Plastic and nonmetallic article shaping or treating: processes – Forming structural installations in situ – Arched – domed – or vertical-cylindrical structure
Reexamination Certificate
1998-10-16
2001-06-05
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming structural installations in situ
Arched, domed, or vertical-cylindrical structure
C264S036150, C264S269000, C264S035000
Reexamination Certificate
active
06241924
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for inverting the convex configuration of the bottom of a tank intended for the storage of liquid products, specifically petroleum and its derivatives, and is intended to provide a more effective shape to promote flow of the undesirable liquids and sediments which normally accumulate there. It is particularly applicable to large capacity tanks.
PRIOR ART
Storage tanks are widely used in the oil industry and are essential to the functioning of an operational unit. They may be intended for the storage of e.g. crude oil, intermediate products or final products.
During product storage it is normal for there to appear undesirable liquids and sediments which might have accompanied the product when it was placed in the tank. The undesirable liquids and sediments (dense residues, sludges, water, sand, etc.) are generally heavier than the product stored, and because of this they tend to deposit out at the bottom of a tank.
The storage tanks currently in use, especially those of medium and large capacity, have a side wall of cylindrical shape mounted on a foundation ring. The bottom is generally in the shape of a dome or cone, with the convex part pointing upwards.
Cleaning ports and inspection ports are provided in the lower part of the cylindrical side wall. The cleaning ports have hatches which are located on the side wall, supported on the foundation ring. This location enables the cleaning port to be used to monitor the level of the undesirable liquids deposited in the bottom of the tank.
This monitoring is performed using a liquid level indicator placed on the hatch of the cleaning port. The level indicator is used to inform an operator when it necessary to drain a tank to remove the undesirable liquids and sediments or when it is necessary to stop drainage, so as to prevent the stored product being inadvertently drained off.
Various drainage pipes are located at the lower part of a tank in order to permit the undesirable liquids and sediments to pass outside the storage tanks. At the outlet from each drainage pipe there is a shut-off valve which, when open, allows the undesirable liquids and is sediments to flow out of the tank.
Commonly, mixers are installed within the tank to homogenize the stored product. The number of mixers depends on the storage capacity and the type of product stored.
The mixers have the disadvantage that they also cause the undesirable liquids and sediments to move towards the center concave bottom of the tank, away from the drainage ports, which makes it more difficult to cause these undesirable liquids and sediments to flow outside the tank.
As the bottoms of tanks currently in use generally have a center at a higher level than their edge, there is a tendency for the undesirable liquids and sediments to accumulate at the edge, forming an annular layer. However, because the edge of the tank is at a uniform level and there is no difference in level to encourage these undesirable liquids and sediments to drain off through the drainage pipes, flow is slow, and this encourages the undesirable liquids and sediments to accumulate at the bottoms of the tanks, mainly in the areas furthest from the drains.
WO-A-98/04479 discloses an improvement for liquid product storage tanks, especially those of large capacity, which solves the problems described above, and comprises inverting the convex configuration of the bottom into a conical configuration, with the center of the tank bottom at a level lower than the level of the edges.
A drainage gutter runs from the center of the tank bottom to the edge, with a sufficient gradient to encourage flow of the undesirable liquids and sediments to the edge of the tank, from where they can be easily drained off to a location outside the tank.
This improvement can easily be applied to the construction of new tanks, since it is easy to get the drainage gutter for undesirable residues to reach the level of the threshold of a cleaning port, immediately above the foundation ring, while at the same time maintaining a gradient in the drainage gutter or channel which encourages the flow of undesirable liquids and sediments to the edge of the tank.
However, when the same principle of inverting the base is applied to an existing tank various difficulties are encountered. If for example the edge of the tank bottom is kept at the level of the foundation ring, it will be seen that the drainage gutters reach the side edges of the tank at a level below the top of the foundation ring, which makes it necessary to make openings in the foundation ring in order to allow flow through the drainage pipes, which implies the need to revise the structural design in order to check the strength of the ring. It may even be necessary to adopt new structural solutions.
In order to change the convex configuration of the bottom of an existing tank so that the drainage gutter ends precisely at the threshold of the inspection port, the edges of the bottom must be at a level higher than the level of the top of the foundation ring. This change makes it necessary to revise the structural design of the tank and also to raise the level of the base supporting the bottom of the tank, which makes the cost of the alteration quite expensive.
It should also be mentioned that the alternatives described above are not envisaged in the petroleum industry standard API 650 which governs the construction of tanks.
OBJECT OF THE INVENTION
It is an object of this invention to provide a method of modifying an existing liquid storage tank so as to improve the efficiency in the operation of draining off the undesirable liquids and sediments which normally accumulate at the bottom of a storage tank.
It is a further object of the invention to provide a simple method of modifying a tank bottom to improve the drainage of such undesirable liquids and sediments.
SUMMARY OF THE INVENTION
The present invention provides a method for inverting the upwardly convex configuration of the bottom of a liquid-product storage tank having at least one cleaning port by applying a material having plastic properties, characterized in that it comprises the following stages:
first arranging on the bottom a plurality of containment formwork units distributed over the surface of the bottom, except in at least one section extending from the center of the tank bottom to its periphery opposite a said cleaning port, which units form containment areas for the placing of material having plastic properties;
then placing in the containment areas formed by the containment formwork a material having plastic properties which hardens over time and which has dimensions such that, other than in said at least one section extending from the tank bottom center to its periphery, the quantity of said hardenable plastic material which has to be placed in the containment areas increases with increasing radial distance from the center of the conventional bottom; and
converting said at least one section into a drainage gutter which has a ramp whose gradient is sufficient to encourage flow of the undesirable liquids and sediments to the region, opposite the cleaning port from which they can more easily be drained off.
This method provides a low-cost and simply-applied method which can be used to invert the concave configuration of an existing tank having a conventional bottom without the need to remove the bottom, simply by depositing over the conventional bottom a material having plastic properties, which hardens over time.
Using this method the original conventional bottom continues to meet the existing standards, any small imperfections in their surface resulting from the process of construction are corrected, and any small leaks which may be occurring are stopped, thus also preventing corrosion and the consequent contamination of groundwater.
This method can also be used when constructing new tanks, in that the bottom may be constructed in accordance with current standards, and then its convex configuration may be inverted by the subsequent application of a material having
Pradel Luis Adolfo Velloso
Witter Aldyr
Witter Roger Vieira
Nixon & Vanderhye PC
Petroleo Brasileiro S.A. - Petrobras
Vargot Mathieu D.
Witter Roger Vieira
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