Leak test

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In the leak test , according to DIN EN 1779, a distinction is made between the vacuum method and the overpressure method to prove the tightness . In addition, according to DIN EN 1779, the test methods are classified according to their sensitivity.

The leak test is one of the non-destructive testing methods . Leak tests are used on a large scale to prove the tightness of test items and to locate leaks. With regard to the respective application (requirements, boundary conditions), it must be carefully checked which test method is most suitable.

Classic areas of application of the test methods according to DIN EN 1779 are:

  1. Leak testing / leak detection on installed systems ( pipelines , storage tanks , cooling systems , vacuum systems , packaging, ...)
  2. Leak test on components and systems in series production (e.g. radiators (elements), gas meters , fuel tanks, fuel filters , brake lines , light alloy wheels, injection pump housings, etc.)

Further terms for leak detection are leak test or leak test.

The various methods of DIN EN 1779 for leak testing

Worker water bath test or bubble test

Probably the oldest and still most widespread technology for leak testing in series production worldwide is the so-called worker water bath test or the “bicycle tube” method. In the case of the C1 bubble detection method of DIN EN 1779, the part to be tested is sealed, (over) pressurized and immersed in a water basin. If there is a leak, (air) bubbles will form. The resulting air bubbles are "detected" by the worker / inspector and the leakage is localized at the same time. The localization of the leak is still a great advantage of this test method today. However, the method is subjective, since here the decision about tight / leaky is made by humans. Therefore, despite its advantages, it is less and less accepted in industrial production. If you see an air bubble with a volume of 3 mm³ every 30 seconds, this corresponds to a sensitivity of approx. 1 · 10 −4 mbar l / s. In reality, a worker only sees much larger leaks. Another disadvantage for industrial use is the necessary drying time for the test item.

Water bath test or bubble test with ultrasonic sensor

In the case of ultrasonic gas bubble detection, the C1 bubble detection method of DIN EN 1779, an ultrasonic system detects the air bubbles that emerge in the event of a leak in the water bath test described above. This is the above. Process objective and sensitive. By evaluating the transit time of the ultrasonic signal from the transmitter / receiver to the leakage bubbles, these can be localized by the ultrasonic gas bubble detection system. This procedure is one of the few automatic leak testing procedures with localization properties.

Differential pressure test

In the differential pressure test (→  pressure test ), the pressure change test D3 of DIN EN 1779, pressure is applied to the test part and a reference part or reference volume. After a short settling time, the part is disconnected from the compressed air supply (valve closed) and after a further measuring time the pressure difference between the test part and reference part is measured. The differential pressure test is one of the most cost-effective methods of manufacturer-independent leak testing methods and is accordingly widespread. The differential pressure test is not to be confused with the simpler pressure differential test, the overpressure method D1 of DIN EN 1779, in which no comparison measurement is carried out on a reference part, but only an absolute pressure drop is measured. The pressure difference measurement is a little cheaper, but also has significant disadvantages to the differential pressure measurement. The differential pressure test is an indirect method, since it is not the amount of substance or substance that emerges from the leak that is detected, but the change in pressure caused by it. The disadvantages of the procedure include:

  1. Volume dependency: the same leakage causes different pressure changes for different volumes. While with a very small volume a small leak already causes a larger pressure difference, a small leak with a large measuring volume causes almost no pressure change.
  2. Temperature dependence: A temperature change also causes a pressure change in an enclosed volume. So z. B. an increase in temperature lead to a pressure increase, although there is a leak. Likewise, a drop in temperature can lead to a drop in pressure even though there is no leakage.

Advantages:

  • low acquisition costs;
  • low operating costs (no expensive test gases);
  • with small volumes quickly and precisely

Ultrasonic detection

Since the ultrasound device previously used in space shuttles required up to a week to detect a leak in the outer skin, a new sensor is now being tested. It consists of 64 small measuring sensors and measures the acoustic oscillations (vibrations) in the spacecraft itself, which are generated by the outflowing air. A computer calculates the position of the flaw within a minute.

Leak test with test gases

The test gas leak test with test gases can be used both qualitatively and quantitatively. Together with measuring methods based on radioactivity, it is the most sensitive non-destructive test method and also offers a wide dynamic range, i.e. it is able to detect both very small and very large leaks. Hydrogen (mostly as forming gas), sulfur hexafluoride and above all helium are used as test gases . The test gas helium dominates the test gas methods, as it only occurs in low natural concentrations of around 5 ppm in the atmosphere. As a noble gas , it does not enter into any chemical reactions and is therefore very reliable. During the test, it can be detected with a high degree of selectivity and without cross-sensitivity . This one firmly on the will as proof device helium mass 4 adjusted mass spectrometer used which forms a compact leak detector with easy operation together with the associated gas distribution and vacuum system. In commercial helium leak detectors, magnetic sector field mass spectrometers are preferably used. In the leak test within the framework of (high) vacuum technology , only helium is used as the test gas to detect leakage points.

The leak test with a test gas is based on the generation of a pressure difference between the test object and the detection device . This can be achieved by increasing or decreasing the pressure on one side of the device under test compared to the other side or by a combination of pressure decrease and pressure increase. In the event of a leak, there is a constant gas flow from the high pressure side to the low pressure side. If there is a detection device for the flowing test gas on the low pressure side, the leakage can be detected qualitatively or quantitatively.

Helium leak detectors

Traditional leak detectors

The traditional detection device is a magnetic sector field mass spectrometer. A filament or filament emits high temperature electrons which is accelerated and in an ionisation chamber are directed. The gas molecules present in the ionization chamber are ionized by electron impacts. This turns electrically neutral gas particles into positive ions. These positive ions can now be extracted electrically by a drawing electrode and injected into a magnetic field using a high acceleration voltage. The ions describe a circular path in the magnetic field , the radius of which depends on the ion mass.

The working pressure of the mass spectrometer is <10 −4 mbar. In commercial leak detection devices, the mass spectrometer is always combined with a vacuum system, which consists of at least a mechanical backing pump, a high vacuum pump , pressure measuring devices and a number of pressure-dependent controlled valves . The state of the art is also an integrated test leak with which the leak detector can be calibrated .

Before starting the leak test it must be ensured that the test item is clean and dry. Outgassing due to vapors or liquid residues extend the pumping time and thus the test time. The entered impurities shorten the service life of the measuring device.

The individual functions and operating states are explained below using a vacuum test as an example. After switching on the leak detector, the spectrometer cell is permanently pumped empty. This removes residual helium from the measuring device and ensures the best possible signal-to-noise ratio .

After flanging of the specimen and the start of the test cycle, the test specimen is first evacuated. In this operating state, the high vacuum pump and mass spectrometer are shut off.

Gross leak or counterflow mode

After a certain pressure threshold has been reached, the high vacuum pump and mass spectrometer are switched on again. The test gas can now reach the analysis cell in countercurrent through the high vacuum pump.

The pressure threshold for this so-called gross leak or countercurrent mode is between 1 and 25 mbar for commercial devices and up to 200 mbar for special devices. To achieve short pumping times, a high pressure threshold is advantageous. However, this must be weighed against any contamination (water vapor, residues of liquids used in the machining of parts) that still have to be emanated from the test object and pumped out.

The gross leak or countercurrent method is characterized by a low time requirement due to short pumping times and good protection of the mass spectrometer in the event of air ingress (e.g. implosion of the test object). Disadvantages are the limited sensitivity of approx. 10 −7 to 10 −8 mbar l / s and a slow response time.

Fine leak or main flow mode

In order to be able to utilize the maximum sensitivity of the leak detector, the pumping must continue. In the so-called fine leak or main flow mode , a direct connection is established between the inlet of the leak detector and the mass spectrometer.

In order not to exceed the working pressure of the mass spectrometer, the inlet pressure must be very low for this method. For most commercial devices, the maximum inlet pressure in the fine leak mode is around 10 −2 mbar. Some devices reach their most sensitive measuring mode with maximum sensitivity at a pressure of approx. 0.5 mbar.

The fine leak or main flow method is characterized by the highest sensitivity of approx. 10 −11 to 10 −12 mbar l / s and a fast response time. The test results are extremely reproducible and the test item is well protected against back-diffusing gases from the backing pump (important e.g. for cryogenic test items). Disadvantages are the higher time required due to longer pumping times and poorer protection of the analysis cell in the event of air ingress.

In addition to the vacuum method described above, the test gas is also fed to the mass spectrometer in countercurrent during the sniffing test.

Novel helium leak detectors

The latest generation of leak detectors separate the helium from the air by means of a heated silicon dioxide membrane. This membrane is permeable to helium, but impermeable to all other gases. Behind the membrane there is a permanent vacuum with an ion getter pump , the discharge current of which is a measure of the total pressure and thus of the helium concentration. This new type of helium sensor does not require a high vacuum system and is therefore particularly suitable for the sniff test , but can also be operated in a vacuum. The test chamber no longer has to be laboriously evacuated, which is why the demands on the chamber and thus its costs are reduced, and moisture does not interfere with the measurement. Atmospheric helium in the air limits the sensitivity to 1 · 10 −7 mbar l / s. In reality, a worker only sees much larger leaks. Such a helium sensor from INFICON is currently used in the Protec P3000 and T-Guard, as well as in another version from Varian in the Helitest.

Investigations at the Max Planck Institute for Plasma Physics showed that a determination of the leakage rate in a test chamber filled with a helium-free technical gas such as If, for example, helium-free nitrogen (10 ppt) or argon (10 ppt) is applied, the sensitivity can be increased by a factor of 500,000 to up to 1 · 10−11 mbar · l / sec with a helium leak detector of the latest generation from Pfeiffer Vacuum can. It is thus possible for a worker to determine the tightness of a component with up to 1 · 10 −10 mbar l / s or to locate a leak in the component.

Novel helium leak detection devices

Devices for leak testing of the latest generation make it possible to test components without vacuum with a sensitivity of up to 1 · 10 −11 mbar l / s in a test environment without vacuum. In contrast to the vacuum test gas method, in which the helium background is reduced by evacuating a test chamber, in the latest generation of helium leak detection devices the helium background is no longer reduced by evacuating the test chamber, but by exposing the test chamber to a helium-free gas (10ppt ). The new generation of devices makes it possible to test a component with a 1 · 10 −11 mbar l / s. The leak rate is determined either by a helium leak detector with a mass spectrometer or with the T-Guard.

Test method with helium

Overprint process
Leak location with the overpressure method B3 of DIN EN 1779, in which the test item is exposed to helium

The test item is filled with the noble gas helium or a gas mixture containing helium, sealed and possibly pressurized.

In the event of a leak, e.g. B. a leaky weld, the pressure gradient from the inside of the test object to the ambient air generates a gas flow through the leakage channel. With the overpressure method B3 of DIN EN 1779, the test object is sniffed on the outer wall of the test object with a sniffer probe . If the sniffer probe passes a leak , the sucked in helium is detected by the leak detector.

The flow resistance of the sniffer probe and the vacuum system of the leak detector ensure the necessary pressure reduction from atmospheric pressure to the operating pressure of the mass spectrometer, which is below 10 −4 mbar.

The sniffing method allows a high spatial resolution in the search for leaks and thus unequivocal identification of faulty locations on the test item. A low overpressure also means only a small amount of force exerted on thin walls of a test object and enables fragile workpieces to be tested.

In a variation of the method, the overpressure method B3 of DIN EN 1779, the increase in the concentration of helium in an envelope around the test object can also be measured and evaluated. Although this method does not offer a spatial resolution, it does provide a quantitative statement as to whether there are leaks.

The helium concentration in the test gas, the test pressure and the natural helium background in the air limit the sensitivity of the analysis method to 5 * 10 −06 mbar l / s, with the detection limit being around 10 −07 mbar l / s. It is therefore possible to use the overpressure method B4 of DIN EN 1779 to detect leaks with a leakage rate of 1 cm 3 per day.

The efficiency of the procedure is operator dependent and difficult to calibrate. Therefore, the sniffing method is mostly used as a qualitative method.

Partial vacuum process
Leak detection with the UST method in which the test item is exposed to helium and the envelope is exposed to a helium-free gas

In contrast to the overpressure method B3 and B4 of DIN EN 1779, the partial vacuum method uses the partial vacuum effect, so that the gas tightness of test objects can be proven at normal pressure with the same sensitivity as with the vacuum method with the noble gas helium . The so-called ultra-sniffer test gas method (UST method) has a sensitivity of 1 * 10 −11 mbarˑl / s and was developed at the Max Planck Institute for Plasma Physics . Similar to the classic pressure method B3 of DIN EN 1779 the specimen in a shell is taken but unlike the traditional pressure methods B3 charged with a helium-liberated gas, so that the sensitivity of the classic pressure process B3 by a factor of 500 000 of 5 × 10 -06 to 1 · 10 −11 mbar · l / sec is increased. This sensitivity corresponds to a theoretical gas loss of 1 cm 3 in 3000 years.

The UST procedure can be used very economically for the ad hoc examination of test items. The entire test system can be set up with normal pneumatic items such as valves and plastic hoses. A simple plastic cover is sufficient to enclose the test item. No special precautions need to be taken against gross leaks, as the detection chamber of the test system can be flushed at atmospheric pressure.

Investigations during the construction of the Wendelstein 7-X also showed that the integral helium tightness of test objects can be proven very well at normal pressure up to 1 · 10 −09 mbar · l / sec. In addition, the UST method, compared to the sniffing method B3 and B4 of DIN EN 1779, even allows the location of leaks with a leak rate of 1 cm 3 in 30 years. In addition, the high cost efficiency of the UST process compared to the vacuum process could be demonstrated.

Vacuum process
Leak test with the A1 vacuum method of DIN EN 1779, in which the test object is evacuated and the envelope is exposed to helium
Leak test with the vacuum method B2.1 of DIN EN 1779, in which the test object is exposed to helium and the chamber that surrounds the test object is evacuated

In the simplest case, the test item is connected to the leak detector and evacuated. This creates a negative pressure of around one bar. This pressure gradient generates a gas flow from the ambient pressure into the interior of the test object and thus to the detection device.

With this method, the A3 vacuum method of DIN EN 1779, the operator blows the helium at potential leak points and can locate leaks.

In another variation of the method, the A1 vacuum method of DIN EN 1779, the test item is evacuated and the gas is let into a bell or a cover that surrounds the test item.

Although this method does not allow spatial resolution, it does provide a quick statement about the presence of leaks and their quantification. This variant is therefore the method of choice for automated test procedures.

During the leak test, the direction of pressure that prevails in practical use of the test object should always be simulated. A method is thus also conceivable in which the interior of the test object is exposed to a helium-containing test gas and the enveloping test chamber is connected to the leak detector.

Bombing

Many test items are built into a closed housing that can neither be connected to a test gas supply nor to a leak detector. Examples are lamps and electronic components such as surface waveguides or oscillating crystals . These test items are either sealed in a helium-containing atmosphere or subsequently exposed to an overpressure of helium in accordance with method B5 of DIN EN 1779. The latter happens in a pressure chamber and is called "bombing". The test gas penetrates the test item through any leaks.

The test specimen exposed to gas in this way is now placed in a vacuum chamber which is directly connected to the detection device.

The internal volume of the test specimen, the impression time, the differential or bombing pressure, the waiting time between the impression and the test and the size of the leak determine the information content of the test. If the waiting times are too long, the helium can escape from the test object again through diffusion and the measurement result becomes irrelevant.

Application examples of leak testing with helium

The helium leak test is used in a wide variety of applications. These include:

SF6 leak detectors

As an alternative to the helium leak test, there is a leak test with sulfur hexafluoride (SF 6 ), which has significant advantages over helium in terms of handling. So this gas z. B. not before in the normal ambient air. In addition, the permeation rate of (SF 6 ) with some plastics is significantly lower than that of helium.

The laser-optical leak detection systems work optically by means of laser radiation. The inert gas sulfur hexafluoride (SF 6 ) is often used as the test gas in these optical leak detection systems, and a CO 2 waveguide laser tailored to this is used as the laser .

Laser-optical leak detection systems are ultimately, due to the completely uncritical final vacuum pressure or the possibility of being able to operate the detection system even at atmospheric pressure, very economical and robust against soiled or damp test objects. The entire test system can usually be set up with normal pneumatic items such as valves and plastic hoses. Ordinary O-rings are sufficient for sealing; a simple oil-sealed rotary vane pump is usually sufficient as a vacuum pump. No special precautions need to be taken against gross leaks, since the detection chamber of the laser system can be flushed very quickly and effectively at atmospheric pressure.

Leak test of private sewage systems

The testing of private sewage systems for leaks is regulated by state laws. For example, § 61a of the Water Act for the State of North Rhine-Westphalia - Landeswassergesetz (LWG) - of June 25, 1995 prescribes: "The owner of a property has sewer pipes laid in the ground or inaccessible to collect or convey dirty water or rainwater mixed with it Have the property checked for leaks by experts after it has been erected. ”The first leak test must be carried out during construction or a change, but by December 31, 2015 at the latest. “Notwithstanding this, the municipalities can specify property-related deadlines in their statutes. This applies if the municipality has stipulated remedial measures on public sewage systems or if the municipality couples the leak test with the inspection of the public sewers. In these cases, the leak test must be carried out by 2023 at the latest. "

In view of the costs associated with leak testing, there are protests against this rule. In North Rhine-Westphalia the regulation is therefore being discussed again at the political level; a meeting of the State Parliament's Environment Committee scheduled for November 9, 2011 was postponed to December 2011. Minister Johannes Remmel then announced that the regulation would be suspended.

With the entry into force of the law amending the state water law on March 16, 2013 (GV. NRW. P. 133), in addition to other changes, § 61 a LWG NRW was repealed. This means that December 31, 2015, the general deadline for testing existing private sewage systems in North Rhine-Westphalia, was no longer applicable. Nationwide, however, pursuant to Sections 60 and 61 of the Water Management Act , wastewater systems must be built, operated and maintained in such a way that the requirements for wastewater disposal are met and the generally recognized rules of technology are observed. In the case of wastewater systems that fall below these requirements, the necessary measures must be carried out within a reasonable period. In order to be able to assess the condition of sewage systems, the operators of the systems are obliged to monitor their condition, functionality, maintenance and their operation themselves. Without support, laypeople can hardly classify the results of this self-monitoring correctly. An easily understandable aid for the assessment of abnormalities and damage is the picture reference catalog - private sewer pipes - of the Ministry for Climate Protection, Environment, Agriculture, Nature and Consumer Protection of the State of North Rhine-Westphalia. The current version of the picture reference catalog represents the status: May 2011, there also damage classes and renovation deadlines are mentioned, these refer to the draft of the now valid DIN 1986-30: 2012-02. This DIN 1986-30: 2012-02 in Section 13, Table 2 regulates the time spans, occasions, type of test and areas of sewage origin. The regular interval for testing private sewage systems is 20 years; this interval can be used for new systems with a verifiable pressure test (e.g. minimum overpressure when testing with water = 1 mWS over the top of the pipe) can be extended once to 30 years. For non-domestic sewage or in protection zones, significantly shorter test intervals apply. In North Rhine-Westphalia, the municipalities are obliged to inform and advise property owners about their obligations under Sections 60 and 61 of the WHG in accordance with Section 53 Paragraph 1e Sentence 3 of the State Water Act.

Possible unconstitutionality

On the basis of two reports by the judiciary of the state parliament of North Rhine-Westphalia and one report under private law, the leak test for so-called substance or system-related regulations is suspected of being unconstitutional according to Art. 72 Para. 3 No. 5 Basic Law. Specifically, due to the classification of the leak test from a constitutional point of view, due to the competing legislation that takes priority and the already issued and required federal regulations ( § 61 WHG), the federal states have no legislative competence .

The federal states do have the power to deviate in accordance with Article 72, Paragraph 3 of the Basic Law, but this does not apply to plant-related systems (see No. 5). According to the regulation, a leak test is provided for a private sewage system, which is why the regulation does not fall under the exception of Art. 72, Paragraph 3, but in Art. 74, No. 32 and, in principle, in competing legislation.

With regard to § 61a LWG NRW it says:

"The state regulations on leak testing of wastewater systems therefore violate Art. 72 Paragraph 1 and Art. 74 Paragraph 1 No. 32 GG in conjunction with Section 61 WHG."

The legal service of the state parliament also determined that the state law from 2007 is older than the federal law (2009) and thus the federal regulation takes precedence. A basic obligation to test for leaks has been removed in the latest reform of the so-called "Canal TÜV".

Three weeks after the 2013 federal election , the coalition factions in the NRW state parliament (SPD NRW and Greens NRW) approved an ordinance of the red-green government on October 17, 2013, which amends the state water law and implements federal regulations.

See also

for the story see: Gasriecher

literature

  • Jobst H. Kerspe et al .: Vacuum technology in industrial practice (= contact & study. Vol. 204 Energy technology. ). 2nd, revised and expanded edition. expert-Verlag, Ehningen near Böblingen 1993, ISBN 3-8169-0936-1 .
  • Max Wutz et al .: Handbook Vacuum Technology. Theory and practice. 7th expanded edition. Vieweg, Braunschweig et al. 2000, ISBN 3-528-54884-3 .
  • Louis Maurice: Practice of leak testing with helium. = Helium leak test manual. German translation by L. Hütten. ALCATEL-Hochvakuum-GmbH, Wertheim 1974.
  • Siegfried Genreith: Ignored, laughed at and withdrawn. When politics and citizens live on different planets, Books on Demand, Norderstedt 2019, ISBN 978-3-7412-9433-4 .

Web links

supporting documents

  1. ↑ New standard for the selection of a suitable method for leak detection and tightness testing , January 7, 2013.
  2. ^ Dale Chimenti, University of Iowa, 2007.
  3. ↑ Ultra-sniffer test gas method . Retrieved October 21, 2016.
  4. ↑ Robert Brockmann: UST method . researchgate.
  5. ↑ Spin-off: 1st prize for highly sensitive leak detection method from "Lambda Leak Testing" , March 19, 2013.
  6. ↑ Portal for the UST procedure , August 26, 2014.
  7. ↑ Water Act for the State of North Rhine-Westphalia of June 25, 1995 (PDF; 796 kB).
  8. ↑ Leak testing of private house connections , State Office for Nature, Environment and Consumer Protection North Rhine-Westphalia, accessed on June 13, 2012.
  9. ↑ See e.g. B. Initiative “Everything in NRW” .
  10. ↑ Leak test in NRW - debate postponed , RP Online , June 10, 2011.
  11. ↑ landtag.nrw.de
  12. ↑ Brief report on the legislative competence of the State of North Rhine-Westphalia for the regulation on leak testing of private sewage systems in § 61a Paragraph 3 to 7 LWG NRW by Prof. Dr. iur. Stefan Muckel University of Cologne
  13. ↑ Ralf Michalowsky (Die Linke): On the leak test: State government acts unconstitutionally ( memento from February 11, 2013 in the web archive archive.today ), February 27, 2012.
  14. ^ FDP Solingen: Canal TÜV finally off the table , Solinger Bote, October 29, 2012.
  15. ↑ Rheinische Post: Homeowners in water protection areas have a duty: Canal TÜV is a done deal