What is a pressure vessel, and how many categories are there?

Among category II and category III pressure vessels, there are always low-pressure vessels that contain highly toxic substances. Is this a mistake, or is there some difference?

1. Overview Equipment in industrial production that has specific functional roles and is subjected to certain pressures is referred to as a pressure vessel. Storage and transportation containers, reaction vessels, heat exchange vessels, and separation vessels all fall under pressure vessels. To distinguish them from ordinary containers (atmospheric pressure containers), only those containers that meet all of the following three conditions can be referred to as pressure vessels: (1) The maximum operating pressure (Pw) (Note 1) is greater than or equal to 0.1 Mpa (excluding hydrostatic pressure; the same applies hereafter); (2) The inner diameter (for non-circular cross-sections, this refers to their largest dimension) is greater than or equal to 0.15 m, and the volume (V) (Note 2) is greater than or equal to 0.25 m3; (3) The medium contained within is a gas, a liquefied gas, or a liquid whose maximum operating temperature is greater than or equal to its standard boiling point. Pressure vessels have a wide range of applications. It is a device that plays an important role in various sectors of the national economy, such as the petrochemical industry, energy industry, scientific research, and the military industry. A pressure vessel generally consists of six main parts that make up its body: the cylinder, the head, flanges, sealing elements, openings and nozzles, and supports. In addition, it is equipped with safety devices, meters, and internal components that carry out various manufacturing processes. Due to reasons such as sealing, pressure resistance, and the type of medium contained, pressure vessels are prone to explosions and fires, which can endanger the safety of people, equipment, and property as well as cause environmental pollution. Currently, countries around the world consider it an important product subject to supervision and inspection, with **designated specialized agencies carrying out such supervision, inspection, and technical testing in accordance with** established regulations and standards. 2. Classification 　　There are many methods for classifying pressure vessels; from the perspectives of use, manufacturing, and inspection, the following classifications exist. 　　(1) Classified by the pressure level they can withstand: low-pressure vessels, medium-pressure vessels, high-pressure vessels, and ultra-high-pressure vessels. 　　(2) Classified by the medium contained: non-flammable, non-toxic ； Flammable or toxic ； Highly toxic. 　　(3) Classified according to their functions in the process: 　　① Reaction vessel: A container used to carry out physical and chemical reactions of the medium. 　　②Heat exchange vessel: A vessel used to facilitate heat exchange between fluids. 　　③Separation vessel: A vessel used to facilitate mass exchange of media, gas purification, and the separation of solids, liquids, and gases. 　　④Storage and transport containers: Containers used to hold liquid or gaseous materials, storage and transport media, or to provide pressure-balancing and buffering functions. 　　(4) To implement scientific management and safety inspections more effectively, China’s \"Regulations on the Safety Inspection of Pressure Vessels\" classifies pressure vessels into three categories based on working pressure, the hazard level of the medium, and its role in production. Different regulations are set for pressure vessels in each category regarding their design and manufacturing processes, as well as the inspection items, contents, and methods. A safety and quality licensing system for imported goods has been implemented for pressure vessels; goods without an import safety and quality license are not allowed to be imported. 　　1. Class III pressure vessels are those that fall under one of the following categories: High-pressure vessels ； 　　Medium-pressure vessels (only for media with extremely high and high toxicity) ； 　　Medium-pressure storage vessels (only for flammable or moderately hazardous media with a pV product of 10 MPa·m3 or greater) ； 　　Medium-pressure reaction vessel (only for flammable or moderately hazardous media with a pV product of 0.5 Pa·m3 or greater) ； 　　Low-pressure vessels (only for media with extremely high or high toxicity levels, and a product of at least 0.2 MPa·m3) ； 　　High-pressure and medium-pressure shell-and-tube waste heat boilers ； 　　Medium-pressure glass-lined pressure vessel ； 　　Pressure vessels manufactured from materials with a high strength level (meaning that the minimum specified value for tensile strength in the relevant standards is 540 MPa or higher) ； 　　Mobile pressure vessels, including railway tank cars (for liquefied gases and cryogenic liquids), tank trucks, and tank containers (for liquefied gases and cryogenic liquids), etc ； 　　Spherical storage tank (volume greater than or equal to 50 m3) ； Low-temperature liquid storage containers (capacity greater than 5 m3). 　　Low-temperature liquid storage containers (with a volume greater than 5 m3). 2. Pressure vessels of Category II: Such vessels are considered to be of Category II if they meet one of the following conditions: Medium-pressure vessels ； 　　Low-pressure vessels (only for media with extremely high and high toxicity) ； 　　Low-pressure reaction vessels and low-pressure storage vessels (only for flammable media or media with moderate toxicity) ； 　　Low-pressure shell-and-tube waste heat boiler ； 　　Low-pressure glass-lined pressure vessels. 　　3. Class 1 pressure vessels: Low-pressure vessels other than those specified above are classified as Class 1 pressure vessels. 3. Others 　Sealed containers that are subjected to gas or liquid pressure, either internally or externally, and require high levels of safety. In the early days, it was mainly used in the chemical industry, with pressures usually below 10 MPa. With the emergence of high-pressure production processes such as ammonia synthesis and high-pressure polyethylene, pressure vessels are required to have pressures of over 100 megapascals. With the development of industries such as chemicals and petrochemicals, pressure vessels are operating in increasingly wide temperature ranges, have larger capacities, and some are also required to be resistant to medium corrosion. Starting in the 1960s, the development of nuclear power plants imposed higher safety and technical requirements on reactor pressure vessels, which in turn promoted further advancements in these vessels and led to their widespread use in various industrial sectors. Pressure vessels are mainly cylindrical, but they can also be spherical or of other shapes. Based on their structural design, they can be classified into multi-layer pressure vessels, wrapped-plate pressure vessels, grooved-wrapped-band pressure vessels, heat-shrinked pressure vessels, forged-and-welded pressure vessels, and thick-plate rolled-and-welded pressure vessels, among others. Most pressure vessels are made of steel, but some are also constructed from non-ferrous metals such as aluminum and titanium, as well as from non-metallic materials like fiberglass and prestressed concrete. If a pressure vessel explodes during use, it can cause a catastrophic accident. In order to ensure that pressure vessels are designed to be advanced, have a rational structure, are easy to manufacture, reliable in operation, and cost-effective while maintaining safety, various countries have established standards, specifications, and technical requirements for pressure vessels based on their own specific circumstances, setting out specific and mandatory rules regarding the design, manufacturing, inspection, and use of such vessels. 　　Inspection of pressure vessels 1. The external inspection of pressure vessels, also known as inspection during operation, involves checking for abnormalities such as cracks, deformation, leaks, and localized overheating on the outer surface of the pressure vessel ； Are the safety accessories complete, sensitive, and reliable? ； Are the fastening bolts in good condition and fully tightened? ； Check for any abnormalities such as foundation settlement, tilting, or damage to the anti-corrosion layer. External inspection is both the task of inspectors and one of the routine inspection items for operators. If any safety-threatening conditions are detected (such as cracks, deformation, or severe leakage in the stressed components), the operation should be stopped and the relevant personnel should be informed promptly. 2. Internal and external inspection of pressure vessels: This type of inspection must be carried out after the vessel has been shut down and its interior cleaned thoroughly. In addition to all the items covered in the external inspection, the main aspects of inspection also include checking for corrosion and wear on the inner and outer surfaces ； Inspect all welds, head transition areas, and other stress concentration zones for cracks using the naked eye and a magnifying glass; if necessary, use ultrasonic or radiographic testing to examine the internal quality of the welds ； Measure the wall thickness. If the measured wall thickness is less than the minimum wall thickness of the container, a strength check should be conducted again, and measures such as operating at reduced pressure or carrying out repairs should be proposed ； Containers that may cause changes in the microstructure of metal materials should be subjected to metallurgical testing when necessary ； The main bolts of high-pressure and ultra-high-pressure vessels should be inspected for cracks using magnetic particle testing or dye penetrant testing. Through internal and external inspections, the causes of the identified defects are analyzed, and recommendations for handling them are proposed. Reinspection is required after repair. The inspection cycle for the interior and exterior of pressure vessels is once every three years, but this cycle should be shortened for vessels handling highly corrosive or highly toxic substances. Containers with serious defects identified during operation, as well as those with poor welding quality or whose material’s resistance to corrosion by the medium is unknown, should also have their inspection intervals shortened. 3. Comprehensive inspection of pressure vessels: In addition to the inspection items mentioned above, a pressure test is also required for comprehensive inspections of pressure vessels (usually a hydrostatic test). Conduct random non-destructive testing on the main welds or inspect all welds. However, for containers holding media with very low pressure that are non-flammable, non-toxic, and non-corrosive, and in which no defects are detected, non-destructive testing may be omitted after gaining sufficient experience in their use. The comprehensive inspection cycle for containers is generally at least once every six years. For manufactured containers filled with air and inert gases, once experience in use has been gained and internal and external inspections have confirmed the absence of corrosion, the comprehensive inspection interval can be appropriately extended.

This post was last edited by Guangming De Xiao Cao on 2019-8-20 at 09:44. The regulations TSG 21-2016 specify the requirements for pressure vessels: their volume must be 0.03 m3 or more, not 0.25 m3 or more.