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1. Concept of pressure vessels A pressure vessel is a sealed device used to hold gases or liquids and withstand certain pressures. It refers to fixed or mobile containers in which the maximum operating pressure is greater than or equal to 0.1 MPa (gauge pressure), the product of pressure and volume is greater than or equal to 2.5 MPa•L, and the maximum operating temperature is greater than or equal to the standard boiling point of the liquids contained； Gas cylinders designed for gases, liquefied gases, and liquids with a standard boiling point of 60°C or lower, having a nominal pressure of 0.2 MPa (gauge pressure) or higher, and a product of pressure and volume of 1.0 MPa•L or higher ； Oxygen chambers, etc. Common pressure vessels include gas storage tanks, steam distribution cylinders, accumulators, water tanks in air separation equipment, spiral plate heat exchangers, compressors, gas cylinders, separators, preheaters, heaters, shell-and-tube waste heat boilers, and steam cooking pots. 2. Relevant regulations (1) Regulations on the Safety Technical Inspection of Pressure Vessels (No. 154 issued by the Quality and Technology Supervision Bureau). (2) Regulations on the Safety Supervision of Gas Cylinders (Quality and Technical Supervision Bureau Order No. 250). 3. Classification of pressure vessels Pressure vessels are classified into three categories based on their safety level. The requirements for the qualification assessment of manufacturers that design and manufacture pressure vessels, as well as those related to pressure window inspection and management, are also connected to the safety classification levels of pressure vessels. The complete classification of pressure vessels depends on the pressure they are subjected to, the hazard level of the medium stored within them, and their volume. The specific classifications are as follows: (1) Category III pressure vessels: ① High-pressure vessels (10 MPa ≤ p < 100 MPa). ②Medium pressure (1.6 MPa ≤ p < 10 MPa), and containers storing extremely hazardous or highly hazardous substances. ③Medium-pressure storage vessels, limited to flammable or moderately hazardous media with a pV product greater than 100 MPa•m3. ④Medium-pressure reaction vessels, limited to media that are flammable or have a moderate level of toxicity, and for which the pV product is greater than or equal to 0.5 Mpa•m3. ⑤Low-pressure vessels, limited to media that are flammable or highly toxic, with a pV product of 0.2 MPa•m3 or greater. ⑥High-pressure and medium-pressure shell-and-tube waste heat boilers. ⑦Medium-pressure glass-lined pressure vessels. ⑧Pressure vessels manufactured from materials with an extremely high strength level (tensile strength ≥ 54 MPa). ⑨Mobile pressure vessels, including railway and road tank cars, trailers, and tank containers. ⑩Spherical storage tanks (capacity ≥ 50 m3) and cryogenic liquid storage vessels (capacity > 5 m3). (2) Class II pressure vessels ① Medium-pressure vessels. ②Low-pressure vessels, limited to media with an extreme and high degree of toxicity. ③Low-pressure reaction vessels and low-pressure storage vessels are limited to flammable media or media with a moderate level of toxicity. ④Low-pressure shell-and-tube waste heat boiler. ⑤Low-pressure glass-lined pressure vessels. (3) Class 1 pressure vessels: other low-pressure vessels other than Class 2 and Class 3 pressure vessels. 4. Requirements for regular inspection of pressure vessels and accessories: (1) Regular on-site inspections of operating pressure vessels shall be carried out at least once a year. (2) Inspection of in-service pressure vessels when shut down: For those with safety grades 1 and 2, internal and external inspections shall be carried out at least once every six years ； For those with a safety level of category three, it should be done at least once every three years. (3) For fixed pressure vessels, a pressure test must be carried out at least once during each internal inspection. For mobile pressure vessels, a pressure test must be carried out at least once every six years. (4) The safety valve must be inspected at least once a year. (5) For pressure vessels with a fast corrosion rate, long service life, having been out of use for more than two years, showing obvious defects, or those whose operating medium is frequently changed, the inspection interval should be shortened. ①Cylinders containing general gases are inspected every three years. ②Cylinders containing corrosive gases, diving cylinders, and those that are frequently in contact with seawater must be inspected every two years