Safety Assessment of Tee Structures in Hydrogenated Gasoline Pipelines

Literature Overview

Tian Haiyan and Zhang Weiyi from Beijing University of Chemical Technology conducted a safety assessment of tee branch structures in hydrogenated gasoline pipelines using three-dimensional finite element analysis and stress intensity evaluation in accordance with JB 4732-1995, the Chinese standard for analytical design of steel pressure vessels. The research was funded by the Beijing Outstanding Talent Training Program, reflecting the importance of pipeline integrity assessment in petrochemical applications.

Finite Element Modeling and Stress Evaluation

The authors employed a regularized mesh of 8-node hexahedral elements to construct the three-dimensional finite element model of the tee branch structure. The choice of hexahedral elements is significant because they provide superior accuracy for stress calculations near geometric discontinuities compared to tetrahedral elements, particularly in regions of high stress gradient such as the tee intersection.

The stress intensity evaluation followed the provisions of JB 4732-1995, which requires classification of stresses into primary membrane, primary bending, and secondary categories, each with its own allowable limit. The analysis demonstrated that the tee branch structure satisfies the stress intensity requirements with a substantial safety margin, and that minor uniform corrosion thinning can be neglected in the assessment.

The conclusion that minor uniform corrosion can be neglected is practically significant because it simplifies the inspection and maintenance program for hydrogenated gasoline pipelines. In petrochemical service, corrosion monitoring is a major operational expense, and establishing that the structure has adequate reserve capacity to tolerate minor wall thinning reduces the frequency and extent of required inspections. However, this conclusion applies only to uniform corrosion and does not extend to localized damage such as pitting, crevice corrosion, or stress corrosion cracking, which can be far more detrimental to structural integrity.

Engineering Practice Considerations

The hydrogenated gasoline pipeline environment presents unique challenges due to the presence of hydrogen sulfide, which can induce sulfide stress cracking (SSC) in susceptible materials. The stress assessment performed in this paper addresses mechanical integrity under design loads but does not explicitly evaluate susceptibility to hydrogen-induced cracking. In engineering practice, a complete safety assessment should combine stress analysis with material selection criteria that account for environmental cracking mechanisms, such as those specified in NACE MR0175 or ISO 15156.

The use of regularized hexahedral meshing is a best practice that should be adopted in all tee structure assessments. Irregular meshes, particularly those with high aspect ratio elements or sharp transitions in element size, can produce spurious stress concentrations that lead to conservative or non-conservative conclusions. The regularization approach ensures that stress results near the intersection are reliable and comparable across different analyses.

This paper provides a valuable example of how analytical design methods, when properly applied, can demonstrate structural adequacy with quantifiable safety margins. The approach is directly transferable to other pipeline configurations and operating conditions, provided that the loading scenarios, material properties, and acceptance criteria are appropriately defined.