Calculation of Maximum Allowable Working Pressure

The actual thicknesses of the various vessel components will usually be thicker than the thickness calculated using the component design pressure (P). It is usually more economic to obtain the required thickness plus corrosion allowance by purchasing the next thicker commercial size of plate, pipe, or ANSI B16.5 flange, than to have the components specially fabricated to the exact thicknesses required. Therefore, the MAWP permitted (without exceeding the maximum allowable design stress) for the material at the design temperature, will usually be somewhat higher than the design pressure (Pd). The Code allows calculating a MAWP based on this extra thickness, adjusted for the hydrostatic head (Ph), for each vessel component, using the lowest MAWP for any component as the MAWP for the vessel.

If a MAWP is not calculated for the actual component thicknesses in this described manner, the design pressure (Pd) must be used for the MAWP on the nameplate. When the design pressure (Pd) is used for the MAWP on the nameplate, the extra thickness should be added to the corrosion allowance for each component of the vessel.

The MAWP of a vessel should not normally be limited by the MAWP of a minor component, such as a flange or nozzle. For example, if an ANSI B16.5 flange has a lower pressure rating than the MAWP for the shell and head components, the flange should be upgraded to the next higher class. However, this upgrading can cause complications if the associated piping class calls for lower pressure flanges, and a nonstandard flange must be added to the pipe mating to the vessel. These factors must be evaluated for each specific circumstance.

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