This case study focuses on the vibration assessment of a pentane line, specifically evaluating vibration stresses and the risk of fatigue issues at the nozzle/shell interfaces. Vibration measurement data provided by the client was analyzed. Initially, it was hypothesized that the vibration problem stemmed from a mechanical resonance phenomenon, implying that the system was oscillating at its natural mode shapes. Detailed modeling of the system was conducted to determine the mechanical resonance frequencies. By applying a harmonic force, the maximum stresses were calculated, which then facilitated a fatigue check.
Fatigue Assessment
The process for calculating fatigue stress values involved several key steps:
Stress Intensification Factors (SIF) and Flexibilities Calculation: For each nozzle/shell interface, Stress Intensification Factors and flexibilities were determined using Finite Element Analysis (FEA).
Pipe Stress Model Development: A comprehensive pipe stress model was created, incorporating accurate routing and support structures, as well as content and insulation weight. This model also considered the flexibility of support structures and nozzle/vessel interfaces.
Natural Frequency Calibration: The friction of the supports was adjusted to align the calculated natural frequency with the measured value.
Harmonic Force Application: A harmonic force was introduced into the model with driving frequencies matching the relevant natural frequencies of the system. The amplitude of this force was adjusted until the model’s displacements matched the measured values.
Dynamic Stress Amplitude Retrieval: From the calibrated model, dynamic stress amplitudes were extracted at locations of maximum stress, such as nozzle/shell interfaces and welds of the supporting beam. These amplitudes were then multiplied by the applicable Stress Intensification Factors.
Fatigue Damage Calculation: The calculated stresses were combined for a fatigue assessment. A conservative approach was taken by summing the absolute stress values for each natural frequency. This approach assumes that all natural modes are in phase, which is typically a conservative estimate.
Conclusions
The analysis revealed that, over an operational period of 80 years, the allowable stress amplitude exceeded the conservative stress values obtained. Consequently, it was concluded that no modifications were necessary from an integrity standpoint.
