Maximum allowable operating pressure

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Maximum Allowable Operating Pressure or MAOP is a pressure limit set, usually by a government body, which applies to compressed gas pressure vessels, pipelines, and storage tanks. For pipelines, this value is derived from Barlow's Formula, which takes into account wall thickness, diameter, allowable stress (which is a function of the material used), and a Safety factor.

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 pressurized equipment, and hence are able to withstand pressures above their design pressure.

Design pressure is the maximum pressure a pressurized 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 pressurized item and sized to prevent the item under pressure from being over-pressurized. Depending on the design code that the pressurized item is designed, an over-pressure 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.

Maximum expected operating pressure (MEOP) is the high end of what is expected.