The Morrison Bros 245M-0300 AV is a lightweight, spring-loaded emergency vent (pressure relief only), used on aboveground storage tanks, as a code requirement, to help prevent the tank from becoming over-pressurized and possibly rupturing if ever exposed to fire. This 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.