Maximum allowable operating pressure or MAOP refers to the wall strength of a pressurized cylinder such as a pipeline or storage tank and how much pressure the walls may safely hold in normal operation. The MAOP is less than the MAWP (maximum allowable working pressure). MAWP being the maximum pressure based on the design codes that the weakest component of a pressure vessel can handle. Commonly standard wall thickness components are used in fabricating pressurised equipment, and hence are able to withstand pressures above their design pressure. Design pressure is the maximum pressure a pressurised item can be exposed to. Due to the availability of standard wall thickness materials, many components will have a MAWP higher than the required design pressure. Relief valves are set at the design pressure of the pressurised item and sized to prevent the pressurised item being overpressured. Depending on the design code that the pressurised item is designed, an overpressure allowance can be used when sizing the relief valve. This is +10% for PD 5500, and ASME Section VIII div 1 & 2 (with an additional +10% allowance in ASME Section VIII for a fire relief case). ASME have different criteria for steam boilers.
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