Patent Application
20070272692
The invention may be better understood by referring to the accompanying drawings, in which:
Examples of storage tanks with the new self-draining floating roof can be seen in the figures. Each illustrated roof 10 is part of a storage tank 12 that can be used for storing liquids at atmospheric pressure. Each illustrated floating roof has a deck 14 with a top surface 16, a bottom surface 18, an outer rim 20, and a plurality of drains 22.
The roof 10 of the tank 12 illustrated in
The use of lightweight stainless steel materials may be economical when storing corrosive products. A stainless steel roof offers several advantages over a carbon steel roof. For example, a stainless steel roof should not require the corrosion allowance required for a carbon steel roof. It also should not require underside seal welding or painting. In those cases, it may be possible to install a reduced-thickness stainless steel roof at a cost that is comparable to the cost of a traditional carbon steel roof.
With the invention, other arrangements can also be used for a floating roof. For example, the roof 10 of the tank illustrated in
The rim on a conventional pan roof on a 150′-diameter tank may be 21″ or higher. A floating roof 10 using the new design that is made of stainless steel with a thickness in the range of 0.105″ to 0.135″ would allow the height of the rim 20 to be lowered to 18″ or less for a 150′-diameter tank. With a pontoon roof, 18″ may be viewed as a minimum height needed to allow access for internal welding and inspection of the pontoon 34. In other cases, the rim height could be even further lowered, reducing the roof weight even further and adding even more effective capacity to the tank.
The roofs 10 illustrated in the figures move vertically within a shell wall 50 of the tank 12, floating upon the surface 40 of the product stored in the tank. As seen in
As best seen in
The illustrated drains 22 each have a conduit 60 that projects through the roof 10. The illustrated conduit is about 3 inches in diameter, and has a bottom opening 62 that is located several inches below the bottom surface 18 of the deck. Other arrangements could be used. An emission control device such as a flap valve or a ball float can be used with the drain to limit gas emissions through the conduit while allowing liquids from above to drain.
Liquid can be drained from the illustrated roof 10 by tilting the deck 14 toward the drains 22. The slope of the tilted deck causes the liquid to pool toward the drains. Once the level of the pooled liquid reaches the level of the drain opening 56, the liquid begins to drain through the conduit 60.
A variety of different kinds of structure can be used to tilt the roof 10. In the examples seen in
In these examples, cables 70 are used to help tilt the deck 14. This arrangement relies on the ability of the deck to strain under load. The cables are connected so that their lower-most ends 72 have different lower-most elevations within the tank. As the level of the product nears the bottom, the cables attached to the central portion 26 of the deck reach their lowermost elevation, holding the central portion at that position. Meanwhile, the periphery of the deck can continue to lower, causing the deck to begin to slope to the outside, in a conical shape seen in
The cables 70 can be attached, for example, to an overhead fixed roof 88 or to various parts of the shell wall 50. In these examples, the cables 90 that are connected to central portion 26 of the roof 10 are arranged so that the lowermost elevations of their lowermost ends 72 are higher than the lowermost elevations of the lowermost ends of the outer cables 92 connected to peripheral parts 94 of the roof. The length of some cables can also be varied, or their upper attachment points can be moved laterally so that some of the cables extend at an angle, depicted by 70′, shortening the effective vertical length of those angled cables.
Supporting the floating roof 10 from the shell wall 50 or from a fixed roof 88 can provide another benefit. In the event the roof becomes imbalanced and starts to sink, the connection of the cables 70 to suspension points near the sinking side of the rim 20 may tend to level the roof, reducing damage from the incident and restoration costs.
Stainless steel aircraft cable may be a good choice for the cables 70 because it coils easily. Multi-strand coated steel cable may also be used. If self-coiling cables are used, the cables may self-coil when the roof rises. This reduces the chance of problems arising from slack cables, without the need for winches or reels.
The number of cables 70 depends on the strength of the floating roof 10 and of the shell wall 50 or the fixed roof 88 where the cables are attached. Supports for conventional floating-roof tanks are often spaced 18′-20′ apart to keep roof stresses at acceptable levels for the dead load plus an assumed 12.5 psf live load. Similar or even more distant spacing may be sufficient using the new design, since the assumed live load can be as low as 5 psf.
The illustrated deck 14 can also be tilted by using landing supports 100 such as the ones seen in
The landing supports 100 can be remotely activated so that they do not extend into the stored product except during emptying operations. The supports seen in
As seen in
As with other floating roofs, a breather vent 114 (
Supporting the roof in these ways can reduce the need for providing access to the top of the floating roof. This, in turn, can eliminate the need for vertical ladders on the inside wall of the tank. Eliminating ladders lowers emission possibilities.
This description of various embodiments of the invention has been provided for illustrative purposes. Revisions or modifications may be apparent to those of ordinary skill in the art without departing from the invention. The full scope of the invention is set forth in the following claims.
