We have a leak in a network section that cannot be isolated, and so I am considering using helium.
Can you tell me under what conditions this technique is effective, and more importantly when it is not?
What restrictions might there be?
To use helium you need to know the exact layout of your pipe network, since detection is done above ground following the linear trajectory.
You will need an injection point upstream of the leak.
For watertight ground surfaces (cement or recently asphalted), you have to drill using a 10-mm bit every two to three meters in order to vacuum the ground and recover any helium that has escaped via the leak.
The operation requires that you inject the helium the night before, and remember that it persists in the ground for two to five days.
The injection time is usually about 1-2 hours (that's a function of the linear configuration and the pipe volume).
Helium detection can identify tiny leaks that can't be found using traditional methods, e.g., acoustic.
The procedure is performed on pipes in use without disruption of water supply or inconvenience to end users, e.g., food-grade helium.
Basically there are few restrictions but leak detection is time consuming.
Have you ever done this before? I'd be curious to learn more details if you're available - please let me know and I can call/email off fluks. I can only imagine the volume of helium can get to be quite big in water piping... although the detectors are fairly sensitive.
I would like to clarify a few points in fifi's reply, namely:
This technique is completely ineffective in clay, swampy soil or frozen ground.
Among the limitations, note that detection equipment is of a substantial size and is not standalone, so it requires a power supply for the vacuum pump and the mass spectrometer. This means the equipment has to be driven to the site; if you have a remote location not accessible by vehicle, this is going to be a problem.
And it can be expensive.
The detection unit itself is quite fragile and demands skilled and careful use. It is not the sort of equipment that can be handled by just anyone, given that maintenance of a mass spectrometer is pretty pricey as well.
Basically this technique requires special training to both master the injection of the gas and to preserve the quality of the water distributed to users during the operation.
If you want to invest in this equipment, be careful, don't forget that helium is a noble gas, the supply of which is running out. Its future for this sort of application is limited.