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. 

Two types of process vessel are likely to be subjected to external pressure those operated under vacuum, where the maximum pressure will be 1 bar (atm) and jacketed vessels, where the inner vessel will be under the jacket pressure. For jacketed vessels, the maximum pressure difference should be taken as the full jacket pressure, as a situation may arise in which the pressure in the inner vessel is lost. Thin-walled vessels subject to external pressure are liable to failure through elastic instability (buckling) and it is this mode of failure that determines the wall thickness required. 

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 may fail at well below the vield strength of the material. The geometry of the part is... 

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. 

Any out-of-roundimss of the shells of vessels subjected to external pressuie reduces the strength of the vessel. This problem was discussed in section 8.7 of Chapter 8. The maximum permissible deviation, e, from a circular form permitted by the code for vessels under external pressure is given by Fig. 13.4. 

Vessels of noncircular cross section may be subjected to external pressure Membrane and bending stresses are considered the same as for internal pressure unless the resulting stresses are compressive where stability may be a possible mode of failure. Interaction equations are used to examine die various plates for stability. Calculated stresses are compared with critical buckling stresses with ii factor of safety applied. This is described in Article 13-14 of the ASME Code,... 

A tall, vertically supported vessel which is subjected to external pressure and external loading contains stresses similar to fliose in a vessel with external pressure only. The method of combining loadings as developed by Bergman and described in Section I6.S is used to obtain used in the analysis. 

In any pressure vessel subjected to internal or external pressure, stresses are set up in the shell wall. The state of stress is triaxial and the three principal stresses are ... 

LPG has strong volatOity, lower flash point and wide explosion limits. Its minimum ignition energy is 0.2-0.3 mJ, and it is easy to get fire and even lead to secondary disasters once encounter fire. Subjected to external heat pressure vessels wall will destructed suddenly and hquid is overheated and then released and hghted therefore the fireball emerged. Consequence analysis of first accidents. 

Tanks that could be subjected to vacuum should be provided with vacuum-breaking valves or be designed for vacuum (external pressure). The ASME Pressure Vessel Code contains design procedures. 

Pressure vessels (including heat exchangers and air coolers) in a plant handling flammable fluids are subject to potential exposure to external fire. A vessel or group of vessels which could be exposed to a... 

However, it is clear that slight variations in vessel shape, etched markings, or external pressure can lead to disagreements as to which thermometer gives the true temperature. Moreover, the reference points chosen to standardize the readings between different thermometers could be subject to disagreements (see Sidebar 2.4), as could the choice of thermometric fluid (e.g., Hg vs. water, each of which has different values of aP in different temperature ranges). Under these circumstances, the choice of the true temperature scale may become subject to non-scientific influences. We therefore seek a universal standard that avoids such arbitrary choices. 

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