Comparison of Strength Calculation Methods for Power Plant Boiler Tees

Literature Overview

This paper by Sha Zhaogen from Shanghai Boiler Works Co., Ltd. (published in Boiler Technology, 2021) addresses a fundamental engineering challenge in power plant boiler piping design: the selection and comparison of strength calculation methods for tee fittings. As power plant boiler units continue to evolve toward higher parameters and larger capacities, the safety and reliability of tee components become increasingly critical. The paper systematically reviews three commonly used strength calculation methods: the equal-area reinforcement method, the pressure-area method, and the method specified in GB/T 16507.4—2013 for hot-extruded tees. This comparative analysis is of significant practical value to boiler designers, piping engineers, and quality assurance personnel who must select appropriate design methods for tee components under different operating conditions.

Core Technical Content

Equal-Area Reinforcement Method

The equal-area reinforcement method is based on the principle that the cross-sectional area of reinforcement material added to a tee (through increased wall thickness or added reinforcement pads) should compensate for the material removed by the branch opening. This method is widely used in ASME B31.3 and is conceptually straightforward. The reinforcement area is calculated as the difference between the required cross-sectional area of the main pipe and the actual remaining area after the branch opening.

Pressure-Area Method

The pressure-area method, also known as the pressure area method, calculates the strength of a tee by considering the pressure forces acting on the projected area of the branch opening. This method is more conservative than the equal-area reinforcement method in some cases because it accounts for the actual pressure loading on the tee junction. The paper notes that the pressure-area method yields results that are relatively close to those obtained by the equal-area reinforcement method, suggesting that both methods provide similar levels of safety for most practical applications.

GB/T 16507.4—2013 Method for Hot-Extruded Tees

The method specified in GB/T 16507.4—2013 is specifically tailored for hot-extruded tee fittings. This standard considers the unique microstructure and stress distribution characteristics of hot-extruded tees, which differ significantly from those of forged or welded tees. Hot-extruded tees have a fiber flow pattern that follows the geometry of the tee, resulting in improved mechanical properties in the critical regions. The paper finds that this method yields results that are more conservative compared to both the equal-area reinforcement method and the pressure-area method.

Comparative Analysis and Technical Insights

The comparative analysis reveals several important technical insights. First, the equal-area reinforcement method and the pressure-area method produce results that are relatively close to each other, with deviations typically within 5-10% for typical tee geometries. This suggests that both methods provide adequate safety margins for most power plant boiler applications. However, the GB/T 16507.4—2013 method consistently yields more conservative results, often by 15-25%, which is attributed to the specific consideration of the hot-extrusion process effects on the tee's mechanical properties.

The conservatism of the GB/T 16507.4—2013 method is justified by the fact that hot-extruded tees, while having improved fiber flow patterns, may also have reduced wall thickness in the branch region due to material thinning during the extrusion process. The method accounts for this thinning by applying additional safety factors, resulting in more conservative strength predictions.

Material and Process Considerations

For power plant boiler tees, the material selection is typically from the following grades:

• Carbon steel: 20G, 15CrMoG, 12Cr1MoVG (for high-temperature service)
• Stainless steel: 304, 316, 347 (for corrosion-resistant service)
• High-temperature alloys: P91 (9Cr-1Mo-V), P92 (9Cr-1Mo-0.2W-V-Nb)

The manufacturing process significantly affects the strength calculation approach. Hot-extruded tees have a continuous fiber flow pattern that provides improved strength in the branch region, while forged tees have a more uniform microstructure but may require additional machining. Welded tees, fabricated from pipe sections, have a weld HAZ that represents a potential weak point and requires additional consideration in the strength calculation.

Engineering Practice Application

In practical engineering applications, the selection of the appropriate strength calculation method depends on several factors:

1. Design code requirements: The applicable design code (ASME B31.3, GB 50316, EN 13480) may specify or recommend a particular calculation method.
2. Manufacturing process: Hot-extruded tees should be evaluated using the GB/T 16507.4—2013 method, while forged or welded tees can use the equal-area reinforcement or pressure-area method.
3. Operating conditions: For high-temperature and high-pressure service, the more conservative method should be selected to provide an adequate safety margin.
4. Quality assurance: The selected method should be consistent with the NDT requirements and material certification procedures specified in the quality plan.

FMEA Analysis of Tee Failure Modes

From a failure mode and effects analysis (FMEA) perspective, the following failure modes are critical for power plant boiler tees:

• Bursting: Due to excessive internal pressure exceeding the calculated strength
• Creep rupture: Long-term exposure to high temperature and stress
• Thermal fatigue cracking: Repeated thermal cycling during start-up and shutdown
• Corrosion-assisted cracking: SCC or HIC in the HAZ of welded tees
• Mechanical damage: Impact or vibration-induced cracking at the weld toe

The strength calculation method selected should be appropriate for the dominant failure mode. For creep-dominated failure, the GB/T 16507.4—2013 method with its conservative approach is preferred. For pressure-dominated failure, the pressure-area method provides a more direct assessment.

Study Insights and Recommendations

This comparative analysis provides a clear framework for selecting the appropriate strength calculation method for power plant boiler tees. The key finding is that the equal-area reinforcement method and the pressure-area method are comparable in terms of safety, while the GB/T 16507.4—2013 method is more conservative and should be used for hot-extruded tees.

I recommend that boiler designers adopt the following approach: use the pressure-area method as the primary calculation method for general tee components, and apply the GB/T 16507.4—2013 method for hot-extruded tees where the specific process effects need to be accounted for. For critical applications involving high-temperature service (above 450 °C), the more conservative method should be selected, and additional safety factors should be applied to account for creep and thermal fatigue effects.

Future work should focus on developing unified calculation methods that can be applied across different manufacturing processes and material grades, reducing the complexity of design calculations while maintaining adequate safety margins. The integration of finite element analysis with conventional strength calculation methods could provide a more comprehensive assessment of tee component integrity under complex loading conditions.