CORNEY
The thing I can't figure is why once I get past the beer in the line, it is fine.
If you have an equilibrium of 15psi in the headspace balanced with the gas dissolved in the liquid. Then 15psi pressure is required to keep the gas in the beer.
when you crack the cap on a plastic pop bottle you can see the bubbles immediately nucleate and come rushing to the surface because there is no longer sufficient pressure to hold the gas in the pop. If you re tighten the cap the bubbles will continue to break out until the pressure in the headspace establishes a new equilibrium.
You do not have enough counterpressure at 12psi to keep the gas from breaking out. Only the gas that breaks out in the line causes foam. the gas breaking out into the headspace of your keg is not affecting the pour. When it sits, the bubbles accumulate in the line. The bubbles then in turn cause turbulence when you pour and cause more breakout and foam.
As far as your dispensing system is concerned... I would advise against using 1/4" tubing. The industry standard is 3/16" ID vinyl tubing ('choke line', or "restriction line') which provides 3lb. of flow restriction per foot. You can dispense a higher carbonation beer by adding more restriction and increasing your dispense pressure to keep the gas from breaking out in the line.
Additionally, if you are slightly over carbonated, the smaller diameter tubing will purge more quickly, and you may still get a reasonable pour, because there will be significantly less volume of foam (ie: 10' of 1/4" tubing has a volume of 5.9 cubic inches, whereas 4' of 3/16" tubing has a volume of 1.3 cubic inches)
There are other restriction values for the hardware in the system (poppet valve, cornelius QD, faucet, # of feet of vertical rise etc. but the quick test is with the appropriate counter-pressure applied to your keg (sufficient to prevent breakout) your product should pour at 2oz. per second (7 seconds for a 16oz. glass yielding 14 oz of beer and 2oz. of head for example).
Carbonation values are closely linked to temperature as well. 2.4vol of CO2 at 38F only requires about 12psi (I could be off slightly I would need to consult a chart to verify precisely) but at 64F the same carbonation value requires about 26psi (previously mentioned caveat)
charts with the temperature to pressure relationship used to be available at mcdantim.com you may have better luck finding them than I've just had, but they do make a nifty FREE tool called the "McDantim Slide Rule" that you can also use for selecting your carbonation pressure for different temperatures (I use this thing all the time at work, and have since started using it for calculating my homebrew carbonation values) Send an e-mail to
[email protected] to request one.
I mention all of this in case you want to make a more efervescent beer in the future, or if you wish to serve a beer at a warmer temperature, but as previously suggested for your porter... shake the piss out of it, rest briefly then bleed off the pressure... do it 3 times, put 12psi back on after it settles and see how it pours. Repeat as necessary.