National standard flanges or end caps are divided into convex flanges, conical flanges (used only for certain special-purpose containers), and flat flanges (used as manhole and handhole covers in pressure vessels). Compared to other types of flanges, spherical flanges have better load-bearing properties, require less thickness at the same diameter and pressure, saving materials, and are often used in high-pressure vessels. At the same time, in all types of flange connections with cylinders, the thickness difference between spherical flanges and cylinders is significant, and the connection structure between them not only affects equipment manufacturing but is also a crucial factor for the vessel's operation. National standard flanges are a component of the vessel (as shown in the diagram) and can be classified into spherical, elliptical, dish-shaped, hemispherical, conical shell, and flat caps, among others. Spherical, elliptical, dish-shaped, and hemispherical flanges are collectively referred to as convex flanges. In terms of welding, they are divided into butt-welded flanges and socket-welded flanges. They are used in various vessel equipment such as storage tanks, heat exchangers, towers, reactors, boilers, and separation equipment, etc. Materials include carbon steel (A3, 20#, Q235), stainless steel, alloy steel, aluminum, titanium, copper, nickel, and nickel alloys.