Do you purge the keg at serving pressure?

[Joeymacca]

Hi everyone. Kind of a random question here. I carbonate my beer at around 30 psi for a few days and then reduce the pressure to serving pressure, usually around 12 psi. What I’m not quite sure of is whether I should be purging the keg when I reduce the pressure.

My common sense is telling me that if I reduce the pressure but I don’t purge the keg I still have 30 psi of pressure in the keg. I see a lot of posts about “reducing to serving pressure” but it’s never mentioned whether the keg should be purged at that time to release the higher pressure so that it is replaced with the lower pressure. Again, this does feel a bit like common sense, but I just thought I would post here to see if I’m thinking about this the right way. Thank you.

 

[Franktalk]

I think the assumption is that only the head space in the keg is 30psi, and that after you reduce the pressure to 12psi eventually the keg will reach equilibrium by the beer absorbing some of the pressure in the head space. If your serving pressure is 12psi then the whole keg will level out at 12psi. So probably no need to purge unless you want to pour a beer right after you drop the pressure.

 

[VikeMan]

I think the assumption is that only the head space in the keg is 30psi, and that after you reduce the pressure to 12psi eventually the keg will reach equilibrium by the beer absorbing some of the pressure in the head space. If your serving pressure is 12psi then the whole keg will level out at 12psi. So probably no need to purge unless you want to pour a beer right after you drop the pressure.

What matters at this point is not really the momentary pressure in the headspace, but rather the volumes of CO2 already in the beer. 30 PSI for "a few days" (like, more than 2 or maybe 3) at typical serving temperatures may well have overcarbonated the beer. In that case, you'd need to release pressure repeatedly (or use a spunding valve) to get rid of the excess CO2.

 

[day_trippr]

I do not believe the point of releasing head space pressure has anything at all to do with carbonation. Instead, it's to avoid possibly backflowing beer up the gas line when the pressure is reduced to chart pressure (aka "serving pressure").

Best practice is to close the regulator shutoff valve, release the keg head pressure, dial down the regulator setting to "chart pressure", then open the regulator shutoff valve...

Cheers!