Fidelis Industry Insights is a series of posts focused on where Fidelis—and the industry-leading SIMULIA software we provide—can change the way teams approach engineering problems. In this installment, we’re looking at pressure vessels and how finite element analysis (FEA) supports decisions across the full lifecycle: from initial design to fitness for service (FFS) once the vessel is operating in the real world.
Why Pressure Vessels Demand Strong Design and Ongoing Monitoring
Pressure vessels sit at the intersection of high stored energy and long service life. Even “routine” operating conditions can drive complex structural response: internal pressure coupled with thermal gradients, cyclic start/stop events, nozzle and support loads from connected piping, and localized effects from weld geometry and discontinuities. When something goes wrong, the consequences are rarely localized—loss of containment can mean unplanned downtime, expensive repairs, regulatory exposure and, most importantly, safety risk.
That’s why pressure vessel integrity is fundamentally a lifecycle problem. Getting the initial design right reduces risk and rework before fabrication, but the job doesn’t end at commissioning. Real equipment accumulates history: corrosion/erosion, repairs, process changes, vibration, and upset events. Monitoring and periodic reassessment ensure that “as-operated” reality stays aligned with “as-designed” assumptions—and when it doesn’t, teams need a defensible path to rerate, repair, or continue operating under defined limits.
Two Questions, One Toolset
Pressure vessel teams usually face one of two high-impact questions:
- Initial design: Will this vessel meet its construction code requirements and performance expectations under the full set of load cases?
- In-service operation: Given what we’ve actually seen in the field—corrosion, repairs, load changes, or indications—can it continue operating safely, and under what limits?
FEA is invaluable in both cases, but for different reasons: in design it helps ensure the geometry and details are right before fabrication; in service it helps convert inspection findings into defensible run/repair/rerate decisions.
FEA in Initial Pressure Vessel Design
For many vessels, design-by-rule methods provide an efficient baseline. But as soon as the geometry becomes more complex or the loading is multi-axial, design-by-analysis becomes the practical path—especially for nozzle-to-shell junctions, head-to-shell discontinuities, external loads from piping/supports, instability concerns under external pressure, or cyclic service where fatigue and ratcheting can govern.
This is where a well-constructed FEA model becomes relevant: it resolves the local stress/strain fields that rule-based formulas can’t represent, and it allows results to be interpreted in the failure-mode framework used in ASME Section VIII, Division 2 (plastic collapse, local failure, buckling, cyclic loading). The output isn’t just “stress plots”—it’s a code-aligned technical basis for design decisions, detail changes, and margin identification.
FEA for Fitness for Service (FFS)
Once a vessel is in operation, the problem shifts. Pressure vessels experience pressure and thermal cycles, process upsets, corrosion/erosion, repairs, and changing boundary conditions. Over time, the question becomes less about as-designed compliance and more about as-is integrity.
A fitness-for-service (FFS) assessment is a structured way to justify continued operation of in-service equipment that may contain degradation or flaws. A common framework is API 579-1/ASME FFS-1, which connects inspection data (thickness readings, NDE indications, materials) to acceptance criteria and operating limits. When screening methods aren’t enough—because the damage is local, the geometry is complex, or loads interact—FEA becomes the engine that provides the stress/strain fields needed to support higher-fidelity evaluations.
How Fidelis Can Help Across the Lifecycle
Many organizations can run an analysis, but fewer have a repeatable workflow that ties modeling choices to code intent and decision needs. Fidelis helps close that gap—both through engineering services and through the SIMULIA toolkit we provide.
For initial design, we help develop FEA models and post-processing that map cleanly to your design basis—whether that’s confirming performance, resolving discontinuity stresses, evaluating buckling or cyclic service concerns, or supporting ASME VIII-2 style design-by-analysis where appropriate.
For FFS, we help translate inspection findings into analysis-ready inputs and then build an approach that matches the physics and the decision being made. That often means combining a global model (for realistic load paths and boundary conditions) with local refinement or submodeling (to accurately capture nozzle hotspots, weld transitions, or localized wall-loss regions). Where needed, we incorporate contact, geometric nonlinearity, and elastic-plastic behavior so the analysis reflects the actual risk drivers rather than idealized assumptions.
Most importantly, we focus on decision-grade outputs: operating envelopes tied to the technical basis, rerate/repair recommendations supported by sensitivity checks, and documentation that stands up to internal review and external scrutiny.
Final Thoughts
Pressure vessels don’t stop being engineering problems once they’re fabricated—they evolve. The same FEA discipline that helps get a vessel designed correctly can later help determine whether it can operate safely after years of service.
If you’re working on a new vessel design, a rerate, an integrity question tied to inspection data, or anything in between, Fidelis can help you turn requirements and findings into a clear, defensible path forward. Get in touch now to learn more!
