In the chemical processing industry, equipment integrity under extreme conditions is paramount for safety and efficiency. This case study examines the thermal and mechanical analysis of a 2” equal tee subjected to high-temperature fluidized catalyst streams. The project’s objective was to assess the tee’s integrity where a 690°C catalyst stream intersects with room-temperature plant air, creating significant thermal stresses.
Addressing Thermal and Mechanical Challenges
The primary challenge was to evaluate the tee’s ability to withstand the severe thermal gradients and mechanical loads without failure. This assessment was critical due to the potential for creep, thermal fatigue, and structural integrity issues in high-temperature applications. The analysis needed to comply with industry standards, specifically the ASME Boiler and Pressure Vessel Code Section VIII Division 2 (2023) and API 579-1 Section 10.5 for creep stress assessment.
CFD and FEA for Accurate Simulations
The project employed a comprehensive approach combining Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA). Initially, a multi-phase CFD model simulated the interaction between the high-temperature catalyst stream and the plant air. This model calculated temperature and velocity fields, as well as heat transfer coefficients between the flow and the pipe wall.
The CFD results then served as boundary conditions for the FEA model. Two mechanical models were developed: one to assess primary mechanical stresses from internal pressure and sustained forces, and another to evaluate the combination of primary and secondary loads, including thermal stresses.
A mesh sensitivity study was conducted to ensure the accuracy of the simulations, with results showing less than 2% variation in maximum temperatures across different mesh configurations. This step was crucial in validating the reliability of the numerical models.
Evaluation Performance Under Various Conditions
The analysis focused on three critical scenarios: maximum flow, winter conditions, and upset conditions. For each case, the study evaluated plastic collapse, local failure, and cyclic loading in accordance with ASME VIII Division 2 Part 5 guidelines.
The tee, constructed from SA-312 TP321H material, operates in the creep range above 550°C. Consequently, allowable stresses were determined based on local metal wall temperatures, significantly reducing the material’s strength capacity.
Thermal analysis revealed that the maximum wall temperature at the catalyst inlet, intersection, and downstream zones matched the catalyst inlet temperature of 690°C. Heat transfer coefficients were consistent across cases, with the highest heat transfer observed during maximum catalyst flow.
Structural integrity assessments showed that the tee passed the plastic collapse and local failure checks for all three cases. For the maximum flow and winter conditions, the tee also complied with the elastic stress ratcheting check. However, the upset condition case failed the elastic analysis, necessitating an additional elastic-plastic analysis.
Proposed Design Solution: Validating the Tee's Long-Term Performance
Based on the comprehensive analysis, the proposed solution was to maintain the current tee design, as it demonstrated adequate performance under the specified conditions. The evaluation confirmed that:
- The tee successfully withstood the primary mechanical stresses from internal pressure and sustained loads.
- For normal operating conditions (maximum flow and winter cases), the tee exhibited satisfactory performance in terms of elastic stress ratcheting.
- Although the upset condition initially failed the elastic analysis, subsequent elastic-plastic analysis confirmed no plastic stress buildup over three additional cycles, ensuring the tee’s integrity even under these extreme conditions.
Creep analysis, performed using API 579-1 Section 10.5, validated the tee’s performance in the high-temperature creep range. The analysis confirmed a lifespan of 12 years (two cycles of 6 years) for all cases, providing confidence in the long-term reliability of the component.
Conclusion: Implications for High-Temperature Equipment
This case study demonstrates the critical importance of rigorous thermal and mechanical analysis in ensuring the integrity of high-temperature components in chemical processing environments. The combined CFD and FEA approach provided a comprehensive understanding of the tee’s behavior under extreme conditions.
Key findings include:
- The tee design is capable of withstanding the severe thermal gradients and mechanical loads present in the system.
- Creep analysis confirms the tee’s suitability for long-term operation in high-temperature conditions.
- The importance of considering upset conditions and performing additional analyses when standard elastic assessments fail.
The success of this analysis highlights the value of advanced simulation techniques in predicting and mitigating potential failures in critical equipment. It also underscores the necessity of adhering to industry standards and employing conservative approaches when dealing with high-temperature applications in chemical engineering.
Future considerations might include exploring the impact of increased crotch thickness on thermal stress reduction, as the current analysis assumed a thickness equal to the nominal pipe thickness minus a 1mm corrosion allowance. This case study serves as a valuable reference for engineers facing similar challenges in high-temperature equipment design and analysis within the chemical processing industry.
