Question about fermenting under pressure without a spunding valve

[Sunfire96]

I'm currently fermenting ~4 gallons of Golden ale with an OG of 1.047. It's currently at 1.017, it likely has another 5-10 gravity points to go. It's in a corny keg with the PRV removed for gas release.

What would happen if I put the PRV back on today? Would the last bit of attenuation cause pressure issues for the batch? If I remember correctly, the standard pressure for a keg PRV is way above typical spunding pressures, so I don't think it would harm the keg at all.

The keg is currently at 65F and I'll keep it there for several more days, maybe a week, to condition before lowering the temp steadily over a few days. Just curious about adding pressure at this stage in the ferment. Thanks!

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[Brooothru]

Realistically you'd 'probably' not be at risk of creating your own IED in a keg. But any time you deal with pressurized fluids without a PRV, you're skating on thin ice. Just on principle alone, I wouldn't do it.

 

[SanPancho]

why cant you put the prv back on? its probably going to be fine, but god forbid your corny has hairline crack somewhere you cant see or some threads are gonna shear and something turns into a missile..... i wouldnt do it.

 

[Sunfire96]

Thanks for your responses.

If the fermentation is wrapping up and there's only a few gravity points left to go, is that realistically enough pressure to blow up my keg? Isn't that the purpose of the PRV? To release pressure at 50-60 psi? Would a few gravity point generate that much CO2?

 

[SanPancho]

a few points, as in less than 5, is something i do often. but 5-10 is asking for trouble.

 

[StayThirsty]

If the fermentation is wrapping up and there's only a few gravity points left to go, is that realistically enough pressure to blow up my keg?

Looking at it simplisticly, yes it could blow it up. You said there were as many as 10 gravity points left which would produce as many as 5 volumes of CO2 (a rough rule of thumb is 1 volume for every 2 gravity points). You have 4 gallons of Golden Ale so 5 volumes of CO2 is 20 gallons of CO2. Since there's 1 gallon of headspace in the keg on top of the 4 gallons of beer, adding 20 gallons of CO2 to the 1 gallon headspace would increase the pressure there to 21 atmospheres or about 315 PSI. Your keg is probably rated for about 120 PSI.

The story gets a bit better if you look at more details. Your beer at 68F would absorb more than 5 volumes of CO2 if it ever got over 66 PSI. That would keep the pressure in the headspace from rising to the extreme calculated above, assuming the CO2 would be dissolved at the same rate it was generated (and I happen to believe it would be dissolved at the same rate, but I wouldn't bet safety on that belief).

Finally, you also have a PRV which is designed to vent the extra pressure. That might be fine but I don't know if it's safe to create a situation where you expect the PRV to vent pressure (as opposed to a situation where the PRV is only used in exceptional situations in case something else goes wrong). I just don't know how keg PRV's were designed to be used.

 

[Sunfire96]

a rough rule of thumb is 1 volume for every 2 gravity points

This is the type of information I'm looking for, thank you. Where can I find the original source for this so that I can understand this better?

 

[Sunfire96]

Realistically you'd 'probably' not be at risk of creating your own IED in a keg. But any time you deal with pressurized fluids without a PRV, you're skating on thin ice. Just on principle alone, I wouldn't do it.

I'm not sure you understood my question; I would either have the PRV off allowing all the gas to escape, or the PRV installed capturing CO2 but releasing it when it reaches the limit rating of the PRV. At no point in time would the keg be sealed without a release valve

 

[Sunfire96]

why cant you put the prv back on? its probably going to be fine, but god forbid your corny has hairline crack somewhere you cant see or some threads are gonna shear and something turns into a missile..... i wouldnt do it.

So you're saying it would be generating far more pressure than what's needed for drinking/serving. Do you know how I can calculate the CO2 that would be generated based on how much attenuation is left to go?

 

[Sunfire96]

a few points, as in less than 5, is something i do often. but 5-10 is asking for trouble.

Do you know a way to determine how many points are ideal for generating ~2.5 vol of CO2?

 

[Brooothru]

I'm not sure you understood my question; I would either have the PRV off allowing all the gas to escape, or the PRV installed capturing CO2 but releasing it when it reaches the limit rating of the PRV. At no point in time would the keg be sealed without a release valve

Yeah, my bad. I see where @StayThirsty gave you the indepth and correct answer. A spunding valve on the gas-in post set to a pressure~15 psig or less with the PRV in place but closed is probably your best solution, and certainly the most safe.

Sorry for the confusion on my part. I get a tad agitated when I think that people may get a bit cavalier about handling compressed gases or other fluids under pressure. Safety first.

 

[ba-brewer]

Do you know a way to determine how many points are ideal for generating ~2.5 vol of CO2?

Gyle and Krausen Priming Calculator - Brewer's Friend

I used it as a guide a few times and it seems to get pretty close. trick is having a good estimate of the FG. I normally seem get more pressure or volumes but spunding help fix that.

 

[StayThirsty]

Where can I find the original source for this so that I can understand this better?

One source is Beer and Brewing Magazine: "Then, for each additional volume of carbon dioxide, you need to add about two gravity points (1.002) worth of fermentable sugar. "

So for 2.0 to 2.5 volumes you would want to close your tank when you are 4-5 points from final gravity.

The problem is not knowing what your final gravity will actually be. If the FG turns out higher than you expect the beer won't be fully carbonated. If it is lower than you expect it could massively overcarbonate and become a safety issue. A spunding valve eliminates the uncertainty for a pretty low price. For example, MoreBeer currently has the BlowTie spunding valve on sale now for $14 (this is the spunding valve I use the most).

 

[ba-brewer]

The first few naturally/primed carbonated kegs I just sealed it up and let it go. I did use a pressure gauge on a ball lock disconnect to check progress though. When the pressure reached the level I wanted I would let some escape with the PRV.

Pressure builds slow at 70F and if I checked once or twice a day I could keep things in check. Normally see about 1 to 2 psi per days when it was getting close to the target level.

I assume my PRV will open at the stated level, I guess most people do the same. If that is the only safety check it might not be a bad thing to test your valve to see if it will open correctly at the stated level. If you are connected to a regulator I view the PRV as a secondary safety check.

 

[Sunfire96]

Thanks so much for the info!!

 

[Smudgey]

Definitly second the BlowTie suggestion. Simple, effective, cheap peace of mind. Unless you always finish at exactly the same sg with this recipe you'll always be vulnerable to surprises.

 

[odie]

The stock "grey" PRVs are around 100 psi. That is not gonna blow up any keg. The kegs are rated higher. yeast will multiply under pressure until around 30-ish I think. But yeast will supposedly not "express" properly under pressure beyond a couple PSI. So no PRV at the start is good.

Go ahead and put the PRV back on at this point. What will happen is the keg is gonna start pressurizing until the fermentation is 100% complete. Depending on how much fermentables are left, it may or may not reach 100 psi. But likely you will have to manually vent the finished keg several times to get to your target PSI.

You can attach a pressure gauge to the gas post and observed the PSI and manually vent it down if it's higher than target. At fermentation temps you are probably looking for 20-25 PSI final to hit desired volume of CO2.

You can buy lower PSI PRVs. The red ones are around 25-30 PSI. Makes for easy pressure ferment without a spunding valve. You can basically just forget about it. Once it's done, you are close to the carb you are looking for anyway.

 

[Jim R]

If someone was really trying to avoid the expense of a spunding valve (not that expensive), it isn't hard to make a makeshift spunding type valve with a cheap push to connect plastic valve (few dollars at the hardware store) and a little polyethylene tubing. I use one of these for pressure transfers to keep beer out of the pressure gauge on my regular spunding valve.

I put one end of the tubing past the valve into a cup of water (or sanitizer) to monitor the bubbles, pressurize my keg to my goal pressure and slowly open the plastic valve till bubbles just start to be released telling me that the plastic valve release point is at that keg pressure. It is pretty accurate for maintaining that set pressure release point and can easily be adjusted up or down.

It is really nice though to have a high quality spunding valve with a good guage.