Housing for compensating for thermal expansion on optical fibre lines

Abstract

The present invention relates to a housing for compensating for thermal expansion on optical fibre lines, characterized in that it comprises a metal cylindrical casing (1) defining an inner surface of diameter DB, closed by two end faces (6, 7), and a winding of at least one optical fibre (2) of outside diameter dF, the diameter DB being between 4 times the outside diameter dF of said at least one optical fibre (2) and 15 times the outside diameter dF of said at least one optical fibre (2).

Description

“Thermal expansion compensation box on fiber optic lines” Technical field of the invention

[0001] The present invention relates to the field of protection of optical fibers used in high temperature, high pressure or high radiation environments, for example in a nuclear reactor vessel, a boiler, a furnace, a combustion chamber.

[0002] The invention relates in particular to optical sensors or detectors based on optical fibers of objects, position, IR radiation or temperature, by Raman effect or by other non-linear optical effects, introduced into extreme environments (high temperature, high pressure, strong radiation) where the active detectors as well as their electronics cannot be present. State of the art

[0003] The demand for optical sensors is increasing in extreme environments. This is possible only by using fiber optic lines protected by metal tubes with sensor ends or connectors.

[0004] Known in the prior art is US patent US9091834 relating to a method and apparatus for releasing thermal stress on optical fibers in fiber optic splice closures used in hostile thermal environments. The optical fibers are deployed through the splice tray and are woven around the removable pins in a wavy pattern during assembly of the fiber optic splice closure before the removable pins are removed so that the enclosed fibers are longer than the closure and are not under tension in the fluctuating thermal environment of a wellbore.

[0005] US patent US6829424 relates to a cable sleeve for a fiber optic cable, preferably for a minicable or microcable with fiber optics, having cable insertion units perpendicular to the axis of the sleeve body. This cable sleeve can be inserted vertically into a core drilled in the subsoil or in the road surface. Splice cassettes located in the sleeve body can be brought out upwards for maintenance work, thanks to the buried excess optical fiber lengths. The excess optical fiber lengths are preferably guided in a protective tubular hose deposited in the form of loops in the sleeve. Disadvantages of the prior art

[0006] The prior art solutions are not satisfactory because at very high temperatures the metal tube, which has a higher coefficient of expansion than silica, will expand more than the fiber. This difference in expansion will cause breakage of the optical fiber, knowing that it is fixed to the tube at its ends.

[0007] For example, the expansion coefficient of a 316L stainless steel tube: 17.5 10 -6 °K -1 (from 20°C to 500°C)

[0008] The expansion coefficient of a silica optical fiber: 4.8 10 -7 °K -1 (from 0°C to 900°C)

[0009] For a distance of 10 m (measured at 25°C), the tube expands, in state-of-the-art designs, by 50 mm more in length than the optical fiber at 360°C.

[0010] Furthermore, the solution proposed by patent US9091834 is also not satisfactory because the undulations formed by the fiber have areas of very small radius of curvature likely to alter the quality of the transmission and create areas of fragility.

[0011] The present invention aims to address these drawbacks. To this end, the invention relates to a thermal compensation box having the technical characteristics set out in claim 1.

[0012] This thermal compensation box comprises a cylindrical metal casing defining an interior surface of diameter D B , closed by two front faces and a winding of at least one optical fiber with an external diameter of d F , the diameter D B being between 4 times the external diameter d F of said less than one optical fiber and 15 times the outer diameter of F of said at least one optical fiber.

[0013] The object of the present invention may also have one or a compatible combination of the following characteristics: The cylindrical envelope is preferably between 7.5 times the outer diameter of F of said less than one optical fiber and 10 times the outer diameter of F of said at least one optical fiber the faces of the cylinder have inputs and outputs of the optical fibers offset radially relative to the axis of the cylinder the faces of the cylinder (6, 7) have inputs and outputs of the optical fibers (1) are not aligned longitudinally

[0014] The invention also relates to a method for installing a line of at least one optical fiber protected by a metal tube in a high-temperature environment, characterized in that at least one thermal compensation box is integrated on said line comprising a cylindrical metal casing defining an interior surface of diameter DB , closed by two front faces and a winding of at least one optical fiber with an external diameter of d F , the diameter D B being between 4 times the external diameter d F of said at least one optical fiber (2) and 15 times the outer diameter d F of said at least one optical fiber.

[0015] Preferably, said tubes (4, 5) are welded or brazed to the faces (6, 7) of said cylinder.

[0016] Advantageously, said lines of optical fiber tubes are assembled and sealed in production at room temperature.

[0017] Description of non-limiting examples of implementation

[0018] The present invention will be better understood on reading the following description, concerning a non-limiting example of embodiment illustrated by the appended drawings where:

[0019] The figure represents a perspective view of a compensation box according to the invention.

[0020] The figure represents a longitudinal sectional view of a compensation box according to the invention.

[0021] The figure represents a transparent sectional view of a compensation box according to the invention

[0022] The figure represents a schematic view of the winding of a fiber in the compensation box according to the invention.

[0023] The figure represents a perspective view of a variant of a compensation box according to the invention. General principle of the invention

[0024] The invention relates to the compensation of the differential expansion occurring between the optical fiber and the metal protective sheath by a thermal expansion compensation box (1) inserted on an optical fiber line protected by metal tubes (4), operating at high temperature high pressure under radiation.

[0025] This compensation box (1) is integrated in the middle of a line of optical fibers protected by tubes. The tubes are welded or brazed to the box and the fibers (2) are wound on the internal face (3) of the cylinder before emerging in the tubes (4). The number of turns (3) is calculated according to the length of the tubes and their thermal expansion. The box (1) and the tubes (4, 5) are welded or brazed together and therefore watertight.

[0026] The fiber winding is of the helical type, the fiber(s) expanding to form a spiral coming into contact with the inner surface of the compensation box, forming loops filling the cross-section of the box. This configuration makes it possible to very significantly increase the maximum bending radius undergone by the fiber, unlike the solution proposed in the prior art.

[0027] The ends of the optical fibers are attached to connectors or probe heads, which are soldered or welded to the metal tubes (4, 5).

[0028] The compensation box (1) according to the invention is a cylinder with an internal diameter significantly greater than the breaking radius of curvature of the optical fiber (2) and less than or equal to the nominal radius of curvature. This nominal radius of curvature is recommended by the fiber manufacturer.

[0029] In the absence of a manufacturer's recommendation, the nominal bending radius corresponds to 10 times the outer diameter of the cable without tensile load and, in a tensile or "loaded" cable situation, the nominal bending radius corresponds to 15 times the outer diameter of the cable. A reference nominal bending radius will typically be 12 times the outer diameter of the cable.

[0030] This nominal radius of curvature is typically between 2 and 25 millimeters.

[0031] The breaking bend radius is the minimum allowable radius that can bend the optical fiber without damaging, kinking, or shortening its lifespan and is called the bending radius. It should be between 4 and 7.5 times the outer diameter.

[0032] The optical fiber (2) is wound several times on the inner face of the cylinder. The number of turns depends on the difference in expansion between the tube (4, 5) and the optical fiber (2), i.e. the length of the optical line tube. Thus, the volume of this housing (1) allows the fiber to wind or unwind depending on the expansion of the tubes (temperature of the medium).

[0033] The optical fiber tube lines (4, 5) are assembled and sealed in production at room temperature. Their high temperature uses require a transfer box (1) to absorb the difference in expansion between the metal tubes and the optical fibers.

[0034] The material and thickness of the cylindrical housing are designed to withstand pressure and thermal expansion.

[0035] The metal tube optical lines can contain a single optical fiber or a group of optical fibers (a bundle). The compensation box (1) can accommodate several fibers at the input and output.

[0036] It is also possible to place several compensation boxes (1) on a long optical fiber line protected by a tube.

[0037] The inputs and outputs of the optical fibers (1) are made through the faces of the cylinder (6, 7). These inputs and outputs are off the longitudinal axis (9) of the cylinder and never face to face in order to push the fibers (1) to wind inside. Another example of achievement

[0038] This illustrates another example of an implementation with an excess length of fiber in order to overcome the problem of fiber slippage induced by the thermal expansion differential at 360°C between the 1 / 16'' stainless steel protection tube of a length of 2 m and the four 200 / 220 µm copper-coated optical fibers.

[0039] The theoretical thermal expansion differential at 360°C between the 316L stainless steel tube, external diameter: 1.58 mm, internal diameter: 0.9 mm, length: 2 m, and a silica optical fiber is 14 mm.

[0040] The fibers are copper coated or 200 / 220 µm aluminum coated fibers for their corrosion resistance properties.

[0041] The differential of the expansion coefficients is, for an optical fiber ΔTSi= 0.2 x 10-5 / °C longitudinally and 0.5 x 10-6 / °C radially Silica and for stainless steel ΔTInox= 18.5 x 10- 6 / °C between 20 and 400 °C.

[0042] The excess length of fiber to be integrated into the cylinder is therefore at least 14 mm, we add a safety margin to this length to increase it to 20 mm. It was decided to distribute this excess length of 20 mm in two compensation cylinders (10 mm per cylinder) placed at each end of the cord near the ferrules, in order to facilitate the sliding of the fiber behind the ferrule during expansion of the tube.

[0043] In order to ensure the compactness of the cord and its integration into a reduced footprint, the ratio: diameter / length of the compensation cylinder must be proportionate as closely as possible to the size of the 1 / 16'' stainless steel tube.

[0044] 4 copper-coated 200 µm fibers are introduced into a tube with an external diameter of 12 mm, an internal diameter of 10 mm and a length of 150 mm, to allow the introduction of an excess fiber length of 10 mm per fiber.

[0045] the radius of curvature caused by the helical shape of the fibers gives a radius of curvature of 40 mm compatible with the manufacturer's recommendations.

[0046] For a helix of wires housed inside a spiral cylinder, the inverse of its radius of curvature is defined by cos²α / r, where r represents the radius of the circular cylinder around which the wire is wound, and α denotes the angle formed by the tangent to the curve with a plane perpendicular to the axis of the spring, as illustrated in the. The angle α is determined by theoretical calculation (sinα= h / L, see the) and amounts to 1.215 rad (representing the angle between the length of the cylinder and the length of the fiber inside the spiral cylinder).

[0047] In order to have regular and synchronous turns, the four fibers are pushed into the housing at the same time. If the fibers are inserted asynchronously, the behavior remains identical to that of synchronous turns; when the pressure is released, the fibers return to their initial positions.

[0048] For example, the fiber is of the Exail™ / Photonics™ type, reference IXF-MM-L-200-220-022-AL with a core diameter of 202.9µm, a cladding diameter of 221.5µm and an aluminum cladding diameter of 271µm. The numerical aperture is 0.219°

Claims

Thermal compensation box characterized in that it comprises a metallic cylindrical casing (1) defining an interior surface of diameter D B , closed by two front faces (6, 7) and a helical winding of at least one optical fiber (2) with an external diameter d F , the diameter D B being between 4 times the external diameter d F of said at least one optical fiber (2) and 15 times the outer diameter d F of said at least one optical fiber (2). Thermal compensation box according to claim 1 characterized in that it comprises a cylindrical casing and preferably between 7.5 times the external diameter d F of said at least one optical fiber (2) and 10 times the outer diameter d F of said at least one optical fiber (2). Thermal compensation box according to claim 1 or 2 characterized in that said front faces (6, 7) have inputs and outputs of the optical fibers (2) offset radially relative to the axis (9) of said cylindrical casing (1). Thermal compensation box according to claim 1 or 2 characterized in that said front faces (6, 7) of said cylindrical casing (1) have inputs and outputs of the optical fibers (2) which are not aligned longitudinally. Method for installing a line of at least one optical fiber protected by a metal tube in a high temperature environment characterized in that at least one thermal compensation box is integrated on said line comprising a cylindrical metal casing (1) defining an interior surface of diameter D B , closed by two front faces (6, 7) and a winding of at least one optical fiber with an external diameter dF , the diameter D B being between 4 times the external diameter d F of said less than one optical fiber and 15 times the outer diameter of F of said at least one optical fiber. Method for installing a line of at least one optical fiber protected by a metal tube according to claim 5, characterized in that tubes (4, 5) are welded or brazed on said front faces (6, 7). Method for installing a line of at least one optical fiber protected by a metal tube according to claim 5 characterized in that said lines of optical fiber tubes are assembled and sealed in production at room temperature.

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