Material for use in lining pipes

Abstract

A material for use in lining pipes, the material comprising: a first set of inlay yarns (28, 30, 82, 94), each yarn of which has a path that, relative to a machine direction of the material, extends

Description

This invention relates to flexible lining material, in particular to a material that is used in the repair or rehabilitation of pipes, conduits and the like. Such a material is expected to find particular application in the rehabilitation of domestic and industrial water pipes, sewage pipes, steam pipes and others that are commonly found underground and involved in the delivery of services to urban locations. The network of pipes in the modern environment is extensive. Mains pipes carry large volumes of water to a neighbourhood; other pipes lead from the mains to, for example, a street and from there smaller pipes divide out and convey fresh water for use in individual buildings and houses. Drainage pipes carry in reverse: from within buildings they take domestic waste to underground drains and thereafter to public sewers. It is invariably the case that a pipeline leak is problematic. It allows not only the leak of material out of the pipe (exfiltration) but also, in the case of underground or underwater pipes, the flow of substances into the pipe (infiltration). Both are undesirable. Sewage pipes may, for example, leak harmful waste products into the soil. A freshwater pipe on the other hand must remain watertight to prevent contamination of the water supply from the environment in which the pipe is laid. Leaks from domestic pipes can cause damp, mould and noxious smells. A burst water main can severely disrupt activity in its locality. A pipeline leak must therefore be repaired quickly. The cure in place pipe (CIPP) methods of repair involve insertion of a resin-soaked tubular textile material into a pipeline. The resin is then cured in situ to form a composite structure of textile-reinforced hardened resin. There are a number of known CIPP installation procedures, which can be broadly categorised into pull-in-place and inversion installations. Both procedures are commonly used in the repair and rehabilitation of underground pipes as it avoids the environmental impact of excavation trenches that are otherwise needed to access the damaged pipeline. The relatively light weight of the fabric lining material and general ease in handling means that the CIPP method of repair is increasingly being used to repair larger diameter pipes and offshore pipelines such as found in the oil and gas industry. A pull-in-place installation requires access at both ends of the pipe to be repaired. A cable is run along the length of the pipe between access points. A resin-soaked liner is attached to one end of the cable and then pulled carefully from one access point towards the other until it reaches the required repair position. An inflatable balloon, known as a calibration hose or tube, is used to sequentially inflate the resin-saturated liner from one access point to another and to press it tightly against the existing pipe in preparation for curing. In carrying out an inversion installation use is primarily made of a single access point The liner for this installation will have a foil or film coating on the outside and is impregnated with resin on its inner surface. The liner is wound into an inversion unit, with its trailing end attached around the exit nozzle. A high-pressure driving mechanism such as compressed air or water is blown into the liner, driving it out of the nozzle, along the pipe and inverting it in the process. Simultaneously, the driving fluid presses the now inverted liner against the pipe inner surface. Limited access is required at the sealed end once the liner is in place. This must only be sufficient however for the end to be opened and avoid blocking the pipe. Each cure-in-place installation process has its own advantages and disadvantages. Regardless of which is used however, the liner is held against the pipeline as the resin is cured. After curing, the pressure is released and any insertion devices, bladders or calibration tubes removed, leaving a cured-resin tubular repair on the inner surface of the original pipeline structure. A variety of materials can be used to form the textile base layer. This textile layer must be able to retain the resin as the liner is manoeuvred into position for curing. It should also be flexible, chemically-resistant and durable to enable it to withstand the installation process. Liners based on knitted, felt, and woven materials have all been successfully used in pipe repair. In addition to the installation process, the environment in which the liner is to be located also places demands on the physical properties of the fabric. Increasingly, CIPP installations are being used in high pressure systems, with a consequent need for a strong fabric material that adds substantial reinforcement to the cured resin. For example, in many countries rising mains convey water at an average 12 bar pressure, which is beyond the reinforcing capability of many fabrics. Moreover, pipes and liners located underground will experience repeated compression if vehicles pass along a road above. The pipe’s structural integrity is weakened under compression, and the ability of lining materials to tolerate such wear is another demand that may be placed on their performance. For other repairs, it is the flexibility of the liner that is paramount. A typical pipeline network includes many bends, often of up to 90°, and numerous junctions. Not only are these more difficult to repair, but they tend also to be sources of pipe weakness, rendering them more prone to leaks in the first place. Pipe material is generally weakened as it is formed into a bend and junctions or couplings must be correctly fitted to avoid leakage at the outset, in addition to being susceptible to damage through use. Many liner materials lack the flexibility to line around bends: they tend to wrinkle severely at the inside bend, which disrupts flow, and to detach from the pipe wall at the outside bend. A flexible, expandable liner is better able to conform to pipe bends and changing diameters within a complex pipe network. There is a fundamental conflict however between material strength and flexibility: improving one, for example by adding glass fibres, tends to adversely affect the other, for example by increasing rigidity. Compromise is therefore inevitable. Moreover, additional challenges arise when physical requirements of an installation procedure are taken into account. Minimal, or at least predictable, elongation is desirable, particularly in a close-end installation. Accurate location of the far end of the fitted liner is important when limited access may be available to open it up. Another factor to be considered is the resilience of the liner material: it should retain structural uniformity as far as possible, despite the forces involved in the installation process. This avoids points of weakness developing within the repaired section. Fabric properties may dictate the installation process that is used for cure-in-place pipe repairs. The stronger liner materials are generally limited to pull-in-place installations, as these fabrics tend to lack the flexibility to undergo the inversion process. One current material that is installed with a pull-in-place process is a fabric of polyester fibres needled into a dense felt. As alternatives, polypropylene or polyethylene fibres may be used, either in combination with polyester or alone, and the felt may further be reinforced with a strengthening material such as glass fibre. On the other hand, fabrics that are designed to line around bends and to cope with junctions between different diameter pipes are more likely to be installed using an inversion process. Inversion installations have fewer restrictions on the pipe network they are used to repair: principal access is required only at one end and the pipe network can be more complex with a variety of bends and junctions. A warp-knitted fabric that is designed for use in this application is described in WO2015 / 166268. A knitted fabric, such as this, exhibits high flexibility and conformability, which in turn allows it to be more readily inverted when carrying out an installation procedure. Composite materials in which a polymer matrix is reinforced with a fibre material are well known in, for example, the automotive, aerospace and sports sectors. In these industries it is known that reinforcement with a multiaxial fabric provides a number of advantages as regards tailoring the fabric properties towards the application. A multiaxial fabric is one in which multiple layers of unidirectional fibres are built up, with each layer having fibres oriented along a particular axis. The layers are typically stitched to form a fabric. Because the reinforcing fibres can be oriented along different axes, the strength and stiffness contributed by the fabric to composite performance is similarly multi-directional. Moreover, in comparison to woven fabrics, multiaxial fabrics avoid the “crimp” of going over and under perpendicular fibres. This makes the fibres inherently straighter and therefore stiffer under tension, resisting elongation. A fabric that is used to reinforce a hose structure is described in WO 2014 / 030018. This material is based on sets of inlay yarns that extend in a zig-zag pattern and which are held together by knitted stitches to form a fabric. This fabric reduces the compromise made between strength and conformability in the hose structure: the inlay yarns are responsible for the strength of the reinforcing fabric and they are held in place by knitted yarns that offer the potential for good conformability. The specific application of this material is to reinforcement of a hose that is to carry high-pressure fluid, such as in a vehicle engine. In such applications, the cylindrical body of the hose experiences a stress in the hoop direction that is twice that experienced in the axial direction. In this particular fabric, the orientation of the inlay yarns can be set such that when configured in a cylindrical geometry, the hoop reinforcement is twice that of the axial reinforcement. That is, the inlay yarns extend at an angle to the longitudinal axis of the cylinder that is close to 54.7°, the critical balanced angle for fibre wound pressure vessels. This corresponds to the situation in which fibres are inclined just enough away from the circumferential direction to make the hose twice as strong circumferentially as it is axially. This helps prevent snaking as well as excessive wear and tear on clamped or otherwise weakened regions of the hose as it is subject to repeated pressurisation. To date, liners reinforced by multiaxial materials lack the flexibility for inversion installations and known inversion liners lack the strength required for use in high-pressure and other more extreme pipeline environments. There is accordingly a requirement for a new type of liner material that better combines the attributes of strength and flexibility. Such a material would lend itself to repair applications in high-pressure pipelines that are not suitable for pull-in-place installations, as well as forming a robust structure that may be used for any CIPP repair. At present, there is an increasing demand for liners that are capable of lining pipes of longer length and larger diameter. However scaling up production of tubular liners has proven problematic. In particular, reinforcing fabrics are machine produced and the size of the machine limits at least one dimension of the resultant fabric. Increasing machine size is possible but such retooling is generally prohibitively expensive. There is therefore a further need for a fabrication process that is capable of producing tubular liners with an increased range of diameters and that may be fabricated in longer lengths. According to a first aspect of the invention, there is provided a material for use in lining pipes, the material comprising: a first set of inlay yarns, each yarn of which has a path that, relative to a machine direction of the material, extends at a first oblique angle (+a) to the machine direction for a first distance (L) and then at a second oblique angle (-a) to the machine direction for a second distance (L),; a second set of inlay yarns each yarn of which has a path that is similar to a yarn path of the first set of inlay yarns but displaced a first proportion of the first distance (L) in a direction parallel to the machine direction; and a set of warp-knitted yarns that interlink with the inlay yarns; wherein the magnitude of the oblique angle (±a) is less than 54.7°. In this aspect, the present invention provides a fabric that is strong, flexible and conformable, such that it can be incorporated in a liner for cure-in-place pipeline repairs. In particular it is sufficiently flexible to be suitable for pipe lining using an inversion installation. Preferably, the first and second distances (L) are substantially equal and the first (+a) and second (-a) oblique angles are of similar magnitude and in opposite directions. In a second aspect, the present invention provides a liner for pipe repair or rehabilitation, the liner comprising: a warp-knitted carrier tube; wrapped by one or more layers of an inlay material, each layer of the inlay material being aligned such that its machine direction corresponds with a longitudinal direction of the carrier tube, and comprising a first set of inlay yarns, each yarn of which has a path that, relative to the machine direction of the inlay material, extends generally linearly at a first oblique angle (+a) to the machine direction for a first distance (L) and then generally linearly at a second oblique angle (-a) to the machine direction for a second distance (L), the first and second distances (L) being substantially equal and the first (+a) and second (-a) oblique angles being of similar magnitude and in opposite directions; a second set of inlay yarns each yarn of which has a path that is similar to a yarn path of the first set of inlay yarns but displaced a first proportion of the first distance in a direction parallel to the machine direction; and a set of warp-knitted yarns that interlink with the inlay yarns. In a further aspect, the present invention provides a method of producing a fabric to reinforce a resin, the method comprising: (a) Determining required hoop expansion of the fabric such that when in a tubular structure, the fabric is expandable from a first diameter to a second diameter; (b) Calculating an inlay angle a for the fabric, from a = sin-1 ....... where e is the ratio of the second diameter to the first diameter; (c) Setting a warp knitting machine to knit a fabric such that a number of rows per unit length in the machine direction approximates a gauge of the machine multiplied by tana; (d) Setting a first bar of the knitting machine to produce a first inlay in which yarn paths move one needle position to the right for each successive stitch-forming process for a first number of such processes and then one needle position to the left for each successive stitch-forming process for a second number of such processes, the first and second numbers being equal; (e) Setting a second bar of the knitting machine to produce a second inlay of the same pattern as the first inlay and that is displaced in the machine direction from the first inlay by a first proportion of the first number of successive stitch forming processes; (f) Setting a further bar of the warp knitting machine to produce a warp-knitted yarn that interlinks with the first and second inlays in an arrangement sufficiently loose to allow yarns within the first and second inlays to move relative to each other; (g) Loading the first and second bars on the warp knitting machine with an inlay yarn and the further bar with an interlinking yarn; and (h) Operating the machine to produce a fabric. The invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figures 1a, 1b and 1c illustrate schematically a method of construction of a multilayer fabric in accordance with this invention; Figure 2 is a representation of a multilayered liner that incorporates at least one layer of a textile material in accordance with this invention; Figure 3 is a representation of a fibre inlay structure that is laid down using one bar of a warp-knitting machine during fabrication of a textile material in accordance with this invention; Figures 4a, 4b, 4c, 4d and 4e illustrate schematically different layers in a fabric as they are built up in a 5-bar knit in accordance with this invention; Figures 5a and 5b are photographs of a layer of the textile material of this invention in, respectively, an unstretched configuration and a configuration adopted when stretched along a machine direction; Figure 6 is a flow chart illustrating the steps involved in designing an embodiment of a fabric in accordance with this invention, the fabric having particular expansion characteristics; Figure 7a is a graph indicating the results of modelling selected physical characteristics of one embodiment of a tubular liner fabric in accordance with this invention, this embodiment being suitable for lining pipes of relatively uniform diameter; Figure 7b is a graph indicating the results of modelling selected physical characteristics of an alternative embodiment of a tubular liner fabric in accordance with this invention, this embodiment being suitable for lining pipes with diameter changes of up to 50%; Figure 8 is a simplified binding diagram illustrating a knitting structure used on each bar for a preferred embodiment of the textile material in accordance with this invention; Figure 9 is a simplified binding diagram illustrating a knitting structure used on each bar for a second preferred embodiment of the textile material in accordance with this invention; and Figure 10 is further simplified binding diagram illustrating a knitting structure. With reference to Figures 1a to 1c, there is shown a method by which a multilayered tubular liner is assembled in a manner that can be adapted to fabricate longer lengths and larger diameters. A carrier tube 12 is used as a base support layer. The carrier tube 12 may be any of a number of materials that can be formed into a tube of an appropriate size. That is, it may be a tubular film, tubular fabric or a fabric sheet that is made tubular by sewing a longitudinal seam. It may further have additional properties that contribute to the performance of a finished liner. For example, the carrier tube 12 may be an extruded plastic tube that provides a waterproof layer to the finished liner. What is important though is that the carrier tube 12 should not hinder any performance requirements of the finished liner product. In particular, as this present disclosure is specifically concerned with liners to be installed by an inversion process, the carrier tube should be flexible and conformable to at least the requirements of the finished product. In the present embodiment, the liner is a flexible glass fibre knitted tube supplied by Scott &Fyfe Limited, described in WO2015 / 166268, and that is used in their AlphaDuct™ product. This product is a seamless liner that is designed with the flexibility necessary for inversion installations and that is conformable so as to line multiple 90° bends and I or diameter changes of up to 50% in a pipeline network. The carrier tube 12 must also be capable of supporting the liner as it is manipulated through the fabrication process. It is dimensioned such that its length and (inner) diameter correspond with those of the finished liner product. The liner itself is fabricated from flat sheets 14a, 14b of fabric material. Each sheet is machine produced to a width that is slightly greater than half the circumference of the finished liner product. The length of each fabric sheet corresponds with that of the carrier tube and of the finished product. The fabric material 14a, 14b may be any of a number of fabrics that are appropriate for use in lining pipes. The fabric may be selected with regard to installation method or application. In this embodiment, it is preferred that the fabric is a novel material that is designed to satisfy the strength requirements for use in high-pressure pipelines, such as those that convey potable water, as well as possessing sufficient flexibility and conformability to be installed in complex networks by an inversion process. The structure of this fabric will be described later but it is emphasised that the presently-described method is not limited with regard to liner fabric material. A first stage of the liner assembly process in shown in Figure 1a. The carrier tube 12 is flattened, as shown in the cross-sectional view of this figure. One side of each fabric sheet 14a and 14b is coated with adhesive and the sheets 14a, 14b are positioned such that one 14a is above the tube 12 and the other 14b below, with the adhesive between sheet 14a, 14b and tube 12. Each fabric sheet 14a, 14b is aligned along one side longitudinally with a respective side of the flattened carrier tube 12. This leaves a first short length 16a of fabric extending to one side below the tube 12 and a second short length 16b extending to the other side above. In this configuration, the layered material is passed through a flatbed laminator. The laminator uses pressurised heating and cooling of the structure to melt the adhesive and to seal the fabric sheets 14a, 14b to the carrier tube 12. On leaving the laminator, the combined structure is re-folded using a former. The former is a pyramidal-shaped element that separates the closed faces of the flattened carrier tube 12 whilst the original longitudinally folded edges are pressed inwards. The result is shown in cross-section in Figure 1b. A new fold is made at what was formerly a central position on the flattened tube (labelled with B and D on Figures 1a and 1b) and the tube is re-flattened at its original fold (labelled with A and C in Figures 1a and 1b). The tube 12 has been reoriented from being flattened in a horizontal direction (Figure 1a) to being flattened in a vertical direction (Figure 1b). As is apparent from Figure 1b, this leaves the first 16a and second 16b short lengths of each fabric liner sheet 14a, 14b overlapping edge portions of the other fabric liner sheet 14b, 14a. In this refolded orientation, the combined structure is rotated through 90° and a second pair of liner sheets 18a, 18b, one side coated with adhesive, are placed respectively above and below. Alignment of this second pair 18a, 18b of fabric sheets is the same as that of the first pair 14a, 14b. That is, one edge of each sheet 18a, 18b is aligned with a respective (new) folded longitudinal edge of the combined structure, leaving short overlaps of the fabric at the opposite folded edge. This configuration is shown, viewed along the length of the carrier tube 12 and liner structure, in Figure 1c. As is apparent in the Figure, the short lengths 16a, 16b of the first pair of fabric sheets 14a, 14b form a region of the liner structure that has an additional layer of thickness that extends longitudinally along the flat faces of the flattened liner. The assembly as shown in Figure 1c is then passed again through a flatbed laminator. This may be the same flat-bed laminator as for the previous pass, or it may be a different laminator positioned in line with the first. After this process, the second pair of fabric sheets 18a, 18b is sealed to the first pair 14a, 14b and the combined liner structure has been built up by an additional fabric layer. The second pair of fabric layers 18a, 18b may be the same or different from the first pair of fabric layers 14a, 14b, either in material or structure. This feature provides the opportunity for tailoring the structure of the assembled liner product to best suit the requirements of any particular application. The above process is then repeated with this laminated 2-layer liner structure. It is passed through a former, which may or may not be the same former as carried out the previous refolding. Regardless of whether it is reused or not, the former this time refolds along a different fold line that, ideally, is displaced from the original fold line. A further pair of fabric sheets 20a, 20b (see Figure 2), coated with adhesive, is positioned above and below the flattened 2-layer structure, with the assembly then being passed again through the laminator. By these steps, a third fabric layer is added to the liner structure. By repeating the steps of forming, layering and laminating, the liner structure can be built up to the thickness required, with properties tailored by adjusting individual layer fabric type and characteristics, if desired. An exemplary completed 4-layer liner structure 10 is shown in Figure 2. As indicated, pairs of fabric sheets 14a, 14b; 18a, 18b; 20a, 20b; 22a, 22b are sequentially built up on the carrier tube 12. In each case, the fold position of the carrier tube extends longitudinally along the length of the tube, but is positioned differently about its circumference. In consequence, the overlap formed where the short length 16a, 16b of one sheet 14a, 14b overlaps the corresponding second sheet of its pair is also staggered around the circumference. The resultant liner structure therefore deviates from uniformity only insofar as it includes a single additional layer at certain, separated regions of the liner. The variation in liner profile is therefore kept small, and the liner is relatively symmetric physically. This, in turn, leads to more uniformity in liner performance. Importantly, the fabric properties are not affected by a singular seam: although a sewn seam may be relatively strong, it is still significantly weaker than the rest of the fabric. With this method of fabrication, the inherent structural weakness created by the seam is mitigated by its distribution around the liner. In an alternative arrangement, the four liner layers may be applied by alternating between two fold lines, oriented at 90°. In order to minimise the non-uniformity in cross section, the overlaps should extend in opposite directions from the fold line. That is, the fifth and sixth sheets should be arranged such that their overlap extends from the fold line away from the overlap between the first and second sheets. Similarly, the overlap between seventh and eighth sheets should extend away from that formed between third and fourth sheets. In alternative embodiments, adhesive is not applied to the fabric sheets 14a, 14b; 18a, 18b; 20a, 20b; 22a, 22b prior to assembly. Instead, the fabric sheets are bonded to the structure by means of an adhesive web that is placed above and below the refolded structure, intermediate the structure and fabric sheet that is next to be added. This layered structure is then passed through the laminator and, as before, heat melts the adhesive, bonding the two layers together. The primary advantage of this method of building up the liner structure 10 is that it simplifies the fabrication of liners with longer lengths and larger diameters when compared with previously known production methods. The problem with constructing longer and wider tubular fabrics is that as the size of the tube increases, the fabric becomes increasingly difficult to handle. Not only the dimensions, but also the weight of fabric increases, making it extremely difficult to manipulate though the machinery required to build up each layer. Moreover, such problems are exacerbated in high-strength applications in which glass may be used in increasing proportions. Although glass fibre is particularly strong, it is also a heavier material than alternatives such as polyester, nylon and polypropylene. With this present method however, manipulation of the liner is limited. In particular, if each layer is added using a former and laminator dedicated to that specific layer, the fabrication machinery may all be placed inline and the liner has only to be conveyed from one station to the next as additional layers are added. In one exemplary application, 300 m of a 200 mm diameter tubular fabric may weigh in the region of 888 kg. By using the method of the present invention, this size tubular structure may be built up of a glass carrier tube, weighing 120 kg for the length required and eight fabric sheets, each weighing 96 kg. The individual components are smaller and considerably lighter than the finished product, each therefore being more easily handled. With the method described herein, manipulation of the final tubular product is limited to conveying the fabric, possibly without even changing its direction of travel. This method is further advantageous in that it enables larger-diameter liner products to be fabricated without retooling any machinery. In general, if a flat fabric sheet is formed into a tubular shape, the diameter of the tube is limited by the width of fabric that can be produced on the relevant knitting (or other) machine. Larger machines could be used to fabricate larger fabric sheets, but this would require significant financial investment, which would likely be prohibitive. With this present invention however, tubes of twice the diameter may be obtained without changing the production machinery. In another embodiment, the short lengths 16a, 16b that form the overlap may derive from the same sheet. Or they may simply not be present and the sheet pairs 14a, 14b that are used to build the liner structure 10 are sized such that their longitudinal edges abut each other, rather than forming an overlap. This embodiment retains the feature of the weaker seam region being distributed about the liner structure, without introducing the same degree of asymmetry in liner thickness. It is therefore, similarly, less likely to fail when the liner is subjected to pressure or continued use. In other embodiments, the former may be configured to refold the structure such that the fold points are moved a set number of degrees between each stage of the process. That is, not to start with the 90° reorientation shown between Figures 1a and 1b. For example, successive refolds with a 60° rotation may be used to distribute six fold points around the liner circumference, which is preferred if three layers are to be built upon the carrier tube. With a 45° movement, four layers may be built up with eight distributed fold points. The different refolding positions may be achieved with suitable designs of former. Another significant advantage of the method described herein is that it offers flexibility in building up a layered structure. There is no requirement for all the fabric sheets to be identical, and incorporating different layers may improve overall performance in some way. For example, a structure fabricated from a series of high-strength layers may be particularly strong but it may also have a dense yarn structure that inhibits resin permeation. Ease of resination is, of course, an important factor in overall liner performance. To improve the flow of resin into the liner walls, the structure may include an intermediate layer of a spacer fabric, which more readily draws in resin than the high-strength layers. In an example of a liner constructed for pull-in-place installation, the outer layer may be a film or material that blocks ultraviolet (UV) radiation. Many resins are cured by application of UV light. In pipe repair applications, it is a relatively straightforward process to insert a UV light inside a liner once it is in place and to cure the resin in a controlled manner. By way of contrast, the application of heat to a heat-cured resin is more difficult to control. In particular, many epoxy resins are prepared at the point of application by mixing two components, which have been stored separately. Once mixed, curing will start and this imposes a time-limitation on getting the resin into the liner and the liner into position within the pipe. An outer UV barrier, as incorporated in this embodiment, therefore allows a liner to be pre-resinated with a UV-curable resin, prior to a pull-in-place installation. The barrier layer prevents undesired curing by daylight exposure during transportation to an installation site. Moreover, once at the site, no time limits are imposed on the installation procedure. A first embodiment of a novel design of warp-knitted fabric that better combines the attributes of strength and flexibility that are desirable for pipe liner reinforcement is shown in Figures 3 to 6. This fabric may advantageously be incorporated as one or more fabric layers in a liner structure produced by the method previously described. Figure 3 is a binding diagram illustrating the laying down of an inlay layer 24 using a warp knitting machine. This inlay layer 24, which may be varied from that shown in the Figure, forms the basis of the novel fabric of this invention. As is well known in the field, a binding diagram is a symbolic representation of the movements of the guide bars of a knitting machine as it draws the yarn about needle positions to create a fabric. From this, a skilled operator can produce the same fabric pattern with a suitably configured machine. Each dot within the diagram represents one needle head at one point in time. Each horizontal row of dots represents a series of needles during one stitch forming process. The indicated pattern is replicated across the width of the fabric. The rows of dots from bottom to top represent a succession of stitch-forming processes. The vertical direction of the page thus corresponds with the machine direction of the knitting process and the horizontal axis with the cross direction, as indicted by axes 26. In general, a knitting machine will knit on multiple guide bars simultaneously. The stitching on one bar may be arranged to hold fabric layers produced on different bars together. Alternatively, yarn may pass between the guide bar needle beds, creating an interlinked fabric, without layers. The structure 24 indicated in Figure 3 is not a knitted structure as such, rather it is an inlay layer that is formed on one bar of the knitting machine. Other inlay layers are formed on additional bars, with a final bar forming stitching that extends across all guidebars, linking the inlay layers together. The base layer structure 24 comprises a plurality of inlay yarns 28a - 28j arranged as a 12-1 needle inlay. A 12-1 inlay is one in which the yam moves continually one needle across the course of the fabric for each successive stitch for a total of 12 needles. After moving 12 needles (taking therefore 12 rows) in one direction, the lateral movement is reversed and the inlay proceeds at a reverse angle. The result is that the plurality 28a -28j of inlay yarns follow a sharply alternating course generally proceeding along the machine direction of the fabric. Although only a section of yarns 28a - 28j are shown, this pattern is repeated in yarns that cover the width of the fabric and the zig-zag path followed by each yarn extends fully along the fabric length. In the specific case of exemplary yarn 28i, one guide bar of the knitting machine feeds the yarn 28i along a path that proceeds upwards (in the figure) at an angle of around 45° clockwise to the machine direction for a length L (12 rows), then turns sharply (through about 90°) at point a, and proceeds again generally vertically but at an angle of around 45° anticlockwise to the machine direction again for length L, where another turn is executed and the pattern is repeated. That is, a pattern of triangular symmetry of length 2L and internal angle about 90° is repeated for each yarn along the machine direction. In this embodiment, the oblique angle of the inlays (at point a) is aligned at + 45° or - 45° to the machine direction of the material as it is fabricated, depending on its lateral direction of movement. As the inlay moves one needle laterally for each row stitched, this angle is achieved by setting the knitting machine such that the needle spacing (gauge) and stitch length are equal. For example, a typical setting is 12 stitches per inch in the cross direction and 12 rows per inch in the machine direction. This angle can be varied, for example by changing either gauge or stitch length, and so in order to avoid loss of generality, reference will be made to an angle ± a, which is the angle made by the linear portions of the inlay yarns with the fabric machine direction. It has been found that the preferred value for a is in the range 30° to 50°. Figures 4a through to 4e represent one embodiment of a textile material in accordance with this invention, the Figures each showing a guide bar layer that, together, form the textile structure. With reference to Figure 4a, bar 1 is arranged to form a first inlay layer 30 of ECR glass yarn, following the same 12-1 needle inlay shown in Figure 3. A second inlay layer 32 is, at the same time, formed on bar 2, as shown in Figure 4b. In this embodiment, the second inlay layer 32 is also a 12-1 needle inlay of ECR glass yarn but it is displaced by 6 needle positions in a machine direction. Effectively, its zig-zag pattern is Vi L out of phase with that of the bar 1 inlay 30. In Figure 4c, a third inlay layer 34, also of ECR glass, that is formed on bar 3 is shown as it is positioned relative to the inlays 30, 32 of bars 1 and 2. The third inlay layer 34 is another 12-1 needle inlay that is displaced a further 6 needle positions in the machine direction. Figure 4d shows a fourth ECR glass yarn 12-1 needle inlay layer 36 displaced a further 6 needle positions in the machine direction. Within each layer, the yam bends (e.g. point a in Figure 3) align in linear fashion across the width of the material. In the finished fabric therefore, the four inlay layers 30, 32, 34, 36 are stacked above each other in a symmetrical pattern such that the linear bend regions are evenly stepped in the machine direction. For this embodiment with 4 bars of a 12-needle inlay therefore, one quarter of the yarns are bent every 6 needle positions. Otherwise, the yarns are straight. Figure 4e shows the full structure of this first embodiment 38 of a textile material in accordance with this invention. Bar 5 follows a path 40 that knits a pillar stitch that loops across all four bars to hold the various layers of the structure together. In this fabric 38 therefore, the bar 5 stitches form essentially one-dimensional chains of knitted fabric, each pillar stitch looping round four inlay yarns, one from each layer 30, 32, 34, 36. The oblique inlay yarns interlink the columns of stitching 40. In its application to a pipe-lining material, the fabric 38 described in relation to Figures 3 and 4 is formed into a tubular shape, by the method described above or otherwise, such that the fabric machine direction is aligned with the longitudinal direction of the tube. In this way, at fabrication, the inlay yarns extend at angles ± 45° to the tube’s longitudinal axis. This fabric 38 has a number of properties that are beneficial to inversion installation liner applications. First is its strength. Away from their bends, the yarns in the inlay layers 30, 32, 34, 36, run in a straight line. This is a very strong configuration, as evidenced by multiaxial textiles in which the alignment of unidirectional fibres defines the strongest direction for that particular layer. In contrast to a multiaxial textile, this fabric does not build up multiple layers each with parallel fibres running in a particular direction. Instead, the fibres of each inlay layer 30, 32, 34, 36 are oriented at a selectable angle a to the machine direction. This angle a can be varied in accordance with the directional requirements for strength of the intended application. In comparison with multiaxial fabrics, the fabric of this present invention is not so strong as it does include fibre bends, which are a source of weakness. In particular, glass fibre is a material known for its strength in a straight line and weakness in a bend. However, it has to be borne in mind that it is only when the liner is installed in a pipe that its full strength is required. At this point, the fabric will be embedded in cured resin. The resin tends to support the glass at each bend, which mitigates its weakness. The use of glass fibre inlay is further advantageous in that glass fibres also tend to link more effectively than the alternatives with resin and so are particularly strong when used as reinforcement in a resin composite. When the installation procedure is complete therefore, and the resin-soaked liner cured, the glass fibres will preferentially take up any load placed on the liner material. The strength of this material, when cured, therefore approaches that of multiaxial liners. The orientation of the straight component of the inlay yarns is significant. They provide strength in this direction and resist extension. In the cylindrical geometry of liner applications, strength and resistance to extension can be resolved into longitudinal (axial) and hoop (radial) components. The angle a therefore determines the relative resistance of the cylindrical liner to axial and hoop extension. This is similar to multiaxial fabrics in which the orientation of the fibres within different layers can be used to tune the relative strengths in axial and hoop directions. Despite a slight reduction in its strength when compared with a multiaxial textile, the fabric of this present invention has a number of advantages that multiaxial textiles lack. In particular, it will be understood that a fabric used to reinforce a liner must satisfy different performance requirements during installation than must be satisfied during its subsequent role in lining a pipe. In brief, the liner must be flexible during installation and strong while in use. For the installation procedure, the liner must only be strong enough not to break. In this respect, the weak points represented by the bends are distributed circumferentially within each inlay layer and between layers are staggered in the longitudinal direction. That is, the weaknesses are not concentrated at one point but distributed throughout the liner to reduce the likelihood of any one fracturing. This is sufficient for the liner to be strong enough to withstand the inversion procedure. Flexibility is however of prime importance during installation: the liner must be flexible enough to invert as it is forced into the pipe; it must be able to navigate and conform to bends; and it must be expandable to accommodate changes in pipe diameter. In contrast to multiaxial fabric lining materials, these features are all provided by the fabric of the present invention. The inlay layers 30, 32, 34, 36 are only loosely held by the warp-knit pillar stitching. There is therefore some freedom for the layers to move over one another. Each guide bar produces a planar inlay structure and the surfaces defined by these structures are relatively free to slide over each other, held only by the loose pillar stitching. This feature renders the fabric very flexible, enabling it to undergo inversion. The third property of this fabric 38 that makes a liner so suitable for inversion installations is its expandability. Expansion of a fabric in accordance with this aspect of the invention is shown in Figures 5a and 5b. Fundamental to this property is the fact that the inlay yarns within the fabric are relatively free to reorientate by rotation with respect to the angle a that they make with the machine direction. In Figure 5a, the 5-bar fabric 38 is shown in its unexpanded configuration 44. In this figure, the oblique structure of the inlay yarns is visible, for example as highlighted by dotted lines 46. The inlay yarns can further be seen extending substantially linearly in two directions: a first direction extending from the bottom right to top left of the figure indicated by a first dashed line 48 and a second direction extending from the bottom left to top right indicated by a second dashed line 50. These two directions are aligned at angles of around ±38° to the machine direction. Exemplary parts of the pillar stitching 52 holding the inlay layers 30, 32, 34, 36 together are highlighted with continuous lines 52. Figure 5b shows the fabric 38 in its expanded configuration 56, with parts of the structure corresponding to the highlighted parts 46, 48, 50, 52 of Figure 5a similarly indicated. As can be seen, the inlay yarns 48, 50 have rotated away from their earlier alignment and are now oriented closer to the fabric cross direction. The two inlay directions now intersect of an angle of around 120°. This movement is facilitated by the loose pillar stitching 52, each stitch expanding to allow the inlay yarns that it holds to move relative to each other. This reorientation of the inlay yarns 48, 50 has two significant effects. First, with the fabric oriented with its machine direction corresponding to a longitudinal direction of a cylindrical liner, the expansion shown in Figures 5a and 5b correspond to a radial expansion of the cylinder. Glass fibre itself is strong and not compressible. The increase in circumference is accommodated by a realignment of the glass inlay fibres. A rotation from an angle of 38° to the axial direction to an angle of 60°, results in around a 60% increase in the circumference of the cylinder. This degree of expansion allows a liner incorporating this fabric 38 to maintain its position adjacent a pipe wall even if the pipe diameter increases by a similar amount. It is certainly more than enough for the liner once positioned in the pipe to be expanded against the pipe inner surface in preparation for and during curing. As a result of this expansion mechanism, the inlay fibres are aligned more closely with the circumferential direction of the cylinder than they were prior to the expansion. In fact, it is found that a cylindrical liner made with this fabric as set out above will expand radially until the angular alignment of the inlay fibres with the machine direction reaches around 55°. After this, stresses placed on the liner will produce a much reduced expansion. This property arises through the cylindrical geometry of the liner and is found to be particularly advantageous to the application envisaged. An inlay alignment of 54.7° means that the reinforcement provided has reached the critical balanced angle. At this point, the cylinder is reinforced twice as much in the hoop direction as it is axially. The induced strains within the fabric will be uniformly distributed and so will not cause any further skewing of the inlay yarns. Expansion therefore can no longer take place via the reorientation mechanism and so will be met with much increased resistance. Moreover if, when in use, the fabric liner is expanded against the pipe such that it adopts a critical balanced angle configuration, the process of curing will fix and hold the inlay fibres at this angle. The cured liner will therefore have a uniform resistance to stresses experienced within the pipe it lines, for example, transporting high-pressure fluids such as potable water. This ideal distribution of stresses will make the liner less liable to failure. The yarn used to form the pillar stitching 40 is not required to make any contribution to the strength of the liner and so could be any suitable material such as polyester or glass. Ideally, the pillar stitch will be able to hold adhesive that is used to adhere different material layers together, as described in the process illustrated in Figures 1 and 2. Given that the inlay layers 30, 32, 34, 36 of the fabric of this invention are required to move with respect to one another in order to ensure that the liner remains flexible during an installation procedure, the ability to concentrate adhesive on the pillar stitching is an important consideration. Without this property, there is a danger that adhesive may permeate through the fabric and bind the inlay layers together. This basic fabric structure 38 of this present invention is known from WO 2014 / 030018, although the prior art does not teach the particular values of the angle a that provide the advantageous properties set out above and that can be exploited in the field of liner reinforcement. In particular, the prior art makes no suggestion that this fabric may be adapted to be made suitable for inversion installations. The prior art application is to hose reinforcement, with the prior art fabric being wrapped around a mandrel circumferentially in the machine direction. The length of the hose that can be reinforced by a single fabric sheet is therefore limited to the width of the machine on which the fabric is produced. By way of contrast, in this present application, the longitudinal direction of the liner is aligned with the machine direction of the fabric. The length of fabric that can be produced is not limited by machine size, rather it will, as noted above, be limited by the weight and handling ability of the fabric produced. That is, fabric orientation for this present application is more amenable for incorporation in long tubular structures. Moreover, although the prior art notes that the angle a can be varied, the embodiment described teaches that ideally the fabric is produced such that the inlay yarns are oriented at the critical balanced angle for fibre wound pressure vessels. That is, a ~ 54.7°. With this present invention however, to be most effective it is necessary to produce the fabric with a value of a that is less than this critical balanced angle. This is for the important reason that a distinction is drawn between the properties required for liner insertion (flexibility, conformability) and those required for its function in pipe repair when it may be required to carry high-pressure fluids. Once installed, the liner will have an increased lifetime if it is strong and ideally structured for cylindrical reinforcement. By way of contrast, during installation it should be flexible, conformable and with a relatively small cross section to aid insertion into the pipe. Prior to the realisation than the hose reinforcing fabric described in WO 2014 / 030018 could be adapted as indicated above, no prior art materials have been able to satisfy such conflicting requirements for a liner. The expansion mechanism of this material provides additional advantageous properties to a pipe lining material. Typically, when a cylindrical structure formed of a woven or knitted fabric is expanded radially, the structure will contract longitudinally. When fitting a liner to a pipe therefore, allowance has to be made for a contraction in its length as the liner is expanded against the inside of the pipe. The expansion mechanism of this fabric, via reorientation of inlay yarns, allows an accurate prediction of the degree of liner contraction that will occur. This is particularly important if, after fitting, access is required to the end of the liner. In addition, the fact that expansion ceases when the inlay yarns are aligned at the critical balanced angle means that a maximum contraction can also be anticipated. Finally, it is noted that as the fabric expands, reorientation of the inlays means that loading is reduced on the pillar stitches. This, in turn, increases the freedom that the surfaces defined by the inlay layers have to slide over each other, further increasing the fabric’s flexibility. That is, as the cylindrical fabric is expanded radially, it becomes more flexible. This may further assist in ensuring the liner conforms to a pipe bend. In accordance with this invention, the liner structure disclosed herein may be fabricated to the size and with expansivity required to line a particular pipework structure. With reference to Figure 6, a set of rules that provide a guide to this aspect of the invention is set out. It is emphasised that these steps are guidelines only as, in practice, a fabricated textile may not match exactly its predicted behaviour as a number of assumptions are inherent in the model that is used. However, without being bound by any theory, the following provides a good approximation to a reasonable starting point. A first step S10 of a method 60 of designing the liner is to consider the size of pipe, Dinitiai, that is to be lined. Pipes used for pipeline transportation generally come in standard sizes to ensure interconnectivity. These are stated in terms of a Diameter Nominal (DN), which is followed by a figure that references the internal diameter of the pipeline in mm. For example, for the intended application of this invention, pipe networks are generally made up of components from the DN 50, DN 70, DN 100, DN 150, DN 200, DN 250, DN 300 sizes, although it is envisaged that pipes up to a size of DN 800 may be lined using a liner constructed from fabric layers in accordance with the method described above. DN 200 pipes are frequently used in the transport of potable water . If a DN 200 pipe is to be lined, then a liner of smaller diameter will be inverted into the pipe and, once in position, expanded to a diameter of 200 mm. If on the other hand, the section of pipe to be lined includes a diameter change, for example from DN 200 to DN 300, then a greater degree of expansion is required. A second step S12 of the method therefore is to consider whether the section of pipework that is to be lined contains a diameter change. Standard connectors are available that enable one size of pipe to connect to the next size up or down. It can be seen that the greatest expansivity is required for lining a pipe that changes from a DN 100 to DN 150 sizing (50% expansion). Although a more common transition in a water pipeline is a change from DN 200 to DN 300 If the answer to this question S12 is “no”, then the expansivity that needs to be exhibited by the fabric has only to be sufficient for a liner of this fabric to be inserted into the pipeline by an inversion method and then expanded against the inside surface in preparation for curing. This is readily satisfied with an expansivity of around 15%. At Step S14 therefore, the expansivity e is set at 0.15. That is, the liner must be able to expand from an initial diameter dinitiai to a final diameter dfinai = 1.15 x dinitiai, where the liner final diameter dfinai is equal to the size of pipe to be lined, Dinitiai. On the other hand, if at Step S12, it is determined that the liner does need to conform to a change in pipe diameter, then the expansion required to adjust from the smaller (Dinitiai) to the larger (Dfinai) diameter is calculated at Step S16: Dfinai Dinitiai expansion = ——-------- ^initial At Step 18 it is noted that in lining the smaller diameter pipe, the liner still needs to be inserted, ideally by an inversion installation, and then expanded to press against the inner surface of the pipe. That is, the expansivity required of this liner has to be slightly greater than the expansion required to conform to the pipe diameter change. At this Step S18, the expansivity e is therefore set at, for example, 10% over the expansion necessary to accommodate the pipe diameter change. Once a value for the expansivity e is determined S14, S18, the method proceeds to calculate S20 the angle ± a that the inlay yarns should make with the fabric machine direction as it is produced on the knitting machine. This is calculated using the assumption that at its greatest expansion, the inlay yarns should be ideally oriented at the critical balanced angle. This is the orientation at which the yarns will be fixed in the cured liner and it is by this mechanism therefore that the strength of a liner in accordance with this invention approaches that of a multiaxial liner fabric when both are in use. Initially however, as the fabric is produced by the warp knitting machine, the inlay yarns will be at an angle a. This initial angle is found by calculating the increase in cylinder circumference resulting from the reorientation of the inlays from a to the critical balanced angle and setting this to be equal to the expansivity that was determined at Steps S14, S18. That is: This ensures that the hoop expansion that is required for a liner of this material to fulfil its intended function is equivalent to the maximum hoop expansion that can be facilitated by reorientation of the inlays. Once this initial angle a is calculated S20, additional fabric parameters are determined at Steps S22 and S24. At Step S22 the total width of the fabric that has to be formed into a cylinder to create the liner is calculated. Depending on the size required, the tubular fabric for this liner may be produced on a single machine or it may be built up using pairs of fabric sheets as described herein with reference to Figures 1 and 2. By way of a first example, consider the design of a fabric that is required to line a section of a DN 200 pipe. From the method described above, the expansivity needed is 15% and the initial inlay angle is 45° (sin-1 (sin 54.7° I 1.15)). The fabric width required on production is accordingly 55 cm, assuming it is to be rolled into a cylinder to form the liner. Alternatively, the fabric may be produced directly in tubular form by the use of a circular knitting machine. As a second example, consider a design of fabric that is to be incorporated in a liner intended to line a section of pipe in which the diameter increases from DN 200 to DN 300. The liner is therefore required (Step S16) to undergo a 50% expansion. At Step S18, the expansivity is set slightly larger than this, say to 0.55 (a 10% uplift). For these characteristics, the initial inlay angle a should be set at 32°. That is, it is required to skew through a greater angle than for the previous example. This fabric is therefore produced as a sheet of width 41 cm, or directly as a tubular structure with 41 cm circumference. With reference again to Figure 6, at Step S24 the gauge and stitch length required to produce an inlay oriented at the determined values of angles ±a are set. As noted previously, for a 45° inlay these should be symmetrical: for example, a gauge of 12 stitches per inch and a stitch length such that 12 rows per inch extend in the machine direction. The gauge tends to be specific to the machine used to create the fabric. Adjusting the gauge will therefore require reconfiguring the machine or using another machine. In practice, it will therefore be the stitch length that is more readily adjusted to set the orientation angle of the inlay yarns. In order to fabricate the more expandable (55%) fabric, the inlay angle a is around 30°. A suitable setting on the knitting machine for this fabric therefore is a gauge of 12 stitches per inch and a stitch length of 7 rows per inch. These figures and method set out in Figure 6 are included by way of guidance only. Actual fabric performance will deviate from the theory, for example on application of a load, it is expected that any looseness in the fabric will take up this load before any inlay reorientation occurs. This practical observation is not built into the theory set out above. Moreover knitting machine parameters are often set and there is more limited freedom to tune the fabric parameters than assumed above. Accordingly, fabric behaviour and selection of parameters provide a good starting point and the ideal angles and sizes should be incorporated to a best approximation. Actual performance should be verified by subsequent testing. It is believed that fabric constructed to the design set out herein and using glass fibre yarn as the inlay material will satisfy the stringent regulations that apply to the lining of pipes that carry potable or drinking water. Water in such a pipe network is generally carried at high pressures, in the region of 12 bar for pipes ranging from 100 to 800 mm diameter. In addition, these pipes must also be able to withstand compressive forces from vehicles passing above their underground location. The current standard that must be met by most potable water pipes is that they must have a modulus in compression of at least 5 MPa. For some applications, this rises to 10 MPa. There are therefore both performance requirements and safety considerations to be met by a material that is used to line drinking water pipes. To date, these requirements mean that CIPP is not currently a widely-used technique for rehabilitation in this environment. The fabric described herein however would appear to exhibit that combination of strength and flexibility that allow it to be used in a liner material for CIPP repair of drinking water pipes. Moreover, flexibility is such that inversion installation of these liners will be possible. Modelling has been carried out on the following fabric constructions. Sample A Carrier tube: 1st fabric layer: 2nd fabric layer: 3rd fabric layer: 4th layer: Warp-knitted glass tubular fabric 5 bar 12-1 needle 45° glass fibre inlay 5 bar 12-1 needle 45° glass fibre inlay 5 bar 12-1 needle 45° glass fibre inlay Flexible TPU film This sample is designed for installation in a single-diameter pipeline section, for example DN 200. It is envisaged that this Sample A structure can be assembled, by the method described above in relation to Figures 1 - 2 or otherwise, to provide a liner structure suitable for lining pipes with a diameter ranging from DN100 to DN800. The warp-knitted glass tubular fabric is a product supplied by Scott &Fyfe Limited. It is a seamless glass knitted product that is highly flexible and conformable and may itself be incorporated in a liner for inversion installations. This tubular fabric is predominantly glass, which ensures that the carrier tube is able to bond well to the glass material in the inlay fabrics. This assists with construction if the inlay fabrics are added sequentially to the carrier tube, in accordance with the method described herein. Moreover the flexibility and conformability of this prior art tubular fabric are such that movement of the inlay yarns of the reinforcing fabric is not hindered, despite bonding the fabrics, and the carrier tube is able to expand furthest to form the outermost layer of the fitted liner. Alternative tubular structures could be used but these should not be less flexible than the inlay layers that form the bulk of the liner structure. In incorporating the novel fabrics of this invention in a liner structure, resin saturation of the liner is of course part of the installation procedure. Ideally, the liner material is supplied in the form of a sleeve and the resin is first poured into the inside of the sleeve. Air is then removed from the liner which draws the resin along its length. At the same time, the saturated liner is compressed by passing it through fixed-gap metering rollers, which ensures an even distribution of resin throughout the liner. The resin itself can be any of a number of suitable materials that are currently used in the art, for example epoxy resins, polyester, vinylester, silica resins as well as UV-cured resins. In Sample A, both the inlay layers and the warp-knitted carrier layer are good drawers of resin: the relatively large gaps between the loosely-held stitching assist in drawing up the resin throughout the layered structure. If the inlay layers had, for example, a more dense structure, then the inner layer may be included to assist with drawing up the resin, regardless of whether or not is needed to support the structure through the liner construction process. Although glass is not a toxic substance and so, on the face of it, eminently suitable for use in these applications, there is some reluctance to risk glass fibres coming into contact with drinking water. For this reason, the outermost layer of Sample A is an extruded polymer (for example thermoplastic polyurethane (TPU)) film tube that presents a waterproof barrier. Following inversion, this barrier will line the inside of the repaired pipe, preventing contact between glass and water. This polymer film is also required to be flexible and to contain the resin as the liner is fed into an inversion drum. In other examples, the film could be a removeable UV-blocking film layer. This layer is advantageous to the installation process in that it permits pre-resination of the liner by a UV-cured resin. In order to reduce work required at the installation site, it is desirable to be able to supply the liner material pre-resinated, leaving only the positioning and curing parts of the procedure to be carried out on site. This is not easily achievable with 2-part epoxy resins, which will cure under ambient conditions. In general, such resins are mixed and applied at the installation site or if applied offsite, the resinated liner has to be transported in a refrigerated van. A UV-cured resin however can be pre-applied if exposure to sunlight can be avoided. One way in which this may be achieved is by the addition of a UV-barrier layer to the outside of the liner, which therefore prevents premature curing of resin contained within the barrier. After the liner is inverted by insertion into a pipe, the film is removed prior to illumination by a UV source, which cures the resin. Alternatively, the liner may be additionally wrapped in a UV-blocking film for transportation to the installation site. The 5 bar 12-1 inlay structure that is described in relation to Figures 3-5 has a thickness of 1 to 2 mm. If a thicker liner product is required therefore, one of the inlay layers e.g. the 2nd fabric layer may be substituted with a thick glass spacer or felt layer. Sample B Carrier tube: 1st fabric layer: 2nd fabric layer: 3rd fabric layer: 4th layer: Warp-knitted glass tubular fabric 5 bar 12-1 needle 30° glass fibre inlay 5 bar 12-1 needle 30° glass fibre inlay 5 bar 12-1 needle 30° glass fibre inlay Flexible TPU film This Sample B is the more expandable version of this fabric, for example accommodating a 50% expansion to line a junction between DN 200 and DN 300 pipes within a network. The carrier tube and film layer are selected as per Sample A. One prior art fabric that is currently used in certain pipe repair applications that require a high degree of strength and flexibility is a glass fibre matting. In this structure, 50 mm lengths of glass fibre are randomly oriented through the mat. This provides the prior art liner with a strong fibre reinforcement. The embodiment of inlay fabric described above i.e. with 30° inlay yarns and that is used to prepare Sample B provides an alternative glass reinforcement structure. If the 12-1 inlay embodiment is fabricated to a specification of 12 stitches per inch and 7 rows per inch, then the distance between direction reversals of the glass fibre yarn is around 50 mm. This is therefore very similar to the fibre lengths in the glass matting that is currently used. The present fabric however improves on the glass matting arrangement in that the inlay lengths are arranged in parallel, and not randomly oriented, and so target the reinforcement in the desired direction. Figures 7a and 7b show the results of modelling the expansion (and contraction) characteristics of a) a 5 bar 12-1 needle 45° glass fibre inlay fabric and b) a 5 bar 12-1 needle 30° glass fibre inlay fabrics when assembled into a cylindrical configuration, as set out respectively as Sample A and Sample B. In both graphs, the angle that the inlay yarns make with the machine direction of the fabric (± a) is plotted along the x axis. Lines 64a, 64b follow the variation in circumference of the cylinder as the inlay angle a changes. Lines 66a, 66b plot the variation in length of the cylinder. For the 45° inlay (Figure 7a), the fabrication state (i.e. zero hoop and axial expansion) is indicated at position 68. As can be seen from this diagram, for a hoop expansion of around 16%, the inlay angle increases up to the critical balanced angle and the expansion is accommodated by a longitudinal contraction of almost 20%. Once expanded to this angle, the cylindrical fabric ceases to be able to expand further by this mechanism. Conversely, as the cylinder is expanded longitudinally, the orientation angle a decreases and the inlays align more towards the longitudinal direction. This longitudinal expansion is accompanied by a contraction in the hoop direction. For the 30° inlay, similar behaviour is expected: from a starting point 70, hoop expansion is facilitated by an increase in the inlay angle a up to the critical balanced angle. This expansion is accompanied by a longitudinal contraction. For this embodiment, expansion in the hoop direction may be up to 60%, as required to line a junction between DN 200 and DN 300 pipes, and the accompanying longitudinal contraction is in the region of 35%. If the expansion and contraction are reversed, the inlay angle is correspondingly reduced. Also shown in these Figures is a line 72a, 72b that illustrates the variation in the cross-sectional area of the cylindrical fabric layer. It is noted that this falls by only 5% for the intended expansion of the 45° inlay and increases by little over 10% for the intended expansion of the 30° inlay. This minimal change in cross-section area is helpful in maintaining an even distribution of resin as the liner is expanded within the pipeline. Finally, the modelled variation in cylinder diameter is also shown 74a, 74b on the plots. In order to ensure an inlay orientation that is in line with the modelled value, reference has been made to adjusting the stitch length, and possibly the gauge of the knitting machine. Further parameters of the fabric and I or knitting machine may additionally be adjusted, increasing still further the flexibility in fabric design. For example, adjusting the following features will all affect the properties of the finished product. 1. Number of knitting machine bars One bar of the machine is assigned to knit the binding chain or pillar stitching. Some or all of the remainder may each be used to include an inlay. Figure 8 illustrates a simplified binding diagram 78 for the fabric that has been described previously in relation to figures 3-5 and 7a. This Figure shows a pillar stitch 80 knitted on Bar 1 at the far left. Note that this is a single yarn representation, the structure repeats laterally at stitch positions extending to both the left and right with many longitudinally-extending yarns. The inlay that is fabricated on Bar 2 is indicated by a single zig-zag line 82 that extends from a bottom left of the figure, diagonally upwards and to the right (one position laterally for each successive stitch) until 12 stitches are formed. Direction is then reversed and the inlay continues upwards but with a leftward lateral shift. Again, this pattern is replicated laterally by yarns at all stitch positions. The inlay that is fabricated on Bar 3 is indicated by a similar zig-zag line 84 that is displaced 6 stitch positions in a vertical direction (½ L). Bars 4 and 5 are similarly represented by a yarn of the zig-zag inlays 86, 88 that are fabricated on these bars, respectively displaced 12 and 18 stitch positions vertically. An alternative embodiment 90 is shown in the simplified binding diagram of Figure 9. In this embodiment, Bar 1 is again used to knit the connecting pillar stitch 92. Bar 2 generates a 9-needle inlay 94. That is, an inlay that is moved 1 stitch position laterally for each successive row, for a total of 9 stitches. The lateral direction is then reversed. Bar 3 generates the same inlay structure 96 but displaced 6 needle positions vertically and Bar 4 generates the same inlay structure 98, this time displaced 12 needle positions. In this embodiment therefore, the fabric is a 4-bar inlay structure. If stitch length and gauge are symmetric then it will again be a 45° inlay. The effect of reducing the number of bars is that the fabric becomes less dense. It is preferred that the inlays are configured symmetrically: each inlay that is included within the fabric structure should be displaced an integral number of stitch positions from the remainder. That is, if the inlay yarns are placed by b bars in an n-needle pattern, then 2n / b should be an integer. This symmetry enables the inlay yarns to fill the fabric area more evenly, producing a fabric with more uniform properties. The 12-needle inlay is accordingly highly adaptable: 2, 3, 4 or 6 bars could be used to add inlay yarns to the fabric. The more bars, the denser and stronger the fabric. The following Sample C has been fabricated and initial tests show promising results that support the disclosure herein. Sample C Carrier tube: Warp-knitted glass tubular fabric 1st fabric layer: 4 bar 9-1 needle 45° glass fibre inlay 2nd fabric layer: 4 bar 9-1 needle 45° glass fibre inlay 3rd fabric layer: 4 bar 9-1 needle 45° glass fibre inlay 4th layer: Flexible TPU film 2. Number of rows in the repeat (2L) As noted above, this is constrained to some degree by the number of inlay bars. Expanding the pattern to increase the number of needles between each lateral reversal, reduces the number of corners for each inlaid yarn. This reduces the number of potential failure points but also reduces the density of inlay yarns. As the inlay yarns predominantly bear any load applied to the fabric, reducing their density reduces the burst strength of the fabric. On the other hand, increasing the density will make resination more difficult Fabrics according to Figures 8 and 9 have densities within an acceptable range. 3. Yarn weight Varying the yarn weight offers another mechanism by which to adjust the strength of the inlay yarns and hence of the fabric. Yarn weight is measured in Tex (g km-1). Yarns however are available commercially only at certain Tex values, which limits the scope for adjustment. Moving to the next Tex level will provide a marked increase in yarn strength, but this may be more than is required and other mechanisms may provide better options for fine-tuning. Of course one or more of these listed adjustments may be combined to achieve the required result. 4. Inlay yarn material As made clear above, the fabric is designed such that its inlay yarns are oriented to provide a balance between strength in the axial and hoop directions of a tubular configuration. For the preferred application of this tubular fabric, it is desirable to have as high a strength in these directions as possible and so the yarn material is selected for its strength. Ideally therefore, the yarn used within all inlay layers 30, 32, 34, 36 is glass. Moreover, glass fibres tend to link more effectively with resin and so are particularly strong when used as reinforcement in a resin composite. However, other applications may not require the strength of glass fibre and different materials may be more suited to these applications. For example, the inlay layers could be formed from a material such as aramid, nylon, polyester, polypropylene, rayon, cotton, basalt, metals, PEEK (polyether ether ketone) and Dyneema®. Increased strength, if required, may be obtained by increasing the weight (Tex) of the yarn. 5. Stitching yarn material The yarn used to form the chain stitching 40 does not make any contribution to the strength of the liner and so could be any suitable material such as polyester or glass. Ideally, the yarn has the ability to draw in adhesive. This ensures that in a multi-layered material, for example that fabricated in accordance with the process described in relation to Figures 1 and 2, adhesive that is required to bind the layers together is concentrated on the chain stitching. This, in turn, leaves the inlay yarns free to reorientate in order to facilitate expansion and also ensures that the inlay yarns of different fabric layers 30, 32, 34, 36 are mobile with respect to one another such that the liner remains flexible during an installation procedure. In summary, the invention described herein relates to a fabric that is strong, flexible and conformable, such that it can be incorporated in a liner for cure-in-place pipeline repairs with inversion installation. Design and fabrication parameters are disclosed to enable the fabric to be constructed to a specification set out for a number of pipe applications. Although the properties of liners described herein are best suited to inversion installations, it is expected that their strength would make them suitable for many pull-in-place installations. That is, the fabrics are new and useful in themselves, independently of the application to which they may subsequently be put. Figure 10 illustrates a binding diagram showing how different configurations can be used in positioning of the inlay yarns. In this Figure, 10 needles are used to generate the material and there is an offset of 5 needles. Different values for the number of needles and the offset are possible. In terms of the general principle, the number of needles equals the number of inlay bars multiplied by the stitch offset divided by two. Since the number of needles has to be a whole number, if the number of inlay bars is odd, then 5 the offset has to be an even number. An even number of inlay bars gives a larger choice of constructions. The number of needles and the gauge of machining determines the size of the feathering of the fabric and also determines the length of the straight section of the inlay. Using extra inlay bars leads to a ticker and heavier material. The table below provides some 10 examples of different configurations of inlay bars and offset and the resulting number of needles required as a result. Inlay bars Offset Needles 3 2 3 3 4 6 3 6 9 3 8 12 3 10 15 4 1 2 4 2 4 4 3 6 4 4 8 4 5 10 4 6 12 4 7 14 4 8 16 4 9 18 4 10 20 5 2 5 5 4 10 5 6 15 5 8 20 5 10 25 6 1 3 6 2 6 6 3 9 6 4 12 6 5 15 6 6 18 6 7 21 6 8 24 6 9 27 6 10 30

Claims

1. A material for use in lining pipes, the material comprising:a first set of inlay yarns (28, 30, 82, 94), each yarn of which has a path that, relative to a machine direction of the material, extends at a first oblique angle (+a) to the machine direction for a first distance (L) and then at a second oblique angle (-a) to the machine direction for a second distance (L);a second set of inlay yarns (32, 84, 96) each yarn of which has a path that is similar to a yarn path of the first set of inlay yarns but displaced a first proportion of the first distance (L) in a direction parallel to the machine direction; anda set of warp-knitted yarns (40, 80, 92) that interlink with the inlay yarns (28, 30, 32; 82, 84; 94, 96);wherein the magnitude of the oblique angle (±a) is less than 54.7°.

2. A material according to claim 1, wherein the first and second distances (L) are substantially equal.

3. A material according to claim 1 or 2, wherein the first oblique angle (+a) and second oblique angle (-a) are of similar magnitude and in opposite directions.

4. A material according to claim 1, 2 or 3 wherein the material also includes:a third set of inlay yarns (36, 86, 98) each yarn of which has a path that is similar to a yarn path of the first set of inlay yarns but displaced a second proportion of the first distance in a direction parallel to the machine direction.

5. A material according to claim 4 wherein the material also includes: a fourth set of inlay yarns (38, 88) each yarn of which has a path that is similar to a yarn path of the first set of inlay yarns but displaced a third proportion of the first distance in a direction parallel to the machine direction.

6. A material according to any one of claims 1 to 5 wherein the path of each yarn of the each set of inlay yarns (28 - 38; 82 - 88; 94, 96); is followed for the first distance (L) by moving the yarn one needle position to the right for each successive stitch-forming process for a first number of such processes and for the second distance (L) by moving the yarn one needle position to the left for each successive stitch-forming process for a second number of such processes, the first and second numbers being equal.

7. A material according to claim 6 wherein for b sets of inlay yarns and the number s of successive stitch-forming processes for which each of the first and second distances (L) is followed, s and b are such that s + b is an integer and each set of inlay yarns is displaced a proportion 2 + b of the first distance (L) from another set of inlay yarns.

8. A material according to claim 7 that includes first, second and third sets of inlay yarns, each forming a 9-needle inlay and displaced relative to each other by 6 stitch-forming processes in the machine direction.

9. A material according to claim 7 that includes first, second, third and fourth sets of inlay yarns, each forming a 12-needle inlay and displaced relative to each other by 6 stitch-forming processes in the machine direction.

10. A material according to any preceding claim wherein each set of inlay yarns comprises glass fibre.

11. A material according to claim 10 wherein the set of warp-knitted yarns comprises glass fibre.

12. A material according to any preceding claim wherein the magnitude of the first and second oblique angles (a) is, in unstretched material, in the range 30° to 50°.

13. A material according to claim 12 wherein the magnitude of the first and second oblique angles (a) is around 45°.

14. A material according to claim 12 wherein the magnitude of the first and second oblique angles (a) is around 30° - 35°.

15. A liner for pipe repair or rehabilitation, the liner comprising: a warp-knitted carrier tube (12); wrapped by one or more layers of an inlay material (14), each layer (14) of the inlay material being aligned such that its machine direction corresponds with a longitudinal direction of the carrier tube (12), and comprising:a first set of inlay yarns (28, 30, 82, 94), each yarn of which has a path that, relative to the machine direction of the inlay material (14), extends generally linearly at a first oblique angle (+a) to the machine direction for a first distance (L) and then generally linearly at a second oblique angle (-a) to the machine direction for a second distance (L), the first and second distances (L) being substantially equal and the first (+a) and second (-a) oblique angles being of similar magnitude and in opposite directions;a second set of inlay yarns (32, 84, 96) each yarn of which has a path that is similar to a yarn path of the first set of inlay yarns but displaced a first proportion of the first distance in a direction parallel to the machine direction; anda set of warp-knitted yarns (40, 80, 92) that interlink with the inlay yarns (28, 30, 32; 82, 84; 94, 96).

16. A liner for pipe repair or rehabilitation, the liner comprising: a warp-knitted carrier tube (12); wrapped by one or more layers of an inlay material (14) as set out in any one of claims 1 to 14, each layer (14) of the inlay material being aligned such that its machine direction corresponds with a longitudinal direction of the carrier tube (12).

17. A liner according to claim 15 or 16 wherein the carrier tube (12) and one or more layers of inlay material (14) are coated with a flexible film.

18. A liner according to any one of claims 15 to 17 wherein the liner is infused with resin.

19. A method of producing a fabric to reinforce a resin, the method comprising:(a) Determining required hoop expansion of the fabric such that when in a tubular structure, the fabric is expandable from a first diameter to a second diameter;(b) Calculating an inlay angle a for the fabric, from a = sin-1 where e is the ratio of the second diameter to the first diameter;(c) Setting a warp knitting machine to knit a fabric such that a number of rows per unit length in the machine direction approximates a gauge of the machine multiplied by tana;(d) Setting a first bar of the knitting machine to produce a first inlay in which yarn paths move one needle position to the right for each successive stitch-forming process for a first number of such processes and then one needle position to the left for each successive stitch-forming process for a second number of such processes, the first and second numbers being equal;(e) Setting a second bar of the knitting machine to produce a second inlay of the same pattern as the first inlay and that is displaced in the machine direction from the first inlay by a first proportion of thefirst number of successive stitch forming processes;(f) Setting a further bar of the warp knitting machine to produce a warp-knitted yarn that interlinks with the first and second inlays in an arrangement sufficiently loose to allow yarns within the first and second inlays to move relative to each other;(g) Loading the first and second bars on the warp knitting machine with an inlay yarn and the further bar with an interlinking yarn; and(h) Operating the machine to produce a fabric.

20. A method according to claim 19 wherein the method includes the additional steps of:Setting a third bar of the knitting machine to produce a third inlay of the same pattern as the first inlay and that is displaced in the machine direction from the first inlay by a second proportion of the first number of successive stitch forming processes; andLoading the third bar on the warp knitting machine with an inlay yarn.

21. A method according to claim 20 wherein the method includes the additional steps of:Setting a fourth bar of the knitting machine to produce a fourth inlay of the same pattern as the first inlay and that is displaced in the machine direction from the first inlay by a third proportion of the first number of successive stitch forming processes; andLoading the fourth bar on the warp knitting machine with an inlay yarn.

22. A method according to any one of claims 19 to 21 wherein the method includes the step of selecting the inlay yarn from the group consisting of: glass fibre, polyester, aramid, Dyneema®, peek and basalt yarns; glass or basalt rovings.

23. A method according to any one of claims 19 to 22 wherein the method includes the step of selecting the interlinking yarn from the group consisting of: glass fibre, polyester, polypropylene and nylon yarns.48

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