What is the difference between design pressure and calculated pressure, and how is it determined?

The design pressure applies to each chamber of the vessel; it serves as the basis for selecting materials for the vessel, classifying it, specifying manufacturing and inspection requirements, and determining the test pressure. It is also used to determine the calculated pressures for various pressure-bearing components of the vessel. The design pressure of each chamber in the container is determined based on its operating pressure, the opening pressure of the safety valve, or the burst pressure of the rupture disc, among other factors. The design pressure shall not be lower than the operating pressure; when equipped with a safety relief device, it shall not be lower than the opening pressure of the safety valve or the burst pressure of the rupture disc. The calculated pressure applies to each pressure-bearing element of the container, and is used solely to determine the thickness required for those elements to meet the requirements regarding strength, stability, and stiffness. The calculated pressure for each stressed element of the container is determined based on the design pressure of each chamber in the container, along with the static pressure of the liquid column, taking into account both the individual effects and the combined effects on those elements. For a single-chamber container with the medium entirely being gas, the calculated pressure of each pressurized element on the container is equal to the design pressure of that container ； When there is a liquid in the medium, the calculated pressure on the compressed element due to the static pressure of the liquid column is the design pressure of the container plus the static pressure of the liquid column. For the pressure-bearing components in multi-chamber containers that are subject to pressures from multiple chambers, the calculated pressure should be determined based on the scenarios that may arise during production. For example, when determining the calculated pressure for a heat exchanger tube sheet, it is necessary to consider the effects of pressure acting alone on the shell side, pressure acting alone on the tube side, and the combined effect of both ； When determining the calculated pressure of the pressurized element enclosed by the jacket on the inner container within a jacketed vessel, it is necessary to consider the effects of the pressure acting on the inner container alone, the pressure acting on the jacket alone, and their combined effect, as well as the stability of the element under the jacket test pressure.

Differences between calculated pressure and design pressure: 1. The design pressure represents the loading conditions for the entire equipment, whereas the calculated pressure is a parameter derived from the calculations of specific stressed components; the nature of the two is different. 2. The design pressure is determined based on the operating pressure of the vessel, that is, the highest pressure that may occur at the top of the vessel under normal operating conditions, taking into account certain safety margins as well as any safety devices installed for protection. 3. A device or chamber has only one design pressure. 4. The design pressure defined in GB-150 is both the pressure used to determine the wall thickness of the container shell and the design pressure indicated on the nameplate; it also stipulates that when the static liquid pressure acting on the component reaches 5% of the design pressure, the design pressure together with this value should be used to calculate the component’s thickness. 5. For containers with non-circular cross-sections, the pressure to be used in calculations should be the pressure from the hydrostatic test, that is, the maximum load-bearing pressure during testing and operation. 6. The calculated pressure is not listed in the technical specifications table of the pressure vessel layout, but only appears in the calculation booklet. Last edited by Nairaho on 2009-3-11 15:55]

The calculated pressure is obtained through comprehensive calculations from various aspects. Its value is much higher than the design pressure; otherwise, the calculation report would not be approved.