Building a higher-pressure pressure cooker
February 3, 2012 11:52 AM   Subscribe

I need to make a small, high-pressure chamber. Pressure cookers only go up to 15 PSI, I'm looking for more like 50+. Can I just swap the valve out?

I need a small container that's high-pressure (CO2 being the gas of choice). It will not be heated.

This video about carbonated fruit is pretty much what I'm looking to do, in that you drop some dry ice into a pressure cooker and leave it overnight. It's perfect, in fact, except for the fact that pressure cookers only get up to around 15 PSI. I'm game for just replacing the valve if it's that easy, but is there a way to increase the allowable pressure while making sure I don't end up with an explosion?

A Sodastream carbonator supposedly gets up to around 50, but the containers are impossible to get things in and out of.
posted by soma lkzx to Grab Bag (32 answers total) 2 users marked this as a favorite
This gets dangerous pretty fast, especially if the container is reasonably large (basically multiply the surface area by the pressure to get a sense of the forces at play).

I would not just replace the valve...

At the very least, you do not have heat, a hot liquid, or a phase change involved here (as with steam, for example) but still, I would not do it unless you can find a properly rated vessel.

I am not the nagging-Nancy, overly-cautious type but this definitely has the potential to damage property or hurt someone.
posted by milqman at 12:01 PM on February 3, 2012 [2 favorites]

You run the risk of your pressure cooker exploding at a pressure so much higher than what it was designed for.
posted by atrazine at 12:01 PM on February 3, 2012

Trying to push a pressure cooker that's rated for 15 PSI up to 50 PSI is an easy way to win a darwin award.
posted by Jairus at 12:04 PM on February 3, 2012 [12 favorites]

Have you considered a paint pot?
posted by buggzzee23 at 12:09 PM on February 3, 2012 [1 favorite]

Well, I don't know if you'll be able to find pressure rated stainless steel vessels off the shelf. What I would probably do is get a small length of large diameter pressure rated stainless steel pipe and put threaded caps on it with a pressure release valve on the top cap.

It won't be food grade stainless, so you may want to keep the food out of direct contact with the pipe wall, but that isn't particularly hard to do.
posted by atrazine at 12:14 PM on February 3, 2012

Response by poster: OK, let's revise right quick: I am probably not going to replace the valve until Dr. Pressure Scientist tells me it is the most perfect, best idea, so I'd love to hear possible other options.

properly rated vessel

Do we have ideas about what kind of vessels these might be? Googling for "high pressure" containers yields huge contraptions trying to get me to like 40 billion PSI, which is not exactly my goal. I'm guessing they're all industry-specific things that don't have descriptive, generic names.

Have you considered a paint pot?

This is looking more promising! Kinda expensive, but more promising.
posted by soma lkzx at 12:15 PM on February 3, 2012

I have a commercial pressure cooker, and its gauge shows CAUTION above 20 psi. You're going to need to talk to a materials scientist, probably, but you definitely do not want to try to 'hack' a food pressure cooker.
posted by Blazecock Pileon at 12:23 PM on February 3, 2012

Actually for 50 psi you can build this out of ABS pipe which is much cheaper and easier to work with than stainless steel. I've seen it in 8 inch diameters which plenty big for fruit, you can probably get everything you need in a plumbing supply store or by ordering it from the McMaster Carr catalog.
posted by atrazine at 12:29 PM on February 3, 2012 [1 favorite]

I wouldn't expect the rubber gasket in a pressure cooker to hold out at super high pressure. Sounds like a really bad idea.
posted by bonobothegreat at 12:36 PM on February 3, 2012

I almost like atrazine's idea, except that I wonder whether ABS might get brittle when very cold.
posted by jon1270 at 12:37 PM on February 3, 2012

Just to clarify that the "no heat" condition is something of a red herring. There is still a massive phase change happening here - straight from solid to gas. This is something that can be super, super dangerous - the increase in volume is tremendous.

The reason that pressure cookers have a 15 PSI valve is because that's what the cooker can safely support - not because there's something magic about 15 PSI for cooking. Now, there's probably some safety margin built in to these things, so maybe you could go up to 20 or 25 PSI, but 50 PSI is likely to be really, really dangerous.
posted by Betelgeuse at 12:40 PM on February 3, 2012 [1 favorite]

I'd probably do this outside and use the ABS or other piping. My first step would be to go to the plumbing supply store and ask them. They love this kind of stuff*, be prepared to be there a while as they hash it out though.

*when I was building a tiny solar water heater they got so into it I thought they were going to show up at my house and build it for me.
posted by fshgrl at 12:48 PM on February 3, 2012

What about a N2O cream whipper?
posted by neroli at 1:01 PM on February 3, 2012

Now, there's probably some safety margin built in to these things

That, or even more accurately the factor of safety, is what you're looking for regarding a given pressure cooker. The simple math is if your pressure cooker is rated to 15 psi maximum and the factor of safety is 2 then the result is that it SHOULD hold 30 psi without failure.

A glance at wkipedia mentions that "Buildings commonly use a factor of safety of 2.0 for each structural member. The value for buildings is relatively low because the loads are well understood and most structures are redundant. Pressure vessels use 3.5 to 4.0, automobiles use 3.0, and aircraft and spacecraft use 1.2 to 3.0 depending on the application and materials. "

The question is when they say "pressure vessels" are they referring to boilers in industrial settings or are they including household items. If the latter is indeed the case then you might be ok to do this a few times for kicks [remember 15psi * 4 = 60 psi] but NOT once a day for an extended period of time. Cyclical stresses in an (aluminum?) pressure cooker would add up and small, microscopic cracks that wouldn't be an issue for years in normal 15 psi use could become a problem much more quickly.

Margin of safety is Factor of safety minus 1 by the way if you happen to see that vocab thrown around.

Perhaps you're looking for an stovetop autoclave (scroll down a bit). I've seen these on ebay for ~reasonable~ amounts before but it might take some doing.

Disclaimer: I've got a degree in Mechanical Engineering but anything you see above is speculation. What you're proposing is dangerous and it's going to be fairly impossible to get Mr. PressureProfessional to say "this is OK" or "this is NOT OK". It's tough calculations where alot of stuff would have to be known for this to be anything beyond a theory problem for a 300 level class problem. ...and we know how applicable those are in real life situations... oh wait...
posted by RolandOfEld at 1:07 PM on February 3, 2012 [2 favorites]

Oh, and if you end up doing this please let me know how it turns out. Carbonated fruit sounds pretty freaking cool.
posted by RolandOfEld at 1:10 PM on February 3, 2012

I've never seen ABS pipe that was safe to 50PSI. Maybe you could get it from a specialty plumbing supplier. But most ABS that you'll find at home centers is "DVW" grade only -- not safe for any sort of pressure.

Pressure-rated PVC is somewhat easier to find, but a lot of the larger stuff at home centers (3"+) is again DVW only. You may need to go to a plumbing supplier to get bigger sizes that can handle pressure. You really don't want to put pressure into DVW anything, especially PVC anything -- PVC fails pretty spectacularly.

But the problem with plastic pipe is going to be having a removable end-cap. The screw-on cleanout plugs that might seem tempting to use are typically DVW; there's an assumption that for pressure use, you'll have solvent welds and valves, I think. While there are PVC pressure-rated valves in very large sizes, they're expensive. (Even 1" or 2" pressure-rated ball valves are not cheap.)

Anyway... you could probably use black iron pipe in a largeish size with a screw-on cap on both ends. You might get some funny looks because it might seem like you're buying pipe-bomb supplies, though. And you'd have to use teflon tape or pipe dope on the threads to make it gastight. And I'd definitely recommend drilling and tapping one of the ends to put some sort of safety valve in there, because if you did put in too much dry ice (you are planning on calculating the right amount of CO2 and weighing it out precisely or something, right?), you would have a pipe bomb.

This article on building pressure / vacuum vessels may be of interest. They recommend using paint pots as well. Harbor Freight is the noted low-cost source.
posted by Kadin2048 at 1:13 PM on February 3, 2012

At the very least, you do not have heat, a hot liquid, or a phase change involved here

No offense, but this is misleading (as Beteleguese said above). Do the math and plug the numbers into Pv=nRT (I forget which process this is, isochoric? Somebody help me here) to see the potential changes. Assume an insulated container to get one approximation instead of calculating heat loss (kinda ugly) or run an experiment with ice (lengthy).
posted by RolandOfEld at 1:17 PM on February 3, 2012

Kadin has it with the pipebomb method.

I'd trust something like this if

a) it had a safety check valve with a reasonable blow-off pressure,
b) it had a bleeder valve to let the pressure be bled off safely before opening access cap(s),
c) had a gauge embedded to display the current pressure in the vessel such that you don't have a disaster when your bleeder valve malfunctions, you think the vessel is safe to open, you unscrew the end cap *boom* and c) it's overengineered by a fair margin, and
d) the user understands how this sucker works and that putting in too much dry ice is a VERY bad idea... like Darwin Award bad idea.
posted by RolandOfEld at 1:22 PM on February 3, 2012

Response by poster: What about a N2O cream whipper?

A link on that page pointed to some guy infusing chicken in a bottle with a hand-held CO2... thing that came from here. I think I'm going to start off from that thanks to ease-of-access and less danger (exploding soda bottles are a little less deadly than a pressure cooker).

Oh, and if you end up doing this please let me know how it turns out. Carbonated fruit sounds pretty freaking cool.

Oh boy, carbonated fruit is only the beginning. I'll definitely update the thread as to what happens, or have the executor of my will do it for me.

This article on building pressure / vacuum vessels may be of interest. They recommend using paint pots as well. Harbor Freight is the noted low-cost source.

Thanks, this looks like a really good read.
posted by soma lkzx at 1:23 PM on February 3, 2012

Response by poster: Kadin has it with the pipebomb method.

d) the user understands how this sucker works and that putting in too much dry ice is a VERY bad idea... like Darwin Award bad idea.

If we're already doing this much construction, what do we think about CO2 tanks instead of dry ice? Seems like it'd be a more controllable route.
posted by soma lkzx at 1:29 PM on February 3, 2012

Yes, easier to manage but more capital cost for tank/regulator. Calculating longterm cost (dry ice vs tank is tougher but likely to still be better/cheaper to use a tank. You might even be able to rig a paintball CO2 tank to do a few runs for you. The larger you go with your tank the more economical it will be in the long run.

PS - I'm assuming that using industrial C02 is ok for your application. I'm not sure that's the case. Getting foodgrade tanks/C02 in your desired volumes might be prohibitive/impossible.
posted by RolandOfEld at 1:33 PM on February 3, 2012

With dry ice, I'd be very concerned about uncontrolled pressure build-up with time as the ice sublimes. With that paint pot rig, it would be fairly easy to control the pressure with a liquid CO2 cylinder, quite hard with dry ice.

Cost is an issue. A 50lb (20kg) bag of dry ice is about $20. You would need an insulated container to hold it. Coolers work pretty well for this. A liquid CO2 handling rig can be had from eBay for ~$100 (tank + regulator). Places like Matheson, Linde, Praxair or BOC gases can fill the tanks for you, for a few dollars. Marginal cost for liquid CO2 is lower, but the startup costs are a lot higher.

Another concern is having stuff fly around in pot. I just spent a couple of hundred dollars getting a fleck of gunk out of a drain valve on an 120psi compressor at work. I'd want some way of controlling the movement of the materials you would be infusing in the pot, a tight fitting filter or screen that could cover your stuff. You don't want a valve getting jammed.
posted by bonehead at 1:43 PM on February 3, 2012

I've never seen ABS pipe that was safe to 50PSI.

Class B ABS pipe is rated to 6 bar @ 20 C, that's 80 psi. (Class E is rated to 15 bar / 217 psi). You're right though that most ABS sold is not pressure rated and that threaded ABS might not be pressure rated without solvent.
posted by atrazine at 2:33 PM on February 3, 2012 [1 favorite]

If you're just trying to carbonate fruit, Nthing the iSi whip; (amazon page): what the modernist cuisine folks recommend in the book, and I've had success using it for other applications like this (from Cooking Issues, an excellent resource for these types of things) using nitrous oxide or carbon dioxide chargers depending on the desired result.
posted by zingiberene at 2:42 PM on February 3, 2012

Also, for reference, when I need a constant temperature bath, I use dry ice, but that's at atmosphere. When I need low temperature at a regulated pressure, I always use liquid CO2 (or N2, depending on the application). I never use dry CO2 when pressure is important.
posted by bonehead at 2:43 PM on February 3, 2012

Maybe look into a cornelius keg.
posted by RobotHero at 2:55 PM on February 3, 2012

Oh, also, it goes without saying that you should have redundant safety measures in place.

-I suggest a regulator (that will bleed when pressure goes over a user adjustable specified operational level (but not dump it all))
-And a burst valve that will "burst", bleeding all pressure when it gets unsafe (purchased according to whatever vessel you build or buy) You want this to be higher than any pressure you expect to experience during normal operation, but less than the "rated" pressure of the vessel.

I would also pay some attention to the expected fracture pattern of the vessel. You want something that will "yield" before "failure". Points for something that deforms and leaks before exploding... That is the key item that will keep you out of the Darwin awards.

Lastly, the smaller you make it, the smaller the potential tragedy. Figure out the maximum-sized object you are going to put in it and make the device as small as possible to accommodate that...
posted by milqman at 3:12 PM on February 3, 2012

The vapor pressure of dry ice is huge, you need to make sure you have at least two pressure relief devices in case one fails. Just look for videos of soda bottles being blown up with CO2.
posted by gjc at 3:49 PM on February 3, 2012

Get a cornelius keg. I routinely pressurize mine to 60psi to force-carbonate home-brew. The plastic tubing leading to it will blow out before the keg will.
posted by DaveP at 3:51 PM on February 3, 2012

My 5 gallon cornelius keg, which I bought used from a home brewing supply house for $20, is rated at 130 psi.

The opening is big enough for my hand not balled into a fist, and because it's used, I don't ever plan to push it beyond about 80 psi.
posted by jamjam at 7:04 PM on February 3, 2012

I think I've seen people do this in wide-mouth nalgene bottles. There's no pressure control - you just measure out the correct amount of dry ice into the bottle in the first place.

Having said that, leave it somewhere where the potential explosion won't cause much damage.

Ah yes - At Instructables.
posted by GuyZero at 8:31 PM on February 3, 2012

I'm going to chime in with the other homebrewers here. You want to carbonate fruit, using only CO2 pressure. Spend the money and use a corny keg because it is designed to handle the stresses of using CO2 to force carbonate things.

I bet you could find a homebrewer near you who would love to experiment with this, I know I would if I was still near my brewing equipment. I bet they would let you try stuff just by having you bring the fruit and they will have the equipment. If it works well and you like it then you can spend the $100 or so setup costs to have your own setup.

First step would be walking down the street and talking with someone who works in Brooklyn Homebrew and I bet they would think it is a cool idea and would be willing to give it a try.
posted by koolkat at 4:11 AM on February 4, 2012

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