Abstract: A lid is described for floating on the surface of a liquid in a storage tank. The lid spans the tank interior and includes an inflatable peripheral seal arrangement selectively operable to seal the liquid within the tank upon inflation and, on deflation, will allow the lid to float freely on the surface of the liquid. The lid is formed of two plates spaced apart by a circumferential rim defining a flotation cavity. Buoyance of the lid is such that the inflatable seal is carried above the liquid surface. The bottom plate is arched toward the top plate of the lid by a tension member extending between the two. The tension member is hollow and open to the area below the lid. The pressure relief valve is mounted in the hollow passageway to selectively allow escape of pressurized gas from below the lid. The seal arrangement is made so that the liquid level always remains below the sealed area between the lid and tank walls.