Welded pipe caps come in various structural forms, mainly including convex, conical, reducing, flat, and constricted designs. Convex caps further include hemispherical, elliptical, dished, and spherical caps.
From a stress perspective, hemispherical convex caps are progressively less desirable, but from a manufacturing perspective, they become progressively easier to manufacture. Classified by pressure rating, there are approximately seventeen types, corresponding to the pressure ratings of American pipe standards, such as Sch5s, Sch10s, Sch10, Sch20, Sch30, Sch40s, STD, Sch40, Sch60, Sch80s, XS, Sch80, SCH100, Sch120, Sch140, Sch160, and XXS. Among these, STD and XS are the most commonly used.