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

In pressure vessels, spherical heads are superior in load-bearing compared to other types of heads. They require less thickness at 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 differs significantly from that of the cylindrical body in all types of head connections. The structural integrity of this connection not only affects the manufacturing of the equipment but is also a crucial factor in the operation of the container.

National standard flanges are a component of containers (as shown in the figure above). They can be categorized into several types based on their geometric shapes, such as spherical, elliptical, dish-shaped, hemispherical, conical shell, and flat covers. Among them, spherical, elliptical, dish-shaped, and hemispherical flanges are collectively referred to as convex flanges. In terms of welding, they are divided into socket weld flanges and insert weld flanges. They are used in various types of 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.