FWIW, this is how I purge a keg and closed transfer beer into it:
To reduce O2 intake at packaging:
1. Fill the keg with no foam sanitizer as much as possible and install the lid. Top off completely by using a bottling bucket with a spigot. Run the hose from the spigot to the out post using a black beer disconnect. Ensure the keg is tilted so the gas in post is the highest point. Attach a spare grey gas QD. Open the spigot on the bottling bucket until sanitizer comes out the gas QD. The keg is now full of sanitizer.
2. Use the same hose you filled the keg with but close the spigot, detach it from the bottling bucket spigot, and place the loose end in the bottling bucket. Pressurize the keg with 2-3 psi from a CO2 tank. Leave the QD attached to the gas in post to push out the sanitizer. Install the black beer QD. Once CO2 begins to bubble in the now full bucket of sanitizer disconnect the black QD. Disconnect the gas in QD from the keg.
3. Turn the keg upside down — this is important— by turning it towards the gas in post with the gas in post at the lowest point (slightly tilted). Any remaining liquid should be puddled at the gas in port. Using a spare gas in grey QD, install the QD to allow residual sanitizer to escape. (Shortened gas in dip tube helps with getting all the residual out) When all the liquid is drained quickly pull off the QD. Do not allow all the CO2 to escape. You just purged your keg with CO2.
4. Using the same hose you used to fill and drain the keg, attach the black QD onto the liquid out post and while gas is escaping quickly attach it to the spigot of your fermenter. Tighten hose clamp. You just purged the line with CO2.
5. Attach a hose to the spare grey QD and install it on the gas in post of your purged keg. As CO2 is escaping attach it to the top of your fermenter. You just purged the line with CO2 and created a pretty darn close oxygen free closed loop.
6. Open drain on the fermenter, have a beer. Wait until beer comes out the gas in line. You might have to burp the line at the top of the FV to get things moving. I like to lay my keg on its side with the gas in post high for this. Once the FV has completely drained or beer comes out the gas in post, you now have a full keg of beer that displaced the CO2 in the keg by pushing it into the fermenter. Remove the gas in QD then the beer out QD.
7. Once full, with keg upright and level, apply a small amount of CO2 pressure to the keg and install a tap. Drain a bit of beer until you no longer hear CO2 bubbling in beer (~ a pint or so). You just created CO2 filled headspace.
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