Heads, vessel thickness, formulas

Although spherical vessels have a limited process application, the majority of pressure vessels are made with cylindrical shells. The heads may be flat if they are suitably buttressed, but preferably they are some curved shape. The more common types of heads are illustrated on Figure 18.16. Formulas for wall thicknesses are in Table 18.3. Other data relating to heads and shells are collected in Table 18.5. Included are the full volume V0 and surface S as well as the volume fraction V/V0 corresponding to a fractional depth H/D in a horizontal vessel. Figure 18.17 graphs this last relationship. For ellipsoidal and dished heads the formulas for V/V0 are not exact but are within 2% over the whole range. 

The ASME design formula from UG-32 for ellipsoidal heads having a major-to-minor axis ratio of 2 1 and being subjected to pressure on the concave side is t = PD/ 2SE — 0.2P), where t is the minimum thickness of the head (in.), P is the MAWP (psi), D is the internal diameter of the major diameter of the ellipsoid (in.) (and equal to the inside diameter of the vessel), S is the allowable stress of the material, as listed in ASME Section II, and E is the weld joint efficiency. ... 

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