Pressure vessel flanges are divided into integral flanges, looping flanges and optional flanges according to the connection of flange rings. There are three types of standard pressure vessel flanges in common use in China: a flat welding flange, B flat welding flange and long-neck butt welding flange, among which A and B flat welding flange belong to arbitrary type flange. Flanges and gaskets, fasteners are collectively called flange joints, which are widely used in engineering design and a kind of spare parts, and are also the key parts prone to problems in engineering design and use. The design of flange joint is one of the important contents in the design of pressure vessel. , select the classification diagram according to the following requirements, and then mark the coordinate points according to the design pressure P (unit MPa) and volume V(unit L) to determine the classification of pressure vessels. Classification of multi-cavity pressure vessels: Multi-cavity pressure vessels (such as tube side and shell side of heat exchangers, jacket vessels, etc.) shall be classified as pressure vessels of high category and shall be used and managed according to the category. However, design and manufacturing technical requirements should be proposed according to the respective category of each pressure chamber.