Pressure Vessel Discontinuity Stresses

Pressure vessels consist of axially symmetrical elements of different geometries and thicknesses, and sometimes different materials. If these individual components were allowed to expand freely as separate sections under internal pressure, each element would have an edge radial displacement and an edge rotation that would differ from those of the adjacent component. However, since all these components form a continuous structure and must deflect and rotate together, the differences in movement at junctions result in local deformations and induce local stresses. Other items, such as stiffening rings and internal bulkheads, also affect the cylinder deformation and introduce local stresses.

Stresses created by the interaction of two shell components at their junction (i.e., an abrupt change in geometry of the vessel shell, or a structural discontinuity) are called discontinuity stresses. Under static loads, such as constant internal pressure, and with ductile materials, discontinuity stresses can be kept low by proper design. They become important, however, under cyclic loads, or at low temperatures where the ductility of the material is reduced. Discontinuity stresses must be added to membrane stresses developed by other loads, as discussed in Section 113 above.

There are two categories of structural discontinuities: gross and local (Figure 100-12).
• Gross structural discontinuities affect a relatively large portion of a structure and have significant effect on the overall stress pattern. All of the junctions between shell components fall into this category.
• Local structural discontinuities are sources of stress or strain intensification that affect only a small volume of material and do not have a significant effect upon the overall stress pattern. They usually produce peak stresses.

Structural Discontinuities in a Pressure Vessel From Structural Analysis and Design of Process Equipment

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