This case study examines the thermal stress analysis conducted on three critical components of a high-pressure heat exchanger:
- Stationary tubesheet
- Cone-shaped channel
- Shell including cone at 1 meter from tubesheet
The heat exchanger was characterized by extreme design parameters, including a tube-side pressure of 690 barg and a shell-side pressure of 10 barg. While the initial temperature difference between tube and shell sides was only 20°C under normal design conditions, an upset scenario with a 100°C temperature difference across the tubesheet was also considered. These extreme conditions necessitated a tubesheet thickness exceeding 300 mm, with the channel side similarly dimensioned to withstand the high pressure differential.
Analysis Methodology
Engineers performed the stress analysis in accordance with ASME Boiler and Pressure Vessel (B&PV) Code Section VIII Division 2. A comprehensive finite element model was constructed, incorporating both temperature and pressure effects. The limited number of tubes allowed for a detailed model that included individual tube holes in the tubesheet.
To accurately represent the system’s behavior, spring elements were integrated into the model to account for the stiffness contributed by the tubes, which was particularly relevant due to the differential expansion between tubes and shell.
Parallel Analysis and Results Interpretation
To gain additional insights, engineers conducted a parallel analysis of the tubesheet using ASME Section VIII, Division 1, paragraph UHX.
This calculation incorporated the effective stiffness and Poisson’s ratio of the perforated tubesheet.The combination of finite element analysis and UHX calculation provided enhanced understanding of stress distribution, particularly bending and shear stresses, through the exceptionally thick tubesheet.
This additional insight was crucial for correctly interpreting the computed results, as the extreme thickness of the tubesheet made standard stress classification according to the ASME B&PV code challenging.
Weld Analysis
The tube-to-tubesheet welds were analyzed using a separate, focused finite element model. Boundary conditions for this smaller model, primarily consisting of tensile loads, were derived from the results of the main finite element analysis.
Key Takeaways
- Extreme pressure differentials in heat exchangers require specialized analysis techniques beyond standard code interpretations.
- Combining multiple analysis methods (FEA and code-based calculations) provides more comprehensive insights into complex stress patterns.
- Detailed modeling of individual tube holes can significantly enhance the accuracy of tubesheet stress analysis.
- Consideration of upset scenarios, such as extreme temperature differentials, is crucial for robust heat exchanger design.
- Separate, focused analyses of critical components like tube-to-tubesheet welds are essential for ensuring overall system integrity.
This case study highlights the importance of advanced stress analysis techniques in designing and evaluating high-pressure heat exchangers. By employing a combination of finite element modeling and code-based calculations, engineers can effectively address the challenges posed by extreme operating conditions and non-standard component geometries.
