WO2007072078A2

WO2007072078A2 - Improvements in ceramic tubular bodies and methods of forming same

Info

Publication number: WO2007072078A2
Application number: PCT/GB2006/050470
Authority: WO
Inventors: James Ian Edwards
Original assignee: Iti Scotland Limited
Priority date: 2005-12-23
Filing date: 2006-12-21
Publication date: 2007-06-28
Also published as: WO2007072078A3; GB2433569A; GB0526410D0

Abstract

A ceramic tubular body (10) is provided with a ceramic core (12) 0ver which is wrapped a strengthening outer casing (16) formed from a helically wound material which may be self overlapping and/or wound under tension and has a greater strength than that of the ceramic core (12). The ceramic may be reinforced by the use of microspheres added to the ceramic mixture. In operation, the ceramic, when subjected to an internal pressure, will be supported by the stronger outer casing and strains at the same rate thereas. In such an arrangement the ceramic is able to operate at internal pressures that might otherwise case tensile cracking thereof and subsequent failure.

Description

DESCRIPTION

This invention relates to ceramic tubular bodies and methods of forming same and more particularly but not exclusively to the production of pipes having a ceramic core for use in pipe systems such as pipelines for carrying material at high temperature and possibly at elevated pressure. Other forms of tubular bodies such as reactor vessels, treatment chambers and furnaces to name but a few may also be manufactured according to the invention described herein.

Although ceramics are generally considered as weak and brittle materials, they have many other irreplaceable properties, including: resistance to oxidation; excellent wear and abrasive resistance; chemically durable in strong acid and alkaline environments, even at high temperature; transparency to most frequencies; excellent thermal insulation in porous or textile forms; and extremely high surface areas in micro-porous or nano-porous form, providing enormous sites for attaching catalysts or certain functional chemicals. As a result, Ceramics in various forms are used in every discipline of science and technology.

Presently, it is known to use ceramic materials such as Alumina or Zirconia in the production of pipes for the transportation of fluids or gasses at temperatures of up to 1500⁰C. Generally, such pipes are made from a ceramic slurry or paste which is pressed or otherwise formed into a desired pipe form before being cured and allowed to harden in the normal manner to produce a pipe with excellent thermal insulation properties and often very good electrical insulation properties.

Unfortunately, due to their lack of strength and their very brittle nature ceramics are not suitable for use in applications where mechanical loads are anything more than nominal and are certainly not suitable for use as pipes carrying products under high pressure. As a consequence of this performance failure, ceramic pipes as such have hitherto been generally excluded from use in applications where mechanical strain of any form is a performance requirement and/or high internal and/or external pressure is experienced. One solution to the above problem is to coat a ceramic onto the inner surface of a mechanically strong pipe so as to insulate that pipe from the internal temperature associated with any product being transferred therethrough. Whilst this approach provides a perfectly acceptable solution for certain applications, it is often difficult to ensure the ceramic adheres to the interior of the pipe and such layers are easily damaged and so great care must be taken during manufacture, assembly and use of such pipes.

Another use of ceramics in high performance tubular structures is discussed in US5160802 which discloses a composite gun tube having a ceramic liner pre- stressed in compression by a surrounding cylinder formed from a memory alloy such as nickel-titanium and nickel-titanium-cobalt alloys, otherwise known as Nitinol alloys. Such memory alloys poses the unique property of remembering what shape and size they are at a given temperature and it is this property that is employed to apply the compressive strain to the ceramic core. In order to manufacture the product the inside diameter of the Nitinol tube is initially made about 8% smaller than the outside diameter of the liner and then both are ten placed in a cold room at -6O⁰C. Under these conditions the Nitinol is stretched over a mandrel such that the ceramic core can be inserted thereinto before removing the combination from the cold environment and allowing it to return to normal temperature. At normal temperature the memory effect of the metal takes over and effectively returns the Nitinol tube back to the pre stretched diameter and it then grips the ceramic and places the ceramic into compression such that, in combination with the Nitinol as a structural backing, the ceramic is able to withstand the internal pressures created within the gun barrel. This is an expensive process and employs very expensive materials and, as such, would not be suitable for the production of low cost pipes. Additionally, the chilling and insertion steps do not lend themselves to use in the production of long lengths of pipe or the use of a continuous manufacturing process. Also known in the prior art is US 5881775 which discloses a heat exchanger tube made from using a hollow, impermeable, monolithic ceramic inner tube surrounded by a circumferentially extending reinforcing material impregnated with a slurry of ceramic particles. In the preferred embodiment the strength of the final product is dependent more on the ceramic inner tube than the outer wrap of reinforcing material. In addition, a sintering process is required to consolidate the final product and this is described as taking place at an elevated temperature. The ceramic core does not appear to be placed under any compressive load by the outer reinforcing material.

In view of the above, there is, therefore, a need for a high performance temperature resistant pipe and for a pipe which is temperature resistant whilst also being resistant to internal pressures, which is economical to produce and relatively easy to transport and install.

It is an object of the present invention to provide a tubular structure that is able to withstand high temperatures in the temperature range hitherto reserved for ceramic pipes whilst also being able to withstand mechanical strain in the range hitherto reserved for metal structures.

Accordingly, the present invention provides ceramic tubular element comprising an inner hollow ceramic core and an outer casing, wherein the outer casing has one or more strips of mechanically inter-engaging helically wound material having a yield strength higher than that of the inner core.

Preferably, an inner surface of the outer casing is in continuous contact with an outer surface of the core and the outer casing is under tension so as to be capable of exerting a compressive force on the core. Advantageously, the ceramic core includes a plurality of discrete lengths of ceramic pipe and the discrete lengths may have proximal ends and distal ends for abutting up against each other and each proximal end may have a first engagement feature and each distal end may have a corresponding second engagement feature for engaging with said first engagement feature.

Conveniently, the first and second engagement features are respectively male and female shaped features for engagement of one within the other and the features comprise respectively circumferentially extending projections and circumferentially extending grooves.

Alternatively, said first and second engagement features comprise circumferentially extending steps in their respective ends of said discrete lengths of ceramic material and may comprise an external taper on one end of said discrete length and said female feature may comprise a corresponding internal taper on another end of said discrete length.

Advantageously, the arrangement further includes a seal between respective ends of said lengths of core and, if provided, said seal may comprise a temperature resistant seal such as a refractory material.

Preferably, the core material comprises a ceramic selected from the list consisting or comprising : Alumina, Zirconia, Mullite, Silicon Carbide, Silica, Boron and Nitride.

Advantageously, said ceramic has a coefficient of expansion of zero.

Advantageously, said ceramic core includes a plurality of micro-spheres within the structure thereof, said spheres being sufficient to transmit at least a portion of any pressure load experienced at an inner surface of said core to an outer surface of said core where it may be reacted by the outer casing. Preferably, the strip has a transverse cross-sectional step, which, in each convolution of the strip accommodates the overlapping portion of the next convolution.

Advantageously, the strip has on one edge a longitudinally extending projection and on another edge a longitudinally extending groove, which, in each convolution of the strip accommodates the adjacent edge.

Advantageously, the strip has a chamfer on each edge, which, in each convolution of the strip accommodates the overlapping portion of the next convolution.

Preferably, the strip includes chamfered edges shaped to accommodate a step in the strip and the strip may be of a metal, such as a metal selected from the group comprising or consisting of steel, stainless steel, titanium and aluminium. Particularly, it may be Martinsite.

Advantageously, the strip includes an indent and detent, which co-operate with one another in successive convolutions.

Preferably, the indent comprises a longitudinally extending indent formed on one side of the strip and the detent comprises a longitudinally extending detent on an opposite side of said strip. Said indent and detent may include flat mutually confronting contact surfaces.

Advantageously, the indent comprises mutually confronting inclined surfaces and the detent includes corresponding surfaces for engagement with said confronting surfaces on said indent.

Conveniently, the mutually confronting surfaces form a saw tooth. Preferably, the mutually confronting surfaces are perpendicular to a longitudinal axis of the tubular member.

Advantageously, the strip includes two edges and one edge is longer than the other.

In some arrangements the element may include an adhesive layer between the inner core and the outer casing.

Advantageously, the body further includes an adhesive layer between overlapping portions of the outer casing.

Preferably, the adhesive layer comprises a strip of adhesive applied to the core or the strip and the adhesive or adhesives comprises a curable polymer.

Conveniently, the adhesive comprises a single part film based epoxy having a textile carrier and may be Cytec FM 8210-1.

According to a further aspect of the present invention there is provided a method of forming a ceramic tubular element having a tubular core and an outer casing having the steps of:

a) providing a hollow ceramic tubular core having a first yield strength; b) providing a strip of material having a second yield strength greater than that of the core; and c) winding said strip onto said core in a mechanically inter-engaging relationship, thereby to form an outer casing surrounding said core. Advantageously, the method includes the step of tensioning said strip prior to winding it onto said core and maintaining said tension as it is wound onto said core.

Conveniently, the method includes the step of forming the ceramic core as a plurality of discrete lengths of ceramic pipe.

Preferably, the method includes the step of forming a first engagement feature on proximal ends of said discrete lengths and a second engagement feature on distal ends thereof for engaging with said first engagement feature when assembled into said core.

Conveniently, the method includes the step of forming the engagement features as respective male and female features.

In one arrangement the method includes the step of forming the first engagement feature as a circumferential Iy extending projection and said second engagement feature as a circumferential Iy extending groove.

The method may include the step of forming the first and second engagement features as circumferentially extending steps in the respective ends of said discrete lengths of ceramic material.

Conveniently, the method may incude the step of forming the first and second engagement features as respective external and internal tapers on respective ends of said discrete lengths.

Advantageously, the method includes the step of forming the ceramic material with a plurality of micro-spheres within its structure. Conveniently, the method includes the step of assembling a plurality of said discrete lengths together to form said core.

Preferably, the method includes the step of forming the strip having a transverse cross-sectional step and winding said strip onto said core such that each convolution of the strip accommodates an overlapping portion of a next convolution of said strip.

Advantageously, the method includes the step of forming an indent and detent on said strip and winding said strip onto said core such as to cause said indent or detent to engage with a corresponding indent or detent on another portion of said strip adjacent thereto.

In one arrangement the method includes the step of forming the indent and detent as a longitudinally extending indent on one side of the strip and a longitudinally extending detent on an opposite side of said strip.

Preferably, the method includes the step of forming the strip having one edge longer than the other edge.

Advantageously, the method includes the step of applying an adhesive layer between the inner core and the outer casing.

Preferably, the method includes the step of applying an adhesive layer between overlapping portions of the strip forming the outer casing.

Conveniently, the method includes the step of providing the adhesive in the form of a strip of adhesive applied to the strip prior to it being over wound with a successive layer of said strip. Conveniently, the method includes the step of applying the adhesive to the strip prior to said strip being wound onto said core.

Advantageously, the method includes the further step of applying an anti- corrosion coating to the outside of the outer casing which may be provided in the form of a plastic material spirally wound onto the body.

The present invention also provides a tubular structure made in accordance with the method described herein.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings of which:

Figure 1 is a schematic longitudinal view, partially cut-away and partially in section, of a tubular member; Figure 2 is a schematic longitudinal view, partially cut-away and partially in section, of an alternative outer casing of the tubular member;

Figure 3 is a schematic longitudinal view, partially cut-away and partially sectioned of a still further arrangement of the present invention;

Figures 4 to 6 illustrate different interlocking arrangements for the outer casing; Figures 7 to 10 are cross-sectional views of alternative forms of mechanically interlocking arrangements for the outer casing; and

Figures 11 to 13 are cross-sectional views of end fittings for the discrete lengths of ceramic core discussed in the present specification;

Before referring to the detail of the present invention we draw the reader's attention to the following information in respect of ceramics which is provided by way of illustration of how ceramics may be used in such arrangements and in order to assist the reader understand the material properties that ceramics poses and how such materials may be used in the production of a product according to the present invention. Other technical details associated with ceramics materials will present themselves to those skilled in the art. Generally, ceramic materials are inorganic and non-metallic. They are generally molded from a mass of raw material at room temperature, and gain their typical physical properties through a high temperature firing process. Ceramics are inorganic non-metallic materials. Metal oxides (AI203, FeO, etc.) are common examples of ceramics, but other compounds such as carbides and nitrides are also included. Porcelain, glass, bricks and refractory materials are some examples of traditional ceramics. In the last 30 years, advances in material science have transformed formerly brittle ceramics into materials tough enough to withstand engine environments but not many ceramics are able to withstand elevated pressures. The properties for which ceramics are most often selected include:

• High-temperature resistance (High melting temperatures.)

• High electrical resistivity (Although some ceramics are superconductors.)

• Broad range of thermal conductivity (Some ceramics are excellent insulators)

• High hardness (Many ceramics are brittle.)

• Good chemical and corrosion resistance.

• Low cost of raw materials and fabrication for some ceramics.

Ceramics are generally more brittle than metals but can have similar stiffness (modulus of elasticity) and similar strength in compression. However, in a tensile test they are likely to fail at a much lower applied stress. This is because the surfaces of ceramics nearly always contain minute cracks ("Griffith cracks"), which magnify the applied stress. It is this very important limitation of such materials that has hitherto limited their applications within high pressure environments. This problem, amongst others s addressed by one aspect of the present invention.

Since ceramics often have very high wear-resistance and hardness, most ceramic parts are formed as near net final shape as possible. Ceramics are most often produced by compacting powders into a body which is then sintered at high temperatures. During sintering the body shrinks, the grains bond together and a solid material is produced. Other ceramic forming processes include: Dry Pressing, lsostatic Pressing, Roll Compaction, Continuous Tape Casting, Slip Casting, Extrusion, Injection Molding, Pre-Sinter Machining, Hot-Pressing, Hot lsostatic Pressing, Grinding, Lapping and Polishing.

Whilst there are a number of ceramics that lend themselves to use in a ceramic product according to the present invention it will be appreciated that some are more suitable than others and the following examples are provided simply by way of illustration of some that may be used. Alumina, for example, is the most versatile engineered ceramic because of its high temperature service limit along with its chemical, electrical and mechanical properties. It is also relatively low cost, is easily formed and finished using a number of fabrication methods. It is often compounded with silica or trace elements to enhance its properties or fabrication. Alumina engineered ceramic parts can be formed by single axis pressing, isostatic pressing, injection molding, slip casting, or extrusion. Parts can be "green machined" to near net size before firing and then "hard" ground using diamond tooling to tolerances less than 0.0002" (0.005 mm). Although the final properties of alumina ceramics will be determined by the manufacturing method, they generally will have the following properties For Alumina Ceramics:

Zirconium Oxide is widely used and has superior high temperature properties. When combined with compounds of yttria, strength and fracture toughness can be significantly increased. This makes some grades of zirconia superior to most materials for abrasion and wear resistance, brittleness, and thermal shock resistance. Zirconia is electrical conductive above 900 ⁰C.

Zirconia engineered ceramic parts can be formed by single axis pressing, isostatic pressing, injection molding, slip casting, or extrusion. Parts can "green machined" to near net size before firing and then "hard" ground using diamond tooling to tolerances less than 0.0002" (0.005 mm). Special grades of zirconia can be metallized and/or brazed to metal parts. Although the final properties of alumina ceramics will be determined by the manufacturing method, it is generally true to suggest that they will have the following properties:

Another possibility resides with MacGlass Ceramic, which is the traditional machinable ceramic used for years for making prototype and low volume production of ceramic parts. It can be milled, drilled, tapped, turned, sawed, ground or polished using conventional carbide tooling to tolerance of 0.0005". MacGlass Ceramic, usable to 1800⁰F (1000⁰C), is non porous, electrically insulating, and is used in a range of electrical, mechanical and thermal applications. Ultra High Temperature Machinable Ceramic also present themselves for use at very high temperatures or where thermal conductivity or thermal shock is important. MC-LD is a lower density material suitable for use where thermal shock resistance is important. MC-MD is a higher density material that is superior where better abrasion and wear resistance are desired. These materials can be used to 2700⁰F (1500⁰C), can be water quenched from 2550°F (1400⁰C), and have a very low thermal conductivity of just 1.0 W-mK°.

Other materials such as Beryllium Oxides may also be useful. Such oxides have exceptionally high thermal conductivities at low to moderate temperatures. Additionally, Glass Ceramics have Low, medium or high thermal expansion depending on composition type, are good electrical insulators, transparent and can be machined with steel tools.

Nitrides and Carbides such as Silicon Nitrides are resistant to high temperatures, to thermal stress and shock whilst being of high strength, oxidation resistant and are good electrical insulators. Boron Carbide high hardness and low density and the best abrasion resistance of any ceramic, but, unfortunately, it has little strength at high temperatures.

Silicon Carbides may also be considered for their low electrical resistivity, high strength and resistance to chemical attack, high temperature and thermal stress. Glass may also be considered, particularly as a protective coating on a surface of a more structural ceramic. Glass which has a good resistance to thermal shock and may be transparent, has a good resistance to chemical attack.

Referring now to Figure 1 of the drawings, a tubular body indicated generally at 10 forms a pipe for use in a pipe system such as a pipeline carrying hot fluids

(which may also be under pressure). The tubular body comprises an inner portion in the form of an inner hollow ceramic core 12 which may be formed by any one of a number of forming processes, as discussed above and an outer load carrying casing discussed in detail later herein. In the preferred process the inner pipe comprises a number of discrete lengths, as will also be discussed in detail later herein however, one may have a single length if so desired. The outer casing indicated generally at 14 is formed on the inner hollow core 12 by helically winding a strip 16 of material onto the outer surface 12a of the core 12 in a self- overlapping fashion similar to the manner which is described in detail for the formation of a pipe on a mandrel in the specific descriptions of the applicants U.K. Patent No. 2,280,889 and U.S. Patent No. 5,837,083. In accordance with one important aspect of the present invention the strip may be wound under tension. The strips 16 which form the outer casing may have one or more transverse cross-sectional steps 18 and 20 each of which is preferably of a depth corresponding to the thickness of the strip 16. The steps 18, 20 are preferably preformed within the strip 16, each extending from one end of the strip 16 to the other to facilitate an over-lapping centreless winding operation in which each convolution of the strip accommodates the overlapping portion of the next convolution. Whilst the strip may comprise any one of a number of materials such as plastic, composites or indeed metal, it has been found that metal is particularly suitable in view of its generally high strength capability and ease of forming and joining as will be described later herein. Examples of suitable metals include steel, stainless steel, titanium and aluminium, some of which are particularly suitable due to their anti-corrosion capabilities. The internal surface 16i of the strip 16 and the outer surface of the pipe 12o may be secured together by a structural adhesive, as may the overlapping portions 16a of the strip. The use of an adhesive helps ensure that all individual components of the tubular member 10 strain at a similar rate.

A further advantage may be gained from the application of a protective primer to the metal strip. Martinsite, for example, although high strength and low carbon, is still mild steel and, hence, subject to corrosion. One suitable primer is BR127, available from Cyrec Engineering materials of 1300 revolution St, Hrvre de Grace, MD 21078 USA from whom a full data sheet may be obtained. This primer is compatible with a wide variety of adhesives, has established corrosion resistance properties and is also a bonding adhesion promoter. Incorporation of this primer, in conjunction with an outer protective wrap of BP's CURVE ™ material (CyCURV), as described later herein provides a feasible, high performance protection system that may easily be applied to the present invention. Application of the CURVE ™ may be by adhesive bonding if so desired but as this material can be pre-formed having a desired radius of curvature adhesive may not be necessary.

An important enabling feature of the Cytec primer is that it can be applied to a flat Martinsite strip and is resistant to the rib forming process without cracking or reduction in properties. Each of the above discussed materials may be applied to each of the examples described herein.

Referring now to Figure 2 of the drawings, a tubular body indicated generally at 22 has an alternative outer casing 24 formed as previously described from a steel strip 26 having only a single step 28 but which is preformed with a projection 30 formed on one side a detent 30a and on another an indent 30b extending longitudinally along the strip 26. The indent and detent, in effect, form a helical thread on the external surface of this alternative outer casing 24. It will be appreciated that this alternative form of casing may be wound onto the core 12 in the same manner as described above, save for the fact that the strip is wound in an overlapping relationship such that the indent 30b on any second layer cooperates with the detent 30b on a previously deposited portion of said strip 26, thereby to locate the layers relative to each other and form said external helical thread. In an alternative configuration (not show), the strip may be formed with a plurality of such steps and indent/detent combinations so as to allow multiple overlapping layers to be accommodated and, thereby, strengthen the pipe still further.

A still further arrangement is shown in Figure 3, in which the ceramic core is over-wrapped with a single or multiple layer arrangement of outer casing 14 formed by helically wound strips. The strips 16 abut up against each other as shown and may be joined to each other by an adhesive or a suitable welding process performed along the adjoining edges 16e. If desired, multiple layers may be provided and each may be bonded to its neighbour. The edges of the strip may be chamfered (as shown) or may be flat sided (not shown) or inter- engaging, as discussed in detail later herein.

In either of the above arrangements the strip 16 or 26 may advantageously be provided with one edge 16a, 26a longer than the other 16b, 26b, thereby to provide a curve to said strip which upon winding onto the core 12 helps secure the strip to the core with a degree of clamping and / or facilitate correct overlapping. Additionally, the adhesives referred to above may take the form of a strip of adhesive applied to the core 12 or the strip 16, 26 prior to or during winding of said strip 16, 26 onto said core 12. The adhesive may, for example comprise a curable polymer and conveniently comprises a single part film based epoxy having a textile liner, such as to facilitate the easy application of the adhesive and the easy curing thereof once it has been deposited. If desired the adhesive may be provided with an anti-bacterial capability or with radiation resistant properties to name but two examples of properties that may be provided. One may employ Crytec FM 8210-1 as the adhesive. This adhesive may be cured in just two minutes at 18O⁰C which is in stark contrast with some other adhesives which, in order to be cured in 2 minutes require a temperature of 25O⁰C which can have a detrimental effect on the adhesive properties. In order to eliminate quality control problems during any manufacturing stage it may be desirable that the Martinsite strip be cleaned/ shot blasted/ mechanically or chemically etched, degreased, primed and pre-coated with the adhesive in factory conditions and supplied as a roll of production prepared product. It will be appreciated that other forms of adhesive may also be used and their selection and suitability will depend on the final use of the product concerned. For example, it may be desirable to provide a highly flexible adhesive if the pipe is to be rolled onto a drum for transportation or a very high strength and rigid adhesive when the pipe is used in high strain applications such as high pressure pipelines and support arms.

If desired a further protective coating in the form of a layer of CURVE ™ may be provided as a layer of wrapped material around the outside of the pipe. Curve is a low weight, high strength polypropylene material invented by Professor Ian Ward of Leeds University, England, developed by BP and now available from PROPEX of Groneau, Germany. The product comprises a plurality of high tensile fibres of polypropylene woven into a mat and then heated under pressure such that the outer portions of each fibre melts and bonds with its adjacent neighbour whilst maintaining a core of high tensile material. Other forms of protective coating may be used and the present invention should not be considered as being limited to the use of CURVE ™. When CURVE™ is employed it may be provided as a long strip and wound onto the outer portion of the tubular body 10 in overlapping or abutting relationship. It may, if desired be adhesively bonded to the tubular body by means of any suitable adhesive such as the Cytec adhesive mentioned above.

Figures 4 to 6 illustrate alternative forms of mechanical engagement for the outer casing 14. In Figure 4, the engagement is by means of a longitudinally extending groove 50 provided on one edge of the strip 16 and into which, in operation, a corresponding projection 52 formed on the other side of the strip is fed whilst the strip is laid down onto the ceramic core 12. In the arrangement of Figure 5, a simple step 54 is provided on each edge of the strip such that, in operation, the steps engage with each other upon the strip being applied to the core 12. Figure 6 illustrates a simpler arrangement in which the edge of strip 16 is simply chamfered at 56 so as to provide an overlapping portion as each revolution of the strip 16 is laid down on the core 12. Each of these mechanical engagements provides an interlock between the edges of the strip and helps strengthen the joint, as will be well appreciated by those skilled in the art. Each of these inter- engagement features may benefit form a chamfered edge shown generally by dotted lines 58 throughout the drawings. It will also be appreciated that the chamfered arrangement of Figure 6 may be provided by sloping the surfaces in either direction.

In order to provide an enhanced degree of axial location or restraint in the outer casing it may be desirable to provide an axial lock in the form of inter-engaging members as shown in Figures 7 and 8. Referring to Figure 7, a first form of lock comprises a channel 60 formed in one edge of the strip 16 and extending along the edge of the strip together with a corresponding longitudinally extending projection 62 provided in an overlapping portion of the opposite edge of the strip 16. In operation, the projection 62 is laid down into the channel 60 as the strip 16 is wound onto the inner casing 12 and interlocks therewith such as to resist any axial load that may be placed on the tubular structure. An alternative arrangement is shown in Figure 8 in which a saw tooth design is employed. A saw tooth 66a, 66b is provided as a longitudinally extending feature on confronting edges of each side of the strip 16 such that they cooperate with each other as the strip is laid down onto the inner casing 12. In operation the confronting surfaces 68a, 68b of the teeth engage with each other to resist any axial load that may be applied to the ceramic tubular element 10. The stress concentration is much lower in this latter option.

It may be desirable to improve still further the load carrying capacity of the outer casing 16, in which case a design as shown in Figures 9 and 10 may be employed. In Figure 9 the edges of the strip 16 forming the outer casing are chamfered or tapered at 70a, 70b such that, in operation, they more closely fit over the step 20 discussed in detail with reference to Figure 1 above. It will be appreciated that this design modification increases the thickness of the casing in the region of the overlap which might otherwise be only one layer thick if the strip was bluff ended, as shown by the dotted lines. By increasing the thickness in this manner it is possible to increase the load capacity in this region. Figure 10 illustrates another modification in which the outer casing 16 is formed from a strip having matching inclined or sloping surfaces rather than the curved surfaces shown in Figure 2. In essence, a longitudinally extending indent 72 provided along the strip accommodates a longitudinally extending detent 74 provided in a confronting surface of the adjacent convolution of the strip. The sloping surfaces 76, 78 of the indent and detent confront each other and engage with each other when the tubular member is subjected to an axial load. The surfaces 76 are mutually confronting whilst surfaces 78 face away from each other. The contact between the sloping surfaces is such as to more evenly distribute the load than in the embodiment of Figure 2. A strip of material may be inserted between the indent 72 of an inner portion of the winding and the inner casing 12 so as space fill any void and restrict any adverse stretching of the inner casing when subjected to radial load. This strip may, advantageously be Martinsite, so as to assist with the load carrying capacity of the casing.

We now turn to the ceramic core which is best illustrated in Figures 11 to 13. Whilst there are a number of ways of forming an inner core of ceramic material the process itself is not central to the present invention and is not, therefore, discussed further herein. For guidance the reader's attention is drawn to the above general discussion on ceramics which describe a number of manufacturing processes including moulding, pressing and machining. It will be appreciated that one might first mould an entire length or discrete lengths of ceramic core (as shown in Figures 11 to 13) and assembling a plurality thereof to form a length of ceramic core 12.

Figure 11 illustrates a first core arrangement in which discrete lengths 12a, 12b etc are formed with respective male and female engagement features 90, 92 formed on proximal and distal ends 94, 96 respectively. The engagement features form a circumferential Iy extending projection 90 and groove 92 which, in operation, fit one within the other to cause the discrete lengths of ceramic material to form a longer length thereof. In order to assist with assembly one may taper the confronting surfaces, as shown by dotted lines 94. Figure 12 illustrates an alternative arrangement of the ceramic portions 12 in which the engagement is by means of circumferential Iy extending steps 98 and 100 provided on respective distal and proximal ends 94, 96. Again, a tapered portion 102 may be provided to assist with assembly. A still further arrangement is shown in Figure 13 in which a simple internal taper is used to form the male portion 104 and a corresponding internal taper 106 is used to form the female portion 108. One of the advantages of this arrangement resides in the fact that one need not provide the chamfered or tapered portions of Figures 11 and 12. It will be appreciated that one of the functions of the engagement feature is to provide a tortuous or lengthened pathway for any hot gasses which might be contained within the finally formed structure so as to reduce the possibilities of any hot gas reaching the outer casing where they could cause some local and undesirable heating of the load carrying structure. A ceramic material or refractory material 110 may be added to the confronting surfaces of the ceramic portions so as to seal any gap that there might be therebetween. Indeed, this material could be fired in situ by the very hot gasses that it is designed to seal against. Also shown in each of Figures 11 to 13 is an internal coating 112 applied to the inner surface 12a of the ceramic core. Such a coating may comprise a fine ceramic material such as glass which, in operation, provides a chemical and vapour barrier between the fluid within the final product and the ceramic inner core 12.

The form of the ceramic core 12 may be altered to suit a particular use to which it is to be put. For example, whilst the provision of a strong outer casing 14 capable of carrying any mechanical load the core 12 might otherwise experience may be sufficient for many applications, this might not be sufficient for all applications. One particularly advantageous addition to the above described ceramic core resides in the addition of microspheres 130 or other materials to the mixture used to form the ceramic core. When added during the mixing process the ceramic microspheres 130 are dispersed evenly amongst the bulk of the ceramics material and once setting takes place the microspheres form a load carrying structure within the ceramic material which may be employed to transfer any mechanical load experienced by the interior of the core directly to the load bearing outer casing, thus reducing the possibility of damaging the ceramic itself. Such an arrangement is not necessary for low load applications where the ceramic is maintained within its normal performance limits and is not subjected to cracking but may be used in applications requiring exposure to elevated pressures and therefore tensile loads.

For the readers benefit we provide the following list of properties in connection with microspheres and the mixture therewith with ceramic having a thermal conductivity of 2.2 W/m°C (Y203):

Table 1 attached hereto provides a chart illustrating the combined K factor for the above range of microspheres at 600⁰C and a ceramic having a K factor of 2.2. From this chart it will be appreciated that microspheres have a very low thermal conductivity so may be added to the ceramics material in order to provide a good degree of strengthening thereof without significantly adversely affecting the insulation properties of the ceramic core. The additional microspheres help transfer any mechanical load on the ceramic through to the stronger outer casing which then acts as a reaction point against such loading. Clearly one will need to match the ratio of microspheres and ceramic material to that of the required purpose to which the product is put. It will be further appreciated that as the microspheres transfer load to the casing any internal pressure experienced by a ceramic core or liner in a pipe having an outer protective casing as described herein will be shared between the ceramic material and the outer casing. The outer casing is provided as a material having a higher elastic limit than the ceramic and, therefore, the ceramic will strain along the elastic limit line of the casing material and is simply subjected to compressive loading rather than tensile loading. Under such circumstances the ceramic which is normally unable to withstand much in the way of tensile loading is maintained within its tensile limits by the high strength outer casing which reacts the load that would otherwise be experienced by the relatively brittle ceramic.

Once formed, the inner ceramic core 12 forms the base onto which one may form the outer casing by wrapping a strip 14 of material therearound, such that each revolution of the strip 14 engages with the previous revolution in the manner described with reference to Figures 1 to 3 above.

In a particularly preferred arrangement of the present invention the outer casing is formed by winding the strip 16 onto the core 12 whilst maintaining the strip under tension such that, once the final product is formed, the strip exerts a compressive force on the ceramic core 12. This feature may be pressed into service for structures used in environments where the fluid passing through the ceramic core is under pressure and therefore exerts a radial load on the ceramic material itself which places the ceramic under tension. As mentioned above, ceramics are generally very poor at accommodating tensile forces and, under normal circumstances, one would expect a simple un-reinforced ceramic to break when subjected to such loading and allow the fluid to escape into the environment. Any escape of such a fluid could have catastrophic consequences if the fluid is of a chemically aggressive type or at an elevated temperature. In effect, this process applies the equivalent of an "auto-frettage" process to the structure and allows the outer casing to carry any mechanical load whilst allowing the ceramic material to carry the thermal load whilst insulating the metal casing from any heat within the core 12.

Claims

CLAIMS:

1. A ceramic tubular element comprising an inner hollow ceramic core and an outer casing, wherein the outer casing has one or more strips of mechanically inter-engaging helically wound material having a yield strength higher than that of the inner core.

2. A ceramic tubular element as claimed in claim 1 wherein an inner surface of the outer casing is in continuous contact with an outer surface of the core and the outer casing is under tension so as to be capable of exerting a compressive force on the core.

3. A ceramic tubular element as claimed in claim 1 or claim 2 in which the ceramic core includes a plurality of discrete lengths of ceramic pipe.

4. A ceramic tubular element as claimed in claim 3 in which the discrete lengths have proximal ends and distal ends for abutting up against each other and in which each proximal end has a first engagement feature and each distal end has a corresponding second engagement feature for engaging with said first engagement feature.

5. A ceramic tubular element as clamed in claim 4 in which the first and second engagement features are respectively male and female shaped features for engagement of one within the other.

6. A ceramic tubular element as claimed in claim 4 or claim 5 in which the features comprise respectively circumferential Iy extending projections and circumferential Iy extending grooves.

7. A ceramic tubular element as claimed in claim 5 or claim 6 in which said first and second engagement features comprise circumferentially extending steps in their respective ends of said discrete lengths of ceramic material.

8. A ceramic tubular element as claimed in claim 5 in which the male feature comprises an external taper on one end of said discrete length and said female feature comprises a corresponding internal taper on another end of said discrete length.

9. A ceramic tubular element as claimed in any one of claims 4 to 8 and further including a seal between respective ends of said lengths of core.

10. A ceramic tubular element as claimed in claim 9 in which said seal comprises a temperature resistant seal.

11. A ceramic tubular element as claimed in claim 10 in which said temperature resistant seal comprises a refractory material.

12. A ceramic tubular element as claimed in any one of claims 1 to 11 wherein the core material comprises a ceramic selected from the list consisting or comprising : Alumina, Zirconia, Mullite, Silicon Carbide, Silica, Boron and

Nitride.

13. A ceramic tubular element as claimed in claim 12 wherein said ceramic has a coefficient of expansion of zero.

14. A ceramic tubular element as claimed in any one of claims 1 to 13 wherein said ceramic core includes a plurality of micro-spheres within the structure thereof, said spheres being sufficient to transmit at least a portion of any pressure load experienced at an inner surface of said core to an outer surface of said core where it may be reacted by the outer casing.

15. A ceramic tubular element as claimed in any one of claims 1 to 14 wherein the strip has a transverse cross-sectional step, which, in each convolution of the strip accommodates the overlapping portion of the next convolution.

16. A ceramic tubular element as claimed in any one of claims 1 to 15 wherein the strip has on one edge a longitudinally extending projection and on another edge a longitudinally extending groove, which, in each convolution of the strip accommodates the adjacent edge.

17. A ceramic tubular element as claimed in any one of claims 1 to 16 wherein the strip has a chamfer on each edge, which, in each convolution of the strip accommodates the overlapping portion of the next convolution.

18. A ceramic tubular element as claimed in any one of claims 1 to 17 wherein the strip includes chamfered edges shaped to accommodate a step in the strip.

19. A ceramic tubular element as claimed in any one of claims 1 to 18 wherein the outer casing is metal.

20. A ceramic tubular element as claimed in claim 1 to 19 wherein the outer casing is selected from the group comprising or consisting of steel, stainless steel, titanium and aluminium.

21. A ceramic tubular element as claimed in claim 20 wherein the strip comprises Martinsite.

22. A ceramic tubular element as claimed in any one of claims 1 to 21 wherein the strip includes an indent and detent, which co-operate with one another in successive convolutions.

23. A ceramic tubular element as claimed in claim 22 in which the indent comprises a longitudinally extending indent formed on one side of the strip and the detent comprises a longitudinally extending detent on an opposite side of said strip.

24. A ceramic tubular element as claimed in claim 22 or 23 wherein the indent and detent include flat mutually confronting contact surfaces.

25. A tubular member as claimed in claim 22 wherein the indent comprises mutually confronting inclined surfaces and the detent includes corresponding surfaces for engagement with said confronting surfaces on said indent.

26. A tubular member as claimed in claim 25 wherein the mutually confronting surfaces form a saw tooth.

27. A tubular member as claimed in claim 25 or 26 in which the mutually confronting surfaces are perpendicular to a longitudinal axis of the tubular member.

28. A ceramic tubular element as claimed in any one of claims 1 to 27 wherein the strip includes two edges and one edge is longer than the other.

29. A ceramic tubular element as claimed in any one of claims 1 to 28 wherein the body further includes an adhesive layer between the inner core and the outer casing.

30. A ceramic tubular element as claimed in any one of claims 1 to 29 wherein the body further includes an adhesive layer between overlapping portions of the outer casing.

31. A ceramic tubular element as claimed in claim 30 wherein the adhesive layer comprises a strip of adhesive applied to the core or the strip.

32. A ceramic tubular element as claimed in any one of claims 29 to 31 wherein the adhesive or adhesives comprises a curable polymer.

33. A ceramic tubular element as claimed in any one of claims 29 to claim 32 wherein the adhesive comprises a single part film based epoxy having a textile carrier.

34. A ceramic tubular element as claimed in any one of claims 29 to 33 wherein the adhesive comprises Cytec FM 8210-1.

35. A method of forming a ceramic tubular element having a tubular core and an outer casing having the steps of:

a) providing a hollow ceramic tubular core having a first yield strength; b) providing a strip of material having a second yield strength greater than that of the core; and c) winding said strip onto said core in a mechanically inter-engaging relationship, thereby to form an outer casing surrounding said core.

36. A method as claimed in claim 35 including the step of tensioning said strip prior to winding it onto said core and maintaining said tension as it is wound onto said core.

37. A method as claimed in claim 35 or claim 36 including the step of forming the ceramic core as a plurality of discrete lengths of ceramic pipe.

38. A method as claimed in claim 37 including the step of forming a first engagement feature on proximal ends of said discrete lengths and a second engagement feature on distal ends thereof for engaging with said first engagement feature when assembled into said core.

39. A method as claimed in claim 38 including the step of forming the engagement features as respective male and female features.

40. A method as claimed in claim 39 including the step of forming the first engagement feature as a circumferentially extending projection and said second engagement feature as a circumferentially extending groove.

41. A method as claimed in any one of claims 37 to 40 including the step of forming the first and second engagement features as circumferentially extending steps in the respective ends of said discrete lengths of ceramic material.

42. A method as claimed in any one of claims 37 to 41 including the step of forming the first and second engagement features as respective external and internal tapers on respective ends of said discrete lengths.

43. A method as claimed in any one of claims 37 to 42 including the step of forming the ceramic material with a plurality of micro-spheres within its structure.

44. A method as claimed in ay one of claims 37 to 43 including the step of assembling a plurality of said discrete lengths together to form said core.

45. A method as claimed in any one of claims 37 to 44 including the step of forming the strip having a transverse cross-sectional step and winding said strip onto said core such that each convolution of the strip accommodates an overlapping portion of a next convolution of said strip.

46. A method as claimed in any one of claims 37 to 45 including the step of forming an indent and detent on said strip and winding said strip onto said core such as to cause said indent or detent to engage with a corresponding indent or detent on another portion of said strip adjacent thereto.

47. A method as claimed in claim 46 including the step of forming the indent and detent as a longitudinally extending indent on one side of the strip and a longitudinally extending detent on an opposite side of said strip.

48. A method as claimed in any one of claims 37 to 47 including the step of forming the strip having one edge longer than the other edge.

49. A method as claimed in any one of claims 37 to 48 including the step of applying an adhesive layer between the inner core and the outer casing.

50. A method as claimed in any one of claims 37 to 49 including the step of applying an adhesive layer between overlapping portions of the strip forming the outer casing.

51. A method as claimed in claim 50 including the step of providing the adhesive in the form of a strip of adhesive applied to the strip prior to it being over wound with a successive layer of said strip.

52. A method as claimed in any one of claims 49 to 51 including the step of applying the adhesive to the strip prior to said strip being wound onto said core.

53. A method as claimed in any one of claims 37 to 52 including the further step of applying an anti-corrosion coating to the outside of the outer casing.

54. A method as claimed in claim 53 including the step of applying the anti- corrosion coating in the form of a plastic material spirally wound onto the body.

55. A tubular structure made by the method of any one of claims 35 to 54.