Zhuojin Pipe Fitting Co., LtdApplication of Common Opening Reinforcement Methods in Plugging Tees
Literature Overview
This paper by Wu Jiangqiao and colleagues from China Petroleum Natural Gas Pipeline Research Institute and Southwest Petroleum University was published in Oil and Gas Storage and Transportation (2016, Vol. 35, No. 7, pp. 793-798). The study addresses the design and strength calculation of plugging tees, which are specialized cylindrical shell radial nozzles with large opening ratios used in pipeline plugging operations. The authors compared three reinforcement calculation methods using a 1422 mm diameter plugging tee as the case study.
Core Technical Points
Plugging tees represent a unique engineering challenge because their opening ratio (branch diameter to main pipe diameter) often exceeds the limits specified in conventional pressure vessel codes. The study evaluated three calculation approaches:
| Method | Standard Reference | Opening Ratio Limit | Result for 1422 mm Case | Applicability |
|---|---|---|---|---|
| Pressure-area method | ASME BPVC VIII-1 | d/D <= 0.5 | Insufficient safety margin | Not recommended |
| Pressure-area stress method (VIII-1) | ASME BPVC VIII-1-2010 | d/D <= 0.5 | Deviates from FEA results | Limited applicability |
| Pressure-area stress method (VIII-2) | ASME BPVC VIII-2-2010 | No limit | Closest to FEA results | Recommended |
| FEA stress analysis | Section VIII-2 Part 5 | No limit | Reference benchmark | Verification tool |
The critical finding is that the pressure-area method (MMA) specified in ASME BPVC Section VIII-1 provides insufficient safety margins for large opening ratios typical of plugging tees. The pressure-area stress method in Section VIII-2, which does not restrict the opening ratio and offers simplified calculation procedures, produces results most consistent with finite element analysis.
Process and Standards Analysis
The design of plugging tees must account for several unique loading conditions:
- Internal pressure during pipeline plugging operations (typically up to 10 MPa for high-pressure gas pipelines)
- External mechanical loads from pipeline weight and soil loads
- Thermal stresses during hot tapping operations
- Dynamic loads during pipeline depressurization
The ASME BPVC Section VIII-2 approach employs a more sophisticated stress evaluation methodology based on the concept of primary membrane stress (Pm), primary bending stress (Pb), and primary plus secondary stress (Pm+Pb), with corresponding allowable stress limits:
| Stress Category | Allowable Limit | Physical Significance |
|---|---|---|
| Pm | S (allowable stress) | Prevents gross plastic deformation |
| Pb | 1.5S | Prevents excessive bending deformation |
| Pm+Pb | 1.5S | Combined primary stress limit |
| Pm+Pb+Q | 2.0S | Includes thermal stress |
Integration with Engineering Practice
In pipeline plugging operations, the plugging tee is installed on a live pipeline to create a branch connection without depressurizing the main line. A practical case from a 1422 mm diameter gas pipeline project in Southwest China involved the design of a plugging tee with a 400 mm branch opening (opening ratio = 0.28). The design process involved:
- Initial sizing using the pressure-area method per ASME VIII-1
- Verification using FEA per Section VIII-2 Part 5
- Discovery that the MMA result was non-conservative by approximately 25%
- Redesign using the Section VIII-2 pressure-area stress method
- Final verification through detailed FEA with appropriate boundary conditions
The welding of the reinforcement ring to the main pipe shell requires careful control of:
- Preheat temperature (typically 100-150 degrees C for carbon steel)
- Interpass temperature (limited to 250 degrees C maximum)
- Weld procedure qualification per AWS D10.9 or ISO 15614-1
- Post-weld heat treatment for materials above specified thickness thresholds
Key Questions and Reflections
The preference for ASME BPVC Section VIII-2 in plugging tee design raises questions about the applicability of Chinese national standards (GB/T 150) for similar applications. The GB/T 150 pressure-area method follows the same fundamental approach as ASME VIII-1 and would likely produce similar non-conservative results for large opening ratios. There is a clear need for Chinese standardization bodies to develop or adopt methodologies equivalent to ASME VIII-2 for large opening reinforcement calculations.
Study Insights and Implications
This paper provides a clear recommendation for the use of ASME BPVC Section VIII-2 pressure-area stress method in plugging tee design calculations. The finding that this method produces results consistent with FEA while maintaining computational simplicity makes it the preferred approach for engineering practice. Design engineers should adopt this methodology as the primary calculation method for all large-opening tee applications, with FEA reserved for verification of critical or unconventional configurations.