FR2671184A1 - Method and a device for inspecting a filtering element by infrared radiation - Google Patents

Abstract

The device for inspecting a filtering element (7) by infrared radiation comprises: a) a support (10) for a filtering element (7) to be inspected, kept at a substantially constant temperature, b) a fluid source (9), the pressure or the delivery rate of which is adjustable, c) means (3) for heating the fluid to a predetermined temperature substantially different from that of the filtering element (7) and that of its support (10), d) means (8) for circulating the fluid at the said predetermined temperature, through the filtering element (7), e) an infrared detection device (5) arranged in front of the filtering element (7) to be inspected, for detecting inequalities in the flow of the fluid (73) which has passed through the filtering element (7), and f) image processing means for determining, from the detected infrared radiation, the quality of the filtering element (7) to be inspected and locating possible defects at the surface of the element (7).

Description

Method and device for controlling a filter element by
infrared radiation
Field of the invention
The present invention relates to a method and a device for controlling filter elements and applies in particular to expert reports and verifications in the manufacture of filters intended for fine filtration, such as metallic fabrics, sintered elements, composite elements or ceramic.

 The invention applies more generally to the study and control of the characteristics of porous materials.

Prior art
In industry, filter elements are defined, among other things, by their filtration power. This characteristic is currently measured and controlled either by the so-called n glass ball method or by the so-called "bubble point" method.

 The so-called "glass ball" method, according to which a calibrated glass ball having passed through the filter mixed with a liquid under pressure is counted, can only be applied in production and is used only for the purpose of because it is destructive.

 The so-called "bubble point" method consists of immersing the filter in a liquid such as alcohol or oil, and applying a gas, the pressure of which is gradually increased, through the filter. The appearance of the first bubble characterizes the weakest point of the filter, that is to say the widest mesh. Such a method is not destructive and can be used in production. However, it has the disadvantage, taking into account the fluids used, liquid and gas, of being polluting for the filter elements and therefore of requiring an element cleaning operation which can be very delicate and expensive. In particular, in most applications in the space field, it is essential that the filters contain no trace of grease, since these are incompatible with filtered fluids such as oxygen.

 Furthermore, methods of studying and analyzing materials are known which use infrared spectrography or are based on the application, on the material, of a beam of visible or infrared light radiation, of predetermined characteristics and on observation of the light reflected by the material.

 Such methods do not lend themselves to industrial operation and are not specifically designed to control the integrity of filter elements.

Subject and brief description of the invention
The present invention aims to remedy the aforementioned drawbacks and to make it possible to carry out, in a simple, convenient, non-polluting and non-destructive manner, a control or a measurement of the filtration power of filter elements.

 The invention aims in particular to make it possible to control the integrity of a filter element at the end of its manufacturing and control cycle, and thus to guarantee a filtration power in a safe manner by the fact that no mechanical or subsequent hydraulics will only change the characteristics of the filter element.

 These goals are achieved by a method of controlling a filter element by infrared radiation, characterized in that it comprises the following steps a) circulating through the filter element to control a fluid at a predetermined temperature, the predetermined temperature being substantially different from that of the filter element, b) the infrared radiation having passed through the filter element to be checked is observed and detected, and c) an image processing is carried out using the detected infrared radiation in order to highlight evidence and locate possible flow inequalities revealing defects on the surface of the filter element.

 According to the invention, the filter element is simply passed through by a non-polluting fluid such as a liquid, or preferably a gas, such as nitrogen or a neutral gas, the essential characteristic of which resides in the temperature, which is significantly different from that of the filter element. There is therefore a wide range of suitable non-polluting fluids, since the only important factor is a temperature difference.

 Furthermore, the observation using an infrared detection device is carried out in a purely passive manner and it is not necessary to apply any radiation to the filter element.

 According to a particular embodiment, the observation is carried out by gradually increasing the pressure of the gas or the flow rate of the liquid applied to the filter element.

 In the event of a filtration fault, a fluid flow rate higher or lower than that on average is observed over the entire surface of the filter. By increasing the pressure or the flow, the entire exterior of the filter appears to let the gas or liquid pass.

 By analogy with the "bubble point" method, and after calibration, the pressure or flow value, for which the first fault appears, constitutes a threshold which indicates the filtering power of the measuring element.

 According to an advantageous embodiment, easy to implement, the filter element is maintained at a temperature close to room temperature, and the fluid is constituted by a hot fluid brought to a temperature greater than or equal to about 500C.

 However, according to another embodiment, the filter element is maintained at a temperature substantially lower than approximately -500C and the fluid consists of a cryogenic fluid brought to a temperature substantially higher than -500C and lower than OOC. In this case, the infrared detection device must naturally be suitable for detection in this temperature range.

 In general, data processing can take place in several different ways.

 According to a first embodiment, during image processing, the areas of the surface of the element in which a temperature above a predetermined temperature has been detected are highlighted.

 According to another embodiment, during image processing, the map of temperatures observed is expressed according to a predetermined temperature scale, by an index relating to the quality of the filter element.

 The invention is particularly applicable to filter elements consisting of filters intended for fine filtration capable of retaining particles of between a few micrometers and a hundred micrometers.

 Another subject of the invention is a device for controlling a filter element by infrared radiation, characterized in that it comprises a) a support for a filter element to be controlled maintained at a substantially constant temperature, b) a source of fluid the pressure or flow of which is adjustable, c) means for bringing the fluid to a predetermined temperature substantially different from that of the filter element and its support, d) means for circulating the fluid at said predetermined temperature, through the filter element, e) an infrared detection device arranged opposite the filter element to be checked for detecting unequal flow rates of the fluid having passed through the filter element, and f) image processing means for determining , from the infrared radiation detected, the quality of the filter element to be checked.

 The filter element support may include a sealed cell, but not thermally insulated, placed in a thermostat.

 The source of fluid may be a source of gas, provided with means for adjusting the pressure, or a source of liquid provided with means for adjusting the flow rate.

 Advantageously, the device comprises, upstream of the filtering element, an additional filter whose filtering power is much lower than the filtering power of the filtering element to be checked.

According to a particular embodiment, the infrared detection device consists of a strip of elements
CCD having a relative movement with respect to the filter element to scan the entire surface thereof.

 According to another particular embodiment, the infrared detection device is constituted by an infrared video camera.

 In general, a flat filter can be observed over its entire surface.

 . a planar or cylindrical filter can be observed according to a rectangular surface of small width which sweeps the surface of this filter.

 a flat, cylindrical, or any shape filter can be observed according to a spot that scans the surface of this filter.

In all cases, the direction of observation is approximately perpendicular to the surface of the filter in the observation area.

 The infrared detection device may comprise an infrared video camera, and a screen provided with an observation slot interposed between the infrared video camera and the filter element, means being provided for causing relative movement between the filter element and the slit so that the slit constitutes an observation window sweeping the entire surface of the filter element.

In the case of a cylindrical filter, this can for example be rotated around its axis in front of the observation slot which remains fixed
Brief description of the drawings
Other characteristics and advantages of the invention will emerge from the description of particular embodiments, given by way of examples, with reference to the appended drawings, in which - Figure 1 is a schematic view of the assembly of a filter control device according to a first embodiment of the invention, - Figure 2 is a schematic view of the assembly of a filter control device according to a second embodiment of the invention, - Figure 3 is an elevational view showing a detail of a particular embodiment with observation slot, - Figure 4 is a sectional view along the line IV-IV of Figure 3, - Figure 5 is a variant of embodiment of FIGS. 3 and 4, showing an infrared detection device constituted by a strip of CCD elements (charge coupled elements), - FIG. 6 shows a simplified image of a filter with defect obtained by the process in accordance with l 'in vention, and - FIG. 7 shows curves giving as a function of time the evolution of the temperature at different points of the filter, the image of which is represented in FIG. 6.

Detailed description of particular embodiments
Figure 1 shows schematically the whole of a filter control installation according to the invention.

 A filter element to be checked 7 is arranged opposite an infrared detection device 5 which is advantageously constituted by an infrared video camera, itself associated with an image analysis system 6 making it possible to quantify the result of the measurement. .

 A pipe 8 is arranged so as to be able to circulate through the filtering element 7 a fluid, which in the example considered is a neutral gas.

 The circuit for applying the neutral gas to the filter 7 comprises an adjustable pressure regulator 1 making it possible to vary the pressure of the gas within a predetermined range of values which can range, for example, from O to a few tens of thousands of pascals.

 A filter 2, whose filtering power is much lower than the filtering power of the element 7 to be measured, is arranged on the neutral gas application circuit, for example downstream of the pressure reducer 1.

 The pressure reducing gas is brought to a predetermined temperature in a heat exchanger 3 located downstream of the filter 2.

 A pressure gauge 4 makes it possible to measure the pressure of the gas at the inlet of the element to be measured 7.

 In the case where the filter element 7 is maintained at a temperature close to ambient temperature, for example at a temperature of the order of 20 C, the expanded fluid applied by the line 8 can advantageously be heated in the heat exchanger 3 at a temperature above about 50 G, for example of the order of 800C to 1000C, so as to be introduced into the filter 7 with a substantial temperature difference, so that the infrared camera 5 can detect leaks from hot fluids through the wall of the filter 5 as the gas pressure increases from zero pressure.

 Figure 1 shows the use of a gaseous fluid. A non-polluting liquid can be used as a fluid allowing infrared detection as soon as the temperature gradient is sufficient.

 The device according to the invention is more particularly suitable for checking filters at the end of a manufacturing process, before the implementation of these filters.

 However, in the case where the filter does not include a casing opaque to infrared radiation or if this casing can be removed, the device according to the invention can also be adapted to in situ checks.

 In this case, for example, the control can be carried out using sanitizing gas (heated nitrogen) in the case of propellant assemblies or using a lubricant in the case of a thermal motor.

 FIG. 2 shows an assembly quite similar to that of FIG. 1, which can be applied in particular to the control of plane samples of porous material 7.

 In the case of FIG. 2, the sample 7 to be checked is maintained between two front plates 71, 72 which leave only the passage necessary for the application of a fluid from a pipe 8 perpendicular to the plane of the 'sample 7, and for the evacuation of this fluid 73 by the face of the sample 7 facing the infrared observation camera 5. The measurement cell has a side wall 74 ensuring a seal with respect to the outside without constituting a thermal barrier.

 The measuring cell is placed in a tank 10 filled with liquid maintained at a constant temperature to constitute a thermostat, pipes 11, 12 ensuring a circulation of the liquid inside the tank 10.

 The fluid applied in the pipe 8 passes through a device 9 for controlling the flow and then through an oven 3 in which it is heated to a temperature substantially higher than that of the liquid contained in the tank 10, which destroys the temperature of the 'sample 7. The furnace 3 may comprise a central pipe connected by flanges 31, 32 to the pipe elements 8.

 The hot fluid used to pass through the sample 7 can for example be nitrogen.

 Temperature measuring devices such as thermocouples are placed on the surface of the sample and in the flow of the gaseous fluid. However, to carry out a fine and precise analysis of the behavior of the material of the sample 7, the infrared camera 5 ensures the visualization of the thermal gradients on the surface of the sample 7, and allows, thanks to the associated image processing device , to measure the temperature distribution as a function of the passage of the hot fluid through the surface of the sample, which contributes to characterizing the porosity properties.

 The measuring device according to the invention is particularly suitable for expertise and verification in the manufacture of filters of the metal fabric type, sintered elements (for example sintered nickel), composite or ceramic elements, the filters being able to have dimensions of meshes such that they are capable of retaining particles of a few microns to about a hundred microns.

 The infrared detection device can advantageously be constituted by a camera provided with a zoom, which allows precise examinations in the plane of the filter. Thus, the invention is well suited to the examination of porosities and a suction of the gas intended to obtain a unidirectional flow is not useful.

 The resolution of the process is only limited by that of the infrared detector.

 In a number of cases, the infrared detector may consist of a simple array of charge coupled elements (CCD). For example, in Figure 5, there is shown a strip 5 'of CCD elements which are placed parallel to the axis of a cylindrical filter 7 traversed by a fluid having a temperature gradient relative to the filter 7. Thanks to a rotation of the filter 7 about its axis, the detector 5 'can scan the entire surface of the filter 7 which passes successively in front of the detector 5'.

 In the case of using an infrared camera 5, it is also possible to reduce the observation to a limited area of the cylindrical filter 7, for example by interposing a plate 50 between the camera 5 and the filter 7, the plate 50 being provided with a slot 51 parallel to a generator of the cylinder 7 so that the observation is limited to the surface portion of the filter which is substantially normal to the plate 50, that is to say in the axis of the camera 5. By rotation of the culindric filter 7, the entire surface of this filter can be scanned successively (FIGS. 3 and 4).

 The case of using a fluid brought to a temperature markedly above ambient temperature has been described above, which constitutes the simplest embodiment to implement.

 Insofar as certain infrared cameras are themselves capable of withstanding temperatures below OOC, it is also possible to cool the apparatus which supports the filter 7, for example the tank 10 of FIG. 2, at very low temperatures. , for example at the temperature of liquid nitrogen or at temperatures of the order of -900C, and applying a fluid consisting of a cryogenic gas which can be, for example, at a temperature of the order of -5O0C.

 In certain cases, instead of detecting the fluid passing through the filter 7, it is possible to maintain the filter 7 for example at ambient temperature, to circulate through the filter 7 a cryogenic fluid for example at around -500C, and observe the higher temperature zones formed by the filter 7 itself.

 The information delivered by the infrared detection device 5 can be processed so as to provide measurements which, after calibration, can be correlated with that obtained by conventional methods of filter control (bubble point, glass beads).

 Measurements can be deduced from the observations made by an infrared detection device for example by - exceeding a predefined temperature threshold, at a predefined number of points on the surface of the filter, - calculating an index allowing express the temperature map.

 It will be noted that the device according to the invention makes it possible to carry out qualitative observations and quantitative measurements taking into account a single basic factor, namely the temperature. In fact, when the fluid pressure increases, the fluid flow is greater and this simply results in a faster temperature rise.

 The method and the device according to the invention make it possible to observe a transient evolution, for example when the temperature at the level of the filter increases by increasing the pressure of the fluid from an initial instant of start of verification, but also d '' carry out checks under stabilized conditions under reproducible measurement conditions.

 Insofar as the emissivity of the material considered is itself known, it is possible to obtain a temperature map directly with the image processing system associated with the camera.

 The presence of a defect is characterized by a stain corresponding to a high temperature. The larger the defect, the larger the stain obtained and corresponds to a product of high temperature.

 From a calibration, the minimum tolerable fault is evaluated. The different filters checked can receive a quality index depending on the faults observed.

 FIG. 6 shows an example of infrared mapping obtained using an infrared camera 5 placed in front of a filter 7 traversed by a hot fluid in accordance with the method according to the invention, and which makes it possible to deduce information therefrom. on the quality of the filters observed.

 It is recalled that thermal imaging is a method which allows the acquisition of phenomena related to the spatial distribution of heat on the bodies examined. It also allows the observation of variations in this distribution over time.

 The system transforms an infrared image into a visible image. The infrared distribution of the object, explored by scanning a surface, gives the detector a video signal whose amplitude varies over time as a function of the variations in luminance encountered.

 Figure 6 shows a thermogram corresponding to the digital processing of the thermal image from the camera. In the example illustrated, the distribution of the temperature map which is to be considered in relative value has been simplified to show only four different temperature ranges. In reality, with color thermograms, it is possible to present a much larger number of temperature ranges, for example at least 16 different temperature levels.

 FIG. 6 shows a filter with a defect 101 on the front face, and five points 111 to 115 distributed over the surface of the filter and for which the evolution of the temperature is followed.

It can be seen in FIG. 7 which shows the evolution of the temperature at points 111 to 115 (curves 121 to 125 respectively) that there is at point 114 where the fault is located a much faster temperature evolution (curve 124) than for the other points.

 In the case of FIG. 6, the defect being located on the front face of the filter, this defect results in a greater passage of hot fluid and therefore in an increase in temperature.

A fault located on the rear panel shows a different temperature distribution. Part of the heat energy is evacuated by the defect, which makes the front panel appear to be cooler.

Claims (20)

1. Method for controlling a filter element by infrared radiation, characterized in that it comprises the following steps a) a fluid is circulated through the filter element to be controlled (7) at a predetermined temperature, the predetermined temperature being substantially different from that of the filter element (7), b) observing and detecting the infrared radiation having passed through the filter element to be checked (7), c) an image processing is carried out from the infrared radiation detected to highlight and locate possible flow inequalities, revealing defects on the surface of the filter element (7). 2. Method according to claim 1, characterized in that said fluid is a non-polluting gas, such as nitrogen or a neutral gas. 3. Method according to claim 1, characterized in that said fluid is a non-polluting liquid. 4. Method according to claim 2, characterized in that the observation is carried out by gradually increasing the pressure of the gas. 5. Method according to claim 3, characterized in that the observation is carried out by gradually increasing the flow rate of the liquid. 6. Method according to any one of claims 1 to 5, characterized in that the filter element (7) is maintained at a temperature close to room temperature, and the fluid consists of a hot fluid brought to a higher temperature or equal to about 500C. 7. Method according to any one of claims 1 to 5, characterized in that the filter element (7) is maintained at a temperature substantially below about -500C and the fluid consists of a cryogenic fluid brought to a temperature substantially higher than -500C and lower than OOC. 8. Method according to any one of claims 1 to 7, characterized in that during image processing, the areas of the surface of the element in which a temperature higher than a predetermined temperature has been detected are highlighted . 9. Method according to any one of claims 1 to 7, characterized in that during image processing, the map of temperatures observed is expressed according to a predetermined temperature scale, by an index relating to the quality of the element filter (7). 10. Method according to any one of claims 1 to 9, characterized in that the filter element (7) consists of a filter intended for fine filtration capable of retaining particles between a few micrometers and a hundred micrometers. 11. Method according to any one of claims 1 to 10, characterized in that the filter element (7) is constituted by a metallic fabric, a sintered element, a composite element or a ceramic. 12. Method according to any one of claims 1 to 10, characterized in that the filter element (7) consists of a porous material. 13. Device for controlling a filter element (7) by infrared radiation, characterized in that it comprises a) a support (10) of the filter element (7) to be controlled maintained at a substantially constant temperature, b) a source of fluid (1; 9) whose pressure or flow is adjustable, c) means (3) for bringing the fluid (1; 9) to a predetermined temperature substantially different from that of the filter element (7) and its support (10), d) means (8) for circulating the fluid (1 ;; 9) at said predetermined temperature, through the filter element (7), e) an infrared detection device (5) disposed opposite the filter element (7) to be checked in order to detect inequalities in the flow rate of the fluid (73) having passed through the filter element (7), and f) image processing means (6) for determining, from the detected infrared radiation, the quality of the filter element (7) to be checked. 14. Device according to claim 13, characterized in that the support (10) of the filter element (7) comprises a sealed cell (71,72,74), but not thermally insulated, placed in a thermostat (10). 15. Device according to claim 13 or 14, characterized in that the source of fluid (l; 9) is a source of gas and comprises means for adjusting the pressure. 16. Device according to claim 13 or 14, characterized in that the source of fluid (1; 9) is a source of liquid and comprises means for adjusting the flow rate. 17. Device according to any one of claims 13 to 16, characterized in that it comprises, upstream of the filter element (7) an additional filter (2) whose filtering power is much lower than the filtering power of l filter element (7) to be checked. 18. Device according to any one of claims 13 to 17, characterized in that the infrared detection device (5 ') consists of a strip of CCD elements having a relative movement relative to the filter element (7) to sweep the entire surface of it.  19. Device according to any one of claims 13 to 17, characterized in that the infrared detection device (5) consists of an infrared video camera. 20. Device according to any one of claims 13 to 17, characterized in that the infrared detection device comprises an infrared video camera (5) and a screen (50) provided with an observation slot (51) interposed between the infrared video camera (5) and the filter element (7) and in that means are provided for causing relative movement between the filter element (7) and the slot (51) so that the slot (51) constitutes an observation window scanning the entire surface of the filter element (7).