Vessel with a supported flat head

For more complex geometry and load cases a linear-elastic analysis is typically completed to validate the design. The displacements and stresses from the analysis are reviewed and acceptability determined. The results of a linear-elastic analysis however are based on the material being perfectly elastic and as a result do not account for failure due to elastic instability or buckling. As a linear-elastic analysis will not report buckling it is the designer’s responsibility to determine if it is a potential failure mechanism. For components subject to vacuum or under direct compressive load such as legs it is fairly obvious that buckling should be investigated. For other components it may be less obvious and neglected. For instance supports on a flat head subjected to internal or external pressure may buckle. A side load is imposed on the supports as the flat head balloons out under pressure. It is quite possible the flat head will meet code calculations and be shown as acceptable with a linear-elastic analysis but in reality it may fail prematurely due to buckling of the supports.

A view showing buckling of the flat head supports

To ensure buckling does not occur Pressure Vessel Engineering completes a buckling analysis. All material properties, loads, and boundary conditions from a linear-elastic analysis are imported into a buckling solver. The buckling solver then provides several possible buckling failure modes and the respective safety factor of each. Results less than 1 indicate the design will fail. For ASME code compliance a safety factor of 3 is typically required.