Drowning in foam...

[The-Engineer-That-Brews]

I've heard about this 'balancing lines' business - is that what I need to do here, or am I doing something else wrong, e.g. over-carbonating?

I'm conditioning my bitter in a Corny under 20psi of external CO2. The problem is that when I drawing off a bit for 'testing' purposes (with a Pluto beer gun) I get 60-80% foam. When this settles out after a minute or two I get more like 30% foam and the beer itself seems fine - just a few bubbles on the tongue which is just right as far as I'm concerned.

What I do is turn off the gas, and vent the PRV. Then I pull the trigger on the beer gun (nothing comes out of course) and open the cylinder just a crack... This results in a nice slow pour, but I can see the beer changing to foam inside the liquid line and that's what it's like when it comes out of the tap. I can get a very nice pint by over-filling and letting the foam overflow, but it seems pretty wasteful...

Help...?!

 

[MickDundee]

How long have you been force carbonating it? 20PSI is quite high if you are using the “set and forget” method, especially for a bitter. I have my bitter sitting at 2.5PSI, and my NEIPA and lager at about 10PSI.

This carbonation chart might help you:

 

[The-Engineer-That-Brews]

Awesome! Great chart - very useful.
I've had it under 20psi for around a week I guess - I'll knock it down to 15psi maybe now you've said that tho, as it's about 17ºC in there

 

[Buffers brewery]

https://www.brewersfriend.com/2009/...our-kegged-beer-co2-line-length-and-pressure/

 

[The-Engineer-That-Brews]

https://www.brewersfriend.com/2009/...our-kegged-beer-co2-line-length-and-pressure/

Hmmm there's a lot of sense in that article. But IMHO it doesn't have the whole story. I look at it like this:

The objective is to transport beer from the pressurised environment of the keg (let's say 20psi) to the glass (atmospheric), reasonably quickly and without bringing too much CO2 out of solution. More specifically not bringing too much CO2 coming out of solution in one place - because if that happens then you've got foam in the line from that point onward.

Basically the beer is travelling along a tube. One end of the tube is at keg pressure and the other at atmospheric pressure. The tube may not have constant diameter, and in particular it may have localised pinch points (taps) to control the flow.

So as beer moves along the tube its pressure steadily reduces. As this happens, small bubbles of dissolved gas form and are carried along in the flow. This is not a problem however, so long as the bubbles are reasonably well spaced out.

One way that the problem can be caused, is to have a pinch point (part open tap) part way along the line. You can very easily observe this, if you have transparent lines: put an inline tap halfway along, close it almost completely and then let the beer flow. Immediately you will see the line downstream of the tap is full of foam. What's happening?

What's happening is that the tap localises the vast majority of the pressure drop to a single location. The beer in the line upstream of the tap is pretty much at keg pressure, and downstream of it everything's pretty much at atmospheric. At the tap itself, beer at keg pressure is squeezing itself through a small appeture and consequently moving at very high speed - therefore (Bernoulli's principle) it suffers a very rapid localised pressure drop and ALL the CO2 comes out at once... foam-tastic. Same thing happens when you have a beer gun at the end of the line and open it just a crack: glass full of foam.

So what's the antidote? Well we need to spread the pressure drop out, so that there's no big drop at any one point. That includes when it emerges from the serving spout: we need the pressure to be already pretty close to atmospheric before this point, so that the end tap can be reasonably wide open to avoid the Bernoulli effect.

This clarifies for me why most of the good advice in the article works; in particular putting a length of thin diameter line at the serving end of the line: in effect this is working like a distributed tap. However it also implies that you probably don't want to make the line too thin otherwise the bubbles may be forced too close together and create foaming.

Now I need to cogitate on all that for a bit, and decide how to design a good beer line.

Maybe what is needed is a way to control the flow rate that does not rely on fluid resistance at all - perhaps something like a peristaltic pump, but running in reverse. Hmmm....

 

[terrym]

The pressure drop through a few metres of small bore pipe or tube will not be significant at the dispensing rates normally experienced by homebrewers. What is important however are the constrictions in the line like flow control valves, and via the Venturi effect, pressure is lowered dramatically though the constriction allowing CO2 to comes out of solution, and although most of the pressure is recovered downstream of the constriction, the CO2 does not all return to solution quick enough, resulting in two phase flow otherwise known as foam at the point at which it leaves the dispensing nozzle.

 

[The-Engineer-That-Brews]

The pressure drop through a few metres of small bore pipe or tube will not be significant at the dispensing rates normally experienced by homebrewers.

Ah, so those people using a lenth of smaller diameter pipe to balance their line are wrong then..?

 

[Buffers brewery]

So, as you know, I don't have a corny. My King Keg currently has a pale ale in it (carbonated using priming sugar) and started at a pressure of 9 psi. The dip tube in the KK is 8mm ID silicone and the beer runs through a corny post. From the post I have a 90cm length of 3/8 beer line running into a beer tap. When the barrel pressure is 9psi the beer poured is 'lively'. As the pressure drops to 4-6psi the pour improved significantly with acceptable head and carbonation.
The question I'm asking is why do you need your corny pressure so high having completed carbonation?

 

[terrym]

Ah, so those people using a lenth of smaller diameter pipe to balance their line are wrong then..?

I didn't say that. The majority of the dp will be through the constrictions. As it usually is through most piping or tubing systems, unless you have high flow, and/or lots of sharp changes of direction as in a 90* bend or fitting, or a long line or a seriously restricted pipe diameter. Take your pick

 

[The-Engineer-That-Brews]

The question I'm asking is why do you need your corny pressure so high having completed carbonation?
Well to be honest before today I wouldn't have said I had yet completed carbonation... it's only been in there a few days and I've just let it stand (no rocking or rolling), and the surface area exposed to the gas is relatively small.
However the beer I've had out of it today has definitely got a slight petillance to it so I'm thinking it may be about 'done', and indeed I've now lowered the pressure :-)

Mind you, all the above being said I'm still a little perplexed why I've had so much foam out of it on the occasions where I completely de-pressurised the keg before serving...

 

[Buffers brewery]

The question I'm asking is why do you need your corny pressure so high having completed carbonation?
Well to be honest before today I wouldn't have said I had yet completed carbonation... it's only been in there a few days and I've just let it stand (no rocking or rolling), and the surface area exposed to the gas is relatively small.
However the beer I've had out of it today has definitely got a slight petillance to it so I'm thinking it may be about 'done', and indeed I've now lowered the pressure :-)

Mind you, all the above being said I'm still a little perplexed why I've had so much foam out of it on the occasions where I completely de-pressurised the keg before serving...

Sorry. I've just re-read your original post :tinhat:
Could it be temperature related? Is your bitter chilled or at room temperature?

 

[The-Engineer-That-Brews]

Sorry. I've just re-read your original post :tinhat:
Could it be temperature related? Is your bitter chilled or at room temperature?

not chilled - well, everything is in the garage at the same temp IYKWIM... I‘m pretty sure it will, as you say, improve as I reduce the keg pressure :-) thanks!

 

[Drunkula]

Only drop the pressure if it's to match the carbonation you want at the temperature you want. Then you use some sort of flow control to get a good pour for those settings. Easiest is 3/16" tube.

I like 2.3 vols of co2 and had the beer at 13c so I'd need 17 psi for that. After I got 2 metres of 3/16" choker line there was never a problem with foam even using a little plakky party tap.

If you're dropping the pressure to serve you've got a problem.