The Following are few of the Common Mistakes Engineers make in Pressure Vessel Design & Analysis
Improper Application of Design Codes and Standards
Standard fatigue evaluation under ASME Section VIII, Division 2, Part 5 assumes the material behaves elastically and that time has no bearing on the result — the same stress cycle produces the same damage whether it happens today or in ten years. That assumption holds only below a material-specific temperature threshold. Once a component operates above that threshold, the material enters the creep regime: stress relaxes over time under sustained load, and deformation becomes a function of duration as well as magnitude. At that point, a standard S-N curve no longer describes what the component is actually experiencing, and the Code requires a different evaluation path.
This is not limited to fatigue. In a buckling analysis O’Donnell Consulting performed on a 304H stainless steel vessel used in styrene production, the vessel was subjected to vacuum loading at a design temperature of 1150°F — well within the creep regime for that material. Because of this, the analysis could not stop at instantaneous elastic or elastic-plastic buckling. Long-term creep buckling was evaluated separately, using incremental analysis with isochronous stress-strain data and time-based creep modeling, per ASME Code Case N-284-1. The same principle that governs fatigue at elevated temperature governs buckling: once creep is active, a time-independent method understates what the component will actually do in service.
The transition gets missed more often than it gets caught. Heat exchangers, reactor vessels, and piping runs near furnaces or other high-temperature process equipment routinely operate above the creep threshold for their material of construction, and the shift is often missed because the equipment looks, on paper, like an ordinary pressure vessel evaluation.
The underlying methods here are not new. Dr. O’Donnell’s 1983 chapter, “Vessels for Elevated Temperature Service” (with J.S. Porowski, in Developments in Pressure Vessel Technology:4), laid out why design-by-formula methods adequate below the creep regime stop being adequate above it, since membrane-stress formulas do not account for the local strain conditions that drive fatigue and creep damage. Later work extended this into current Code development: “Future Code Needs for Very High Temperature Generation IV Reactors” (O’Donnell and Griffen, ASME Companion Guide to the Boiler and Pressure Vessel Code, 2009) addresses how Section III, Subsection NH and related Code Cases need to evolve to cover very-high-temperature service, including the structural integrity issues that arise once creep and fatigue interact.
Recognizing that a component has crossed into elevated-temperature service is the first decision point — the second is how fatigue and creep damage are evaluated together once it has. That combined evaluation, under ASME Section III, Subsection NH, is the subject of our companion article on creep-fatigue interaction.
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