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The licensing levels for pressure vessel manufacturing are divided into four categories, A, B, C, and D, based on the range of products that can be manufactured. Each of these categories is further subdivided into several grades, as follows: (1) Category A: Ultra-high pressure vessels, high-pressure vessels (A1)； Other high-pressure vessels (A2) ； On-site welding of spherical storage tanks or manufacturing of spherical shell plates (A3) ； Non-metallic pressure vessels (A4) ； Medical oxygen chamber (A5) ； Ultra-high pressure vessel (A6). A1, Large high-pressure vessels (A1): High pressure: 10 MPa ≤ P < 100 MPa. Note: High-pressure vessels with an inner diameter of ≥ 2 meters. Coverage: Grades A2 and D. A2, Other high-pressure vessels (A2): High pressure: 10 MPa ≤ P < 100 MPa. Note: High-pressure vessels with an inner diameter of less than 2 meters. Coverage: Grade D. A3, Ultra-large pressure vessels, spherical tanks (A3): Medium and low pressure: 0.1MPa ≤ P < 10MPa. Note: Ultra-large pressure vessels refer to those whose diameter is too large to be transported by road or rail. Units specialized in the on-site fabrication of extra-large medium and low-pressure aspherical pressure vessels must obtain a pressure vessel manufacturing license corresponding to that category (the license shall specify on-site fabrication of extra-large medium and low-pressure aspherical pressure vessels). Manufacturing units holding an A3-level pressure vessel manufacturing license are eligible to carry out the on-site fabrication of such vessels. A4, Non-metallic pressure vessels (A4) A5, Medical oxygen chambers (A5) A6, Ultra-high pressure vessels (A6) Ultra-high pressure: P≥100MPa (2) Class B: Seamless gas cylinders (B1) ； Welding gas cylinder (B2) ； Special gas cylinders (B3). B2 includes (is limited to) cylinders containing dissolved acetylene gas or liquefied petroleum gas ； B3 includes cylinders for motor vehicles, wound-type cylinders, non-rechargeable cylinders, and vacuum-insulated cryogenic cylinders. (3) Class C: Railway tank cars (C1) ； Motorized tank truck or long-tube trailer (C2) ； Tank container (C3). (4) Class D: Category I pressure vessels (D1) ； Category II low and medium pressure vessels (D2). (5) Expansion joints for pressure vessels: They can be manufactured by units that have obtained the qualification for manufacturing pressure vessels, or by units that have obtained the qualification for manufacturing expansion joints for pressure pipelines. (6) For pressure vessels designed according to analytical design standards, the manufacturing enterprise must hold an A or C class license. (7) Heads: The manufacturing license for boiler and pressure vessel heads is divided into two categories, namely heads with a diameter of 1800 mm or more, and those with a diameter less than 1800 mm. Units that have obtained the qualification to manufacture heads with a diameter of 1800 mm or more also possess the qualification to manufacture heads with a diameter less than 1800 mm. Manufacturers of end caps with a diameter of 1800 mm or more must meet the basic requirements for manufacturing pressure vessels of Class A3, while manufacturers of end caps with a diameter less than 1800 mm must meet the basic requirements for manufacturing pressure vessels of Class D2 (and also comply with the specific requirements specified in the safety technical regulations).