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Leak testing method is needed to check fabricated components and systems
for nuclear reactors, pressure vessels, electronic valves, vacuum equipments,
gas containers at pressures etc. 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 outside, or vice versa, as a result of pressure differential
between the two regions.
Leak test methods are employed either as 'proving tests' i.e. to determine the job
leaks or not and/or as 'location tests' to pin point the location of leak.
Depending upon the range of leak detection capability, a number of test methods
are available.
Leak detection may be carried out by visual inspection using soap bubbles or mass
spectrometers etc. Among the various methods of detection available, bubble testing
is widely used for less sensitive applications and mass spectrometer is used for
high sensitive detection.
BUBBLE TESTING -
Applications - The technique can most advantageously be applied for checking
the integrity of pressure vessels, valves, instruments, piping circuits, containments,
condensers, heat exchanges, pumps, cylinders etc.
Bubble forming solution is applied to the surface of a pressurized vessel
if it is too large or not possible to submerge. The ideal liquid for bubble testing
should have low surface tension and low viscosity. The bubble size
depends on the viscosity of the liquid, pressure and diameter of leak. Sometimes,
vibration or hammering of the pressurized component under test is
applied for enhanced detection sensitivity.
The best combination of pressurizing medium is hydrogen (has
a fast flow through small leaks) & ether (has a low surface tension). Methanol
& helium are better than hydrogen, and can be used for
detection of smaller leaks.
Advantages - The advantages of bubble testing are that
it is inexpensive, can be carried out by relatively less experienced personnel,
is rapid, gives accurate location of leak, and the whole
specimen is inspected simultaneously.
Limitations - The technique cannot locate very
small leaks. In some cases, leaks have been known to pass gas in one direction only,
and if this is inward, the bubble technique will
not locate such leaks.
HELIUM LEAK DETECTOR -
For very sensitive leak testing, mass spectrometer
based helium leak detector is used. The commercial helium leak detector can detect
the presence of less than one part of helium in
10 million parts of air. The helium leak detector is a portable mass-spectrometer
especially designed to be highly sensitive to
helium gas. A mass spectrometer is an instrument for separating or sorting atoms
of different mass. The helium leak detector is adjusted
so that only helium ions are collected. Helium is generally used for
leak detection because it is an inert gas and does not react with other gases and
materials in the system. Helium, having a light
mass, passes through small leaks more readily as compared to heavier gases.
There are five different techniques in using helium
leak detector. They are -
- Probe technique
- Envelope Vacuum technique
- Sniffer technique
- Envelope pressure technique
- Pressurization technique
In all these techniques, it is necessary to have clean test specimens/components
since dirt, moisture, scale and oil may easily seal comparatively large leaks.
- Probe technique - In this technique, a fine jet of helium such
as that obtained from a hypodermic needle, 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 desired to determine
only the presence of leaks or the total magnitude of all the 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 leak.
- 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
atmosphere. The surface of the test object is then 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 Pressure technique - In this technique, the test system
is surrounded by a hood containing helium. The test system is then evacuated. Helium
will flow through any leaks into the evacuated test system and then to 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.
STANDARDS -
- ASTM E 493 - Test methods for leaks using mass spectrometer
leak detector in the inside-outside testing mode.
- ASTM E 1003 - Methods of testing for hydrostatic leak testing
- ASTM E 515 - Methods of testing for leaks using bubble emission
technique
- ASTM E 499 - Methods of testing for Leaks using the mass spectrometer
leak detector in the detector probe method
- ASTM E 498 - Methods of testing for leaks using the mass spectrometer
or residual gas analyzer in the tracer probe mode.
- ASTM E 1002 - Methods of testing for leaks using ultrasonics
- ASTM E 908 - Practices for calibrating leaks, gaseous reference
- ASTM E 432 - Practices for guide for selection of leak testing
methods
- ASTM E 479 - Practices for guide for preparation of a leak testing
specification.
- ASTM E 425 - Definitions of terms relating to leak testing
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