Design of vessels subject to external pressure

The design methods and design curves given in the standards and codes should be used for the detailed design of vessels subject to external pressure. 

The method outlined here is not recognized by the ASME BPV Code and can be used only for a preliminary estimate of wall thickness required under external pressure. The method recommended by the BPV Code is substantially more complex and takes into account the fact that the maximum allowable stress in compression is different from that in tension. The ASME BPV Code Sec. VIIID. 1 Part UG-28 should be consulted for the approved method for detailed design of vessels subject to external pressure. 

Vessels subject to external pressure should be designed to resist the maximum differential pressure that is likely to occur in service. Vessels likely to be subjected to vacuum should be designed for a full negative pressure of 1 bar, unless fitted with an effective, and reliable, vacuum breaker. 

The design for the internal pressure condition of vessels usually is straightforward and well understood. Under vacuum conditions, equipment is subject to external pressure from the atmosphere and the design for external pressures is more difficult than that for internal pressures. The devious ways in which external pressure can be applied often may be overlooked. 

Additionally, tolerances for cylindrical and conical shells are provided when subjected to external pressure, and uniform axial compression and bending. If the tolerances are exceeded, the allowable buckling stresses must be adjusted. In the case of vessels with large diameter over thickness ratios (approximately 300 or higher), it may be prudent to discuss shell tolerances with the fabricator to ensure that if tolerances cannot be met, the reduced allowable buckhng stresses can be used for checking tlie design of the vessel. 

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. 

As expected, the analysis performed in ordinary conditions has shown that the tank is not subjected to excessive stresses and strains when everything operates normally both Von Mises stresses and displacements are smaller than the maximum allowable limit set by the designers. Moreover, the analysis has highlighted the weakness of the structure in correspondence of the conjunction between the wall and the dome of the inner vessel. This is due to the fact that the stresses concentrated in this portion of the structure are very high, as exemplified in Figure 1 which shows the stress level in the inner tank under external pressure, in nominal conditions. 

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. 

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