Leak detection

Industrial and laboratory applications of leak detection

Any system that is to be filled with a gas or liquid is generally tested for leaks as part of the quality control procedure. If the system is to be used for extended periods of time without re-charging with the contained gas or liquid, for example a domestic refrigerator or car air conditioning system, it is essential to have absolute leak integrity. Other systems, such as car radiators which can be topped up with coolant fluid during regular service check ups, also require high levels of leak integrity despite the possibility to recharge the system when required. Nobody wants to be faced with an empty car cooling radiator on a long trip in regional areas.

Analytical instrumentation is also highly sensitive to leaks and will often be tested prior to use. Whilst it is obvious that leaking gas will flow out from a high pressure system to low pressure surrounds, it is less known that ambient air from the surroundings also enters the high pressure system due to a process of back diffusion and eddies that form around the leak. Surprisingly, higher pressure leaks can cause more ingress of air due to the formation of more turbulent eddies around the leak. This ingress of ambient air into the analytical system may be highly undesirable because it will introduce oxygen or moisture, which may react with some of the sample chemicals. It will also introduce chemicals present in the air which may contaminate the sample and interfere with the analytical results.

Leak detection methods

There are many non-destructive leak testing methods that can be applied to manufactured items or gas piping systems. A common technique is to fill the test piece with compressed air and brush or spray a soap solution over the test piece. If there is a significant leak, bubbles form to identify the leak. This method is simple, but not effective at detecting the smallest of leaks. It also results in a wet test piece which may be highly undesirable for high precision or chemically sensitive components.

A popular leak test method for complex geometries and longer piping systems is to pressurise the system, then shut off the gas supply. The system pressure is then recorded and the system left for a period of hours or days, before recording the pressure again. If there is a leak there will be a drop in pressure, which gives this test its name: the 'drop test'. Whilst this leak assessment method is good at quantifying whether or not there is a leak, it offers no help in locating the leak. Once it has been established that there is a problem, a secondary method must be used to track down and eliminate the leak itself.

Perhaps the most practical technique to balance both cost and reliability is to use a gas detection tracer gas. In this method the test piece is filled with a gas mixture containing, for example, helium or hydrogen. These two tracer gases are easy to detect using an electronic gas sensor device. Both helium and hydrogen are small, highly diffusive molecules with low viscosity. This means that they will quickly find and flow through the smallest of pinhole leaks.

Product selection tips

Helium has been the gas of choice in this application for many decades because it is inert. It has often been used as a pure gas or in a mixture of 10% helium in a balance of nitrogen. However, in recent years hydrogen has become more popular due to the significant cost savings versus helium. A mixture of 5% hydrogen in nitrogen is dilute enough to be classified as non-flammable, but contains enough tracer gas to detect the smallest of leaks. This mixture therefore combines good economics with safety and is an optimal solution. Note that the detectors for helium and hydrogen are not the same, so if a switch from one gas to the other is made it is important to re-invest in a new gas detector instrument.

The application of the leak detection gas mixture can be done with a single or a two-stage chrome-plated brass specialty gases regulator. For heavy industrial applications, industrial grade regulators may be specified if there is a tight budget to work to and rough leak detection is required. For high-tech applications or cases when very fine leaks must be found, industrial gas pressure regulators should be avoided because over time the diaphragm inside will release particles, which may contaminate sensitive materials and may also block fine leaks and so prevent their detection.

Product use: evacuation or pulse purging with leak detection gas

It is recommended to either pulse purge or evacuate the test part before filling it with the leak detection tracer gas. This is particularly critical for long and narrow shapes. If you do not evacuate prior to filling, the residual air in the test part will be pushed into the shape and the gas detection tracer gas will only slowly diffuse to that area. This means any potential leaks at that end of the shape will emit air, which can not be detected by the leak detector.

Evacuation is especially important if low pressures of leak detection tracer gas are used because the trapped air will dilute the tracer gas as it fills the test piece. Consider this case: if the part is initially full of air at atmospheric pressure and one additional atmosphere (100 kPa) of tracer gas is added, the gas detection tracer gas concentration will be 50%. If two atmospheres (200 kPa) of tracer gas are added, the concentration of tracer gas will be 66%. At these concentrations of tracer gas, it may not be possible for the leak detector to pick up leaks of the gas detection tracer.

An alternative to evacuation is pulse purging with the leak detection tracer gas mixture. In this technique the gas detection mixture is applied to the test part and the pressure is allowed to build up. Then, after a short period of waiting for the gases to diffuse together, perhaps a minute, the pressure is released the gas allowed to flow free from the test part. The mixture inside the test part is now a combination of the gas detection gas mixture and the original air. If this pulse purge procedure is repeated three or four times it is possible to eliminate most of the air from the test piece and achieve a high concentration of the gas detection tracer in the test part. This technique will ensure that any leaks will be sniffed successfully by the gas detector device, even when the test part is only filled to low pressures.