This article delves into the finite element (FE) analysis of the flange and flued expansion joint within a reboiler. The focus is on both low and high cycle fatigue, as well as the primary stress assessment, in compliance with the ASME Section VIII Division 2 Code. According to Appendix 5 of the ASME Section VIII Division 1 Code, FE analysis is a permissible method for calculating the primary and secondary stresses in the expansion joint.
Assessment
The assessment involved six static stress cases applied to the reboiler body, ranging from idle operation to design conditions. These cases provided a conservative estimate of the stress range within the reboiler, with Von Mises stress calculations performed at seventeen different locations within the expansion joint body.
Two scenarios were considered during the fatigue and primary stress assessments:
- Uniform mean wall thickness as specified in the manufacturing drawings.
- Uniform mean wall thickness with minimum wall thickness specified at the expansion joint.
This dual-scenario approach ensures that wall thinning during the forming process does not affect the assessment results.
A primary loading assessment of the reboiler body was conducted to prevent plastic collapse and rupture during operation. Stress linearization was performed in high stress concentration regions of the expansion joint to evaluate membrane and bending stresses. These values were compared to the allowable stress as defined by ASME Section VIII Division 1. The primary stress results were validated using a second finite element method program, Pro-Mechanica, to ensure accuracy. A similar method was applied for the low and high cycle fatigue assessment.
Results
The analysis revealed that the reboiler body is subjected to medium cyclic stress levels, within 85% of the code allowable limits, meeting the low and high cycle fatigue criteria of ASME Section VIII Division 2. The high cycle fatigue analysis indicated that the reboiler could withstand over 100,000 cycles throughout its operational lifetime. The primary stress on the reboiler body and expansion joint remained within 78% of the material’s allowable code stress.
Conclusions
The finite element analysis of the flange and flued expansion joint of a reboiler demonstrates that the design adheres to the fatigue and primary stress criteria specified in the ASME Section VIII Division 2 Code. The thorough assessment confirms the reboiler’s durability and operational safety, capable of enduring significant cyclic stresses within the permissible limits.
