Optimization of Unequal Wall Thickness Connection Structure for Natural Gas Station Tees

Literature Overview

This study, published in Pressure Vessel (Vol. 39, No. 9, 2022, pp. 32–41) by Yang Ming and colleagues from the Western Pipeline Co., Ltd. of National Pipeline Network Group and China Petroleum Natural Gas Pipeline Engineering Co., addresses a critical engineering challenge in high-pressure natural gas transmission systems. The paper focuses on the largest station tee specification currently in engineering application — P 12 MPa, DN1400 mm × 1200 mm, X80 steel grade — and proposes two novel connection solutions to achieve equal internal diameter joining between the tee and process pipelines. The research employs elastoplastic finite element analysis combined with response surface methodology (RSM) for parameter optimization, ultimately validated by hydrostatic burst testing.

Core Technical Problem and Design Philosophy

The fundamental challenge in large-diameter natural gas station tees lies in the substantial wall thickness difference between the tee body and the connected process pipeline. Traditional butt-weld connections between components of differing wall thicknesses create significant stress concentration at the weld root, leading to premature fatigue failure under cyclic loading conditions. At 12 MPa operating pressure with X80 grade steel (minimum yield strength 552 MPa), the hoop stress in the pipeline wall approaches 600 MPa, leaving limited margin for additional localized stress peaks.

The authors propose two conceptual approaches:

1. Enlarging the tee internal diameter to match the process pipeline ID, thereby reducing the wall thickness differential at the connection interface.
2. Adopting a novel internal tapered groove (inner bevel groove) preparation on the traditional tee, creating a gradual wall thickness transition zone that eliminates abrupt geometric discontinuities.

The second approach is particularly innovative because it preserves the tee's structural integrity while modifying only the local geometry at the connection point, making it more practical for field fabrication and retrofitted installations.

Finite Element Analysis and Optimization Methodology

The elastoplastic finite element model was constructed using a bilinear kinematic hardening model to capture the cyclic plastic behavior of X80 steel under operating loads. The response surface methodology was applied to optimize three key geometric parameters:

The optimization criterion was the elimination of stress concentration points within the connection region. The response surface model achieved a high R² value (implied >0.95 based on the successful validation), indicating that the quadratic approximation adequately captured the nonlinear relationship between geometric parameters and maximum von Mises stress.

Hydrostatic Burst Test Validation

The optimized structure was fabricated and subjected to hydrostatic burst testing, which serves as the ultimate proof of structural adequacy. The burst pressure exceeded the design pressure with an adequate safety factor, confirming that the optimized geometry successfully eliminated the stress concentration that would otherwise have initiated crack propagation at the weld toe. This experimental validation is particularly important because finite element predictions of burst behavior in thick-walled pressure vessels are known to be sensitive to mesh refinement and material model accuracy.

Engineering Practice Implications and Standards Considerations

From a standards perspective, this work addresses a gap in current Chinese national standards (GB/T 20801, SY/T 0413) and international codes (ASME B31.8, ISO 15590) regarding the connection design of large-diameter station tees with unequal wall thicknesses. The ASME B31.8 Code does not explicitly cover the internal bevel groove approach proposed here, meaning that any implementation would require a code case or alternative design justification under ASME Section VIII, Division 2 or the Chinese equivalent TSG 21.

In practice, the proposed 59 mm wall thickness with 35 mm internal bevel groove depth represents a practical fabrication solution. The groove preparation can be accomplished by CNC machining or plasma cutting followed by grinding, with the groove surface requiring visual inspection (VT) and magnetic particle testing (MT) per ASME B16.9 and SY/T 0413 requirements. The welding procedure should follow a multi-pass sequence with preheat temperature of 100–150°C for X80 steel at this wall thickness, using low-hydrogen electrodes (E71T-8 or equivalent) with interpass temperature control below 250°C.

Key Technical Insights and Reflections

After careful study of this paper, several important observations emerge. First, the use of response surface methodology for structural optimization in pressure vessel design is a relatively novel approach in the Chinese pipeline engineering community, and its successful application here suggests broader applicability to other complex geometric optimization problems in station equipment design. Second, the hydrostatic burst test, while definitive, is a destructive method that does not reveal the fatigue performance under cyclic loading — a concern for station tees subjected to pressure transients during compressor start/stop operations. Future work should incorporate fatigue life assessment using the API 579 or ASME FFS-1 framework.

The paper also highlights an important principle in pressure vessel design: that geometric continuity, not just material strength, governs structural performance at connection interfaces. This philosophy aligns with the "stress-based design" approach advocated in ASME BPV Code Section VIII, Division 2, where the primary concern is not exceeding the allowable stress at any point rather than ensuring adequate cross-sectional area.

This research provides a valuable reference for the design and fabrication of future large-diameter natural gas tees in China's expanding pipeline network, particularly for projects involving X80 and higher grade steels at pressures exceeding 10 MPa. The methodology demonstrated — combining numerical optimization with experimental validation — sets a benchmark for evidence-based design in critical pipeline components.