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Why is Leak Detection Important?
Whether it’s a gas leak or a liquid leak, leak detection is important to ensuring the safe and proper operation of equipment, and can help prevent other major problems. Even a minor leak can substantially reduce performance, and can often lead to complete system failure. A leak also equates to lost media (whatever it may be), which can increase operating costs. In pressurized applications, small leaks can quickly become large ones, causing damage to equipment, user injury, or worse. And, depending on what media you’re working with, a leak can present a serious health hazard.
Ultimately, leak detection will save you time and work, improve the quality of the end result, and reduce operating costs, among other benefits.
The Importance of Detecting Leaks Early
For the above reasons and more, it is always best to detect a leak early. In almost all cases, the longer a leak goes undetected, the worse it—and its effects—will get. Left untreated, a small drip-drip-dripping of water, for example, can cause significant damage over an extended period of time. Even if no direct damage is caused, the performance of the device or system that is experiencing the leak will be negatively affected. The longer that your equipment runs at diminished capacity, the more time, effort, and money will be expended on operation, while the results will continue to lag.
What is An Air Leakage?
An air leakage can best be described as the unwanted flow of air through the wall(s) of a sealed container or vessel. The difference in pressure between the inside of the vessel and the ambient atmosphere will cause the air to flow from higher pressure to lower pressure. While this usually happens from the “inside out”—i.e., from inside an object with higher interior pressure to outside the object where pressure is lower—it can also occur from the “outside in” when outside atmosphere flows into an evacuated/vacuum container.
What Are Leak Detection Tools?
Air leak detection tools are devices designed specifically to detect leaks within a pressurized vessel or system. The most effective air leak detection tools are highly sensitive instruments that use any of a number of leak testing methods to identify, measure, and, in some cases, provide the precise location of air leaks. Some air leak detection devices can also be used to detect other gas leaks (not just air).
Detecting A Leak
The right way to detect a leak depends on the type leak you’re dealing with. Different media—liquids, fluids, and gases—may leak in different ways, and at very different rates. The following sections will provide insights into a number of leak testing methods. Read on to find the right solution for your specific needs.
Air Leak Detection Methods/Techniques for Pressure Vessels, Etc.
Bubble Testing/Submersion Testing
This is very basic method of air leak detection. Simply pressurize the part or vessel, then submerge it in water and watch for bubbles created by escaping air or gas. If the test part cannot be fully submerged for whatever reason, soapy water can be sprayed over the part—any air leak will create bubbles in the soap. The intensity and size of the bubbles created will be directly proportional to the size of the leak in the test vessel. Bubble testing is effective for locating a leak, but does not provide accurate measurement of the leak.
Helium Accumulation Leak Detection
The test part is connected to a pump and a helium detector and sealed within an airtight chamber. The part is then evacuated; a gross leak will prevent full vacuum from being reached. If full vacuum is reached, the vessel is charged with helium to a predetermined pressure level for a set length of time. Ambient atmosphere within the enclosure is constantly mixed by fans and blowers to maintain uniform air concentration. After the prescribed length of time, a mass spectrometer samples the air inside the test chamber and monitors helium pressure to detect any change from the initial atmospheric level; any pressure variation indicates a leak. Helium accumulation leak detection is highly sensitive (down to 10-5 scc/s), very reliable and repeatable, and is not affected by temperature changes.
Helium Sniff Leak Detection
First, the tested part is charged to predetermined pressure level using helium. Then, a helium sniffer device is used to scan the part’s surface to detect leaks. A leak in the vessel will result in a higher concentration of helium at the leak point (the earth’s atmosphere naturally contains trace amounts of helium). Helium sniff testing is highly sensitive (down to 10-6 scc/s) and can accurately locate the leak point. It is often performed manually, however, which reduces throughput compared to automated air leak detection processes. Additionally, due to the nature of the testing process itself, there may be multiple individual leaks that fall below testing sensitivity but combine to create a total leak rate that is beyond acceptable limits.
Helium Spray Leak Detection
Helium spray testing is performed in two phases. The test piece is first evacuated and sealed with using a pumping with an integrated helium detector; a gross leak will prevent the vessel from reaching full vacuum. If full vacuum is achieved, the test part is then spray with helium; a fine leak will pull helium into the part, where the helium detector will sense the gas and measure the incoming leak rate. This leak testing process is highly accurate (down to 10-6 scc/s) and can accurately locate leaks. However, due to the nature of the testing process, if multiple leaks are too close together, they may be “masked” and only register as a single leak.
Pressure Decay Leak Detection
The pressure decay air leak detection method measures the internal pressure of a sealed and pressurized part or vessel. Using dry air, nitrogen, or other inert gas, the vessel is charged (pressurized) to a predetermined level and monitored over a set length of time. Any decrease in the part’s pressure indicates a leak. The sensitivity of this testing process is directly related to the volume of the vessel under test (as larger parts take longer to pressurize), the required test time, and the resolution of the pressure transducer. Pressure decay air leak testing is the most common and user friendly method available.
Ultrasonic Leak Detection
All air leaks/gas leaks produce some sound, the frequency of which will be higher or lower in relation to the size of the leak. Smaller leaks producing higher frequency sounds, often at frequencies too high for the human ear to detect. Using an ultrasonic air leak detection device, leaks up to 10-2 mbar l/s can be located. These (usually) handheld instruments are, in general, better suited to finding larger leaks rather than fine ones. It should be noted that ultrasonic air leak detection provides only an estimate of the leak rate based on the frequency of the sound, rather than an accurate measurement.
Vacuum Decay Leak Detection
In vacuum decay air leak testing, the test piece is evacuated (brought to vacuum), sealed, and measured for any pressure increases. Vacuum decay testing is more or less the opposite of pressure decay testing, and offers a number of advantages.
- Vacuum decay air leak detection is more sensitive than pressure decay testing
- Vacuum decay is more precise than pressure decay
- Vacuum decay is less susceptible to temperature changes and other environmental factors
However, the pressure difference applied to the tested vessel cannot be greater than 14.7 psiv. Vacuum decay can also be affected by liquid evaporation and surface outgassing, which can reduce sensitivity and generate false leaks.
Hard Vacuum Leak Detection
In hard vacuum leak testing, a part or vessel is placed inside a sealed chamber. A vacuum pump removes all the atmosphere from the chamber and the part. The part is filled with tracer gas, such as helium, then the mass spectrometer samples the air inside the test chamber for tracer gas escaping the part. A highly sensitive air leak testing process (down to 10-8 scc/s) that provides reliable and quantifiable results, vacuum testing can be fully automated and is easy to integrate into production lines. However, it is among the more expensive air leak detection methods.
Tracer Gas Leak Detection
A tracer gas, such as hydrogen or helium, is injected into an isolated section of the water distribution pipe. If any leaks are present, the gas will escape and, being lighter than air, will permeate the surface of the soil or pavement above the leak. A high-sensitivity gas detector is used to scan the ground area to locate the leaking gas.
What Gases Can Be Leak Tested?
There are a number of tracer gases that can be used for air leak detection. Common examples of tracer gases include helium, nitrogen, and argon, among others. An effective and safe tracer gas must:
- Be non-toxic to humans, animals, and the environment
- Be inert (not react chemically nor be readily flammable)
- Be quantifiable through leak testing
- Not be present in ambient air, if possible (helium being the notable exception)
- Not be easily mistaken for other gases
- Not displace air, as this could lead to suffocation or other hazards for the test operator
Cost of Air Leak Detection
The cost of air leak detection can vary greatly, depending on the process used and the technology required. A simple hand test costs nothing, but is not as effective as other methods and cannot be used in every instance; vacuum leak detection is one of the most sensitive and accurate air leak detection methods, but requires specialized equipment and can be very costly.
However, even the most expensive air leak testing process can be far less costly than an undetected leak. Leaks can lead to increased operating costs, equipment failure, property damage, personnel injury, or worse.