Maximum Allowable Working Pressure (MAWP)

Definition - What does Maximum Allowable Working Pressure (MAWP) mean?

Maximum allowable working pressure (MAWP) is the maximum pressure at which the weakest point of the equipment, system or a vessel can handle at a specific temperature when in normal operation. This is an important factor that should be stated on all piping or vessels to help determine the safe internal or external pressures that the equipment can withstand.

Corrosionpedia explains Maximum Allowable Working Pressure (MAWP)

Maximum allowable working pressure is used when rating pressure-relief devices that protect vessels. It is also used to rate components such as piping, steam drums, tubes, boilers and any pressure vessels in hydraulic systems.

The operating pressure is the pressure that a vessel is subjected to when in service. The design pressure, which is the maximum pressure the vessel can be exposed to, is typically between 10 and 25 percent above the maximum operating pressure, and usually equal or slightly lower than, the MAWP.

The MAWP ratings help to reduce chances of explosions in steam systems due to unexpected pressure changes. The boiling temperature of a liquid increases with pressure and decreases when pressure decreases. A crack in a boiler would lead to the pressure dropping to the atmospheric one. This would cause water to boil at 212°F (100°C), which can cause superheating (heating water past its boiling point), which can in turn lead to an explosion.

The MAWP value is affected by operating temperatures and each value should therefore have an associated temperature. The value is higher at normal or lower temperatures and decreases at higher temperatures. Very low temperatures may give a higher MAWP, but caution should be taken since there is a risk of sudden and catastrophic embrittlement.

The MAWP value does not remain constant throughout the lifespan of the equipment. The value reduces due to wear, fatigue and corrosion in the case of the carbon steel. Storage tanks and pressure vessels should undergo periodic inspections and re-rating or calibrations to allow for changes due to corrosion and wear. These should be well documented and a safe set of pressures implemented, as well as be reflected on the vessel’s pressure release devices.

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