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Ideal Keg PSI for Pressurized Transfer after Spunding

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jbschuyler

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Hi All,

I am preparing for my first pressurized transfer after spunding a 5 gallon batch in my Spike Flex+ fermentor. Currently, the tank is sitting at 15psi. In the next couple of days I will cold crash down to about 35-40 degrees, and I'm guessing the tank pressure will drop to around 8-10psi as a result. After the cold crash I will be transferring into a keg that has been freshly purged of Starsan.

My question is about whether there is an ideal differential between the the fermentor pressure and the keg pressure to ensure a fairly quick transfer without foaming (which I have read about happening to some)?

For example if the fermentor is about 10psi should the keg be around 8? I know I don't want to go too low and have foam, thereby losing the natural carbonation (and the whole idea of spunding).

Thanks for any help on this!
 

Vale71

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No need to guess, just use the attached spreadsheet to predict final equilibrium pressure.

As to avoiding foaming issues, the receiving vessel should always mantain a pressure value slightly higher than the beer's equilibrium pressure. Let's say your FV is at 10 PSI with a maximum working pressure of 15 PSI. I'd pressurize it to 15 PSI and set the spunding valve in the receiving ket to at least 12 PSI.
 

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Thanks for the quick reply and the spreadsheet too - Very helpful info! I will check with Spike to get the total volume of the fermentor including the domed pressure lid.
 

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Thanks for the quick reply and the spreadsheet too - Very helpful info! I will check with Spike to get the total volume of the fermentor including the domed pressure lid.
Might be over-thinking a bit. The info that @Vale71 sent is interesting and valuable but probably not entirely necessary to do the transfer. I normally crash from 68F (for an ale) at the end of fermentation down to ~38F and transfer after several days at 38F. Pressure is usually 1 atm/14.7psig at 68F and "something less" at 38F. Since that's the temperature of my kegerator, I'm looking for a pressure that gives me the desired CO2 volume at that temperature. As long as the pressure initially in the keg and the pressure on the spunding valve is 1.5-2.0 psig less than the fermenter pressure I can complete the transfer fairly quickly without excessive foaming. Although any differential will cause flow, obviously more pressure differential will cause faster flow whereas less pressure differential results in less foaming. It's a balancing act which is as much art as it is science.

Pressurize the receiving keg slightly below your desired pressure at serving temperature. Then hook up your gas supply to the fermenter and set it to slightly above your desired keg pressure at serving temperature. Open the butterfly valve on your fermenter slowly to start the transfer. When the fermenter and keg pressures approach equilibrium, slowly open the gas supply of the CO2 tank attached to the fermenter so that "slightly above" serving pressure continues to transfer beer to the keg.

As long as the delta in pressures between the fermenter and the receiving keg are within 1 psig you'll have transfer flow within a reasonable timeframe. As long as the pressure differential is around 1½-2 psi your foaming should be minimal, assuming you are transferring in via the Fluid Out port on the keg into and underneath the increasing column of beer in the keg. When the transfer is complete connect the gas line to the IN post on the keg to top off the headspace pressure. Condition for 2-3 weeks and serve.

Which reminds me, I have to replace the keg that kicked last night before Covid Happy Hour with the IPA that's been conditioning for three weeks. There's always that level of anticipation when you tap a new keg. Cheers!
 
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Might be over-thinking a bit. The info that @Vale71 sent is interesting and valuable but probably not entirely necessary to do the transfer. I normally crash from 68F (for an ale) at the end of fermentation down to ~38F and transfer after several days at 38F. Pressure is usually 1 atm/14.7psig at 68F and "something less" at 38F. Since that's the temperature of my kegerator, I'm looking for a pressure that gives me the desired CO2 volume at that temperature. As long as the pressure initially in the keg and the pressure on the spunding valve is 1.5-2.0 psig less than the fermenter pressure I can complete the transfer fairly quickly without excessive foaming. Although any differential will cause flow, obviously more pressure differential will cause faster flow whereas less pressure differential results in less foaming. It's a balancing act which is as much art as it is science.

Pressurize the receiving keg slightly below your desired pressure at serving temperature. Then hook up your gas supply to the fermenter and set it to slightly above your desired keg pressure at serving temperature. Open the butterfly valve on your fermenter slowly to start the transfer. When the fermenter and keg pressures approach equilibrium, slowly open the gas supply of the CO2 tank attached to the fermenter so that "slightly above" serving pressure continues to transfer beer to the keg.

As long as the delta in pressures between the fermenter and the receiving keg are within 1 psig you'll have transfer flow within a reasonable timeframe. As long as the pressure differential is around 1½-2 psi your foaming should be minimal, assuming you are transferring in via the Fluid Out port on the keg into and underneath the increasing column of beer in the keg. When the transfer is complete connect the gas line to the IN post on the keg to top off the headspace pressure. Condition for 2-3 weeks and serve.

Which reminds me, I have to replace the keg that kicked last night before Covid Happy Hour with the IPA that's been conditioning for three weeks. There's always that level of anticipation when you tap a new keg. Cheers!
Thanks Brooo Brother. This is very helpful too! And Yes- I can't wait to tap this new keg either!
 
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@Vale71 is there any way to do closed loop transfer of carbonated beer from unitank? Having that unitank completely full of 15 PSI CO2 at end of transfer while I'm just bleeding off CO2 from my purged kegs always seems wasteful.
Hey, @bracconiere, how many ounces /grams of CO2 is Eric worrying about?
 

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Hey, @bracconiere, how many ounces /grams of CO2 is Eric worrying about?

well according to @doug293cz you use psi absolute, which is psi gauge, added to ATM pressure for your elevation.. then like multiply that by the weight of co2 at normal pressure..


hold on let me reference the calc some one else tiped me too for 5 gallons of co2...wait i remeber it off-hand, 1.23 oz's so assume ballpark 14psi ATM, 15+14=29...ah, **** dougs, post was kinda long winded going to have to look it up quick!

so then you dived the psia by the psig so 15/14=1.07 volumes co2, multiply that by 1.23oz's to get weight, hope i didn't reteach my lesson wrong

edit: and at my $27 for a 20lb swap, which usually only has 18lb's of co2 in it. 18x16=288oz's, so 1.3/288=4 tenths of a cent, but it's really the inconvience of having to stay sober long enough to swap the tank that i care about!

edit: i had to reread it again, temp matters too! did you cold crash? unfortantly my weight calc doesn't give me an option to enter temp?
 
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eric19312

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Hey, @bracconiere, how many ounces /grams of CO2 is Eric worrying about?
I tried this calculator with following assumptions

volume = 15 gallons (just counting the beer pushed into the kegs)
pressure = 30 psi (15 psi on the gauge plus 15 psi for atmospheric pressure)
temperature = 60 F

comes to 4.89 mols of CO2
44 grams per mol
= 215 grams = 0.47 pounds

that is just the gas needed to push the beer out of the fermentor
doesn't count the gas needed to purge the kegs it is going into - would be similar amount I push that star san out at 15 psi on the gauge

so I'm using about a pound of CO2 during kegging
I guess it is why I love my 20# tank
 

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well according to @doug293cz you use psi absolute, which is psi gauge, added to ATM pressure for your elevation.. then like multiply that by the weight of co2 at normal pressure..


hold on let me reference the calc some one else tiped me too for 5 gallons of co2...wait i remeber it off-hand, 1.23 oz's so assume ballpark 14psi ATM, 15+14=29...ah, **** dougs, post was kinda long winded going to have to look it up quick!

so then you dived the psia by the psig so 15/14=1.07 volumes co2, multiply that by 1.23oz's to get weight, hope i didn't reteach my lesson wrong

edit: and at my $27 for a 20lb swap, which usually only has 18lb's of co2 in it. 18x16=288oz's, so 1.3/288=4 tenths of a cent, but it's really the inconvience of having to stay sober long enough to swap the tank that i care about!

edit: i had to reread it again, temp matters too! did you cold crash? unfortantly my weight calc doesn't give me an option to enter temp?
I think you have to double the psi to count for filling it first to atmospheric pressure and then filling it again to get to 15 psi on the gauge
also in my area price of CO2 went up recently some silly story about shortages due to covid-19, I spent over $50 recently to swap out a 20# and 5#
 

bracconiere

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and man this scale is great for stuff like this! only $27 off amazon! acurate to the 1/10th ounce....



just put your co2 tank on it....

edit: and i assure you it only take me 0.2oz's to push a 12 pack out my keg at 10psi.. takes about 0.7oz to purge at 50psi 3-4 times.
 

bracconiere

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well i'll quote another friendly HBT'er...here's the calc they gave me.


and no my question to doug was, how to convert pressure from that calc, at ATM, to 50 psi....
edited: for the right thread! :(
 
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eric19312

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just put your co2 tank on it....

edit: and i assure you it only take me 0.2oz's to push a 12 pack out my keg at 10psi.. takes about 0.7oz to purge at 50psi 3-4 times.
If I remember next time I'm brewing I will weigh it. I do like it when there is a way to actually measure something to verify the math is right.
 

bracconiere

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this was doug's post if you want a more complicated explination. maybe that's easier for smart people to understand, ;)


edit: sorry didn't do that right....
 

eric19312

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Ok copied from @doug293cz post:
1 volume of CO2 is 1.977 g/L, 0.264 oz/gl, or 0.033 oz/pint. So, 1 pint of CO2 at 0°C (32°F) and 1 atm pressure (14.7 psi) weighs 0.033 oz. At 50 psig the CO2 absolute pressure is 50 + 14.7 = 64.7 psia, so at 32°F and 50 psig, 1 pint of CO2 would contain 64.7 / 14.7 = 4.4 volumes or 0.33 * 4.4 = 0.145 oz.

So to push 15 gallons of beer out of my conical while maintaining 15 PSI in the headspace, I need 1 volume to replace the beer leaving the conical and an additional 15 PSI (about 1 more volume) to maintain headspace pressure.

so
2 x 0.264 oz.gl x 15 gl = 7.92 oz

in addition I need a similar amount of CO2 to push 15 gallons of sanitizer solution out of 3 corny kegs at same pressure.

so I’m still at about a pound of CO2 for the process. This is in addition to the CO2 I’m using to carbonate and serve the beer. I guess using fermentation gas to purge kegs would save a quarter pound.

back to (my question sorry if off track OP) original question...do you actually need pressure differential between headspace and keg of the conical and headspace of the keg to transfer? If both headspace are at 15 psi and I hook gas in on the keg to the conical headspace to equalize them and then let gravity push the beer from conical to keg shouldn’t I get a foam free transfer? Maybe a bit slower than with pressure differential?
 

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@Vale71 is there any way to do closed loop transfer of carbonated beer from unitank? Having that unitank completely full of 15 PSI CO2 at end of transfer while I'm just bleeding off CO2 from my purged kegs always seems wasteful.
Theoretically yes but since you need quite a bit of height differential to keep the flow going all the way to the end it might turn out that your keg must be in the basement with your FV ending up in the living room for it to work. Jokes aside HB-sized unitanks usually sit too low, even with leg extensions, for that to work. You have to consider that the level of beer in the keg is rising while the level in the FV is sinking and that is the height differential driving the transfer. With my 14g SSB Unitank a 5.5 gal corny is taller than the cone portion so in order to be able to transfer all the beer I'd have to elevate the conical so much that it would actually punch a hole in the ceiling... :(
 

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Theoretically yes but since you need quite a bit of height differential to keep the flow going all the way to the end it might turn out that your keg must be in the basement with your FV ending up in the living room for it to work. Jokes aside HB-sized unitanks usually sit too low, even with leg extensions, for that to work. You have to consider that the level of beer in the keg is rising while the level in the FV is sinking and that is the height differential driving the transfer. With my 14g SSB Unitank a 5.5 gal corny is taller than the cone portion so in order to be able to transfer all the beer I'd have to elevate the conical so much that it would actually punch a hole in the ceiling... :(
^^^^Exactly my experience. I once thought about relocating my unitank upstairs and running a long transfer line to a keg in the basement. Would have worked, but SWMBO'd wouldn't sign-off on the idea.

Brooo Brother
 

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Theoretically yes but since you need quite a bit of height differential to keep the flow going all the way to the end it might turn out that your keg must be in the basement with your FV ending up in the living room for it to work. Jokes aside HB-sized unitanks usually sit too low, even with leg extensions, for that to work. You have to consider that the level of beer in the keg is rising while the level in the FV is sinking and that is the height differential driving the transfer. With my 14g SSB Unitank a 5.5 gal corny is taller than the cone portion so in order to be able to transfer all the beer I'd have to elevate the conical so much that it would actually punch a hole in the ceiling... :(
Spike notes: "The long leg extensions were specifically designed to bring the racking port to a height where the conical can be gravity drained into a 5 gallon corny keg."

I have the short extensions but my fermentor feet are about a foot off the ground in my freezer so probably a bit higher than it would be with long extensions. I guess no harm in giving it a try, worst case is the third keg takes too long to fill and I get impatient and go back pushing it out with a pressure differential.
 

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I'm sure you'll get some beer to transfer but I doubt it will go to completion if you're draining the FV completely.
Do you have a scale to put the keg on? One issue with gravity transfer is that it's a closed system and there's really no way of telling whether it has stopped or not without the reassuring hiss of pressure releasing from the keg...
 

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I'm sure you'll get some beer to transfer but I doubt it will go to completion if you're draining the FV completely.
Do you have a scale to put the keg on? One issue with gravity transfer is that it's a closed system and there's really no way of telling whether it has stopped or not without the reassuring hiss of pressure releasing from the keg...
yes I monitor transfer on a scale. thinking I could time the flow rate at different parts of the transfer (seconds per pound) and see how bad it gets near the end. I'm usually brewing during the transfer so its ok if it takes a little longer than my current rate of about 10 min per keg...but if first keg took 20 min and second took 30 min and third took 90 min that might be an issue.
 

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I'm currently regretting having sold all my 2.5g kegs. Those might have been worth a try as they were much shorter...
 

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not sure this is still reavent to the conversation, but what about a bladder tank for water, repurposed to catch the co2 at pressure?


when i lived at a place that had a well and pump, we used one so that the water pressure wouldn't drop as soon as you turn the faucet on?


maybe hook it up with a check valve for one way flow, then use the fermentation co2 to do all your pushing?

off the wall idea i know, but at 8oz's of co2?
 

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Spike notes: "The long leg extensions were specifically designed to bring the racking port to a height where the conical can be gravity drained into a 5 gallon corny keg."

I have the short extensions but my fermentor feet are about a foot off the ground in my freezer so probably a bit higher than it would be with long extensions. I guess no harm in giving it a try, worst case is the third keg takes too long to fill and I get impatient and go back pushing it out with a pressure differential.
A more critical factor is the high center of gravity when the fermenter is on 'stilts', especially when filled to capacity. I once had a near disaster rolling a 7 gallon Chronical from my outside brewing area on a patio to my indoors fermentation space. I was carefully attempting to lift the small casters over the metal threshold of a sliding glass door when gravity and basic Newtonian physics intervened to spoil my day. The good news is the lid seals held and the slight crease in the side of the Chronical is hidden by the neoprene jacket, and since it's not technically a pressure vessel I'm not fearful that structural integrity was compromised.

Now I roll the boil vessel inside to transfer wort to the fermenter (firmly bungeed to the bulkhead) since the BV has a much wider base and is not nearly as tall. I also custom built a wider, more stable base on larger casters for my Unitank, but still move the wort to the fermenters instead of the other way around. You'd have thought an old sailor would have had a greater appreciation of metacentric height and freeboard movement of liquids above a moving vertical axis. You'd have been wrong.
 

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I'm currently regretting having sold all my 2.5g kegs. Those might have been worth a try as they were much shorter...
I've actually done that a time or two when transferring a batch to split with my son from a 7 gallon Chronical on 'stilts' to (2) 2½ gallon or 6L Torpedo kegs. The closed loop transfer at ~2 psi works well, but still slower than venting through a spunding valve. So usually (and since I transfer mostly to 5 gallon kegs) I take the lazy man's way out and hook up a CO2 supply to save some time, since that what I have to do when transferring from my unitank which is mounted lower on its casters.
 

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A more critical factor is the high center of gravity when the fermenter is on 'stilts', especially when filled to capacity. I once had a near disaster rolling a 7 gallon Chronical from my outside brewing area on a patio to my indoors fermentation space. I was carefully attempting to lift the small casters over the metal threshold of a sliding glass door when gravity and basic Newtonian physics intervened to spoil my day. The good news is the lid seals held and the slight crease in the side of the Chronical is hidden by the neoprene jacket, and since it's not technically a pressure vessel I'm not fearful that structural integrity was compromised.

Now I roll the boil vessel inside to transfer wort to the fermenter (firmly bungeed to the bulkhead) since the BV has a much wider base and is not nearly as tall. I also custom built a wider, more stable base on larger casters for my Unitank, but still move the wort to the fermenters instead of the other way around. You'd have thought an old sailor would have had a greater appreciation of metacentric height and freeboard movement of liquids above a moving vertical axis. You'd have been wrong.
My CF15 does not move when it is full. No casters on the tank. I am going to put casters on the freezer it sits in but they will be locked while the fermentor is full. It's just there to be able to move the freezer from the wall to access some plumbing back there and clean underneath it. I pump from brewstand through CFC into the fermentor.
 

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My CF15 does not move when it is full. No casters on the tank. I am going to put casters on the freezer it sits in but they will be locked while the fermentor is full. It's just there to be able to move the freezer from the wall to access some plumbing back there and clean underneath it. I pump from brewstand through CFC into the fermentor.
Wise decision.
 

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Does anyone else just lay the keg on its side for closed transfers? If I don't have my unitank/FV on a bench I just lay the corny down, business end elevated slightly on a shoe or something with the gas post up. It takes an hour or two to gravity feed in. The condensation line is a good progress indicator, otherwise if there's beer in the gas line heading back to the fv its full and I run a glass or two off. It doesn't seem to matter what the psi is, because its equalised gravity gets it in there eventually and without foam. The keg just has to be below the final beer level in the fermenter.

My process: connect the fermentor to a keg full of deoxygenated sanitiser and displace it into next month's keg, then splund on the now empty kegs liquid post. Crash a couple days with back pressure from the keg to keep O2 out, harvest yeast, and transfer. Ideally I only use my CO2 to serve and purge lines.
 

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Does anyone else just lay the keg on its side for closed transfers? If I don't have my unitank/FV on a bench I just lay the corny down, business end elevated slightly on a shoe or something with the gas post up. It takes an hour or two to gravity feed in. The condensation line is a good progress indicator, otherwise if there's beer in the gas line heading back to the fv its full and I run a glass or two off. It doesn't seem to matter what the psi is, because its equalised gravity gets it in there eventually and without foam. The keg just has to be below the final beer level in the fermenter.

My process: connect the fermentor to a keg full of deoxygenated sanitiser and displace it into next month's keg, then splund on the now empty kegs liquid post. Crash a couple days with back pressure from the keg to keep O2 out, harvest yeast, and transfer. Ideally I only use my CO2 to serve and purge lines.
That's an interesting idea that should in theory work since the beer would always be flowing 'downhill' from the transfer valve of the fermenter to the Beer Out post of the keg (which would be on the down facing side of the keg). Then run a return CO2 line from the Gas In post on the keg (which would be the high side of the keg) to a port on the fermenter that is somewhere above the level of the beer being transferred from the FV. You probably would need a pre-charge of CO2 in the keg of about 0.5-1.0 psi below the pressure in the FV. Firstly, that would ensure the keg lid was sealed and wouldn't leak beer from its slightly elevated side, and secondly it would allow beer to start flowing when you opened the FV transfer butterfly valve. Then you would slowly open the CO2 return pathway from the keg to the FV, and thus would establish pressure equilibrium between the two, hopefully preventing foaming. If my thinking is correct, that would establish a gravity flow and siphon effect from the FV to the keg.

At some point, with the keg tilted on its side (and the surface of the beer in the keg being parallel to the ground), transferred beer will reach the tailpiece of the Gas In port and attempt to flow back to the FV due to hydrostatic pressure but wouldn't rise any further than the level of any residual beer in the FV or the transfer line. You would, however, be limited as to how much beer you could transfer into the keg, limited by the length of the Gas In dip tube and the angle of tilt of the receiving keg. Finding the optimum angle to tilt the keg would be the key to transferring the greatest amount of beer. The best angle would be something in the vicinity of 45 degrees, but since the dip tube isn't exactly parallel to side of the keg the exact angle would be the compliment of whatever angular difference there was between the dip tube and the side of the keg. Probably would also be a good idea to place the keg in some sort of stable cradle for the transfer to prevent it from rolling away and toppling the FV when it starts to fill.

Since the pressure in both vessels is at equilibrium, the motive force would seem to be a gravity siphon rather than pressurized CO2 in the FV and the rate of flow would vary with and be dependent on the height differential and hydrostatic pressure between the FV and the receiving keg.

In my simple mind, it should work. Let me know, 'cause I have 7 gallons of a Blonde Ale to move from from a Chronical FV to kegs today, and would like to not waste 2.6+ volumes of bottled CO2 doing a pressurized transfer. I need someone to be a Guinea pig.
 

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It works for me. I use a liquid line thats short as practicable and low flow-resistance ~8mm internal but I don't know how much difference the line makes, the flows restricted most by the liquid post. Sometimes I finish off under pressure from my regulator if it gets too slow (thick stout and very little left in the fermenter) and just bleed by depressing the poppet in the gas line's connect, but usually it over-fills on its own and without a steep keg angle. I have normal gas dip-tubes and find a cradle of 2 Steelcap boots facing off does the trick. The lid is sealed well already as its all still at splunding pressure ~12psi etc. If you use a floating dip in your FV and need a siphon like with a fermntasaurus etc., disconnecting the gas return from the FV and bleeding the connect or prv would probably do, or connect your reg and 'up' the pressure in the FV a little.
 
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To close the loop on my original question, I wanted to share the details of my pressurized transfer, which I did last night.

The Flex+ FV, after cooling down to about 35 degrees, had a gauge pressure of about 10.5psi. I then charged up the purged keg, which was also chilling in the fridge to 8psi. After connecting the CO2 line to the gas manifold on the FV, I opened the butterfly valve on the racking arm and the beer flowed nicely. I also had a pressure gauge attached to the tap on the keg, which I continuously monitored. I also kept using the PRV to vent the keg and keep it as close to 8psi as I could. This process overall was fairly successful, although I did have a bit of beer leaking from one of the connections on the tap, which I will isolate and fix prior to next time.
 
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