Design of vessels subject to combined loading

Pressure vessels are subjected to other loads in addition to pressure (see Section 13.4.7) and must be designed to withstand the worst combination of loading without failure It is not practical to give an explicit relationship for the vessel thickness to resist combined loads. A trial thickness must be assumed (based on that calculated for pressure alone) and the resultant stress from all loads determined to ensure that the maximum allowable stress intensity is not exceeded at any point. 

External loads imposed by piping and attached equipment. 

The longitudinal and circumferential stresses due to pressure (internal or external), given by  

The direct stress ow due to the weight of the vessel, its contents, and any attachments. 

The stress will be tensile (positive) for points below the plane of the vessel supports, 

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Example 11.15. A cylindrical shell that is 84-in. ID by 1.0-in. nominal thickness contains a nozzle 8-in. ID by 1.0-in. nominal thickness. The design pressure is 400 psi and the allowable stress of the material is 17.5 ksi. The nozzle is subjected to an inward radial loading of 12,000 lb and an applied moment in the longitudinal direction of 150,000 in.-lb. What are the combined stresses on the longitudinal axis due to these two external loadings using the Mershon method and the curves in Appendix K The vessel is not subjected to cyclic loading, and therefore no stress concentration factors need be considered. 

A vessel will not be subject to all these loads simultaneously. The designer must determine what combination of possible loads gives the worst situation, and design for that loading condition. 

Coils inside pressure vessels may be subjected to the internal pressure of the vessel acting as an external pressure on the coil. In addition, steam coils should be designed for full vacuum or the worst combination of external loads as well as the internal pressure condition. The coil must either be designed for the vessel hydrotest, externally, or be pressurized during the test to prevent collapse. 

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