Problem
In-service equipment can degrade over time due to factors such as local material loss or cracking. Ensuring continued safe operation throughout the equipment’s design life is critical. In this case, a crack-like flaw was identified at the interior weld connecting a nozzle to the domed end of a pressure vessel. To determine whether the component could safely remain in service or required repair or retirement, an assessment of the flaw’s criticality was necessary.

Solution
Fitness-For-Service (FFS) codes, such as API 579-1 and BS 7910, provide methodologies for evaluating the structural integrity and remaining life of in-service equipment. These guidelines help engineers assess the safety of pressure vessels, piping, and tanks. Fracture mechanics assessments, which consider crack-like flaws at increasing levels of complexity, are an essential tool in this process.

Result
A Finite Element (FE) model of the pressure vessel was developed to evaluate through-thickness stresses at the crack location. An initial Level 2 FFS assessment placed the flaw just outside the acceptable region of the Failure Assessment Diagram (FAD), suggesting a need for further analysis.

To reduce conservatism, a more advanced Level 3 assessment was conducted using an elastic-plastic FE model with a cracked-mesh approach. The J-integral and corresponding stress intensity factor were directly evaluated and compared against the material’s fracture toughness. Additionally, protection against plastic collapse for the nozzle and domed end was assessed using ASME VIII Part 5 elastic-plastic analysis methods.

The Level 3 FFS assessment confirmed that the crack was stable and that the equipment could continue operating safely. This case highlights how increasing assessment complexity can refine predictions, sometimes demonstrating that damage is acceptable when lower-level assessments (Level 1 or 2) suggest otherwise.