Abstract:
The present invention relates to a method of detecting deposits comprising at least one ferromagnetic material, such as nickel, magnetite or the like, on or near the external wall of a tube, notable in that it comprises at least the following steps of: moving a magnetized source inside the tube in the lengthwise direction using an electric motor, measuring the strength of the current in the electric motor, and determining the position and/or the thickness and/or the volume of the said deposit as a function of the variations in the strength of the current measured in the electric motor. Another subject of the invention is a device implementing the said method.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the general field of processes and devices for magnetic detection and more particularly the field of processes and devices for detection fouling or clogging by deposits of ferromagnetic materials on or near cooling tubes of a steam generator of a pressurised nuclear water reactor known as PWR. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of electronuclear plants of PWR type according to the acronym “Pressurised Water Reactor”, it is well known that heat produced in the core of the reactor is transmitted by means of a closed circuit known as the primary circuit in which water circulates to a so-called secondary circuit whereof the water transformed into steam powers turbines to produce electricity. 
         [0003]    In reference to  FIG. 1  which illustrates a steam generator in exploded perspective, each electronuclear plant of PR type generally comprising three or four steam generators, said steam generator is constituted by a confinement enclosure  5  receiving the primary circuit  10  and the secondary circuit  15 . The thermal exchange between the primary circuit  10  and the secondary circuit  15  occurs via plurality of tubes  20  in an inverted U. Said tubes  20  are held in place by spacer plates  25  immobilised by ties fixed in the lower part of the steam generator. 
         [0004]    In reference to  FIG. 2  which illustrates a perspective view of a detail of the spacer plates  25  and the tubes  20 , said spacer plates  25  comprise holes in the form of so-called quadrifoliage hollows through which said cylindrical tubes  20  pass. 
         [0005]    It is known that clogging deposits  35  form at the level of the quadrifoliages  25  ( FIG. 2 ) between the tubes  20  and the spacer plates  25 . The consequence of these deposits  35  on the one hand, as a function of normal behaviour, is to modify the mechanical stresses on the tubes  4  and on the other hand, in case of incident or accident, to boost forces on the spacer plates  25  thus increasing the risk of rupturing the tubes  20 . 
         [0006]    Also, it is likewise known that so-called fouling deposits form on the external surface of tubes  20  causing a drop in performance of thermal exchange in the steam generator. 
         [0007]    To eliminate these clogging or fouling deposits, it is known to clean the tubes and spacer plates by chemical cleaning processes. These processes consist of injecting chemical reagents into the secondary circuit of steam generators to demolish and dissolve these oxide deposits such as magnetites. 
         [0008]    However, the quantity of reagents to be injected depends on the quantity of oxides present in the steam generators. 
         [0009]    Consequently, the quantity of oxides needs to be determined in advance. 
         [0010]    For this purpose, processes and devices are well known which detect deposits of magnetites using an axial probe with low-frequency Foucault current, said probe being introduced into the tubes of the steam generator, whereof the measurements are correlated with televisual images or online standards representative of deposits encountered. 
         [0011]    The drawback to this type of process is that it needs analysis time of around 1 month for acquisition of data, considerably driving up costs. Also, measurements obtained by this type of process exhibit low precision. 
         [0012]    The process and device for detecting deposits described in US patent U.S. Pat. No. 4,088,946 are also known. Said device comprises a probe with Foucault current which is moved at a constant speed in a tube to detect deposits. 
         [0013]    In the same way as earlier, this probe exhibits low precision and needs acquisition of video images. 
         [0014]    Other processes and devices for detection of deposit on the external wall of tubes having the same drawbacks are described especially in French patent application FR 2 459 490 and in US patent U.S. Pat. No. 4,700,134. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0015]    One of the aims of the invention is therefore to rectify these drawbacks by proposing a process and a detection device for deposits comprising at least one ferromagnetic material on or near the external wall of a tube, more particularly designed for detection of deposit on or near tubes of a steam generator of a electronuclear plant of PWR type, which is simple in design and minimally costly and has high precision and is substantially reliable. 
         [0016]    For this purpose and in keeping with the invention, a process for detecting deposits is proposed, comprising at least one ferromagnetic material, such as nickel, magnetite or similar, on or near the external wall of a tube, significant in that it comprises at least the following steps of moving a magnetised source inside the tube in the direction of its length by means of an electric motor, measuring the intensity of the current in the electric motor, and determining the position and/or thickness and/or volume of said deposit as a function of the variations in intensity of the current measured in the electric motor. 
         [0017]    The magnetised source preferably consists of at least one permanent magnet. 
         [0018]    Also, said magnetised source is moved in the tube at a constant speed, said magnetised source being moved in a first direction and then in its opposite direction. 
         [0019]    Also, the step for determining the position and/or thickness and/or volume of said deposit comprises a comparison step of the variation in intensity of the current measured in the motor with a reference model and/or a calibrated model. 
         [0020]    Another object of the invention relates to a detection device executing the process significant in that it comprises at least one magnetised source, moving means of said magnetised source inside of said tube in the direction of the length comprising an electric motor, means for measuring the intensity of the current in said electric motor and means for determining the position and/or thickness and/or volume of said deposit as a function of variations in intensity of the current measured in the electric motor. 
         [0021]    Said magnetised source preferably consists of at least one permanent magnet. 
         [0022]    Said moving means of the magnetised source consist of a piston whereof one of the ends bears said magnetised source and whereof the opposite end comprises a bolt cooperating with an endless screw solid with the rotary drive axle. 
         [0023]    Also, the rotary drive axle consists of the output axle of a reducer coupled to an electric motor. 
         [0024]    By way of advantage, the device comprises blocking means of drive means inside a tube. 
     
    
     
       BRIEF DESCRIPTION OF THE DIAGRAMS 
         [0025]    Other advantages and characteristics will emerge from the following description of several variant embodiments, given by way of non-limiting examples, of the device for detecting magnetic deposits on or near an amagnetic tube according to the invention, from the attached diagrams, in which: 
           [0026]      FIG. 1  is an exploded perspective view of a steam generator of electronuclear plants of PWR type, 
           [0027]      FIG. 2  is a perspective view of a detail of the tubes passing in the quadrifoliages of spacer plates, said quadrifoliages comprising clogging deposits, 
           [0028]      FIG. 3  is a schematic representation, in longitudinal section, of the detection device according to the invention introduced into a tube comprising a fouling deposit, 
           [0029]      FIG. 4  is a schematic representation of the different positions of the probe of the device according to the invention relative to deposit, attraction forces as a function of the position of the probe and variations in intensity of the current motor, 
           [0030]      FIG. 5  is a schematic representation of the different positions of the probe of the device according to the invention relative to deposit, attraction forces as a function of the position of the probe, the balance of forces, and variations in intensity of the current motor, 
           [0031]      FIG. 6  is a schematic representation of the different positions of the probe of the device according to the invention relative to deposit, attraction forces as a function of the position of the probe, forces exerted on the threads of the screw and the bolt of the moving means of the probe, and variations in intensity of the current motor, 
           [0032]      FIG. 7  is a graphic representation of the variations in intensity of the current motor when the probe of the device according to the invention is moved in a tube comprising fouling deposits of different thickness and length, 
           [0033]      FIG. 8  is a view in longitudinal section of the probe of the device according to the invention introduced into a tube at the level of a quadrifoliage of a spacer plate, 
           [0034]      FIG. 9  is a view in partial perspective of a tube passing into the quadrifoliage of a spacer plate, 
           [0035]      FIG. 10  is a view in section along the section line X-X′ of a tube and a quadrifoliage illustrated in  FIG. 9 , 
           [0036]      FIG. 11  is a graphic representation of the variations in intensity of the current motor when the probe of the device is moved in a tube, in a first direction, at the level of a quadrifoliage not comprising clogging deposit, 
           [0037]      FIG. 12  is a graphic representation of the variations in intensity of the current motor when the probe of the device is moved in a tube, in an opposite direction, at the level of a quadrifoliage not comprising a clogging deposit, 
           [0038]      FIG. 13  is a graphic representation of the variations in intensity of the current motor when the probe of the device is moved in a tube, in a first direction and then in the opposite direction, at the level of a quadrifoliage not comprising clogging deposit, 
           [0039]      FIG. 14  is a view in longitudinal section of the probe of the device according to the invention introduced into a tube at the level of a quadrifoliage of a spacer plate comprising a clogging deposit, 
           [0040]      FIG. 15  is a graphic representation of the variation in intensity of the current motor of the device according to the invention when the probe is moved, in a first direction, in a tube at the level of a quadrifoliage comprising a clogging deposit, 
           [0041]      FIG. 16  is a graphic representation of the variation in intensity of the current motor of the device according to the invention when the probe is moved, in an opposite direction, in a tube at the level of a quadrifoliage comprising a clogging deposit, 
           [0042]      FIG. 17  is a schematic representation of the different positions of the probe of the device according to the invention relative to a deposit, attraction forces as a function of the position of the probe, and variations in intensity of the current motor when the probe is moved, in a first direction, in a tube at the level of a quadrifoliage comprising a clogging deposit, 
           [0043]      FIG. 18  is a schematic representation of the different positions of the probe of the device according to the invention relative to deposit, attraction forces as a function of the position of the probe, and variations in intensity of the current motor when the probe is moved, in an opposite direction, in a tube at the level of a quadrifoliage comprising a clogging deposit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    In reference to  FIG. 1 , the detection device  100  according to the invention comprises a magnetised source  105  comprising one or more permanent magnets, said magnetised source  105  forming a probe, moving means  110  of said magnetised source  105  inside a tube  115  in the direction of its length, said moving means  110  comprising at least one electric motor  120 , as will be detailed hereinbelow, means for measuring the intensity of the current  125  in said electric motor  120  and means  130  for determining the position and/or thickness and/or volume of a deposit on or near the external wall of the tube  115 , as a function of variations in intensity of the current measured in the electric motor  120 . 
         [0045]    The moving means  110  of the magnetised source  105  consist of a piston  135  whereof one of the ends bears said magnetised source  105  and whereof the opposite end comprises a bolt  140  cooperating with an endless screw  145  solid with a rotary drive axle  150 . Said rotary drive axle  150  consists of the output axle of a moto-reducer  155  constituted by a reducer  160  coupled to an electric motor  120 . Said electric motor  120  is powered by fixed voltage regulated such that, to compensate for the increase in power necessary to pass the hard points associated with attraction of the magnets of the magnetised source  105 , the intensity of the current motor increases. 
         [0046]    In fact, having a magnetised source  105  called a probe, constituted by one or more permanent magnets, near a deposit  165  comprising at least one ferromagnetic material, such as nickel, magnetite or similar, on or near the external wall of the tube  115 , results in attraction forces which vary as a function of the volume of the deposit  165 , the distance between the probe  105  and the deposit  165 , and the relative position between the probe  105  and the deposit  165 . Moving this probe  105  is motorised at a constant speed, under the effect of attraction forces, results in variation in intensity of the supply current of the motor  120  which calibrates, detects the presence and estimates the volume of the deposit  165 . Maximal force is attained when the minimum volume of the magnetic material is equal to the volume of the magnet of the probe  105 . The attraction forces generated by the permanent magnets of the probe can be active or resistant, can favour displacement or oppose displacement. The control process by magnetic probe of the fouling and clogging consists of motorising in the tube  115  displacement of a probe  105  at a constant speed, comprising one or more permanent magnets, and making acquisition of the current motor whereof the intensity varies as a function of the presence, distance and volume of deposits. 
         [0047]    Also, acquired signals are compared to the signal tube or to calibrated reference signals representative of dimensional data of deposit forms. 
         [0048]    Also, to keep the device in place inside the tube  115  during displacement of the probe, the device comprises blocking means  165  of the moving means  110  inside said tube  115 . Said blocking means  115  will be able to consist of all means well known to the person skilled in the art, such as mechanical means or plastic deformation means for example. 
         [0049]    Also, the means for measuring the intensity of the current  125  consist of a device of ammeter type connected to a computer  170  of PC type by means of an acquisition card USB  175 . An algorithm in the form of registered software on a physical medium, such as a hard drive and/or computer memory  165 , the position and/or the thickness and/or volume of a deposit on or near the external wall of the tube  115 , as a function of variations in intensity of the current measured in the electric motor  120 , measurements of the intensity being transmitted to the computer  170  by means of the acquisition card USB  175 . 
         [0050]    An explanation now follows of the function of the device for detecting deposits comprising at least one ferromagnetic material, such as nickel, magnetite or similar, on or near the external wall of an amagnetic tube, in reference to  FIGS. 3 to 18 . 
         [0051]    The device according to the invention, in reference to  FIG. 3 , is blocked in a tube  115  comprising a fouling deposit  165  simulated by a magnetic ring. 
         [0052]    When the probe  105  is moved at constant speed in the amagnetic tube  115 , it is necessary to exert motorisation force of the probe depending on its position relative to the deposit  165 . 
         [0053]    The probe  105  is moved at constant speed, the voltage being regulated, the power function of the electric motor  120  with continuous current written as P(t)=U×l(t). 
         [0054]    The intensity l(t) varies as a function of the relative position between the permanent magnets of the probe  105  and the deposit  165 , said relative position generating a variation in attraction forces. 
         [0055]    In some phases (input and output of the zone of the deposit  165 ), the attraction forces tend to draw on mechanics and generate axial restrictions which are compensated by the current motor. 
         [0056]    The marker points on the curves of  FIG. 4  give information on the start of ring (A) and finish of ring (C). The length of the deposit  165  is equal in this case to X(C)-X(A), where X is equal to the base time multiplied by the speed of displacement of the probe  105 . 
         [0057]    A highly characteristic point of these curves is the point of inflection (B) which indicates that the probe is in material equilibrium, that is, that there is as much magnetic material on either side of the normal axis of the probe  105 . 
         [0058]      FIG. 5  discloses the behaviour of the probe  105  when it is subjected to attraction forces and graphic elements are taken therefrom to structure analysis. For each relative position of the probe  105  relative to the deposit  165 , this  FIG. 5  shows the diagram of the attraction forces, the balance of forces in play, that is, the balance of the motor force and the attraction forces, the impact on mechanical transmission and their link to the motor intensity curve. 
         [0059]    It is evident that the value of these forces depends on the volume of deposits encountered. 
         [0060]    To retain displacement at constant speed the motor  120  must compensate for the effect of attraction forces when they are powered or resistant. 
         [0061]    When they are powered, they are more substantial than the advance force. In reference to  FIG. 6 , their effect is to draw on the screw/bolt transmission and consequently generate stress on the threads. 
         [0062]    In the case of the particular embodiment of the invention in which transmission consists of a transmission via screw/bolt, magnetic forces generate an axial force on the axle of the moto-reducer. These magnetic forces are sometimes so powered that they become resistant for mechanics. 
         [0063]    In reference to  FIG. 7 , which illustrates the variation in intensity of the current motor when the probe of the device is moved inside a tube  115  comprising two rings of different thickness and length, said rings simulating deposits of different thickness and length, it appears that analysis of the form of the acquired signal produces data relative to the start of the deposit, to the length of the deposit, correlated with the speed of advance, and to the thickness of the deposit (e 1  or e 2 ), correlated with the amplitude of the intensity of standard rings. 
         [0064]    Consequently, the device according to the invention detects the presence of a deposit around the tube and determines the length of these deposits and their thickness. 
         [0065]    Also, the device according to the invention enables direct on-screen reading of results by comparing the acquired signals to the reference tube signal or to calibrated signals. 
         [0066]    In reference to  FIGS. 8 to 10 , the process according to the invention can be applied to the plate tube/spacer link. Analysis of moving forces (function of the position of the probe relative to the spacer plate) helps determine whether there is a variation in volume of material. 
         [0067]    Tests conducted using the device according to the invention have shown that the variation in volume of deposits could be detected by comparison to a reference signal, the reference signal consisting of variations in intensity of the current motor when the probe is moved in a tube comprising no deposit, or by the difference in plate input and output signals. If there is a clogging deposit, it is present on one side of the spacer plate only, which compares the corresponding signals. 
         [0068]    In the case of comparative input/output analysis, to boost precision for detection and characterisation of deposits, it is necessary to make a double acquisition (out and back) since the behaviour of the probe is different if entry is made via the clogged side or exit is made via the clogged side. 
         [0069]    The first assay, in reference to  FIGS. 11 and 12 , consists of the device according to the invention acquiring the intensity of reciprocal displacement of the probe in a tube mounted in a spacer plate whereof the quadrifoliage passages are not obstructed. 
         [0070]    In reference to  FIG. 13 , this shows the relation between the position of the probe relative to the spacer plate and the motor acquisition curve. 
         [0071]    To analyse the signals, characteristic points must be located on the curve. A variation in intensity of the current motor at these points will reveal the presence of a variation in volume of material, that is, the presence of a clogging deposit. 
         [0072]    In reference to  FIG. 14 , a clogging deposit  180  was simulated by positioning a magnetic ring in the quadrifoliage passage of a spacer plate. 
         [0073]    In reference to  FIGS. 15 and 17 , an evolution in signals for the two reference points (A) and (B) is evident. At the plate inlet (A), there is more material as the attraction forces are higher, and they draw on the transmission generating a resistant force which the motor compensates by the increase in current. 
         [0074]    In the current part (B), the attraction forces due to deposit at the plate intake hold the probe, leading to an increase in intensity. 
         [0075]    In reference to  FIGS. 16 and 18 , an evolution in signals for the two reference points A and B is evident. At the plate outlet (A), there is more material as the attraction forces are higher, and they push on the transmission generating a resistant force which the motor compensates by the increase in current. In the current part (B), the attraction forces due to deposit at the plate outlet attract the probe and keep it in equilibrium for some time, resulting in a decrease in intensity. 
         [0076]    Consequently, the device according to the invention detects clogging of the quadrifoliage passages of the spacer plates with fine sensitivity, and determines the depth of this clogging and its thickness. 
         [0077]    Also, the device according to the invention enables direct on-screen reading of results by comparing signals acquired at the reference tube signal or at calibrated signals. 
         [0078]    Finally, it is understood that the examples given hereinabove are only non-limiting particular illustrations as to fields of application of the invention.