An aboveground tank in which a metal deck or pan, the “roof,” floats directly on the surface of the liquid. The roof is flush with the wall of the tank and moves up and down with the level of the liquid, much like in a piston in a cylinder.
In some installations, the floating pan is open to the environment and designed in a way that will cause rainwater to drain from the top of the pan. Such an installation is referred to as an “open floating roof tank.” In other installations, the tank may have a fixed roof or cover to protect the floating pan. These installations are referred to as “covered floating roof tanks.”
The purpose of a floating roof system is to minimize the volume of vapor present in the tank, as well as to reduce emission of vapors from the tank.
See also Fixed roof tank.