I have a question concerning simultaneous sampling of water and gas phase.

The situation is a pipe (diameter somewhat around 15 cm) where a mixture of water and gas is flowing through (two-phase flow). Assuming that the flow is separated and the gas phase is moving on top, what will happen if I open a sampling tap at the bottom of the pipe? The flow is induced by a pump and the pressure in the system is above ambient (20 bar).

My assumption is, that I only sample the water and the gas phase on top will not leave the pipe unless I allow the gas phase to "connect" with the tap by extracting a big amount of fluid.

The problem is maybe comparable to a bottle of sparkling water, where I drill a small hole at the bottom. Will I only sample the fluid phase or will I sample a mixture of both?

I hope somebody has an answer. Thanks in advance!

  • $\begingroup$ In your sparkling water bottle example, assuming it is capped tight, some water would leak out and somewhat of a vacuum would form. Would that be the case in the flowing system or is there some pressure associated with (i.e. causing) the flow of the fluids? In other words, is this an open flow system, or sealed, or under some pressure, etc. Some clarification of the physical setup with respect to this issue would be helpful. $\endgroup$
    – airhuff
    Commented Sep 28, 2017 at 18:26
  • $\begingroup$ Thanks. Added the conditions of the physical setup. Concerning the vacuum: wouldn't the gas phase on top just expand as the volume for the gas phase increases? $\endgroup$
    – Thorsten
    Commented Sep 29, 2017 at 6:41
  • $\begingroup$ Of course it would. $\endgroup$ Commented Sep 29, 2017 at 7:27

1 Answer 1


That's a tricky question.

Given your setup, it follows that you will only be sampling water (not simply "fluid", mind you; fluid is a common term for both gas and liquid). But gases might be more or less soluble in liquids. Chances are that your water is saturated with some gas (say, $\ce{CO2}$) and pressurized with the same gas in the pure form. Then, as soon as you take your sample and put it in a sealed (or worse yet, unsealed) container, it will release some gas, and so the composition of the sample will become different from that of the liquid in the pipe.

Whether or not this issue is relevant to your situation, is up to you to assess.

  • $\begingroup$ Thanks for the answer and the clarification about "fluid". As a non-native I was not aware of this difference. The current sampling setup is taking account of the gas release which is occuring due to sampling by a gas counting meter (it's the volume I am most interested in). I was just asking myself if the sample is representative for the whole system, which I can now deny. $\endgroup$
    – Thorsten
    Commented Sep 29, 2017 at 16:13
  • $\begingroup$ Well, if you take account of the released gases, then your sample is OK as long as we are talking of the liquid. Then again, the very intention manifested in counting the gas implies that you're interested in it, hence you might want to know the gas/liquid ratio, which you won't be able to know by sampling the liquid alone. $\endgroup$ Commented Sep 29, 2017 at 18:35

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