Fatigue Life Assessment of a Blow-Case Pressure Vessel

Location
Structure Type
Software Used

Alberta
Horizontal Pressure Vessel
Abaqus

Objective

To assess the fatigue life of a horizontal Blow-Case pressure vessel relocated for new service conditions. Finite Element Analysis (FEA) was used in compliance with ASME Section VIII, Division 2 and API 579 standards to evaluate stress amplitude and determine suitability for continued service.

Pressure Vessel

Linearized Stresses at Critical Location

Stress Contours Pressure Vessel

Challenges

Need for stress linearization at critical locations (especially nozzles) to extract through-thickness stress parameters (membrane, bending, and peak stress). Modelling complex weld geometries and applying fatigue strength reduction factors (FSRF). Achieving convergence in highly refined local 3D solid models for accurate fatigue evaluation.

Solution

ENA2 developed a detailed FEA workflow: A shell model was first used to identify global stress distribution and highlight critical zones. The most critical region (N2 nozzle) was then modeled using 3D brick elements to perform linearized stress extraction. Fatigue assessment followed ASME VIII-2, Part 5 procedures and used fatigue curves from Figure 3-F.1M. Sensitivity analysis was conducted to determine allowable pressure limits while maintaining fatigue life criteria.

Result

The vessel was found suitable for long-term operation under the anticipated pressure cycles, with a safety margin aligned with industry standards. A fatigue life in excess of 100 million cycles was demonstrated for the target pressure range. Pressure limits were established to ensure continued compliance until the next inspection cycle. Recommendations included routine non-destructive inspection and operational pressure monitoring to manage future fatigue damage.

Key Results

Validated for Long-Term Operation: Vessel structure shown to safely withstand cyclic loads over an extended period.
Fatigue Life Capability: Demonstrated >100 million cycles in current pressure range using ASME design criteria.
Established Pressure Limit: A safe upper pressure bound was determined to preserve fatigue life and inspection intervals.
Inspection Interval Alignment: Fatigue performance validated to align with planned 5-year inspection cycles.
Conservative Assumptions Used: Ensured safety in absence of load history or weld defect data.