Pressure. The pressure acting on a pressure vessel can originate from two sources: externally or internally.
The maximum working pressure of a pressure vessel refers to the highest pressure likely to occur at the top of the vessel during normal operation for vessels subject to internal pressure; for vessels subject to external pressure, it refers to the highest pressure likely to occur at the top of the jacket during normal operation.
The design pressure of a pressure vessel refers to the pressure used to determine the thickness of the vessel shell at the corresponding design temperature-specifically, the pressure marked on the vessel's nameplate; this value must not be less than the maximum working pressure. When the static pressure of the liquid column acting on specific parts of the vessel or on pressure-bearing components reaches 5% of the design pressure, the sum of the design pressure and the liquid-column static pressure must be used for the design calculations of those parts or components. For pressure vessels equipped with safety relief devices, the design pressure must not be lower than the opening pressure or bursting pressure of the safety relief device. The vessel's design pressure shall be determined in accordance with the relevant provisions of GB150.
Temperature. Metal temperature refers to the average temperature of a pressure-bearing component across its cross-sectional thickness. Under no circumstances shall the surface temperature of the component metal exceed the allowable operating temperature of the steel material.
Design temperature refers to the metal temperature of a pressure-bearing component established for normal operating conditions at the corresponding design pressure; this value must not be lower than the maximum metal temperature the component is likely to reach. For metal temperatures below 0°C, the design temperature must not be higher than the minimum metal temperature the component is likely to reach. The vessel design temperature (i.e., the design medium temperature marked on the vessel's nameplate) refers to the design temperature of the shell.
Medium. There is a wide variety of process media involved in production processes, and various classification methods exist. Based on the state of matter, they are classified into gases, liquids, liquefied gases, pure substances, and mixtures. Based on chemical properties, they are classified into four categories: combustible, flammable, inert, and combustion-supporting. Based on the degree of toxicity to humans, they are further classified into four hazard levels: extremely hazardous (I), highly hazardous (II), moderately hazardous (III), and mildly hazardous (IV). Flammable media: Refers to gases that, when mixed with air, have a lower explosive limit of less than 10%, or a difference between the upper and lower explosive limits of 20% or greater (e.g., monomethylamine, ethane, ethylene).
Toxic media: The classification of media toxicity levels follows the "Safety Technical Supervision Regulations for Pressure Vessels" (hereinafter referred to as the "Regulations"), which references GB 5044, "Classification of Hazard Levels for Occupational Exposure to Toxic Substances." The maximum allowable concentrations are: extremely hazardous (Class I) < 0.1 mg/m³; highly hazardous (Class II) 0.1 to < 1.0 mg/m³; moderately hazardous (Class III) 1.0 to < 10 mg/m³; and mildly hazardous (Class IV) ≥ 10 mg/m³.
When the medium within a pressure vessel is a mixture, the process design department of the design organization or the production technology department of the user organization shall determine the toxicity level or flammability status based on the medium's composition and the established classification principles.
Corrosive media: Petrochemical media impose specific corrosion-resistance requirements on materials used for pressure vessels. In some cases, the presence of impurities in the medium can drastically accelerate corrosion. As the types and properties of corrosive media vary-and process conditions differ-the corrosivity of the media also varies. Consequently, when selecting materials for pressure vessels, one must ensure not only that mechanical property requirements for the operating conditions are met, but also that the materials possess adequate corrosion resistance; where necessary, specific anti-corrosion measures must be implemented.