LEAK TESTING (LT)
1. Principle
Any lack of material that allows leakage is called a leak. By definition, a leak is the passage of a fluid from one side of a wall to the other. This only occurs when the leak is subject to a pressure difference. Depending on the fluid in question (liquid or gaseous), the measurable quantity characterising a leak is called flow rate or leakage rate. As liquids cannot be compressed, the term used is volume flow rate or mass flow rate. As gases occupy the volume available to them, the quantification takes account of the pressure. The leakage rate is measured in Pascal cubic metres per second (symbol Pa m 3s-1, where the expression in IS units is m² kg s-3).
The flow of a leak can be altered by:
- The type of fluid; air, tracer gases (helium, ammonia, halogens, process gas);
- the pressure difference between the internal and external walls;
- the environment of the defect that is influencing, more or less, the ability of the gaseous current to pass through.
The leak tightness of an object is determined by measuring its leakage rate, namely the amount of fluid that enters or exits, in operating or test conditions.
2. Test method
Leak testing is an NDT method applied to finished parts.
They must be clean, dry and free of grease.
Five techniques are used to implement these tests:
- Bubble test;
- pressure variation test;
- halogenated tracer test;
- test by chemical reaction of the ammonia gas;
- helium test using mass spectrometry.
3. Implementation and sensitivity
3.1 Bubble test
Pressurised
The part is pressurised with gas and submerged in a liquid (global test) or sprayed with a surfactant solution (local test). The leak is shown by the regular appearance of bubbles or foam in line with the through-wall defect.
Sensitivity ≈ 1 x 10-4 Pa m3 s-1 (depending on the pressure).
Vacuum box
The outside surface of the part is exposed to a surfactant solution, the vacuum box is applied to the surface and a vacuum is created; foam or bubbles appear in line with the leak.
Local test
Sensitivity ≈ 1 x 10-3 Pa m3 s-1.
3.2 Pressure variation test
The part is pressurised or depressurised in relation to the atmospheric pressure. If there is leakage, the internal pressure tends towards the atmospheric pressure.
Global test
Sensitivity ≈ 1 x 10-5 Pa m3 s-1 (depending on the volume, time and pressure sensor).
3.3 Halogenated tracer test
The part is pressurised using a halogenated gas (or a mix). The areas to be tested are inspected manually using an internal sniffer probe connected to the leak detector. The gas that flows through the leak is analysed and quantified by the detector. Calibration is carried out using a calibrated leak arranged in the test conditions.
Global test (sniffing with accumulation)
Sensitivity ≈ 1 x 10-7 Pa m3 s-1 (depending on the time and volume of the pocket).
Local test (direct sniffing)
Sensitivity ≈ 1 x 10-7 Pa m3 s-1.
Note: Discharging halogenated gases into the atmosphere is PROHIBITED.
3.4 Test by chemical reaction of the ammonia gas
The part is placed in a vacuum before being pressurised with ammonia. The external area to be tested is passivated before painting with reactive paint (yellow). The ammonia gas reacts with the reagent (bromophenol blue) in line with the through-wall defect which changes locally, from yellow to blue.
Local test
Sensitivity ≈ 1 x 10-7 Pa m3 s-1
Note: ammonia gas is a toxic, flammable gas. The discharged ammonia-based solution must be neutralised.
3.5 Helium test using mass spectrometry
In vacuum
The part is placed in a vacuum before coming into contact with the leak detector (mass spectrometer, set on the helium). The entire part (global test) or certain areas (partial test) are placed in a helium atmosphere. The helium that flows through the leak is analysed and quantified by the leak detector. Calibration is carried out using a reference leak connected to the part.
Sensitivity ≈ 1 x10-10 Pa m3 s-1.
The leak is located using the helium jet technique (spraying the tracer gas on the suspect places).
Vacuum box
The vacuum box is applied to the area to be tested. After the vacuum is created, it is connected to the leak detector. The opposite wall is placed under helium atmosphere. The helium that flows through the leak is analysed and quantified by the detector. Calibration is carried out using a reference leak connected to the vacuum box.
Local test
Sensitivity ≈ 1 x 10-9 Pa m3 s-1.
Pressurised
The part is pressurised with helium. The areas to be tested are inspected by an internal sniffer probe connected to the leak detector. The helium that flows through the leak is analysed and quantified by the detector. Calibration is carried out using a calibrated leak arranged in the test conditions or using the natural helium in the air.
Global test (sniffing with accumulation)
Sensitivity ≈ 1 x 10-7 Pa m3 s-1 (depending on the time and volume of the pocket).
Local test (direct sniffing)
Sensitivity ≈ 1 x 10-7 Pa m3 s-1.
Bleedout
This technique applies to small sealed parts. The part is pressurised with helium; after ventilation, it is placed in a vacuum chamber connected to the leak detector. The helium that penetrated the part during the immersion phase flows into the vacuum chamber and is then analysed and quantified by the detector. Calibration is carried out using a reference leak connected to the vacuum chamber.
Global test
Sensitivity ≈ 1 x 10-9 Pa m3 s-1 (depending on the immersion time and pressure).
4. Scope
The leak test takes place as a final test. It guarantees:
- The quality of a product by preventing fluids from entering in it;
- the safety of equipment by containing the hazardous products in the petrochemical and nuclear power sectors and the transport of liquefied gas;
- the safety of means of transport: automobile, aeronautics;
- the permanence of facilities devoted to spatial (vacuum chambers
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