Limit Pressure Testing and Numerical Simulation of Extruded Tees

Literature Overview

The study by Xuan Fuzhen and Li Peining from East China University of Science and Technology, published in Oil and Gas Storage and Transportation in 2001, presents a systematic investigation of the limit pressure capacity of cold-extruded tees through both experimental testing and finite element analysis. This research, supported by the National "9th Five-Year Plan" Key Science and Technology Project, addresses a fundamental question in pressure vessel and piping engineering: whether extruded tees can achieve equivalent or superior pressure-bearing capacity compared to seamless pipes of the same material and dimensions. The findings have direct implications for the design, qualification, and acceptance of extruded fittings in oil and gas pipeline systems.

Technical Background and Manufacturing Process

Cold extrusion is a metal forming process that produces tees by forcing a heated or cold billet through a die to create the tee geometry. The process offers several advantages over welding or seamless pipe machining:

• Grain flow continuity: The grain structure follows the component geometry, enhancing mechanical properties
• Material utilization: High material utilization rates compared to machining from solid bar stock
• Dimensional accuracy: Excellent dimensional control and repeatability
• Mechanical properties: Superior mechanical properties due to work hardening during extrusion

For oil and gas applications, extruded tees must meet the requirements of:

• API 5L: Specification for line pipe, applicable to the base pipe material
• API 5CT: Specification for casing and tubing, for reference on material requirements
• ASME B31.4: Code for liquid hydrocarbon pipelines
• ASME B31.8: Code for gas transmission and distribution pipelines
• SY/T 5257: Chinese standard for steel pipe fittings in oil and gas industries

Material Selection for Extruded Tees

Typical materials for cold-extruded tees in oil and gas applications include:

• Carbon steel: API 5L Gr. B, X42, X52, X60, X65, X70
• Low-alloy steel: API 5L PSL2 grades with improved toughness
• Stainless steel: For sour service or corrosive environments
• CRA (Corrosion Resistant Alloy): For sour gas service with H2S and CO2

The material selection must consider the service environment, including temperature, pressure, corrosion potential, and mechanical loading conditions.

Experimental Methodology and Results

Overall Yield Test Procedure

The study conducted overall yield tests on two typical domestic cold-extruded tees, measuring wall thickness dimensions at multiple locations and determining the limit pressure through hydrostatic pressure loading until yielding occurred. The test procedure included:

1. Dimensional measurement: Wall thickness measurements at branch root, branch crown, main pipe crown, and transition regions
2. Hydrostatic loading: Gradual pressure increase with strain measurement at critical locations
3. Yield determination: Identification of yield pressure based on strain measurement criteria (typically 0.2% offset method)
4. Post-test examination: Fracture surface analysis and dimensional verification after test failure

Key Findings on Limit Pressure Capacity

The experimental results demonstrated that:

The most significant finding is that the extruded tees' limit pressure capacity is not less than the theoretical value for equivalent seamless pipes. This result validates the use of extruded tees in pressure-containing applications and provides confidence in their structural integrity.

Finite Element Analysis Validation

The study also performed nonlinear finite element analysis (FEA) on simplified models of the extruded tees to verify the experimental results. The FEA model incorporated:

• Material nonlinearity: Bilinear or multilinear isotropic hardening material model
• Geometric nonlinearity: Large deformation analysis accounting for wall thinning and bulging
• Contact conditions: Appropriate boundary conditions at pipe ends

The FEA results showed good agreement with experimental measurements, validating the numerical model as a useful tool for predicting tee performance under pressure loading.

Engineering Practice Integration

Design and Acceptance Criteria

Based on the study's findings, the following design and acceptance criteria should be considered for extruded tees:

1. Limit pressure requirement: The limit pressure should be at least 1.5 times the design pressure, providing adequate safety margin
2. Wall thickness tolerance: Wall thickness variations should be within ±10% of nominal, with minimum wall thickness verified at critical locations
3. Material certification: Full material certification including chemical composition, mechanical properties, and heat treatment records
4. NDT requirements: UT or RT inspection of critical regions, particularly the branch root, to detect internal defects

Quality Control Procedures

The following quality control procedures are recommended for extruded tee manufacturing:

• Incoming material inspection: Verification of raw material chemical composition and mechanical properties against specification requirements
• Process monitoring: Control of extrusion temperature, speed, and lubrication to ensure consistent product quality
• Dimensional inspection: Verification of critical dimensions including wall thickness, branch root radius, and overall geometry
• Mechanical property testing: Tensile testing of coupon specimens from each production lot to verify mechanical properties
• Non-destructive testing: UT or RT inspection of all critical welds and branch root regions
• Hydrostatic testing: 100% hydrostatic pressure testing at 1.5 times design pressure for minimum 5 minutes

Comparison with Welded Tees

The study's findings provide a basis for comparing extruded tees with welded tees:

The extruded tee offers superior structural integrity but at higher manufacturing cost, making it suitable for critical applications where reliability is paramount.

Key Questions and Reflections

The study's finding that extruded tees achieve limit pressure capacity equal to or exceeding equivalent seamless pipes raises important questions about current design codes and standards. Many codes apply uniform design factors to all fittings regardless of manufacturing method, potentially underestimating the capacity of extruded fittings. This suggests an opportunity for code harmonization that recognizes the superior structural integrity of extruded fittings through reduced design factors or increased allowable stresses.

Another consideration is the effect of long-term creep and relaxation on extruded tee performance. The study's tests were conducted at ambient or moderate temperatures, but oil and gas pipeline applications may involve elevated temperatures where creep becomes significant. The superior grain flow of extruded fittings should provide better creep resistance, but this benefit should be quantified through long-term creep testing for critical applications.

Study Insights and Engineering Recommendations

This paper provides strong experimental and analytical evidence supporting the use of cold-extruded tees in oil and gas pipeline systems. The key recommendation is to adopt extruded tees for critical applications where structural integrity is paramount, particularly in sour service, high-pressure applications, and fatigue-critical locations. The study's FEA methodology should be adopted as a standard verification tool for extruded tee design, complementing experimental testing with numerical analysis for detailed stress evaluation. Additionally, the findings support advocacy for code modifications that recognize the superior performance of extruded fittings through appropriate design factor adjustments. The integration of these findings into engineering practice will enhance the reliability and safety of oil and gas pipeline systems while optimizing material utilization and reducing lifecycle costs.