Fitness-for-service (FFS) is an approach to asset management that helps owners determine whether equipment, structures or components are safe to continue operation after the discovery of damage or degradation.
By considering how an asset’s current state responds to the loads we require it to support, the FFS approach helps extend the lifespan of valuable assets while demonstrating ongoing compliance with industry standards and safety regulations.
Wiss, Janney, Elstner Associates (WJE) was engaged by a major energy provider to perform a Level 3 FFS assessment on a high- temperature vessel head experiencing localized corrosion. The vessel, designed to ASME Section VIII, Division 2, operates above 700 psig and 850°F and is internally lined with refractory to protect the steel shell from the high process temperatures.
Asset overview and corrosion challenges
The assessment focused on the top hemispherical head of the vessel, where dewpoint corrosion developed near the primary inlet nozzle. This corrosion, attributed to vapor migration through the refractory, had progressed significantly since commissioning. A prior Level 2 FFS assessment addressed the nozzle region, but continued material loss prompted the client to employ a more advanced evaluation. Two ultrasonic thickness surveys conducted over the past two years informed the project team the details of the corrosion loss pattern. The corrosion was determined to be a localized phenomenon that nonetheless raised concerns about the vessel’s long-term integrity.
Technical approach
WJE utilized ABAQUS/Standard, a commercially available solver for highly accurate simulations in the static and low-speed regimes to simulate the vessel’s behavior under actual operating conditions. The model incorporated key factors such as internal pressure, bolt preload and detailed contact interactions between flanges, gaskets and bolts. Additionally, non-uniform corrosion patterns identified through inspection data were integrated into the analysis to reflect the vessel’s existing and future conditions. The analysis modeled four corrosion states: as-designed, as-found and two forward-projected scenarios based on linear corrosion rates. The most critical projection suggested the vessel could reach a minimum allowable thickness within 14 months from the date of assessment.
Modeling and results
The finite element model included detailed representations of the vessel head, nozzle and bolting. Material properties were defined for the vessel steel and bolting materials at elevated temperatures. The analysis revealed that the bolting configuration was the limiting factor in pressure retention. Stress and strain distributions were evaluated for each corrosion state. Even under the most severely projected corrosion, the vessel maintained sufficient capacity to avoid plastic collapse. Local failure criteria were also satisfied, with weld regions experiencing less than 12% of the allowable strain.
Ensuring safety with ongoing surveillance
The corrosion at the top of this vessel was deemed fit for continued service, provided wall thickness remained above the limits established in this study. However, the client was advised that if corrosion continues at historical rates, this threshold could be breached in less than two years. WJE recommended that proactive measures be evaluated, including repair or replacement planning and corrosion mitigation strategies. Continued surveillance will enable the client to monitor the effectiveness of selected interventions and plan for repairs at an opportune time.
WJE is a nationally recognized engineering, architecture and materials science firm known for solving some of the most challenging problems in the built environment. The firm’s multidisciplinary approach and deep experience in FFS evaluations help clients make informed, datadriven decisions that extend asset life and uphold the highest safety standards.
For more information, visit wje.com.
