National standard flanges or end caps are categorized into convex flanges, conical flanges (used only for certain special-purpose containers), and flat flanges (used as manhole and handhole covers in pressure vessels).

In pressure vessels, spherical heads have better load-bearing properties compared to other types of heads. They require less thickness for the same diameter and pressure, saving materials, and are thus commonly used in containers with higher pressure. At the same time, the thickness of spherical heads is significantly different from that of the cylindrical body in all types of head-to-body connections. This difference not only affects the manufacturing of the equipment but is also a crucial factor in the operation of the container.

National standard caps are a component of containers (as shown in the figure above) and can be divided into several types according to different geometric shapes, such as spherical, elliptical, disc-shaped, hemispherical, conical shell, and flat cover. Among them, spherical, elliptical, disc-shaped, and hemispherical caps are collectively referred to as convex caps. In terms of welding, they are classified into butt-welded caps and socket-welded caps. They are used in various container equipment, such as storage tanks, heat exchangers, towers, reactors, boilers, and separation equipment, etc. The materials include carbon steel (A3, 20#, Q235), stainless steel, alloy steel, aluminum, titanium, copper, nickel, and nickel alloys, etc.