Devices of the prior art, utilized to decrease the evaporation losses emanating from liquids contained in storage tanks, and which are mainly due to filling, emptying and "breathing" of these tanks, comprise floating roofs, blankets, variable vapor space enclosures and conservation vent valves. The first class consists of, as its name implies, a metallic structure which is essentially a pan floating on the surface of the liquid that has sealing means between the periphery of the pan and the tank's shell. Because it is practically impossible to effect a tight closure of the gap between the pan's pheriphery and the tank's shell, due to the shell out of roundness and other imperfections, the evaporative losses experienced with this type of devices are very substantial.
Blankets are thin, plastic sheets usually built with a cellular structure to decrease their density, which allows them to float on the liquid surface. Here again, the coverage of the liquid surface by the blankets is never complete and consequently the evaporative losses sustained are substantial. The variable vapor space class consists of a flexible diaphragm of suitable materials (elastomers impregnated fabrics, as an instance) contained in a metallic tank, which in turn is sometimes located over the roof of the storage tank or is located independently outside of the storage tank. Means are provided to connect the vapor space in the storage tank with the variable space container. The storage tank vapor space now linked with the variable vapor container define a variable vapor space.
The variation in volume allowed by the movement of the flexible diaphragm controls most of the tank's breathing losses, but does not control the venting loss due to filling and emptying operations.
Vent conservation devices are essentially check valves set at somewhat higher and lower pressures than the normal atmospheric pressure, the setting depending on the allowable stresses for the tank's shell.
Here again, the main function of these devices is to control a certain extent the breathing losses, but are not suitable for the control of the venting loss due to filling and emptying operations.
Other devices of the prior art, which were intended to decrease substantially the losses experienced by the floating roofs, consists of cylindrical curtains attached to the periphery of a floating disc, pan, or similar structure and to the tank's shell, usually to the top portion of the shell.
These cylindrical curtains are concentrical to the tank's shell, and are kept apart from it by a relatively narrow annular gap.
Although this type of devices would provide a very efficient control of the evaporative loss, they are seldom, if ever, used in practice, mainly for reasons that will become apparent below.
Glass: U.S. Pat. No. 1,631,959, Plummer: U.S. Pat. No. 2,578,090, Champagnat: French Pat. No. 1,116,444 exemplify this class of devices. In the devices of Glass and Plummer the gas contained in the annular gap tends to be compressed when the liquid level in the tank rises. The compressed gas will exert pressure on the curtains which, by collapsing inwardly, keep the gas volume constant.
In the device of Champagnat, the compressed gas inflates an expandable container, thereby keeping the total volume of gas constant. Theoretically the total amount of gas should remain substantially constant at all times. But in practice, due to unavoidable losses caused by imperfect sealing at the attachment of the vertical liners or curtains to the tank and floating disc, pin-holes in the materials of construction for this liners, etc, the gas which is under a slight pressure leaks out and thus gradually decreases the amount of gas therein contained.
When the liquid level in the tank descends, the gas available to fill the gap is insufficient and consequently a vacuum is created.
The atmospheric pressure acting on the liners pushes and holds them tightly against the tank's shell and as the liquid continues to descend, the forces holding the liners against the shell restrain the free descent of the floating disc, which literally hangs up. Part or all of the weight of the floating disc is thereby supported by the liners which generates stresses of enough magnitude to induce failure of the same.