1. Analysis and Conclusions for High-Pressure Manifolds
1) From the stress distribution law of High-Pressure Manifolds, whether the component is a high-pressure straight pipe assembly, Swivel Joint, tee pipe or Hammer Union, the stress level gradually decreases from the inner pipe wall to the outer pipe wall. The stress level is highest at the inner wall because the working load acts directly on the inner wall.
2) Distribution of high-stress areas. In a high-pressure straight pipe assembly, the high-stress area is the inner wall of the pipe. In a Swivel Joint, the high-stress area is on the inner-wall surface of the elbow. In a tee, the high-stress area is at the crotch where the main pipe and branch pipe intersect. The crotch of the tee is a stress-concentration area. This is completely consistent with qualitative analysis and also conforms to structural mechanical performance.
3) Stress exceeding the standard. From finite element calculation results, under a working load of 100 MPa, the stress at the crotch where the main pipe and branch pipe of an imported tee intersect reaches 759.633 MPa. In fact, this is not the true stress at the crotch. That is, the real stress of the tee cannot reach such a high value. This stress level is caused by the finite element model of the tee. The model forms a sharp point at the tee crotch, causing stress concentration, while the actual structure is not a sharp point.
4) Pipeline design life can generally be considered as 10 to 15 years. In addition to meeting strength requirements, the wall thickness of manifold components should include allowance for corrosion, wear and negative manufacturing deviation. The annual corrosion rate may be taken as 0.1 mm. When stress is calculated for all High-Pressure Manifold components with 2 mm wear thickness, normal service requirements are still satisfied.
5) Experimental results for both domestic and imported High-Pressure Manifold materials show that High-Pressure Manifolds have strong rigidity.
6) The high-stress area of a tee is located at the crotch where the two pipes intersect. Under a working load of 100 MPa, Mises stress of most manifold components is between 300 MPa and 370 MPa, and strength reserve is relatively low.
7) The number of alternating-load cycles experienced by High-Pressure Manifolds is relatively low. Therefore, whether using fatigue theory or crack-growth theory, the analysis results show that safe service life of High-Pressure Manifolds is greater than 20 years.
2. Recommendations and Outlook for High-Pressure Manifolds
1) Establish files for in-service manifolds and carry out comprehensive maintenance and inspection every 2 to 3 years. Inspection should include service condition, wear level, corrosion and sudden incidents. When cracks are found, compare them with the research results. If the crack is in a safe state, the component can continue to be used, but crack-growth change should be observed so that problems can be diagnosed in time and High-Pressure Manifolds can operate safely and reliably.
2) The theory and methods of this research can be extended to damage prediction for various types of High-Pressure Manifolds in the petrochemical industry. When working pressure increases, number of alternating load cycles increases, accidental damage occurs, or many other factors change, and for other high-pressure parts not covered in this article, this research method can still be used to make damage predictions for corresponding high-pressure parts.
3) Research on fatigue fracture of high-pressure pipelines and manifolds is still at the linear-elastic stage. Calculation of the crack-tip plastic zone is also based on elastic analysis, with corresponding corrections to obtain approximate answers. Many formulas in fracture mechanics are empirical formulas, and their theoretical foundation is still not solid enough. Therefore, macro and micro fracture mechanics should be combined. Starting from the mechanism of crack formation, fracture mechanisms should be studied to obtain more accurate answers and gradually apply the results to fracture analysis of High-Pressure Manifolds and other pressure vessels. Therefore, this research still needs deeper development.
