Pressure Boundary Condition Best Practices
Principles for boundary conditions in pressure vessel FEA — supports, symmetry, pipe interfaces, closed-end effects and avoiding artificial restraints.
Do not over-constrain
Fully fixed boundary conditions (all six DOF constrained) create artificial stress concentrations at the constraint edge. Use realistic support stiffness or spring elements where possible. Allow the model to deform naturally at the supports.
Symmetry conditions
Use symmetry only when the geometry, loading and expected response are all symmetric. A half model with symmetry boundary conditions is valid for a nozzle on the centreline of a cylinder under axisymmetric pressure. It is not valid for a nozzle with non-axisymmetric piping loads.
Pipe interface boundaries
At pipe connections, model sufficient pipe length for the stress field to decay to the nominal pipe stress. Do not cut the pipe too close to the nozzle. Apply the piping loads at the flange face, not at the cut boundary.
Closed-end effects
If the model does not include the end caps, apply the pressure end load as an axial force at the cut boundary: F = p * pi * r^2. Without this force, the longitudinal stress will be zero (open-ended) instead of p * r / (2 * t) (closed-ended).
Check for artificial restraints
Review the boundary conditions for any constraint that does not exist in the real structure. A symmetry plane that prevents a deformation mode that would occur in practice is an artificial restraint. Compare the displacement pattern with the expected physical behaviour.