The picture below shows a dished / domed / torespherical pressure vessel head having a weld preparation machined on. The vertical borer at Abbotts can spin material up to 1300 mm diameter. The prep might also involve scarfing the thickness down in a shallow taper – this is sometimes necessary when the end needs to be thicker than the shell on the dish element to meet design pressure requirements. The difference in thickness between the end and the shell is not acceptable for many approved welds and therefore the end is skimmed to match thickness and a gentle change in cross sectional area, and a nicely weldable joint.
The v prep weld joint may be from one or two sides depending upon the access ability of the two sides of the weld (you probably don’t want a welder to weld himself into the vessel !)