Pressure Load Application Best Practices
Principles for correctly applying pressure loads in FEA — surface identification, normal direction, end-cap pressure, gauge vs absolute and resultant verification.
Identify the correct surface
Apply pressure to the wetted surface — the surface in contact with the pressurised fluid. For internal pressure, this is the inner surface. For external pressure, this is the outer surface. Applying pressure to the wrong surface is a common and serious error.
Verify the normal direction
Pressure in FEA is applied normal to the element face, directed into the structure. Check that the element normals point outward from the pressurised surface. If the normals are reversed, the pressure will act in the wrong direction — pulling instead of pushing.
Include end-cap pressure
If the model includes end caps or heads, apply pressure to those surfaces. If the model uses symmetry or cuts the cylinder, apply equivalent axial forces at the cut boundary to represent the end-cap pressure. Without the end-cap pressure, the longitudinal stress will be incorrect.
Use the correct pressure convention
Use gauge pressure for the structural analysis. The differential pressure across the wall is the structurally significant quantity. For vacuum or subsea cases, explicitly include the external pressure. State the pressure convention in the analysis documentation.
Verify the resultant
After solving, check the pressure resultant against the analytical value. For a cylinder: axial resultant = p * pi * r^2, radial resultant per unit length = p * 2 * pi * r. If the resultants do not match, the pressure was applied incorrectly.