Fatigue Life Assessment
Fatigue failure is a critical issue in industrial equipment, often resulting in unexpected breakdowns and costly repairs. The Health and Safety Executive in the UK found that 25% of all severe failures were caused by fatigue, often occurring in welds and other stress-prone areas.
At Dynaflow Research Group, we specialize in fatigue analysis and prevention using advanced techniques and industry-standard codes (ASME B&PV VIII-2, BS-5500, EN – 13445, ASME B31 Piping Codes) to analyze high cycle fatigue (HCF) and low cycle fatigue (LCF), focusing on critical areas such as welds, notches, and stress concentrations. Our team uses state-of-the-art finite element analysis (FEA) methods to capture peak stresses and structural stresses, ensuring accurate predictions of fatigue life.
Challenges We Solve
Caused by repeated stress cycles at lower strain levels, typically affecting notches and welds. Over time, it leads to crack initiation and propagation, which can result in component failure if not addressed.
Occurs under high strain and fewer cycles, often from thermal expansion or mechanical loading. It involves gradual material deformation, leading to fatigue failure due to stress accumulation in critical areas.
Repeated thermal cycling in equipment causes expansion and contraction, resulting in fatigue cracks. This is common in heat exchangers, boilers, and dryers, where fluctuating temperatures induce stresses.
Fatigue failures frequently originate at welded joints due to stress intensification. Weld imperfections act as starting points for crack initiation under cyclic loading.
Pressure vessels, separators, and similar equipment are subjected to varying pressure cycles, which can lead to fatigue. Repeated changes in pressure stress critical areas over time.
Once a crack initiates, cyclic tensile stress can cause it to propagate, ultimately leading to fracture. Early identification and management of crack growth are essential to prevent failure.
Once a crack initiates, cyclic tensile stress can cause it to propagate, ultimately leading to fracture. Early identification and management of crack growth are essential to prevent failure.
Fatigue failures often initiate at welds due to stress concentrations and variations in material properties, leading to cracks and potential failure under repeated loading cycles.
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Applications we conduct fatigue analysis for
Fatigue assessments are conducted for components such as oil separators, compressors, and boilers under cyclic pressure loading, particularly following ASME VIII-2 standards. The analysis identifies potential fatigue risks in welded and bolted joints, where stress concentrations are most prominent.
Specialized analysis of thermal fatigue in air coolers and hairpin heat exchangers addresses the significant stress caused by fluctuating temperatures. By assessing these systems’ fatigue life, the analysis ensures that thermal cycling does not lead to premature failure.
Agitators subjected to constant motion are prone to fatigue. Fatigue analysis, performed per ASME VIII-2 guidelines, ensures that these critical components remain operational over extended periods.
Fatigue services extend to systems like charge gas dryers and jacketed structures, where cyclic loading can cause fatigue cracks, leading to potential system failures. Precise fatigue assessments help predict and prevent these issues.
CFD Case Studies
A hairpin heat exchanger is a specific type of heat exchanger consisting of one or more U-shaped tubes. The unit we analyzed is exposed to a maximum temperature of 400°C and experiences cyclic pressure and nozzle loading.
These cycles occur throughout the heat exchanger’s 25-year lifespan, alternating between standby and operation every 1 to 2.5 days. To assess its performance, a finite element analysis (FEA) was conducted. Both a fatigue and ratcheting check were performed in accordance with ASME B&PV Section VIII, Division 2.
The fatigue analysis of the heat exchanger integrates multiple disciplines. First, a CFD simulation of the flow through the heat exchanger was conducted, and the heat transfer coefficients (HTC) for the different compartments were determined. Next, bolt pretension and nozzle loads were calculated to incorporate all external conditions into the FEA model.
For the fatigue and ratcheting assessment, the most conservative fatigue stress values were used. Stress classification lines (SCL) were drawn at critical locations to linearize the stress and separate it into its different components. Additionally, fatigue strength reduction factors (FSRF) were included, depending on the inspection method used at the stress concentration points.
The combined effects of pressure cycles, temperature cycles, and other external loads were incorporated into the model and compared against the ASME allowable limits. The analysis revealed that only the T-rings required a 75% increase in crotch radius. These rings connect the tube bundles to the supporting bracket. All other critical locations complied with both the fatigue and ratcheting checks as specified by the code, hence no mitigation measures were required.
Vibrations are a common cause of fatigue and have multiple sources. In this analysis, a large desorber vessel was vibrating at a frequency close to its natural frequency. This resulted in alternating forces acting on the vessel’s supporting skirt and the connected equipment. Three different locations were identified that show a high risk of fatigue failure: 2 welds and a nozzle connection.
First, a structural model was created in Caesar II in order to determine the forces and moments associated with the vibrations. The model includes the support locations, and the weight of the vessel contents, including internal equipment, was approximated by point masses. This approximation ensures that the calculated natural frequency of the model matches the measured natural frequency.
A harmonic force was applied at the top of the vessel, oscillating at its natural frequency. The magnitude of this force was derived from the measured displacements of the vessel. The internal forces in the vessel generated by the periodic movement served as an input for the FEA analysis.
The actual fatigue analysis was conducted with FEPipe, a piping-specific FEA software. The vessel was assessed according to the EN13445 code, which prescribes an allowable stress amplitude based on the type of joint. Since no information on weld testing was available, the most conservative weld quality was assumed.
The analysis showed that the stress amplitude was well below the allowable limit set by the EN code, with a maximum of 70%. This indicates that the stresses generated during vibration do not affect the vessel’s longevity, making mitigation measures unnecessary.
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Software Solutions For Fatigue Assessment
Helyx OpenFOAM
CFD analyses are done with HELYX OpenFOAM: a general-purpose CFD software package, based on an advanced open-source simulation engine developed by ENGYS using OpenFOAM technology. The CFD simulation engine, HELYX features an advanced hex-dominant automatic mesh algorithm with polyhydra support which can run in parallel to generate large computational grids.
