Engineering task and calculation objective
The FELI module is the interface between the analytical calculation modules and finite element analysis in COMSOL Multiphysics. It transfers the geometry and calculation data of an LV module to COMSOL, so that an FEM model built there is fed directly with the parameters of the code calculation – without manually re-entering dimensions, pressures or material data.
The element size controls the mesh refinement of the imported model: a finer mesh resolves stress peaks at notches, transitions and openings better, but increases the computing time. In practice, the path from formula-based calculation to FEM is used whenever a detail lies outside the scope of the code formulas, when local stresses are needed for a fatigue or design-by-analysis verification (e.g. per EN 13445-3, Annex B or Clause 18), or when an analytical design is to be backed up by an independent comparison calculation.
Calculation workflow
- Calculate the source module: First, the calculation module concerned – for example a shell, head or flange calculation – is fully computed with its geometry, loads and material data. These data form the basis of the FEM model.
- Transfer to COMSOL: FELI exports the module parameters to the COMSOL model. Dimensions, pressures and material properties are available there as model parameters and remain consistent with the original calculation.
- Control the meshing: The mesh refinement is set via the element size. For reliable stress results at notches and transitions, a convergence study is advisable: the mesh is refined step by step until the governing result quantities no longer change appreciably.
- FEM analysis and assessment: The finite element calculation delivers stress and deformation fields, which are compared with the analytical results or evaluated for further verifications – such as stress categorization or fatigue assessment.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Element size | El | mm |
| Material number | 1 | – |
| Calculation temperature loadcase 1 | T1 | °C |
| Calculation pressure 1 loadcase 1 | p1,1 | MPa(p) |
| Nominal design strength | K | N/mm² |
| Safety factor | S | - |
| Wall thickness manufacturing tolerance | c1 | mm |
| Corrosion allowance | c2 | mm |
| Allowable stress | f | N/mm² |
| Modulus of elasticity at 20°C | E20 | N/mm² |
| Material number | 2 | – |
| Calculation temperature loadcase 2 | T2 | °C |
| Calculation pressure 1 loadcase 2 | p1,2 | MPa(p) |
| Nominal design strength | K | N/mm² |
| Safety factor | S | - |
| Wall thickness manufacturing tolerance | c1 | mm |
| Corrosion allowance | c2 | mm |
| Allowable stress | f | N/mm² |
| Modulus of elasticity at 20°C | E20 | N/mm² |
| Relevant calculation temperature | T* | °C |
| Relevant calculation pressure | p* | MPa(p) |
| LF1 | Fx | N |
| LF1 | Fy | N |
| LF1 | Fz | N |
Frequently asked questions
When is the step from the code formula to an FEM calculation worthwhile?
Code formulas cover defined standard geometries and load cases. As soon as a detail lies outside these limits of applicability – unusual nozzle arrangements, closely spaced openings, special designs, combined local loads – or a verification based on local stresses is required (design by analysis, detailed fatigue analysis), FEM is the tool of choice. For standard cases, however, the formula-based calculation remains faster, conservatively backed and directly traceable for inspection bodies.
What element size should you choose?
A good starting point is an element size that resolves the wall thickness with several elements; at notches, weld toes and openings, the mesh must be locally much finer. The result only becomes reliable through a mesh convergence study: the calculation is repeated with a progressively refined mesh until the assessment-relevant stresses stabilize. A mesh that is too coarse systematically underestimates stress peaks – a common and dangerous error in FEM verifications.
Does the FEM calculation replace the code verification?
Not automatically. An FEM result must be assessed according to the rules of the applicable code – for example via stress categorization or the direct route per EN 13445-3 Annex B/C. The very advantage of the interface is keeping both worlds consistent: the analytical calculation provides the documented basic design, while the FEM provides the deeper look at the details, with identical input data.