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 stress distribution 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 difference between spherical heads and cylindrical bodies in all types of head connections is significant. The connection structure between them not only affects the equipment's manufacturing but is also a crucial factor in the operation of the container.

National standard head covers are a component of containers (as shown in the above figure). They can be divided into several types according to their geometric shapes, including spherical, elliptical, dish-shaped, spherical crown, conical shell, and flat covers, among which the spherical, elliptical, dish-shaped, and spherical crown head covers are collectively referred to as convex head covers. They are classified into weld neck covers and socket weld covers in terms of welding. These are used in various container equipment such as storage tanks, heat exchangers, towers, reactors, boilers, and separation equipment. Materials include carbon steel (A3, 20#, Q235), stainless steel, alloy steel, aluminum, titanium, copper, nickel, and nickel alloys.