch-welded approach represents a cost-effective compromise for large sizes, but it requires rigorous quality control of both the forming operation (to ensure proper wall thickness distribution) and the welding operation (to ensure sound weld joints). In practice, the intersection area of formed-welded tees often shows the greatest variation in wall thickness, which can affect pressure containment capability.

Key Questions and Reflections

A critical question that arises is how to ensure consistent quality across different manufacturing methods when all three types are used in the same boiler system. The material properties, particularly creep resistance and thermal fatigue performance, must be equivalent regardless of fabrication method. This requires careful matching of heat treatment schedules and verification through comprehensive mechanical testing.

The paper's analysis highlights the importance of understanding the relationship between manufacturing method and in-service performance. Engineers should not select tee types based solely on cost but should consider the entire lifecycle, including fabrication quality assurance, in-service inspection requirements, and expected maintenance intervals.