This question has been asked many times in this forum. Read ASME B&PV Code, Section VIII, Div 1, UG-21 carefully, especially Footnote 8. MAWP is defined in UG-98.
There is a difference between Design pressure and MAWP.
Design Pressure-This will be higher than the operating pressure with 30 psi or 10%, whichever is the greater, will satisfy this requirement.This is the pressure used in the design of a vessel.
MAP-Maximum allowable pressure means the pressure at which the weakest element of the vessel is loaded to the ultimate permissible point, when the vessel is not corroded,the temperature does not affect its strength.
But when calculations are not made, the design pressure may be used as the Maximum allowable Working Pressure-MAWP
Well AravindSujay, what you state requires some corrections.
There is no margin specified by code for the design pressure with respect to the most severe condition of operating pressure and corresponding temperature. The design pressure is simply the envelope of the operating conditions of the vessel with respect to stresses developed and strength at the corresponding temperature (including any transient conditions).
For completeness it may be noted that a vessel may have more than one design conditions: for example one pressure and temperature and a higher pressure at a lower temperature: the severer will govern.
The MAWP is considered the same as the design pressure when calculations are not performed to determine it.
However the MAWP does depend on temperature and will be determined at the same operating temperature(s) used for the design conditions. The code requires to determine the MAWP in the corroded condition, but often the design specification will require to determine it for both the corroded and uncorroded conditions.
To summarize the difference between the two, one may state that, for a heavy or large vessel, a design is good (as far as cost is concerned) when the calculated MAWP is just the same as the design pressure.
