Device and method for detecting a test gas escaping from a leak

Abstract

The invention relates to a device for detecting a test gas escaping from a leak in a test object (12), and a method for detecting a test gas escaping from a leak in a test object (12). The device (10) for detecting a test gas escaping from a leak in a test specimen (12), comprising a test chamber (14) for receiving the test specimen (12), a vacuum pump (26), at least one 2 / 3-way valve (20), and a gas detector (24), has a first line (16) and a second line (18), each of which is connected to the test chamber (14) and can be selectively connected to a gas line (22) via the 2 / 3-way valve (20), wherein the first line (16), the second line (18), and the gas line (22) are permeable to gas flow in a flow direction (28), and the gas line (22) is connected to the gas detector (24) and the vacuum pump (26).It is characterized in that the first line (16) is designed such that a complete flow takes a first time interval, the second line (18) is designed such that a complete flow takes a second time interval, the second time interval is longer than the first time interval, and the gas detector is designed to detect at least one parameter of gas from the first line (16) and the second line (18).

Description

[0001] The invention relates to a device and a method for detecting a test gas escaping from a leak in a test specimen.

[0002] In accumulation leak detection, a test specimen containing a test gas, such as helium or hydrogen, is monitored for leakage over a specific period. To induce leakage and thus detect the presence of a leak, a pressure difference is created between the interior of the test specimen and its immediate surroundings. This can be achieved, for example, by applying overpressure inside the specimen. Alternatively, the test specimen can be placed in a test chamber, which is then connected to a vacuum pump that evacuates the chamber. Due to the higher pressure inside the test specimen compared to its immediate surroundings, the test gas will escape from the specimen in the event of a leak and can be detected by a measuring device in its vicinity.

[0003] Since only very small amounts of test gas escape, especially in the case of small leaks, and measuring devices have error rates, reliable identification of leaks is often difficult to achieve.

[0004] In particular, it is a known problem that gas detectors, such as mass spectrometers, are subject to signal drift. In the context of measurement technology, the term "drift" refers to undesirable changes or shifts in the measurement results without a change in the measured parameter.

[0005] Drift can have various causes and occur in different types of measuring devices, including sensors, instruments, and measurement systems. Causes of signal drift can include temperature fluctuations, component aging, environmental conditions such as humidity, air pressure, vibrations, or the influence of electric or magnetic fields.

[0006] Signal drift occurs over time. The longer the interval between two measurements of the same type, the greater the influence of signal drift, which distorts the measurement results. This influence can therefore be reduced by shortening the time between measurements. The invention aims to provide a method and a device for reducing the influence of signal drift. A further objective of the invention is to enable the reliable detection of even small leaks in test specimens.

[0007] The problem is solved by a device having the features of one of claims 1 and 2 and a method having the features of claim 13.

[0008] A key aspect of the invention is that, starting from a test chamber, two gas-carrying lines can be selectively connected to a gas detector. This allows two parameters to be recorded in immediate succession, the comparison of which can reveal the presence of a leak. In the event of a leak, the lines contain gas samples that escaped from the test object at different times.

[0009] The device according to the invention comprises a test chamber for receiving a test specimen. The test specimen can be, for example, a hydrogen tank that is to be examined immediately after production or as part of a maintenance procedure. First, it must be ensured that a minimum quantity of test gas, for example helium or hydrogen, is present inside the test specimen. This can be introduced manually into the test specimen.

[0010] The test chamber is connected to at least one first and one second line, each of which can be selectively connected to a gas line via a changeover valve. The lines are designed such that the flow of gas from the second line takes longer than the flow of gas from the first line. The changeover valve can, for example, be a 2 / 3-way valve.

[0011] Alternatively or additionally, the switching valve can be designed to switch back and forth between the first and second lines in order to connect the two lines alternately and sequentially to the gas supply. The switching valve is configured, for example, by a suitably designed control device for switching the valve, to connect the first line to the gas supply for a first period of time and the second line for a second period of time, the first period being longer than the second period.

[0012] The changeover valve, or its control device, can be designed to switch the changeover valve back and forth in a periodic cycle. The switching of the changeover valve occurs at such a frequency that the total time the first line is connected to the gas pipeline is longer than the total time the second line is connected to the gas pipeline.

[0013] In a standard position of the changeover valve, the first line is connected to the gas supply. In a test position of the changeover valve, the second line is connected to the gas supply. The gas supply is connected to a gas pump, which may be a vacuum pump used to evacuate the test chamber. Evacuation in the context of this document does not necessarily require the creation of an absolute vacuum. The crucial factor is the creation of a pressure differential between the interior of the test specimen and the interior of the test chamber outside the specimen. Alternatively, this can also be achieved by maintaining overpressure in the test specimen while atmospheric pressure prevails in the test chamber.

[0014] If the test specimen has a leak, test gas, such as helium or hydrogen, flows from the specimen first into the test chamber and then into the first and second lines. For this to occur, a pressure difference must exist between the inside of the test chamber and the gas detector such that the pressure in the test chamber is higher than in the gas detector, in order to draw gas from the test chamber to the gas detector. In the standard setting, the test gas flows through the first line into the gas line. A gas detector is connected to the gas line and is designed to detect the test gas and determine its quantity. In the standard setting, the second line is closed by the changeover valve. Test gas that would also enter the second line in the event of a leak collects in the second line as long as the changeover valve remains in the standard position.When the changeover valve is moved into the test position, the test gas flows from the second line through the gas line path to the gas detector.

[0015] The design of the device ensures that test gas that has completely flowed through the second line and reached the gas detector via the gas line escapes from the test object significantly earlier than test gas that has reached the gas detector via the first line. It is important that the gas flows through the lines at different times; the pressure and / or length are of secondary importance.

[0016] Therefore, two parameters can be recorded in a very short time, one representing a current or recent release quantity of test gas and the second representing a release quantity that lies further in the past.

[0017] For example, the first line could be designed such that a complete flow through the first line takes 10 seconds, and the second line could be designed such that a complete flow through the second line takes 120 seconds.

[0018] One parameter to be measured could be the volumetric flow rate of a specific gas, for example, helium. If this parameter is measured in the standard position of the switching valve, it indicates how much helium escaped from the test specimen 10 seconds ago. In this case, the gas flowed through the first line.

[0019] If this parameter is recorded in the test position of the switching valve, it allows conclusions to be drawn about how much helium escaped from the test specimen 120 seconds ago, since the gas to be tested must have flowed through the second line, the flow through which took considerably longer.

[0020] Accordingly, it is possible to determine by how much the volume flow of helium that has escaped from the test object has changed within 110 seconds, without actually 110 seconds passing in which the signal drift to which the gas detector is subject would distort the measurement results.

[0021] According to an advantageous embodiment, the device according to the invention comprises an evaluation device. This device is configured to provide measured values ​​from the parameters, to compare these values, and, for example, to calculate a leakage rate.

[0022] According to a further advantageous embodiment, the second pipe has a larger volume than the first pipe. This means that more gas must flow into the second pipe before it reaches one end. A complete flow through the second pipe therefore takes longer than a complete flow through the first pipe.

[0023] This can be implemented, for example, by making the second line longer than the first line. In this case, the second line is preferably at least 3 times longer, and more preferably at least 5 times longer, than the first line.

[0024] Alternatively or additionally, the second pipe can have a larger cross-section than the first pipe in order to achieve the described effect of a longer flow time.

[0025] This connection A1*v1=A2*v2 is described by the continuity equation for steady flows of incompressible fluids.

[0026] Here, A1 is the cross-sectional area of ​​the first pipe, v1 is the flow velocity in the first pipe, A2 is the cross-sectional area of ​​the second pipe, and v2 is the flow velocity in the second pipe.

[0027] The ratio of the products is constant. If the cross-section of the first pipe is smaller than that of the second pipe, this means that the gas flows faster in the first pipe than in the second. This results in a longer flow time through the second pipe compared to the first.

[0028] Furthermore, there are other possibilities, such as designing the system so that the gas flowing through the second pipe is subject to a higher flow resistance than the gas flowing through the first pipe. This can be achieved by selecting a material for the second pipe with a higher roughness than the material used for the first pipe.

[0029] To increase the flow resistance in the second line compared to the first, the second line can, for example, be filled with a porous material. Alternatively, both lines could be filled with a material, with the one in the second line creating greater flow resistance. Depending on the application, the gas to be analyzed is conveyed through the lines at a flow rate of approximately 1 sccm to approximately 10,000 sccm.

[0030] According to a further advantageous embodiment, a restrictor is arranged on the gas line upstream of the gas detector in the direction of flow. The restrictor is located downstream of the changeover valve in the direction of flow. Preferably, the restrictor is positioned as close as possible to the gas detector, without any other components between the restrictor and the gas detector. The restrictor serves to regulate the flow of gas to the gas detector by constricting the cross-section of the gas line.

[0031] According to a further advantageous embodiment, the vacuum pump is arranged downstream of the gas detector along the gas line. This ensures that, throughout the system, from the gas detector and the gas line to the first and second lines and into the test chamber, fundamentally similar pressure conditions prevail. This can contribute to increased measurement accuracy.

[0032] The gas detector can be provided, for example, by a mass spectrometer.

[0033] According to a further advantageous embodiment, a test gas containing helium and / or hydrogen is used. In particular, the test gas can be helium or hydrogen or a mixture of both.

[0034] The invention is not limited to the previously described embodiment in which a volumetric flow rate of a specific gas is measured. It is equally possible to measure the total pressure of the gas flowing from the first or second line, to derive measured values ​​from these parameters, and to compare them. In this process, the presence of a specific gas is not detected, but rather the general escape of gas from the test object, which causes an accumulation of gas and thus a pressure increase.

[0035] Furthermore, it is possible to monitor the relative pressure of a gas component, for example helium, and to detect an accumulation of helium that has escaped from the test specimen by comparing several relative pressure measurements. This also allows conclusions to be drawn about the presence of a leak in the test specimen.

[0036] According to a further inventive concept, a method for detecting a test gas escaping from a leak in a test specimen is presented. The method comprises the following steps: a) Creating a pressure difference between the interior of the test specimen and the interior of the test chamber outside the test specimen, b) Connecting the first line to the gas pipeline route, c) Detection of at least one first parameter of the gas from the first line by the gas detector, d) Switching the changeover valve and connecting the second line to the gas pipeline, e) Detection of at least one second parameter of the gas from the second line by the gas detector, and f) Providing measured values ​​from the recorded parameters and comparing the measured values.

[0037] The procedure can be carried out using the device described above.

[0038] Procedure step a) can be carried out, in particular, by evacuating the test chamber using the vacuum pump. It is also possible to generate the pressure difference by pressurizing the test specimen, for example, by supplying air or test gas to the specimen using a compressor.

[0039] According to an advantageous embodiment, a time of at least 30, preferably at least 60 seconds, elapses between process steps a) and e). In the event of a leak, this ensures that any gas from the second line, from which a parameter is being measured, escapes from the test specimen at least 30 or at least 60 seconds earlier than any gas from the first line. The measurements themselves (process steps c) and e) are taken in close succession. Only the switching valve needs to be changed between them. The time interval between the measurements is crucial for minimizing the influence of signal drift. Since this interval is kept short, the influence of signal drift is comparatively small.

[0040] According to a further advantageous embodiment, in process step f) a difference is determined between the measured values ​​recorded in process steps c) and e). This difference provides information about the presence of a leak in the test specimen.

[0041] A leak is present, in particular, when there is a difference between the measured values. To compensate for measurement inaccuracies, a threshold value can be determined that incorporates tolerable fluctuations. In this case, a leak is only considered proven if the measured difference exceeds the threshold value. The magnitude of the difference can also provide information about the size of a leak.

[0042] According to a further advantageous embodiment, an optical and / or acoustic signal is emitted when the difference exceeds a threshold value. This indicates to the user the presence of a leak. The user can then initiate measures such as sorting out the test piece or checking a production process.

[0043] The invention will be explained in more detail below with reference to two figures. They show: Fig. 1 a schematic representation of an embodiment with the switching valve in the standard position, Fig. 2 a schematic representation of the exemplary embodiment from Fig. 1 with the changeover valve in the test position.

[0044] Fig. 1 and Fig. Figure 2 shows a schematic representation of a device 10 for detecting a test gas escaping from a leak in a test specimen 12. The device includes a test chamber 14. The test chamber 14 can be a rigid test chamber or a flexible test chamber, for example in the form of a foil chamber.

[0045] The test specimen 12 is located in the test chamber 14. The test specimen 12 contains helium as the test gas. The test chamber 14 is connected to both a first line 16 and a second line 18. In the illustrated embodiment, the second line 18 is significantly longer than the first line 16. This is indicated by two parallel lines in the second line 18.

[0046] The first line 16 and the second line 18 can each be selectively connected to a gas line 22 via a changeover valve 20 in the form of a 2 / 3-way valve, so that either the first line 16 or the second line 18 is connected to the gas line 22. A switching position in which both lines 16 and 18 are connected to the gas line 22 is not possible. In a standard position of the changeover valve 20, the first line 16 is connected to the gas line 22. In a test position of the changeover valve 20, the second line 18 is connected to the gas line. In the present illustration, the changeover valve 20 is located in Fig. 1 in the standard position and in Fig. 2 in the test position.

[0047] A gas detector 24 is arranged along the gas pipeline 22. In the present embodiment, this is a quadrupole mass spectrometer. It is configured to detect helium.

[0048] A vacuum pump 26 is arranged downstream on the gas line 22. The device is permeable to gas flow in a flow direction 28 from the test chamber 14 to the gas line 22.

[0049] The vacuum pump 26 generates a negative pressure in the test chamber 14 via the gas line 22 and optionally the first line 16 or the second line 18. For example, the vacuum pump 26 can generate a negative pressure in the test chamber 14, lines 16 and 18, and the gas line 22. The vacuum pump 26 is then switched off, or its connection to the gas line 22 is closed. If a leak occurs and a certain period of time is allowed, test gas from the test specimen 12 will initially collect in the test chamber 14 and subsequently spread into lines 16 and 18 and the gas line 22. Alternatively, it is conceivable that the test chamber 14 is pressurized to atmospheric pressure for the accumulation of test gas, while the pressure in the test specimen 12 is higher than in the test chamber. The crucial factor in the accumulation method is that the pressure in the test specimen 12 is higher than within the test chamber 14.

[0050] Now, a parameter of gas from the first line 16 can be measured using the switching valve 20 in the standard position (shown in Fig. 1) by switching the changeover valve 20 to the test position (shown in Fig. 2) Immediately following the first measurement, a parameter of gas from the second line 18 can be measured. Since the second line 18 is significantly longer than the first line 16, gas will now flow into the gas detector 24 that escaped from the test object 12 much earlier and subsequently collected in the second line 18.

[0051] In this example, the first line 16 is chosen to be long enough that, at the set pressure conditions, gas takes approximately 10 seconds to flow from the test specimen 12 to the second line 16 and completely through it. The second line 18 is chosen to be long enough that, at the set pressure conditions, gas takes approximately 120 seconds to flow from the test specimen 12 to the second line 18 and completely through it. This results in a difference of approximately 110 seconds for the gas sample collection time. The lengths and diameters of the lines can be varied as desired to achieve a specific difference.

[0052] The device includes an evaluation unit, which is not shown here. This unit can be connected to the gas detector 24, for example, wirelessly or via cable. It is configured to process the parameters and determine measured values, in this example a volumetric flow rate of helium.

[0053] By comparing the two measured values, a difference in volume flow rates can be calculated, whereby the gas samples used for the measurement originate from different times. In the present example, the interval between the times is 110 seconds. The length of this time interval can be determined by the geometry of the first and second lines 16, 18.

[0054] If the calculated difference exceeds a predefined threshold, this indicates a leak in the test specimen. The evaluation device can also determine a leak rate from this difference, which allows conclusions to be drawn about the size of the leak.

[0055] Information about the presence and size of a leak can be displayed to a user by means of a visual and / or acoustic signal.

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