FR2749060A1 - TANK FOR LIQUEFIED OIL GAS - Google Patents

Abstract

The cistern consists of an airtight tank (4) made of sheet steel having on its outer surface a layer of passive anticorrosion sheathing (10) which itself is covered by a layer of hydraulic mortar sheathing (23) which is designed to come in direct contact with the natural soil (13) in the ground without requiring the use of a layer of sand (17) but offering the same possibilities of cathodic protection for the cistern (22) by means of sacrificial anodes (15).

Description

 The present invention relates to a tank of the family of pressure vessels not subjected to flame and designed to contain liquefied petroleum gases, intended to be buried by being combined with a cathodic protection system by at least one sacrificial anode and comprising this effect - a sealed tank made of sheet steel, having an inner peripheral surface delimiting a gas receiving chamber and an outer peripheral surface, and - a layer of electrically insulating passive anti-corrosion coating continuously covering the outer peripheral surface of the tank.

 Conventionally, the protection of such tanks, once they are buried, is ensured by their layer of anticorrosion coating, in practice in a passive organic material, an active cathodic protection by sacrificial anodes and an embankment of the tank in sand, ensuring mechanical protection of the tank by sheltering its anti-corrosion coating layer from direct contact with the natural soil and creating around the tank a homogeneous and continuous environment conducive to the proper functioning of cathodic protection. The sacrificial anodes, generally two in number, electrically connected to the reservoir through the anticorrosion coating layer, are in turn embedded in the surrounding natural soil.

 The sand embankment has a drawback in that it requires, after digging a pit of dimensions much larger than those of the tank to receive the latter, the evacuation of the corresponding cuttings; it also requires the supply of the necessary quantities of sand, which are significant. As a result, the installation of an underground tank for liquefied petroleum gases is a long and costly operation, immobilizing significant personnel and equipment for a relatively long time. In addition, until the tank is buried, its anti-corrosion coating layer is exposed to shocks, especially during storage, transport and handling of the tank.

 The object of the present invention is to remedy these drawbacks and, to this end, the present invention provides a tank of the type indicated in the preamble, characterized in that it also comprises a layer of over-coating in hydraulic mortar, covering continuously the anticorrosion coating layer intended to come into contact with natural soil.

 Surprisingly, it has been found that such a layer of hydraulic mortar over-coating, preferably of substantially constant thickness, is sufficient to protect the anti-corrosion coating layer against impact and allows backfilling with the ground. natural likely to contain stones, without interposition of sand between this natural soil and the over-coating layer, even if the thickness of the latter was small, for example at most equal to approximately 20 mm, and did not cause this does both a low expenditure on concrete and a small increase in the weight of the tank, that is to say an additional weight suitable for keeping in the finished tank, bearing its overcoating in hydraulic mortar , good transport and installation conditions.

 It appeared that the overcoating in hydraulic mortar brings impact resistance by being at the same time hard enough to protect the coating layer from corrosion, for example from a passive organic material chosen from a group comprising epoxy paints, punctures and nicks caused by falling stones and other elements contained in the natural soil during backfilling, as well as during slow ground movements related to stabilization, and flexible enough to absorb shocks without retransmitting the impact energy to the layer of anticorrosion coating.

 It contributes itself to the protection of the tank itself against corrosion insofar as, by its alkaline nature, it generates near the steel a range of plI in which the latter is deemed insensitive corrosion.

 In addition, it behaves favorably with respect to the cathodic protection system insofar as it behaves as a good electrolyte, in the sense that its electrochemical behavior is close to that of sand conventionally used as backfill, and to the extent where it ensures a homogeneous and continuous contact, comparable to that of sand, with the anticorrosion coating layer itself in homogeneous and continuous contact with the tank. It does not add an electrical insulation barrier in addition to the organic coating layer whose resistivity is controlled, which makes it possible to maintain sufficient cathodic protection currents and therefore easily measurable on site and directly comparable to the values measurable in the sand. Thus, not only is the overcoating in hydraulic mortar compatible with cathodic protection systems using sacrificial anodes, in particular the previously known system, but the experience of the installations set up according to the prior art can be directly used; in other words, there is no break with regard to the cathodic protection control values between a tank installation in accordance with the prior art and an installation using a tank in accordance with the present invention .

 By its conductive nature, linked to the fact that it contains water, and continuous, concrete avoids any effect of "tent", that is to say air occlusion, which could exist with sur -insulating coatings.

 Finally, it keeps the tank's vaporization capacities for liquefied petroleum gas thanks to favorable heat exchange coefficients.

 Naturally, the hydraulic mortar constituting the overcoating layer can be reinforced, and any known type of reinforcement can be used for this purpose, for example fibers, which are drowned in the hydraulic mortar during the manufacture of the tank, and more precisely the production of the over-coating layer, for example by spraying onto the anticorrosion coating layer.

 However, it is also possible, preferably, to provide that the tank itself constitutes at least essentially the reinforcement of the overcoating layer in hydraulic mortar through the anticorrosion coating layer by ensuring that the overcoat layer is bonded at all points to the coating layer. To this end, the adhesion of the overcoat layer on the coating layer can be reinforced by means of a continuous film of adhesion interposed therebetween and bonded at all points to both of these. layers ; it may, for example, be certain resin compositions such as epoxies, acrylics, styrenes-butadienes, which are sprayed in the liquid state on the anticorrosion coating layer but which are liable to harden subsequently and on which we project, while they still have a sticky or viscous feel during their hardening process, the fresh hydraulic mortar intended to constitute the overcoat layer. However, it is also possible to dispense with such an adhesion layer and to ensure that the overcoating layer is anchored directly, at any point, on the coating layer, in particular by using an epoxy resin for the latter, or any another resin suitable for enabling it to fulfill its anticorrosion function, which is thus projected in the liquid state onto the reservoir and onto which, while it still exhibits a tacky or viscous feel during its hardening process, directly projects the fresh hydraulic mortar intended to constitute the over-coating layer. In either case, the hydraulic mortar is set or allowed to set, then hardens, before burying the tank.

Insofar as a tank produced in accordance with the present invention no longer requires backfilling with sand, the construction of a buried installation for the storage of liquefied petroleum gas is thereby modified and the present invention also provides a method for producing '' such an installation, consisting in particular of burying a tank in the family of pressure vessels not subjected to flame and designed to contain liquefied petroleum gases, comprising: - a sealed tank of sheet steel, having an inner peripheral surface delimiting a gas receiving chamber and an outer peripheral surface, and - a layer of electrically insulating passive anti-corrosion coating, continuously covering the outer peripheral surface of the tank, by jointly burying at least one sacrificial anode, placed directly in contact with the natural soil, and by electrically connecting the re serve at the sacrificial anode through the anticorrosion coating layer,
characterized in that the tank is a tank according to the invention, in that the reservoir is electrically connected to the sacrificial anode through the layer of overcoating in hydraulic mortar and also the possible adhesion film, and in that this tank is buried by placing the over-coating layer in hydraulic mortar directly in contact with the natural ground.

The installation itself has an original character compared to the installations using tanks of the prior art, and the present invention further provides a buried installation for storing liquefied petroleum gas, comprising a family tank. pressure vessels not subjected to flame and designed to contain liquefied petroleum gases, itself comprising: - a sealed tank made of sheet steel, having an inner peripheral surface delimiting a gas receiving chamber and a peripheral surface external, - a layer of electrically insulating passive anti-corrosion coating, continuously covering the external peripheral surface of the tank, and - electrical connection means between the tank and at least one sacrificial anode also buried placed directly in contact with natural soil, these electrical connection means passing through the coating layer anti-corrosion,
characterized in that the tank is a tank according to the invention, the overlay layer of hydraulic mortar is also crossed by the electrical connection means, as well as any adhesion film, and is placed directly in contact natural soil.

 Other characteristics and advantages of these various aspects of the present invention will emerge from the description below, relating to a nonlimiting example, as well as from the appended drawings which form an integral part of this description.

 FIG. 1 shows a buried installation for the storage of liquefied petroleum gas, using a tank of the prior art, seen in section through a vertical plane, transverse to a longitudinal axis of the tank.

 FIG. 2 shows, in a similar view, such an installation produced by means of a tank in accordance with the present invention.

 In these two figures, we have exaggerated certain dimensions of the tank compared to others in order to better illustrate the different components.

 Referring first to Figure 1, we will recall the traditional design of a buried tank 1, intended to contain liquefied petroleum gases 2 in variable quantities, in the presence of a gaseous sky, in the presence of a sky gaseous 3 consisting of the gas phase of these same petroleum gases, obtained by natural vaporization of the liquid product, the overpressure with respect to the ambient atmosphere is equal to the relative saturated vapor pressure of petroleum gas and the value of which depends the temperature of the upper layer of the liquid product.

 In such a tank 1, the sealed confinement of the liquefied gas 2 and of the gas phase 3 is ensured by a sealed tank 4 made of sheet steel, which alone ensures the mechanical resistance of the tank 1 with respect to the overpressure of the gas with respect to the ambient atmosphere and generally comprising, as illustrated, an elongated tubular wall 5, internally and externally cylindrical of revolution around a longitudinal axis 6 generally horizontal, and by two end walls 7 in the form of caps, internally and externally hemispherical of revolution around axis 6 and concave towards each other along this axis.

The walls 5 and 7 thus have an inner peripheral surface 8 which delimits a chamber 29 for receiving the liquefied gas 2 and the gaseous phase 3, and an outer peripheral surface 9 fully covered, continuously, with a coating layer 10 passive anticorrosion, electrically insulating, made of a passive organic material chosen from a group comprising epoxy paints. The coating layer 10 thus has an inner peripheral surface 11 in intimate, continuous contact with the outer peripheral surface 9 of the reservoir 4, both at the tubular wall 5 thereof and at its end walls 7 , and an outer peripheral surface 12 which, in the case of this tank 1 of traditional design, constitutes the outer peripheral surface of the tank.

 To bury such a traditional tank 1, a pit 14 is dug in the natural soil 13 having, transversely to a longitudinal, horizontal axis, which merges with the axis 6 in FIG. 1, dimensions considerably greater than those of the surface external device 12 which should be avoided contact with natural soil 13. The pit 14 has for example the shape of a rectangular parallelepiped. The tank 1 is then placed in the pit 14 thus produced, by suspending or wedging it so that it does not come into contact with the natural soil 13 and it is buried in the latter, respectively by hand and by 'other of the pit 14, two sacrificial anodes 15 for example placed approximately at the same level as the axis 6, parallel to the latter, and which is electrically connected to the tank 4 by a respective electrical conductor 16 connected by a part at one, respectively, of the sacrificial anodes 15 and on the other hand at the reservoir 4 locally crossing the coating layer 10. Then, the pit 14 is filled, around the tank 1, with sand 17 which closely follows the outer peripheral surface 12 of the covering layer 10 and prevents any contact of this covering layer 10 with the natural soil 13.

 Not shown, the internal chamber 29 of the reservoir 4 is connected to user devices by means of a system of pipes and valves housed in a manhole 18 comprising in particular a sealed tubular wall 19, connected in a sealed manner to the reservoir 4 through the covering layer 10, and having a generally tubular shape of revolution about an axis 20 perpendicular to the axis 6; during the installation of the tank 1 inside the pit 14, this axis 20 is oriented vertically by turning the sight 18 upwards so that the wall 19 emerges from the sand 17 after filling the pit 14 The manhole 18 is provided with a removable cover 21 then located above the level of the sand 17.

 Referring now to FIG. 2, a tank according to the invention 22 will be described, in which we find identically, and under the same reference numerals, the constituents or parts of constituent 4 to 12 and 18 to 21 and 29, for the same purpose of containing a liquefied petroleum gas 2 in the presence of a gaseous sky 3 in overpressure relative to the ambient atmosphere.

 However, in this case, the outer peripheral surface 12 of the anticorrosion coating layer 10 does not constitute the outer peripheral surface of the tank 22 but it itself carries an overcoating layer 23 of hydraulic mortar, which covers it completely , continuously, and thus has an internal peripheral surface 24 in intimate contact with the external peripheral surface 12 of the coating layer 10 and an external peripheral surface 25 constituting in this case the external peripheral face of the tank 22.

 By its inner peripheral face 24, the overcoating layer 23, obtained for example by spraying hydraulic mortar, is closely integral, in a uniformly distributed manner, with the outer peripheral surface 24 of the coating layer 10, by adhesion at any point on the latter. If necessary, this adhesion can be improved by interposition, between the outer peripheral surface 12 of the coating layer 10 and the inner peripheral face 24 of the overcoating layer 23, of a continuous adhesion film 26 bonded in any point at either of layers 10 and 23; this continuous adhesion film 26 can be produced for example from a material chosen from the group comprising epoxy resins, acrylic resins, butadiene styrenes, these examples being in no way limiting, by projecting such a material in the fluid state on the outer peripheral surface 12 of the coating layer 10 and by spraying fresh hydraulic mortar, that is to say containing its mixing water, on the film thus formed while it still has a tacky or viscous feel.

 Tests have shown that with such an adhesion film 26, or even in the absence of such an adhesion film 26, for example by spraying the fresh hydraulic mortar on the coating layer 10 while the constituent resin of the latter, projected in the liquid state onto the outer peripheral surface 9 of the tank 4, still exhibits a tacky or viscous feel, in the course of its hardening process, it was possible to obtain a sufficiently effective anchoring of the mortar hydraulic 23 on the covering layer 10, itself anchored sufficiently effectively on the tank 4, so that the latter can constitute, for the layer of mortar 23, after its hardening, a reinforcement making it possible to dispense with any other reinforcement for the latter this. The mechanical characteristics which are communicated to the reservoir 4 in order to enable it to resist the overpressure of the gases which it contains, in comparison with the ambient atmosphere, are sufficient in this regard. However, one can also provide an internal reinforcement in the layer 23 of hydraulic mortar, for example in the form of fibers integrated into the mortar during its projection.

 Preferably, the over-coating layer 23 has a substantially constant thickness e, which has been shown in tests to be at most equal to 20 mm while providing the coating layer 10 with protection comparable to that of the sand 17 usually used to mechanically isolate a tank 1 of traditional design, from natural soil 13.

 Under these conditions, the tank according to the invention 22 can be buried in a manner which will now be described.

 A pit 27 is dug in the natural soil 16 in every way comparable to the pit 19, that is to say in particular having dimensions of the same order as the dimensions of the latter, which are sufficient to receive the tank 22, provided from its manufacture with the over-coating layer 23 in hydraulic mortar, and the tank 22 is deposited in the pit 27 thus dug, respecting an orientation such that the sight 19 is turned upwards and its axis 20 oriented vertically. In parallel, two sacrificial anodes 15 are found buried in the natural soil 13, respectively on either side of the pit 27, which are found identically, and which are advantageously oriented parallel to the axis 6. electrically connects each of these sacrificial anodes 15, by an electrical conductor 16 passing through not only the coating layer 10 but also the overcoating layer 23 and the possible adhesion film 26, to the reservoir 4 then the pit 27 is filled by means natural soil 13. The tank 22 is then directly in contact, through the external peripheral surface 25 of the over-coating layer 23, with natural soil 13.

 A person skilled in the art will readily understand, by comparing Figures 1 and 2 and the corresponding comments, above, that the earthworks necessary for the installation, buried, of a tank 22 according to the invention are much less restrictive than those imposed by the installation of a tank 1 of the prior art; in fact, to the extent that part of the natural soil 13 which has been extracted to prepare this pit 27 is used to fill the pit 27, the spoils to be removed are much less bulky, which adds to the fact that it is not necessary to bring backfill sand 17 to considerably reduce transport around the site.

 Naturally, the embodiment of the invention which has just been described, however, constitutes only a nonlimiting example and, in particular, all the dimensional characteristics of the constituents common to a tank according to the invention 22 and to a tank 1 of the prior art can be used in the same way as the prior art, in particular as regards the wall thicknesses of the tank 4 and the thickness of the coating layer 10; in addition, it would not go beyond the scope of the present invention to give the over-coating layer 23 a thickness greater than 20 mm, it being understood that this value was chosen to be both sufficient to provide the desired mechanical protection while not constituting a hindrance for the handling of tank 22; The optional adhesion film 26 may have a much smaller thickness, for example of the order of a few micrometers, but this figure is also given only by way of non-limiting example.

Claims (10)

1. Cistern of the family of pressure vessels not subjected to flame and designed to contain liquefied petroleum gases, intended to be buried by being combined with a cathodic protection system by at least one sacrificial anode (15) and comprising this effect: - a sealed tank (4) made of sheet steel, having an internal peripheral surface (8) delimiting a chamber (29) for receiving the gas (2, 3) and an external peripheral surface (9), and - a layer (10) of electrically insulating passive anti-corrosion coating, continuously covering the external peripheral surface (9) of the tank (4),  characterized in that it further comprises a layer (23) of overcoating in hydraulic mortar, continuously covering the layer (10) of anticorrosion coating and intended to come into contact with natural soil (13). 2. Tank according to claim 1, characterized in that the layer (23) of over-coating has a thickness (e) substantially constant. 3. Tank according to claim 2, characterized in that said thickness (e) is at most equal to 20 mm. 4. Tank according to any one of claims 1 to 3, characterized in that the over-coating layer (23) is bonded at all points to the coating layer (10). 5. Tank according to claim 4, characterized in that the over-coating layer (23) is anchored directly, at all points, on the coating layer (10). 6. Tank according to claim 4, characterized in that it comprises a continuous film (26) of adhesion interposed between the layer (10) of coating and the layer (23) of over-coating and bonded at all points to the 'one and the other of these layers. 7. Tank according to any one of claims 1 to 6, characterized in that the coating layer (10) is made of a passive organic material chosen from a group comprising epoxy paints. 8. Tank according to claim 7 in combination with claim 6, characterized in that the adhesion film (26) is made of a material chosen from a group comprising epoxy resins, acrylic resins, butadiene styrenes. 9. Method for producing a buried installation for the storage of liquefied petroleum gas, comprising in particular the burial of a tank from the family of pressure vessels not subjected to flame and designed to contain liquefied petroleum gases, comprising: - a sealed tank (4) made of sheet steel, having an inner peripheral surface (8) defining a chamber (9) for receiving the gas (23) and an outer peripheral surface (9), and - a coating layer (10) passive anticorrosion, electrically insulating, continuously covering the external peripheral surface (9a of the tank (4), by jointly burying at least one sacrificial anode (15), placed directly in contact with natural soil (13) and by electrically connecting (16 ) the reservoir (4) at the sacrificial anode (15) through the layer (10) of anticorrosion coating,  characterized in that the tank is a tank (22) according to any one of claims 1 to 8, in that the reservoir (4) is electrically connected to the sacrificial anode (15) through the layer (23 ) of hydraulic mortar over-coating and the possible adhesion film (26) as well, and in that this tank (22) is buried by placing the layer of hydraulic mortar over-coating directly in contact with the ground natural (13). 10. Buried installation for the storage of liquefied petroleum gas, comprising a tank from the family of pressure vessels not subjected to flame and designed to contain liquefied petroleum gases, itself comprising - a tight tank (4) made of sheet metal of steel, having an internal peripheral surface (8) delimiting a chamber (9) for receiving the gas (2, 3) and an external peripheral surface (9), - a layer (10) of passive anticorrosion coating, electrically insulating, continuously covering the external peripheral surface of the tank (4), and - means (16) for electrical connection between the tank (4) and at least one sacrificial anode also buried (15), placed directly in contact with the natural ground ( 13), these means (16) of electrical connection passing through the layer (10) of anti-corrosion coating,  characterized in that the tank is a tank (22) according to any one of claims 1 to 8, the layer (23) of which is coated with hydraulic mortar is also traversed by the means (16) of electrical connection, as well as the possible adhesion film (26), and is placed directly in contact with the natural ground (13).