In GB150, why is it specified that a folded edge structure with a transition section should be used when the half apex angle is 30°?

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Stress analysis shows that the larger end of the flangeless conical shell has the highest membrane stress; meanwhile, at the junction with the cylindrical shell, significant edge stresses and lateral thrusts arise due to boundary effects. Under internal pressure, due to the combined effect of the aforementioned stresses, the inner surface at the large-end connection edge will experience a maximum overall radial tensile stress, with bending stress playing a dominant role. Both the combined stress and the lateral thrust increase as the half-vertex angle α increases, with the increase being particularly significant when α > 30°. Therefore, GB150 stipulates that flangeless conical heads are limited to use in cases where α ≤ 30°. When a conical head is equipped with a transition section, its connecting edges can be changed from being connected by non-tangent lines to being connected by tangent lines. This not only completely eliminates the lateral thrust, but also reduces or eliminates edge stress; furthermore, it keeps the welds of the connecting ring away from the areas where peak edge stress occurs, thereby significantly improving the stress conditions at the connection edges. To this end, GB150 stipulates that when α>30°, a transition section with folded edges must be provided for the conical head, and the radius of the transition section’s corner shall be no less than 10% of the inner diameter of the larger end of the head; otherwise, design shall be carried out using the stress classification method.