How are pressure, maximum operating pressure, and design pressure defined and determined?

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Pressure, unless otherwise specified, refers to gauge pressure. The maximum operating pressure refers to the highest pressure that may occur at the top of the container under normal operating conditions. The design pressure refers to the pressure used to determine the thickness of the container shell at the corresponding design temperature; it is also the design pressure of the container indicated on the nameplate, and this value must not be less than the maximum operating pressure. When the hydrostatic pressure exerted by the liquid column on various parts of the container or on the stressed components reaches 5% of the design pressure, the sum of the design pressure and the hydrostatic pressure should be used for the design calculations of those parts or components. When a pressure relief device is installed on the vessel, the design pressure of the vessel shall be determined in accordance with the corresponding provisions in Appendix B of GB150.1. For containers holding liquefied gases, within the specified filling coefficient range, the design pressure shall be determined based on the highest metal temperature that may be reached under operating conditions. The design pressure of a pressure vessel shall be taken as not less than the maximum difference between internal and external pressures that may occur under normal operating conditions. Vacuum vessels are designed to withstand external pressure. When equipped with safety control devices such as vacuum relief valves, the design pressure is set at 1.25 times the maximum difference between internal and external pressures, or the smaller of these two values, i.e., 0.1 MPa; in the absence of such safety control devices, the value of 0.1 MPa is used. For jacketed vessels, when calculating the inner vessel with a jacket, the maximum pressure difference between the inside and outside that may occur under normal operating conditions should be taken into account.