Abstract:
The tank is made up of a plurality of elementary tanks such as tubes ( 20 ) connected in parallel to at least one manifold device ( 30, 32 ), and it includes closure valves enabling any one of the elementary tanks to be isolated in response to a drop in the pressure contained therein.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a tank for fluid under pressure, more particularly a fluid under high pressure, i.e. much greater than 1 MPa, typically greater than 5 MPa. 
     A particular, although not exclusive, field of application of the invention is that of tanks for liquefied gas, in particular for liquefied propane gas (LPG) used in motor vehicles. 
     BACKGROUND OF THE INVENTION 
     The presence of tanks under pressure close to people or sensitive goods, or in a confined space, gives rise to problems of safety. The usual solution consists in using a container that is strong, and thus heavy. In addition, the optimum shape for such a container enabling it to withstand internal pressure well often limits the ways in which it can be installed, in particular on a motor vehicle. This takes up a large amount of the available volume in the vehicle. In addition, safety standards mean that vehicles fitted with such tanks can be banned from having access to road tunnels. 
     BRIEF DESCRIPTION OF THE INVENTION 
     An object of the invention is to provide a tank for fluid under high pressure that does not present those drawbacks, and to this end the invention provides a tank comprising a plurality of elementary tanks connected in parallel to at least one manifold device, and closure means suitable for isolating any one of the elementary tanks in response to the pressure therein dropping. 
     Advantageously, the elementary tanks are constituted by tubes. 
     A first advantage of the invention lies in the great ease with which it can be adapted to the space available. Ability to withstand pressure is determined by the section and the wall thickness of each elementary tank, regardless of the overall shape of the tank as a whole. It is therefore possible to distribute the volume of the tank in the space available while using elementary tanks of different lengths or by placing them in rows with varying numbers per row, or grouping them together in distinct subassemblies which are interconnected. This option is particularly advantageous for motor vehicles to ensure that the housing for the tank does not penalize available volume. 
     In addition, because of its modular design, the tank is simple to make and of low cost. This applies in particular with elementary tanks that are in the form of tubes since the same tubes, when cut to desired lengths, can be used to make tanks of any shape and of any volume. 
     Furthermore, such elementary tanks present the ability to contain external pressure, typically due to a relative loss of pressure in the elementary tank, that would not be possible with a container of any complex shape without special architecture and dimensioning. 
     The present invention also makes it possible to use various materials for the elementary tanks, for example metals, metal alloys, or composite materials. Composite materials can be reinforced with carbon fibers, “Kevlar” (registered trademark) or glass, and they can have a matrix made of resin, e.g. epoxy resin. 
     In addition, the requisite conditions, particularly in terms of wall thickness, are much less severe for each elementary tank than they are for a single-body tank having the volume of the tank that is to be provided, and the mass saving compared with a single body tank can be significant. 
     Another advantage of the tank of the invention is safety. Because of the closure means, damage to an elementary tank does not endanger the entire tank together with its content, and thus limits nuisance to the environment in the event of a leak. The low rate of flow and the small total volume of fluid that escapes when only one elementary tank is damaged mean that certain restrictions on use, such as banning access to road tunnels, for example, need no longer be justified. 
     Advantageously, the closure means are in the form of respective valves, for example in the form of flexible membrane means, which are mounted at each end of each elementary tank that is connected to a manifold, a valve closing the end of an elementary tank in response to the pressure in said elementary tank dropping relative to the pressure in the manifold. A same flexible membrane can be mounted in a manifold connected to a plurality of elementary tank ends so as to be common to a plurality of elementary tanks. 
     The tank can be provided with a protective shield covering at least each exposed surface of the tank. 
     The shield advantageously has an armored structure made up of a rigid covering sheet and a thick underlying layer of cellular material in foam or honeycomb form. The covering sheet, e.g. of composite material, is capable of absorbing part of the energy of an impact or of a projectile, and of transmitting the energy it does not absorb to the foam material which is suitable for spreading it over a large area of the tank so as to avoid deforming the underlying structures. The magnitude of the impacts that are to be absorbed without functionally damaging the tank overall will determine the dimensioning of the shield. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages for the tank of the invention will appear on reading the following description given below by way of non-limiting indication with reference to the accompanying drawings, in which: 
     FIGS. 1 and 2 are highly diagrammatic views, respectively an end view and a side view, showing an embodiment of a tank of the invention; 
     FIG. 3 is a detail view showing how an elementary tube is connected to a manifold in the tank of FIGS. 1 and 2; 
     FIGS. 4 and 5 are highly diagrammatic section views showing means for closing elementary tubes in the tank of FIGS. 1 and 2; 
     FIG. 6 is a section view through a tank of the invention which is fitted with a shield for protecting it against impacts and projectiles; and 
     FIG. 7 is a highly diagrammatic view showing an embodiment of a tank of the invention in a plurality of portions. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The description below relates to making a tank for liquefied gas under high pressure, and more particularly an LPG tank for a motor vehicle. The person skilled in the art will understand that the principles described are immediately applicable to other uses of tanks for gas or liquid under pressure, for example tanks containing toxic substances on industrial sites or tanks containing halon gases. 
     FIGS. 1 and 2 show a tank  10  made up of a plurality of elementary tubes  20  connected in parallel to manifolds  30 . 
     The tubes  20  are disposed parallel to one another in a plurality of superposed rows, i.e. in a “bundle” type of disposition. The tubes all have the same diameter and the same wall thickness. By way of example they are made of metal, such as steel, or of a composite material, such as epoxy resin reinforced with fibers of carbon or “Kevlar” (registered trademark). 
     The lengths of the tubes  20  and the numbers of the tubes in each of the rows are selected so as to occupy in optimum manner the volume available for housing the tank, e.g. beneath the structure of a vehicle. In FIGS. 1 and 2, the limits on the available volume are represented by chain-dotted lines. As can be seen immediately, building up a tank in modular form using elementary tubes is particularly well suited to adapting the tank to a variety of shapes. 
     At each of its two ends, each tube  20  is connected to a manifold  30 . In the example shown, each manifold  30  is in the form of a tube to which the elementary tubes  20  in a given row of tubes are all connected. Additional manifolds  32  and  34  interconnect the manifolds  30  in parallel at each of the two ends of the tank. The manifolds  32  and  34  are connected to a duct  36  connecting the tank  10  to an outlet for use and to an inlet for filling (not shown). 
     Each end of an elementary tank  20  is connected to a manifold  30  by means of a coupling  22  which is screwed or welded to an end  22   a  of the elementary tank  20  and which is screwed or welded at its opposite end to the manifold tube  30 . The coupling  22  penetrates a short distance into the manifold tube  30  at its opposite end  22   b , so that this end projects inside the tube (FIGS. 3,  4 , and  5 ). 
     Although an array of manifolds is described at each end of the elementary tubes, it is naturally possible to provide only one array of manifolds at one end of the tube, in which case the opposite ends are closed. 
     In addition, instead of using manifolds  30  in the form of tubes themselves interconnected in parallel to manifold tubes  32  or  34 , each manifold assembly at each end of the tank could be constituted by a sinuous tube passing along all of the rows of tubes, or it could be constituted by a hollow end plate. In which case the end plate would be formed by two spaced-apart parallel walls interconnected in gastight manner around their periphery, with one of the walls being provided with holes into which the couplings of the elementary tubes penetrate. 
     FIGS. 4 and 5 show a flexible membrane  40  constituting closure means for the elementary tubes  20  in the event of the pressure within any elementary tube dropping, e.g. because of a break or damage resulting from a collision or the impact of a projectile. 
     In the example shown, the membrane  40  is in the form of a strip of flexible material extending along the entire length of the manifold  30  with its faces perpendicular to the axes of the elementary tubes  20  of the row associated with the manifold. As a result, the membrane  40  constitutes closure means that are shared by all of the tubes  20  in the row. At its ends, the membrane  40  is fixed to the closed ends of the manifold  30 , e.g. by adhesive or by mechanical means. 
     By way of example the flexible membrane  40  is made of a composite material constituted by a fiber-reinforced elastomer. In normal operation, the membrane  40  is not deformed and allows free access to the tubes  20  that are connected to the manifold tube  30  (FIG.  4 ). Equal pressures are maintained on both faces of the membrane since it does not split the manifold  30  into two longitudinal volumes that are isolated from each other in sealed manner. 
     In the event of a sudden pressure drop in an elementary tube  20  (FIG.  5 ), the membrane  40  automatically deforms and closes the end  22   a  of the coupling  22  corresponding to said tube. The same phenomenon occurs at both ends of the elementary tube if it is connected to a manifold assembly at each of its ends. As a result, the faulty portion of the tank is rapidly isolated from the remainder of the tank, which remainder can continue to be used, with any losses of fluid being very limited. 
     The use of a flexible membrane is advantageous because of its low cost and its reliability, given that no moving parts are required. Nevertheless, other embodiments of the closure means could be used, for example non-return valves associated with each end of each elementary tube, with the non-return valves then being optionally subjected to a small return force that keeps them open in the event of no pressure drop in the corresponding elementary tubes. 
     As already mentioned, the use of elementary tubes, each of which can be quite small in diameter, typically less than 5 cm, or even less than 1 cm, makes it possible to withstand pressures that are very high while using walls that are not very thick. For example, tubes made of carbon/epoxy composite material having an outside diameter of 8 mm and a wall thickness of 1 mm can withstand an internal pressure of 100 MPa. 
     A tank, even for a fluid under high pressure, can thus be substantially lighter than a single-body tank of the same capacity. 
     In order to protect the elementary tubes against impact and against projectiles, it is desirable to provide the tank with a protective shield, at least over each exposed face. 
     Such a shield is shown in FIG.  6 . In this example, it comprises a layer of foam material  42 , in particular polyurethane foam, surrounding the bundle of elementary tubes  20  and the manifold tubes. The layer  42  is coated by a rigid shell or structure  44 , e.g. made of a composite comprising an epoxy resin matrix reinforced with aramid fibers. The shell  44  can be formed by draping resin-impregnated fiber fabric over the foam  42  and then polymerizing the resin. In a variant, the impregnated fiber fabric can be draped over the inside face of a mold in which the tank is inserted for the purpose of forming the covering foam layer. The shield may also be constituted by a case of fibers, e.g. aramid fibers, and a thickness of honeycomb structure material taking the place of and performing the function of the foam material. 
     The use of a protective shield is particularly desirable when the elementary tubes (and the manifold tubes) are made of a material other than metal, for example a composite material, since such materials often present lower impact resistance and provide less plasticity than metals. 
     In the event of a collision or an impact, the rigid shell  44  distributes the pressure over the surface of the foam  42 . This distributes the pressure even more, such that no deformation is imparted to the rear face of the foam which is in contact with the elementary tubes of the tank. 
     In the above, it is assumed that the tank  10  can be complex in shape while being implemented as a single set of elementary tubes  20 . 
     When separate spaces are available for receiving the tank and none of them offers sufficient volume on its own, it is possible as shown in FIG. 7 to make the tank  10  as a plurality of subassemblies  12 ,  14 . Each subassembly comprises a plurality of elementary tubes of lengths and dispositions which are selected as a function of the space available. The subassemblies  12 ,  14  are interconnected by one or more pipes  38 .