Leak Technique
The leak testing method collates a number of conventional and unconventional NDT methods to address a common function—the detection, location and measurement of leaks. Each of these methods is considered to be a technique within the LT method, and each has characteristics that dictate its use in different applications. It is convenient to compare sensitivity, and to this end, a standard unit of leak rate has been adopted for this publication.
Overpressure tests (hydrostatic tests) are regularly performed on pressure vessels, hydraulic components and a wide variety of pressure containing equipment. If the test is performed with a liquid, one can examine for leakage or wetting due to the escape of the pressurizing fluid. If the test is performed with a gas, then it is common to coat the area of inspection with a suitable fluid and to watch for the formation of bubbles.
A helium mass spectrometer leak detection instrument can be used to determine leak existence and location for many industrial components. For units under pressure, helium is added to the vessel at a suitable test pressure.
The bubble emission method requires a gas pressure differential to be established across the pressure boundary to be tested. A test liquid is placed in contact with the lower pressure side of the pressure boundary. Gas leakage through the pressure boundary can then be detected by observation of bubbles formed in the detection liquid or liquid film at the exit points of leakage through the pressure boundary.
The systematic observation of pressure change can reveal leakage of a pressurized vessel or a vacuum system. The system can use on-line pressure gauges to record pressure drops, or a test liquid can be coated on the outside of a vacuum container, and its effect on internal pressure, as it is drawn through any leak, can be noted.
It may be convenient to use a change in a chemical reagent as an indicator of a leak. For example, leaking ammonia gas may be detected by its reaction with hydrogen chloride producing dense white fumes of ammonia chloride.
Fluorescent or visible dye liquid penetrants are widely used for the detection of flaws which are open to the surface. They can equally applied to the detection of leaks. A low viscosity fluid has a high degree of surface migration; it can be painted on one side of a surface and the opposite side is observed for seepage. It can also be added to a service liquid as a tracer. The technique is simple, low cost
Leaks emit sound of a frequency that is related to the pressure, hole size and geometry of the breach. Acoustical leak detection uses the sonic or ultrasonic signal generated by the escaping
fluid as it expands through the leak orifice. The technique uses a sensitive microphone or an accelerometer. It is simple, fast and non-contacting, however, it is sensitive to other environmental noises.
Radiotracers provide a powerful alternative to other leak detection techniques. They are the most sensitive tools available and they can be used on-line (while the system or equipment is operating). The addition of very small concentrations of radioactive isotope provides an extremely detectable level of measureable and detectable radiation.
Pressure leak test in which the leakage of a component, pressurized with a halogen rich mixture, is detected by scanning over the test object boundary surface with a probe connected to a halogen leak detector. Halogen gas is pulled from the leak through the probe inlet to the sensing element to cause a visible or audible signal on the indicator of the leak test instrument.
Technique of bubble testing where a vacuum box is used to create a pressure differential across a boundary. A viewing window allows observation of bubble formation
