How to Choose a Leak Detection Method M. L. Vinogradov* (IEEE Member), D. K. Kostrin, M. V. Karganov, V. Yu. Tiskovich Department of electronic instruments and devices Saint Petersburg Electrotechnical University "LETI" Saint Petersburg, Russia *
[email protected] Standard conditions for leak detection are: differential pressure between the inner and outer side of the product ΔP = 105 Pa (1 atm); test gas is air; temperature is 298 K. The leakage flow is measured in Pa·m3/s.
Abstract — This article describes a calculation procedure of a permissible leakage flow for sealed products. It is shown how a leak detector sensitivity can be improved by changing a pressure drop and a test gas. Optimal leak rate ranges for a mass spectrometric leak detector, a portable helium leak detector and a pressure decay leak detector are suggested in order to achieve high accuracy, repeatability and minimum test expenses.
When specifying requirements on the product tightness, the maximum permissible leak flow (leak detection threshold) have to be defined in standard conditions. This makes it easy to recalculate the leakage flow for the application of other leak detection methods and various test conditions: gas, pressure and temperature.
Keywords — helium leak detection; mass spectrometric leak detector; portable helium leak detector, pressure decay leak detector; non-destructive testing
Non-standard conditions of leak detection are often used to increase the sensitivity and improve the reliability of testing. Leak flow indicated by a leak detector in conditions different from standard is denoted by Q symbol.
I. INTRODUCTION For tightness control of industrial products three main technologies are used: – measuring a helium flow penetrating through a place of leakage; – monitoring pressure change over time in a pre-pressed or evacuated object; – evaluation of leakage flow by volume and intensity of bubbles from a product immersed into liquid.
A ratio between the amount of leakage flow B in standard conditions and the flow Q in arbitrary conditions in the viscous gas flow regime is expressed from the Poiseuille equation [1]:
B
It is often difficult to quantitatively express permissible leakage flow of the product. Ideal product has an absolute integrity, but in reality complete leakage absence is impossible due to the diffusion of gases through the material, presence of microcracks and imperfections of vacuum seals.
Q
Kgas
Pat2
Kair P22 P12
,
(2)
where ηair – coefficient of dynamic viscosity of air; ηgas – coefficient of dynamic viscosity of test gas; P2 and P1 – partial pressure of the test gas from the outside and inside of the object; Pat – atmospheric pressure.
The task is to determine a permissible leakage flow. According to the definition, it is an established by normative and technical documentation the highest total flow of matter through leaks of a sealed product, ensuring its working state and taking into account the purpose, design, service life and operating conditions of the sealed object. Permissible leakage flow (leak detection threshold) is taken into account when choosing methods and means of control based on the sensitivity, reliability and performance of testing.
For molecular flow regime the ratio between B and Q is determined by the ratio of molecular weights of gases [2]:
B
Q
М gas
Pat
М air P2 P1
,
(3)
where Mair – molecular weight of air; Мgas – molecular weight of test gas.
(1)
To select the gas flow nature the following general provisions are used: for the leakage flow less than 10–8 Pa·m3/s molecular flow regime takes place; for the leakage flow more than 10–5 Pa·m3/s – viscous regime is observed. In the staging area mixed mode regime is used. Calculations in this mode are made using both equations and then prevailing leakage flow is determined.
where V – volume occupied by gas; ΔP/Δt – pressure change rate after pumping cessation.
For products with internal test gas pressure above 105 Pa when a gas flows to the atmosphere, the leakage flow regime is
II. PERMISSIBLE LEAKAGE FLOW AND CONDITIONS OF LEAK DETECTION The leakage flow for the product is characterized by the amount of gas coming through the leak per unit of time. The leakage flow at standard conditions is equal to:
B V 'P 't ,
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To achieve test sensitivity higher by two orders of magnitude than provides a leak detector in standard conditions it is necessary to change test parameters. Let us choose helium as a test gas. The differential partial pressure drop of the test gas should be calculated from (3) for the molecular regime of gas flow:
considered to be viscous. This is true for the leak detection using sniffer probe, manometric or bubble methods. III. CALCULATION OF PERMISSIBLE LEAKAGE FLOW FOR PRODUCT AND METHOD FOR INCREASING THE SENSITIVITY OF LEAK TESTING Time of a pressure change in the pumped device ∆t can be calculated by the following equation: 't
2.3
V § Pinit lg ¨ S ¨© Pperm
· ¸ , ¸ ¹
P2 P1
If the flow has a viscous nature then the desired partial pressure drop should be calculated according to (2), by the following equation:
When pumping of the product is stopped and degassing flow from the walls can be neglected, a gas flow into the product will be determined only by leaks. Thus, the effective pumping speed S can be expressed from a leak flow Q:
P22 P12
(5)
Q Pinit .
2.3Pinit
V § Pinit ln ¨ 't ¨© Pperm
· ¸. ¸ ¹
Pat2
Q Kgas . B Kair
(8)
For this task the partial pressure of helium outside the object P1, i.e. in atmosphere, can be considered negligible compared to the pressure of helium inside the product P2. Then, with regard to the ratio of viscosities of air and helium, calculated differential partial pressure of helium will be 9.4 atm (9.4·105 Pa).
The calculation is made according to an operation gas, which is the gas that fills the sealed product during operation or storage. Then the maximum permissible leak flow Q according to the operation gas for a device is determined by the ratio: Q
(7)
Equation (7) allows calculating the differential value of the partial pressure of helium, which compensates the lack of leak detector sensitivity. The result for the molecular regime of gas flow is 30 atm (3·106 Pa).
(4)
where V – pumped volume of the object; S – effective pumping speed; Pinit – initial gas pressure; Pperm – permissible maximum gas pressure.
S
М gas Q Pat . B М air
(6)
Thus, in the viscous gas flow regime the increase of the pressure during testing on one order of magnitude enlarges sensitivity of leak test procedure by two orders of magnitude. For recalculation of the leakage flow from one method of testing to another, as in the example, at first step the leakage flow should be conversed to standard conditions for leak detection.
The calculation of the maximum permissible flow B (leak detection threshold) at standard conditions based on the obtained flow Q in operation gas is made according to (2) and (3), taking into account the gas flow regime. For example, it is necessary to calculate the leak detection threshold for miniature quartz oscillator with volume V = 10–5 m3. Initial pressure of the operation gas (argon) Pinit in the product is 1 atm. Over a five-year service life it should not drop more then to Pperm = 0.7 atm.
IV. INDUSTRIAL METHODS AND EQUIPMENT FOR LEAK TESTING To compare the results of the leakage flow measurements obtained using different methods, let us examine a tightness of three products with leaks in a welded joint. The product is a copper terminal, two parts of which must be vacuum-tightly welded to each other (Fig. 1).
The leakage flow Q for argon, according to (6), is equal to 10–8 Pa·m3/s, which corresponds to the molecular regime of gas flow. Then by (2) the leak detection threshold B for the device, taking into account ratio between molecular masses of argon (39.95 u) and air (28.98 u), is equal to 1.2·10–8 Pa·m3/s. Using ratio (2) and (3) between the magnitude of leak in standard conditions B and the flow in an arbitrary conditions Q, it is possible to choose the parameters of a leak testing to ensure the required sensitivity. Let us consider the method of test parameters selection on the example of device discussed above. A registration of gas flow under standard conditions should not be worse than 1.2·10–8 Pa·m3/s; available leak detector is a portable helium leak detector with minimum detected leakage flow Q = 10–6 Pa·m3/s.
Fig. 1. Test objects to control the tightness of welds by various methods.
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detector. The leak detector indicates a leak in a numeric value and also as a graph of the helium flow in time.
A. Mass Spectrometric Method As a device, detecting the test gas flow in the conducted experiment, a helium mass spectrometric leak detector Heliot was used (Fig. 2). The minimum reliably detected helium flow with this detector is 5·10–13 Pa·m3/s, the effective pumping speed of the product – 5 l/s for helium. The feature of this leak detector is that it provides a specified sensitivity in the counterflow mode. Because of this, a chamber of mass spectrometric analyzer and a turbomolecular pump are always protected from contamination and from pressure shock increase at the breakdown of the tested object.
Fig. 3. Portable helium leak detector for estimating the leakage in the tested object.
E. Methods for Leakage Testing by Pressure Change or Gas Consumption Rate For leak detection by this method in experiments was used a manometric leak detector S9. It is an automated device for batch inspection of products using high-sensitivity differential pressure sensor. The minimum reliably detected leak flow for air is 10–4 Pa·m3/s [4].
Fig. 2. Testing samples for leaks using the mass spectrometric leak detector Heliot.
In the leak detector S9 a reference sealed volume separated from the measured object by a sensitive to differential pressure membrane is installed. For the method of leak detection by measuring differential pressure the object and the reference volume are pumped or filled with gas to the same pressure.
B. Helium Chamber The product is installed on the inlet of the leak detector, its interior is evacuated. From the outside to the product a sealed chamber is connected, which is evacuated by a forepump and then filled with helium to the required pressure [3]. The flow of helium penetrating through leaks into the product is registered by the leak detector.
In the presence of a leak in the test object, the pressure balance is broken and the membrane separating the volumes deforms. By the change of the capacitance of the capacitor, one electrode of which is the said membrane, the magnitude of a leak in the test object is calculated.
C. Vacuum Chamber Inside of the product is evacuated by a forepump and then connected to the helium tank. The helium pressure in the product will be set using a reducer on the tank. The product is fixed in a vacuum chamber connected to a vacuum system of a helium leak detector. Helium flowing from the internal part of the product into a vacuum chamber is detected with a mass spectrometric leak detector.
The leak detector with integrated gas flow meter can also be used for leakage testing by measuring the flow of gas emerging from the object. F. Method of Tightness Control by Gas Consumption Rate Measurement The leak detector creates a set overpressure in the test object. Then measurement of the air flow which exits the product in case of a leak is conducted. Tests are made using a gas flow meter installed in the measurement system of the leak detector. The device can be calibrated using a control leak and an external gas flow meter.
D. Electronic Sniffer Probe Helium Leak Detector Easier to apply and 4–6 times cheaper, compared with mass spectrometric devices, is a portable helium leak detector X1 (Fig. 3). The minimum reliably detected helium flow with this instrument is 10–6 Pa·m3/s, the weight of the device in the shape of a gun is 300 g.
Leak detectors of manometric type display results of the measurement in non-system units [mm3/s] or [cm3/s], which explicitly does not specify the pressure drop. In this case, standard conditions for leak detection are used [5]. When comparing this units of measure with used in SI units of flow
In the sniffer probe method an object is filled with helium from a tank, and from the outside the helium flow through the defects in the welds is controlled by the probe of the leak
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[Pa·m3/s], received result should be multiplied on the pressure drop of 1 atm.
example, if the operator every second sees the appearance of an air bubble with 2 mm diameter (4·10–9 m3), at a differential pressure of 1 atm, according to expression (1), flow of air that passes through the leak is 8·10–4 Pa·m3/s.
G. Method of Leakage Testing by Pressure Drop The inner part of the product is connected to a pneumatic system leak detector S9. Inside the product is fed excess air pressure. Then product is isolated from the system supplying air for subsequent control of the pressure drop, which is caused by the presence of leaks.
V. THE RESULTS OF EXPERIMENTAL COMPARISON OF LEAK DETECTION METHODS The experiment to determine the leakage flow with described methods of leakage control was conducted for the three controlled products. The tests were performed at four values of test gas differential pressure (1, 2, 3 and 4 atm) for all tests except for the method of measurement of pressure rise. Built-in ejector pump of the pressure change leak detector allows creating a differential pressure of 0.1 atm.
H. Method of Tightness Control by Pressure Increase Vacuum pump of the leak detector S9 creates a vacuum in the inner part of the controlled product. Then the pump stops and the pressure rise occurring in the presence of leaks is registered by the measuring system of the leak detector.
The results of the flow measurements converted to standard conditions of testing, according to (2) and (3) taking into account regime of gas flow and averaged are presented for each method in TABLE I. TABLE I.
LEAKAGE FLOW FOR MEASUREMENT USING DIFFERENT METHODS OF TIGHTNESS CONTROL
Control method
Fig. 4. Manometric leak detector for measuring leakage flow through the test object.
Leakage flow for standard conditions, Pa·m3/s Test object 1
Test object 2
Test object 3
Helium chamber method
6·10–4
9·10–10
4·10–5
Vacuum chamber method
9·10–4
2·10–9
7·10–5
Electronic sniffer probe helium leak detector
7·10–4
–
4·10–5
Gas flow rate method
1·10–3
–
8·10–5
Pressure drop method
2·10–3
–
6·10–5
Pressure increase method
7·10–4
–
–
Bubble method
2·10–3
–
–
When implementing the methods of leak detection by pressure change a highly sensitive differential pressure sensor is used. Therefore, result of measurement is represented in units of pressure. For conversion of the results to units of flow (1) is used, where ΔP is the pressure change obtained as a result of test and Δt is the measurement time.
Test object 1 had a major leak with a flow of about 10–3 Pa·m3/s, which was identified by all methods of leak detection. The application of highly sensitive mass spectrometric leak detector is not desirable to evaluate such leaks. Leakage of this sort can be found on the preliminary control phase by using an automated manometric leak detector or bubble method.
I. Bubble Method
The advantage of manometric leak detector over bubble method appeared in easiness and repeatability of leakage flow measurement. Bubble method should be used primarily for the localization of leaks, but measurement of the intensity of leakage flow should be carried out by manometric leak detector.
In the bubble method, test object is filled with test gas to excess pressure and then immersed in a liquid. Gas, escaping from cracks in the product, forms bubbles that can be registered by operator. Advantages of the bubble method are simplicity and low cost of its implementation; its disadvantages include the ability to miss leak because of the strong influence of the operator in the measurement process and the likelihood of leaks blockage due to capillary forces.
Test object 2 had a relatively small leakage flow of about 10–9 Pa·m3/s. This leak was registered only with the help of mass spectrometric leak detector. Even at a differential pressure of test gas of 4 atm, the flow was not sufficiently intense to be indicated using a portable helium leak detector and other methods.
For the conducted experiment the test gas is air and liquid is water. The minimum reliably detectable leak flow for air in this method is limited to 10–3…10–4 Pa·m3/s, but can be improved by changing the type of fluid and pressure. For
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The leakage flow of 5·10–5 Pa·m3/s in test object 3 was confidently measured by helium leak detector. Measurement of gas flow rate and pressure drop gave the result only at elevated differential pressure (3–4 atm) when the leakage flow in the experiment exceeded 10–4 Pa·m3/s. The optimal low-cost unit to find leakage of such intensity is a portable helium leak detector.
For reasons of accuracy, repeatability and minimum cost of testing for leaks it is recommended to use the following compliance of leak detection threshold for products and testing equipment: – less than 10–6 Pa·m3/s – mass spectrometric helium leak detector; – 10–6…10–3 Pa·m3/s – electronic sniffer probe helium leak detector; – more than 10–3 Pa·m3/s – manometric leak detector.
VI. RESULTS AND CONCLUSION Objects with the same structure of leaks were checked using different methods. Given equations allow comparing the results obtained during measurements under conditions different from the standard conditions of leak detection.
REFERENCES [1]
It is shown how a leak detector sensitivity can be improved by changing a pressure drop and a test gas. Pressure increase method proved to be less sensitive than pressure drop method, due to the relatively small pressure difference created by the pump of the leak detector during the tests.
[2]
[3]
To increase the sensitivity for this method, the S9 leak tester should be connected to an additional forepump, and from the external side of the product excessive pressure should be applied.
[4] [5]
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J. Welty, C. E. Wicks, G. L. Rorrer, R. E. Wilson, Fundamentals of Momentum, Heat and Mass Transfer. New Jersey: John Wiley & Sons, 2008. G. Schröder, “Neue Norm zur Auswahl eines geeigneten Verfahrens zur Lecksuche und Dichtheitsprüfung”, ZfP-Zeitung, vol. 74, pp. 31–39, 2001. M. L. Vinogradov, V. T. Barchenko, A. A. Lisenkov, D. K. Kostrin, N. A. Babinov, “Gas Permeation through Vacuum Materials”, Vakuum in Forschung und Praxis, vol. 27, no. 3, pp. 26–29, 2015. EN 1779:1999. Non-destructive testing. Leak Testing. Criteria for method and technique selection. EN 13184:2001. Non-destructive testing. Leak testing. Pressure change method.
