Leak detection in pressurized devices the sensitive test
September 3, 2020
4 MIN READ
Devices operating above atmospheric pressure, such as valves for the oil and gas industry, are an important application of leak detection. Other uses include checking the leak integrity of containers holding biological and nuclear substances, and even the performance of car tyres. Here's a quick overview of the sensitive test of a helium checker — fundamental to leak detection in pressurized devices.
Sensitive test of a helium checker
There are many methods of leak detection, such as a bubble test or pressure drop test, but none can compete with the sensitivity and the ability to pinpoint a leak that a Helium leak checker can offer. Additionally, a helium instrument allows for both quantitative and qualitative measurements.
A helium or, more precisely, a Helium 4 leak detector is essentially a tuned mass spectrometer which detects this gas and has the further option of identifying Helium 3 (used in cryogenics) and hydrogen. The unit has appropriate pumps supplied making it portable, with only mains electricity required. The unit is extremely sensitive to helium.
Investigating leaks in these circumstances can involve both a qualitative and a quantitative approach and just as in vacuum applications a helium leak detector is essential. Depending on the specification, leaks rates below 10-6 mbar l/s are frequently required. Helium leak checkers with an ultimate sensitivity of 10-12 mbar l/s offer the most accurate solution.
Why we use helium as the search gas
There are several key benefits of using Helium as the search gas:
It is non-toxic
Relatively low cost
Only 5 ppm occurs naturally in air
It is one of the smallest atoms, so small leaks can be investigated
It is a low-density gas which disperses easily
Related: Vacuum Science World shared a great article on Helium leak detection methods. Find the blog post here, Four Ways of Finding Vacuum Leaks Using Helium.
Two ways of using the helium leak detector
The schematic below shows the leak checker operating in a quantitative, or integral mode, to determine the total leak rate of a component. This value will determine the overall lifetime, say, of containers for hazardous gases and liquids. In the case of car tyres, in general their pressure is not checked on a regular basis. Thus, the correct leak rate will ensure the pressure loss is sufficiently low over a period of several months to guarantee that there are no safety issues.
You will notice that depending on the size of the test chamber, additional pumps might be required to pump the chamber to below 10-3 mbar and to ensure the unit is operating at its most sensitive range. However, this means not all the helium is detected by the leak checker. To determine the correct leak rate, the pumping ratios of the leak detector and chamber pumps must be factored in to give an accurate reading.
If the component is of sufficient value to warrant re-working, the actual leaks can be determined by operating the detector in what is known as “sniffer mode,” using a sniffer attached to the inlet port of the leak detector and pressuring the component with helium. The illustration below gives a typical set up:
In this mode, a probe with a small orifice is connected to the inlet of the leak detector by a flexible vacuum hose, typically 4 m in length. The size of the orifice governs the flow of detected helium so that the inlet pressure to the instruments is kept below 10-3 mbar, allowing accurate measurements of leak rates. This has the following advantages:
Exact location of leaks is determined
It performs in a qualitative mode
No additional pumps are required
Actual flow out of the object is measured, simulating real leakage from the device
A sniffer with an additional pump allows a hose length of 20 m in length
You will notice that the minimum detection level in this case is 1x10-7 mbar l/s. This is a consequence of air having a natural concentration of 5 ppm of helium, which gives a less sensitive detection level than the local mode described previously. However, this can be acceptable for many applications.
Two ways of using the helium leak detector
There's more to leak detection than just a Helium leak checker. Dive into more detail on our resource page (which you can also download as a free ebook) by clicking the link below.
Did you know leaks could be costing you thousands per day in lost process efficiency? Download out eBook on the fundamentals of leak detection and save yourself time and money.
