Pressure vessel refers to the closed equipment holding gas or liquid and carrying a certain pressure. The pressure vessels can be classified using many ways from the perspective of use, manufacture and monitoring. (1) According to the level of pressure - bearing: low - pressure vessels, medium - pressure vessels, high - pressure vessels and ultra high - pressure vessels. (2) According to the media: non-flammable and non-toxic; flammable or toxic; highly toxic. (3) According to the role in the technological process: ① reaction container to complete the physical and chemical reaction of media. ② heat - exchange container to complete the heat exchange of media . ③ separation containersto complete the quality exchange, gas purification and separation of solid, liquid and gas . ④ storage and transportation containers to hold liquid or gas materials, store and transport media or to balance and buffer the pressure.