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II. Structural Features and Main Parameters of Pressure Vessels 1. Structural Features Pressure vessels generally consist of a cylinder (also known as the shell), end caps (also known as terminal covers), flanges, sealing elements, openings and connections (manholes, access holes, sight glass holes, inlet and outlet connections for materials, level gauges, flow meters, temperature probes, safety valves, etc.), supports, as well as various other internal components. Based on the type of support, they can be classified as: 1) Horizontal containers ； 2) Vertical containers ； 3) Suspended containers. Based on their end caps, they can be classified as: 1) Elliptical end cap ； 2) Butterfly head ； 3) Conical head ； 4) Spherical head, hemispherical head ； 5) Flat head. Based on the overall shape of the container, they can be classified as: 1) Cylindrical pressure vessels ； 2) Spherical vessel ； 3) Rectangular container. 2. Main parameters 1) Design pressure: refers to the pressure used, at the corresponding design temperature, to determine the calculated wall thickness of the container shell as well as the dimensions of its components. The design pressure of a pressure vessel shall not be lower than the maximum operating pressure. For pressure vessels equipped with safety relief devices, their design pressure shall not be lower than the opening pressure of the safety valve or the burst pressure of the rupture disc. 2) Maximum operating pressure: refers to the highest gauge pressure that may occur at the top of the container during normal operation. 3) Operating pressure: refers to the gauge pressure generated by the container under conditions that meet the process requirements. 4) Test pressure: refers to the pressure at the top of the container during a pressure test. 5) Design temperature: It refers to the metal temperature of the components as specified under normal operating conditions. The design temperature indicated on the nameplate should be either the maximum or minimum value of the shell’s design temperature. 6) Test temperature: refers to the metal temperature of the shell of a pressure vessel during pressure testing. 7) Calculated thickness: refers to the thickness of various components of a pressure vessel, as calculated using formulas. 8) Design thickness: refers to the sum of the calculated thickness and the corrosion margin. 9) Nominal thickness: refers to the design thickness, adjusted upward by the negative deviation of the steel, and rounded to the standard specification of the steel. 10) Effective thickness: refers to the thickness obtained by subtracting the negative deviation of the steel material and the corrosion allowance from the nominal thickness. 11) Measured thickness: refers to the actual thickness of a pressure vessel as determined by a thickness gauge during inspection. 12) Outer diameter: refers to the outer diameter of cylindrical or spherical pressure vessels. 13) Inner diameter: refers to the inner diameter of cylindrical or spherical pressure vessels. 14) Representation of container specifications: inner diameter X wall thickness X length (height), unit: mm, represented by symbols: ф, &L.