Leak testing is employed to detect leaks and determine the rate at which a liquid or gas will penetrate from inside a tight component or assembly to the outer regions, or vice versa, as a result of differential pressure between the two regions. The term "minimum detectable leak" refers to the smallest detectable fluid flow rate. An item cannot be guaranteed to be leak proof if it has not been tested.
Leak rates are measured in standard cubic centimeters per second or atmospheric cubic meters per second. Leakage is measured by how much fluid passes through a leak under a given set of conditions. Since leakage varies with conditions, it is necessary to state both the leak rate and the prevailing conditions to define a leak properly. There are two types of leaks (1) real leaks and (2) virtual leaks. A real leak is an essentially localized leak, (i.e.) a discrete passage through which the fluid may flow (hole, crack etc.). Virtual leaks are leaks that involve the gradual absorption of gases from surfaces or components within vacuum systems.
Leak detection can be carried out by visual inspection using soap bubbles or mass spectrometer. Among the various methods of detection available, bubble testing is widely used for less sensitive applications and the mass spectrometer for highly sensitive detection.
Bubble leak testing: Immersion of a pressurized component in water is used as a crude test in some cases to check for leak tightness. Sensitivity can be made very high if very high pressures are used. Before bubble tests are applied all test surfaces must be cleaned and inspected. When components are to be bubble leak tested, weld slag should be removed and the weld joint should be cleaned with a wire brush. Before pressure bubble tests are performed the calibration of the pressure/vacuum gauge should be checked. The range of the pressure gauge should be roughly double the desired system pressurization. Sealed test specimen is immersed in a preheated liquid having low surface tension. Care must be taken to ensure that the test specimen is not over pressurized as it can rupture or explode. For this reason hydrogen gas must never be used for pressurization. A continuous flow of bubbles would be produced by a hole or crack in the test boundary.
The advantage of bubble testing is that it is inexpensive and can be carried out by relatively less experienced personnel. The test is rapid and gives accurate location of leak and the whole specimen can be inspected simultaneously.
The limitation of this technique is that it cannot detect very small leaks. This technique can be applied to check the integrity of pressure vessels, valves, instruments and piping circuits.
Mass spectrometer helium leak detector: For sensitive leak testing, a mass spectrometer helium leak detector is used. The helium detector is a portable mass spectrometer highly sensitive to helium gas. A mass spectrometer is an instrument for separating or sorting atoms of different mass. Electrons emitted by a heated element bombard gas molecules entering the mass spectrometer. The ion beam produced by the electronic bombardment is accelerated in the form of a narrow beam by means of an electric field. The ions then pass between the pole pieces of a permanent magnet. The magnetic field deflects the ions in a circular path. Ions having equal mass will all emerge from the magnetic field at a certain position. The helium leak detector is adjusted in such a way that only helium ions are collected. The flow of helium ions to the collector induces a minute electrical current, which can be detected and amplified. Helium is used for leak detection because it is an inert gas. Helium is not present in significant quantities in the atmosphere thus causing little interference in sensitive leak detection work. Helium, having a light mass, passes through small leaks more readily as compared to heavier gases. The following are the different techniques in using the helium leak detector:
Probe Technique: In this technique a fine jet of helium gas is passed over the exterior surface of the specimen. Helium gas will be drawn into any opening through the walls of the specimen and register on the leak detector as a visible or audible indication.
Envelope vacuum technique: Sometimes it is desirable only to determine the presence of leaks or the total magnitude of all leaks. In such a case, the specimen containing helium air mixture is put into an evacuated chamber. The chamber is evacuated using an auxiliary pump and the pump outlet is connected to the leak detector for measuring the extent of leaks.
Sniffer Technique: In this technique, the specimen to be tested is filled with helium, or a mixture of helium and air, to a pressure greater than the atmospheric pressure. The surface of the test object is scanned with a sniffer connected to the leak detector. Helium flowing out through any opening will be sucked into the leak detector system by the sniffer and the leak rate is indicated.
Envelope pressurization technique: In this technique, a hood containing helium surrounds the test system. The test system is then evacuated. Helium will flow through any leaks into the evacuated test system and then into the leak detector. This technique gives the overall leak rate of the component.
Pressurization technique: In applying this technique, the component is first placed in a helium pressurization vessel and exposed to a helium atmosphere. The component is removed from the pressurization vessel and transferred to a second chamber, which is connected to a vacuum pump and helium leak detector.