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The Fluid Systems Engineering and Management Blog

by Swagelok Northern California

Three Kinds of Leaks and How to Track Them Down

by Jeff Hopkins, on 9/21/17 8:45 AM

There's no single best way, but each method has its own advantages


Learn more about the Swagelok Northern California Compressed Gas Leak Detection Program and other Energy Management Services Here >>

Real and virtual leaks and permeation

Have you priced a gallon of hydraulic fluid lately? Now think of a leak that wastes six drops of fluid per minute. That's more than 40 gallons a year in needless replacement costs. Leaks can also cause safety issues, ruin equipment and knock you off schedule. It's worth the time and effort to find those leaks and deal with them.

When we think of fluid leaks, we usually picture a real leak caused by cracks, gaps between sealing surfaces, or permeation through seal materials. Basically, this category covers any time a pressure barrier fails to contain or isolate a system fluid from the surrounding environment.

But a system also can have virtual leaks, which release internally trapped fluid into a fluid system due to material outgassing (escape of gas from a material under test in a vacuum), absorbed or adsorbed fluids, entrapment in cracks, or deadlegs.

There's also permeation, the passage of fluid into, through, and out of a pressure barrier having no holes large enough to permit more than a small fraction of the molecules to pass through any one hole.

How to find the three kinds of leaks

There are several ways to find a leak, each with its own set of advantages and disadvantages.

Bubble test

Bubble Testing is fast, simple and inexpensive test uses immersion or a film solution. The unit being tested is pressurized to create a pressure differential, and any leaks create bubbles in the solution. It's a fairly sensitive test, and allows you to evaluate an entire component at once. If you know there's a leak but haven't discovered whether it's real or virtual, this test can tell you. On the downside, smaller leaks are harder to detect, you won't be able to measure the specific leak rate, and the unit you are testing will have to be cleaned or dried when you are done. 


It's worth the time and effort to find those leaks and deal with them.


Pressure leak testing

Pressure Leak Testing is an excellent proof test that is best for leak location. The two techniques for best results are hydrostatic and pneumatic tests. The unit under test is gradually pressurized with water or air to a specified mark and held for a set length of time. Pressure is then reduced to design pressure and the unit is observed for leakage. Like Bubble Testing, it's inexpensive, simple and clean, and you can evaluate the entire assembly at once. On the other hand, it's a slow test, and it might even be dangerous if air is not completely evacuated from the assembly. Once again, assemblies must be cleaned and dried following testing.

Pressure Change Measurement Testing

Pressure Change Measurement Testing determines total leakage. It looks a change in pressure across a pressure boundary caused by leakage. A decrease in pressure indicates leakage. The leak rate can then be calculated and evaluated to see whether it is within acceptable limits. The four common techniques for this test are pressure decay, pressure change absolute, pressure change reference, and volume or flow measurement. Pressure decay is the most commonly used technique, but it is best for small systems with volumes less than 7.5 cubic feet. The big advantage with this test is that you can determine total leakage. It requires no special tracer gas, and works in small-volume applications. But with this test you must already know the internal volume. This test can't locate leaks, and it can be less sensitive in large-volume applications.

Mass Spectrometry Testing

Mass Spectrometry Testing is a versatile, reliable method of both measuring leakage and locating leaks. A mass spectrometer is used to measure the amount of tracer gas, usually helium, in the unit being tested. This is done by creating a pressure differential between the mass spectrometer and the unit being tested. The presence of the tracer gas inside the mass spectrometer indicates a leak. There are actually different five techniques for this test, using a hood, a tracer probe, a detector probe, accumulation, and a bell jar. With this test, you can measure a great degree of reliability and sensitivity. You can both measure leakage and locate leaks. But this test does require a skilled operator and a comprehensive test plan when used on large or complex systems. The equipment is also expensive and costly to repair.


Which method should you start with? Talk to us and we can help you decide. We're standing by at 510-933-6200, or you can reach us through our website.

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