Dynaflow Research Group was asked to conduct a comprehensive finite element (FE) analysis on a custom hairpin heat exchanger. The primary objective was to assess the exchanger’s structural integrity under demanding cyclic loading conditions, focusing on fatigue life and ratcheting behavior in compliance with the ASME Boiler and Pressure Vessel Code, Section VIII Division 2.
This case study details the analysis methodology, critical findings, and key engineering takeaways relevant for the design and assessment of pressure equipment subjected to cyclic thermal and mechanical loads.
Project Background and Engineering Challenge
The analyzed unit is an 8 5/8″ hairpin heat exchanger, designed with TAPER-LOK® closures on the tube side and a standard triangular seal ring on the shell side. The right-hand support, closest to the tube sheets, provides full restraint. The left-hand saddle allows axial movement, functioning as a guide restraint. The primary process challenge stemmed from the operational cycle: the shell side is continuously pressurized with ethylene glycol/water, while the tube side, carrying chilled natural gas, is subjected to pressure and temperature cycling — 21 cycles per day over the heat exchanger lifetime.
The analysis required accurate prediction of the allowable number of cycles for all critical regions, ensuring compliance with ASME B&PV VIII-2 fatigue and ratcheting criteria and confirming the robustness of design against fatigue-induced failure.
Methodology
The assessment followed a structured three-step FE analysis approach, utilizing ANSYS Workbench 2020 R1 for all simulations:
- Thermal Boundary Determination:
Heat transfer coefficients for all surfaces were determined using standard thermodynamic relationships and analytical methods. Tube-side coefficients were taken from HTRI calculations and the final heat transfer coefficients were verified with KHT. - Thermal Analysis:
A series of FEA models of the critical regions in the heat exchanger were made. Using the heat transfer coefficients from Step 1, the temperature field in the heat exchanger was determined. Brick elements were used for the critical regions of interest. The holes in the tube sheet were modeled explicitly. A single transient thermal calculation is performed to consider the thermal transient effects during heat up and cooling down. - Mechanical Analysis:
The temperature fields from the thermal analysis were combined with all pressure and nozzle loads. Stresses were linearized (where appropriate) along stress classification lines (SCLs) and assessed for both fatigue (cyclic stress amplitude) and ratcheting (progressive plastic deformation) in accordance with ASME VIII-2 Part 5.5.3 and 5.5.6, respectively.
A separate sub-model was developed for the tube-to-tube sheet weld, as the main FE model did not fully capture the local behavior of these connections. Caesar II software was used to estimate the thermal expansion-induced axial load within the tube bundle, which was then incorporated into the assessment.
Key Analysis Inputs
- Process Cycle: Tube side pressure cycles between ambient and 4214.7 psia, with natural gas entering at 109.7°F and leaving at 7°F. The shell side is constantly at 75 psia and -3.8°F to 16°F. The operational cycle of 21 per day was simulated, with corresponding thermal and pressure transients applied in the FE model.
- Materials: All major pressure components were modeled with temperature-dependent properties from ASME Section II, Part D.
- Loads: All relevant bolt preloads, nozzle loads, and moments were included as per supplied drawings and specifications. Nozzle loads on the tube side were considered fully cyclic, while those on the shell side were treated as non-cyclic.
- Quality Factors: Welds were assessed with appropriate fatigue strength reduction factors (Kf), reflecting actual fabrication quality and in accordance with ASME guidance.
Results and Discussion
The FE analysis highlighted several critical regions for fatigue and ratcheting assessment:
- Bonnet and Shell Closures: Both welds were found to have ratcheting and fatigue stresses well below allowable limits. Estimated fatigue life exceeded 100 years, with maximum fatigue stress ranges at only a fraction of the allowable for a 30-year design life.
- Barrel Welds: The tube sheet barrel butt welds conform with the ASME B&PV VIII-2 allowable values for ratcheting and fatigue at all locations. Fatigue life estimates again exceeded 100 years.
- Nozzle Welds: Both shell and tube nozzle welds were analyzed under combined thermal, pressure, and nozzle loading. All locations passed ASME VIII-2 checks, with the highest fatigue utilization at 79% of allowable for a 30-year design.
- Saddle to Hairpin Connections: The fixed support welds were among the most stressed regions due to the combined effect of cyclic thermal expansion and mechanical loading. Even with conservative Kf factors (1.7 at toe, 4.0 at root), the majority of locations exhibited fatigue lives far beyond 30 years. The most critical points approached allowable fatigue limits, but all remained compliant.
- Tube-to-Tube Sheet Weld: This fillet weld was identified as the most critical location, particularly due to the high cyclic pressure loads. Analysis required a conservative Kf of 3.0, achievable only with post-fabrication volumetric examination as per code. Even in this worst-case scenario, fatigue life estimates met or exceeded the 30-year target, provided quality control procedures are followed.
A mesh dependency study demonstrated that the chosen mesh density did not significantly affect the results, with calculated fatigue lives and ratcheting margins remaining consistent across refined and unrefined meshes.
Conclusions
The FE analysis provided a comprehensive, code-compliant evaluation of the hairpin heat exchanger under severe cyclic loading. All critical regions were demonstrated to conform to ASME B&PV VIII-2 fatigue and ratcheting criteria, with no modifications to the existing design required. The analysis validated both the overall design approach and the detailed fabrication quality requirements (notably, the need for volumetric examination of critical welds).