Zhuojin Pipe Fitting Co., LtdLimit Pressure Testing and Numerical Simulation of Extruded Tees
Literature Overview
This paper by Xuan Fuzhen and Li Peining from East China University of Science and Technology, published in Oil and Gas Storage and Transportation (2001, Vol. 20, No. 7), presents a combined experimental and numerical study of the limit pressure capacity of cold-extruded tees. The work was supported by the National 95th Five-Year Science and Technology Program (96-918-02-03), reflecting its importance in the Chinese oil and gas industry. The study addresses two representative types of domestically produced cold-extruded tees and compares their experimental limit loads with finite element predictions.
Technical Background
Cold extrusion is a widely used manufacturing process for pipe tees in the oil and gas industry. The process involves forcing a heated billet through a die to form the tee geometry in one or more steps. Cold extrusion produces a favorable grain flow pattern that follows the component geometry, providing excellent mechanical properties. However, the complex deformation patterns during extrusion can introduce residual stresses, geometric variations, and material property gradients that affect the limit pressure capacity of the finished component.
Experimental Methodology
Specimen Preparation
Two types of cold-extruded tees were selected for testing. The specimens were manufactured according to standard production practices, and the wall thickness dimensions at various locations were measured to characterize the geometric accuracy of the extrusion process.
| Measurement Location | Parameter Measured | Purpose |
|---|---|---|
| Run pipe (away from junction) | Wall thickness | Verify uniformity of extrusion |
| Branch pipe (centerline) | Wall thickness | Assess material flow into branch |
| Junction (inner surface) | Wall thickness | Identify thinning at stress concentration |
| Junction (outer surface) | Wall thickness | Assess material displacement |
Yield Test Procedure
The tees were subjected to an overall yield test, which involves applying internal pressure until the material yields globally. This is distinct from a burst test in that the yield test identifies the pressure at which the entire cross-section reaches the yield condition, providing a more conservative design basis.
Finite Element Modeling
A nonlinear finite element model was developed to predict the limit load capacity. The model incorporated material nonlinearity (elastic-plastic behavior) and geometric nonlinearity (large deformation effects) to capture the full yield behavior of the tee.
Key Results
Experimental Limit Pressure
The experimental results demonstrated that the limit load capacity of the cold-extruded tees was not less than the theoretical value calculated for the corresponding straight pipe. This is a significant finding because it indicates that the extrusion process, despite introducing geometric complexity, does not compromise the structural capacity of the component.
Finite Element Validation
The finite element model, built on certain simplifying assumptions, was shown to accurately reproduce the overall yield characteristics observed in the experiments. The agreement between predicted and measured limit loads validates the numerical approach for design purposes.
Wall Thickness Analysis
The wall thickness measurements revealed that the extrusion process produces a non-uniform wall thickness distribution:
- The run pipe away from the junction maintains a relatively uniform wall thickness.
- The branch pipe may exhibit slight thinning at the junction due to material flow into the branch.
- The junction region shows the most significant geometric variation, with the inner surface potentially experiencing thinning and the outer surface experiencing thickening.
Engineering Practice Integration
Design Code Compliance
For pressure piping design under ASME B31.4 or GB/T 3507, the limit pressure of a tee must be established through either analytical methods, finite element analysis, or experimental testing. This study provides experimental data that supports the use of finite element analysis as an acceptable design method for cold-extruded tees, subject to appropriate model validation.
Manufacturing Quality Control
The study highlights several quality control aspects for cold-extruded tees:
- Wall thickness inspection: The wall thickness at the junction is the most critical dimension, as thinning in this region directly affects the limit pressure capacity. Ultrasonic thickness measurement should be performed at multiple angles around the junction.
- Material property verification: The cold extrusion process can alter the material properties, particularly the yield strength and elongation. Tensile testing of specimens taken from the as-extruded tee should confirm that the material properties meet the minimum requirements.
- Residual stress assessment: The extrusion process introduces residual stresses that can be either beneficial (compressive surface stresses) or detrimental (tensile stresses in the HAZ-like regions near the junction). Residual stress measurement by hole drilling or X-ray diffraction can provide valuable data for design analysis.
Process Optimization
The study provides a basis for optimizing the cold extrusion process parameters to maximize the limit pressure capacity:
| Process Parameter | Effect on Limit Pressure | Optimization Direction |
|---|---|---|
| Extrusion temperature | Higher temperature reduces flow stress but may reduce yield strength | Balance formability and strength |
| Die geometry | Affects material flow and wall thickness distribution | Optimize for uniform wall thickness |
| Number of extrusion steps | More steps reduce strain but increase cycle time | Minimize steps while maintaining quality |
| Post-extrusion heat treatment | Relieves residual stress and normalizes microstructure | Standardize for consistent properties |
Study Insights and Reflections
This paper provides a valuable contribution to the understanding of cold-extruded tee performance by combining experimental validation with numerical analysis. The finding that the limit pressure of extruded tees meets or exceeds that of the parent pipe is reassuring for designers and provides confidence in the manufacturing process.
The successful validation of the finite element model is particularly important for engineering practice, as it establishes a reliable computational tool for predicting the performance of extruded tees without the need for costly physical testing of every design variant. This is especially valuable for large-diameter tees where test specimens are expensive and time-consuming to manufacture and test.
The wall thickness analysis reveals an important quality control consideration: the junction region is where the extrusion process introduces the most geometric variation, and this is also where the stress concentration is highest. This creates a potential vulnerability that must be addressed through careful process control and inspection.
From a broader perspective, this study exemplifies the value of integrating experimental testing with numerical analysis in the qualification of pressure piping components. The experimental data provides the ground truth against which numerical models are validated, while the numerical models provide the analytical flexibility needed for design optimization and code compliance. This combined approach is essential for the safe and efficient design of pressure piping systems in the oil and gas industry.