EP2706021B1

EP2706021B1 - A method for making an inner metal wall inside a tank and a tank having a double wall

Info

Publication number: EP2706021B1
Application number: EP13183022.6A
Authority: EP
Inventors: Markus Lechthaler; Peter Werth
Original assignee: Wolftank Systems AG
Current assignee: Wolftank Systems AG
Priority date: 2012-09-07
Filing date: 2013-09-04
Publication date: 2016-08-03
Anticipated expiration: 2033-09-04
Also published as: ITVR20120180A1; EP2706021A1

Description

The present disclosure relates to the sector of storage tanks in general. In particular, the present disclosure relates to a method for making an inner wall inside a tank and to a tank having a double wall, i.e., an outer wall and an inner wall.
The storage tanks, both underground and aboveground, have a very broad field of application and use. A particular reference is made here to underground or aboveground tanks for storing potentially polluting liquids. Examples of these tanks are the underground reservoirs for storing fuels for motor vehicles at service stations, the underground reservoirs for storing chemicals in chemical factories, the underground reservoirs for storing diesel for fuel oil for heating buildings.
For many decades these tanks were made with a single wall, which for example is a cylindrical shell of steel, which bounds the inner chamber for storing the liquid.
A common problem with single-walled tanks is that the breaking of the continuity of the wall (for example, the development of holes or cracks due to corrosion) causes a leakage of the liquid contained in the tank and an environmental pollution of the surrounding soil.
This problem is particularly felt for an underground tank: since the underground tank cannot be viewed from the outside, it is practically impossible to promptly notice the development of holes or cracks in the wall of the tank. Therefore, the tank can leak liquid for a long time, so causing a massive pollution, without the keeper of the tank being aware of what is going on.
The operations of removal and replacement of a damaged underground tank are very costly and time-consuming, due to the construction work to be carried out as well as the cost of the new tank and the need of decontaminating the polluted soil. Some processes for transforming a single-walled tank into a double-walled tank have been developed to overcome these drawbacks at least partially.
Japanese patent application No. JP2001323522A relates to a water tank that is made of concrete. This document provides for the inside of the tank to be coated by thin plates of a corrosion-resisting metal, such as titanium or titanium alloy, to avoid water leak. The metal plates completely adhere to the inner wall of the tank and, for example, they are fastened to the inner wall by an adhesive substance or an adhesive tape. The ends of the metal plates are joined together by welding. German patent application No. DE1933955 provides for applying a flexible metal foil around a tank, to create an outer jacket. The two ends of the flexible foil, which is bent to a cylinder-like shape, are joined together by a joining bead of fiberglass. It is evident that this method is not suitable for restoring underground tanks.
Other processes allow avoiding the need of digging up the underground tank.
For example, U.S. patent No. US 4,739,895 provides for making a double-walled bottom inside the tank, by using aluminium foils to create an inner wall. The aluminium foils have ridges that protrude towards the outer wall, so as to leave an interspace between the inner wall and the outer wall.
Since the aluminium foils are not available in size sufficient to lay out the entire tank in one foil, a plurality of foils are placed adjacent to each other with overlapping edges; the overlap is covered by a fiberglass-reinforced tape that bridges the overlap. Then, the whole is coated by a layer of epoxy resin, which embeds the fiberglass tape and retains it in position.
The above-mentioned operations are carried out by workers who enter the tank and work inside it.
Furthermore, as described for example in U.S. Patent No. US 4,613,922 , the interspace between the inner wall and the outer wall can be monitored over time to detect any leakage of the tank contents. For example, vacuum is created inside the interspace, which is then continuously monitored. When vacuum is no longer kept inside the interspace, this means that a leakage of liquid through one of the walls has occurred.
Another technique is disclosed by EP 2 067 719 A1 , which relates to a spacer for a leakage indicating lining of a tank, a lining and tank therewith. The spacer has a spatially formed paper provided with a metal coating, e.g. aluminium. The paper is impregnated and has burls on one side of the paper. The paper has grooves on the side of the paper directed towards a tank wall and provided between the burls. The paper has ridges corresponding to the grooves and provided opposite to the grooves. Recesses corresponding to the burls are separated by the ridges. A synthetic resin coating is provided on the side of the paper.
WO 2008/038052 A1 discloses a double skin tank lining with interstitial spacer. The lining system is suitable for lining a surface and generates a fluid impervious lining and an interstitial space. The system includes a layer comprising a flexible material having an adhesive coating on each side thereof, a second layer of flexible material, and a spacing means to generate a gap between the first and second layers, the gap constituting the interstitial space. The system further includes a fibre glass scrim layer which is incorporated by a coating, the coating comprising a curable material which cures to form a hard fluid impervious coating. According to this system, the gap is an interstitial layer of the flexible material that is used as a lining. Thus, the joints between adjacent sheets of flexible material may prevent the gap from being continuous all over the tank; in order to avoid this, the positioning of the flexible material in the tank is cumbersome.
DE 199 50 466 A1 relates to a leak-resistant lining for tanks used for the storage of water contaminating substances. An angled fixing bracket is fitted in position around the edge between the tank wall and the tank floor. The bracket comprises two faces, running parallel to the tank wall and the tank floor respectively. The face parallel to the tank wall has a joint with the tank wall which is resistant to the contained substance. A cavity exists between the angled fixing bracket and the adjacent sections of the tank wall and tank floor. Said cavity is suitable for leak control. The film or sheet laying on the insert which defines the cavity is fixed in a substance-resistant manner to the face of the angled fixing bracket parallel to the tank floor. This method is mainly aimed at making a leak-resistant joint between the tank wall and the tank floor.
These processes for transforming single-walled tanks into double-walled tanks are advantageous from many points of view, but they have some drawbacks that are still not overcome.
In particular, the inventor of the present disclosure has realized that the joint between the aluminium foils by the above-described manner can give rise to drawbacks during both the making of the inner wall and the subsequent use of the tank.
First of all, the use of fiberglass tape implies a working step that is arduous and sensitive, due to the need of properly placing the tape on all joints; this implies also a prolonged working inside the tank.
A similar drawback is met also in the case where welding is used to join together metal foils or metal plates. This is even more disadvantageous, due to the dangers related with carrying out a welding process within a closed environment with a potentially hazardous atmosphere, as it is found inside a fuel tank.
Moreover, even if the fiberglass tape has an adhesive side that adheres to the aluminium foils, the fiberglass tape is retained in position, as a matter of fact, by the layer of epoxy resin in which the tape is embedded. Basically, the fiberglass tape creates a sort of sealing of the joint, but it does not ensure a firm joint between the aluminium foils over time. The resin layer does not ensure a firm joint either. For example, the aluminium foils, the tape and the resin layer have thermal expansion coefficients that are different from each other and then strains and mutual displacements of the tape relative to the aluminium foils and of the aluminium foils relative to the resin, the repetition of which over time could cause the joint to fail, may occur in the course of time.
The strains and the mutual displacements may also be due to the cycles of loading (with a tank lorry) and unloading of the product in the tank during its use.
A similar problem is met also in the case where the tank is intended to be used under pressure. In fact, the fiberglass tape placed as described above does not prevent a mutual rotation of one aluminium foil relative to another foil around the edge of the overlapped foil: as the tape is placed only on one face of the overlap of aluminium foils (i.e., above the overlap, at the inner side), it is not able to ensure a firm and immobile joint in all directions. Therefore, a high pressure in the tank, sudden changes in temperature or the loading phase could cause mutual movements of the aluminium foils relative to each other.
Since the coating layer of epoxy resin is thin (in fact, it has a thickness of about 1 mm), it could be damaged and perforated by the mutual movements of the aluminium foils, causing a break of the continuity of the inner wall and a formation of passage points for leakages of the liquid contained in the tank.
Thus, the present disclosure moves from the technical problem, identified by the inventor, of providing a method for making an inner wall inside a tank, such a method allowing to overcome the drawbacks mentioned above with reference to the prior art and / or to achieve further advantages.
This is achieved by providing a method for making a second wall according to independent claim 1. The technical problem is solved by a tank according to independent claim 9 as well.
Specific embodiments of the subject of the present disclosure are defined in the corresponding dependent claims.
A first aspect of the solution offered by the present disclosure is to use a structural glue to join together the pieces or foils of sheet metal that compose the second wall of the tank, i.e. the inner wall. Basically, the pieces of sheet metal are glued to each other by glue having a high mechanical strength and that, after setting, creates a non-removable fastening between the glued pieces.
This is useful for making in the tank an inner wall that, although it is composed of several pieces of sheet metal positioned side by side, behaves as if it were a single piece from a mechanical point of view. In particular, the structural glue can be placed on the entire side-by-side region of two adjacent pieces of sheet metal, interposed between one piece and the other, so as to obtain a firm joint that is resistant in all strain directions. This is useful for avoiding any mutual displacements of one piece of sheet metal relative to adjacent pieces of sheet metal, when the tank is under pressure or when it is subjected to sudden changes in temperature or cycles of loading and unloading.
Therefore, the risk of development of cracks in the inner wall during the drying of the resin and subsequently during the cycles of loading / unloading of the tank is greatly reduced or even eliminated thanks to the glueing of the pieces of sheet metal to each other, which creates a very resistant inner wall that behaves as if it were a single piece.
In particular, the pieces of sheet metal are partially overlapping with each other in overlap regions along the edges of the pieces of sheet metal and the structural glue is sandwiched between one piece and the other in the overlap regions. This is useful for obtaining a joint that is particularly strong and resistant, thanks to the extension of the overlapping surface on which the structural glue is placed.
In contrast, the reinforced tape used in the prior art has at most one adhesive side that sticks to the foils of sheet metal, but without creating a resistant joint. In fact, the adhesive of the tape is not very resistant by itself and it creates a removable fastening that is not able to oppose the mutual displacements of the foils and cannot effectively withstand the strains during the cycles of loading / unloading.
Moreover, the making of the joints by structural glue, instead of by a reinforced adhesive tape to be placed over the overlap regions of the foils of sheet metal, is useful for simplifying the working process and for reducing the stay time of the workers inside the tank, thus increasing the safety of the workers themselves.
In fact, the reinforced tape of the prior-art process is a member that is additional to the foils of sheet metal and it should be placed in a very precise manner to ensure that the reinforced tape properly covers the joint and that it is well centred with respect to the joint. Moreover, during the placing of the reinforced tape, the foils of sheet metal (which are not yet bound to each other) may accidentally move relative to each other. Even during the spreading of the coating epoxy resin, mutual displacements of the foils and the reinforced adhesive tape may occur.
Therefore, it is evident that the prior-art process requires a considerable skill from the workers, which must be very careful to avoid any errors in placing the tape or displacements of the foils of sheet metal, that could jeopardize the effectiveness of the obtained inner wall.
On the contrary, with the method according to the present disclosure, the glue can be spread in the glueing regions with a greater tolerance and a lower accuracy (possibly, abounding in glue for sake of reliability and putting the glue in a region wider than that of actual overlapping), without this jeopardizing the final result. Moreover, the setting of glue starts as from the time of contact between the edges of the pieces of sheet metal (i.e., just after the positioning of one piece of sheet metal against another piece of sheet metal), and this is useful for reducing the risk of mutual displacements between the pieces during the carrying out of the process. These operations, that are simpler to carry out, involve a reduction of the time during which the workers stay inside the tank; since the inside of the tank is a potentially hazardous environment, shorter stay times reduce the risk of accidents.
The pieces of sheet metal may be provided with glue even before being put into the tank. In other words, the pieces of sheet metal are prepared for glueing before being positioned in the tank. For example, the supplied pieces of sheet metal are already provided with glue along at least one edge region, in which glue is protected by a removable film or veil. Accordingly, the operations to be performed inside the tank are further reduced because the step of spreading the glue on the pieces of sheet metal is carried out outside the tank rather than inside it.
To be more specific, the pieces or foils of sheet metal are made of a ductile material, in particular aluminium or an aluminium alloy.
According to a secondary aspect of the present disclosure, an adhesive or a glue is used to fasten each piece of sheet metal to the first wall, i.e., to the inner face of the outer wall. In particular, an adhesive that creates a removable fastening or, as an alternative, a glue that creates a non-removable fastening after the drying or setting of the glue, can be used.
Unlike the prior art, in which the aluminium foils are simply laid on the inner face of the outer wall of the tank or, at most, they are stuck to the inner face using a both sides adhesive tape that creates a weak removable fastening, according to the present disclosure the pieces of sheet metal are fastened on the inner face of the outer wall directly via adhesive or glue, in such a way that the inner wall is bound to the outer wall in a firm manner.
This is useful for preventing any unwanted displacements of one piece of sheet metal relative to the other, during the making of the inner wall as well as the use of the so-obtained double-walled tank.
The fastening of the pieces of sheet metal to the first wall (outer wall) can be adopted in combination with the structural bonding of the pieces of sheet metal to each other.
Similarly to what is described above, adhesive or glue for fastening the pieces of sheet metal to the first wall can be prearranged on the pieces of sheet metal before these are put into the tank.
The method according to the present disclosure can be applied also to a tank that was made with a double wall and wherein one of the two walls is damaged and is not fluid-tight any more; thus, it is possible to restore a double-walled tank as well. In this case, the method allows making a further inner wall that lines the already existing inner wall.
Further advantages, characteristic features and the modes of use of the subject of the present disclosure will be made evident in the following detailed description of an embodiment thereof, given by way of example and not for limitative purposes. However, it is evident that each embodiment of the subject of the present disclosure may have one or more of the advantages listed above; in any case, it is not required for each embodiment to concomitantly have all the listed advantages.
Reference will be made to the figures of the annexed drawings, wherein:

Figure 1 shows a partially-sectional perspective view of an underground tank according to the present disclosure;
Figure 2 shows a sectional view of a portion of the tank of Figure 1, according to a section line II-II in Figure 1;
Figure 3 shows an enlarged detail of a piece of sheet metal used in a method according to the present disclosure;
Figures 4 to 6 schematically depict successive steps of a method according to the present disclosure;
Figure 7 shows an enlarged view of a detail VII of Figure 6;
Figure 8 shows a variant embodiment according to the present disclosure;
Figure 9 shows an enlarged view of a detail IX of Figure 8;
Figure 10 shows a front view of the detail of Figure 9;
Figure 11 shows a coil of sheet metal according to the present disclosure, in a step of cutting a piece of sheet metal;
Figure 12 shows a piece of sheet metal obtained from the coil of Figure 11.

A double-walled tank obtained by a method according to the present disclosure is shown in Figure 1, where the tank is denoted by reference number 1. The tank 1 has a substantially cylindrical shape, with a cylindrical side surface 11 and two opposite heads 12, 13.
The tank 1 bounds and encloses an inner chamber 18, or available volume, adapted to receive a liquid product (such as a fuel for motor vehicles or chemicals) or any other substance to be stored in the tank 1. In Figure 1, the side surface 11 is depicted partially in section to show the interior of the tank 1.
A manhole access aperture 14 in communication with the inner chamber 18 is positioned at the top of the tank 1. Pipes (not shown) for loading and unloading the stored product during the use of the tank 1 go through the manhole aperture 14. The workers can enter the inner chamber 18 through the same manhole aperture 14 to carry out the operations of the method of the present disclosure and the maintenance operations.
The tank 1 is buried in a soil 9. In Figure 1, the surface level of the soil 9 is depicted as interrupted to show at least a part of the tank 1. When the tank 1 is installed in the site, the tank 1 is completely underground and surrounded by the soil 9; only the top 140 of the manhole aperture 14 is at the height of the surface level of the soil 9 to allow inspection of the tank 1 and access to the inner chamber 18 when needed. In any case, the method of the present disclosure can also be applied to tanks that are non-underground or only partially underground. In the case of non-underground tanks or only partially underground tanks, the method is preferably applied to the lower part only of the inner chamber 18, i.e., where a leakage of product towards the soil may occur.
At the beginning the tank is single walled, i.e., it has a single shell or outer wall 21 that includes the side surface 11 and said two opposite heads 12, 13 and bounds the inner chamber 18. For example, the outer wall 21 is of steel.
As at the beginning there is the outer wall 21 only, which then would be directly in contact with the product stored in the tank, any hole or crack or slit through the wall 21 would cause an at least partial discharge of the stored product, which then would pour into the surrounding soil 9 and would pollute it.
In order to prevent a risk of uncontrolled pollution of the soil 9, the regulations presently in force in many countries require that the underground tanks should be double-walled and equipped with a leak-detection system.
This raises the problem of transforming the underground single-walled tanks that are already in operation into double-walled tanks that comply with the regulations, without having to dig up the tank itself. Processes for this purpose have already been developed in the prior art, as previously mentioned, for example those described in US 4,613,922 and US 4,739,895 .
The process according to the present disclosure is described in the following; the steps of the process that are made according to the prior art are not described in detail because they are already within the knowledge of the person skilled in the art, whereas the steps of the process that differ from the prior art are described in detail. After the tank has been drained and the pipes for loading and unloading have been closed, a cleaning of the manhole aperture 14 and of the inner chamber 18, as well as a suction of any hazardous vapours, are carried out. The conditions inside the tank are checked by an explosimeter to ascertain that these conditions are not dangerous for workers that have to enter the tank.
The inner surface, i.e., the inner face 211 of the outer wall 21, is cleaned and sandblasted to remove all fouling. After sandblasting, the inner surface 211 is checked to find out any corroded areas, which in case are processed to block corrosion and to restore the completeness of the outer wall 21.
A first layer 25 of resin (for example, epoxy resin) is applied, preferably by spraying, on the entire inner surface 211 of the outer wall 21. A coating layer 25 of resin that perfectly adheres to the inner face 211 of the outer wall 21, coating it completely, is thus obtained after drying of the resin. The first resin layer 25 is checked, for example by a pinhole detector, to verify that it has no holes or irregularities, which in case are repaired.
Suction pipes (not shown), which allow to monitor an interspace 29 that is created between the outer wall 21 (which then is a first wall) and the inner wall 22 (which then is a second wall), are positioned and secured on the first resin layer 25. In particular, the suction pipes are arranged so as to draw from the lowest point in the interspace 29 and are connected to the outside going through the manhole aperture 14.
This is followed by the making of the inner wall 22, which is created using a plurality of pieces 30 of sheet metal. In particular, the sheet metal is made of a ductile metal, which preferably is aluminium or an aluminium alloy. Therefore, reference to aluminium sheet and aluminium pieces 30 will be made in the following; however, it should be understood that other ductile materials, such as copper or iron or alloys, may be used as well.
Basically, the sheet metal pieces 30 are portions of aluminium foil, for example having a thickness S30 of 0.3 mm. For example, each piece 30 has a length L30 ranging between 100 and 2000 cm and a breadth P30 of 70 cm.
The sheet or foil of aluminium has an ashlared or embossed profile, i.e., it has a plurality of protuberances 40 that protrude from a face 31 of the sheet. The protuberances 40 are evenly distributed on the sheet surface. In practice, each sheet metal piece 30 has a first face 31 from which the convexities 401 of the protuberances 40 protrude, and a second face 32 in which the concavities 402 of the protuberances 40 sink. The protuberances 40, which rise from the bottom 310 of the first face 31 and have a top 41, may have the shape of a truncated pyramid, a truncated cone, a paraboloid, a hemisphere, a rib, or another form.
In substance, each protuberance 40 has a convexity 401 that protrudes from the first face 31 of the sheet metal piece 30 and a corresponding concavity 402 that sinks in the second face 32 opposite the first face 31.
The protuberances 40 are made and arranged in such a way that no region of the first face 31 is secluded from the rest of the first face 31. In other words, when the sheet metal 30 is resting on a plane so that the tops 41 of the protuberances 40 touch the resting plane (i.e., with the first face 31 of the sheet metal 30 facing the resting plane), an interspace is defined between the resting plane and the bottom 310 of the first face 31, such an interspace passing around the protuberances 40 and being without any region that is closed (i.e., inaccessible from the other regions of the interspace) with respect to the other regions of the interspace. As it will be clearer in the following, this is useful for obtaining an interspace 29 that is continuous and can be monitored.
The pieces of aluminium sheet 30 are put into the inner chamber 18 and are positioned next to each other on the first resin layer 25. Each sheet metal piece 30 is positioned so that the first face 31 with the convexities 401 of the protuberances 40 is facing the first resin layer 25 itself, i.e., so that the tops 41 of the protuberances 40 rest on the first resin layer 25.
In other words, the first face 31 is intended to be adjacent to the inner face of the outer wall 21, in particular to the first resin layer 25 that coats the face 211 of the outer wall 21 of steel. The second face 32 of the sheet metal piece 30 is instead intended to face the inner chamber 18 of the tank 1.
According to the present disclosure, as shown in Figure 4 for the depicted embodiment, each piece of aluminium sheet 30 is fastened on the first resin layer 25: the first face 31 of the sheet metal piece 30, or at least the tops 41 of the protuberances 40, is spread with glue or adhesive substance 45 (for illustrative purposes, a brush 49 is shown in the figures to denote the step of spreading the glue / adhesive substance); the sheet metal piece 30 is then positioned and pressed towards the first resin layer 25, so as to obtain the fastening of the sheet metal piece 30 on the resin 25 and then to the inner face of the first wall 21.
As an alternative, the glue or the adhesive substance 45 can be spread on the first resin layer 25 instead of the first face 31 of the sheet metal piece 30.
According to a first version, glue 45, i.e., a substance whose drying or setting creates a firm fastening that is not removable (unless to destroy the parts fastened together) is used. Basically, industrial glue is used to create a structural bonding of the sheet metal piece 30 on the first resin layer 25.
According to a second version, an adhesive substance 45, i.e., a substance that creates a removable fastening of the sheet metal piece 30 on the first resin layer 25, is used; for example the fastening can be removed by using suitable solvents or exerting a mechanical action. In this second version, the sheet metal pieces 30 can be more easily detached from the first wall 21, for example during a disposal of the tank 1 at the end of life or during a next restoring of the inner wall 22.
This difference in meaning between "glue" and "adhesive substance" should be regarded as valid throughout the present disclosure.
For example, the glue or the adhesive substance 45 has a very low content of solvents, so that it can be managed according to Directive 94/9/EC about the regulation of equipment intended for use in areas with a risk of explosion; preferably, it requires less than ten minutes to reach 50% of the final mechanical strength and requires less than 24 hours to reach the final mechanical strength.
After a first sheet metal piece 30 has been positioned on the inner face of the first wall 21, other sheet metal pieces 30 are positioned next to the first piece.
According to an aspect of the present disclosure, the sheet metal pieces 30 are partially overlapped with each other (for example, with an overlap of 3-10 cm) and are glued together by glue 35 for bonding aluminium on aluminium. This glue 35 is namely structural glue that creates a non-removable fastening between the sheet metal pieces 30.
As shown in Figure 5 for the depicted embodiment, a strip or edge region 34 of the second face 32 of a first piece of aluminium sheet 30a, that is already positioned (and fastened to the first resin layer 25), is spread with structural glue 35 along its whole length L30. A second piece of aluminium sheet 30b, which has to be positioned, is placed next to the first piece of sheet 30a with a partial overlap, i.e., so that a strip or edge region 33 of the first face 31 of the second piece 30b goes to overlap with the edge region 34 of the first piece 30a and the structural glue 35. Therefore, an overlap region 37 along the edges of the sheet metal pieces 30a, 30b is obtained; the structural glue 35 in the overlap region 37 is sandwiched between the first piece 30a and the second piece 30b. In other words, a sandwich-like structure composed by the edge regions 33, 34 of the pieces 30a, 30b and a layer of glue 35 between them is made in the overlap region 37.
For example, the overlap region 37 has a width L37 ranging between 3 and 10 cm. When the structural glue 35 has set, i.e., it has hardened or dried, a very strong joining between the two sheet metal pieces 30a, 30b, which behave as if they were a single piece from a mechanical point of view, is obtained.
The mechanical strength (e.g., to tensile, shear or bending strain) of the overlap region 37 joined by the structural glue 35 is equivalent to or greater than the mechanical strength of the region of the sheet metal piece 30 that is not overlapping with other sheet metal pieces 30.
In order to further improve the mechanical strength of the joint between the two sheet metal pieces 30a, 30b, the pieces 30a, 30b are overlapped to each other in the overlap region 37 in such a way that the convexities 401 of the protuberances 40 of one piece are inserted and received in the concavities 402 of the protuberances 40 of the other piece. The structural glue 35 is sandwiched between the convexities 401 and the concavities 402 that are mutually received.
For example, the structural glue 35 has a strength to shear strain (i.e., a value of shear failure) that is greater than 10 N/mm², in particular greater than 20 N/mm², even more in particular it is 25 N/mm².
Preferably, glue 35 has a very low solvent content (manageable according to Directive 94/9/EC), takes less than ten minutes to reach 50% of the final mechanical strength and less than 24 hours to reach the final mechanical strength.

For example, glue 35 has methyl-methacrylate as a basis. An example of glue 35, which is commercially available and can be used in the method of the present disclosure, has the features given in the following table. Glue 45 to fasten the sheet metal piece 30 on the resin 25 may have the same features.

NON-POLYMERIZED PRODUCT
Chemical basis:	Methyl-methacrylate
Appearance:	Pasty
Viscosity at +20°C:	Adhesive	21,000 mPa·s
Viscosity at +20°C:	Activator	Very fluid liquid
Evaporation time of the activator at +20°C:	5 minutes
Effectiveness of the activator after application at + 20°C:	30 days maximum
Use temperature:	from +10°C to +30°C
Polymerization temperature:	from +6°C to +40°C

POLYMERIZED PRODUCT
Average shear strength after 7 days at +20°C and with application of the activator on one side only (DIN 53281-83):	Sandblasted Aluminium	25 N/mm²
	Sandblasted Steel	21 N/mm²
	Galvanized Steel	6 N/mm²
	Sandblasted Stainless Steel	26 N/mm²
	Roughened GRP (epoxy resin)	16 N/mm²
Resistance to temperature:	from -50°C to + 130°C, to +180°C for 30 minutes
Peel strength on aluminium:	6 N/mm
Coefficient of linear thermal expansion:	70·10^-6 K^-1

Although it is not shown in the figures, it should be understood that also the first face 31 of the second sheet metal piece 30b (or the corresponding region of the first resin layer 25) is spread with glue or adhesive substance 45 before positioning the second piece 30b next to the first piece 30a, to fasten also the second piece 30b to the outer wall 21.
The positioning and glueing of other sheet metal pieces 30 are carried out in a similar manner, until the entire outer wall 21 is internally lined, both on the inner face of the side surface 11 and on the inner face of the two heads 12, 13.
Basically, each piece of aluminium sheet 30 is bonded at its edges to other pieces of aluminium sheet 30 next to it, so obtaining an inner wall 22 composed of pieces of aluminium sheet 30 that are bonded together through the structural glue 35.
The inner wall 22, or second wall, is a closed shell that has an outer surface (composed of the first faces 31 of the sheet metal pieces 30) adjacent to the inner face of the outer wall 21, and an inner surface (composed of the second faces 32 of the sheet metal pieces 30) that faces the inner chamber 18 bounded and enclosed by the second wall 22 itself.
The use of structural glue 35 to join together the pieces of aluminium sheet 30 allows making a joint having a very high mechanical strength. The tensile strength (along a direction parallel to the surface of the sheet metal pieces 30) in the overlap regions 37 is equivalent to, or even equal to or greater than, the tensile strength of the piece of aluminium sheet 30 in the non-overlapping region. In other words, the joint regions 37 between the sheet metal pieces 30 may be stronger than the rest of the inner wall 22.
In practice, the inner wall 22 behaves substantially as if it were a single body made of a single sheet of aluminium. This is useful for having a double-walled transformed tank that has a long life and a resistance to pressure, as well as a resistance to thermal expansion, which are greater than the double-walled tanks transformed according to the prior art.
An interspace 29 is left between the inner wall 22 and outer wall 21 (more specifically, between the inner wall 22 and the first resin layer 25), thanks to the protuberances 40 of the sheet metal pieces 30 that act as spacers between the first resin layer 25 and the bottom 310 of the first face 31 of the sheet metal pieces 30. In particular, the protuberances 40 are made on the sheet metal pieces 30 in such a way that the interspace 29 is continuous, i.e., it is devoid of any closed or secluded region. The interspace 29 is air-tight with respect to the inner chamber 18.
The inner wall or second wall 22 may be fluid-tight by itself, i.e., a sealing or a second resin layer is not required to ensure the seal against seepage of fluid between one sheet metal piece 30 and another sheet metal piece; in particular, the joining between the sheet metal pieces 30 is made in a fluid-tight manner, i.e., the joint or overlap regions 37 are fluid-tight thanks to the structural glue 35 that creates a seal between the glued sheet metal pieces 30a, 30b: the second wall 22 is air-tight immediately after glueing.
In a variant embodiment, a fluid-tight joint region is obtained by further providing for an aluminium band 39 to be glued on the joint between the two sheet metal pieces 30a, 30b, along the edges 33, 34 joined to each other. Specifically, the aluminium band 39 is glued onto the overlap region 37 between the two sheet metal pieces 30a, 30b and, in particular, the aluminium band 39 has a width L39 that is greater than the width L37 of the overlap region 37.
The aluminium band 39 is glued with a glue layer 395 that is interposed between the aluminium band 39 and the sheet metal pieces 30a, 30b. For example, the glue used for the layer 395 is the same structural glue that is used for glueing the sheet metal pieces 30a, 30b to each other.
The glue layer 395 may have such a thickness that it fills the concavities 402 of the protuberances 40 in the region of the aluminium band 39, whereby obtaining substantially a bead on the overlap region 37.
A second layer 27 of resin (for example, epoxy resin) is applied preferably by spraying on the inner surface of the inner wall 22, thus coating the second faces 32 of the sheet metal pieces 30. As an alternative to spraying resin, the resin could be applied by a lamination process.
A coating resin layer 27, which adheres to the pieces of aluminium sheet 30 and completely coats the interior of the tank 1, is obtained after drying of the resin. This is useful for protecting the sheet metal pieces 30 from the product that will be stored in the tank 1, ensuring compatibility with the product itself and giving a long life to the double-walled system. Moreover, the second coating layer 27 can increase the fluid-tightness of the inner wall 22. The second coating layer 27 may be provided even when the inner wall 22 and the joint regions 37 are already fluid-tight by themselves.
Also the second resin layer 27 is checked, for example by a pinhole detector, to verify that it has no holes or irregularities, which in case are repaired.
The second resin layer 27 is coated in turn by a layer 28 of electrically-conductive material 28, such as a conductive resin. The conductive layer 28 is connected with the earthing of the tank 1 through the manhole aperture 14. The conductive layer 28 allows discharging to earth the static electricity and any potential differences that are produced inside the inner chamber 18 of the tank 1, thus ensuring that there are no sources of ignition inside the tank 1.
Thus, a tank 1 is obtained, in which the inner storage chamber 18 is separate from the soil 9 by a double wall 21, 22, as it is required by the regulations about underground tanks: the first wall 21, i.e. the outer wall, is the original wall of the tank 1 before the operation of transformation, whereas the second wall 22, i.e., the inner wall, has been made using the pieces 30 of aluminium sheet, which have been joined to each other by the structural glue.
A pressure difference is applied in the interspace 29 between the first wall 21 and the second wall 22. In particular, the interspace 29 is preferably put under low pressure condition (via the suction pipes) to about -0.7 bar. The pressure in the interspace 29 is constantly monitored during the use of the tank 1 to verify that it remains stable over time, or in any case within a range between -0.25 bar and -0.7 bar.
For example, the low pressure remaining stable indicates that the interspace 29 is sealed and that there is no seepage or leakage from the inner chamber 18, so that there is no risk of product pouring from the inner chamber 18 to the soil 9.
A significant increase in pressure in the interspace 29, or at least a significant change in it, indicates that there was a seepage or leakage through one of the two walls 21, 22 and, therefore, that maintenance is required before a leakage of product to the soil 9 may occur. In case, the leakage points are searched for and get repaired.
Since, as previously mentioned, the interspace 29 is continuous, it has the same pressure at all points: a leakage through any point of the second wall 22 would increase the pressure in the entire interspace 29. Therefore, measuring pressure in just one point of the interspace 29 is sufficient for the monitoring.
In a variant embodiment, the second wall 22 is made fluid-tight (even before application of the second resin layer 27) and low pressure is created inside the interspace 29 (i.e., vacuum is created) immediately after the second wall 22 itself has been made, i.e., after the structural glueing between the sheet metal pieces 30. This allows checking the air tightness of the second wall 22 and possibly closing any leaks that may emerge from this check.
Furthermore, by creating low pressure inside the interspace 29 and in particular by creating vacuum inside it, the structure of the second wall 22 is pulled towards the first wall 21 and it fits perfectly to the inner surface of the first wall 21, i.e., it uniformly rests against the latter.
Therefore, the drying of the structural glue 35 to get the second wall 22 in final configuration takes place with the second wall 22 that is well positioned on the first wall 21. Thus, the second wall 22 adopts a maximum extension when the glue 35 is not yet dried, allowing some mutual movement between the sheet metal pieces 30 to fit to the first wall 21. In this way, any tensile stress in the second wall 22 during use, arising from vacuum in the interspace 29 during use of the tank 1, is prevented. Tensile stresses arising in the inner wall pose a problem that is encountered in the prior art and is particularly relevant in the case where an inner layer of epoxy resin was applied as well. In fact, the tensile stresses can cause cracks of the inner resin layer.
This problem is solved here by creating low pressure inside the interspace 29 immediately after the glueing of the sheet metal pieces 30, in such a way that the second wall 22 fits to the first wall 21 before the structural glue 35 creates a completely rigid union between the sheet metal pieces 30. Therefore, a second wall 22, which is suitable to stand vacuum in the interspace 29 without creating tensile stresses, is obtained.
Possibly, a second layer 27 of resin is applied on the inner surface of the second wall 22 after the step of creating low pressure inside the interspace 29, i.e., when the problem of the tensile stresses has already been overcome.
In the case of a non-underground tank or only partially underground tank, as already mentioned, the lining of the outer wall 21 by means of the sheet metal pieces 30 may be restricted to the lower part only of the side surface 11 and the heads 12, 13, i.e., where there is a risk of a leakage to the soil. In this case, therefore, the inner wall 22 has an extension smaller than the outer wall 21 and compose an open shell rather than a closed shell; in any case, an interspace 29 is left between the outer wall 21 and the inner wall 22.
In a variant embodiment, the pieces of aluminium sheet 30 are provided with a layer 350 of glue 35 along the strip or edge region 34 before being put into the tank 1. In other words, each sheet metal piece 30 is supplied with a layer 350 of glue 35 already provided on it, so that it is not necessary to spread the glue 35 when the sheet metal piece 30 is positioned inside the tank 1. The use of sheet metal pieces 30 with prearranged glue 35 simplifies the work of the workers and reduces the stay time of the workers inside the tank 1.
If the glue 35 is of a bi-component type, a first component may be prearranged on the edge region 33 of the first face 31 of the sheet metal piece 30 and a second component may be prearranged on the edge region 34 of the second face 32. In this way, during positioning of the sheet metal pieces 30a, 30b with overlapping, the first component on one sheet metal piece 30b would mix with the second component on another sheet metal piece 30a to obtain the activation of the glue 35.
Similarly, also the first face 31 of each sheet metal piece 30 may be provided with a layer 450 of glue or adhesive substance 45 (for fastening to the first wall 21) before being put into the tank 1, i.e., the sheet metal piece 30 is supplied with a prearranged layer 450 of glue or adhesive substance 45.
In order to protect the layer 350 of structural glue 35 and / or the layer 450 of glue or adhesive substance 45 before use, the layers 350, 450 are protected by respective removable films 355, 455, for example of plastic material.
For example, the sheet metal pieces 30 are obtained by cutting from a coil 5 of aluminium sheet that is wound to form the coil 5 itself. The coil 5 has a layer 350 of structural glue 35 along its edge 54 and has a layer 450 of glue or adhesive substance 45 on a first face 51 of the sheet. The layers 350, 450 are protected by respective removable protective films 355, 455.
Each sheet metal piece 30, which is cut from the coil 5, will therefore have an edge 34 provided with structural glue 35, which is a portion of the edge 54 of the coil 5, and a first face 31 provided with glue or adhesive substance 45, which is a portion of the first face 51 of the sheet wound in a coil 5.
In alternative embodiments, the coil 5 and / or the sheet metal pieces 30 are supplied with only one prearranged layer of the two layers 350, 450 of glue or adhesive substance, i.e., only one of the two layers 350, 450 is already provided before the piece 30 is put into the tank 1. The missing layer of glue or adhesive substance may be spread during the operation steps inside the tank 1
In alternative embodiments of the present disclosure, the sheet metal pieces 30 are not fastened by glue or adhesive substance 45 to the inner face of the outer wall 21. Therefore, in these alternative embodiments, the glue / adhesive substance 45 and the layer 450 are neither provided nor used.
The method according to the present disclosure can also be applied to a tank that had been constructed with a double wall and wherein one of the two walls is damaged and no longer fluid-tight. The process is substantially similar to that described above, with the difference that in this case a further inner wall lining the pre-existing inner wall is made. The pre-existing inner wall is then the "first wall" of the above description, whereas the further inner wall (made of glued pieces of sheet metal) is the "second wall".
The subject of the present disclosure has been hereto described with reference to embodiments thereof. It is understood that other embodiments might exist, all relating to the same inventive core and falling within the protection scope of the claims hereinafter.

Claims

A method for making a second wall (22) inside a tank (1) having a first wall (21) that encloses an inner chamber (18), comprising the steps of:
- providing a plurality of pieces of sheet metal (30, 30a, 30b), each piece of sheet metal (30, 30a, 30b) having an embossed profile comprising a plurality of protuberances (40) that protrude from a first face (31) of the piece of sheet metal (30, 30a, 30b);

- putting such a plurality of pieces of sheet metal (30, 30a, 30b) into the inner chamber (18) of the tank (1);

- positioning the pieces of sheet metal (30, 30a, 30b) next to each other to compose an inner lining of the first wall (21), said first face (31) of each piece of sheet metal (30, 30a, 30b) being adjacent to an inner face (211, 25) of the first wall (21);

- joining the pieces of sheet metal (30, 30a, 30b) to each other, to obtain a second wall (22) and an interspace (29) between the first wall (21) and the second wall (22), said protuberances (40) acting as spacers between the first wall (21) and the second wall (22), wherein
in the step of positioning the pieces of sheet metal (30, 30a, 30b) next to each other, the pieces of sheet metal (30, 30a, 30b) are partially overlapped with each other in overlap regions (37) along edges (33, 34) of the pieces of sheet metal (30, 30a, 30b),
characterised in that the pieces of sheet metal (30, 30a, 30b) are joined to each other by
means of a structural glue (35) adapted to create a structural bonding between the pieces of sheet metal (30, 30a, 30b), the structural glue (35) being sandwiched between the pieces of sheet metal (30a, 30b) in said overlap regions (37),
the second wall (22) being composed of said pieces of sheet metal (30, 30a, 30b) that are glued to each other,
the structural glue (35) being a glue having a high mechanical strength such that, after setting, the glue creates a non-removable fastening between the glued pieces, the glued pieces behaving as if they were a single piece from a mechanical point of view.
The method according to claim 1, wherein said pieces of sheet metal (30, 30a, 30b) are provided with a layer (350) of structural glue (35) that is prearranged on at least one edge (33, 34) of each piece of sheet metal (30, 30a, 30b), said layer (350) of structural glue (35) being provided on said at least one edge (33, 34) before putting the piece of sheet metal (30, 30a, 30b) into the inner chamber (18).
The method according to claim 2, wherein, before putting the piece of sheet metal (30, 30a, 30b) into the inner chamber (18), said layer (350) of structural glue (35) is protected by a removable film (355).
The method according to claim 2 or 3, wherein the step of providing a plurality of pieces of sheet metal (30, 30a, 30b) includes the sub-steps of:
- providing a coil (5) of sheet metal, said coil (5) being provided with a layer (350) of structural glue (35) along at least one edge (54) of the coil (5);

- cutting said coil (5) into pieces, to obtain said plurality of pieces of sheet metal (30, 30a, 30b), each piece of sheet metal (30, 30a, 30b) having at least one edge (33, 34) provided with a layer (350) of structural glue (35), which is a portion of said at least one edge (54) of the coil (5).
The method according to any one of claims 1 to 4, wherein each protuberance (40) of said plurality of protuberances (40) of each piece of sheet metal (30, 30a, 30b) has a convexity (401) that protrudes from the first face (31) and a corresponding concavity (402) that sinks in a second face (32) opposite to the first face (31),
wherein, in the step of positioning the pieces of sheet metal (30, 30a, 30b) partially overlapping with each other, the convexities (401) of the protuberances (40) of one piece of sheet metal (30b) are inserted into the concavities (402) of the protuberances (40) of another piece of sheet metal (30b) in the respective overlap region (37), the structural glue (35) being sandwiched between said convexities (401) and said concavities (402).
The method according to any one of claims 1 to 5, comprising a step of glueing an aluminium band (39) onto a joint region (37) between two pieces of sheet metal (30, 30a, 30b), to obtain a fluid-tight joint region (37).
The method according to any one of claims 1 to 6, comprising the further steps of:
- making a second wall (22) that is fluid-tight, the joint regions (37) between the pieces of sheet metal (30, 30a, 30b) being fluid-tight and the interspace (29) being air-tight with respect to the inner chamber (18);

- creating low pressure inside the interspace (29) immediately after the second wall (22) is made.
The method according to claim 7, comprising the further step of applying a resin layer (27) on the inner surface of the second wall (22) to obtain a coating of the pieces of sheet metal (30, 30a, 30b), wherein said step of applying a resin layer (27) is carried out after the step of creating low pressure inside the interspace (29).
A tank (1) having a first wall (21) and a second wall (22), wherein the second wall (22) internally lines the first wall (21) and an interspace (29) is provided between the first wall (21) and the second wall (22),
the second wall (22) being an inner wall that is composed of pieces of sheet metal (30, 30a, 30b) positioned next to each other, each piece of sheet metal (30, 30a, 30b) having an embossed profile comprising a plurality of protuberances (40) that protrude from a first face (31) of the piece of sheet metal (30, 30a, 30b), said first face (31) of each piece of sheet metal (30, 30a, 30b) being adjacent to an inner face (211, 25) of the first wall (21), said protuberances (40) acting as spacers between the first wall (21) and the second wall (22) to define the interspace (29), wherein the pieces of sheet metal (30, 30a, 30b) are partially overlapping with each other in overlap regions (37) along the edges (33, 34) of the pieces of sheet metal (30, 30a, 30b) characterised in that
the pieces of sheet metal (30, 30a, 30b) composing the second wall (22) are joined to each other by a structural glue (35) adapted to create a structural bonding between the pieces of sheet metal (30, 30a, 30b), the structural glue (35) being a glue having a high mechanical strength such that, after setting, the glue creates a non-removable fastening between the glued pieces, the glued pieces behaving as if they were a single piece from a mechanical point of view,
and in that the structural glue (35) being sandwiched between the pieces of sheet metal (30a, 30b) in said overlap regions (37).
The tank (1) according to claim 9, the second wall (22) being a shell in which the mechanical strength of the overlap regions (37) joined by the structural glue (35) is equivalent to or greater than the mechanical strength of the non-overlapping regions of sheet metal.
The tank (1) according to claim 9 or 10, wherein the second wall (22), which is composed of the pieces of sheet metal (30, 30a, 30b), is fluid-tight, the structural glue (35) creating a fluid-tight seal between one piece of sheet metal (30a) and another piece of sheet metal (30b).
The tank (1) according to any one of claims 9 to 11, wherein the interspace (29) is continuous, the interspace (29) being devoid of any closed or secluded region.
The method or the tank (1) according to any one of claims 1 to 12, wherein said pieces of sheet metal (30, 30a, 30b) are made of a ductile metal, in particular they are pieces of sheet of aluminium or aluminium alloy.
The method or the tank (1) according to any one of claims 1 to 13, wherein said structural glue (35) has a shear strength that is greater than 10 N/mm², in particular said structural glue (35) has a shear strength that is greater than 20 N/mm², even more in particular said structural glue (35) has a shear strength that is 25 N/mm².
The method or the tank (1) according to any one of claims 1 to 14, wherein said structural glue (35) has methyl-methacrylate as a basis.