GB2340197A - Flexible pipe for conveying fluids - Google Patents

Description

2340197 IMPROVED PIPE FOR CONVEYING FLUIDS

Field of the Invention

The present invention relates to pipes for conveying fluids. It is particularly applicable to but in no way limited to pipes for use in conveying hydrocarbon liquids and gases. Such pipes may be used to convey petrol from a reservoir to a dispensing pump and a filling station.

Background to the Invention

The use of dual containment piping systems in which an inner supply pipe is coaxially 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 to the fuel supply pipelines. The secondary containment pipeline prevents leaks from 1 the fuel supply pipeline from being discharged into the environment, and also can convey leaked petrol to a remote sensing device.

The pipes forming the secondary containment pipeline may be separate from the fuel supply pipes and are sleeved over the latter as the fuel pipes are installed between the fuel storage tanks and dispensing pumps. Alternatively, the combination of primary and secondary pipes can be created during the manufacturing process, being delivered to site as a complete unit.

A wide priority of secondary containment systems are available. Examples are the ENVIROFLEX (TM) piping system available from Total Containment Inc of Exton PA USA and the PERMA-FLEXX (TM) system available from Containment Technologies Corporation Minneapolis USA. A further example is the UPP (TM) 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.

Secondly, whilst the primary supply pipe contains a liner which is practically impermeable to hydrocarbon fuels, the secondary containment pipe generally does not. In fact, the secondary containment pipe is usually of a single wall, single component construction made typically from polyethylene or fibreglass. It follows therefore that in the event of a serious leak which floods the secondary containment 2 system some fuel may eventually escape into the environment over time because the secondary containment pipe is slightly permeable to hydrocarbons.

Thirdly, the primary pipe tends to be of rigid or semi-rigid construction. A typical example is described in US 5227130 (Webb) which illustrates various multi- layered, straight-sided pipes. Whilst these are described as flexible or semi-flexible, the degree of flexibility is restricted as a direct consequence of their construction. External longitudinal ribbing, where it is present, does nothing to improve flexibility and probably has the opposite effect of stiffening the pipe.

It is the object of the present invention to overcome some or all of these disadvantages.

Summary of the Invention

According to a first aspect of the present invention there is provided a flexible pipe for conveying fluids comprising:

(i) an inner barrier layer substantially impervious to the fluid to be carried which forms an internal surface of the pipe and which is comprised of a first polymeric plastics material; (ii) an outer protective layer forming an outer surface and which is comprised of a second polymeric plastics material; wherein the pipe is corrugated on at least the external surface.

Preferably the pipe is corrugated on both the internal and external surfaces.

This novel configuration gives a pipe with excellent permeability resistance and improved flexibility.

Preferably the inner barrier layer comprises a plastics material selected from the group comprising:- 3 nylon 612 polyamides polyamides 6, 11 or 12 polyethylene terphthalate polyvinyl chloride polyvinylidene chloride/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 layer.

In a particularly preferred embodiment the inner layer comprises Nylon 612 or polyvinylidene fluoride and the outer layer comprises linear low density polyethylene.

4 In a further embodiment an adhesive layer or polymer blend is provided between the inner barrier layer and the outer protective layer. A suitable material is maleic anhydride modified polyethylene. In principle, any suitable adhesive layer can be used as selected by the materials specialist.

According to a second aspect of the invention there is provided a secondary containment pipework system comprising an inner supply pipe and outer containment pipe with an annular gap there between wherein both the supply pipe and the containment pipe comprise flexible corrugated pipes as described herein.

Preferably the pipe comprises flexible pipes wherein the inner layer comprises Nylon 612 and the outer layer comprises linear low density polyethylene.

Description of the Drawings

The invention will be further described, by way of example only, with reference to the accompanying drawings wherein Figure 1 illustrates a typical cross-section of a double walled pipe according to a first embodiment of the invention; Figure 2 illustrates cross-sectional views of various strips from which the pipe of Figure 1 is formed; Figures 3 and 4 illustrate views of sections of various corrugated pipes according to the present invention.

Description of the Preferred Embodiments

The present embodiments represent currently the best ways known to the Applicant of putting the invention into practice. But they are not the only ways in which this could be achieved. They are illustrated, and they will now be described, by way of example only.

The use of spiral wound technology is well known in the pipework industry with typical applications being that of hose products, venting and ducting applications as well as corrugated pipe for land and sewer applications. It has unexpectedly been discovered that this technology can be used to make a new type of pipe for carrying a variety of fluids. In a preferred embodiment adapted for the transport of petroleum products, the pipe consists of just two layers, an inner layer made of nylon and an outer layer of polyethylene. The process consists of the following steps:

Stage (i) Co-extruding a stdp of polyethylene and nylon together to form a strip of polyethylene with a layer of nylon on one side.

Stage (ii) Feeding this strip product into a die head in the form of a pipe which preferably has a spirally wound or corrugated geometry.

Stage (iii) Heat fusing the wound strips to form a pipe optionally with a polymer blend as adhesive between the strips.

The various stages of the process and the materials of construction will now be described in more detail.

Stage (i) The ribbon-like strips from which the pipe will ultimately be formed can take a variety of shapes and forms. Several of these are illustrated in Figure 2. It is intended that the preferred size and shape of the strip will be determined by the 6 materials specialist and will depend in part on the composition of the various polymers used. In these particular examples, the strip consists of just two polymers, polymer 10 which is destined to form the inner, barrier layer of the pipe, and polymer 12 which is destined to form the outer protective layer.

The relative thicknesses of these two layers can vary according to the 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, polypropylene, ethylene/vinyl alcohol copolymers. This list is not intended to be exhaustive or limiting. Rather it gives an indication of the wide type of materials which can be chosen. The choice would be made on the basis of the permeability through the polymer of the material to be conveyed.

Figure 28 illustrates a further form of strip which can be wound and fused into a pipe. These strips have the advantage that they are interlocking and this interlocking arrangement is shown in perspective view in Figure 3A.

Strip 20 consists of an inner layer of nylon 10' and an outer layer of polyethylene 12'formed into a castellated arrangement. This could be regarded as a channel arrangement with a depending lip extending downwards from the top of one of the channel side walls. The upwardly extending side wall of one strip fits snugly within the cavity created by the side wall/lip of another strip. Where spiral wound technology is used these are opposite sides of the same strip wound around a former or mandrel.

7 In certain cases the strip can be fused or heat welded to itself. However, it is more often the case that adhesive or a polymer blend is applied to the areas of contact. This is shown diagrammatically in Figure 3B.

The resultant pipe has a continuous, substantially smooth inner surface formed from one polymer, in this case nylon 10', and a ribbed or corrugated outer surface formed from polyethylene 12'. This interlocking configuration imparts considerable strength. It provides, for the first time, a pipe which is ribbed or corrugated around the circumference on the outer surface but smooth on the inside.

Many variations on this theme are envisaged. One such variation is shown in Figures 2C, 4A and 4B. In this case two strips of different shapes are used. Strip comprises a generally channel-shaped profile with a convoluted central region in the base of the channel where the strip is doubled back on itself. This creates a form of keyhole shape in cross-section which improves flexibility in the finished pipe.

Rather than interlocking, these strips are designed to fit together in a side-by-side arrangement.

Strip 31 takes the form of an inverted channel with the inner part of the channel designed to fit snugly over the edges of two adjacent side-byside strips 30.

Once again, adhesive or polymer can be used to complete the welding or pipe formation process.

This arrangement offers several advantages. Strip 31 can be formed from the same material as the outer region of strip 30. This means that one plastics material is being bonded to the same material during manufacture. This generally results in a stronger bond.

8 Importantly in this context, the flexibility of the pipe is much improved. The convoluted regions can flex apart and together much more readily than plain corrugations and this adds significant elasticity.

It will be appreciated that this is just one of many such arrangements which can achieve this end result. It could also be achieved using the interlocking arrangement shown in Figures 3A and 3B by introducing a convoluted region along the face which will ultimately form the inner pipe wall.

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 1-4g/m2/day.

In the example where a pipe of 110 mm diameter is intended for petroleum applications, then the thickness of the wall 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 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 (14) is provided between the two layers (10 and 12). 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.

Stages (ii) and (iii) 9 The method of heat fusing a spirally wound strip into a pipe is known per se and optimum conditions will be determined by the materials specialist/machine operator.

This technology is not the only way in which pipes according to the present invention can be made. Where the pipe is already formed in a spiral or corrugated form, subsequent processing is not necessary. However, forming corrugations in a straight-sided plastics pipe is a process known per se. One method of achieving this result is to heat the pipe, pass it inside or over a corrugated former or mandrel, then apply pressure to the pipe to force it against the former or mandrel. The resulting pipe adopts the shape of the former.

The term corrugated in this context has a broad meaning. It is intended to encompass any regular or irregular variations in pipe wall cross section which enhances the flexibility of the pipe over and above the uncorrugated form. For example, the shape of the corrugations in the pipe could be spirally wound or ribbed or any configuration where the flexibility of the product is enhanced by the subsequent or performed ribbing.

The present invention is intended to encompass other methods of preparing a pipe of this type. For example, the two polymers could be co-extruded and then corrugated using conventional techniques.

It has unexpectedly been discovered that pipes of the type described above can be used to create a secondary containment system by threading one pipe within another such that they are substantially concentric. In this context concentric and coaxial have broad meanings and include the broad concept of "one pipe substantially within another" as well as usharing a common axis". Where the internal diameter of the containment pipe is significantly greater than the outer diameter of the inner pipe then obviously the two will not be strictly concentric or coaxial. The inner pipe will simply rest on the boftom of the inner surface of the outer pipe.

The resulting combination of the two pipes presents a number of 5 advantages.

(1) The combination is virtually impermeable to fluids.

(2) The internal ribbed effect of the containment pipe presents considerably less frictional resistance when the inner pipe is inserted when compared with a corresponding unribbed pipe. This makes construction easier, particularly when the primary pipe is inserted on site.

It would be appreciated that pipes according to the present invention can be used for conveying gases as well as liquids.

In summary, the pipe product can be made from polyethylene with a nylon barrier layer. A typical product could involve a LLIDPE (linear low density polyethylene) with a nylon 612 barrier layer. The process involves co- extruding a strip of polyethylene and nylon together to form a strip of polyethylene with a thin layer of nylon on one side. This product is fed into a die head which has a spirally wound geometry in the form of a pipe. The pipe product is therefore formed as a result. In a further optional step corrugations may be formed along the pipework if they are not already present.

The advantages of the product is that the combination of the nylon and the polyethylene provides a dynamic relationship in the following properties:

The nylon has excellent resistance to the chemical and permeation of alcohol and alcohol blends 11 0 The polyethylene has excellent permeation resistance to hydrocarbons and chemical resistance to salts, alkalis and other chemical media 0 The flexibility of the polyethylene material in a corrugated form 5 provides a user-friendly product for installing these products on site 0 The addition of the nylon barrier provides additional toughness to the pipe as well as providing the pipe with excellent permeability resistance over traditional corrugated pipe products 0 Permeability figures for secondary pipe products are typically of the order of 12-2ogIM2/day. This product has values less than 5g/M2/day.

1. For a 11 Omm pipe with a wall thickness of 7mm the nylon could be of a thickness of 0.01 - 6.99mm. Conversely the polyethylene could be of a thickness of 0.01 - 6.99mm.

2. The shape of the corrugated pipe could be spirally wound or ribbed or any configuration where the strength of the product is improved over the ribbed version.

3. The shape of the pipe does not have to be circular but may be of any regular cross-section.

4. Any grade of polyethylene is suitable although LLDPE is preferred.

5. Any grade of nylon is suitable, however nylon 612 is preferred.

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Claims (11)

1 A flexible pipe for conveying fluids comprising:- (i) an inner barrier layer substantially impervious to the fluid to be 5 carried which forms an internal surface of the pipe and which is comprised of a first polymeric plastics material; (ii) an outer protective layer forming an outer surface and which is comprised of a second polymeric plastics material; wherein the pipe is corrugated on at least the external surface. 10

2. A flexible pipe according to Claim 1 wherein both internal and external surfaces are corrugated

3. A flexible pipe as claimed in Claim 1 or Claim 2 wherein the inner barrier layer comprises a plastics material selected from the group comprising:nylon 612 15 polyamides polyamides 6, 11 or 12 polyethylene terphthalate polyvinyl chloride polyvinylidene chloride 20 polypropylene ethylene I vinyl alcohol copolymers the selection being based on the nature of the fluid being conveyed.

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4. A flexible pipe as claimed in any preceding claim wherein the outer protective layer comprises a plastics material selected from the group comprising:

polyethylene polypropylene polyvinyl chloride

5. A flexible pipe as claimed in any preceding claim wherein the inner layer comprises Nylon 612 and the outer layer comprises linear low density polyethylene.

6. A flexible pipe as claimed in any preceding claim wherein an adhesive layer is provided between the inner barrier layer and the outer protective layer.

7. A flexible pipe as claimed in Claim 6 wherein the adhesive layer is comprised of polyvinylidene chloride.

8. A flexible pipe substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

9. A secondary containment pipework system comprising an inner supply pipe and outer containment pipe with an annular gap there between wherein both the supply pipe and the containment pipe comprise flexible pipes as claimed in any of Claims 1 to 8 inclusive.

10. A secondary containment pipework system according to Claim 9 wherein the pipe comprises flexible pipes as claimed in Claim 5.

11. A secondary containment pipework system substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

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