Vented pressure barrel

[stevic]

I have just inherited a pressure barrel. The cap is vented to let gas out but not air in. When i come to draw beer from the barrel will i need to loosen the cap to allow beer out or will the build up of pressure allow the beer out?

 

[JapanBrew]

But if the cap is vented, won't that make the beer flat?

 

[terrym]

Most typical homebrew PBs will have a cap that screws on and seals up the PB.
You will then normally pressurise the PB and the beer inside it by priming it with sugar that then ferments forming CO2 which generates an internal pressure in the PB and/or there is a valve in the cap which you use to inject CO2, typically from a cylinder or from one off shots from CO2 bulbs. All PBs should have a pressure relief device of some sort fitted in the cap to release excess pressure which protects the PB and in the worse case prevents it rupturing.
If you do not add priming sugar or CO2 you will not be able to get much beer out of the PB, before you either a) gurgle air in through the tap to balance the inside and outside pressures or b) vent the cap to balance the air pressures. Either way letting air into the barrel especially via a) will cause the beer to go off quite quickly.
If you are still unclear about your PB/cap set up I suggest you post a photo so that people can have a look and advise you.

 

[stevic]

Thanks for the replies. Sorry my question was a bit flaky. The cap is a pressure valve rather than my description of it being a vent. I will be priming the barrel with sugar but did not think this alone would be enough to "force" the beer from the barrel. It makes perfect sense, 24hrs later, not to introduce air to the barrel but last night i could not fathom liquid running out of a barrel without air coming in.

 

[JapanBrew]

At a certain point you'll need to add air pressure or gas to it or you'll being using the carbonation in the beer to release and push the beer eventually making it flat.