Patent Description:
The move away from traditional hydro-carbon fuels in aviation and other vehicle applications may require new cryogenic and high-pressure fuel systems, using both new fuel types and high-pressure levels that increase safety risks such as flammability and detonation. Achieving an equivalent level of safety to existing kerosene or gasoline fuelled vehicles requires new solutions, particularly to contain and detect any leakage from the fuel lines or pipes in the vehicle so that such a failure will not create an unsafe condition. For cryogenic fuels there is the additional hazard of the liquefaction of gases like oxygen from the surrounding air onto fuel pipes that can create a flammable mix which should be avoided.

It is expected that two levels of protection against a single fuel system failure endangering the vehicle/aircraft and a means to detect a failure condition will be needed for many vehicle applications. However, the assembly of structural elements and system components in vehicles like aircraft can make it difficult to physically position and install a second level of leak containment, and fuel pipework is traditionally installed in multiple individual lengths and connected after installation to enable access. This creates a multiplication of joints that not only makes it difficult to create a second layer of protection but can cause the second layer protection design to be very heavy and complex due to the number of joints. The number of joints can also make it difficult to detect and isolate failures. The present invention builds on past aviation and fuel supply industry knowledge to produce a superior fuel line protection system addressing all these issues for use with new aircraft and vehicle designs using alternative fuels such as hydrogen.

<CIT> discloses a vacuum-insulated double-walled fluid transfer conduit.

<CIT> discloses a cryogenic fluid transfer tube. <CIT> discloses a system for supplying an aircraft with inert gas and to the use of a membrane for dehumidifying exhaust gas from a fuel cell.

In general terms, embodiments of the invention provides a protected fuel line including a primary line for transmitting fuel, a sealed volume, and a protective sleeve providing a second layer of containment for fuel transmitted by the primary line. Embodiments are considered to be particularly suitable for vehicles using high pressure or novel fuels such as hydrogen. In some embodiments the protective sleeve comprises a reinforced protective sleeve. The protected fuel line may also comprise insulation within the sealed volume. Moreover, the protected fuel line may comprise a means of leak detection.

In a first aspect, the invention provides a protected fuel line as claimed in claim <NUM>.

In some embodiments the primary fuel conduit is for delivery of a cryogenic fuel to a propulsion system.

The protected fuel line may be for use in any vehicle with a fuel system requiring a supply of a potentially dangerous high-pressure gas or liquids. For example, aircraft, marine vehicles, land vehicles, or space vehicles. That is, embodiments of the invention are particularly suitable for applications in which the fuel delivered by the primary fuel conduit is a cryogenic fuel, and/or has a pressure above the environmental pressure, for example a pressure more than <NUM>-<NUM> percent above atmospheric pressure at sea level (<NUM> bar) or more. Suitable fuels include hydrogen, methane, LPG, CNG. Embodiments of the invention are considered to be particularly applicable to applications where weight and safety are important design factors and applications using cryogenic fuels.

The terms cryogenic fluids and cryogenic fuels used herein have their typical meaning as used in the art. Cryogenic fluids or fuels typically have a boiling point of below <NUM> Kelvin. Cryogenic fuels include liquified gases such as liquid hydrogen.

The primary fuel conduit (or primary fuel line) comprises a plurality of interconnected rigid or semi-rigid fuel pipes and connecting elements that creates the fuel flow path for the fuel on the vehicle from the storage vessel to its consumption point in the propulsion system. The integrity of this primary line forms a first level of protection against leaks and other failures. This is then protected by a secondary flexible outer sleeve that fits around the primary fuel line in usually a roughly concentric manner. This outer sleeve comprises a flexible membrane to contain or retain the fuel, such as hydrogen gas. In some embodiments the flexible membrane may be selectively permeable or impermeable such that the membrane is substantially impermeable to the fuel, but may be permeable to other liquids or gases.

The primary fuel conduit and the outer sleeve may have any shape appropriate to their function, even if in most applications a generally circular cross-section will be used.

A significant advantage of the innovation is due to the flexible nature of the outer sleeve is the ability to install a single outer sleeve across multiple interconnected primary pipes forming the primary fuel conduit, and through multiple penetrations of the vehicle structure. Installation of the outer sleeve will occur after the primary fuel conduit is assembled within the vehicle structure. The primary pipes will typically be rigid and impermeable, by using materials like stainless steel, but other materials like high-density plastics are also possible. Semi-rigid and flexible elements such as bellows may also be used for the primary fuel line to ease the primary line installation and to achieve other requirements like vibration isolation. The connections used in the primary line to connect the constituent pipes and other elements may be of various types, such as swaging or mechanical connections like threaded unions or bolted flanges. The largest connection or feature on the assembled primary line will define the minimum diameter of the outer sleeve so that it can be slid along the primary pipe for installation.

This capability of the outer sleeve to enclose multiple sections of the primary fuel conduit is significantly beneficial for weight reduction and to increase the robustness of the design, as it reduces the need to split the outer sleeve and minimises the number of heavy joints required to join the outer sleeve. This ability to slide the outer sleeve over an assembly of pipes also beneficially makes it possible to repair individual failures in the primary line without having to dispose of the whole assembly as in some traditional double wall pipe designs.

In preferred embodiments the outer sleeve comprises one or more reinforcement members, the reinforcement members optionally comprising woven or coiled reinforcement elements encircling the flexible membrane.

Thus, the outer sleeve may be lightly reinforced with a coil or a weave of strong fibres or monolithic material to contain any failure pressures and avoid excessive sleeve inflation after failure. This reinforcing weave can be any beneficial weave, such as plain or twill, and may be a closed or an open weave. Different materials for the weave are possible, including CFRP, metal or glass fibres. If a non-conductive material is used for the weave, metallic wires can be included to ensure electrical bonding. This reinforced sleeve will contain any build-up of gaseous fuel (or liquid) inside it due to a failure of the primary line. For certain applications the sleeve may also be designed with a specific burst or venting location to vent any leaked fuel safely away from the aircraft or vehicle.

In the innovation the primary fuel conduit will carry the normal operational loads of the fuel transported, including pressure and thermal loads, as well as any induced loads from the vehicle. The outer sleeve will in normal operation only react loads associated with any gas that has been injected into the sealed volume between the primary fuel line and outer sleeve. Due to the flexible nature of the sleeve and the weave of its reinforcing, this will avoid any significant transition of vehicle loads being transmitted by the outer sleeve between clamping points. In the failure case where high-pressure fuel leaks into the sealed volume the sleeve and any external clamping elements will additionally react any loads induced by this fuel entering the volume.

In some embodiments the sealed volume comprises an insulating gas at a pressure above an environmental pressure, optionally an inert gas.

This arrangement provides additional insulation for cryogenic fuel applications. Whilst the secondary impermeable outer sleeve will create additional insulation of the primary pipe, if cryogenic fuel is passing through the primary fuel line, then the presence of the outer sleeve by itself may be insufficient to avoid the outer surface of the sleeve to fall below the liquification point of critical gases like oxygen in the surrounding air, creating a safety risk. The outer sleeve however in most embodiments will be sized bigger than the outer diameter of the primary line creating a sealed volume between the primary fuel line and the outer sleeve, which can be optionally achieved by the use of spacers mounted to the primary pipe and light inflation of the outer sleeve after assembly with a thermally insulating gas. The inflation level (pressure) would be less than that required to trigger any frangible trip wire integrated on the sleeve. This sealed volume will increase the insulation of the fuel conduit increasing the temperature of the outside of the outer sleeve above the critical liquefaction temperature. This inflation may optionally be with an inert gas to provide an additional degree of safety.

Embodiment of the invention also provide a failure protection system as some leaks can become dangerous undetected failures for applications such as aircraft. Thus, a failure of the primary fuel line causing leakage into the sealed volume should be detectable to enable timely maintenance actions to restore both levels of protection as soon as a failure occurs. The protection system enables such detection either by the use of a direct pressure sensor installed inside the sealed volume created by the sleeve, the activation of a frangible portion of the outer sleeve (burst vent), or a frangible trip wire surrounding the sleeve and triggered by expansion of the sleeve after a failure.

Thus, the outer sleeve may comprise a frangible portion configured to burst when a pressure within the sealed volume exceeds a threshold pressure, and optionally wherein the frangible portion is in fluid communication with a venting conduit arranged to deliver fuel from the frangible portion to a safe location.

Similarly, the failure detection system may include a pressure sensor configured to detect changes in pressure in the sealed volume.

In addition, or alternatively, the failure detection system may include a frangible wire encircling the outer sleeve, the frangible wire being configured to break if the outer sleeve expands to or beyond a given threshold, and a detection system for detecting a break of the frangible wire. In some embodiments the failure detection system may include a plurality of frangible wires, each associated with a sealed sub-volume of the sealed volume, wherein detection of a break of a specific one of the frangible wires indicates a failure associated with the respective sealed sub-volume.

Preferred embodiments comprise a clamp for securing the outer sleeve to a structural member having an opening through which the outer sleeve passes, the clamp comprising an inner clamping member and an outer clamping member, the inner clamping member having an inner bearing surface adjacent the outer sleeve, the outer clamping member having an outer bearing surface configured to be installed within the opening of the structural member, and the inner and outer clamping members each comprising one of a pair of cooperating ramped mating surfaces, wherein relative movement of the ramped mating surfaces urges the inner bearing surface away from the outer bearing surface to thereby apply a clamping force to the outer sleeve.

In particularly preferred arrangements the pair of cooperating ramped mating surfaces each comprise an eccentric mating surface defined by a curved plane with a radial axis that is offset from an axis of the primary fuel conduit or outer sleeve. In this way, relative rotation of the inner and outer clamping members creates the clamping force.

The ability to install the outer sleeve after the primary line is assembled through restricting structural features on the vehicle is particularly enabled through the use of this novel clamp. Restricting features could be closed panels, bulkheads, or other structural members of the vehicle. The clamp would typically be fitted over a position where an internal spacer is positioned between the primary fuel line and the outer sleeve, unless the configuration was of tight fit without the sealed volume and not necessitating the introduction of the spacer.

The clamp is made of two elements (inner and outer clamping members), preferably made of a rigid material, but with a degree of flexibility to allow the installation and function of the clamp, such as a medium or medium-high density plastic. The inner clamping member may be designed with its inner bearing surface to fit over the outer sleeve with a minimum clearance and may have as one part of its outer surface forming the inner part of an eccentric mating surface, whilst the other part of its external surface may form part of the mounting feature to one side of the structure it is to be clamped to. The outer clamping member may have as its internal surface the matching outer part of the eccentric mating surface, with another part of the clamp's external surface being the outer bearing surface with a circular shape to match the vehicle structural penetration that the fuel line transitions through. The other part of the outer clamping member's external surface may be the mounting feature to mount to the opposite side of the structure to that of the inner clamping member. Both inner and outer clamping members may have a split and enough flexibility so they can be opened up and placed over the outer sleeve, or a means to install them in two or more parts. Without an opening force applied the inner clamping member will preferably have the minimum clearance to be placed over the outer sleeve with the sleeve installed, and slid into the structural penetration. The outer clamping member may be opened and placed over the sleeve from the other side of the structural penetration, and then pushed into the penetration and over the inner clamping member, with the eccentric mating surfaces of both elements of the clamp mating. The outer clamping member can then be rotated to create a set clamping force through the eccentric surfaces to lock the sleeve and primary line assembly to the mounting point. This clamping level can be set by a defined rotation or by a load level, and then the inner and outer clamping members locked in the clamped position by an integral locking feature or fastener. The clamp may also be locked to the structure, normally by a fastener pattern, which may in some variations of the embodiment be used to lock the clamp to stop it loosening. Alternate embodiments of the clamp design may split the clamp assembly into more than the two parts described.

Alternate configurations of the clamp are envisioned for mounting the fuel line assembly to structure that may not require a penetration, but rather are clipped or attached to free structure. These configurations may instead of the prior description have the outer clamping member designed with a mounting foot to attach the clamp to the structure, with the inner clamping member being fastened and locked directly to the outer clamping member after rotation of the inner clamping member relative to the outer clamping member.

In most configurations the outer sleeve will be of greater diameter than the primary fuel line inside it, so spacers may be needed for the clamping positions. If an effective insulation volume is desired or to avoid the sleeve sagging and influencing spacing margins between the protected line assembly and other features of the vehicle, then intermediate spacers can also be used in-between the clamping positions.

Thus, the sealed volume may comprise one or more spacers arranged to maintain a separation between the primary fuel conduit and the outer sleeve.

In some embodiments at least one of the one or more spacers provides a sealed barrier to divide the sealed volume into first and second adjacent sealed sub-volumes of the sealed volume.

Alternatively, or in addition, at least one of the one or more spacers may be configured to permit fluid to flow across it. spacers, optionally with openings to permit fluid flow.

Intermediate spacers should be lightweight for weight sensitive applications and, in some embodiments, should allow the flow of gas or liquid through them. This can be achieved by the spacer having sufficient holes placed in the direction of the flow, although a sufficient gap from the outer diameter of the spacer and the inner diameter of the sleeve may suffice. These intermediate spacers will in most cases have an inherent clamping load or interference fit to fix their position on the primary fuel line. The intermediate spacer may be single piece and installed on individual primary fuel line pipes before they are assembled, or single piece designed with a split so they can be installed later, or even in multiple parts that are assembled over the primary fuel line after assembly.

In a clamping position, the spacer can be of different configurations depending on the purpose. The spacer should be of sufficient strength that it does not detrimentally crush when a clamp is installed and clamped over the outer sleeve. The inner surface of a clamp spacer may as for the intermediate clamps be an interference fit to restrain the primary line from transitional movement. Alternatively, if translational movement of the primary line is desirable, for instance to avoid stresses being introduced to the primary fuel line such as from thermal expansion or contraction, then the internal surface of the spacer can be of clearance fit to allow such translational movement.

If the sealed volume defined between the primary fuel line and the outer sleeve is to be filled with (an inert) gas to promote safety and insulation properties, then the clamp spacers will have sufficient holes in them to allow the introduced gas to flow along the full length of the installed sleeve. Where a gas is not injected into this sealed volume, a configuration with clamping spacers that completely fill the space between the primary fuel line and the outer sleeve and do not allow flow between one clamping position and another is also a possible configuration to create sub-divisions of the sealed volume. The selection of such a variation of the innovation gives the possibility in the case of a failure of the primary fuel line to allow the identification of the zone where failure has occurred as only the section between the filled clamps will be filled and trigger any sensors.

The outer sleeve extends the full length of the primary fuel line. Each end of an outer sleeve segment will need to be connected to a fitting to create the sealed volume between the primary fuel line and the outer sleeve. This end fitting may be a durable fitting installed into the primary line, but with a second surface or male thread to match the outer sleeve. The outer sleeve may terminate in a matching female threaded union, or alternatively may mate with a close-fitting flat surface with a sealant interface and external clamps to connect the outer sleeve to the fitting. Some fittings, for embodiments that use an inert gas or pressure in the sealed volume between the primary fuel line and the outer sleeve, the end fitting can have a tapping to introduce the gas into the volume. This tapping can be used in periodic maintenance to assess the effective sealing of the sealed volume. Other end fittings may have a fused burst valve connected to the sealed volume that triggers at a defined limit pressure, with a connection that allows any leakage to be safely ejected from the vehicle.

Thus, the protected fuel line may comprise an end fitting for connecting the primary fuel conduit and the outer sleeve to a fuel system component, the end fitting comprising a bore to provide fluid communication between the primary fuel conduit and the fuel system component, a generally ring-shaped interface surface arranged to receive an open end of the outer sleeve to provide a sealed mating connection therebetween, and a web extending between the bore and the interface surface to enclose the sealed volume.

In some embodiments the interface surface may comprise a male thread to cooperate with a female thread at the open end of the outer sleeve.

The end fitting may include a valve operable to provide fluid communication with the sealed volume.

A second aspect of the invention provides a method of assembling a protected fuel line as claimed in claim <NUM>, the method including: (a) installing the primary fuel conduit within a vehicle structure, the primary fuel line having a first end and a second end; (b) installing the outer sleeve by sliding it over the primary fuel conduit from the first end to the second end; and (c) sealing the outer sleeve to the primary fuel conduit at the first and second ends to define the sealed volume.

The method may include the further step of, after step (a) and before step (b), installing one or more spacers on the primary fuel conduit. The spacers may be configured as defined above in relation to the first aspect.

In addition, the method may include the further step of installing one or more clamps to secure the outer sleeve to the vehicle structure.

In embodiments in which the one or more clamps comprise a clamp as defined above in relation to the first aspect of the invention, the method may include: installing the inner clamping member such that its inner bearing surface bears against the outer sleeve; installing the outer clamping member such that its outer bearing surface bears against an opening of the vehicle structure through which the outer sleeve passes; and providing relative movement of the inner and outer clamping members to cause relative movement of the ramped mating surfaces to urge the inner bearing surface away from the outer surface to thereby apply a clamping force to the outer sleeve.

Embodiments of the invention provide a protected fuel line comprising a primary fuel conduit and an outer sleeve encircling the primary fuel conduit to define a sealed volume therebetween, the outer sleeve comprising a frangible portion configured to burst when a pressure within the sealed volume exceeds a threshold pressure, and optionally wherein the frangible portion is in fluid communication with a venting conduit arranged to deliver fuel from the frangible portion to a safe location.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover, the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Preferred features of each aspect of the invention may be as described in connection with any of the other aspects.

In general terms, embodiments of the invention provide a protected fuel line <NUM> for dispensing fuel. For example, cryogenic fluid in the form of liquid hydrogen, or a mixture of liquid and gaseous hydrogen, or gaseous hydrogen at cryogenic or non-cryogenic temperatures. In the illustrated embodiments the protected fuel line <NUM> is for mounting in an aircraft to supply fuel (in the form of gaseous hydrogen) to an aircraft propulsion system or to discharge over-pressure fuel from a storage tank via a safety vent line. However, in other embodiments the protected fuel line <NUM> according to the invention may be applied to marine, land or space vehicles. Moreover, the fuel line <NUM> may have utility in any application where weight, complex installation conditions and safety are important design factors.

The fuel line <NUM> is illustrated schematically in <FIG>, which shows a primary fuel conduit <NUM> through which in normal operation gaseous or liquid fuel may flow <NUM>, surrounding this is a flexible outer sleeve <NUM> which defines a sealed volume <NUM> therebetween. The primary fuel conduit <NUM> may be an individual pipe, or an assembly of pipes and elements such as bellows joined by an appropriate means, including swaging or mechanical connections like threaded unions or bolted flanges. The outer sleeve is made from a selectively permeable or impermeable membrane 20a for the fuel type used in the system and a reinforcing weave or coil 20b made from a suitably strong material such as steel wire or carbon fibres to resist the failure pressures should the primary fuel conduit <NUM> leak or break. In some embodiments there may be an additional layer of material 20c to provide abrasion resistance or additional thermal insulation. For embodiments where the failure pressures are not high, or there is another restraining feature, then the reinforcing weave or coil 20b may be unnecessary and optionally excluded. The minimum internal diameter of the outer sleeve <NUM> will be determined by the maximum diameter of any elements of the assembled primary fuel conduit <NUM>, and optionally for cryogenic applications any insulation needs.

The sealed volume <NUM> may be optionally excluded in some applications, such as for a primary fuel conduit <NUM> that is a single formed pipe, but typically will be present. The sealed volume <NUM> where it is present, may be filled with air at the ambient pressure when the protected pipe assembly is completed, or may alternatively be filled with a pressurised gas, which may also be an inert gas to increase safety. The pressure delta between the sealed volume <NUM> and the external air pressure shall not exceed the design value of any sensors, frangible trip wire <NUM> or frangible burst vent <NUM> connected to the sealed volume <NUM> and the pressure delta shall take into account changes in surrounding ambient air pressure due to normal vehicle operation, such as a change in altitude.

The reinforcing weave or coil 20b embodiment for the outer sleeve <NUM> may be of any appropriate weave or coil tailored to react the failure pressures of the application, whilst ensuring sufficient flexibility and low weight for weight sensitive application embodiments. Alternative embodiments may replace the reinforcing weave or coil 20b with a monolithic material. Preferred embodiments shown in <FIG> may include a tight weave <NUM> for very high failure pressure applications, a loose weave <NUM> for medium failure pressure applications, or a coil <NUM> for low failure pressure applications. Any weave type, for instance twill <NUM> or plain <NUM>, or coil type, such as a single helix <NUM> may be used. The materials for the reinforcing weave or coil 20b in preferred embodiments may be monolithic metallic wire, or bundles of fibres such as carbon fibre. If the embodiment does not use a conductive material for the reinforcing weave or coil 20b, then a supplementary conductive element, such as a copper wire may be woven into the embodiment to enable bonding of the outer sleeve <NUM>.

<FIG> show a sequential series of sectioned views illustrating assembly of a preferred embodiment of a protected fuel line within a vehicle structure <NUM>. This begins in <FIG> with the structure elements of the vehicle <NUM> of which one or more elements make it impossible to install a rigid or semi-rigid fuel conduit in a single piece. There may be other reasons that installation of a single fuel pipe is not possible, such as manufacturing constraints, or the presence of other systems or equipment that similarly make the placement of a single piece conduit impractical and these conditions would be applicable for the embodiment of the invention. The structural elements <NUM> will have penetrations <NUM> designed into them to allow the later installation of the fuel line.

Next in <FIG> the primary fuel conduit <NUM> will be installed in its constituent elements into the vehicle structure <NUM> through the accessible spaces in the structure. The primary fuel conduit <NUM> can be made up of multiple constituent elements, which in preferred embodiments can include connection end fittings <NUM>, plain tubes <NUM>, tubes with fittings <NUM>, swage couplings <NUM> and bellows <NUM>. Some or all of the spacers <NUM> may be pre-installed on the constituent parts of the primary fuel conduit <NUM> before they are installed into the vehicle.

In <FIG> the primary fuel conduit <NUM> has been fully assembled whilst correctly positioned within vehicle structure <NUM> and with all joining operations such as swaging, tightening and locking performed and any installation tests completed.

In <FIG> any spacers <NUM> which had not previously been pre-installed on the primary fuel conduit elements are installed on the primary fuel conduit <NUM>.

In <FIG> the outer sleeve <NUM> is drawn over the primary fuel conduit <NUM> and the installed spacers <NUM> from one or other end of the assembly.

In <FIG> the outer sleeve <NUM> has been fully drawn over the primary fuel conduit <NUM> and is in its final installed position. In preferred embodiments this would include a mating with a matching end fitting <NUM> to create the sealed volume <NUM>. In some embodiments the outer sleeve <NUM> may have one closed end <NUM> to create the closing of the sealed volume <NUM>, but the closed end <NUM> will have an opening for the fuel <NUM> to flow and to clear any specific features on the adjacent primary fuel conduit <NUM> fitting. In other embodiments both ends of the outer sleeve <NUM> may be of open design to mate with an end fitting <NUM>.

In <FIG> the inner element <NUM> of the clamps <NUM> are installed over the outer sleeve <NUM> and positioned inside the structural penetrations <NUM>.

In <FIG> the outer element <NUM> of the clamps <NUM> are installed over the outer sleeve <NUM> and the slid in over the inner element <NUM> of the clamp <NUM> and inside the structural penetrations <NUM>. The outer elements <NUM> are then rotated until they are positioned in their locking position and locked in place. Additionally in some embodiments with a connection end fitting <NUM> that does not have a threaded attachment for the outer sleeve <NUM>, at this stage one or more of the necessary fitting sleeve clamp <NUM> would be installed over the outer sleeve <NUM> and connection fitting <NUM> to complete the seal of sealed volume <NUM>.

<FIG> outlines a preferred embodiment of the clamp <NUM>. The clamp <NUM> consists of an inner element <NUM> and an outer element <NUM>, and these elements are made out of plastic in the preferred embodiment, but various materials may be used. The inner element <NUM> has an inner diameter sized to match the outer sleeve <NUM> of the protection system. The inner bush element <NUM> of the inner element <NUM> locates inside the structural penetration <NUM> and has a mating surface <NUM> that is eccentric to the main axis of the clamp. In one embodiment of the inner element <NUM>, the base of the clamp has a cut out <NUM>, and the bearing surface is split <NUM> and possible to open up, so that the whole clamp can be fitted over the outer sleeve <NUM>. An alternative embodiment of the inner element 41a consists of two or more separate elements that can be connected by a snap fitting or mechanical fasteners over the outer sleeve <NUM>. The outer element <NUM> may in some embodiments be single piece with a cut-out similar to the depiction of the inner element <NUM>, but in the preferred embodiment the outer element <NUM> will be two or more separate elements that can be connected by a snap fitting or mechanical fasteners over the outer sleeve <NUM>. The mating surface <NUM> of the outer element <NUM> that mates to the mating surface <NUM> of the inner element <NUM> will have a matched eccentric offset to the main axis of the clamp so that when the outer element <NUM> is rotated in relation to the inner element <NUM> a closing force will be induced on the inner bush element <NUM> of the inner element <NUM>, creating a clamping force on the outer sleeve <NUM> and the assembled protected fuel line <NUM>. Once the rotation of the outer element <NUM> is sufficient and complete in relation to the inner element <NUM>, both elements will be locked in place to maintain the clamping force. In the preferred embodiment of the clamp <NUM> the inner element <NUM> and outer element <NUM> are shown with mounting holes <NUM> to allow the clamp to be mechanically fastened to the vehicle structure <NUM> and for both elements to be fastened together. Other embodiments may alternatively use snap fittings or other means to locate and lock the inner element <NUM> and outer element <NUM> to the vehicle structure <NUM>.

<FIG> shows two preferred embodiments of a spacer <NUM>. The spacer <NUM> has an internal diameter <NUM> that is sized by the primary fuel conduit <NUM> pipe it is attached to. For intermediate spacers used solely to position the outer sleeve <NUM> and not used for clamping locations then the internal diameter <NUM> will usually be an interference fit to keep the spacer <NUM> in position relative to the primary fuel conduit <NUM>. If the spacer <NUM> is used in conjunction with a clamp <NUM>, then the inner diameter <NUM> may be a clearance fit to allow translational movement of the primary fuel conduit <NUM> to minimise any stresses in the primary fuel conduit <NUM> due to thermal loads or structural movements. Where the inner diameter <NUM> is clearance fit, then the spacer <NUM> will have one or more temporary installation attachments to stop it from moving whilst the outer sleeve is drawn over it. If, however, the spacer <NUM> is used in conjunction with a clamp <NUM> and sub-division of the sealed volume <NUM> is desired then the inner diameter <NUM> may be an interference fit to create a complete seal at the clamping position.

The outer diameter <NUM> of the spacer <NUM> is defined by the inner diameter of the outer sleeve <NUM>. For intermediate spacers the outer diameter <NUM> will be a clearance to the inner diameter of the outer sleeve <NUM> to ease the installation of the outer sleeve over the assembled primary fuel conduit <NUM> and attached spacers <NUM>. In the clamping locations, the outer diameter <NUM> will be a close fit or transition fit so that a good clamp can be formed between the spacer <NUM>, the intermediate outer sleeve <NUM>, and the clamp <NUM>.

For locations where the spacer <NUM> is not used to sub-divide the sealed volume <NUM>, then to promote the ability of leaked fuel to move within the sealed volume and/or to reduce weight then holes <NUM> will be introduced into the spacer <NUM> in preferred embodiments. These holes <NUM> may be circular or any other shape appropriate for weight and strength.

In preferred embodiments the outer corners <NUM> of the spacer <NUM> will be rounded to avoid the outer sleeve <NUM> catching on the spacer during installation or abrading on the spacer <NUM> in service.

The spacer <NUM> may be formed from a single element 50a, where the spacer <NUM> can be installed on the plain tubes <NUM>, tubes with fittings <NUM>, or other elements of the primary fuel conduit <NUM> before it is assembled. Where the spacer 50a is made of a flexible material, some embodiments can have a radial split or cut to enable the spacer 50a to be installed on the assembled primary fuel conduit <NUM>. Where spacers <NUM> of the configuration of a single element 50a cannot be fitted, then a multiple element 50b spacer with a cut <NUM> into two or more elements can be embodied. This multiple element 50b spacer can in some embodiments be connected by a fastener <NUM> or can alternatively use a snap fit between elements.

<FIG> shows and embodiment of a trip wire <NUM>, using a frangible wire <NUM>, to detect failures of the primary fuel conduit <NUM> that cause the sealed volume <NUM> to pressurise beyond the operational design pressure, creating an expansion of the outer sleeve <NUM>. In preferred applications the frangible wire <NUM> will be metal and with the failure expansion of the outer sleeve <NUM> causing the loop 60a of the frangible wire <NUM> to snap and as the frangible wire <NUM> in this case is conductive this loss of conductivity can be detected if monitored. An assembled fuel conduit may have one or more loops 60a to maximise the ability to detect failure, although a single loop will usually be sufficient. For applications where it is chosen to sub-divide the sealed volume <NUM> with sealed spacers <NUM> and clamps <NUM>, then a loop 60a will be required for each sub-divided volume. If these loops 60a are connected in parallel with an individual trip wire <NUM>, then it will be possible to detect exactly in which sealed volume the failure has occurred. In applications where there are more than one loop 60a on an undivided sealed volume <NUM> then these will be interconnected by an extension of the wire 60b with all loops 60a and the connecting wires 60b being a single continuous piece of wire. To form the loops 60a a crimp 60c is used to stop the wire pulling through and expanding the loop 60a rather than failing due to the over pressure of the sealed volume. The frangible wire <NUM> in preferred embodiments will have its outside protected to avoid conduction through the crimp 60c point and to also protect the wire from abrasion. The trip wire <NUM> at its termination 60d may either be connected in series with the trip wire of adjacent assemblies, but giving less fidelity in failure identification, or via an independent connection to the failure sensor. In preferrable embodiments each individual sealed volume <NUM> or sub-division of the sealed volume will have an independent trip wire <NUM> to enable easy maintenance trouble shooting. The frangible wire <NUM> in preferred applications is conductive metal but may be replaced with another media where a break from the outer sleeve <NUM> expanding will result in failure detection, such as a fibre optic wire. An alternate sensing solution to a trip wire <NUM> would be the direct placement of pressure sensors within the sealed volume during assembly, or monitoring of a fused burst vent <NUM> if fitted.

<FIG> shows a preferred embodiment of an end fitting <NUM> to connect the primary fuel conduit <NUM> and the outer sleeve <NUM> to create the sealed volume <NUM>. This end fitting <NUM> in preferred embodiments can have a threaded connection 70a to attach the primary fuel conduit <NUM>, which is not shown in the figure. The end fitting <NUM> has a bore 70b through it to connect the primary fuel conduit <NUM> to an attached equipment, storage vessel or another fuel line, with an interface space 70c for an appropriate attachment, such as a thread, to make the connection. The end fitting <NUM> will also have features such as fastener holes 70d in preferred embodiments to attach the fitting to a mounting structure or to attach it to equipment.

The end fitting <NUM> also has an interface surface 70e to attach the outer sleeve <NUM>. This interface surface may in some embodiments include matching surfaces to enable additional clamping features such as rings <NUM> that are seated within respective annular grooves to securely clamp the selectively permeable or impermeable membrane 20a and seal it to the end fitting <NUM> to avoid leakage from the sealed volume <NUM>. Sealant to enhance the seal between the interface surface 70e and selectively permeable or impermeable membrane 20a may be additionally used in some embodiments. Over the top of the selectively permeable or impermeable membrane 20a is the reinforcing weave or coil 20b, which will be additionally clamped to the fitting by one or more fitting sleeve clamps <NUM>. Alternate embodiments of the invention may replace the interface surface 70e with a male thread and integrate a matching female threaded fitting onto the end of the outer sleeve <NUM> to create the attachment between the end fitting <NUM> and the outer sleeve <NUM>.

If pressurised or inerting gas is used in the sealed volume <NUM>, then in preferred embodiments one of the end fittings <NUM> for an assembled protected fuel line will have a connection and valve <NUM> to allow the introduction of the gas. Alternatively, a valve fitting may be inserted into the outer sleeve <NUM>, but with more difficulty. This valve <NUM> can be used during period maintenance to confirm the contained pressure within the sealed volume and that there are no leaks from the outer sleeve <NUM> and end fittings <NUM>.

Claim 1:
A protected fuel line (<NUM>) configured to be within a vehicle, the fuel line comprising:
a primary fuel conduit (<NUM>) configured to be installed into the structure of the vehicle (<NUM>) for delivery of a fuel to a propulsion system, the primary fuel conduit (<NUM>) having a first end and a second end and comprising a plurality of interconnected rigid or semi-rigid fuel pipes (<NUM>, <NUM>, <NUM>) and connecting elements (<NUM>, <NUM>) therebetween; the protected fuel line further comprising
an outer sleeve (<NUM>) comprising a flexible membrane which is impermeable to fuel, the flexible membrane encircling the primary fuel conduit (<NUM>) and sealed between the first end (<NUM>) and the second end (<NUM>) of the primary fuel conduit to define a sealed volume (<NUM>) between the primary fuel conduit (<NUM>) and the outer sleeve (<NUM>),
such that the fuel line (<NUM>) is protected from escape of fuel due to failure of the primary fuel conduit (<NUM>) by the sealed volume (<NUM>) being configured to restrain fuel leaking from the primary fuel conduit (<NUM>).