Safety Assessment of Tee Branch Structure in Hydrotreated Gasoline Piping

Literature Overview

This paper by Tian Haiyan and Zhang Weiyi from Beijing Institute of Petrochemical Technology, published in Chemical Equipment and Piping in 2009 (Volume 46, Issue 4, pp. 45-47), presents a finite element stress analysis and stress intensity assessment of a tee branch structure in a hydrotreated gasoline pipeline. The work was supported by the Beijing Outstanding Talent Training Project (Grant No. 20041D0500516). The assessment was conducted in accordance with JB 4732-1995, the Chinese standard for analytical design of steel pressure vessels.

This topic is directly relevant to piping engineers working in the petrochemical industry, where hydrotreated gasoline pipelines operate under conditions that require careful attention to structural integrity, particularly at tee junctions where stress concentrations are highest.

Application Context

Hydrotreated gasoline pipelines in petrochemical plants operate under conditions that present specific challenges:

The tee branch structure in such pipelines is a critical component that must withstand combined mechanical and thermal loading while maintaining integrity over the design life. The presence of hydrogen in the medium introduces additional concerns related to hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC), which can compromise the material's resistance to stress corrosion.

Finite Element Modeling

The authors employed a three-dimensional FEA model using regular eight-node hexahedral elements. The choice of element type is significant because:

• Hexahedral elements: Provide superior accuracy for stress analysis compared to tetrahedral elements, particularly in regions with complex stress states.
• Regular mesh: Ensures consistent element quality and reduces numerical artifacts that can affect stress predictions.
• Three-dimensional model: Captures the full stress state at the tee junction, including out-of-plane effects that two-dimensional models cannot represent.

Model Setup

The FEA model incorporated the following elements:

1. Geometry: Accurate representation of the tee branch structure, including pipe dimensions, wall thicknesses, and weld geometry.
2. Material properties: Elastic and plastic material properties for the pipe steel, including yield strength, ultimate tensile strength, and strain-hardening behavior.
3. Loading conditions: Internal pressure, thermal loads, weight of the piping system, and support reactions.
4. Boundary conditions: Realistic representation of support constraints and connection to the rest of the piping system.

Stress Intensity Assessment

The stress intensity assessment was conducted in accordance with JB 4732-1995, which provides a methodology for evaluating the structural integrity of pressure vessels and piping components using limit analysis and fracture mechanics principles. The assessment involves:

Key Results

The assessment concluded that the tee branch structure satisfies the stress intensity requirements specified in JB 4732-1995, with a substantial safety margin. The safety margin is sufficient to accommodate minor uniform corrosion thinning without compromising structural integrity.

Engineering Practice Considerations

The findings of this assessment have important implications for the design and operation of hydrotreated gasoline piping systems:

• Corrosion allowance: The large safety margin allows for the use of a reduced corrosion allowance, which can result in material savings and lighter piping systems.
• In-service inspection: The stress assessment provides a basis for determining the required inspection frequency and acceptance criteria for in-service inspection.
• Retrofit potential: The safety margin may allow for modifications or additions to the piping system without requiring complete redesign.

Welding Quality Requirements

Given the critical nature of the tee branch structure, the welding requirements are stringent:

The welding quality directly affects the stress distribution in the tee structure, as weld defects and residual stresses can significantly reduce the fatigue and fracture resistance of the component.

Study Insights and Reflections

This paper demonstrates the value of rigorous FEA-based stress assessment in establishing confidence in the structural integrity of critical piping components. The use of regular hexahedral elements and the application of a recognized analytical design standard (JB 4732-1995) provide a solid technical foundation for the assessment.

The finding that the tee branch structure has a substantial safety margin is valuable for engineering practice, as it provides flexibility in design decisions related to corrosion allowance, inspection intervals, and retrofit potential. However, it is important to recognize that the safety margin is based on the specific loading conditions and material properties assumed in the analysis, and any changes to these assumptions would require re-assessment.

The work also highlights the importance of considering the full range of loading conditions in the stress assessment, including thermal loads and support reactions, which can significantly affect the stress distribution at tee junctions. A comprehensive loading analysis is essential for a reliable structural assessment.