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Leak Testing to Avoid Losses and Dangers

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Leak-testing is the whole of nondestructive inspection procedures designed to detect and locate loss of fluid.

Containers intended to be hermetically sealed may leak.

In that case the location of the leaking defect must be determined, in order to prepare a repair procedure.

Manufacturers supplying components or systems built for keeping, transporting or supplying fluids, are accountable for providing reliable, sound and leak free equipment.

Therefore any production program of any type of tank, vessel or processing system for fluids, must include tests capable of verifying the sealing capacity.

Leak-testing for Economy and Safety

That is over and above the implementation of traditional non destructive crack detection methods.

Welds of aluminum or stainless steel tanks are usually tested by fluorescent penetrant inspection.

That is done by painting the penetrant liquid, on the welded seams from one side, and inspecting for fluorescence (under black or ultraviolet light) from the other.

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In Leak-testing terminology, leak indicates the passage, through which fluid may escape.

Leakage is the fluid lost through the leak, and leak rate is the amount of fluid passing through the leak per unit time.

Operators managing structures involving fluids, are responsible for Leak testing and for taking all measures to assure that their installations are at all time safe and leak free.

Leakage of dangerous liquids or gases may become a safety issue or a pollution source requiring repair for redressing the situation in the least possible time.

An infamous and enormous oil leak which flowed for three months, started with an explosion of Deepwater Horizon.

That was a semi-submersible mobile offshore drilling unit, in the Gulf of Mexico on April 20, 2010.

It is the largest accidental marine oil spill in history.

In general a pressure difference is required to drive a leakage.

The classic way to perform Leak-testing is the so called Bubble testing as used when looking for leaks in inner tubes of car tires.

Low pressure air is admitted in the tube, submerged in a water tank: leaks are revealed by air bubbles raising in the water.

Leak-testing of pressure vessels is performed by filling them with water and increasing the pressure an agreed upon amount above the nominal working pressure, and sealing the system.

Any pressure decrease in time indicates a possible leak that must be further detected and located visually or by other means.

Safety is assured by using an incompressible liquid, because the overpressure would fall instantly to zero.

That avoids explosion risks should any local failure occur, causing a minimal leak.

An empirical Leak-testing procedure used sometimes consists in imparting hammer blows on the external surface of a pressure vessel full of water and sealed.

If there is no air in the system, pressure peaks caused by hammer blows can drive marginal leaks until they become visible to the naked eye on the vessel external surface.

Gas leaking from a system produces a hissing sound at sonic and ultrasonic frequency.

Those can be detected by acoustic methods, by ear or by more sensitive microphones.

Specific gas detectors are used for Leak-testing, using sniffing sensors responding to minimal quantities of the gas employed.

One of the most sensitive instruments for detecting trace amounts of gases is the mass spectrometer, a device for sorting charged particles.

Helium gas spectrometers are most used for Leak-testing of vacuum systems.

Besides helium used for Leak-testing of vacuum systems, various tracer gases and their specific detectors can be used to detect leaks in pressure containing systems.

Among these are hydrogen, ammonia, halogen gases (chlorine, fluorine, bromine, iodine), sulfur hexafluoride, nitrous oxide and other gases.

Each instrument has a definite sensitivity for given minimum leak rates, below which it cannot be used for Leak-testing with confidence.

The two most commonly used units of leakage rates are standard cubic centimeters per second for pressure leaks and torr liters per second for vacuum leaks.

It is unreal to specify no leakage at all. Smaller leaks are increasingly difficult and costly to determine.

Requirements must be based on safety, contamination and reliability, depending on the fluid type and on the operating conditions of the system.

Gas flow physics is important for comparing flows, as well as for determining the best conditions for Leak-testing.

To increase the flow rate through the leak one should increase the pressure difference.

The less viscous the tracer test gas is, the more sensitive the test because the leakage rate is inversely proportional to the test gas viscosity.

Leak-testing should be conducted in two or more stages, starting with the less sensitive methods, because sensitive methods are useless for detecting serious leaks.

It is also recommended to check the surroundings of a located link, because additional lighter leaks might hide nearby.

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You may wish to download the following publication:
Guide to good leak testing (10 pages)

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Helium Leak Testing


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