WO2021198574A1

WO2021198574A1 - Membrane leak testing method and associated leak detection device

Info

Publication number: WO2021198574A1
Application number: PCT/FR2021/000032
Authority: WO
Inventors: David HASSLER; Bruno Deletre; Olivier Perrot
Original assignee: Gaztransport Et Technigaz
Priority date: 2020-04-02
Filing date: 2021-04-01
Publication date: 2021-10-07
Also published as: KR20220162719A; FR3108980A1; FR3108980B1; CN115461603A

Abstract

The invention relates to a method for leak testing a tank membrane, comprising an intermediate isolation space (87) of the detection chamber (61), the leak detection device (54) further comprising a vacuum pump (57) connected to the detection chamber (61) and a measurement apparatus connected to the detection chamber (61) and configured to measure a variable that is representative of a quantity of at least one test gas present in the gaseous atmospheric phase of the external space, wherein a step of verifying that an isolation threshold Si is attained in the intermediate isolation space (87) comprising a so-called neutral gas, different from the test gas is performed, the attainment of this insulation threshold Si being a necessary condition for the triggering the leak testing of the membrane.

Description

Title of the invention: Method for testing the tightness of a membrane and associated leak detection device

The invention relates to the field of tanks, sealed and thermally insulating, with membranes, for the storage and / or transport of a fluid, such as a cryogenic fluid.

[0002] The invention relates more particularly to a method for testing the tightness of a membrane of such a tank and to a leak detection device for the implementation of such a method.

[0003] Document KR1020100050128 discloses a method for testing the tightness of a membrane of a tight and thermally insulating tank for storing liquefied natural gas (LNG). The tank has a multilayer structure and successively presents, from the outside to the inside, a secondary thermally insulating barrier, a secondary waterproofing membrane, a primary thermally insulating barrier and a primary waterproofing membrane intended to be in contact with it. the LNG contained in the tank. The method aims more particularly to detect leaks through the weld seams which allow the metal sheets of the primary waterproofing membrane to be connected in a sealed manner. The method provides for injecting a tracer gas, such as helium, into the primary thermally insulating barrier and then moving a detection equipment equipped with a tracer gas analyzer, inside the tank, along the lines. primary waterproofing membrane weld beads. Thus, if the detection equipment detects the presence of the tracer gas, it can be concluded that the primary waterproofing membrane is leaking.

In such a process, the injection of the tracer gas into the primary thermally insulating barrier is critical since the detection method can only guarantee reliable results if the tracer gas has diffused homogeneously and at high concentrations in the 'entire primary thermally insulating barrier. Also, the operation of injecting the tracer gas is relatively long to implement in order to achieve a satisfactory level of diffusion of the tracer gas. In addition, the operation of injecting the tracer gas is expensive because of the quantity of tracer gas necessary to achieve a satisfactory concentration in the primary insulating space. In addition, some tracer gases, such as ammonia, are toxic and dangerous. Finally, to ensure the reliability of the test, the tracer gas can only be injected into the primary thermally insulating barrier if a certain level of tightness is ensured between the primary thermally insulating barrier and the interior of the tank. The tightness test cannot therefore be carried out until the primary waterproofing membrane has been fully assembled in order to achieve the seal between the primary thermally insulating barrier and the interior of the tank.

[0005] Furthermore, the detection equipment consists of a tracer gas suction unit and a tracer gas detector. The suction unit is moved by means of a trolley along the length of the weld bead, the trolley being located on a bottom wall of the tank, or a stage of the scaffolding present in the tank, and the 'suction unit being fixed to the carriage so as to be located opposite a weld bead of a wall adjacent to the bottom wall. However, it is difficult using this equipment to check the tightness of all the weld seams of the tank because the equipment is cumbersome and needs to be connected to the cart on the back wall. This equipment is also very slow because the equipment only checks a small portion of the weld bead at a time and it is necessary to modify the assembly of the equipment to the cart each time the bead is changed. weld to be checked.

[0006] The applicant intends to remedy the shortcomings of the current process and equipment by proposing a method for testing the tightness of a membrane and a leak detection device for the implementation of such a method which are reliable, simple and quick to implement.

[0007] In particular, the present invention intends to make such a method of testing the tightness of a membrane in which no tracer gas is necessary to detect a possible leakage.

[0008] After various experiments and tests, the applicant has thus developed a method for testing the tightness of a tank membrane, the method comprising the successive steps of:

- provision of a leak detection device in a tank comprising a external space in an atmospheric gas phase and a membrane comprising an internal face and an external face facing the external space, the membrane having a test zone the tightness of which must be tested, the leak detection device comprising a bell detection device and comprising a main body and a seal connected to the main body and configured to define a detection chamber between the main body and the test area, the seal has a closed contour and the detection device of leak comprising an intermediate space for isolating the detection chamber, the leak detection device further comprising a vacuum pump connected to the detection chamber and a measuring device connected to the detection chamber and configured to measure a variable representative of a quantity of at least one test gas present in the atmospheric gas phase of the external space;

- positioning of the detection bell against the internal face of the membrane facing the test zone, the seal being pressed against the internal face of the membrane around the test zone;

- Depression of the detection chamber by means of the vacuum pump;

- determination by means of the measuring device of a variable cpt representative of the quantity of test gas present in the detection chamber under vacuum; and

- comparison of the variable cpt with a reference threshold cp _r .

The method according to the invention is characterized in that it further comprises a step of verifying that an insulation threshold Si is reached in the intermediate insulation space comprising a so-called neutral gas different from the test gas. , the attainment of this isolation threshold Si being a condition necessary for triggering said steps of determination and comparison.

[0010] Thus, the present invention has the advantage of not using any test gas while allowing an extremely reliable leak detection test, the test measurement being triggered only when the detection device is certain of being able to detect a leak. leakage of the membrane due to the perfect insulation of the test area thanks to the intermediate insulation space. The aforementioned term "trigger" is understood to mean the beginning or the start of said determination or comparison steps.

The expression "inner face" and "outer face" for the membrane to be tested is understood to mean the side of the latter located or oriented respectively towards the contents of the tank and towards the outside of the tank.

[0013] Within the meaning of the description and of the claims, the term “atmospheric gas phase” is understood to mean a gaseous phase having a composition close to ambient dry air, that is to say comprising approximately 78% of dinitrogen, 21 % dioxygen, 0.9% argon as well as rare gases and volatile organic compounds liable to be emitted by an adhesive used in the thermally insulating barrier or coming from insulating solids or even traces of gas, such as for example carbon dioxide or pentane, said to expand (chemical or physical) during the process of forming the thermally insulating foam.

In other words, this atmospheric gas phase consists of the surrounding air. For example, the atmospheric gas phase consists of part of the ambient air in the tank when the thermally insulating barrier is closed by the waterproof membrane or by a supply of air from outside the tank.

In other words, no tracer gas is injected into the external space before the detection bell is placed against the membrane and the detection chamber is not placed under vacuum or even while the chamber detection unit is placed in depression. Therefore, such a tightness test method is simpler and less expensive to implement.

In addition, no tracer gas being used, the method can be implemented without a certain level of sealing being ensured between the external space and the interior of the tank. Therefore, the sealing test process can be implemented before the assembly of the membrane is finalized. Also, the tightness test method can, for example, be implemented on a membrane test area involving a first series of sheets welded together. to the others before and / or while assembling and welding together a second series of sheets of the membrane.

Finally, such a detection device is easy to handle and move, which makes it possible to test all the weld beads of a membrane more quickly.

According to a first embodiment of the invention, the step of verifying the insulation threshold Si consists in verifying that a pre-set concentration threshold of neutral gas in the intermediate insulation space is reached, the insulation threshold Si being verified as soon as the neutral gas concentration is at least equal to said pre-set neutral gas concentration threshold.

The neutral gas concentration in the intermediate insulation space is an important criterion because, given the primary vacuum in the test zone, it is almost inevitable that very slight gas leaks exist between the space intermediate insulation and test area. This is why, in order to make the tightness test measurement more reliable, according to one embodiment, it is crucial to know the neutral gas concentration in the intermediate space and not to launch the aforementioned step of determining a variable cpt representative of the quantity of test gas present in the detection chamber and comparison of this variable cpt with a reference threshold cp _r only on condition that the neutral gas concentration in the intermediate insulation space has reached at least one prefixed / predetermined low threshold value; this pre-fixed / predetermined concentration threshold value obviously being a function of the leakage value that one seeks to detect and of the tightness of the test zone, that is to say of the seal of the latter.

It should be noted here that the leakage of the seal is much greater than the leakage, or lack of sealing, which one seeks to measure so that conventionally the concentration of inert gas in the intermediate insulation space is ideally necessarily close to 100%.

According to one possibility offered by the invention, the verification of a pre-set neutral gas concentration threshold is carried out by injecting neutral gas, under a pressure of at least ten millibars, preferably a pressure at most 20 mbarg, in the intermediate insulation space for a period of predetermined, the insulation threshold Si being verified as soon as the time for injection of neutral gas into the intermediate insulation space reaches at least the predetermined time. Here, knowing the characteristics of the different elements of the leak test device - namely the characteristics of the vacuum pump, the measuring device, the sealing of the test area, the pressure of the injected neutral gas in the intermediate space etc. - and possibly after a few tests, the operator is able to assess that by sending neutral gas under pressure for a certain predetermined time, the conditions, essentially the concentration of neutral gas in the intermediate insulation space, are gathered to authorize the launch of the steps of determining cpt and its comparison with cp _r .

In this case, the step of verifying that the insulation threshold Si is reached is carried out prior to the aforementioned steps of determination and comparison. In general, whether in the first mode of execution, except in the aforementioned case, and in the second mode of execution explained below, the step of verifying that the isolation threshold Si is reached in the intermediate isolation space does not necessarily have to be carried out prior to the determination and comparison steps; this verification step can be performed after these two steps of determination and comparison or only after the determination step. Nevertheless, in the following, this step of verifying that the isolation threshold Si is reached is presented as a prerequisite to the aforementioned steps of determination and comparison, but it is of course understood that the invention is in no way limited to this. succession of specific steps.

Advantageously, the intermediate insulation space comprises at least one purge orifice, not shown in the appended figures, so as to evacuate the gas initially present in said space in order to replace it with the injected neutral gas. The gas initially present in the intermediate insulation space normally consists of ambient air.

In the latter hypothesis, there is advantageously a neutral gas lighter than air, such as for example helium, and the purge orifice is positioned at the bottom, or at a lower height, in the intermediate insulation space and the neutral gas injection orifice, here for example helium, in the upper or upper part of said space. In doing so, the injection of neutral gas into the intermediate insulation space will push the ambient air, which is heavier than helium and therefore lead to occupy the lower space compared to the neutral gas, out of this intermediate space. so as to be evacuated through this purge orifice.

Of course, when the leak detection device is to be used on a vertical face or on the ceiling of the tank, this characteristic of locating the purge, with regard to the relative weight of the neutral gas with respect to the ambient gas (normally atmospheric air), appreciably loses its interest. This is why two purge orifices can be provided, one located in the upper or upper part of the intermediate insulation space and the other located in the lower or lower part of the intermediate insulation space, the 'one and the other of these orifices being closable at will, automatically or not, by the operator so as to adapt to the location of the leak detection device in the tank as well as to the relative weight ratio between the neutral gas and the gas initially present in the intermediate insulation space in order to facilitate replacement of the latter by the neutral gas.

According to a second embodiment of the invention, the step of verifying the insulation threshold Si consists in verifying that a pre-set concentration threshold of test gas in the intermediate insulation space is reached, the insulation threshold Si being verified as soon as the test gas concentration is at most equal to said pre-set test gas concentration threshold.

Preferably, in this hypothesis, the gas the concentration of which is sought to be known in the intermediate insulation space is identical to that which one seeks to measure in the test zone. This time, the residual quantity of test gas, preferably of (di) oxygen and / or (di) nitrogen and / or argon, is measured in order to trigger the above steps / operations of detection and comparison. Of course, this second embodiment of the invention can optionally be combined with the aforesaid first embodiment: in this case, the neutral gas concentration is checked just like the test gas concentration - or one of the gases. test - and it is necessary on the one hand that the neutral gas concentration reaches at least - passes to the level of - its preset concentration value and on the other hand that the test gas concentration reaches at least - drops to the level of - its pre-set concentration value to authorize the launching of the steps for determining cpt and its comparison with cp _r .

In this hypothesis of the second embodiment, according to a possibility offered by the invention, advantageously, the measuring device for the test zone, preferably consisting of a mass spectrometer, measures the concentration of test gas in the intermediate insulation space. Such an arrangement is possible by using, for example, a three-way valve and by connecting the measuring device, advantageously a mass spectrometer, both to the test zone and to the intermediate isolation space. In doing so, a sampling pump can be used which will take a determined volume from the intermediate isolation space and bring said volume to the mass spectrometer with the pressure required for the efficient use of the latter.

In general, to achieve the aforesaid first and second embodiments of the invention, the verification of a pre-set concentration threshold, in neutral gas or in test gas, is carried out using a katharometer, by ultrasound measurement, by infrared radiation or by electrochemistry. Preferably, this verification is carried out using a katharometer.

Concerning the ultrasound measurement, an ultrasound generator is used which is coupled with a receiver of these waves, these two elements being placed in the intermediate insulation space and connected, by wire connection, to a circuit processing of the data collected. This measures the propagation time of the wave from the transmitter to the receiver. Knowing the distance between the transmitter and receiver, we deduce the speed of sound propagation in the intermediate insulation space so that the gas concentration in said space can be determined.

Regarding infrared radiation, one proceeds by placing at two opposite ends, in the intermediate insulation space, respectively an infrared source, conventionally a diode, and a detector, the emission and reception taking place at a wavelength corresponding to the absorption peak of the neutral gas or test gas, the concentration of which is sought to be measured. Thus, the attenuation of the light intensity, in the wavelength considered, allows to deduce the quantity of matter in gas, and therefore by approximation its concentration. According to one possibility, the infrared source is capable of scanning a relatively wide spectrum of wavelengths and the detector comprises a series of phototransistors per band so as to be able to characterize a transmission or reflection spectrum. Thus, knowing the wavelength of the various gases capable of composing the gas mixture present in the intermediate insulation space, it is possible to obtain the precise composition thereof.

Regarding the technique of electrochemistry, the procedure is carried out using a chemical electrolyte, the variation of potential of which, measured directly, reflects the concentration of the gas the concentration of which is sought to be measured. To do this, we have an electrochemical cell, connected by wire or not to a circuit for processing the data collected, in the intermediate isolation space.

[0033] Other advantageous features of the invention are presented succinctly below:

Advantageously, the aforesaid neutral gas consists of helium or carbon dioxide.

[0035] According to one embodiment of the invention, advantageously, the intermediate insulation space comprises a second seal, surrounding the seal, linked to the main body. In this mode, preferably, the second seal has a sealing lip intended to be pressed against the internal face of the membrane around the seal.

[0036] According to a preferred embodiment of the invention, the vessel is a sealed and thermally insulating vessel and the external space is a thermally insulating barrier comprising insulating solids.

According to an advantageous aspect of the invention, the method comprises a phase of establishing the reference threshold cp _r comprising:

- Positioning the detection bell against the internal face of the membrane in a sealed reference area of the membrane so that the detection chamber is placed opposite said sealed reference area;

- pressurize the detection chamber by means of the vacuum pump; and

- determine the reference threshold cp _{r by means of the measuring device} representative of the quantity of test gas in the negative pressure detection chamber.

Advantageously, the measuring device is a mass spectrometer.

Advantageously, the detection chamber is placed in a vacuum until a threshold value Ps is reached.

[0040] According to an advantageous possibility offered by the invention, the seal comprises a peripheral sealing lip which is pressed against the internal face of the membrane when the detection chamber is placed under vacuum.

Advantageously, the measuring device is configured to detect the presence of a plurality of test gases present in the atmospheric gas phase of the thermally insulating barrier and in which, for each test gas, it is determined by means of the apparatus for measuring a variable cpt representative of the quantity of said test gas in the detection chamber under vacuum; and the variable cpt is compared with a respective _{reference threshold cp r.}

Advantageously, the gas phase contained in the detection chamber is led to the measuring device in order to determine the variable cpt.

[0043] According to one embodiment, during the depressurization of the detection chamber, the external space is in an atmospheric gas phase.

[0044] According to one embodiment, the test gas is chosen from dinitrogen, dioxygen and argon.

[0045] According to another embodiment, the test gas is chosen from water vapor, carbon dioxide, neon, crypton or other constituent of air.

According to another embodiment, the test gas is chosen from volatile organic compounds emitted by glues used to bond two of the insulating solids to one another or of the volatile organic compounds emitted by the degassing of an insulating foam forming one of the insulating solids.

It is also possible to envisage detecting a combination of the aforementioned gases, more particularly the increase in quantities for a plurality of these gases, or even all of these gases present in the environment under or behind the zone the tightness of which is tested. The reference threshold cp _r is not necessarily determined by positioning the detection bell on a reference zone of the membrane and can be established directly on the test zone, when the (partial) vacuum created / generated by the vacuum pump is reached. In other words, in this case, the observation of a leak at the level of the test zone is carried out by increasing the quantity of the gas or gases measured by the measuring device.

Thus, the reference threshold cp _r is representative of a quantity of test gas present in the detection chamber when there is no leak. It is understood that this reference threshold cp _r is a function of the vacuum level created or generated in the detection chamber, in other words this reference threshold cp _r is a variable linked to the desired / obtained (partial) vacuum.

According to one embodiment, the mass spectrometer is of the residual gas analyzer type.

According to one embodiment, the vacuum pump, the detection chamber and the measuring device are connected to each other by a vacuum circuit comprising a first channel connected to the detection chamber, a second channel connected to the vacuum pump and a third channel connected to the measuring device, the first, second and third channels being connected to each other, the detection chamber being depressed through the first and the second channels, the third channel being equipped with a metering valve which is open after the step of depressurizing the detection chamber in order to determine the variable cpt representative of the quantity of test gas present in the detection chamber.

Thus, thanks to the metering valve, it is possible to obtain at the inlet of the measuring device a vacuum compatible with the working range of the measuring device, while the pressure level in the detection chamber is greater than said working range of the measuring device.

According to one embodiment, the detection chamber is placed under vacuum until a threshold value Ps is reached.

According to one embodiment, the threshold value Ps is between 10 and 1000 Pa, for example of the order of 25 to 70 Pa absolute. According to one embodiment, the leak detection device comprises a mechanical pressure means comprising at least one pressure element configured to exert on a portion of the sealing lip a pressure directed towards the membrane when the body is arranged opposite the test zone, and, prior to the depressurization of the detection chamber, pressure is applied to the sealing lip using the mechanical pressure means in order to press the sealing lip against the waterproofing membrane. Thus, the mechanical pressure means makes it possible to press the sealing lip on one or more portions, in particular where there is a risk of the seal coming off the sealing membrane, in order to make the detection of '' a possible leak through the detection bell.

According to one embodiment, the mechanical pressure means is carried by the main body.

According to one embodiment, the gas phase contained in the detection chamber is led to the measuring device in order to determine the variable cpt.

[0058] According to one embodiment, the test zone involves a first series of sheets of the membrane which are welded to each other and the tightness test method is carried out before or while a second series of Sheets of the membrane are not welded to each other to ensure the waterproofing of the membrane.

The present invention also relates to a leak detection device for testing the tightness of a tank for the implementation of the method as defined briefly above, comprising a thermally insulating barrier comprising solid insulating materials in a gas phase atmospheric and a membrane comprising an internal face and an external face facing the thermally insulating barrier, the membrane having a test zone the tightness of which must be tested, the leak detection device comprising a detection bell intended to be placed facing the test area and comprising a main body and a seal which is bonded to the main body and is configured to define a detection chamber between the main body and the test area, the seal having a closed contour intended to be pressed against the face internal membrane around the test area, the leak detection device comprising an intermediate space for isolating the detection chamber and further comprising a vacuum pump connected to the detection chamber and a measuring device connected to the detection chamber and configured to measure a variable representative of a quantity of at least one test gas present in the atmospheric gas phase of the thermally insulating barrier.

The leak detection device is characterized in that the intermediate insulation space is connected to a storage tank for a neutral gas different from the test gas so as to allow injection of neutral gas into the space. intermediate isolation and in that the intermediate isolation space and its content define an isolation threshold Si of the detection chamber which, once reached, allows the aforesaid measurement of a variable representative of a quantity of said test gas characterizing the detection of a membrane leak.

Advantageously, the intermediate insulation space includes a second seal, surrounding the seal, linked to the main body.

Preferably, the second seal comprises a sealing lip intended to be pressed against the internal face of the membrane around the seal.

Advantageously, the measuring device is configured to measure a variable representative of a quantity of at least one test gas present in the atmospheric gas phase of the thermally insulating barrier chosen from dinitrogen, dioxygen, argon. and volatile organic compounds capable of being emitted by degassing the insulating solids of the thermally insulating barrier.

According to an advantageous aspect of the invention, the vacuum pump, the detection chamber and the measuring device are connected to each other by a vacuum circuit comprising a first channel connected to the detection chamber , a second channel connected to the vacuum pump and a third channel connected to the measuring device, the first, second and third channels state connected to each other, the third channel being equipped with a metering valve arranged upstream of the measuring device.

[0065] According to one embodiment, the seal comprises a peripheral sealing lip which is intended to be pressed against the internal face of the membrane.

According to one embodiment, the device further comprises a mechanical pressure means comprising at least one pressure element configured to exert on a portion of the sealing lip a pressure directed towards the membrane when the main body is arranged. next to the test area.

[0067] According to one embodiment, the pressure element is an elastically deformable element which exerts pressure on the portion of the sealing lip by elastic deformation. Thus, the elasticity of the pressure element allows during its elastic deformation to exert a return force on the sealing lip towards the sealing membrane.

[0068] According to one embodiment, the pressure element is oriented perpendicular to the contour of the peripheral sealing lip.

According to one embodiment, the mechanical pressure means comprises a plurality of pressure elements configured to exert pressure on a plurality of portions of the sealing lip, portions being located at both ends of the lip of sealing in a longitudinal direction. Thus, the mechanical pressure means applies pressure to different areas where there is a risk of the seal peeling, namely laterally on the seal, at the ends on the lips of the seal and the foot areas of the seal. wave when the detection device is placed on a part of the membrane comprising waves.

[0070] According to one embodiment, the sealing lip comprises at least one notch having a shape corresponding to that of a corrugation of the membrane, the notch being intended to span the corrugation.

According to one embodiment, the membrane comprises at least two metal sheets connected to one another by a weld bead. [0072] According to one embodiment, the membrane test zone comprises a portion of a weld bead.

[0073] According to one embodiment, the peripheral sealing lip is curved outwardly of the detection bell and is configured to flex and press against the membrane when the detection chamber is depressed.

According to one embodiment, the portion of the weld bead is crossed by at least one corrugation of the membrane.

[0075] According to one embodiment, the sealing lip is shaped to adapt to the geometry of said at least one corrugation.

[0076] According to one embodiment, the portion of the weld bead is crossed by at least two corrugations, for example three corrugations, parallel to the membrane and the sealing lip is shaped to adapt to the geometry of said corrugations.

According to one embodiment, the sealing lip comprises at least two notches having a shape corresponding to that of a corrugation of the membrane projecting towards the interior of the tank, said notches being intended to span said corrugation .

[0078] According to one embodiment, the portion of the sealing lip pressed by the mechanical pressure means is located at a base of the notch. Thus, the mechanical pressure means applies pressure to an area where there is a risk of the seal peeling due to the change in slope of the notch. According to one embodiment, the mechanical pressure means comprises a plurality of pressure elements configured to exert pressure on a plurality of portions of the sealing lip which are located at the bases of the notch or notches. Thus, the mechanical pressure means applies pressure to different areas where there is a risk of the seal coming apart, namely the base of the notch (s).

According to one embodiment, the pressure element comprises a curved blade comprising at one of its ends in contact with the sealing lip a pad. According to one embodiment, the pad is cylindrical in shape and has an axis cylinder which extends in a direction which is substantially parallel to the base of the facing indentation. Thus, the pad makes it possible to apply the pressure of the mechanical pressure means uniformly to part of the sealing lip.

According to one embodiment, the detection bell has an elongated shape.

[0081] According to one embodiment, the seal is made of an elastomeric material having a hardness of between 20 and 50 Shore A.

[0082] According to one embodiment, the elastomeric material of the seal is chosen from the elastomeric polyurethane, rubber, ethylene-propylene-diene monomer, silicone, nitrile and Viton ^®.

According to one embodiment, the vacuum pump, the detection chamber and the measuring device are connected to each other by a vacuum circuit comprising a first channel connected to the detection chamber, a second channel connected to the vacuum pump and a third channel connected to the measuring device, the first, second and third channels connected to each other, the third channel being equipped with a metering valve arranged upstream of the measuring device.

Another idea underlying the invention consists of a leak detection device which makes it possible to use measuring devices working at very low pressures.

According to one embodiment, the invention provides a leak detection device for testing the tightness of a tank comprising a membrane, the membrane having a test zone the tightness of which must be tested, the device for leak detection comprising a detection bell intended to be disposed opposite the test area and comprising a main body and a seal which is linked to the main body and is configured to define a detection chamber between the main body and the test zone, the seal comprising a closed contour intended to be pressed against the internal face of the membrane around the test zone, the leak detection device further comprising a vacuum pump connected to the pressure chamber detection and a measuring device connected to the detection chamber and configured to measure a variable representative of a quantity of at least one test gas, the vacuum pump, the detection chamber and the measuring device being connected to each other by a vacuum circuit comprising a first channel connected to the chamber of detection, a second channel connected to the vacuum pump and a third channel connected to the measuring device, the first, second and third channels connected to each other, the third channel being equipped with a metering valve arranged in upstream of the measuring device.

Such a leak detection device has the advantage of making it possible to obtain, at the input of the measuring device, a pressure compatible with a working range of the measuring device, while the pressure level in the detection chamber is greater than said working range. While such a detection device is advantageous when no tracer gas is used, it can also be used when the process uses a tracer gas.

The invention will be better understood, and other aims, details, characteristics and advantages thereof will emerge more clearly during the following description of several particular embodiments of the invention, given solely by way of illustration. and not limiting, with reference to the accompanying drawings.

[0088] [Fig.1] is a schematic illustration of a multilayer structure of a wall of a membrane tank.

[0089] [Fig.2] is a schematic view of a membrane leak detection device according to one embodiment.

[0090] [Fig.3] is a schematic view of a device for detecting a leak of a membrane according to a variant of the embodiment of Figure 2.

[0091] [Fig.4] is a cross-sectional view along the plane II-II of the detection bell of the leak detection device of Figure 1.

[0092] [Fig.5] is a perspective view of a seal according to the embodiment of Figures 2 and 3.

[0093] [Fig.6] is a schematic view of a variant of a leak detection device in which the detection bell is equipped with a mechanical pressure means. [0094] [Fig.7] is a schematic cross-sectional view of the detection bell of Figure 6, before depressurization of the detection chamber.

[0095] [Fig.8] is a schematic cross-sectional view of the detection bell of Figure 6, after depressurizing the detection chamber.

[0096] [Fig.9] schematically illustrates the positioning of the detection bell facing a portion of a weld bead sealing between two adjacent corrugated metal sheets of a membrane.

[0097] [Fig.10] is a schematic illustration of a first embodiment of a membrane leak detection device according to the present invention.

[0098] [Fig.11] is a schematic view of a membrane leak detection device according to another embodiment of the present invention.

[0099] [Fig.12] is a graph illustrating the reference threshold cp _{r as} well as the variable cpt representative of the quantity of test gas present in the detection bell delivered by the measuring device when no sealing defect is detected (curve a) and when a leak is detected (curve b).

[0100] In the following, the present invention is illustrated with a katharometer as an apparatus carrying out the verification of a pre-set concentration threshold in gas, here in neutral gas, but it is of course understood that all the devices and all the The methods presented above, without being exhaustive, are also possible solutions for carrying out the step of verifying that an insulation threshold Si is reached in the intermediate insulation space.

[0101] By convention, the terms "external" and "internal" are used to define the relative position of an element with respect to another, by reference to the interior and exterior of the vessel.

A method for testing the tightness of a membrane of a sealed and thermally insulating membrane tank will be described below. For example, such membrane tanks are notably described in the patent application FR2691520 which concerns a tank type Mark III ^®.

[0103] The membrane tanks have a plurality of walls which have a multilayer structure, as shown in FIG. 1. Each wall 1 comprises, from the outside towards the inside of the tank, a secondary thermally insulating barrier 2 comprising secondary insulating panels 3 anchored to a supporting structure 4, a secondary membrane 5 resting against the secondary thermally insulating barrier 2, a thermally insulating barrier primary 6 comprising primary insulating panels 7 resting against the secondary membrane 2 and anchored to the supporting structure 4 or to the secondary insulating panels 3 and a primary membrane 8 which rests against the primary thermally insulating barrier 6 and which is intended to be in contact with the liquefied gas contained in the tank.

Before putting into service such membrane tanks and failing to inject a tracer gas into the secondary 2 and primary 6 thermally insulating barriers, the gas phase present in said secondary 2 and primary 6 thermally insulating barriers is a phase atmospheric gaseous, that is to say it has a composition close to that of ambient air.

[0105] According to one embodiment, the gas phase also comprises volatile organic compounds emitted by one or more of the glues used in the thermally insulating barrier, for example to glue the insulating materials used for the manufacture of the insulating panels to each other or originating from any other element of the thermally insulating barrier, for example originating from the degassing of the insulating foam of the insulating panels.

[0106] At least one of the primary 8 and secondary 5 membranes comprises a plurality of metal sheets which are welded to each other. The tightness test method which will be described below aims more particularly to test the tightness of the weld beads making it possible to connect the metal sheets to each other, for one and / or the other of the primary membranes 8 and secondary 5. According to one embodiment, the membrane 5, 8 to be tested has undulations which allow it to deform under the effect of the thermal and mechanical stresses generated by the fluid stored in the tank. To do this, each metal sheet has two sets of corrugations perpendicular to each other.

In relation to FIG. 2, there is a leak detection device 54 aimed at testing the tightness of a membrane 5, 8. The leak detection device 54 comprises a detection bell 55 which is intended to be placed against the internal face of the membrane 5, 8 facing a portion of the weld bead to be tested.

[0109] The detection bell 55 has an elongated shape and has a length of between 0.5 and 5 m, for example of the order of 1 m. The length of the detection bell 55 is advantageously as long as possible so as to check the tightness of a larger area during one and the same test.

As shown in Figure 4, the detection bell 55 comprises a main body 100, here rigid, and a flexible seal 60 which are fixed to each other and which are arranged to define with the membrane 5, 8 to be tested, a sealed detection chamber 61, placed opposite the portion of the weld bead 62 to be tested.

Returning to Figure 2, it is observed that the leak detection device 54 also comprises a measuring device 56 which is connected to the detection chamber 61 and a vacuum pump 57 which is associated with said measuring device 56 The vacuum pump 57 is connected, on the one hand, to the detection chamber 61 of the detection bell 55 so as to allow a depressurization of the detection chamber 61 and, on the other hand, to the measuring device 56 so as to conduct the gas contained in the detection chamber 61 to the measuring device 56.

[0112] The vacuum pump 57 is connected to the detection bell 55 via a pipe 58 which is preferably flexible. The pipe 58 is connected to a channel which is formed in the main body 100 and opens into the detection chamber 61.

[0113] In another embodiment shown in FIG. 3, the leak detection device comprises a second vacuum pump 84 which is connected to the pipe 58 via a valve 85 and advantageously comprises a power greater than that of the vacuum pump. vacuum 57 associated with the measuring device 56. In such a case, the second vacuum pump 84 aims to pressurize the detection chamber 61 while the vacuum pump 57 aims to conduct the gas contained in the detection chamber. 61 to the measuring device 56, after the prior depressurization of the detection chamber 61. As shown in Figures 4 and 5, the main body 100 comprises a rigid core 59. The seal 60 comprises a casing 63 matching the shape of the rigid core 59 and a peripheral sealing lip 64 which extends l envelope 63 down. The casing 63 has a bottom 83 which covers the upper surface of the rigid core 59 and a peripheral wall 74 which conforms to the periphery of the rigid core 59. The bottom 83 has at least one hole, not shown, to which the pipe 58 connected to the vacuum pump 57. The rigid core 59 has on its lower surface 80 a recess 79 over the entire length of the rigid core 59. The recess 79 allows during a depressurization of the detection chamber 61 ensure that the test zone 62 is still in fluid contact with the detection chamber 61, despite a lowering of the rigid core 59 towards the membrane 5, 8 due to a deformation of the sealing lip 64. In addition, the rigid core 59 also comprises a channel, not shown in FIG. 2 because it is present only in a plane passing at the level of the pipe 58, which opens out facing the hole made in the bottom 83 of the casing 63, and which thus makes it possible to to connect the room of detection 61 with a pipe 58, as shown in Figures 2 and 3, leading to the vacuum pump (s) 57, 84 and the measuring device 56.

[0115] The sealing lip 64 is curved outwardly of the detection bell 55 and is thus configured to flex and press against the membrane 5, 8 when the detection chamber 61 is depressed. In other words, the sealing lip 64 has a section having a general L shape.

[0116] The outwardly curved portion of the sealing lip 64 has a width of the order of 15 to 40 mm. The sealing lip 64 is shaped to match the geometry of the membrane 5, 8 along the weld bead to be tested. Also, in Figure 5, the sealing lip 64 comprises notches 65 having a shape corresponding to that of the corrugations of the membrane 5, 8 that the detection bell 55 is intended to span when it is in position against the portion. of the weld bead to be tested.

[0117] The seal 60 is advantageously made of an elastomeric material having a hardness of between 20 and 50 Shore A. The seal seal 60 is for example made of polyurethane elastomer, EPDM rubber, silicone, nitrile or Viton ^®.

Figures 6, 7 and 8 show a detection bell 55 according to another embodiment. The detection bell 55 of FIGS. 6 to 8 is designed similarly to the detection bell 55 of FIGS. 4 and 5 but differs in particular in that it comprises a mechanical pressure means 66 capable of pressing the peripheral sealing lip 64 against the membrane to be tested so as to ensure the tightness of the detection chamber 61. The detection bell 55 comprises a main body 100 extending in a longitudinal direction, a flexible seal 60 fixed to the main body 100 and a mechanical pressure means 66 carried by the main body 100 and configured to exert a pressure directed towards the membrane 5, 8 on the seal 60. The rigid core 59 comprises a channel 82 making it possible to connect a lower surface 80 to a upper surface 81 of the rigid core 59. The channel 82 enables the detection chamber 61 to be placed in communication with a gas outlet fitting 78 which is intended to be connected to a pipe 58, as shown in FIGS. ures 2 and 3, leading to the vacuum pump (s) 57, 84 and the measuring device 56.

[0119] The seal 60 comprises a casing 63 fixed to the rigid core 59 by fixing means 110, for example consisting of a hoop surrounding the entire circumference of the rigid core 59 and of the seal 60 and fixing so seals the rigid core 59 and the seal 60 to each other by means of fasteners, such as screws.

[0120] The mechanical pressure means 66 comprises a support member 73 extending over the entire length of the main body 100 above it and fixed to the main body 100. Handles 76 are fixed to the two longitudinal ends of the 'support element 73 so as to allow the manipulation of the detection bell 55 by an operator and possibly to actuate the mechanical pressure means 66 by an effort of the operator.

[0121] The mechanical pressure means 66 is composed of a plurality of pressure elements which are here produced in the form of curved blades 72. The curved blades 72 are distributed over the sealing lip 64 and are fixed by means of fixing means 77 to the support element 73. The curved blades 72 are deformable elastically so as, when they are deformed, to exert an elastic force on the sealing lip 64 in order to press it against the membrane 5, 8. To make the sealing of the detection chamber 61 more reliable, it is necessary to press the sealing lip 64 in areas where the risk of detachment is greater. This is why the curved blades 72 have ends resting against the sealing lip 64, in particular at the base of the notches 65 of the sealing lip 64 and at the longitudinal ends of the detection bell 55, on the lip of the sealing lip. sealing 64.

[0122] Some of the curved blades 72 are fixed at one end to the support element 73 while the other end is placed on the sealing lip 64. These blades 72 are in particular placed on the ends of the detection bell 55. Other curved blades 72 are fixed in their middle to the support element 73 while their two ends are placed on the sealing lip 64 so as to apply pressure to two different zones, these curved blades 72 being in particular placed between two notches 65.

[0123] The curved blades 72 have at each of their end in contact with the sealing lip 64 a pad 75 aimed at limiting the punching phenomena liable to degrade the integrity of the sealing lip 64. To do this, the pad 75 has a larger bearing surface than the section of the curved blades 72. In addition, the bearing surface of the pad 75 is advantageously cylindrical in shape, the axis of which extends in a direction which is substantially parallel to the base of the notches 64. The length of a pad 75 is further substantially equal to the dimension of the portion of the sealing lip 64 protruding from the main body 100, in the direction in which the pad 75 extends. Thus the pad 75 allows the mechanical pressure means 66 to exert pressure in a homogeneous manner on the sealing lip 64.

As shown in FIG. 8, when the vacuum pump 57 or 84 is activated, a vacuum is created in the detection chamber 61 which makes it possible to fix the detection bell 54 against the membrane 5, 8 to be tested. This vacuum force then activates the mechanical pressure means 66 so that it presses the sealing lip 64 against the membrane 5, 8 in certain well-defined areas. In particular, the curved blades 72 are tensioned so that they transmit the force to the sealing lip 64 via the pads 75 to the areas where the separation of the sealing lip 64 is most likely, namely the longitudinal ends of the main body 100 and the bases of the notches 65 .

[0125] According to a particularly interesting aspect of the invention, the method for testing the tightness of a membrane 5, 8 which will be described below does not have a step of injecting a tracer gas into the interior. of the thermally insulating barrier covered 2, 6 by the membrane 5, 8 to be tested. Also, during the sealing test of the membrane 5, 8, it is sought to detect the migration of the atmospheric gas phase present in said thermally insulating barrier 2, 6 towards the detection chamber 61, through a bead. weld to identify a sealing defect.

[0126] Therefore, the tightness test process can optionally be implemented before or after the membrane whose tightness is to be tested is fully assembled. Thus, according to one possibility offered by the invention, the test method is implemented on a test zone of the membrane involving a first series of welded sheets and, after or in parallel with said leak test of said zone, one assembles and welds together a second series of sheets of said waterproofing membrane.

[0127] The measuring device 56, shown in FIG. 1, is configured to measure a variable representative of the quantity, in the detection chamber 61, of one or more test gases present in the atmospheric gas phase of the barrier. thermally insulating 2, 6 covered by the membrane 5, 8 to be tested. Advantageously, the test gas is chosen from gases present in dry air at a content greater than 0.5%, namely dinitrogen, dioxygen and argon. This makes it possible to limit the relative uncertainty of the measurement delivered by the measuring device 56. According to an alternative or complementary embodiment, the test gas is chosen from the volatile organic compounds emitted by the adhesives or any other component of the barrier. thermally insulating.

[0128] According to one embodiment, the measuring device 56 is a mass spectrometer and more particularly a residual gas analyzer. A waste gas analyzer is a mass spectrometer that measures the chemical composition of a gas present in a low-pressure environment. The residual gas analyzer comprises an ionization source which ionizes the molecules of the gas or gases to be analyzed followed by one or more mass analyzers which separate the ions produced according to their mass to charge ratio. The residual gas analyzer further comprises an ion detection system which for each mass to charge ratio measures the corresponding generated electric current, which makes it possible to deduce therefrom the number of molecules for each gas analyzed.

[0129] The procedure for detecting a sealing defect in a weld bead is as follows.

First, the method comprises a step of establishing one or more reference thresholds cp _r . During this step, the detection bell 55 is placed by one or more operators, in a sealed reference zone of the membrane 5, 8, for example a zone without a weld bead.

The vacuum pump, referenced 57 in FIG. 2 or 84 in FIG. 3, is put into operation so as to place the detection chamber 61 in negative pressure and thus ensure the fixing of the detection bell 55 against the membrane. 5, 8 to test. As soon as the pressure inside the detection chamber 61 reaches a pressure threshold Ps, the vacuum pump 57 or 58 is stopped. The pressure threshold Ps is advantageously between 10 and 1000 Pa absolute, for example of the order of 25 to 70 Pa absolute. As soon as the pressure is reached or shortly thereafter, the vacuum pump 57 associated with the measuring device 56 is put into operation so as to conduct the gas phase contained in the detection chamber 61 towards the measuring device 56. , for a period Tm which is less than or equal to 5 seconds and advantageously less than 1 second. The vacuum pump 57 associated with the measuring device 56 is controlled as a function of a pressure setpoint inside the detection chamber or as a function of a volume setpoint.

The measuring device 56 then delivers a reference threshold cp _r which is representative of the quantity of test gas present in the detection chamber 61 when the detection bell 55 is positioned opposite a zone of the membrane. 5, 8 devoid of sealing defect. According to one embodiment, when the measuring device 56 is configured to detect several test gases present in the atmospheric gas phase of the thermally insulating barrier 2, 6, a reference threshold cp _r is measured for each of the test gases.

[0133] Subsequently, when the reference threshold or thresholds cp _r have been established, the detection bell 55 is then placed opposite the portion of the weld bead 62 to be tested, as shown in FIG. 9, the detection bell 55 being suitably centered with respect to the weld bead 62 so that the two lateral parts of the curved portion of the sealing lip 64 are disposed on either side of the weld bead 62.

The intermediate insulation space 87 comprises a katharometer 200, placed in said space 87, for example fixed on the internal face of the wall 190 delimiting, with the seal 86, the intermediate insulation space 87. It should be noted that the intermediate insulation space 87 can also be completely delimited by the seal 86, as could be the case more easily with the intermediate insulation space 87 which has a volume. reduced in the embodiment shown in Figure 11. It is nevertheless preferable that at least a portion of this delimitation is formed by a wall 190 which may be of a more rigid material than that constituting the seal 86, such as, for example, a plastic material or a mixture of plastic materials, a metallic material, preferably consisting of aluminum, or else a composite material combining layers of plastic and metallic materials, or even ceramic.

[0135] The leak detection device 54 further comprises a storage tank 88 for an inert gas which is connected in a sealed manner to the intermediate space 87. The neutral gas is necessarily a gas different from the test gas (s). The neutral gas storage tank 88 is connected to the intermediate space 87 by a valve and / or by a pump.

The katharometer 200 is connected to the control circuit by wire, or not, and warns, visually or by an audible signal, the operator that the insulation threshold Si of the intermediate space 87 has been reached. The warning of this insulation threshold Si reached can constitute simple information for the operator or else this warning or this signal, audible and / or visual, can be unblocking, that is to say that the operator does not can measure the test gas (s) in the area test 62 only once this insulation threshold Si has been reached. Of course, the warning of reaching this insulation threshold Si of the intermediate space 87 can also automatically trigger, without any action by an operator or the like, the measurement of the test gas (s) in the test zone 62. .

Neutral gas is injected into the intermediate space 87 before or after the partial vacuum produced in the test zone 62 by the vacuum pump 84 and preferably as soon as the pressure threshold Ps is reached and during the period of time during which the measuring device 56 determines the reference threshold cp _r or the variable cpt. Advantageously, the neutral gas is also injected during the depressurization of the detection chamber 61 and optionally before said depressurization. Thus, the intermediate space 87 forms a neutral gas barrier preventing or limiting the introduction of ambient air into the detection chamber 61. This makes it possible to maintain excellent reliability of the leak test even when the seal 60 does not provide sufficient sealing.

As soon as the pressure inside the detection chamber 61 reaches a pressure threshold Ps or shortly thereafter, and provided that the insulation threshold Si is reached, the vacuum pump 57 associated with the The measuring device 56 is put into operation so as to conduct the gas phase contained in the detection chamber 61 towards the measuring device 56 for the duration Tm. The measuring device 52 then measures, for the gas or gases for for which a reference threshold cp _r has been established, a variable cpt representative of the quantity of test gas present in the detection chamber 61.

When the portion of the weld bead 62 tested has no sealing defect, the variable cpt delivered by the measuring device 52 has a value substantially equal to that of the reference threshold cp _r . This situation corresponds to the curve a illustrated in figure 12.

[0140] On the contrary, when the portion of the weld bead 62 tested has one or more sealing defects, molecules of the test gas migrate from the gas phase of the thermally insulating barrier to the detection chamber 61 through the defect (s). sealing due to the pressure differential between the pressure of the atmospheric gas phase of the thermally insulating barrier 2, 6 which is equal to or close to atmospheric pressure and that prevailing in the detection chamber 61. Also, in such circumstances, as soon as the pressure threshold Ps is reached, the quantity of the test gas (s) present in the detection chamber 61 increases. Also, the quantity of test gas measured by the measuring device 62 is greater than the quantity of test gas measured when the detection chamber 61 is arranged in the sealed reference area of the membrane 5, 8. This situation corresponds to curve b, illustrated in figure 12.

Therefore, to determine the presence of a sealing defect in the portion of the weld bead 62 tested, the variable cpt is compared with the reference threshold cp _r .

[0142] If the variable cpt is less than or equal to cp _r + D with D a constant or variable value representative of an absolute or relative measurement uncertainty, then it is concluded that the tested portion of the weld bead 62 does not exhibit leakage. In this case, the detection bell 55 is then placed opposite an adjacent portion of the weld bead 62, ensuring an overlap between the two successively tested portions so as to ensure that the tightness of the weld bead 62 has been tested. along its entire length.

On the contrary, if the variable cpt is greater than cp _r + D, then it is concluded that the portion of the weld bead 62 tested has a sealing defect. Corrective welding measures are then implemented to correct the defect.

When several test gases are used, the variable cpt of each of the test gases is compared with the _{corresponding reference threshold cp r of} said test gas. This ensures redundancy of the leak test and further guarantees the reliability of the leak test carried out.

[0145] According to a possibility offered by the invention and visible in FIG. 11, the evacuation circuit advantageously comprises three channels 89, 90, 91 connected to each other, namely a first channel 91 which is connected to the detection chamber 61, a second channel 90 which is connected to an empty pump 84 and a third channel 91 which is connected to the measuring device 56, itself being equipped with a pumping device 57. Advantageously, the pumping device 57 fitted to the measuring apparatus comprises two pumps, namely a main pump and a turbomolecular pump which maintains a high vacuum.

In this embodiment, the third channel 91 is equipped with a metering valve 92, arranged upstream of the measuring device 56. The metering valve 92 makes it possible to take a very low flow of gas from of the detection chamber 61 and send it to the measuring device 56. The metering valve 92 thus makes it possible to obtain, at the inlet of the measuring device 56, a flow of gas having a pressure greater than weak than that prevailing in the detection chamber 61.

[0147] Thus, by using such a metering valve, it is possible to obtain at the inlet of the measuring device a high vacuum compatible with the working range of the measuring device, then that the pressure level in the detection chamber 61 is greater than the working range of the measuring device 56.

According to an advantageous embodiment, when the measuring device 56 is a mass spectrometer of the residual gas analyzer type, the working pressures of such a measuring device 56 are typically less than or equal to 1.10 ⁴ mbar. . Thus, the adjustment of the metering valve 92 is determined as a function of the pressure in the first and second channels 89, 90 so that the pressure, in the third channel, downstream of the metering valve, is lower. or equal to 1.10 ⁴ mbar.

[0149] Advantageously, the adjustment valve has an adjustment range of between 5 × 10 ⁶ mbar and 1000 mbar.l / s.

[0150] Furthermore, the metering valve 92 is equipped with an all-or-nothing valve arranged upstream of the flow rate adjustment means. Thus, when the tightness test method is implemented, the tap of the metering valve 92 is maintained in closed mode as long as the pressure in the detection chamber has not reached a threshold value and then is opened when said threshold value is reached, for the duration Tm.

Although the invention has been described in connection with several particular embodiments, it is obvious that it is in no way limited thereto and that it includes all the technical equivalents of the means described as well as their combinations if these come within the scope of the invention. The use of the verb "to include", "to understand" or "to include" and its conjugated forms does not exclude the presence of other elements or other steps than those stated in a claim.

[0153] In the claims, any reference sign between parentheses cannot be interpreted as a limitation of the claim.

Claims

[Claim 1] ÎProcess for testing the tightness of a tank membrane (5, 8), the process comprising the successive steps of:

- provision of a leak detection device (54) in a tank comprising an external space in an atmospheric gas phase and a membrane (5, 8) comprising an internal face and an external face facing the external space, the membrane (5, 8) having a test zone (62) the tightness of which is to be tested, the leak detection device (54) comprising a detection bell (55) and comprising a main body (100) and a seal sealing

(60) bonded to the main body (100) and configured to define a detection chamber (61) between the main body (100) and the test area (62), the seal (60) has a closed contour and the leak detection device (54) comprising an intermediate space (87) for isolating the detection chamber (61), the leak detection device (54) further comprising a vacuum pump (57, 84) connected to the detection chamber (61) and a measuring apparatus (62) connected to the detection chamber (61) and configured to measure a variable representative of an amount of at least one test gas present in the atmospheric gas phase of external space;

- positioning of the detection bell (55) against the internal face of the membrane (5, 8) facing the test zone (62), the seal (60) being pressed against the internal face of the membrane (5, 8) around the test area (62);

- Depression of the detection chamber (61) by means of the vacuum pump (57, 84);

- determination by means of the measuring device (62) of a variable cpt representative of the quantity of test gas present in the detection chamber

(61) in depression; and

- comparison of the variable cpt with a reference threshold cp _r ; characterized in that it further comprises a step of verifying that an insulation threshold Si is reached in the intermediate insulation space (87) comprising a so-called neutral gas different from the test gas, the attainment of this threshold isolation Si being a necessary condition for triggering said determination and comparison steps.

[Claim 2] A leak test method according to claim 1, wherein the step of verifying the insulation threshold Si consists of verifying that a pre-set concentration threshold of neutral gas in the intermediate insulation space ( 87) is reached, the insulation threshold Si being verified as soon as the neutral gas concentration is at least equal to said pre-set neutral gas concentration threshold.

[Claim 3] A leak test method according to claim 1, wherein the step of verifying the insulation threshold Si comprises verifying that a pre-set concentration threshold of test gas in the intermediate insulation space ( 87) is reached, the insulation threshold Si being verified as soon as the test gas concentration is at most equal to said pre-set test gas concentration threshold.

[Claim 4] Leak test method according to claim 2 or 3, in which the verification of a pre-set concentration threshold, in neutral gas or in test gas, is carried out using a katharometer, by ultrasound measurement, by infrared radiation or by electrochemistry.

[Claim 5] Leak test method according to claim 2, in which the verification of a pre-set neutral gas concentration threshold is carried out by injecting neutral gas, under a pressure of at least ten millibars. , preferably a pressure of at most 20 mbarg, in the intermediate insulation space (87) for a predetermined time, the insulation threshold Si being verified as soon as the time for injecting neutral gas into the intermediate isolation space (87) reaches at least the predetermined time.

[Claim 6] A test method according to claim 5, wherein the step of verifying that the isolation threshold Si is reached is performed prior to the above steps of determining and comparing.

[Claim 7] A test method according to claim 5 or 6, wherein the intermediate insulation space (87) has at least one purge port so as to evacuate the gas initially present in said space (87) in order to remove it. replace with the injected neutral gas. [Claim 8] A leak test method according to any one of the preceding claims, wherein said neutral gas consists of helium.

[Claim 9] A leak test method according to any preceding claim, wherein the insulating intermediate space (87) has a second seal (86), surrounding the seal (60). ), linked to the main body (100).

[Claim 10] A leak test method according to claim 9, wherein the second seal (86) has a sealing lip intended to be pressed against the internal face of the membrane (5, 8) around the seal (60).

[Claim 11] A method of leak testing any one of the preceding claims, wherein the vessel is a sealed and thermally insulating vessel and the outer space is a thermally insulating barrier (2,6) comprising insulating solids.

[Claim 12] Leak test method according to any one of the preceding claims, comprising a phase of establishing the reference threshold cp _r comprising:

- position the detection bell (55) against the internal face of the membrane in a sealed reference zone of the membrane (5, 8) so that the detection chamber (61) is placed opposite said zone of waterproof reference;

- pressurize the detection chamber (61) by means of the vacuum pump (57, 84); and

- Determine by means of the measuring device (62) the reference threshold cp _r representative of the quantity of test gas in the detection chamber (61) in depression.

[Claim 13] A leak test method according to claim 3, wherein the measuring apparatus (62) for the test area (62), preferably consisting of a mass spectrometer, also measures the test gas concentration. in the intermediate insulation space (87). [Claim 14] A test method according to any one of the preceding claims, in which the detection chamber (61) is depressed until a threshold value Ps is reached.

[Claim 15] A leak test method according to any preceding claim, wherein the seal (60) has a peripheral sealing lip (64) which is pressed against the inner face of the membrane ( 5, 8) when the detection chamber is depressed.

[Claim 16] Leak detection device (54) for testing the tightness of a tank for implementing the method according to any one of the preceding claims, comprising a thermally insulating barrier (2, 6) comprising insulating solids (3, 7) in an atmospheric gas phase and a membrane (5, 8) comprising an inner face and an outer face facing the thermally insulating barrier (2, 6), the membrane (5, 8) having a test zone (62) the tightness of which must be tested, the leak detection device (54) comprising a detection bell (55) intended to be placed opposite the test zone (62) and comprising a body main body (100) and a gasket (60) which is bonded to the main body (100) and is configured to define a detection chamber (61) between the main body (100) and the test area (62), the seal (60) having a closed contour intended to be pressed against the internal face of the membrane (5, 8) around the test area (62), the leak detection device (54) comprising an intermediate space (87) for isolating the detection chamber (61) and further comprising a pump vacuum (57, 84) connected to the detection chamber (61) and a measuring apparatus (62) connected to the detection chamber (61) and configured to measure a variable representative of a quantity of at least one gas test present in the atmospheric gas phase of the thermally insulating barrier (2, 6), characterized in that the intermediate insulation space (87) is connected to a storage tank (88) of a neutral gas other than gas test so as to allow injection of neutral gas into the intermediate isolation space (87) and in that the intermediate isolation space (87) and its contents define an isolation threshold Si of the detection chamber (61) which, once reached, authorizes the aforementioned measurement of a variable representative of a quantity of said test gas characterizing the detection of a leak at the level of the membrane (5, 8).

[Claim 17] A leak detection device according to claim 16, wherein the insulating intermediate space (87) has a second seal (86), surrounding the seal (60), bonded to the body. main (100).

[Claim 18] A leak detection device according to claim 17, wherein the second seal (86) has a sealing lip intended to be pressed against the internal face of the membrane (5, 8) around the seal. sealing (60).

[Claim 19] A leak detection device (54) according to one of claims 16 to 18, wherein the measuring apparatus (62) is configured to measure a variable representative of an amount of at least one test gas. present in the atmospheric gas phase of the thermally insulating barrier (2, 6) chosen from dinitrogen, dioxygen, argon and volatile organic compounds liable to be emitted by degassing the insulating solids (3, 7) of the thermally insulating barrier (2, 6).

[Claim 20] A leak detection device (54) according to any one of claims 16 to 19, wherein the vacuum pump (84), the detection chamber (61) and the measuring apparatus (62) being connected to each other by a vacuum circuit comprising a first channel (89) connected to the detection chamber (61), a second channel (90) connected to the vacuum pump (84) and a third channel (91 ) connected to the measuring device (56), the first, second and third channels connected to each other, the third channel (91) being equipped with a metering valve (92) arranged upstream of the device measurement (62).

[Claim 21] A leak detection device (54) according to any one of claims 16 to 20, wherein the measuring device (62) is a mass spectrometer !.