The Morrison Bros 244OM-0020 is a UL Listed emergency vent (pressure relief only) used on aboveground storage tanks, as a code requirement, to help prevent the tank from becoming overpressurized and possibly rupturing if ever exposed to fire. The vent must be used in conjunction with a “normal vent.” Correct application of this vent requires proper vent size and selection for the tank system in order to meet the specific venting capacity.