Plastic Limit Internal Pressure of Welded Tees with Full-Circumference Incomplete Penetration at the Intersection Line

Literature Overview

The paper by Zhang Baogui, Sun Liang, and Shuai Jian, published in the journal Pressure Vessel (Volume 23, Issue 6, 2006, pp. 10–16), addresses one of the most critical integrity concerns in welded tee fittings used in high-pressure piping systems. The study investigates the plastic limit internal pressure of butt-welded tees that contain a full-circumference incomplete penetration defect at the intersection line (the phase-intersection line between the branch and the run pipe). The work was funded under the National "10th Five-Year Plan" Science and Technology Key Project (2004BA803B02-05), reflecting its importance to national energy infrastructure safety. The authors employed finite element analysis (FEA) to systematically calculate and analyze the plastic collapse pressure, identify the dominant factors influencing this limit load, and derive an engineering estimation formula that is both simple in form and accurate in prediction.

Core Technical Content and Methodology

Defect Geometry and FEA Modeling

The study focuses on a specific and severe welding defect: incomplete penetration (IP) that extends around the entire intersection line of a welded tee. In butt-welded tees, the branch pipe is welded to the run pipe, and the intersection line represents the geometric transition where the two cylindrical surfaces meet. A full-circumference IP defect at this location creates a continuous ligament loss around the entire branch, which is far more detrimental than a local IP at a single point. The authors modeled this defect geometry in a three-dimensional FEA framework, considering the elastic-plastic material behavior of the pipe steel under internal pressure loading.

The key technical parameters examined in the FEA study include:

Identification of Dominant Factors

Through parametric FEA studies, the authors identified the principal factors governing the plastic limit pressure of the defective tee. The most significant finding is that the defect depth ratio (a/t) is the dominant factor: as the incomplete penetration depth increases relative to the wall thickness, the plastic limit pressure decreases dramatically, following a nonlinear relationship. The wall thickness ratio and diameter ratio also exert substantial influence, with thinner branch walls and larger diameter ratios leading to lower collapse pressures. The material hardening behavior (strain hardening exponent n) has a secondary but non-negligible effect on the limit load.

Engineering Estimation Formula

The authors fitted the FEA results to derive a practical engineering estimation formula for the plastic limit internal pressure. The formula takes a compact form that captures the essential physics while remaining computationally accessible for field engineers. The formula structure can be expressed as:

P_pl = f(a/t, t_branch/t_run, D_branch/D_run, σ_y)

where P_pl is the plastic limit pressure, and the function f encapsulates the nonlinear interactions between geometric parameters and material properties. The accuracy of this formula was validated against the full FEA dataset, demonstrating high precision across the parameter ranges studied.

Standards and Engineering Practice Implications

Relevance to Fitting Integrity Assessment

This research has direct implications for the integrity assessment of in-service welded tees under standards such as ASME B31.3 (Process Piping), ASME B31.4 (Piping for Liquid Fuels, Oils, and Gases), and the Chinese GB/T and SY/T standards governing pressure piping. When a full-circumference incomplete penetration is detected at the intersection line of a welded tee during inspection (e.g., via radiographic testing or ultrasonic testing), the plastic limit pressure formula derived in this study provides a quantitative basis for fitness-for-service (FFS) evaluation.

Comparison with Existing Assessment Methods

Quality Control Implications for Welding Practice

From a welding quality control perspective, this research underscores the critical importance of ensuring complete penetration at the intersection line of welded tees. The full-circumference nature of the defect means that even a small percentage of IP can create a continuous weak zone that governs the collapse pressure of the entire fitting. This has direct implications for:

• Welding procedure qualification (WPQ) requirements for tee fabrication
• The necessity of post-weld radiographic testing (RT) at the intersection line
• The acceptance criteria for incomplete penetration under standards such as ASME B31.3 Appendix A or SY/T 0413
• The importance of proper weld joint preparation (bevel angle, root gap) at the intersection line

FMEA Perspective on Incomplete Penetration

Applying a Failure Mode and Effects Analysis (FMEA) approach to the welding process of tees:

Study Insights and Reflections

After reviewing this paper, I find the work to be both timely and practically valuable. The systematic approach of combining FEA with parametric studies to derive an engineering formula is a methodology that should be more widely adopted in the Chinese pressure vessel and piping engineering community. The focus on the full-circumference IP defect at the intersection line is particularly astute, as this is precisely the type of defect that is most difficult to detect and most consequential in terms of structural integrity.

One area where I would suggest further development is the extension of this work to consider the combined effects of IP defects with other common welding defects such as undercut, porosity, and slag inclusion. In practice, defects rarely occur in isolation, and the interaction between multiple defect types at the intersection line could further reduce the plastic limit pressure. Additionally, the study could benefit from experimental validation through burst tests on instrumented tee specimens with simulated IP defects, which would provide confidence in the FEA model's accuracy.

Another important consideration is the effect of residual stresses from the welding process on the plastic limit pressure. The FEA model in this study likely assumes a stress-free initial state, whereas in reality, the welding process introduces significant residual stresses that can either increase or decrease the effective collapse pressure depending on their distribution and the loading mode. Incorporating welding residual stresses into the FEA model would provide a more realistic assessment of the in-service performance of defective tees.

The engineering estimation formula derived in this study should be integrated into integrity assessment software used by inspection companies and operators of pressure piping systems. The formula's simplicity makes it suitable for rapid field assessments, while its accuracy provides confidence in the results. This represents a valuable bridge between academic research and industrial practice, and I commend the authors for this contribution to the field of pressure vessel and piping integrity engineering.