Abstract:
A flexible secondarily-contained pipe of unitary or one-piece construction characterized in that it comprises the following combination of features: an inner supply pipe ( 1 ); an inner barrier layer ( 6 ) which forms an internal surface of the supply pipe ( 1 ) and which is formed from a first polymeric plastics material, an outer containment pipe ( 2 ); a plurality of passages associated with the secondary containment pipe, said passages being adapted to receive any fluid which has leaked from the inner supply pipe and to convey said fluid to a remote leak detector.

Description:
FIELD OF THE INVENTION 
     This invention relates to a pipe for conveying fluids. It is particularly applicable, but in no way limited to pipes for conveying organic fluids such as (for example) petroleum products. Such pipes may be used to convey petrol from a reservoir to a dispensing pump and a filling station. The invention also relates to a system for dispensing petrol. 
     BACKGROUND OF THE INVENTION 
     The use of dual containment piping systems in which an inner supply pipe is co-axially placed within an outer secondary containment pipe is known and accepted commercial practice. Typically, such systems are found in the nuclear, oil and gas, petroleum refining and chemical processing industries. The supply pipe is used to transport hazardous or toxic fluid while the secondary containment pipe is used to contain leakage from the supply pipe should it occur. It is also known to provide leakage detectors and drainage systems within the annulus between the carrier and containment pipes. 
     In the design of petroleum forecourt installations, it is regarded as increasingly important to contain and detect any leaks of petrol or diesel fuel from subterranean pipes which connect one or more storage tanks to dispensing pumps in the installation. To that end, many current designs of forecourt installation utilise secondary containment. This involves containing each fuel supply pipeline in a respective secondary containment pipeline which is optionally sealed at its ends tc the fuel supply pipeline. The secondary containment pipeline prevents leaks from the fuel supply pipeline from being discharged into the environment, and also can convey leaked petrol to a remote-sensing device. Typically, the pipes forming the secondary containment pipeline are initially separate from the fuel pipes and are sleeved over the latter as the fuel pipes are installed between the fuel storage tanks and dispensing pumps. 
     A wide variety of secondary containment systems are available. Examples are the ENVIROFLEX™ piping system available from Total Containment Inc of Exton PA USA and the PERMA-FLEXX™ system available from Containment Technologies Corporation Minneapolis USA. A further example is the UPP™ pipework system available from PetroTechnik Limited. These systems all share certain common features. Firstly, the primary supply pipe and the secondary containment pipe are of a different construction. Furthermore, they are each available in a range of different diameters to suit different applications. This has the disadvantage that it requires different manufacturing plant to produce each product, primary and secondary, and a substantial amount of stock is required if orders are to be satisfied quickly. Pipe is very bulky to store and thus takes up a great deal of expensive warehousing space. 
     Such systems therefore require twice as much piping as systems which have no secondary containment, thus increasing the costs of storage and transportation of the components for a forecourt installation. The installation of the secondary containment pipeline increases the amount of time, and hence the cost, of assembling the forecourt installation. In addition, when installing such a system, it can be difficult to ensure that the fuel supply pipes remain spaced from the secondary containment pipes and do not block the passage of leaked fuel to a leak detection system. 
     Certain types of integral secondarily-contained pipe are known. For example, 
     GB1326512 (Institut Francais Du Petrole Des Carburants Et Lubricants) describes an inner pipe separated from an outer envelope by a butyl rubber or other type of foam. According to this specification, the foam will expand on contact with any leaked hydrocarbons from the inner pipe so as to seal that pipe. The foam would not therefore appear to be capable of conveying any leaked liquids to a remote sensor. GB1141014 (Samuel Moore &amp; Company) describes a pipe assembly having an inner metal pipe and a heating line running parallel to and along side it, the whole being contained in an outer casing. Fibrous filler material packs the space between the two inner pipes. The fibrous fillers described would absorb any leakage from the primary pipe rather than conveying it to a remote sensor. 
     U.S. Pat. No. 4,657,050 (Shell Oil Company) illustrates a pipe for conveying hydrocarbons from an offshore location. A steel inner tube runs through a high density plastic sheath formed with circumferential ribs over which a further layer is sleeved. The gaps between the ribs are filled with a foam but the ribs prevent leaked liquid from being conveyed along the pipe. A similar problem exists in the type of pipe disclosed in U.S. Pat. No. 4,644,977 (The Gates Rubber Company) which describes a hose which is formed by co-extrusion and has a foamed layer separating inner and outer non-foamed layers. The foaming process described causes a closed cell foam to form consisting of tiny closed air pockets. Such a foam would positively hamper the progress of any leaked liquid along the pipe. 
     GB1185062 (Francesco Steffenini) describes a pipe with a one-piece wall having an inner and outer surface, the surfaces being spaced apart by integral helical ribs. The helical arrangement of these ribs provides a positive impediment to the passage of any leaked fluid along the pipe to a remote sensor since any fluid has to make many revolutions of the pipe before reaching an end. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a flexible secondarily-contained pipe of unitary or one-piece construction characterised in that it comprises the following combination of features: 
     (i) an inner supply pipe; 
     (ii) an inner barrier layer which forms an internal surface of the supply pipe and which is formed from a first polymeric plastics material; 
     (iii) an outer containment pipe; 
     (iv) a plurality of passages associated with the secondary containment pipe, said passages being adapted to receive any fluid which has leaked from the inner supply pipe and to convey said fluid to a remote leak detector. 
     This arrangement provides for the first time a lightweight, integral secondarily-contained pipe with passageways designed to channel any leaked fluid to a remote leak detector. 
     Preferably the plurality of passages is formed by a foam. This provides significant weight reduction. In a particularly preferred embodiment the foam, the outer containment pipe and the inner supply pipe are formed from the same, second polymeric plastics material. 
     In a further preferred embodiment the foam is an open-celled foam adapted to encourage the flow of any leaked fluid along the pipe. 
     Preferably the foam is resiliently compressible. Thus, when the pipe is bent into a tight radius the foam can give and allow some relative movement of the inner supply pipe with respect to the outer containment pipe. 
     Preferably the foam comprises a polymeric material. 
     Alternatively the foam comprises a silicone foam. The choice of foam will be made by the materials specialist and will depend, in part, on the other plastics material used in the pipe. 
     In an alternative embodiment the outer containment pipe incorporates a series of tubular passages extending along the longitudinal axis of the pipe and angularly spaced from each other about the circumference of the supply pipe. 
     Preferably the plurality of passages are formed from a convoluted wall extending between the inner supply pipe and the outer containment pipe and extending substantially the length of the pipe. 
     Preferably the convoluted wall is substantially corrugated, corrugations providing a particularly favourable weight to strength ratio. 
     Preferably alternate corrugations contact the inner supply pipe and the outer containment pipe, forming a series of substantially parallel passages extending substantially the length of the pipe. 
     In a particularly preferred embodiment the inner supply pipe, inner barrier layer, outer containment pipe and plurality of passages are formed by a process of co-extrusion. 
     The inner barrier layer may comprise a plastics material selected from the group comprising: 
     Nylon 612 
     Polyamides 
     Polyamides 6, 11 or 12 
     Polyethylene terphthalate 
     Polyvinyl chloride 
     Polyvinylidene chloride or fluoride 
     Polypropylene 
     Ethylene/vinyl alcohol copolymers 
     the selection being based on the nature of the fluid being conveyed. 
     The outer containment pipe may comprise a plastics material selected from he group comprising: 
     Polyethylene 
     Polypropylene 
     Polyvinyl chloride 
     By way of example, there is provided a pipe for conveying organic fluids, for example petroleum products such as petrol, the pipe comprising a one-piece wall means having an inner surface which defines a first passage for said fluids, the wall means also including further passage means which is integrally formed with the wall means and extends along the length of pipe, the further passage means being arranged to receive any fluid which has leaked from the first passage and to enable detection of the leaked fluid at a position remote from the leak site. 
     Thus, the detection of leaks can be achieved using a detection system connected to the further passage means without the need for any secondary containment of the pipe. 
     Preferably, the further passage means substantially surrounds said first passage, to ensure that fluid from leaks at any angular position from the first passage is received by the further passage means. 
     Preferably, the pipe is formed from a single extrusion. 
     The further passage means may comprise a series of elongate passages angularly spaced from each other about the first passage. Preferably, however, the wall means comprises an inner wall integrally formed with an outer wall and with one spacer means separating the inner and outer walls to define therebetween an annular space which constitutes, or contains, said further passage means. 
     The spacer means may conveniently comprise a foam which also constitutes a further passage means. To that end, the foam is preferably of an open-celled structure. 
     Preferably, the foam is resiliently compressible. This is particularly advantageous because the foam then enables the pipe to be relatively flexible, the foam maintaining spacing between inner and outer walls even if the pipe is flexed. 
     Preferably, the inner wall, outer wall and foam are all formed by a process of co-extrusion, the walls being formed from a polymeric material such as polyethylene. The foam preferably also comprises a polymeric material. Alternatively, a silicone foam may be used. 
     Preferably, the inner surface of the pipe is lined with a skin, for example, a polyamide such as nylon, which acts as a barrier to the diffusion of liquid from the first passage. Conveniently, the skin is formed by co-extrusion with the other parts of the pipe. 
     The invention also lies in a petroleum forecourt installation comprising a storage tank connected to a dispensing pump through one or more pipes, each manufactured in accordance with the first aspect of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a sectional view of a pipe in accordance with the invention; 
     FIG. 2 is a diagrammatic cut-away view of part of a petroleum forecourt installation which includes a pipeline formed from a series of pipes of the type shown in FIG. 1; and 
     FIGS. 3 and 4 are sectional end views of alternative embodiments of pipe, each in accordance with the invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, a pipe in accordance with the invention is formed from polyolefines, and comprises a supply pipe means  1  separated from an outer protective layer  2  by an intermediate layer of open-celled foam  4 . The supply pipe means  1  comprises an inner barrier layer or cylindrical skin or coating  6  of nylon or a nylon derivative which is supported by an inner supply pipe or cylindrical support layer  8  of polyethylene having a modulus in the range 100-4000 MPa. Alternative construction materials are discussed below. 
     The skin or coating  6  lines the central passage  10  through which petrol is to be conveyed. The layer  6  is constructed from material which is substantially impervious to petrol, and thus presents a barrier to the diffusion of petrol through the inner layer means  1 . The layer  6  is advantageous because the polyethylene layer  8 , although showing some resistance towards the diffusion of petrol, does exhibit some permeability thereto. 
     The foam, which is also formed from polyethylene the modulus of which is in the range of 100-4000 MPa, fills the space between the layer  8  and the layer  2  and is sufficiently resilient to provide support which maintains spacing between those layers. The layer  2  is formed from solid polyethylene which is also of a modulus of 100-4000 MPa. 
     The thicknesses of the various layers of the pipe will, at least in part, be dependent on the size or diameter of the pipe. For example, in a SDR 4  (Standard Diameter Ratio) pipe, having an external diameter of 63 mm (and maximum thickness of 6.5 mm) the thicknesses of each of the skin or coating  6  and the layer  8  lie in the range of 0.01-6.4 mm, that of the foam can be in the range 0.5-6.4 mm whilst the outer layer  2  can have a thickness of between 0.01-6.4 mm. 
     The pipe  1  has a degree of flexibility which facilitates storage (for example in a coiled form) and installation. However, the resilient support provided by the foam  4  helps to maintain spacing between the inner layer means  1  and outer layer  2  even if the pipe is flexed. The layers of the pipe are formed by a process of co-extrusion so that the pipe is of an integral, one-piece construction. 
     Open celled foam structures are formed in a variety of ways. In the extruder an agent is placed into the barrel of the extruder which creates a reaction that foams the basic polymer. Polymer types that are most commonly used for foaming are LDPE (Low-density polyethylene) and EVA (Ethylene Vinyl Acetate). Polyurethanes can be used for foaming as well, however they are not entirely suitable for extruding but excellent for moulding. 
     The percentage of open cell characteristics can be from 100%-0% depending on the agent used to produce the reaction and the die design. 
     The agents used to create the reactions are preferably Azo dicarbonamide compounds with levels of 1-1.5%. Foams can be created with densities ranging from 0.02-0.6 kg/m 3  (Closed cell) and open cell of 0.4-0.8 kg/m 3 . 
     Other agents for producing foamed structures include: 
     Oxybisulphonamide compounds 
     Sodium Citrate 
     Sodium Bicarbonate 
     Methyl Pentane 
     It is intended that this invention should encompass both open and closed cell foam. These can be formed from any suitable material as selected by the materials specialist and formed by any one of a variety of known techniques. 
     In FIG. 1 the outer layer of the secondary containment system is shown as a discrete layer with an inner and an outer surface. This is not necessarily the case and is only one example of what might be possible. 
     For example, a process may be used as described in U.S. Pat. No. 4,644,977(Gates Rubber Company), the entire text of which is incorporated herein by reference and is intended to form an integral part of this disclosure. It follows that there need not be discrete layers or separation between the foamed inner region and unfoamed outer region. They may simply merge into each other with the degree of foaming tending towards substantially zero towards the outer region of the pipe. 
     It will be observed in FIG. 1 that the inner supply pipe  1 , 6  and the outer secondary containment layers are shown as substantially concentric or co-axial. However, this is not strictly necessary and any arrangement of a pipe within a pipe will suffice. Thus the terms “concentric” and “co-axial” have very broad meanings in this context. 
     Referring to FIG. 2, the pipe shown in FIG. 1 is one of a plurality of identical pipes which form a pipeline  12  connecting a subterranean fuel storage tank  14  to dispensers  16  and  18  which include suction pumps (not shown). The pipeline  12  is inclined so that the end adjacent the dispensers  16  and  18  is higher than the end adjacent the tank  14 . The pipes are contiguously arranged and, where necessary, may be joined together by a process of Electro-fusion jointing which provides leak-proof joints between adjacent pipes. However, joints other than at manholes or in chambers are to be avoided. At the upstream end of the pipeline  12 , at or near the lowest point, in a manhole chamber  21  above the tank  14 , there is provided a leak detection sensor  20  which is situated downstream of a suction line  22  which extends into the tank  14 , to allow the pumps  16  and  18  to draw fuel from the tank  14  and along the pipeline  12 . 
     FIG. 2 also depicts a pressure line  23 , which forms part of an alternative form of fuel supply system. That system is known as a pressure system and uses a pump  25 , at the top of the line  23 , to supply fuel along the pipeline  21 . In a pressure system, the pump such as the pump  25  replaces the suction pumps in the dispensers  16  and  18 . 
     In this example, the detector  20  comprises a reservoir (not shown) which communicates with the foam layers of the pipeline  12  and contains a float level switch (not shown). 
     Any fuel which leaks from within the central passage of one of the pipes in the line  12  is captured by the foam layer of that pipe. It can be seen from FIG. 2 that the pipeline  12  is downwardly inclined from the pumps  16  and  18  to the sensor  20 . Consequently, the leaked fuel will tend to flow through the foam layer of the pipeline until it flows into the reservoir in the sensor  20  and, ultimately, triggers the float level switch. An example of a leak detecting device such as the sensing means  20  is shown in GB-A-2304221. 
     The pipeline  12  can be fitted with an alternative form of leak detection system, for example one which pumps an inert gas, such as nitrogen, through the foam layers of the pipeline  12 , so as to displace petroleum vapour from any leaks to a suitable gas detector at the end of the pipeline  12 . 
     Alternative flexible pipes according to the present invention will now be described. 
     The pipe shown in FIG. 3 is a three-layered co-extrusion having an inner nylon skin  26  corresponding to the skin  6  of the pipe shown in FIG.  1 . The pipe also has a polyethylene inner layer or inner supply pipe  27  interposed between the skin and a solid polyethylene wall  28 . Optionally, instead of having an annular foam-filled space, the pipe has twelve identical, equi-angularly spaced cylindrical passages such as passages  29 - 32  which are incorporated into the wall  28 , and which run along the length of the pipe. Those passages constitute the plurality of passages which can receive any petrol which leaks through a breach in the skin  26  and can convey the leaked petrol to a remote sensing device in a similar fashion to the pipe shown in FIG.  1 . 
     There are certain important features concerning this embodiment. In FIG. 3 the outer, secondary layer  28  maybe uniform except for the passageways  29 - 32 . This need not be the case, for example, the inner body  28  of the secondary pipe is foamed, which further assists the passage of fluid into the passageways  29 - 32 . 
     Since layer  28  is foamed it has an unfoamed region or discrete layer  33  which serves as an outer skin around the outer circumference of the pipe. This will act as a protective layer or skin when the pipe is buried. 
     The pipe shown in FIG. 4 is also a co-extrusion having an inner nylon skin  34  and polyethylene wall  35 . However, in this case, the wall for the pipe comprises a solid outer polyethylene layer  36  spaced from the skin  34  by a series of corrugations, for example,  38 - 42  which extend radially from the skin  34  to the layer  36  and define axial channels  43  extending along the length of the pipe, These channels provide the further passage means of this embodiment of pipe. 
     Corrugations are just one way of forming the passageways in this embodiment. In fact, any convoluted wall extending between the inner supply pipe  35  and the outer containment pipe  36  could be employed. 
     The corrugated version is formed through the die design of the tool. This is similar to the product with holes in it. The only difference is that the holes are bigger and shaped differently. The materials can again be any plastic materials that can be extruded. 
     The terms corrugated and convoluted have very broad meanings in this context. They refer to any curvi-linear partition between the inner supply pipe and outer containment pipe. It is not necessary that the corrugations or convolutions touch the pipe walls at every approach. In fact, there may be positive advantages to leaving spaces at alternate corrugations. 
     In the illustrated example, the pipe is of 63 mm in diameter, the thickness of each of the inner and outer layers  34  and  36  is in the range 0.25 mm to 5 mm and the thickness of the web forming the corrugations is in the range 0.5 mm to 4 mm. 
     The relative thicknesses of the various layers can vary according to particular application. The example given below is for the case where petroleum products are to be conveyed by the pipe. In this case, the preferred inner barrier material must have very low permeability to petroleum spirit and the various chemicals found in petrol and diesel fuels. Nylon 612 is one of the preferred materials. However, other materials can be used and these include polyamides, polyamides 6, 11 or 12, polyethylene terphthalate, polyvinyl chloride, polyvinylidene chloride or fluoride, polypropylene, ethylene/vinyl alcohol copolymers, or mixtures thereof, the selection being based on the nature of the fluid being conveyed. 
     This selection is not intended to be limiting but rather demonstrates the flexibility and breadth of the invention. The plastics material with the lowest permeability to the fluid in question will usually be chosen. Furthermore, it is known to use blends of two or more polymers and this invention extends to cover known and yet to be developed blends of plastics material. 
     Preferably the outer protective layer comprises a plastics material selected from the group comprising: 
     Polyethylene 
     Polypropylene 
     Polyvinyl chloride 
     or variations and combinations of plastics material specified for the inner barrier layer. 
     In a particularly preferred embodiment the inner layer comprises Nylon 612 or polyvinylidene fluoride and the outer layer comprises linear low density polyethylene. The choice would be made on the basis of the permeability through the polymer of the material to be conveyed. 
     The relative proportions and thicknesses of the two layers are important to the performance of the pipe. For petroleum based products, permeability should preferably be zero. However, as all plastics are permeable to a degree, a suitable commercially applicable range is 0.2-4 g/m 2 /day. 
     In the example where a pipe of 110 mm diameter is intended for petroleum applications, then the thickness of the wall of the supply pipe is typically in the order of 7 mm. Of this the inner nylon layer can be from 0.01 mm to 6.99 mm. Conversely, the outer polyethylene layer could be of a thickness of 0.01 to 6.99 mm. 
     In a preferred embodiment the nylon layer is 0.05 mm and the polyethylene layer is 6.95 mm thick. 
     In certain cases adhesive is provided between the two layers. Once again, the adhesive can take a variety of forms and will depend largely on the inner and outer polymers or the polymers being joined. One example of a suitable adhesive is maleic anhydride modified polyethylene. Alternatively, it can take the form of a polymer blend of the two polymers to be joined. 
     Pipes according to the present invention can be used to carry a wide variety of fluids, which term includes both gases and liquids.