Q. Define Maximum Allowable Working Pressure?

The MAWP to be marked on the Section VIII nameplate is defined as the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident metal temperature for that pressure. This MAWP may be determined from the design pressure or from calculations, based on the specified nominal component thicknesses, reduced by the specified corrosion allowances.

The maximum permissible set-to-operate pressure of a single safety relief device cannot be higher than the MAWP.

A suitable margin consistent with the uncertainties with which the true maximum mean-metal temperature can be determined should be included. The design temperature for vessels shall be the maximum temperature for which a reduction in maximum allowable stress is not required (650⁰F (343⁰C) for carbon steel). If there is a limiting appurtenance, vessel design metal temperature shall be the maximum temperature allowed for that appurtenance at design pressure.