A pressure vessel is a sealed container that withstands internal or external gas or liquid pressure and has high safety requirements.

　　Pressure vessels are mainly cylindrical, with a few being spherical or other shapes. Cylindrical pressure vessels are usually composed of parts and components such as cylinders, heads, nozzles, and flanges. The higher the working pressure of the pressure vessel, the thicker the wall of the cylinder should be.

　　Pressure Vessel Classification
　　Classification by pressure level: Pressure vessels can be divided into internal pressure vessels and external pressure vessels.

　　Internal pressure vessels can be further divided into four pressure levels according to the design pressure (p), the specific division is as follows:

　　Low-pressure (code L) vessel 0.1 MPa≤p＜1.6 MPa;
　　Medium-pressure (code M) vessel 1.6 MPa≤p＜10.0 MPa;
　　High-pressure (code H) vessel 10 MPa≤p＜100 MPa;
　　Ultra-high pressure (code U) vessel p≥100 MPa.

　　Classification by the role of the container in production:

　　Reaction pressure vessel (code R): Used to complete the physical and chemical reactions of the medium.
　　Heat exchange pressure vessel (code E): Used to complete the heat exchange of the medium.
　　Separation pressure vessel (code S): Used to complete the fluid pressure balance buffering and gas purification separation of the medium.
Storage pressure vessel (code C, where the spherical tank code is B): Used to store and contain gases, liquids, liquefied gases, etc.

In a pressure vessel, if two or more process principles are present at the same time, the type should be classified according to the main function in the process.

　　Classification by installation method:

　　Fixed pressure vessel: A pressure vessel with a fixed installation and use location, relatively fixed process conditions, and operators.
　　Mobile pressure vessel: When in use, it not only bears internal pressure or external pressure loads but also suffers from impact forces caused by the internal medium shaking during handling, as well as external impacts and vibration loads during transportation. Therefore, it has special requirements in terms of structure, use, and safety.

　　The above-mentioned classification methods only consider a certain design parameter or use condition of the pressure vessel and cannot comprehensively reflect the danger level of the pressure vessel.

　　The danger level of a pressure vessel is also related to the hazard of the medium and the product of its design pressure p and full volume V. The larger the pV value, the greater the explosion energy when the container ruptures, the greater the hazard, and the higher the requirements for the design, manufacturing, inspection, use, and management of the container.

　　Classification by safety technology management:

　　The "Pressure Vessel Safety Technology Supervision Regulations" adopts a comprehensive classification method that considers both the product of container pressure and volume, the hazard of the medium, and the role of the container in the production process, which is conducive to safety technology supervision and management. This method divides pressure vessels into three categories:

　　1. Third-class pressure vessels. Any of the following conditions constitute a third-class pressure vessel:

　　High-pressure vessel;
　　Medium-pressure vessel (limited to media with extremely and highly hazardous toxicity);
　　Medium-pressure storage vessel (limited to flammable or moderately hazardous toxic media, and the pV product is greater than or equal to 10 MPa·m3);
　　Medium-pressure reaction vessel (limited to flammable or moderately hazardous toxic media, and the pV product is greater than or equal to 0.5 Pa·m3);
　　Low-pressure vessel (limited to media with extremely and highly hazardous toxicity, and the product is greater than or equal to 0.2 MPa·m3);
　　High-pressure and medium-pressure shell and tube waste heat boilers;
　　Medium-pressure glass-lined pressure vessels;
　　Pressure vessels manufactured from materials with higher strength levels (referring to the lower limit of the tensile strength specified in the corresponding standard being greater than or equal to 540 MPa);
　　Mobile pressure vessels, including railway tank cars (media are liquefied gases, cryogenic liquids), tank trucks [liquefied gas transportation (semi-trailers), cryogenic liquid transportation (semi-trailers), permanent gas transportation (semi-trailers)], and tank containers (media are liquefied gases, cryogenic liquids), etc.;
　　Spherical storage tanks (volume greater than or equal to 50 m3); Cryogenic liquid storage containers (volume greater than 5 m3).
　　Cryogenic liquid storage containers (volume greater than 5 m3)

　　2. Second-class pressure vessels. Any of the following conditions constitute a second-class pressure vessel:

　　Medium-pressure vessel;
　　Low-pressure vessel (limited to media with extremely and highly hazardous toxicity);
　　Low-pressure reaction vessels and low-pressure storage vessels (limited to flammable media or moderately hazardous toxic media);
　　Low-pressure shell and tube waste heat boilers;
　　Low-pressure glass-lined pressure vessels.

　　3. First-class pressure vessels. Low-pressure vessels other than those specified above are first-class pressure vessels.

　　It can be seen that the domestic pressure vessel classification method comprehensively considers design pressure, geometric volume, material strength, application scenarios, and the degree of hazard of the medium.

　　For example, due to different medium characteristics or container functions, that is, according to the size of potential hazards, low-pressure containers can be classified as first-class, second-class, or even third-class pressure vessels