National standard closures or end covers are divided into dome closures, conical closures (used only for certain special-purpose containers), and flat closures (used in pressure vessels, except as covers for manholes and handholes).

In pressure vessels, spherical heads have better load-bearing properties compared to other types of heads. They require less thickness at the same diameter and pressure, saving materials, and are thus commonly used in high-pressure vessels. At the same time, the thickness of spherical heads differs significantly from that of the cylinder in all types of head-to-cylinder connections. This connection structure not only affects the equipment's manufacturing but is also a crucial factor in the vessel's operation.

National standard caps are a component of containers (as shown in the figure above). They can be divided into several types according to their geometric shapes, such as spherical, elliptical, dish-shaped, hemispherical, conical shell, and flat covers. Among them, the spherical, elliptical, dish-shaped, and hemispherical caps are collectively known as convex caps. In terms of welding, they are classified into socket welding caps and plug welding caps. They 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.