When it comes to thermal imaging, temperature is not the only criterion to be considered.
Different gases emit different wavelengths and these wavelengths may or may not be picked up by thermal imaging. Passive or active thermal imaging will detect different wavelengths.
Also these wavelengths may be detected under pressure differently than pressure at a release point.
So, wavelength along with temperature is what is needed.
Pipeline X: Gas at 20°C. It is in wavelength A which cannot be detected by thermal. The test of the thermal would be that the thermal cannot “see” the temperature.
Pipeline Y: Gas at 20°C. It is in wavelength B which can be detected by thermal. The test of the thermal would be that the thermal can “see" the temperature.
MOBOTIX thermal sensors can detect the wavelengths in the following range: 7.5 - 13.5 μm
Most of the common gases emit in the following range: 3 - 5 μm
This is the reason why they are “invisible” to our cameras. Just to use an analogy, it’s like IR light in combination with day sensors.
However, it may be possible to see the “side effect” of a gas leak. For example, when the gas runs in a pipe under pressure and there is a crack on the pipe, the gas tends to expand quite rapidly; by doing that it gets colder and it cools down the surface of the pipe. Hence it may be possible to see a “cold” spot surrounding the leaking point.
Here is a very interesting read by FLIR: