External Pressure
Most vessels are designed to contain positive internal pressure, but it is possible to develop a partial vacuum inside of the vessel during steam-out cleaning, when draining liquids, or during abnormal process conditions. Therefore, it is general practice to check all vessels that are designed for an internal pressure for their resistance to collapse under 7.5 psi external pressure at 450°F. This will normally affect only relatively large diameter vessels that are designed for relatively low internal pressures. If the minimum required thickness for the internal pressure does not provide adequate resistance to collapse under a partial vacuum, it is usually more economic to add stiffening rings to the shell rather than to increase the thickness. On the other hand, if a vessel design exceeds the required minimum external pressure rating of 7.5 psi, the vessel should be stamped for the higher pressure, up to 15 psi.
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