I purge it by loosening the TC between the brink and the butterfly valve and running in gas for a while but a PRV might be easier. But how would you purge the gas out of the downcomer tube? I think that would trap some air that wouldn't be easy to purge without loosening the TC anyway. Maybe the PRV would work if it could be at top of the downcomer pipe near the TC connection to the fermentor.
Still that is not the issue that pushed me to try this setup. The issue is getting sufficient quantity of the hop pellets up into the fermentor. With the 3" brink I've been able to push in 4oz at a time. The first addition is not too bad. Following Jaybird's suggestion to trim half an inch off the bottom of the downcomer pipe I could get the first 4oz into the fermentor in an hour or two. Load the hops, purge the hops, flood the brink, wait 30-45 minutes and then push. That would get maybe 3/4 of the hops into the tank, so I'd flood the brink again, wait another 30 min and push, now most of the hops are into the tank, might need one more cycle. Now for the next 4oz. Clean the brink and let it drip dry. Add 4 more oz hops. Purge and flood. Wait and push. Oops even waiting an hour on the second batch of hops they don't want to be pushed up...what is going on...more pressure...lots more pressure (what is this sight glass rated for anyway? I've gone pretty high but it scares me to do that)...open and close the butterfly...finally they start to move...maybe 1/4 goes up...flood again, wait..push...maybe repeat 6 times to get 80-90% of the hops into the fermentor. Have to bleed CO2 out of the headspace will all that pushing. What is going on? My theory is that while the first slug of hops gets up there ok, when you flood the second slug of hops you are flooding it with a mixture of beer and hops. Maybe the answer is you can push 4oz of hops first go and only 2oz at a time after that.
So I've got 16-17 gallons of beer in the tank. Typical dry hop quantities in current IPA recipes run 1-2 oz per gallon (and I've seen higher than that) I need to get 1-2 pounds into the fermentor. Just not going to happen. I think best I've managed was about 12oz and that took a couple days.