The Morrison Brothers 244OM-0800 AV is a UL Listed emergency vent (pressure relief only) used on above ground storage tanks, as a code requirement, to help prevent the tank from becoming over-pressurized 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 requirements.