1. Field of the Invention
The present invention relates to the field of large storage tank structures.
2. Prior Art
Large above ground storage tanks are frequently used for the storage of various material such as water, petroleum products and the like. The tanks of the type which are the subject of the present invention are the large tanks used either singularly, or more often in plurality, to store large quantities of material such as, by way of example, crude oil and/or processed petroleum products at an oil refinery. In general such tanks are fabricated by welding up a tank floor of steel plate on an appropriate foundation, and then welding curved side plate sections to the floor and butt welding the sections to themselves to build up the cylindrical sidewall of the tank to the desired height. The roof structure is supported either by a post at the center of the tank or alternatively, a center post and a pattern of posts therearound to provide intermediate support for the roof between the center post and the tank circumferential wall, depending upon the size of the tank in question. In general the center post, and any other posts within the ultimate tank enclosure, are used to support a rafter system, typically of I beam construction bolted to the posts and connected to the sidewalls of the tank, with steel plate panels welded thereover to complete the tank enclosure. The roof plates, generally rectangular segments, are continuously welded only on the outside of the tank because of tradition, economics and the inaccessibility of many of the seams, etc., from within the tank, particularly because of the rafter construction. Most tanks are then coated on the inside with a suitable coating material to protect the steel used in the tank construction, and to isolate the steel from the materials to be stored therein.
Various such coating materials are well known in the prior art and readily commercially available to provide a tenacious and highly protective coating on steel to prevent corrosion thereof even by materials known to be very corrosive to unprotected steel. However, there are various characteristics of tanks of the foregoing construction which have been found to overtax the capabilities of such materials, resulting in inadequate coatings and/or the development of cracks therein which allow the passing of corrosive materials within the tank into gaps such as gaps between the roof plate and the rafters, grossly accelerating the deterioration of the roof structure and resulting in a very premature failure thereof. In particular, because the roof rafters are internal, one can reasonably easily coat and inspect the coating on any downward facing surfaces of the I beam rafters and on the upper part of the vertical portion of the I, but can neither conveniently coat nor inspect the lower part of the vertical portion of the I beams of the upward facing surfaces, Further, the portion of the roof plates above the I beams cannot be coated. Obviously corrosion in these regions will quickly weaken the intended support, leading to a very premature sagging or failure of the roof structure.
Also the effects of differential expansion and the working of unwelded gaps and overlaps must be appreciated. By way of example, of one overlaps the edges of two steel plates, such as roof panels, and continuous welds the two panels from the outside, the inside gap may be very low, or even almost zero depending upon how well the plates fit before welding. Such a gap will readily coat with a protective coating to prevent the seepage of any material being stored in the tank into the gap in the region of the overlap of the two plates. If however, tension is applied across the weld, the unwelded gap will tend to open up to perhaps many times its original size, eventually if not immediately causing the cracking or splitting of the coating material at that location to allow penetration of the material within the tank into that gap. In other instances where one member supports another but is not welded thereto, differential expansion may cause lateral motion between the two members, again substantially immediately splitting the protective coating to allow penetration of corrosive materials into the gap between the two members. In that regard it must be recognized that differential expansion in tanks of this type can be quite large, in part because of the cumulative effect of even relatively small temperature differences over the relatively large spans involved, and in part because of the quite substantial temperature differences which frequently exist under certain conditions. By way of example, the sidewalls of a tank are reasonably well sheltered from the midday sun and accordingly, are not significantly heated thereby, particularly when heat sinked against the contents of a substantially filled tank. On the other hand, the roof of a typical tank is fairly directly exposed to the middway sun and not generally heat sinked to the materials therein, and accordingly can experience large temperature fluctuations both daily, and particularly over seasonal extremes. In other situations a tank may be used to store the intermediate products or end products of a particular process which as a result of the process have an elevated temperature whereby the walls and floor of the tank will expand in accordance with that elevated temperature, whereas the roof will tend to be cooled by the outside environment. Consequently, it is common for cracks or splits to occur in the coating in the region of unwelded overlaps, supports, etc. to expose unprotected steel to the corrosive effects of the tank contents.
U.S. Pat. No. 2,395,685 discloses a storage tank constructed from substantially thin plates of corrosion resistant alloy. The edge portions of the thin plates are welded to channels in the manufacturing plant, which channels may be bolted together during erection in a manner to define the tank enclosure. Thereafter strips of corrosion resistant material are welded over the inner seams of the tank to provide a relatively smooth inner tank surface as in the present invention. However, the structure and method of fabrication of the tank of the U.S. Pat. No. 2,395,685 patent is entirely different from that of the present invention, and is not suitable for the construction of large tanks as is the present invention.
U.S. Pat. No. 2,849,143 discloses a tank design for the storage of corrosive liquid materials in which rough edges such as bolts, overlays, exposed I beams and exposed channels are eliminated and which provides a substantially continous interior surface to serve as a base for a corrosion resistant material. For this purpose the various rafters and girders are formed as closed channel members to avoid concave surfaces within the tank. The patent is relevant as addressing some of the same problems with large storage tanks as the present invention, though is quite different from the present invention in that U.S. Pat. No. 2,849,143 addresses the problem by controlling the roof structure within the tank, whereas the present invention essentially removes the roof structure from within the tank.