I’m seeking help from those with more experience regarding the concepts of internal floating roof tanks and external floating roof tanks. .

　　1) Floating roof storage tank. The floating roof of a floating-roof tank is a movable cover that floats on the surface of the liquid stored inside. It moves up and down as liquid is added to or removed from the tank. There is an annular space between the floating roof and the tank walls, and this space is equipped with a sealing mechanism that prevents the liquid in the tank from coming into contact with the atmosphere as the roof moves up and down, thereby **reducing evaporation losses during storage. When using floating roof tanks to store oil, oil losses can be reduced by about 80% compared to fixed roof tanks. 　　2) Internal floating roof storage tanks. An internal floating roof tank is a floating roof tank equipped with a roof; it is also a new type of storage tank that combines the features of a vaulted roof tank and a floating roof tank. The top of an internal floating roof tank is a combination of an arch roof and a floating roof, with the arch roof on the outside and the floating roof on the inside. Internal floating roof storage tanks have unique advantages: first, compared to floating roof tanks, the presence of a fixed roof enables effective prevention of the intrusion of wind, sand, rain, snow, or dust, thus ensuring the quality of the stored liquid. At the same time, the inner floating disk floats on the liquid surface, eliminating any vapor space above the liquid and reducing evaporation losses by 85% to 96% ； Reduces air pollution and the risk of fires and explosions; facilitates maintaining the quality of stored liquids. It is particularly suitable for storing high-grade gasoline and jet fuel, as well as toxic petrochemical products ； Since there is no gas space on the liquid surface, corrosion of the tank walls and roof is reduced, thereby extending the service life of the storage tank. Secondly, under the same sealing conditions, it can further reduce evaporation losses compared to floating roofs. 　　Disadvantages of internal floating roof storage tanks: Compared to dome tanks, they require more steel plates and have higher construction requirements ； Compared to floating roof tanks, maintenance is inconvenient (sealing structure).

I don’t understand; could it be that some external floating roofs are written as internal floating roofs? The features of the external floating roof are not mentioned!!

A floating roof tank is what is commonly referred to as an external floating roof tank, while an internal floating roof tank is a dome-shaped tank equipped with an internal floating disk.

Is there any fellow marine engineer who can explain in detail how a floating roof tank manages to remain sealed as its roof moves up and down? Thank you!