Pressure vessels are defined as enclosed equipment designed to contain gases or liquids under pressure. Their scope encompasses: stationary and mobile vessels with a volume of 30 L or greater and an internal diameter (or maximum geometric dimension of the internal cross-sectional boundary for non-circular cross-sections) of 150 mm or greater, containing gases or liquefied gases with a maximum working pressure of 0.1 MPa (gauge pressure) or higher, or liquids with a maximum working temperature at or above their standard boiling point; gas cylinders containing gases, liquefied gases, or liquids with a standard boiling point of 60°C or lower, having a nominal working pressure of 0.2 MPa (gauge pressure) or higher and a pressure-volume product of 1.0 MPa·L or greater; and hyperbaric oxygen chambers.
The design pressure (p) of pressure vessels is categorized into four pressure classes: Low pressure (code L): 0.1 MPa ≤ p; Medium pressure (code M): 1.6 MPa ≤ p; High pressure (code H): 10.0 MPa ≤ p; (4) Ultra-high pressure (code U): p ≥ 100.0 MPa. Based on their functional role in the production process, pressure vessels are classified into reaction, heat exchange, separation, and storage pressure vessels. The specific classifications are as follows: Reaction pressure vessels (code R) are primarily used to facilitate physical or chemical reactions of the medium (e.g., various reactors, reaction kettles, polymerization kettles, synthesis towers, shift converters, gas producers); heat exchange pressure vessels (code E) are primarily used for heat exchange involving the medium (e.g., various heat exchangers, coolers, condensers, evaporators); separation pressure vessels (code S) are primarily used for fluid pressure balancing/buffering and gas purification/separation (e.g., various separators, filters, oil collectors, scrubbers, absorption towers, copper-wash towers, drying towers, stripping towers, steam headers, deaerators); and storage pressure vessels (code C; spherical tanks are coded B) are primarily used to store or contain media such as gases, liquids, or liquefied gases (e.g., various types of storage tanks).
In accordance with the *Catalogue of Special Equipment*, stationary pressure vessels are categorized into Class I, Class II, and Class III based on parameters such as design pressure, medium hazard level, and volume. The specific classification criteria are as follows: Class I pressure vessels are primarily low-pressure vessels with a design pressure of 0.1 MPa ≤ P ≤ 1.6 MPa; Class II pressure vessels include medium-pressure vessels with a design pressure of 1.6 MPa ≤ P ≤ 10 MPa, as well as certain low-pressure vessels (such as those containing flammable media or media with moderate toxicity, and storage vessels); Class III pressure vessels include high-pressure vessels with a design pressure of 10 MPa ≤ P ≤ 100 MPa, medium-pressure vessels containing media with extreme or high toxicity, and vessels where the product of volume and pressure exceeds a specific threshold (such as low-pressure vessels with PV ≥ 0.2 MPa·m³ and medium-pressure reaction vessels with PV ≥ 0.5 MPa·m³).