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What's Actually Happening When you Pull a PRV? Like Sciencewise, Bro.

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pinemarten

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I'm hoping someone smarter than I can break down exactly what is happening when we pull a PRV on a pressurized keg or fermenter.

It sounds straightforward, but is it? I've been kegging for a few years and recently decided I haven't given this enough thought as I try to hunt down all the barbarian oxygen at the gates.

If I have an empty and CO2 pressurized keg, not hooked up to CO2 and I pull the PRV is the CO2 in the keg just firing out simply due to the pressure I put into the keg? Or, is the CO2 in the keg actually being displaced by the oxygen in the room until they both equalize? Does the actual PSI I'm the keg make a difference in this situation? For example if, just for fun, the keg is pressurized to 100 PSI and I pull the PRV will there be no oxygen entering the keg until the CO2 pressure remaining falls below X PSI?

If that same keg has CO2 attached while the PRV is pulled, is there a certain minimum PSI/BAR needed to overcome the atmospheres ( :)?) of oxygen in the room?

Thanks Folks!
 
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Jayjay1976

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Until the pressures equalize, it will just be CO2 leaving the vessel. If you hold it open past this point, gases will exchange slowly through simple diffusion. Is this really so hard to envision? CO2 under pressure doesn't act like a fart does when ye roll down yer window at 55mph stirring up a tornado of stank. Have ye been to school? Maybe circled the lot for a bit, but did ye go inside?

Edit: oxygen isn't the boogeyman some make it out to be. Hell, some of us even enjoy a wee bit from time to time.
 
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pinemarten

pinemarten

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Until the pressures equalize, it will just be CO2 leaving the vessel. If you hold it open past this point, gases will exchange slowly through simple diffusion. Is this really so hard to envision? CO2 under pressure doesn't act like a fart does when ye roll down yer window at 55mph stirring up a tornado of stank. Have ye been to school? Maybe circled the lot for a bit, but did ye go inside?
So, farting schoolboy whom isn't such a big talker on bike forums where he proves he doesn't grasp the difference between imperial and metric, perhaps ye learned to read at said school ye didn't simply circle the lot of and tonight read "For example if, just for fun, the keg is pressurized to 100 PSI and I pull the PRV will there be no oxygen entering the keg until the CO2 pressure remaining falls below X PSI?"

Solve for X.
 

day_trippr

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Ok, that was entertaining! :mug:
But, unambiguously, O2 is the brewer's bane, to be avoided at even extraordinary cost.

For the OP: why not both? Yes, if you've pressurized the keg with CO2 above its atmospheric partial pressure when you pop the PRV you'll hear the out-rush. At the same time, O2 molecules will try to get through the PRV to equalize it's partial pressure to the atmosphere.

Obviously the former effect will be more pronounced/evident than the other - you're not going to oxidize a keg of beer by popping the PRV once...

Cheers!
 

day_trippr

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I don't know about @Qhrumphf, but I'll say it.
Yes, there may be attenuation of ingress versus a system that started at atmospheric pressure but for sure both Nitrogen and Oxygen (and lesser gases wrt atmospheric content) will try to establish their partial pressure atmospheric equilibriums. This is just basic gas physics...

Cheers!
 

IslandLizard

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This is just basic gas physics...
It's been awhile, so I'd probably need to brush up. But...

I find it very counterintuitive that gasses at atmospheric pressure would move against a strong pressure gradient. Gas (at pressure) from the inside of the keg is spewing out the little holes in the PRV. It makes no sense anything goes back in until that gradient has pretty much leveled off, and I'd say it even has to be well below 12 psi (above atm pressure) for that to start happening.

We're not CO2 flushing a bucket with the lid off...
 

Vale71

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Yes, diffusion works based on partial pressure of single gases so it will work against a pressure gradient too. However, diffusion even in gases is still quite slow and any outflow of gas occurring when the vessel is vented will mechanically prevent any gas ingress. This is especially true if venting through a small opening as flow velocity will then be very high.

Outflow will obviously stop as pressure in the vessel equals outside pressure and that's when diffusion will start prevailing again.
 

Qhrumphf

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Are you saying gas moves against a pressure gradient, entering the keg through the PRV while venting out through it?
The lads who understand it far better than I have already explained it. As I said, completely counterintuitive.

As pointed out, any O2 that might make it through a venting PRV will be far dwarfed by the velocity of gas escaping. But that doesn't mean O2 won't try to squeak its way back in to restablish equilibrium with the atmosphere. But any that did would be negligible to say the least and worth precisely zero worry. Just as boiling doesn't reach 100% sterile, but I'd pay zero mind to whatever minutae of organisms that survive the boil (under normal brewing conditions, not wort canning...)
 
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Jayjay1976

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So, farting schoolboy whom isn't such a big talker on bike forums where he proves he doesn't grasp the difference between imperial and metric, perhaps ye learned to read at said school ye didn't simply circle the lot of and tonight read "For example if, just for fun, the keg is pressurized to 100 PSI and I pull the PRV will there be no oxygen entering the keg until the CO2 pressure remaining falls below X PSI?"

Solve for X.
If you worry that sufficient o2 will enter your keg while venting pressure via PRV (are you going to take the lid off?) that would change change the flavor of your beer in any perceptible way then you must have sorted out and perfected absolutely ever other aspect of your brewing process, every possible variable or source of imperfection, and are now looking to win every last ribbon in any competition you enter.

Bravo, sir.
 

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Humans don't develop common sense at uncommon scales. We can't really visualize how big the universe is or even how big our solar system is. Similarly we can't figure out the scales at the atomic/molecular level, at least not intuitively.

Here's one way to look at gas laws. If the PRV opening was represented by a 4 foot piece of PVC pipe, let's say that pipe is 12" in diameter.

First, let's assume no pressure gradient. The pressure in the keg is at 1ATM and the pressure outside is 1ATM (atmosphere). Assume pure CO2 inside and pure Nitrogen outside. The gas exchange is represented by firing a BB gun through the 12" PVC pipe in both directions. One BB is one gas molecule. Sure, some gases are ever so slightly larger in diameter than others but for this exercise assume they are the same. What are the odds that the two BBs are going to hit each other and rebound back out where they came from? Keep in mind they are not traveling parallel to the walls of the PVC. Okay, it's not just ONE BB each way but given the 12" pipe diameter, the single 1/8" diameter BB is generously representative of the relative difference is empty space we are dealing with. Almost 100% of the time, the BBs just pass each other.

Now add some pressure. Pressure is just MORE of a given molecule in a given space. Let's say you have 14 PSIG in the keg so twice as many CO2 BBs as Nitrogen BBs on the outside. That's like 2 BBs fired through the PVC going out and 1 BB going in. What are the odds they all fly past each other now?

Ok, 28 PSIG... That's 4 BBs to 1. Now the odds of some of the incoming nitrogen getting deflected back out reaches a few percent.

Here's where it gets interesting. We are using a 12" PVC to represent a hole in a tank that is designed to flow gas. This same principal also represents materials that appear to be solid and are designed to hold gas in but actually have porosity larger than the molecules we want to keep in. That's why we talk about keg tubing in terms of how much rejection it has. Picture a length of keg gas tubing more like a perforated tube where some of the perforations are 1/8" diameter. Can the oxygen BB make it in? Sometimes. Do the CO2 BBs bump into them at the perforation and keep them out? Sometimes.
 

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If you worry that sufficient o2 will enter your keg while venting pressure via PRV (are you going to take the lid off?) that would change change the flavor of your beer in any perceptible way then you must have sorted out and perfected absolutely ever other aspect of your brewing process, every possible variable or source of imperfection, and are now looking to win every last ribbon in any competition you enter.

Bravo, sir.
I agree that the concern in this one example is hyperbolic but the question does bring up principals that are very relevant to brewing and the quest for quality and longevity. Don't be too quick to dismiss it.
 

bucketnative

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The gas exchange is represented by firing a BB gun through the 12" PVC pipe in both directions. One BB is one gas molecule. Sure, some gases are ever so slightly larger in diameter than others but for this exercise assume they are the same. What are the odds that the two BBs are going to hit each other and rebound back out where they came from? Keep in mind they are not traveling parallel to the walls of the PVC.
Perhaps drunken gnats may better approximate Brownian motion than BBs?

But, good analogy.
 

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:bravo:
Humans don't develop common sense at uncommon scales. We can't really visualize how big the universe is or even how big our solar system is. Similarly we can't figure out the scales at the atomic/molecular level, at least not intuitively.

Here's one way to look at gas laws. If the PRV opening was represented by a 4 foot piece of PVC pipe, let's say that pipe is 12" in diameter.

First, let's assume no pressure gradient. The pressure in the keg is at 1ATM and the pressure outside is 1ATM (atmosphere). Assume pure CO2 inside and pure Nitrogen outside. The gas exchange is represented by firing a BB gun through the 12" PVC pipe in both directions. One BB is one gas molecule. Sure, some gases are ever so slightly larger in diameter than others but for this exercise assume they are the same. What are the odds that the two BBs are going to hit each other and rebound back out where they came from? Keep in mind they are not traveling parallel to the walls of the PVC. Okay, it's not just ONE BB each way but given the 12" pipe diameter, the single 1/8" diameter BB is generously representative of the relative difference is empty space we are dealing with. Almost 100% of the time, the BBs just pass each other.

Now add some pressure. Pressure is just MORE of a given molecule in a given space. Let's say you have 14 PSIG in the keg so twice as many CO2 BBs as Nitrogen BBs on the outside. That's like 2 BBs fired through the PVC going out and 1 BB going in. What are the odds they all fly past each other now?

Ok, 28 PSIG... That's 4 BBs to 1. Now the odds of some of the incoming nitrogen getting deflected back out reaches a few percent.

Here's where it gets interesting. We are using a 12" PVC to represent a hole in a tank that is designed to flow gas. This same principal also represents materials that appear to be solid and are designed to hold gas in but actually have porosity larger than the molecules we want to keep in. That's why we talk about keg tubing in terms of how much rejection it has. Picture a length of keg gas tubing more like a perforated tube where some of the perforations are 1/8" diameter. Can the oxygen BB make it in? Sometimes. Do the CO2 BBs bump into them at the perforation and keep them out? Sometimes.
Best example I've seen in a very long time.:bravo:
 

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Well in the name of scientific knowledge, bb guns, and PVC pipes, I have a few more questions vital to brewing that I'd like to add:

How fast would I have to hop on a pogo stick to reach the rooftop of a 10 story brewery, if I'm in elevator #7 headed downwards and already quite drunk? And, do they still sell pogo sticks?

I've never been to Hawaii, but if I wanted to plan out my itinerary, how many Bud Lights would I need to imbibe to quell an active volcano with my stream of urine, and yea or nay on the Hawaiian shirt?

Assuming I'm an adept swimmer, how long would it take me to doggy-paddle up Niagara falls, will my hair absolutely get wet, and will this cause my wort to become aerated?
 

Qhrumphf

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Well in the name of scientific knowledge, bb guns, and PVC pipes, I have a few more questions vital to brewing that I'd like to add:

How fast would I have to hop on a pogo stick to reach the rooftop of a 10 story brewery, if I'm in elevator #7 headed downwards and already quite drunk? And, do they still sell pogo sticks?

I've never been to Hawaii, but if I wanted to plan out my itinerary, how many Bud Lights would I need to imbibe to quell an active volcano with my stream of urine, and yea or nay on the Hawaiian shirt?

Assuming I'm an adept swimmer, how long would it take me to doggy-paddle up Niagara falls, will my hair absolutely get wet, and will this cause my wort to become aerated?
42
 

lump42

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Humans don't develop common sense at uncommon scales. We can't really visualize how big the universe is or even how big our solar system is. Similarly we can't figure out the scales at the atomic/molecular level, at least not intuitively.
Space is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.
 

Bobby_M

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Well in the name of scientific knowledge, bb guns, and PVC pipes, I have a few more questions vital to brewing that I'd like to add:

How fast would I have to hop on a pogo stick to reach the rooftop of a 10 story brewery, if I'm in elevator #7 headed downwards and already quite drunk? And, do they still sell pogo sticks?

I've never been to Hawaii, but if I wanted to plan out my itinerary, how many Bud Lights would I need to imbibe to quell an active volcano with my stream of urine, and yea or nay on the Hawaiian shirt?

Assuming I'm an adept swimmer, how long would it take me to doggy-paddle up Niagara falls, will my hair absolutely get wet, and will this cause my wort to become aerated?
 

kh54s10

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Science...:drunk:. This makes my head hurt. Great for the knowledge, but in the scheme of things, so little transfer is going to take place that you couldn't possibly taste any effect...
 

doug293cz

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If the keg is pressurized at higher than atmospheric pressure (gauge pressure greater than 0 psi) then the net mass flow on opening the PRV will be out of the keg. O2 and N2 will try to move into the keg by diffusion (because their partial pressures outside the keg are higher than inside the keg), but if the net outflow is faster than the diffusion flow rate, then no O2 or N2 will get into the keg. So, what kind of flow rate is needed to prevent air ingress into the keg?

I won't go thru the gory math detail, but we can get an idea of the required flow by looking at required flow rates in safety hoods. These flow rates are set so that that gas molecules in the enclosed hood volume will not be able to diffuse out into the room (where people are working.) IIRC the common spec for airflow is 100 linear feet per minute, so if you had an open area of 10 sq ft, the volumetric flow would have to be 1000 cubic ft per minute. Now the hood design spec will have a safety factor built into it, so we only actually need something like a 50 linear ft/min flow rate to prevent backflow by diffusion.

The PRV has a diameter of about 3/16", which is a cross sectional area of 0.00019 sq/ft. To get 50 linear ft/min flow we need 0.0096 cu ft/min flow or 16.6 cu in/min (about 0.275 cu in/sec.) That's not a very high flow rate. But, once the flow rate drops below that, air will start getting into the keg by diffusion.

Brew on :mug:
 

doug293cz

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Humans don't develop common sense at uncommon scales. We can't really visualize how big the universe is or even how big our solar system is. Similarly we can't figure out the scales at the atomic/molecular level, at least not intuitively.

Here's one way to look at gas laws. If the PRV opening was represented by a 4 foot piece of PVC pipe, let's say that pipe is 12" in diameter.

First, let's assume no pressure gradient. The pressure in the keg is at 1ATM and the pressure outside is 1ATM (atmosphere). Assume pure CO2 inside and pure Nitrogen outside. The gas exchange is represented by firing a BB gun through the 12" PVC pipe in both directions. One BB is one gas molecule. Sure, some gases are ever so slightly larger in diameter than others but for this exercise assume they are the same. What are the odds that the two BBs are going to hit each other and rebound back out where they came from? Keep in mind they are not traveling parallel to the walls of the PVC. Okay, it's not just ONE BB each way but given the 12" pipe diameter, the single 1/8" diameter BB is generously representative of the relative difference is empty space we are dealing with. Almost 100% of the time, the BBs just pass each other.

Now add some pressure. Pressure is just MORE of a given molecule in a given space. Let's say you have 14 PSIG in the keg so twice as many CO2 BBs as Nitrogen BBs on the outside. That's like 2 BBs fired through the PVC going out and 1 BB going in. What are the odds they all fly past each other now?

Ok, 28 PSIG... That's 4 BBs to 1. Now the odds of some of the incoming nitrogen getting deflected back out reaches a few percent.

Here's where it gets interesting. We are using a 12" PVC to represent a hole in a tank that is designed to flow gas. This same principal also represents materials that appear to be solid and are designed to hold gas in but actually have porosity larger than the molecules we want to keep in. That's why we talk about keg tubing in terms of how much rejection it has. Picture a length of keg gas tubing more like a perforated tube where some of the perforations are 1/8" diameter. Can the oxygen BB make it in? Sometimes. Do the CO2 BBs bump into them at the perforation and keep them out? Sometimes.
While conceptually your example is correct, it may lead readers to believe that air molecules can travel significant distances before they collide with other molecules. This is not the case. The average distance air molecules travel before colliding with other air molecules is on the order of 30 nano meters (or 30 billionths of a meter) at room temp and atmospheric pressure. Each molecule collides many times per second.

"Having this approximately correct expression, what use is it? Consider the mean free path of an air particle. Atmospheric pressure (sea level) is about 760 Torr. Plugging this into the final expression gives a mean free path of λmfp = 3.4 × 10-6 cm. This is 34 nanometers, which is roughly half of the commonly reported value of 65 nm. Particles in air do not travel very far before they collide with other particles."
Brew on :mug:
 

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While conceptually your example is correct, it may lead readers to believe that air molecules can travel significant distances before they collide with other molecules. This is not the case. The average distance air molecules travel before colliding with other air molecules is on the order of 30 nano meters (or 30 billionths of a meter) at room temp and atmospheric pressure. Each molecule collides many times per second.

"Having this approximately correct expression, what use is it? Consider the mean free path of an air particle. Atmospheric pressure (sea level) is about 760 Torr. Plugging this into the final expression gives a mean free path of λmfp = 3.4 × 10-6 cm. This is 34 nanometers, which is roughly half of the commonly reported value of 65 nm. Particles in air do not travel very far before they collide with other particles."
Brew on :mug:
Touché!
 

Moose_MI

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Is any else wondering why “you” wasn’t capitalized in the thread title?

“What's Actually Happening When you Pull a PRV? Like Sciencewise, Bro.”

I can’t be the only one that couldn’t get anything done all day wondering.
 

Jayjay1976

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Is any else wondering why “you” wasn’t capitalized in the thread title?

“What's Actually Happening When you Pull a PRV? Like Sciencewise, Bro.”

I can’t be the only one that couldn’t get anything done all day wondering.
You know I thought I might have caught a whiff of troll earlier...
 

Moose_MI

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I wonder how much light enters the keg

“When you Pull The PRV? Like Who-Gives-A-Feces, Bro”
 

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I wonder how much light enters the keg

“When you Pull The PRV? ...”
If you're worried about that, paint all of the "dry side" interior components of the PRV flat black. Any light that gets in has to be reflected off of something, since there is no direct path for light to enter. :D

Brew on :mug:
 

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This level of detail is unable to reach the types of folks that my explanationation was attempting to connect with.
While conceptually your example is correct, it may lead readers to believe that air molecules can travel significant distances before they collide with other molecules. This is not the case. The average distance air molecules travel before colliding with other air molecules is on the order of 30 nano meters (or 30 billionths of a meter) at room temp and atmospheric pressure. Each molecule collides many times per second.

"Having this approximately correct expression, what use is it? Consider the mean free path of an air particle. Atmospheric pressure (sea level) is about 760 Torr. Plugging this into the final expression gives a mean free path of λmfp = 3.4 × 10-6 cm. This is 34 nanometers, which is roughly half of the commonly reported value of 65 nm. Particles in air do not travel very far before they collide with other particles."
Brew on :mug:
 

Moose_MI

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It took us until post #32 to be concerned about the level of detail here?

I love this place
 

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O2 and N2 will try to move into the keg by diffusion (because their partial pressures outside the keg are higher than inside the keg)
I've always understood it's entropy (disorder) that's root to equalization/homogenization of different gasses (or mixtures), not selective motion due to "partial pressure" differences. IOW, molecules are not driven by partial pressure differences in bordering systems, randomness of motion is the big equalizer. Therefore equalization of (gas) mixtures requires time, it's a random process, and far from instantaneous.
 

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I've always understood it's entropy (disorder) that's root to equalization/homogenization of different gasses (or mixtures), not selective motion due to "partial pressure" differences. IOW, molecules are not driven by partial pressure differences in bordering systems, randomness of motion is the big equalizer. Therefore equalization of (gas) mixtures requires time, it's a random process, and far from instantaneous.
Partial pressures are a stand-in for concentration (based on ideal gas law: P = nRT/V. Concentration is n/V.) Unless the concentration is zero on one "side", diffusion goes in both directions with the rate determined by the concentration on the side "behind" the direction of diffusion. If the concentration is equal on both "sides" then the diffusion flow is equal in both directions, so there is zero net flow. If the concentrations (partial pressures) are not equal then the diffusion flows are unequal, and the net flow is the difference of the two flows. The closer the two concentrations, the lower the net flow. When the concentrations are close to equal, the diffusion flow in each direction is very much larger than the net flow.

The gas mixing by diffusion does increase the entropy of the system, thus lowering the Gibbs Free Energy, and you can think of this lowering of the system energy as the driving force for diffusion. But, just the statistics of the random motions/collisions of the gas molecules will explain the mixing of the gases.

I'm sure this is more than the vast majority of participants in this thread want to know. :p

Brew on :mug:
 
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