It seems as though there are two predominant methods of kegging: "saturation over time" and "high pressure and shaking". I have also seen only two references to another method: connecting the Gas In line to the Beverage Out post of a ball or pin lock keg and saturating the beer from the bottom over the course of about two days. It seems like a foolproof compromise between control and speed, but the lack of discussion about the method leads me to believe there may be a problem with it. Can someone with more kegging experience could discuss the pros, cons, and potential dangers of this method of force carbonation?
My experience has shown that going through the beer out line doesn't change the rate the beer carbs up. Whether using the 'set and forget' process, or the high PSI and shake method. The bubbles coming out the bottom really aren't increasing the surface ratio enough for it to be significant. The bubbles just rush up to the surface.
The downside to the technique is the real danger of beer coming down the gas line. If your lid loses pressure and the tank is below the keg for some reason if most definitely can happen. Its happened to me before which is why I have since abandoned the practice.
This method operates on the idea that the CO2 will come into contact with more surface area on the way up than it would by just being forced into the headspace and diffusing down. Similar to the idea behind sloshing the keg which also attempts to expose more surface area of the beer to the CO2.
Both this method and the sloshing method technically run the risk of getting beer into your gas line. This one is particularly worse because as you add pressure to the headspace that pressure is actively trying to force the beer up the tube and into the gas line. So if you have an imbalance between the pressure you've set your regulator to and the headspace you can pump beer into your regulator and really break it. There are one-way valves you can get to prevent or at least mitigate this but they're more expensive than standard keg hardware.
I've never had to replace a regulator but I have had to replace a gas line because it had beer in it and I didn't want to risk it.
I do sometimes hook the CO2 to the beer out post at like 30 PSI, but generally I'll disconnect it as soon as the gurgling begins to die down. Then I'll purge the headspace a few times and set the PSI to target and fridge the keg. If I'm antsy I'll leave it at 30 for about 5 minutes while gently rocking the keg...I don't want to risk beer getting into the gas in tube. Sometimes I'll also fill it with 30 PSI, then disconnect the gas line and shake the keg violently. Hook it back up and repeat a few times. Basically I'm paranoid. But it generally gives me passably carbonated beer when I try it after giving it a day or so to cool down.
If you're willing to waste some co2, you should do the following:
Make sure there's no pressure in keg.
Put Beer connector on co2 line.
Turn on co2.25 psi is good.
Connect. Wait for bubbling to stop as co2 enters dip tube and bubbles through to headspace.
Slightly open the relief, so that there is near constant barely audible bubbling.
I'd estimate this method takes 20 minutes or so. No rolling. In situ.
This will not force beer into the co2 line as long as you always make sure the pressure in the co2 line is higher than in the keg.
Side note: this kind of method will scrub some of the aromatics from your beer. If you're making a beer with significant esters/phenolics or big hop character, it's best avoided. Conversely, if you have a small sulfur problem, this can help.
I recently had accidently reversed my gas/liquid posts on my corny keg when I installed my co2 post onto my dip tube without realizing it. I then pressurized my beer at 18 psi for 3 days. After 3 days I went to check the carbonation levels and realized I had my posts reversed. After switching them back to the normal configuration I bumped my co2 pressure down to a serving pressure, about 6 psi. My beer turned out great with perfect carbonation results and I didn't have any issues with beer forcing its way into my gas line.