Design Pressure and Pressure Used for Design of Individual Components

Although the design pressure (Pd) establishes the basis for the design of a vessel, it is not always the pressure used for designing components of the vessel. The pressure at the bottom of a vessel containing a liquid is higher than the pressure at the top due to the hydrostatic head. Therefore, the hydrostatic head (Ph) from the top of the liquid to the component being designed must be added to Pd to establish the component design pressure (P), which is used in the Code design calculations for that component (i.e., P = Pd + Ph for liquids).

The gas pressure at the top of a vessel that contains a liquid can be thought of as the design pressure (Pd). If a vessel contains only gas, the hydrostatic head (Ph) is negligible, and the design pressure (Pd) can be used as the pressure for the design of all of the vessel’s components (i.e., P = Pd + 0 for gases).

Usually, only one value for P is determined for the bottom of the vessel (i.e., for the maximum hydrostatic head). That number is used for the design of all components of the vessel from top-to-bottom. However, for tall vessels at low internal pressure (i.e., Ph is large relative to Pd), it may be advantageous to design individual components for the actual hydrostatic heads that exist at various levels in the vessel. The shell thickness can be reduced toward the top of the vessel, which in turn reduces the material and fabrication costs and the cost of the vessel support structure, because of the reduced vessel weight and wind/earthquake overturning moment.

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