Definition - What does Ullage mean?
Ullage is the volume of empty space left in a container, cargo tank or storage tanks in cargo ships and oil terminal tanks. The space is left so that once the lid is open the liquid does not come out with a lot of pressure due to pressure difference. Ullage is related to the marine industry and oil and gas industry during transportation and storage of crude oil and petroleum products.
Petropedia explains Ullage
Large tanks which carry petroleum products are pressurized containers which should not be filled more than 98% as per the ABS rule of steel vessels and Code of Federal Regulations. All such containers have pressure relief valve that has to remain in contact with the internal gases or petroleum vapors that generally form because of long travel distance or long storage of petroleum products in hot climatic conditions. Thus, this 2-5% space given as ullage is for the safe formation of petroleum vapors/gases that can be released using pressure relief valve. Ullage is widely used by mariners, shippers and petroleum storage tank operators at oil storage terminals.