Receptacles – Closures – Floating closure
Reexamination Certificate
1998-04-08
2001-08-14
Newhouse, Nathan J. (Department: 3727)
Receptacles
Closures
Floating closure
C220S220000
Reexamination Certificate
active
06273287
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device which is to be installed in the floating roofs of tanks for storing liquids and which is intended to prevent the formation of a vacuum inside the tank.
BASIS OF THE INVENTION
There are two possible ways in which the device of the invention can operate. When the floating roof is in operating mode and reaches a minimum operating height, a lower stop of a sliding shaft comes up against a lower stop of the body of the device, causing the vacuum-relief valve to open.
When the floating roof is placed in the maintenance mode, use is made of a locking pin for rigidly fastening the sliding shaft to the body of the device, which enables the vacuum-relief valve to open when the floating roof reaches the maintenance height.
PRIOR ART
Storage tanks are widely used in the petroleum industry and are essential to the functioning of an operational unit. They may be intended, for example, for storing crude oil, intermediate products and final products.
Given the highly volatile nature of these products, in the storage tanks use is made of a roof capable of floating over the stored liquid, as a way of preventing the undesirable accumulation of gases between the layer of liquid and the roof.
Tanks which are currently used, especially those of large capacity, generally have a bottom which is in the form of an upwardly convex cap or cone, i.e. with the centre higher than the edges. Undesirable liquids, which are generally heavier than the products stored, are frequently dispersed within the liquid mass. Because they are heavier, these undesirable liquids have a tendency to be deposited on the bottom of the tanks.
As the structural characteristics of the bottom of the tank do not favour draining-off of these undesirable liquids, an extensive layer of sludge is usually formed on the bottom of the tank.
As the product stored inside the tank is drained off, the floating roof descends. There is a limit on the descent of the floating roof which, in theory, could descend until it touches the bottom of the tank.
However owing to the formation of the layer of sludge, which in some cases may reach as far as one fifth of the way up the tank, it becomes necessary to limit the descent of the floating roof at such a height that contact between the roof and the layer of sludge is prevented, since contact could compromise the integrity of the floating roof. This minimum height which the floating roof may reach is referred to by specialists as the “minimum operating height”.
Support legs which are rigidly fastened to the floating roof are preadjusted to touch the bottom of the tank when the floating roof descends as far as the minimum operating height. Even if the level of liquid falls, the floating roof remains in the operating position, supported by the support legs.
In this situation, under the floating roof a vacuum would be formed which could give rise to the structural collapse of the roof, on account of its large external free surface area being subjected to atmospheric pressure.
To prevent the formation of a vacuum, devices are used which enable the internal and external pressures to be equalized, such devices being referred to by specialists as “vacuum-relief valves”, which are installed in the floating roof.
The vacuum-relief valves which are currently used basically comprise a body which can slide inside a casing. At the top of the body there is a cover which sits on the upper edges of the casing, prevents the assembly from falling inside the tank, and acts as an element for closing the vacuum-relief valve.
At the bottom of the body there is a shaft which extends vertically to the bottom of the tank. The length of the shaft is such that, when the floating roof approaches the minimum operating height, the shaft touches the bottom of the tank and, as a result, the body is forced to move upwards, inside the casing, causing the valve to open.
When it is necessary to place the storage tank in maintenance mode, all the liquid has to be drained off from inside the tank and the floating roof has to descend as far as a specific height which enables maintenance teams to access the inside of the tank, this height being the “maintenance height” as referred to by specialists. In this situation, the adjustment of the support legs has to be changed since the maintenance height is normally slightly higher than the minimum operating height.
The vacuum-relief valves described above operate perfectly with liquid-storage tanks constructed according to current techniques, where the maintenance height and the minimum operating height are very similar. However, the introduction of a new technique for constructing the bottom of the liquid-storage tanks has made this type of vacuum-relief valve unsuitable for this function, as will be demonstrated hereinbelow.
Our International Patent Application PCT/BR97/00022 proposes the use of a tank bottom whose centre is located at a level below the level of the edges, with a view to concentrating, in the central region of the bottom, the undesirable liquids which are to be drained off. A ramp drains these liquids off to the edges, where they are drained off to the outside.
This novel type of tank bottom made it possible for the floating roof of the tank to descend as far as a position much closer to the bottom than had previously been achieved, since the removal of the undesirable liquids from the bottom of the tank practically eliminates the formation of the layer of sludge. As the maintenance height remains the same, the difference between it and the minimum operating height becomes very large. Consequently, it became necessary to revise the design of the support legs and of the vacuum-relief valves.
Our International Patent Application PCT/BR98/00007 proposes the use of an assembly for supporting floating roofs in which a support leg slides inside a guide. The guide acts as support for the floating roof when the roof descends as far as the minimum operating height. The support leg has a structural function only when the floating roof is placed in maintenance mode. In this situation, a locking pin locks the support leg in the position in which it has to remain in order to support the floating roof.
Nevertheless, there remains the problem of the vacuum-relief valves known hitherto being unsuitable for the new type of storage tank. If conventional vacuum-relief valves were to be used in the floating roofs of these tanks, a major problem would arise since the valves would open when the floating roof reached the maintenance height, and would remain open from then on.
When the floating roof reaches the minimum operating height, most of the length of the shaft would be above the roof and a body surrounding the shaft could even be entirely outside the casing, located at a significant height above the top of the floating roof, like a post.
Such an occurrence would cause a great deal of damage, since the shaft is not designed to operate in this way and would possibly buckle. One solution would be to strengthen the shaft and the casing. However, such a measure would give rise to an undesirable increase in the weight of the assembly and, as a result, of the floating roof. It therefore became necessary to design a new type of vacuum-relief valve which would solve the problems described above.
The present invention proposes the use of a vacuum-relief valve which has a double action, which solves the problems described above.
SUMMARY OF THE INVENTION
The present invention relates to a device which is to be installed in the floating roofs of tanks for storing liquids, and is intended to prevent the formation of a vacuum between the floating roof and the layer of liquid.
The vacuum-relief valve of the present invention is characterised by the features of claim
1
.
When the floating roof is in the operating mode and in an intermediate position, the shaft of the vacuum-relief valve rests on its upper shaft stop. The upper body stop rests on the top of the casing and operates as a plug, closing off communication between the inside of the t
Pradel Luis Adolfo Velloso
Witter Aldyr
Witter Roger
Newhouse Nathan J.
Nixon & Vanderhye PC
Petroleo Brasileiro S.A. - Petrobras
Witter Roger
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