Elliptical and spherical heads for pressure vessels can be formed from one piece of plate by “pressing.” Plate of the required thickness is first cut to a circular shape with a diameter large enough to provide all of the material required for the formed head. The circular plate is then pressed between mating dyes that have been manufactured with the inside and outside dimensions and contour of the head, as illustrated in Figure 600-3. Other head forming methods include incremental pressing, and “spinning.” Forming of a head usually reduces the thickness of the plate. This reduction must be estimated by the fabricator, and compensated for by procuring plate material that is thicker than the required thickness of the formed head.
When it is not possible to obtain one piece of plate large enough for the required diameter of the circular shape, it is acceptable to weld two or more pieces of plate together. The welds should receive full-coverage RT examination, and the knuckle of elliptical and torispherical heads should be visually inspected (VT), where the deformation of the plate associated with forming is most severe.
Heads can be formed either cold or hot, like shell ring courses, with similar considerations after forming. The following equation from ASME Code, Section VIII, Division 1, Paragraph UCS-79, for calculating the fiber elongation in formed heads is more restrictive than the equation for shell ring courses because of the double curvature of formed heads.
For double curvature (for example, heads):
Formed heads may therefore require a stress-relief heat treatment when cylindrical components in the same vessel do not. When postweld heat treatment is required by the ASME Code or the purchase specification, this heat treatment can also serve the function of the stress-relief necessitated by the fiber elongation. In fact, it is preferable to perform the stress-relief after the cold formed heads are welded to the cylindrical shell components, because some distortion can accompany stress-relief that could result in poor fit-up for girth welding.
Formed heads can be purchased by the fabricator from a vendor, but they can also be formed from plate material by the fabricator. Heads can also be fabricated from several “pie-shaped” pieces of plate, called gore sections. Each piece is formed to the required contour of the head, and they are then welded together.
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