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I think I know the answer to this, but I'd like to see an answer listing more conclusively the different reasons why this happens. Some causes I've seen listed:

  • High pressure plus shaking can cause beer to get into the CO2 coupler
  • Dual CO2 regulator hooked up to two kegs, one has higher pressure than the other due to switching gas on and off and regulating at different pressures, pushing beer out when both are opened

This just happened to me and I didn't realize my dual regulator didn't have check valves in the shutoff valves. Luckily beer only got as far as the valves, but not all the way up into the regulator, but I just ordered some new valves with check valves and will be swapping them out when they get here.

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Pressure will attempt to equalize. By that I mean if you have one container with PSI of 10 and a PSI of 15 in another, then pressure will become 12.5 in both containers. In doing so, the contents (beer) will end up traveling to the vessel of lower pressure (the CO2 regulator) from the vessel of higher pressure (the keg).

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  • I think something like this is what happened. I later realized that one keg was empty and when I opened the tap to pour a beer, CO2 and a little beer sprayed out violently. I think this spray was a release of CO2 from the keg without beer, which lowered the pressure in the keg and caused beer to flow through to the other keg. I'm not sure which keg it came from, because I do have a carbonation stone with a tube in one keg, but I checked it and it looked clean - didn't seem like beer got sucked into the tube.
    – paul
    Sep 11, 2012 at 3:55
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Beer will come out of the gas line when both of these conditions occur:

  • The pressure in the keg is higher than whatever's on the other side of the gas line.
  • There is beer in the gas dip tube or post (either because you've overfilled the keg or because the keg was shaken or lay on its side).

I once over-filled a keg such that the gas dip tube was partly submerged. I then tried to vent the keg by pressing the gas poppet (pin lock keg) and beer shot everywhere. If I'd hooked up the gas side to a regulator set to a lower pressure than the keg, beer would've flowed up the gas line.

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  • So if you're carbonating with a carbonation stone at 10 psi then lower to 5 psi would that happen?
    – paul
    Sep 7, 2012 at 5:26
  • @paul: only if you have no headroom, so beer is in the gas tube
    – baka
    Sep 7, 2012 at 12:21

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