The head refers to the element used to close the end of the container to isolate the internal and external media, also known as the end cover. The head of a cylindrical container is generally a rotary shell. According to the shape of the head surface can be divided into convex, conical, flat and combined shape. Convex head refers to the outer surface shape of the convex head, such as hemispherical, oval, dish and no flanged spherical head. Some cylinders adopt a convex inward combined bottom head, which can ensure strength and meet the needs of safe use