Design Pressure: ASME Code, Section VIII, Division 2

Under Division 2, the pressures used for the design of each component are very similar to those discussed for Division 1, although terminology differs. The maximum pressure permissible at the top of the vessel at the design temperature is defined as the design pressure (Pd), and is displayed as such on the vessel’s nameplate. However, similar to Division 1 requirements, this pressure may not be the pressure that is used for the design of the components of the vessel. The hydrostatic head (Ph) must be added to Pd to determine the pressure (P) used for the design of each component (i.e., P = Pd + Ph). Division 2 requires that the coincident pressure (i.e., Pd + Ph) at any point should be used for the design of a component.

Ph for a Division 2 vessel is usually very small in relation to the high Pd that makes the use of Division 2 requirements economic, and the highest Ph at the bottom of the vessel can be used for the design of all components, regardless of the liquid level, in the vessel, without cost penalty.

Division 2 does not refer to a MAWP. Each component of a pressure vessel will usually be fabricated from custom-produced materials close to the minimum thickness plus corrosion allowance, with the exception of ANSI B16.5 flanges. Therefore, no benefit will be obtained by calculating a MAWP for the actual thicknesses.

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