leak detection methods and defining the sizes of leaks

LEAK DETECTION METHODS AND DEFINING THE SIZES OF LEAKS

http://www.ndt.net/article/v04n02/slov_30/slov_30.htm[12.08.2013 06:50:51]

TABLE OF CONTENTSAbstractIntroductionLeak Sizes and Measurements of Leak RatesLeak Detection MethodesHelium Mass Spectrometer (Ms-) Leak DetectionConclusionLiterature

NDT.net - February 1999, Vol.4 No.2

LEAK DETECTION METHODS AND DEFINING THE SIZES OFLEAKS

Andrej Pregelj, Marjan Drab, IEVT, Teslova 30, LjubljanaMiran Mozetic, ITPO, Teslova 30, Ljubljana

The 4th International Conference of Slovenian Society for NondestructiveTesting "Application of Contemporary Nondestructive Testing inEngineering" 24 - 25 April 1997, Ljubljana, Slovenia. Table of Contents

ABSTRACT

There is a constantly growing need for products andtechnologies that for their realisation requirehermetically closed elements, vessels and tubes. Envelopes with greater or smaller vacuum tightnesshad to assure a satisfactory isolation between external atmosphere and inside over- or underpressure.Crytical leak spots in closed systems are usually: connections, gaskets, welded and brazed joints,defects in material etc. Regarding to quality demands in different production processes, technicalpeople very often encounter with standards for tightness testing. Wishing to state if an element orsystem meets with corresponding standards we have to be familiar with the procedures enabling thefollowing appointments:

- question if there is leakage or not- settlement of the leak size- detection of leak locality

For solving the mentioned problems different treatments and techniques are known but among themthere is no an universal method, Each testing is suitable only for a selected leak rate or for fixed formsand technologies. Our article presents the leak types, their sizes and various leak detectiontechniques. Keywords: tightness, leakage, trace gas, helium mass spectrometer leak detector, hermetic sealing

1. INTRODUCTION

The words "leak" and "leakage" appear in the field of vessels' hermetical closing and do not confrontonly with vacuum technologists but also engineers working with high pressures. A leak means anunintended crack, hole or porosity in an enveloping wall or joint which must contain or excludedifferent fluids and gases allowing the escape of closed medium. The basic functions of leak detectionare the localization and size measurement of leaks in sealed products and systems For majority ofexamples, a leak test procedure is a quality control step to assure a device integrity, and is one-timenondestructive test.

Tipical products in which the leak detection has to be used are: vacuum chambers, TV- , and othercathode tubes, hermetically sealed electronic components, pressure vessels, aerosol containers,vacuum thermal isolation (e.g. dewars), pumps, refrigerating systems, chemical and nuclear plants,beverage cans, products containing metal bellows, electron microscopes, peace makers, etc.

In spte of modem technologies it is practically impossible to manufacture a sealed enclosure orsystem that can be guaranteed to be leakproof without first being tested. The main question is: whatis the maximum acceptable leak rate consistent with resonable performance life of the product.

We can distinguish different tipes of leak sources.

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Leaks caused by defects in the containing envelope. For example a too thin wall of a plastic bottlebecomes microscopic cracked at enough high pressure difference, or in canning industry if the scoremark is too deep in ring pull-tab can top, or porous cast in machine housing metallurgy, etc. Leaks in newly manufactured products are most commonly imperfect joints or seals by whichvarious parts are assembled to form the final article. There are known demountable and fixed joints.Between them the most often used are welds, brazed and soldered joints glass-to-metal and ceramic-to-metal seals, O-rings and other gaskets, etc. Materials permitting gas diffusion and permeation through the wall. Virtual leak, a special type of leak in vacuum technique, which is not really a leak but is the internalsource of gas or vapor. These are cavities in a chamber wall with thin connections to the inner vesselspace such as improper welds, closed threads and holes etc.

2. LEAK SIZES AND MEASUREMENTS OF LEAK RATES

The shapes of leaks (cracks, fissures, porosity, damages, etc ... ) are very different, unknown andnon-uniform. Therefore it is impossible to measure their sizes with any geometrical dimension exceptof course in the case of an ideal or artificial leak as used for calibration. How then to define the leaksize? A generally accepted method became the observation of gas or fluid flow through it in certainconditions of temperature and pressure difference. Consequently leak rates can be defined in twoways:

in terms of the application, e.g.: 3 g of refrigerant R-12 in 2 years at 5 bar, or 65 m of oil peryear at 60 °C at atmosferic pressure, etc...in terms of vacuum leak detection method, e.g.: 2 mbar pressure increase in I hour or 3x 10-7mbarl/s helium (using He-detection method)

Each of the mentioned examples gives a legitimate description of leak rate but the generally acceptedunit is the last one because of very simple and understandable helium leak detection. Its efficiency isthe result of a fierce and highly competitive developments within the last 15-20 years.

The basic experiment explaining the leakage of hermetic system (increasing pressure method) ispresented in Figure 1. The tested vessel or system with known volume (Vo) is by a valve connected topump. After evacuation the connecting valve should be closed and then the pressure in volumeregistered for a suitable long time periode. There are more possibilities: system is tight and clean (a),fight and not clean (b), not tight and clean, i.e.: ideal leak (c), not tight and not clean, i.e.:combination of leak and degassing.

As we can see in each case (except for a tight vessel) we have pressure increase and regarding to theshape of diagram we can conclude the type of leak. Using the curve inclination data the leak size canbe determined by the following formula:

Q = ^p * Vo/^ t (mbarl/s)

This simple test helps us to describe the situation at the begining of leak tracing procedure. Verysimilar test can be realized by pressurizing the the system (and detecting the pressure decay) butgives only data on leak size and is not used so often.

The leak rate Q does not only depend on the geometric dimensions (diameter, lenght) of the leak butalso on the physical properties of the gas (or the liquid), such as viscosity, relative molecular massand on the pressure difference. For example: in the same environment conditions helium flowsthrough orifices 2.7 times faster than air. Because of different results if the same leak is measured byvarious mediums it must be always noted with which gas a testing was performed.

The maximum acceptable leak rate for a given product depends on the nature of product. Since thecost of leak detection (and manufacturing too hermetic envelopes) increases in inverse proportion toa leak rate, it follows that testing for unnecessary small leaks causes unnecessary rise of production



costs. Some examples of tolerated leaks in different elements and systems are shown in Table 1. Wecan see very wide range of sizes: from great with some tenths mbarl/s in rough vacuum devices, tomilion- and more- times smaller in hermetically closed electronic elements. It is possible to state thereare no ideal products without leakage. We only can demand they have leaks smaller than the specifiedleak.

Table 1. Leak rate specification for various elements and systemsElement or System Max. permissible Leakage Remark

chemical process equipment 10-1 to 1 mbar l/s great process flows

beverage can bottom 10-5 to 10-6 mbar l/s retention of CO2

dynamic pumped vacuum s. 10-5 to 10-7 mbar l/s permanent pumping

IC-package 10-7 to 10-8 mbar l/s

pacemaker cca 10-9 mbar l/s long time implanted in body

closed vacuum elements 10-8 to 10-10 mbar l/s e.g. TV- and Xray- tubes

3. LEAK DETECTION METHODES

A few leak detection techniques are known. Their performances regarding to detecting sesitivity arepresented in Fig 2. Because of their advantages we shall concentrate attention on the helium massspectrometer tecniques but at first a short description of others is presented,

The spark coil technique uses a high voltage or Tesla cod and sparkling point to create theelectromagnetic radiation which causes the generation of glow discharge in neighbouring evacuatedampoules. Normally it is possible only in non metal envelopes, that means first of all in glass andplastic elements or tubing. Drawing the leak antenna along the tested element we can see plasmainside and coming to the leak, a sharp arc passage between plasma and antenna appears. The defectspot is very clearly marked and a skilled person can from the colour of plasma also estimate the innerpressure. This simple metod however has a number drawbacks; since besides the restrictedapplication it is also to be avoided because of radiodisturbances.

Pressure change method uses pressure gauges which are ordinary used to monitor the systemperformance. Suspected leak sites can be squirted with a solvent (i.e. acetone or similar) whilewatching the gauge for a pressure rise that occurs when the solvent enters the leak. This method haslimited sensitivity (depending also on the type of pressure measurement cell) and some shortcomings(possibility of solvent freezing causes temporary stuffing of leak, solvents may attack vacuum greaseand elastomer gaskets).

Figure 1. Testing of tightness by increasing pressure method and various



possible results

Bubble test (soappainting)

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Bubble test (air,water)

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Thermal conductivity -------

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Radioisotope -------

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Halogen detector -------

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Mass spectrometer -------

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mbarl/s 100 10 1 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-

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Figure 2. Sensitivity ranges of various leak detection methods

Overpressure methods can be performed by fluid or gas with which the tested element must befilled. As a fluid usually the water from house installation is used. Observing the outside surface thewetted areas show us great leaks and smaller ones up to approx. I mbarl/s. Testing with gas, thevessel is subjected to overpressure of some bars (depending on material and wall thickness) andimmerged into the water. At leaks the gas bubbles begin to escape. In this manner the leaks up to 1.10-3 mbarl/s can be detected. If the vessel is too great for immersion, the suspected points should bepainted by soap solution and again we can see the bubbles escaping if there is a leak. This methodenables detecting the leakage up to 10-5 mbarl/s and is usable also for very large systems.

Halogen leak detectors are used in the detector-probe mode (to 10-3 mbarl/s), requiring that thesystem be pressurized with a gas containing an organic halide, such as one of the Freons. Theexterior of the system is then scanned with a sniffer probe sensitive to traces of the halogen -bearinggas (Fig. 3). The principle is based on the increased positive ions (K or Na) emission because ofsudden halide composition presence. The ion current is the measure for a leak size. Halogen detectorscan be used also in turned mode: evacuated vessel is connected to detecting instrument and issprayed by freon. In this manner its performance is up to 5.10-7 mbarl/s and is used in rough,medium and high vacuum.



Figure 3. Halogen leak detector

Dye penetrant method is an adaptation of a technique used to find cracks in metals and defectsin welds. It uses a low viscosity fluid that exhibits a high rate of surface migration. This fluid ispainted on one side of a suspected leak site, and after a time, it is detected on the other side of thewall. The test is simple, low cost, it leaves records, the sensitivity can be as high as 10-6 mbarl/s

Acoustical leak detection uses the sonic or ultrasonic energy generated by gas as it expandsthrough an orifice. Pressurized gas proceeds from tested system through leaks which are detectedoutside by sensible microphone (tipically about 40 000 Hz). Acoustical leak detection is widely used intesting high pressure lines, ductworks etc. It requires modest instrumentation; it is simple and fast butis limited to about 10-3 mbarl/s.

Radioisotope method is useful only for testing hermetically sealed components. They are placed ina chamber which is to be evacuated and filled with radioactive tracer gas (tipically krypton 85). Itdifuses through present leaks in the components and after removing it from environs test gas expandsthrough leaks back and can be detected by radiation sensor. The instruments for this type leak tracingare very expensive but they enables the measurement of flows up to 10- 11 mbarl/s.

Mass spectrometers as leak detectors are used as most sensitive instruments for stating leakexistance and presence in vacuum systems. They are adjusted on the atom respectively molecularweight of tracer gas. It is usually helium because of. - its small mass and atom volume assures good supply of gas through a leak -relative great massdistance from neighbour mass enables greater sensitivity - its partial pressure in air is low, approx.4.10-3 mbar

The first next suitable gas for leak detection purposes would be H2 but it is dangerous and residualatmosphere in vacuum systems always contains this gas. There are also spectrometers adjustable toother gases e.g. argon.

4. HELIUM MASS SPECTROMETER (MS-) LEAK DETECTION

Helium leak detection systems work as follows: He is introduced to a test part that is connected tothe leak detector. The helium travels through leak into the leak detector, its partial pressure is senzedrespectively measured and results are displayed on istrument as flow rate.

The main part of the device is mass spectrometer, a sensor for different gas masses. In very lowpressure (or vacuum) the molecules of rest gasses are transformed in ions by electron impact.Separating the ionized particles of different mass to charge ratios (q/m) it is possible to state thepartial pressures of present gases - in our case also the presence of the sought gas helium. For heliumMS- leak detectors, magnetic sector type mass spectrometers are preferably used as gas searchsensors. The low pressure (less than 2x10-4 mbar) required for operation of the mass spectrometersis produced by an integrated high vacuum pump system. The auxiliary vacuum pump required forrough pumping the tested equipment is either incorporated or can be attached via suitableconnection.



Figure 4. Conventional type of the helium leakdetection apparatus

Figure 5. Main leak detection techniqueswith MS-leak detector and He tracing gas. Inone mode (examples a and b) He flows fromoutside into vacuum and in other it expandsfrom inner overpressure in atmosphere

As the necessary pressure for the mass spectrometercannot always be attained in the tested equipment within aresonable pump-down time, various types of MS-leakdetectors with different performances have beendeveloped. The most used are constructions with "full flowoperation" (conventional) and with "contra-flow operation",regarding to the way in which He is supplied towards MS-sensor. For example the figure 4 shows schematically theconventional type MSleak detector. In presented system,vacuum is maintained in the spectrometer tube by use of adiffusion pump in combination with mechanical pump. Acold trap pumps condensable vapors such as oil and water.A roughing pump is provided to evacuate the test port (andthe connected sample to be tested) to a pressure level thatwill not disrupt diffusion pump operation. Reaching asuitable vacuum the sample is valved off from the roughpump and by opening test valve joint to the MS- vacuum

system. Helium sprayed near the leak on the sample surface has now an open way to the MS-tubeand there it is detected. After completing procedure the test port is valved off from the leak detectorand vented to air; than we can change the sample.

How to use a helium MS- leak detector? There are basicallyfour different techniques for finding leaks: two "OUTSIDE-IN"and two "INSIDE-OUT" methods (Fig.5).

In the most commonly used "outside-in" technique, the sampleto be tested is connected to leak detector and evacuated(Fig.5a). Than its surface is "probed" with a pointed jet ofhelium. Coming over a leak detector gives an acoustic or visiblesign (exactly location) and the data about leak size.

The second technique (Fig.5b) consists of evacuating andhooding the sample with something like a plastic sheet andflooding the hood with helium. So it is possible quickly toestablish whether or not a sample leaks and to establishe thetotal leak rate. This technique is most useful on production lineswhere a test piece must be accepted or rejected.

In "inside-out" techniques the test configuration is reversed.Instead of being evacuated, the sample is pressurized with He.In this case the leak detector is equipped with so calleddetector probe and it can be used in two modes presented infigures 5c and 5d. In mode c) the test piece is probed with thedetector probe around suspected leak sites. This methodsensitivity (up to 1. 10-6 mbarl/s) is not as good as by othersbecause the helium in the air is constantly being admitted intothe detector.

Mode d) allows testing of a large number of samplessimultaneously. It is often called the bombing techniquebecause the parts are previously placed in a pressurized Hevessel where the helium leaks into the parts which leak. All parts are than exposed to detector probein closed container. The mentioned leak test techniques are the main methods for establishingwhether leaks exist and for locating where they are. Each has its own advantages and shortcomings,each has also its own individual set of variations.



5. CONCLUSION

Resarchers, technicians, scientists, producers etc who in their work need hermetically closed elementsand vessels, vacuum or only tight seals have to become familiar with measurements and localizationof leaks. In Slovenia there are already exist some groups (first of all on fakulties and institutes) whichhave mentioned knowledge and there are also a few of technologies that use leak detection techniquein manufacturing and in production lines as a 100 % quality control. But in general engineering andalso in important projct organisations this technical field is nearly unknown. Wishing to progress onour way towards more developed countries this situiation must be changed. Department for vacuumelements and systems in Institute for electronics and vacuum technique (IEVT) is, beside others,permanently occupied by tightness testing and leak detection for our own needs and as service forcustomers. In cooperation with the Slovenian Vacuum Society we prepare every year educatingcourses for vacuum techniques, where also this knowledge and skills are represented. Regarding tomentioned circumstances this contribution with basics about leaks and detection methods wasprepared as information.

6. LITERATURE

1. J.L Ryan, D.L.Roper: Process vacuum system, design and operation; McGraw-Hill BookCompany, New York etc-, 1986

2. Nigel Harris: Modem vacuum practice, McGraw-Hill Book Company Europe, Berkshire, England,1989

3. Varian: Introduction to Helium mass spectrometer leak detection; Varian Associates Inc., PaloAlto, 1980

4. E. Kansky: Hermetologija sticnih ploskev in spojev, predavanja na III. st.- Vakuumistika.; FERIMaribor, IEVT, Ljubljana; 1983

5. L.C.Beavis: Real leaks and real leak detection, Vacuum, Vol.20, No.6, p233, Pergamon Press,London,1970

6. M.Wutz, H.Adam, W.Wachler Theorie und Praxis der Vakuumtechnik, F. Vieweg & Sohn VerlagGmbH, Braunschwieg, 198"

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leak detection methods and defining the sizes of leaks

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