External loads applied to a pressure vessel are usually of a local nature. WRC Bulletin 107 provides the almost universally accepted methodology for calculating the stresses that these loads develop in the shell of a vessel, and for determining the effect that they will have upon the design (i.e., the minimum required thicknesses for the affected components of the vessel).
The stresses developed in the vessel’s shell by local external loads must be added to the stresses attributable to the internal pressure. It is important to recognize that they are normally either local primary membrane stresses, or bending stresses. Therefore, as explained in Section 100, the total stress obtained by adding the stresses developed by local external loads to those attributable to the internal pressure is permitted to reach 1.5 times the maximum allowable design stress given for the material of construction in the Code. Consequently, only relatively high external loads are likely to affect the design of a vessel.
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