External pressure, vessels subject

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 ... 

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. 

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. 

When large quantities of a substance are handled, sensitivity of the material to heating under confinement may need to be considered to demonstrate the effect on the stored/handled, and probably confined, substance in the event of an external heat load. Tests such as the steel sleeve test or Koenen test [24, 137], the Dutch pressure vessel test (DPVT) [143], and the United States pressure vessel test (US-PVT) [143] may be applicable. These tests are used mostly for transportation considerations. The tests generally subject the sample substances to very high energy inputs under confined conditions, and thus are more severe than the deflagration and autoclave tests previously discussed in Section 2.3.3.2. As an example, the Koenen test, used mainly in Europe, is illustrated in Figure 2.32. 

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. 

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. 

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. 

Two fundamentally different types of failure may occur in vessels operated under vacuum (as opposed to high pressure). The problem in vacuum operation is the elastic stability of the vessel shell when it is under an external pressure loading. In general, elastic instability is a problem that must be considered in all structures having limited rigidity when subjected to bending, torsion, compression or a combination of these loadings. In failure by elastic instability, the structure buckles or collapses like an evacuated thin-shelled vessel. 

Lymphatic flow is known to be increased by heat, massage, inflammation, movement of the body part, and increases in hydrostatic pressure within the lumen of lymphatic collecting vessels and decreased by cold, lack of movement, and decreased external pressure (Uren et al., 1999). In addition, the normal lymphatic drainage of the skin has been shown to be highly variable from subject to subject, even when the same region of the body is examined (Uren, 2004). Average flow rates reported by Uren et al. (2004) are shown in Table 13.1. Further, studies have shown that the... 

Vessels subject to external pressure may fail at well below the vield strength of the material. The geometry of the part is... 

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