When Does a Pressure Vessel Need FEA? ASME VIII Div. 2 Part 5 Explained

Design-by-rule and design-by-analysis

ASME Section VIII Division 1 and the rules of Division 2 Part 4 give formulas for shells, heads, nozzles and flanges. They are fast and reliable for standard geometry and loading. Design-by-analysis — ASME VIII Div. 2 Part 5 — uses numerical analysis, normally FEA, to show that a component is safe when the rules do not cover it or are too conservative.

Typical triggers for FEA

• High external nozzle loads from piping that exceed WRC 107 / WRC 537 applicability or give results close to the limits.
• Non-standard geometry — large openings, closely spaced nozzles, conical transitions, non-circular sections and unusual supports.
• Cyclic service — pressure swings, thermal cycles or batch processes that need a fatigue assessment. Adsorbers and switching vessels are common examples.
• Local loads from lugs, trunnions, saddles, skirts, platforms and lifting.
• Thermal stress where temperature gradients through the wall or between parts are significant.
• Existing equipment that must be rerated, modified or checked after damage — often combined with API 579 fitness-for-service.

What Part 5 checks

The analyst must also document the model idealisation, mesh refinement at stress concentrations, material properties, boundary conditions, load combinations and the acceptance criteria used.

A practical example: adsorber fatigue

In one SLETEC project an adsorber had to demonstrate its fatigue life under repeated pressure cycling and nozzle loads. SLETEC carried out a fatigue assessment to ASME VIII Division 2 using FEA of the shell, nozzles and welds under cyclic pressure. The fatigue evaluation was documented against the agreed criteria and supported the vessel's cyclic-service approval. A similar workflow applies to switching vessels, pressure swing adsorption units and batch reactors.

What a good FEA report contains

• Design basis: code edition, design conditions, materials, allowable stresses and load cases.
• Model description and mesh study, with plots showing refinement at critical areas.
• Boundary conditions and how external loads are applied.
• Stress classification lines and linearised results in tabular form.
• Pass / fail summary against each failure mode, with margins.
• Conclusions and any required design changes, plus an independent check record.

Building a reliable model

The quality of a Part 5 assessment depends more on modelling decisions than on the software. Points we check on every vessel model:

• Geometry and thickness. Use the nominal thickness minus corrosion allowance and fabrication tolerance as required by the code, not the ordered plate thickness.
• Mesh at discontinuities. Nozzle-to-shell junctions, head-to-shell joints and support attachments need a refined mesh with enough elements through the thickness for stress linearisation.
• Boundary conditions. Supports must allow the free thermal and pressure growth the real vessel has. Over-restraint creates artificial stress.
• Load combinations. Pressure, weight, nozzle loads, wind or seismic, and thermal loads are combined as the code requires, including the hydrotest case.
• Stress classification lines. They are placed through the thickness at the locations of highest stress, and the results are separated into membrane, bending and peak components.

Common nozzle-load problem and solutions

The most frequent reason a vessel goes to FEA is a nozzle load table from the piping designer that is higher than the vessel was designed for. Options, in order of cost, are:

1. Confirm the loads are realistic — pipe stress models sometimes over-restrain the nozzle.
2. Run FEA to replace conservative WRC estimates with an accurate local stress result.
3. Re-route or re-support the piping to reduce the load.
4. Increase the reinforcement pad or nozzle neck thickness.
5. Increase the shell thickness locally with an insert plate.

FEA often shows the original design is acceptable, which avoids redesign of the vessel or the piping.

How to start

Send the vessel drawing or data sheet, design conditions, nozzle load table and the cycle history or duty for fatigue cases to enquiry@sletec.com. See our ASME pressure vessel and FEA analysis services.