Patent ID: 7073530

Claim:
In a method for removing an oxidizable liquid and related liquid vapor from a storage tank having a closed interior defined by at least one upstanding wall having an exterior side and an interior side, a bottom, and an open top opposing said bottom, said open top being fitted with a floatable external roof that is slidingly sealed to said interior side to keep said vapor from escaping to the ambient atmosphere over said roof and outside said tank interior, said roof carrying a plurality of support legs that extend essentially equidistantly into said tank interior which legs stop said roof and hold it stationary a finite distance apart from said bottom thereby leaving liquid and related vapor remaining between said stationary floating roof and said bottom, said floating roof carrying at least two vents which are in fluid communication through said roof between said tank interior that remains under said stationary floating roof and said ambient atmosphere over said roof, the improvement comprising a method of removing essentially all said remaining liquid from said tank interior under said stationary external roof without creating a vacuum within said interior while removing said related vapor from said interior with minimal emissions to said ambient atmosphere comprising the steps of removing from said interior said remaining liquid, enclosing at least one of said vents in said roof with a first housing, removing through said at least one vent with a first housing from said interior below said stationary external roof said vapor, said vapor removal being effected in a contained manner, transporting said contained vapor to at least one thermal oxidizer, oxidizing said contained vapor to a substantial extent in said at least one oxidizer, emitting to said ambient atmosphere the exhaust of said at least one oxidizer, enclosing at least one other of said vents in said roof with a second housing so that all said at least two vents are closed to said ambient atmosphere, introducing through at least one other of said vents with a second housing at least one inert gas in an amount sufficient to maintain a gaseous pressure between said stationary roof and said liquid in said tank sufficient to prevent the formation of a vacuum in said tank interior as said liquid is removed there from while said floating roof remains stationary, whereby said tank is emptied of essentially all its liquid content without appreciable loss of related vapor to said ambient atmosphere, and without damage to said tank.