[0001] The invention relates to a method for determining deformation of an equipment in contact with hot material and to the associated equipment.

[0002] As known by the man skilled in the art, some equipment comprises an inner (or hot) face in contact with hot material and an outer (or cold) face opposite to this inner face. This is notably the case of a blast furnace that generally comprises an inner wall partly covered with equipment defining cooling plates (or staves). In the rest of the text, the term cooling plates or staves will be used indifferently.

[0003] For instance, in the case of a cooling plate the inner face may comprise ribs, parallel therebetween, and grooves, also parallel therebetween and each separating two neighboring ribs, that allow anchorage of a refractory brick guniting or lining or of an accretion layer inside the blast furnace. The cooling plate is also equipped with cooling tubes spreading all along the outer face of the stave to keep it cooled while in operation. [0004] This stave body undergoes deformations due to differential dilatation linked to the fixation of the stave to the blast furnace wall and to the thermal constraints the stave is subjected to. These deformations, which increase with time, mainly results in a curvature (or bending) in a vertical cross section that is often called "banana effect". The magnitude of this curvature may vary from place to place in the same stave body.

[0005] The stave deformations may induce a protrusion of a part of the stave into the furnace which will be preferentially hit by materials charged into the furnace, it will induce a premature wear of the stave and may lead to a breakage in the furnace wall which would no longer be protected. So, it would be useful to monitor the deformations of a stave in order to replace it when the magnitude of its deformations reaches a predefined threshold. [0006] But, it is very difficult to perform such a monitoring due to the very constraining environment and also due to the lack of available space on or in the stave, and more generally in an equipment subjected to hot conditions on its inner face.

[0007] So, an objective of the invention is to allow determination and monitoring of equipment deformations.

[0008] To this end, the invention relates to a method of monitoring of deformation along a first direction of an equipment in contact with hot material and comprising an inner face in contact with this hot material and an outer face opposite to this inner face.

[0009] This method comprises the steps of: - providing on the equipment outer face: o at least one first optical fibre set along the first direction and comprising at least two first Bragg gratings having different grating periods for reflecting photons having different Bragg wavelengths, and o at least two second optical fibres set along at least one second direction that crosses a first optical fibre at a crossing point located in the vicinity of one of the first Bragg gratings, and each comprising, in the vicinity of a crossing point, a second Bragg grating having a grating period for reflecting photons having a Bragg wavelength,

- inputting photons having wavelengths belonging to a wavelength group comprising all of these Bragg wavelengths, into respective first ends of the first and second optical fibres,

- determining wavelengths of photons reflected by a corresponding first or second Bragg grating, and - determining a deformation of the equipment along the first direction from corresponding couples each comprising a determined wavelength and a corresponding Bragg wavelength.

The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations:

- the step of providing may comprise providing :

o two first optical fibres set along parallel first directions and comprising at least three first Bragg gratings having different grating periods for reflecting photons having different Bragg wavelengths, and o three second optical fibres set along second directions that each cross these two first optical fibres at crossing points located in the vicinity of three of the first Bragg gratings, and each comprising at least two second Bragg gratings having different grating periods for reflecting photons having different Bragg wavelengths and located in the vicinity of corresponding crossing points;

- in a first alternative, the step of determining a deformation may comprise the subsequent sub steps of : o determining a strain variation experienced by each first Bragg grating, considering there is no temperature variation, from a corresponding couple comprising a determined wavelength of photons reflected by this first Bragg grating and the Bragg wavelength of this first Bragg grating and from a strain reference, o determining a temperature variation into each second Bragg grating from a corresponding couple comprising a determined wavelength of photons reflected by this second Bragg grating and the Bragg wavelength of this second Bragg grating, o correcting the determined strain variation of each first Bragg grating according to the determined temperature variation of at least a second Bragg grating located into the vicinity of this first Bragg grating, and o determining a deformation of the equipment along the first direction from the corresponding corrected strain variations of the at least first optical fibre;

- in a second alternative, the step of determining a deformation may comprise the subsequent sub steps of: o determining a temperature variation into each second Bragg grating from a corresponding couple comprising a determined wavelength of photons reflected by the second Bragg grating and the Bragg wavelength of the second Bragg grating, o determining a corrected strain into each first Bragg grating using the previously determined temperature variation, o determining a corrected strain variation experienced by each first Bragg grating from the previously determined corrected strain and from a strain reference, and o determining a deformation of the equipment along the first direction from the corresponding corrected strain variations of the at least first optical fibre.

[0010] The invention also relates to an equipment intended for being in contact with hot material and comprising an inner face in contact with this hot material and an outer face opposite to this inner face.

[0011] This equipment is characterized in that its outer face comprises:

at least one first optical fibre set along a first direction and comprising at least two first Bragg gratings having different grating periods for reflecting photons having different Bragg wavelengths, at least two second optical fibres set along at least one second direction that crosses this first optical fibre at a crossing point located in the vicinity of one of the first Bragg gratings, and each comprising, in the vicinity of a crossing point, a second Bragg grating having a grating period for reflecting photons having a Bragg wavelength,

photon sources arranged for inputting photons, having wavelengths belonging to a wavelength group comprising all of these Bragg wavelengths, into respective first ends of these first and second optical fibres,

sensors coupled respectively to these first ends and arranged for determining wavelengths of photons reflected by a corresponding first or second Bragg grating, and

processing means arranged for determining a deformation of the equipment along the first direction from corresponding couples each comprising a determined wavelength and a corresponding Bragg wavelength.

[0012] The equipment of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations: it may comprise two first optical fibres set along parallel first directions and comprising at least three first Bragg gratings having different grating periods for reflecting photons having different Bragg wavelengths, and three second optical fibres set along second directions that each cross these two first optical fibres at crossing points located in the vicinity of three of the first Bragg gratings, and each comprising at least two second Bragg gratings having different grating periods for reflecting photons having different Bragg wavelengths and located in the vicinity of corresponding crossing points; in a first embodiment, the processing means may be arranged for determining a strain variation experienced by each first Bragg grating from a corresponding couple comprising a determined wavelength of photons reflected by this first Bragg grating and the Bragg wavelength of this first Bragg grating, and a temperature variation into each second Bragg grating from a corresponding couple comprising a determined wavelength of photons reflected by this second Bragg grating and the Bragg wavelength of this second Bragg grating, then for correcting' the determined strain variation of each first Bragg grating as according to the determined temperature variation of at least a second Bragg grating located into the vicinity of this first Bragg grating, then for determining a deformation of the equipment along each first direction of a first optical fibre from the corresponding corrected strain variations of this first optical fibre; each second optical fibre may be installed in a thermal-conducting tube that is set along one of the second directions;

> each thermal-conducting tube may be installed fixedly in a horizontal groove defined into the outer face along one of the second directions;

• each thermal-conducting tube may be installed fixedly in a horizontal groove by means of a glue;

> each thermal-conducting tube may comprise an exterior-facing part covered with a copper paste;

• the copper paste may comprise an exterior-facing part covered with a copper cover; all the second directions may be parallel therebetween; each first optical fibre may be installed fixedly in a vertical groove defined into the outer face along a first direction;

> each first optical fibre may be installed fixedly in a vertical groove by means of a glue; each first optical fibre may comprise an exterior-facing part covered with a seal material;

• the seal material may comprise an exterior-facing part covered with a protection material intended for protecting it against heat shocks; it may define a cooling plate of a blast furnace; > the first direction may be a vertical direction of the cooling plate and the second direction may be an horizontal direction of the cooling plate.

[0013] The invention also relates to a blast furnace comprising at least one equipment such as the one above introduced.

[0014] Other characteristics and advantages of the invention will emerge clearly from the description of it that is given below by way of an indication and which is in no way restrictive, with reference to the appended figures in which:

- figure 1 illustrates schematically, in a perspective view, an example of embodiment of a cooling plate according to the invention,

- figure 2 illustrates schematically, in a perspective view, a part of an example of embodiment of a first optical fibre of the cooling plate illustrated in figure 1 ,

- figure 3 illustrates schematically, in a first cross section view, a part of the cooling plate illustrated in figure 1 in the area of a crossing point, - figure 4 illustrates schematically, in a second cross section view (in a plane perpendicular to the plane of the first cross section view), a part of the cooling plate illustrated in figure 1 in an area comprising a horizontal groove which comprises a thermal-conducting tube comprising a second optical fibre, and - figure 5 illustrates schematically, in a perspective view, another example of embodiment of a cooling plate according to the invention.

- Figure 6 illustrates schematically the process steps to determine deformation of the equipment according to one embodiment of the invention

[0015] The invention aims, notably, at proposing a method for determining deformation of an equipment 1 that can be used in a system or apparatus or installation and comprising determination means intended for determining its deformation along a first direction D1.

[0016] In the following description it will be considered as an example that the equipment 1 is a cooling plate that can be used in a blast furnace and which comprises monitoring means intended for monitoring its deformation along a first direction D1. But the equipment 1 may be also a continuous casting roller or a galvanization snout for instance. [0017] An example of embodiment of an equipment 1 (here a cooling plate) according to the invention is illustrated in figure 1. Such a cooling plate 1 is intended to be mounted on an inner wall of a blast furnace.

[0018] As illustrated, a cooling plate 1 according to the invention comprises a copper body 2 having an inner (or hot) face 3 and an outer (or cold) face 4 opposite to its inner face 3. The body 2 may be made of copper, for instance, or of pig iron.

[0019] The inner face comprises several ribs 22 parallel there between and separated by grooves 23. Once the cooling plate 1 is mounted on the blast furnace inner wall, its ribs 22 and grooves 23 are arranged (or set) horizontally.

[0020] The outer face 4 is fixed to the blast furnace inner wall. So, the inner face 3 is the body face in contact with the very hot material and gas present inside the blast furnace.

[0021] The ribs 22 and grooves 23 may have a dovetail cross-section in order to optimize anchorage of a process generated accretion layer. [0022] The outer face 4 comprises at least one first optical fibre 5j, at least two second optical fibres 7 _k , photon sources 9, sensors 10 and processing means 11 which define together determination means.

[0023] The (each) first optical fibre 5, is set along a first direction D1 (perpendicular to the ribs 22 and grooves 23) and comprises at least two first Bragg gratings 6, _j having different grating periods for reflecting photons having different Bragg wavelengths

[0024] A non-limiting example of a part of a first optical fibre 5, comprising three first Bragg gratings 6M to 6 _i3 (j = 1 to 3) is illustrated in figure 2. The first Bragg grating 6 has a first grating period for reflecting photons having a first Bragg wavelength, the second Bragg grating 6 _i2 has a second grating period (different from the first one) for reflecting photons having a second Bragg wavelength (different from the first one), and the third Bragg grating 6 _i3 has a third grating period (different from the first and second ones) for reflecting photons having a third Bragg wavelength (different from the first and second ones).

[0025] It is recalled that an optical fibre Bragg grating is a distributed Bragg reflector defines in a short portion of an optical fibre and intended for reflecting photons having at least one particular wavelength λ _Β , named Bragg wavelength, while transmitting photons having wavelengths different from each Bragg wavelength λ _Β it reflects. Such a Bragg grating may be defined, for instance, by creating a periodic variation in the refractive index of the optical fibre core, which generates a wavelength-specific dielectric mirror.

[0026] The reflected Bragg wavelength λ _Β is defined by the following equation: λ _Β = 2*η*Λ, where n is the effective refractive index of the Bragg grating in the optical fibre core (which depends on the photon wavelength and on the photon propagation mode (in multimode waveguides)), and Λ is the grating period.

[0027] In figure 2, the photons, having different wavelengths and that enter the first optical fibre 5, on the left side (arrow F0), propagate into the latter (5,) and when they reach the first Bragg grating 6M those having the first Bragg wavelength are reflected (arrow F1), while those having wavelengths different from the first Bragg wavelength are transmitted (arrow FT) and keep on propagating towards the second Bragg grating 6 _i2 . When the transmitted photons reach the second Bragg grating 6 _i2 , those having the second Bragg wavelength are reflected (arrow F2), while those having wavelengths different from the second Bragg wavelength are transmitted (arrow F2') and keep on propagating towards the third Bragg grating 6 _i3 . When the transmitted photons reach the third Bragg grating 6 _|3 , those having the third Bragg wavelength are reflected (arrow F3), while those having wavelengths different from the third Bragg wavelength are transmitted (arrow F3') and keep on propagating towards the first optical fibre 5,.

[0028] It is important to understand that when an optical fibre comprises a Bragg grating and is not "stressed" (by a strain variation or a temperature variation) its Bragg wavelength is predefined. But when this optical fibre is stressed in a portion comprising this Bragg grating, its Bragg wavelength is modified and therefore is no more the predefined one. So, a local stress variation or temperature variation of an optical fibre can be detected by a modification of the Bragg wavelength of the photons reflected by its local Bragg grating. [0029] Each second optical fibre 7 _k is set along a second direction D2 that crosses each first optical fibre 5, at a crossing point 21 _ki located in the vicinity of one of the first Bragg gratings 6,j. So, the different second optical fibres 7 _k are set along at least one second direction D2. More, each second optical fibre 7 _k comprises, in the vicinity of each crossing point 21 _ki , a second Bragg grating 8 _kn (not illustrated but similar to the ones illustrated in figure 2) having a grating period for reflecting photons having a Bragg wavelength. So when the determination means comprises only one first optical fibre 5,, each second optical fibre 7 _k comprises at least one second Bragg grating, and when the monitoring means comprises two first optical fibres 5, (i = 1 or 2, as illustrated in figure 1), each second optical fibre 7 _k comprises at least two second Bragg gratings.

[0030] In the case where the equipment 1 is a cooling plate (or stave), the first direction D.1 may be a vertical direction of this cooling plate 1 and the second direction D2 may be an horizontal direction of this cooling plate 1. But the first D1 and second D2 directions of the equipment 1 depend of its arrangement. What is important is that the first direction D1 is a direction along which the equipment 1 is subjected to a deformation due to the hot conditions on its inner face 3. In the non- limiting example illustrated in figure 1 all the second directions D2 are parallel therebetween. But in another type of equipment 1 the second directions D2 could be different therebetween. [0031 ] In the non-limiting example illustrated in figure 1 , the monitoring means comprises three second optical fibres 1 to 7 ₃ (k = 1 to 3) for the determination of the temperature variations with at least two second Bragg gratings 8 _kn> and two first optical fibres 5j (i = 1 or 2) for the determination of the strain variations, using said temperature variations with at least three first Bragg gratings 6y,. So, each second optical fibres 7 _k crosses the two first optical fibres 5, at two crossing points 21 _kj located respectively in the vicinity of two of their (at least) three first Bragg gratings 6j _j , and comprises at least two second Bragg gratings 8 _kn located respectively in the vicinity of these two crossing points 21 _ki . There are always a first Bragg grating 6 and a corresponding second Bragg grating 8 _kn located in the vicinity of a crossing point 21 _k i.

[0032] But in variants of embodiment the determination means could comprise one first optical fibre 5i(with at least two first Bragg gratings 6i _j ) and two second optical fibres 7 _k (k = 1 or 2, with at least one second Bragg grating 8 _kn ), or two first optical fibres 5, (i= 1 or 2, with at least two first Bragg gratings 6 ) and two second optical fibres 7 ₁ (k= 1 or 2with at least two second Bragg gratings 8 _kn ), or else three or more first optical fibres 5, and two or more second optical fibres 7 _k .

[0033] The number of second optical fibers 7 _k is advantageously chosen according to the known areas submitted to substantially different temperatures. In the case of an implementation on the cold face of a stave, it is known that the upper and lower edges are colder than the central portion, so three second optical fibers 7 _k are therefore adapted. [0034] It is important to notice that a Bragg wavelength λβ(8κ _η ) of a second Bragg grating 8 _kn defined into a second optical fibre 7 _k may be equal to a Bragg wavelength _B (6j _j ) of a first Bragg grating defined into a first optical fibre 5,, because they concern photons propagating into different optical fibres. [0035] The photon sources 9 are arranged for inputting photons, having wavelengths belonging to a wavelength group comprising all of the Bragg wavelengths, into respective first ends of the first 5, and second 7 _k optical fibres. For instance, a photon source 9 may comprise at least one light-emitting diode (or LED) providing a white light (i.e. with a broad spectrum comprising the different Bragg wavelengths of the first 6 or second Bragg grating(s) 8| _< n defined into the first 5| or second 7 _k optical fibre it feeds. [0036] In the non-limiting example illustrated in figure 1 , the photon sources 9 are located respectively near the first ends of the first 5j and second 7 _k optical fibres. But they could be located away from these first ends and coupled to the latter via waveguides or other optical fibers. [0037] The sensors 10 are coupled respectively to the first ends (of the first 5, and second 7 _k optical fibres) and arranged for determining wavelengths of photons reflected by a corresponding first 6 or second Bragg grating 8 _kn . As explained above, when a Bragg grating is not stressed, the determined wavelength of the photons reflected by this Bragg grating is equal to the Bragg wavelength of the latter.

[0038] For instance, each sensor 10 may be a compact spectrometer.

[0039] In the non-limiting example illustrated in figure 1 , the sensors 10 are located respectively near the first ends of the first 5j and second 7 _k optical fibres. But they could be located away from these first ends and coupled to the latter via waveguides or other optical fibres.

[0040] Also in the non-limiting example illustrated in figure 1 , the cooling plate 1 comprises a protected groove 27 which may comprise, for instance, cables for electrically feeding and/or controlling the photon sources 9 and sensors 10. In this case, these cables may extend up to a pipe 24 where they may be grouped together, for instance into a sheath intended for protecting them against heat and mechanical stress.

[0041 ] In a variant of embodiment illustrated in figure 5, the first ends of the first 5, and second 7 _k optical fibres are located away from the cooling plate 1 . In fact, the first 5i and second. 7k optical fibres goes into a protected groove 28 and extend up to a pipe 24 where they are grouped together into a sheath 25 intended for protecting them against heat and mechanical stress, till they reach a distant box 26 where their respective first ends are connected to the corresponding photon sources 9 and sensors 10.

[0042] The processing means 1 1 is arranged for determining a deformation of the cooling plate 1 along a first direction D1 from corresponding couples each comprising a determined wavelength X _R and a corresponding Bragg wavelength λ _Β . So, the processing means 1 1 receives the wavelengths detected by the different sensors 10 and determines a cooling plate deformation along the first direction D1 from wavelengths. For instance, if one considers the first direction D1 of the first first optical fibre 5i of the example of figure 1 , then the corresponding wavelength couples are:

- (the determined wavelength R(6H) of photons reflected by the first first Bragg grating 6n of the first first optical fibre 5-i in the vicinity of the first crossing point 2111 , and the Bragg wavelength λ _Β (6ι ι) of this first first Bragg grating 6n),

- (the determined wavelength R(8H) of photons reflected by the first second Bragg grating of the first second optical fibre 7i in the vicinity of the first crossing point 21-11 , and the Bragg wavelength λ _Β (8n) of this first second Bragg grating 8i i),

- (the determined wavelength λ _κ (6ι ₂ ) of photons reflected by the second first Bragg grating 612 of the first first optical fibre 5i in the vicinity of the third crossing point 21 ₂ i , and the Bragg wavelength λβ(6ι ₂ ) of this second first Bragg grating 612),

- (the determined wavelength _R (8 ₂ i) of photons reflected by the first second Bragg grating 8 ₂ i of the second second optical fibre 7 ₂ in the vicinity of the third crossing point 21 ₂ i , and the Bragg wavelength λ _Β (8 ₂ ι) of this first second Bragg grating),

- (the determined wavelength _R (6i ₃ ) of photons reflected by the third first Bragg grating 6i ₃ of the first first optical fibre 5i in the vicinity of the fifth crossing point 2131 , and the Bragg wavelength λ _Β (6ι ₃ ) of this third first Bragg grating 6i ₃ ), and

- (the determined wavelength λ _κ (8 ₃ ι) of photons reflected by the first second Bragg grating 8 ₃ i of the third second optical fibre 7 ₃ in the vicinity of the fifth crossing point 2 ₃ , and the Bragg wavelength λ _Β (8 ₃ ι) of this first second Bragg grating).

[0043] The determination of a cooling plate deformation along the first direction D1 from corresponding wavelength couples can be performed by the processing means 1 1 according to at least two different manners. [0044] In general terms, the processing means 1 1 may be arranged for determining :

- strain σ(6^) and strain variation Δσ(6ί ) experienced by each first Bragg grating 6j _j from a corresponding couple comprising a determined wavelength R(6 i) of photons reflected by this first Bragg grating 6y and the Bragg wavelength λ _Β (6ί]) of this first Bragg grating 6jj, and

- a temperature T(8 _kn ) and temperature variation AT(8 _kn ) into each second Bragg grating 8 _kn from a corresponding couple ^ _R (8 _kn ), _B (8 _kn )) comprising a determined wavelength _R (8 _kn ) of photons reflected by this second Bragg grating 8 _kn and the Bragg wavelength _B (8 _kn ) of this second Bragg grating 8 _kn .

[0045] Then, the processing means 1 1 are arranged for correcting the determined strain variation Δσ(6^) or the strain a(6jj) of each first Bragg grating 6y according to the determined temperature variation AT(8 _kn ) of at least a second Bragg grating 8 _kn located into the vicinity of this first Bragg grating 6jj. Then, the processing means 1 1 is arranged for determining a deformation of the cooling plate 1 along first direction D1 of a first optical fibre 5, from the corresponding corrected strain variations Aa(6 ) of a first optical fibre 5,.

[0046] In other words, the processing means 1 1 correct each strain variation Aa(6y) or strain o(6ij) determined into a first optical fibre 5, in the vicinity of a crossing point 21 _ki according to the temperature variation AT(8 _kn ) determined into at least a second optical fibre 7 _k in the vicinity of this crossing point 21 ki-

[0047] The determination of a temperature variation AT(8 _kn ) experienced by a second Bragg grating (8 _kn ) can be performed by using an equation giving the temperature T(8 _kn ) in a second Bragg grating 8 _kn as a function of a temperature reference T _re f considered when there is no deformation, the Bragg wavelength λβ(8κη) in this second Bragg grating 8 _kn considered when there is no deformation and when the temperature is equal to the reference temperature T _ref , and the determined wavelength A _R (8 _k n) of the photons reflected by this second Bragg grating 8 _kn .. For instance, T _ref = 22,5°C, and n = 1 to 3 when each second optical fibre 7 _k comprises three second Bragg grating 8kn-

where ST and S ₂ are constants relative to the second optical fibre 7 _k .

Such an equation can be used when the considered second optical fibre 7k is not strained as it will be detailed below in reference to figures 3 and 4. [0049] The temperature variation AT(8kn) may also be calculated by considering that this temperature variation AT(8 _kn ) is proportional to a wavelength variation. We then first determine a difference between each determined wavelength of photons AR(8 _kn ) reflected by a Bragg grating 8kn and corresponding Bragg wavelength A _B (8kn), and then applying the proportional factor to calculate the temperature variation AT(8 _kn ) in the second Bragg grating 8 _kn . [0050] In a first manner, the determination of a corrected strain variation Aa(6y) experienced by a first Bragg grating 6¾ is performed by using an equation giving the corrected strain σ(6¾) in a first Bragg grating 6y and according to a strain reference a _re f(6j _j ) in this first Bragg grating 6^ considered when there is no deformation along D1 and when the temperature is equal to a reference temperature T _re f, for instance 22,5°C).

[0051] For instance, this equation B may be :

where _R (6^ is the determined wavelength in the corresponding first Bragg grating 6ij,

where _B (6y) is the Bragg wavelength in the corresponding first Bragg grating 6, _, where a _s is the coefficient of thermal expansion of the cooling plate 1 ,

Of is the coefficient of thermal expansion of the first optical fibre 5, comprising the first Bragg grating 6j (for instance equal to 0.5 με/ ^ο 0),

k, Si and S ₂ are strain gauge parameters determined in calibration sheets,

ΔΤι-ef = T(8kn) - ref,

and ATo.ref = To - T _re f, (with T ₀ the temperature at the beginning of the measurement and T _re f = 22.5 °C, for instance). [0052] In a second manner and as illustrated at figure 6, the determination of the deformation along for instance the first optical fibre 5i between a first first Bragg grating 6n and a second first Bragg grating 6i ₂ is performed by :

determining the uncorrected strain variations Δσ(6ι ι) and Δσ(6ι ₂ ) from the corresponding couples ( R(6H), B(6H)), ( R(6I ₂ ), λ _Β (6ι ₂ ) using the above equation B when T ₀ is equal to T _rer and when AT(8 _kn ) is equal to 0 (no temperature variation),

determining the temperature variations ΔΤ(8 ₁₂ ) and ΔΤ(8 ₂₂ ) from the corresponding couples ( _R (8 ₁₂ ), λ _Β (8ι ₂ )), ( _R (8 ₂₂ ), λ _Β (8 ₂₂ ) of a first second Bragg grating 8 ₁₂ and of a second second Bragg grating 8 ₂₂ (see figure 1 ) using the above equation A when there is no deformation applied to the second optical fibres 7-1 , 7 ₂ at least in the vicinity of the corresponding crossing points 21 n and 21 ₂ i, correcting the previously determined strain variations Δσ(6π) and Δσ(6-| ₂ ) with the previously determined temperature variations ΔΤ(8ι ₂ ) and ΔΤ(8 ₂ ι), and - evaluating the deformation along the first optical fibre 5i between the first first Bragg grating 6n and the second first Bragg grating 6i ₂ .

The same determination method is applied for the other Bragg grating 6i _j of the first optical fibre 5i and for the Bragg gratings 6¾ of the second optical fibre 5 ₂ in order to evaluate the general deformation of the outer face 4 of the cooling plate 1.

[0053] In the case where a first optical fibre 5j comprises several first Bragg gratings 6y near a crossing point 21 _k i, the strain variation Aa(6 ) of each of these first Bragg gratings 6, _j may be determined by means of the determined temperature variation AT(8 _kn ) of the second Bragg grating 8kn that is the nearest of this crossing point 21 _k i. In a variant, in the case where a first optical fibre 5, comprises several first Bragg gratings 6 located between two crossing points 21 _k i and 21 _k 'i, and where there exists a temperature gradient between these two crossing points 21 _k i and 21 _k 'i, the strain variation Δσ(6, ) of each first Bragg grating θ¾ (located between these two crossing points 21 _k i and 21 _{k i} ) may be determined by means of an estimated temperature variation ΔΤ _Ε (8^) depending on the corresponding value of the temperature gradient at the location of this first Bragg grating 6, _j .

[0054] Referring to figures 3 and 4, in order to avoid any strain on the second optical fibres 7k and to perform the determination of the temperature variation AT(8 _kn ), this second optical fibre 7 _k is arranged in a thermal-conducting tube 12 that is set along one of the second directions D2 and whose diameter is substantially greater than that of the fibre 7k. For instance, each thermal-conducting tube 12 may be made of inox.

[0055] Each thermal-conducting tube 12 may be installed fixedly in a horizontal groove 13 that is defined into the outer face 4 of the body 2 along one of the second directions D2. In this case, each thermal-conducting tube 12 may be installed fixedly in a horizontal groove 13 by means of a glue 14, for instance. This glue 14 may be the one produced by Vishay under reference M-bond 600 (and usable up to +260°C), for instance.

[0056] The depth of each horizontal groove 13 depends on the diameter of the thermal-conducting tube 12 and on its shape. In the non-limiting example illustrated in figure 4, each horizontal groove 13 has a V-shape in cross section. But, it may have other shapes in cross section, such as a U-shape or a rectangular-shape. For instance, when the second optical fibre 7 _k has a diameter comprised between 240 pm and 250 pm, the diameter of the thermal-conducting tube 12 may be equal to 1 mm and the depth of the horizontal groove 13 may be equal to 1 ,2 mm so that the second optical fibre 7 _k is not strained [0057] Also for instance, and as illustrated in figure 4, each thermal-conducting tube 12 may comprise an exterior-facing part covered with a copper paste 15. The latter 15 is intended for avoiding the presence of a layer of air against the exterior- facing part of the thermal-conducting tube 12, because of bad thermal conductivity of air. It could be also possible to use a stainless-steel material.

[0058] Also for instance, and as illustrated in figure 4, each copper paste may comprise an exterior-facing part covered with a copper cover 16. The latter (16) is intended for protecting the thermal-conducting tube 12 against shocks. [0059] As illustrated in figure 3, each first optical fibre 5j may be installed fixedly in a vertical groove 17 defined into the outer face 4 of the body 2 along a first direction D1. In each crossing point 21 _k j, the first optical fibre 5, is preferably nearer from the exterior than the second optical fibre 7 _k . Therefore, when the latter (7 _k ) are located inside thermal-conducting tubes 12, the latter (12) may comprise deformations in each corresponding crossing point 21 _ki in order to pass "below" the vertical grooves 17, or else the horizontal grooves 13 must have a much more important depth so that the second optical fibres 7 _k be everywhere "under" the level of the vertical grooves 17 (for instance this depth may be equal to 1.2 cm or 2 cm). In these cases, the thermal-conducting tubes 2 must be installed fixedly inside the horizontal grooves 13 before the first optical fibre(s) 5, is/are installed fixedly in vertical groove(s) 17. In a variant of embodiment, instead of defining horizontal grooves 13 into the outer face 4, it is possible to define drills into the body 2, "under" the level of the vertical grooves 17, and then to introduce the thermal- conducting tubes 12 inside these body drills.

[0060] The depth of each vertical groove 7 depends on the diameter of the first optical fibre 5,. In the non-limiting example illustrated in figure 3, each vertical groove 17 has a V-shape in cross section. But, it may have other shapes in cross section, such as a U-shape or a rectangular-shape. For instance, when the first optical fibre 5, has a diameter comprised between 240 pm and 250 pm, the depth of the vertical groove 17 may be equal to 1 cm.

[0061] For instance, and as illustrated ih figure 3, each first optical fibre 5, may be installed fixedly in a vertical groove 17 by means of a glue 18. This glue 18 may be the one produced by Vishay under reference M-bond 600 (and usable up to +260°C), for instance.

[0062] Also for instance, and as illustrated in figure 3, each first optical fibre 5, may comprise an exterior-facing part covered with a seal material 19 intended for protecting against mechanical shocks. This seal material 19 may be a silicon rubber produced by Vishay under reference RTV3145 (and usable up to +260°C), for instance. It is also possible to insert a varnish between each first optical fibre 5, and the seal material 19, for protecting each first optical fibre 5, and the glue 18. For instance, this varnish may be the one produced by Vishay under reference M- COAT A. [0063] Also for instance, and as illustrated in figure 4, the seal material 19 may comprise an exterior-facing part covered with a protection material 20 intended for protecting it against heat shocks. For instance, this protection material 20 may be a silicone.

[0064] The processing means 11 may be localized into a computer, away from the cooling plate 1 , for instance. In this case, it is preferably made of software modules, at least partly. But it could be also made of a combination of electronic

[0065] circuit(s) (or hardware modules) and software modules (which also requires a software interface allowing interworking between the hardware and software modules). So, it could be a computer. In case where it is made only of software modules it can be stored in a memory of a computer or in any computer software product, such as a CD-ROM, for instance, which can be read by a computer or the like. [0066] Thanks to the permanent determination of the wavelengths of photons reflected by the Bragg gratings defined into the optical fibres (dedicated to strain measurement and temperature measurement), it is now possible to monitor in real time the evolution of the vertical deformation of the cooling plates (or staves) of a blast furnace, and more generally of determining deformation in a given direction of any equipment in contact with hot material.

[0067] Optionally, the inner face 3 of the cooling plate 1 may also comprise one or more optical fibres (non illustrated) for the determination of temperature variations on this face 3 which are set along the first direction D1 in an arrangement corresponding to the optical fibres 5i of the cold face 4.