Better be careful here and use the proper definitions. The design pressure is the maximum allowable working pressure (MAWP)of the boiler based on ASME Boiler and Pressure Vessel Code. What does this mean? By definition in Section I, Note (9) MWAP is the maximum pressure and temperature conditions that could be sustained above normal operating conditions (working pressure). Please review ASME Boiler and Pressure Vessel Code, Section I, PG-21.
Actually, I agree with desertfox. The Design Pressure is the Pressure that the vessel is to be designed to. The MAWP cannot be determined until the components of that vessel have geometry. After the unit has been completely designed the MAWP can be determined or, per UG-99(b) "The MAWP may be assumed to be the same as the design pressure when calculations are not made to determine the MAWP".
