Flushing keg with fermentation CO2 - what am I doing wrong?

[IanMC]

Hi everyone,

I'm trying out something new for the first time, and having some unexpected results. I've read about people flushing their kegs with CO2 produced during fermentation by hooking the blowoff into the keg gas in, and then connecting the liquid out to a tube in a jar of StarSan. Basically, it's just an extended blowoff tube with the keg in between, which gets continuously flushed with CO2. Since I'm also planning on spunding it at the end of fermentation, this should also make the transfer super easy as almost everything is already hooked up and the keg and transfer tube are already sanitized.

That said, something seems to be going wrong. After 2 days of fermentation, my SG is dropping as expected, but I'm getting zero bubbles in the sanitizer jar. The gas must be escaping somewhere, as it hasn't blown up, but it doesn't seem to be coming out of the keg. I've checked all the connections and they seem to be fine. For anyone else who's done this before, is there something I'm missing? Have you had similar experiences? Could the increased back-pressure of the keg-blowoff-tube-jar system be too much, and it's forcing the CO2 to vent elsewhere (around the seals, perhaps)?

Thanks!

 

[IanMC]

For reference, here's a picture:

 

[Vale71]

Corny kegs often don't seal well unless you apply some positive pressure to firmly seat the lid. If you didn't do that then it's likely the CO2 is escaping from an improperly sealed lid.

 

[IanMC]

Good idea. I believe it should be sealed, as I pushed StarSan out of the keg (with a CO2 tank) before hooking it up here. It's possible that the seal got lost at some point. Would it remain sealed even after the pressure is removed?

 

[Vale71]

Good idea. I believe it should be sealed, as I pushed StarSan out of the keg (with a CO2 tank) before hooking it up here. It's possible that the seal got lost at some point. Would it remain sealed even after the pressure is removed?

Hard to say. This type of lid is really designed to seal with some amount of positive pressure as the latching mechanisms itself exerts relativiely little force so it's all a bit hit-and-miss.

 

[Mike_kever_kombi]

Good idea. I believe it should be sealed, as I pushed StarSan out of the keg (with a CO2 tank) before hooking it up here. It's possible that the seal got lost at some point. Would it remain sealed even after the pressure is removed?

If you filled the keg with starsan and pushed it out with CO2 then you already have a purged keg (provided you filled keg completely with starsan), and you are not accomplishing anything more by hooking it up to fermentation vessel.

 

[McKnuckle]

One reason to bother with this setup in the first place is so you don't have to purge the keg in advance, using CO2 from your tank. The fermentation does it for you. Rinse with Star-San, dump it out, seal up the keg, and go from there.

Gas will always flow to the path of least resistance, so it's likely there's a leak. I've had bad luck with MFL connectors in my setup not sealing, so I replaced them with QDs that just have a barb on them. Make sure your poppets and posts are all installed correctly and screwed down tight.

Drop the line coming out of the fermenter into that blow-off jar to confirm the seal on the pre-keg parts of the system.

Also, a minor point that probably doesn't matter; next time you do this, pipe the CO2 into the liquid post so it enters via the bottom, and put the blow-off on the gas post.

 

[Baglorious]

(Edit: I replied at the same time McKnuckle did regarding the 'order'.)

I doubt this is the critical failure point here, but I believe you want to run the blowoff tube (from the fermenter) to the liquid port, so that the gas is entering the keg from the bottom. The flow of the gas in the keg is bottom (entering) and exiting out the top (gas port).

I won't re-open the "CO2 is heaver than oxygen" debate (though it is true) as they apparently mix. However... still better, in my opinion, to introduce the gas from the bottom.

I'd check the seals issue first (as others have suggested), but would also switch your gas flow.

Also... have you tried, just for testing purposes, running the blowoff tube into a vessel of starsan (just like normal) to ensure the fermentation is, in fact, active? I know you said the samples show the gravity is dropping... but I'd still check that to ensure all appears normal. The leak could be from the fermenter lid itself. (I've heard the gaskets on the screw on style fermenters can occasionally be problematic? I've got one of those, and am always a bit concerned about it.)

 

[Deadalus]

How well does that rubber stopper sit in the fermenter lid? Might have bumped it.

 

[mattdee1]

I love this concept and am currently trying it out with a plastic bucket fermentor. Honestly, I have low confidence it will work with my setup. Since the flow path of the CO2 from the fermentor to the jar of starsan includes a few feet of tubing, a liquid dip tube, and a bunch of fittings, I'm afraid this will create enough "back pressure" to cause the fermentation pressure to leak out elsewhere - such as around the lid seals on the bucket. If I don't see bubbles in that jar in the next day or so, I guess I'll just have to stick with liquid purging, which wouldn't be the end of the world because it has worked very well for me so far.

 

[IanMC]

Thanks, everyone, for your thoughts and advice. Here are my replies/updates:

1) What I said before was misleading - I didn't fully purge the keg. I just filled it with a bit of StarSan, sloshed it around to coat everything, and then blew it all out to sanitize the liquid out dip tube, post, quick disconnect, and transfer tube. I'm sure there was still a lot of regular air in there afterwards.
2) It seems that the seal was compromised around the rubber stopper. I'm considering buying a solid Fermonster lid and using a weldless MFL bulkhead (already have this, so no extra cost) instead. Not sure yet what I'll do to accommodate the thermowell.
3) Regarding the CO2 to gas/liquid post: I believe you are correct that it would be best to do it that way. My rationale was that since CO2 and air mix fairly freely, and since fermentation creates such a large volume of CO2, it would fully flush out the keg regardless of orientation. This may be ill founded.
4) I have a Tilt inside the fermenter which was confirmation fermentation proceeding as planned. The SG dropped from 82 to 28 over 2 days, so I'm not too worried about that.

With my SG approaching expected FG, I decided to rack to the keg for spunding this morning. That didn't go too well, as I'm pretty sure the fermenter spigot clogged up right away and I only got about a quart of it out before I had to switch to my autosiphon. I tried using CO2 from my tank to force it out, but I was afraid to take it above 2 PSI or so, given that the Fermonster isn't rated for pressure. So much for a closed transfer... I'm hoping that the still-active yeast will take care of whatever oxygen got in, however. Maybe I should have called this thread "Misadventures in oxygen mitigation" instead.

 

[IanMC]

I love this concept and am currently trying it out with a plastic bucket fermentor. Honestly, I have low confidence it will work with my setup. Since the flow path of the CO2 from the fermentor to the jar of starsan includes a few feet of tubing, a liquid dip tube, and a bunch of fittings, I'm afraid this will create enough "back pressure" to cause the fermentation pressure to leak out elsewhere - such as around the lid seals on the bucket. If I don't see bubbles in that jar in the next day or so, I guess I'll just have to stick with liquid purging, which wouldn't be the end of the world because it has worked very well for me so far.

Please keep us updated on how this goes! It sounds like we're in much the same boat. Would you mind posting a picture of your setup? I think my gas out connection was a bit too convoluted. I had a 3/8" barb pushed into a hole in my #10 silicone stopper. This barb was then connected to a 3/16" silicone tube (the only thing I had on hand, despite the odd size), then to a 1/4" barb on a metal carbonation cap, connected to a gas QD, through a braided vinyl gas line to another gas QD on the keg post. I think, next time, I will just try the gas QD on the keg to a flare connection directly on a solid lid.

 

[mattdee1]

Please keep us updated on how this goes! It sounds like we're in much the same boat. Would you mind posting a picture of your setup? I think my gas out connection was a bit too convoluted. I had a 3/8" barb pushed into a hole in my #10 silicone stopper. This barb was then connected to a 3/16" silicone tube (the only thing I had on hand, despite the odd size), then to a 1/4" barb on a metal carbonation cap, connected to a gas QD, through a braided vinyl gas line to another gas QD on the keg post. I think, next time, I will just try the gas QD on the keg to a flare connection directly on a solid lid.

I'll definitely let you know what happens, but like I said, confidence is not very high it will work.

I have a rubber airlock grommet on my plastic bucket fermentor lid (the same kind commonly used for airlocks.) Inserted in that grommet, I have the bottom piece of a standard 3-piece airlock, with a short length of 1/2" I.D. silicone tubing connected to the top of it. From the 1/2" tubing I transition to some red 5/16" tubing (the type normally used for CO2), which terminates with a QD attached to the serving keg liquid post. The gas post of the serving keg has a QD attached with a very short (8" or so) jog over to a small flask of starsan. In other words, all tubing is at least 5/16" I.D. and the runs are not too long, but intuitively I still feel like it's too much resistance for this to work as intended. I hope I'm wrong, though. I'll post an update in the next day or so once I've got positive ID on a fermentation in progress.

 

[IanMC]

I'll definitely let you know what happens, but like I said, confidence is not very high it will work.

I have a rubber airlock grommet on my plastic bucket fermentor lid (the same kind commonly used for airlocks.) Inserted in that grommet, I have the bottom piece of a standard 3-piece airlock, with a short length of 1/2" I.D. silicone tubing connected to the top of it. From the 1/2" tubing I transition to some red 5/16" tubing (the type normally used for CO2), which terminates with a QD attached to the serving keg liquid post. The gas post of the serving keg has a QD attached with a very short (8" or so) jog over to a small flask of starsan. In other words, all tubing is at least 5/16" I.D. and the runs are not too long, but intuitively I still feel like it's too much resistance for this to work as intended. I hope I'm wrong, though. I'll post an update in the next day or so once I've got positive ID on a fermentation in progress.

Awesome. Your setup definitely sounds more direct and less restrictive than mine. Good luck!

One question: how do you transition from the 1/2" silicone to 5/16" gas tubing?

 

[mattdee1]

Awesome. Your setup definitely sounds more direct and less restrictive than mine. Good luck!

One question: how do you transition from the 1/2" silicone to 5/16" gas tubing?

The red 5/16" tubing has a 1/2" O.D. and "friction fits" right into the 1/2" I.D. silicone tubing. I make sure I have 1.5" - 2" overlap and use a gently tightened hose clamp. Actually, the hose clamp hardly seems necessary; the fit is pretty snug.

I use this trick elsewhere as well, for example, on the suction side of a self-priming pump I use to move star san and line cleaner around. Works great, no visible air bubbles getting in. It sounds cheesy to just connect 2 kinds of tubing this way but it works just fine so long as you're not having to deal with any significant pressures, which we are definitely not in this application. I suppose the "correct" way to do it would be to find some kind of 5/16" x 1/2" barbed reducer, but those kinds of obscure fittings are always expensive for what they are, and it seems like a waste of $10 - $15 for this use case.

 

[IanMC]

The red 5/16" tubing has a 1/2" O.D. and "friction fits" right into the 1/2" I.D. silicone tubing. I make sure I have 1.5" - 2" overlap and use a gently tightened hose clamp. Actually, the hose clamp hardly seems necessary; the fit is pretty snug.

I use this trick elsewhere as well, for example, on the suction side of a self-priming pump I use to move star san and line cleaner around. Works great, no visible air bubbles getting in. It sounds cheesy to just connect 2 kinds of tubing this way but it works just fine so long as you're not having to deal with any significant pressures, which we are definitely not in this application. I suppose the "correct" way to do it would be to find some kind of 5/16" x 1/2" barbed reducer, but those kinds of obscure fittings are always expensive for what they are, and it seems like a waste of $10 - $15 for this use case.

Ah, I see. I've done something similar to this before, with PE tubing pushed into vinyl, and it was extremely airtight. Good idea!

 

[IanMC]

That pic was taken when I was experimenting with this rig, I tried it both ways. I've been wondering if anyone was going to ask about that picture. What I settled on was having the fermentation gas come in the black/beer post, and the grey/gas post being used for the airlock. Like this:

View attachment 632556

The reason I came to that conclusion was not due to concerns about gases mixing. The gases are going to naturally diffuse and mix -- whatever you do. CO2 will not stay as a separate layer on the bottom of the keg.

If the keg is completely dry and empty, it should not matter which post is used.

But the inside of the keg will not be dry and completely empty. It will have been freshly washed and rinsed with StarSan. What I found was that even with a thorough draining, there were enough drops of moisture to pool into the bottom of the keg and cover the end of the long (beer) dip tube. When I had fermentation gas going in through the grey/gas post, the gas would try to push that liquid up the long dip tube. That made enough back pressure to pop open the lid of my Big Mouth Bubbler fermenter (which is notorious for having a slippery lid when it is wet with StarSan).

When the gas comes in through the black/beer post, it just bubbles through that small amount of liquid and there are no back pressure issues, so that's the way to go.

I found the above piece of advice from @LittleRiver in the Pressurized transfers from Fermonster to corny keg thread (thank you!). This may have been my problem. I pushed all the StarSan out of the keg with gas, but left all the residue/bubbles inside, which would have certainly pooled at the bottom over time. I will definitely try piping the CO2 in the liquid out port next time.

 

[mattdee1]

Please keep us updated on how this goes!

Hey dude, just a heads up - I think I'm officially declaring my experiment a failure. The yeast are going crazy (WLP090) but no trace of bubbles coming out of the serving keg. It could very well be that CO2 is getting into that keg, but I'm not going to assume anything. Liquid purge it is.

In that thread you linked to, some guys were talking about using keg lube to improve the seal on their fermentors. I might try that just for the hell of it, but I'm pretty much resigned to the fact that if I want to use this method then I need to switch to a fermentor that can handle pressure better than these buckets.

Good luck

 

[McKnuckle]

Keg to keg is really the best, since there's a minimum of mismatched fittings and tubing involved (i.e. none). You just hook them up with QDs and hoses and you're good to go. Save your other fermenters for less oxygen-sensitive brews.

I think I've posted this pic a couple of times on other threads, so forgive me, but... fermenter is on the right (3 gal keg), eventual serving vessel on the left (2.5 gal keg):

 

[IanMC]

Hey dude, just a heads up - I think I'm officially declaring my experiment a failure. The yeast are going crazy (WLP090) but no trace of bubbles coming out of the serving keg. It could very well be that CO2 is getting into that keg, but I'm not going to assume anything. Liquid purge it is.

In that thread you linked to, some guys were talking about using keg lube to improve the seal on their fermentors. I might try that just for the hell of it, but I'm pretty much resigned to the fact that if I want to use this method then I need to switch to a fermentor that can handle pressure better than these buckets.

Good luck

Damn. Two strikes against this method. I'm pretty sure I've seen other people do it with these kinds of fermentors, which gives me reason to continue trying, except for...

Keg to keg is really the best, since there's a minimum of mismatched fittings and tubing involved (i.e. none). You just hook them up with QDs and hoses and you're good to go. Save your other fermenters for less oxygen-sensitive brews.

I think I've posted this pic a couple of times on other threads, so forgive me, but... fermenter is on the right (3 gal keg), eventual serving vessel on the left (2.5 gal keg):

View attachment 668150

Maybe this is the way to go. I only do small (1-3 gallon) batches these days anyway, and I have a 5 gallon keg that doesn't get much use. I could ferment in that and use my 2.5 gal keg for serving.

McKnuckle, in the picture you showed here, are you purging the serving keg with fermentation CO2? What is going on with your gas out on the serving keg? Is that a spunding valve where the overpressure output goes into an airlock of sanitizing solution? If so, I've never seen one like that. The one I have is just the standard kind with a circle of holes around the circumference of the main shaft. Is a liquid airlock even necessary in this situation, as the >1 atm pressure in the kegs should keep anything from entering? Do you ferment under pressure the whole time? What do you do to mitigate trub pickup when transferring? I'm sorry for all the questions. I'm getting excited about the possibilities and simplicity of this system.

Thanks!

 

[Dland]

As has already been mentioned, corny kegs lids seal best with positive pressure inside. But since you can't do that and have fermentation CO2 flow in, perhaps try some food grade grease or lube such as CIP film on main keg seal, it does not take much.

 

[mattdee1]

As has already been mentioned, corny kegs lids seal best with positive pressure inside. But since you can't do that and have fermentation CO2 flow in, perhaps try some food grade grease or lube such as CIP film on main keg seal, it does not take much.

I find it works well if you put some keg lube on the big o-ring for the keg lid, close up the keg, and hit it with a little bit of CO2 pressure to "seat" the lid. Once you do that, even after you release the pressure from the keg, the lid seems to maintain a decent seal.

 

[bleme]

Williams Brewing also sells extra thick and squishy lid rings that can make getting a seal easier.

 

[McKnuckle]

McKnuckle, in the picture you showed here, are you purging the serving keg with fermentation CO2? What is going on with your gas out on the serving keg? Is that a spunding valve where the overpressure output goes into an airlock of sanitizing solution?...is a liquid airlock even necessary in this situation, as the >1 atm pressure in the kegs should keep anything from entering? Do you ferment under pressure the whole time? What do you do to mitigate trub pickup when transferring? I'm sorry for all the questions. I'm getting excited about the possibilities and simplicity of this system.

@IanMC Yes, I am pumping ferm CO2 into the serving keg from the beginning of fermentation. I rinse the serving keg with sanitizer, dump it out, clamp on the lid, and hook up the hoses as shown.

One caveat; so far I have only brewed lagers with this setup (at 67F). If I was brewing with ale yeast and wanted to encourage esters, I might delay setting pressure for 2-3 days.

The spunding valve pictured is a SPUNDit, a really nicely engineered gadget sold on Ebay by a guy in California. The airlock is included, and it attaches to a special connector on the end. This valve is of the diaphragm type, which is more precise to set than the more common variety.

Is the airlock necessary? No, but it shows you when pressure is exceeding the set value, and gives some insight into the activity level just like a normal airlock does.

Yes, I set pressure to 12 psi and leave it. Fermentation takes off as it does, and a little bit of time elapses before pressure builds up, so technically there are some hours fermenting with little to no pressure. But then I keep it pressurized until it's time to transfer.

For trub pickup avoidance in the fermenter, I have the dip tube passing through the lid of a stainless hop canister. The canister's bottom provides maybe 1/2" of clearance from the keg floor, limiting the amount of muck that comes over. Here's a pic of what's left after transfer:

 

[IanMC]

Williams Brewing also sells extra thick and squishy lid rings that can make getting a seal easier.

Nice! I may get one of these for one of my older kegs with a slightly warped lid opening. Thanks for the tip.

@IanMC Yes, I am pumping ferm CO2 into the serving keg from the beginning of fermentation. I rinse the serving keg with sanitizer, dump it out, clamp on the lid, and hook up the hoses as shown.

One caveat; so far I have only brewed lagers with this setup (at 67F). If I was brewing with ale yeast and wanted to encourage esters, I might delay setting pressure for 2-3 days.

The spunding valve pictured is a SPUNDit, a really nicely engineered gadget sold on Ebay by a guy in California. The airlock is included, and it attaches to a special connector on the end. This valve is of the diaphragm type, which is more precise to set than the more common variety.

Is the airlock necessary? No, but it shows you when pressure is exceeding the set value, and gives some insight into the activity level just like a normal airlock does.

Yes, I set pressure to 12 psi and leave it. Fermentation takes off as it does, and a little bit of time elapses before pressure builds up, so technically there are some hours fermenting with little to no pressure. But then I keep it pressurized until it's time to transfer.

For trub pickup avoidance in the fermenter, I have the dip tube passing through the lid of a stainless hop canister. The canister's bottom provides maybe 1/2" of clearance from the keg floor, limiting the amount of muck that comes over. Here's a pic of what's left after transfer:

View attachment 668195

Thanks for explaining everything, @McKnuckle. This is all very helpful and exciting. I can't believe I didn't realize previously the benefits of fermenting in a keg... it just seems so easy and precise. The only bad thing is that now I feel bad for my Fermonsters... I just got them, and don't think they'll be seeing a lot of use from now on. Also, I wish I knew about the SPUNDit before I built my one, but what are you going to do? There will always be something better out there.

Thanks again!

 

[McKnuckle]

There will always be something better out there.

Ain't it the truth? But perfect is the enemy of good! So there's that.

Kegs just need a few relatively low cost components to convert them into fermenters. I am really glad to have discovered them, too. Can't say that every attempt leading to this point was flawless for me - hardly - but I seem to have landed on a setup that works (knock wood).