Sciencemadness Discussion Board

How to make/obtain a pressure release valve

entheologist - 24-3-2017 at 07:10

For reactions in which you need to maintain a relatively high pressure, but have no way of controlling how much gas gets generated in the vessel, you need to release the pressure before it builds up to the point that it explodes. I know I could buy some expensive device that lets you set how many bars of pressure it takes before the release valve opens up and that would be ideal, but I'm running low on funds atm so have to come up with a more ghetto approach.

Using a pressure cooker as a reaction vessel is one idea because they have safety release valves built into them. But they're pretty expensive too. So I'm brainstorming on what viable way there is to do this. I was contemplating a system where I run tubing out the top of the flask, and it runs into the bottom of a container which has a heavy weight on top of it, as the pressure builds up to dangerous levels, it will lift the weight, and when the weight gets lifted to a certain point, the container is no longer sealed so the pressure is released. But thats why easier said than done, making a sealed container like that would be really difficult. Its more likely the epoxy used to seal the tubing in place would blow out.

I checked out the hardware store for some cheap valves, but all I could find were inlet+outlet valves for radiatiors which aren't suitable for this. Also another issue is I don't know what pressure it takes to explode the flask, so it would be nice to have a pressure guage but I don't have on unfortunately.

Has anyone here tried coming up with a ghetto approach to regulating pressure in reactions that require high pressure?

yobbo II - 24-3-2017 at 10:28

What rough pressure range are you needing?

Praxichys - 24-3-2017 at 11:42

A pressure cooker is typically limited to 15 or 20 psi.

https://www.grainger.com/product/CONTROL-DEVICES-Brass-Press...

This valve seems to be adjustable from 0 to 20 psi and it's only $10.

JJay - 24-3-2017 at 12:12

I find it concerning when people want to ghetto high-pressure setups. Generally, people who attempt to do that sort of thing should know what they are doing rather than asking random people on the Internet how to do it.

You could, of course, simply clamp additional weights to your pressure cooker weight, but at some point that will cause the pressure release valve to blow out. If you block that (highly recommended against), you risk warping your pressure cooker, not to mention the spraying superheated steam around the kitchen and so forth. This could lead to serious consequences like blindness and getting grounded and probably won't do what you want.

Melgar - 24-3-2017 at 12:25

It depends on the pressure, obviously. If you only need 20-30 psi or so, a 500 mL swing-top bottle can work:

https://images-na.ssl-images-amazon.com/images/I/31D59Um8AfL...

It will leak gas rather than burst if the contents exceed a pressure that's roughly 20-30 psi. The smaller the bottle, the higher the pressure can get inside and still be safe. I've even heated these types of bottles on a hotplate before, since they're designed to be able to withstand high temperatures for sterilization. However, you do have to make sure not to move them when they're hot, or when the surface that you're moving them to is hot. So, you put it on the hotplate with both cold, turn on the hotplate, allow it to heat to the temperature you want and do whatever, then turn the hotplate off, and wait for it to cool to where you can hold your hand on it without burning yourself before taking it off.

These bottles are a few bucks each in homebrewing stores. Or if you're in the NYC area, you can pick one up from me in midtown for like $5. (I bought a six-pack and have too many)

edit: Also, NO FLAMES, only electric hotplates, and not ones where they're sitting directly on the element. Also, if your hotplate is more than 1000 watts, then only increase the temperature gradually, and not to more than about 150˚C. This type of top will only be able to release pressure that's caused by a liquid reaching its boiling temperature; if there's also pressure caused by gas forming from an exothermic reaction, then it CAN generate gas fast enough to explode. When I used one to dissolve platinum in aqua regia, I did everything from outside the garage by looking in the window, with two extension cords so I could plug in and unplug the hotplate from outside.

[Edited on 3/24/17 by Melgar]

Melgar - 24-3-2017 at 12:43

Quote: Originally posted by JJay  
I find it concerning when people want to ghetto high-pressure setups. Generally, people who attempt to do that sort of thing should know what they are doing rather than asking random people on the Internet how to do it.

I guess that depends on what ranges of pressures he's referring to. If it's 1-2 atm, that's not really in the realm of what any chemist would consider "high-pressure". Hell, ye olde shake-and-bake meth recipe uses a soda bottle, since they're rated to a surprising 100 psi.

violet sin - 24-3-2017 at 14:04

So, the weak link here is the flask generating your gas... If it is just hose attached to the pressure unit it is under pressure as well and fighting to produce the gas. Wbat kind of gas would be helpful, NH3, Cl2, ? Corrosive, flammable, toxic? Your building blocks here depend on what you are doing and chemical compatibilities.

PVC is easy to get, will hold pressure and is tolerant to some things. It is also cheap and easily worked with. A pressure cooker will either be stainless steel or aluminum, harder to work or modify and Al has some issues with corrosion. What kind of tubing??? Not all created equal, not all going to love your gas most likely and under pressure while deteriorating sounds LAME...

Step one, think as if this was some one elses Idea and shoot holes in the theory of its construction as if it weren't your project... Re-think or reconsider. You are not likely to walk into a hardware store and walk out with a working project worth of parts. It will cost more than anticipated or it will fail immediately due to sbortcuts taken.

Think worst case scenario in all areas of construction. Lid rupture, what are you spraying on the ceiling and is there a better place to do that? Hose failure, gas is pouring out, can it be bent in half and clamped untill out doors or are you carrying a pressure box full of things you dont want to wear? Flask break, is the rxn in your chamber going to boil over when solubilized gas comes out of solution - will this make it to the inlet hose and spray your mix back all over broken gas generating flask? If the two solutions come i.to contact is there more to worry about? Basically imagine you just got this thing up and running, and have some nightmare happening, what is going to happen to you, your pets/family, the surroundings, neighbors, your lab. A depressurizing cooker is loud, will the sound plus gas smell get cops called... Think, or rethink you plans

Not saying don't do it, but I'm saying if its not safe, then its not worth the risk. Wait for more money and a better job at constructing it. If it truely is important to you, then save towards a good set up

JJay - 24-3-2017 at 14:08

Quote: Originally posted by Melgar  

I guess that depends on what ranges of pressures he's referring to. If it's 1-2 atm, that's not really in the realm of what any chemist would consider "high-pressure". Hell, ye olde shake-and-bake meth recipe uses a soda bottle, since they're rated to a surprising 100 psi.


Oh, and of course, making shake-and-bake meth is the acme of safety with high pressure. I think meth cooks typically release pressure from their soda bottle "labs" periodically to prevent them from blowing apart.

And is the shake-and-bake meth recipe ye olde? I thought it was more like blasé. IDK, I don't make meth, so I'm not really up on the most avant-garde technique for reducing benzylic alcohols without dissolving off any of your most important fingers.

I don't think 2 ATM is particularly concerning, but the OP said "relatively high pressure" and wants a pressure gauge but doesn't know what pressure will destroy the flask.





[Edited on 24-3-2017 by JJay]

violet sin - 24-3-2017 at 14:12

Naw they just leave the soda bottle on the side of the road that evening and come back later to see if it survived the rxn.. if not, not their problem..

Melgar - 25-3-2017 at 10:47

Quote: Originally posted by JJay  
Oh, and of course, making shake-and-bake meth is the acme of safety with high pressure. I think meth cooks typically release pressure from their soda bottle "labs" periodically to prevent them from blowing apart.

And is the shake-and-bake meth recipe ye olde? I thought it was more like blasé. IDK, I don't make meth, so I'm not really up on the most avant-garde technique for reducing benzylic alcohols without dissolving off any of your most important fingers.

I don't think 2 ATM is particularly concerning, but the OP said "relatively high pressure" and wants a pressure gauge but doesn't know what pressure will destroy the flask.

I've never used the aforementioned reaction either, and always wondered why meth cooks spent so much effort trying to get phosphorus by scraping matchbooks, rather than just brominating the alcohol with HBr, then removing it with magnesium metal in alcohol or something. I mean, removing a secondary -OH group, on the benzylic position no less, seems like it'd be so ridiculously simple that there would be no point in the DEA prohibiting the sale of two of the most useful elements on the periodic table. However, I did hear that instant cold packs were somehow being used to make methamphetamine, and was curious how, so I looked into it. Apparently it's a Birch reduction with ammonia and lithium dissolved in diethyl ether, done under high enough pressures for the ammonia to remain a liquid. I guess the benzene ring isn't reduced because lithium is used and not sodium? In any case, it's comically atom-inefficient.

By "ye olde", I was just using a hyperbole to mean "blasé", though I've been trying to use fewer figures of speech and puns on this forum because of the number of people for whom English isn't their first language.

And yeah, OP hasn't responded, so no need to waste any more time here unless he does.

[Edited on 3/25/17 by Melgar]

entheologist - 25-3-2017 at 11:59

Quote: Originally posted by Melgar  
It depends on the pressure, obviously. If you only need 20-30 psi or so, a 500 mL swing-top bottle can work:

https://images-na.ssl-images-amazon.com/images/I/31D59Um8AfL...

It will leak gas rather than burst if the contents exceed a pressure that's roughly 20-30 psi. The smaller the bottle, the higher the pressure can get inside and still be safe. I've even heated these types of bottles on a hotplate before, since they're designed to be able to withstand high temperatures for sterilization. However, you do have to make sure not to move them when they're hot, or when the surface that you're moving them to is hot. So, you put it on the hotplate with both cold, turn on the hotplate, allow it to heat to the temperature you want and do whatever, then turn the hotplate off, and wait for it to cool to where you can hold your hand on it without burning yourself before taking it off.

These bottles are a few bucks each in homebrewing stores. Or if you're in the NYC area, you can pick one up from me in midtown for like $5. (I bought a six-pack and have too many)

edit: Also, NO FLAMES, only electric hotplates, and not ones where they're sitting directly on the element. Also, if your hotplate is more than 1000 watts, then only increase the temperature gradually, and not to more than about 150˚C. This type of top will only be able to release pressure that's caused by a liquid reaching its boiling temperature; if there's also pressure caused by gas forming from an exothermic reaction, then it CAN generate gas fast enough to explode. When I used one to dissolve platinum in aqua regia, I did everything from outside the garage by looking in the window, with two extension cords so I could plug in and unplug the hotplate from outside.

[Edited on 3/24/17 by Melgar]


I thought about that but wasn't fully sure if the cap would release before the bottle explodes. I'm looking for a multipurpose vessel so some of the reactions are definitely gonna be exothermic. I heard of someone using a stainless steel fire extinguisher as a reaction vessel. The advantage this is you can watch the pressure gauge and discharge it if gets too high. Disadvantage is you can't see into the vessel, and a magnetic stirrer might not work so well since the magnet will stick to the steel. I'm not sure how you would open it up to get the reactants in there though, all the fire extinguishers I've encountered are sealed.

Quote: Originally posted by violet sin  
So, the weak link here is the flask generating your gas... If it is just hose attached to the pressure unit it is under pressure as well and fighting to produce the gas. Wbat kind of gas would be helpful, NH3, Cl2, ? Corrosive, flammable, toxic? Your building blocks here depend on what you are doing and chemical compatibilities.

PVC is easy to get, will hold pressure and is tolerant to some things. It is also cheap and easily worked with. A pressure cooker will either be stainless steel or aluminum, harder to work or modify and Al has some issues with corrosion. What kind of tubing??? Not all created equal, not all going to love your gas most likely and under pressure while deteriorating sounds LAME...


Yeah Cl2 is one of the gases I have in mind. SO2 is another. I agree about not doing it until I'm 100% sure its safe. A fire extinguisher seems like a pretty good approach since you know exactly what pressure it can withstand, although temperature is a different matter. A heavy duty stainless steel extinguisher can withstand high temperatures as well as a pressure cooker can. Corrosion is definitely an issue to be considered. Cl2 in the presence of water can corrode stainless steel. Lining the inside of the vessel with teflon spray will solve that problem:
https://www.amazon.com/DuPont-Teflon-Non-Stick-Dry-Film-Lubr...
Quote: Originally posted by Melgar  

I've never used the aforementioned reaction either, and always wondered why meth cooks spent so much effort trying to get phosphorus by scraping matchbooks, rather than just brominating the alcohol with HBr, then removing it with magnesium metal in alcohol or something. I mean, removing a secondary -OH group, on the benzylic position no less, seems like it'd be so ridiculously simple that there would be no point in the DEA prohibiting the sale of two of the most useful elements on the periodic table. However, I did hear that instant cold packs were somehow being used to make methamphetamine, and was curious how, so I looked into it. Apparently it's a Birch reduction with ammonia and lithium dissolved in diethyl ether, done under high enough pressures for the ammonia to remain a liquid. I guess the benzene ring isn't reduced because lithium is used and not sodium? In any case, it's comically atom-inefficient.
[Edited on 3/25/17 by Melgar]


Theres a documentary on youtube where they show a couple of people performing the reaction. They didn't use diethyl ether, they used lighter fluid (C6-C10 I'm guessing), from what I could see they filled a plastic bottle with lighter fluid, added ammonium nitrate, NaOH and crushed tablets. Then added lithium strips. And then the part that boggles my mind. They added a cap full of water and started shaking the bottle violently, and somehow they don't blow themselves up. I'm guessing if they used sodium it'd be a flaming inferno the moment they add that cap of water.

[Edited on 25-3-2017 by entheologist]

Jstuyfzand - 25-3-2017 at 12:10

I like how this thread went from "Looking for a pressure release valve" to "how to cook meth".

Y'all on a list now! :)

Melgar - 25-3-2017 at 12:46

Quote: Originally posted by entheologist  
I thought about that but wasn't fully sure if the cap would release before the bottle explodes. I'm looking for a multipurpose vessel so some of the reactions are definitely gonna be exothermic. I heard of someone using a stainless steel fire extinguisher as a reaction vessel. The advantage this is you can watch the pressure gauge and discharge it if gets too high. Disadvantage is you can't see into the vessel, and a magnetic stirrer might not work so well since the magnet will stick to the steel. I'm not sure how you would open it up to get the reactants in there though, all the fire extinguishers I've encountered are sealed.

It seems the general rule for cheap pressure vessels is: "They can be corrosion-resistant, heat-resistant, and high-pressure-resistant. Pick two." If it's cheap and seems to meet all three criteria, you'll probably have a bomb on your hands. Virtually every metal will corrode appreciably with acidic conditions in the presence of chloride ions (which are very difficult to avoid), and your odds of successfully coating the inside of one with PTFE or enamel are slim to none. However, if your reaction temperatures stay below 100C or so, Sodastream carbonation bottles are rated for a whopping 1000 psi, and they're only about $10 each. I think they're PETE, which has only deteriorated for me personally in the presence of hot sulfuric acid. If your goal is condensing a gas like SO2, a Sodastream bottle would probably do the trick, since you could put it in acetone/dry ice and send your gas from your SO2 generator into it, where it would condense. Then just keep bubbling until you have what you want, and cap it with a cap that's been outfitted with a needle valve. Pressure can be calculated from temperature and vapor pressure formula, if you know for sure what's in int.

Exothermic reactions aren't really a problem for the swing-top bottles, but exothermic reactions that also generate gas are, because of the potential of thermal runaway. Think, KNO3 + sugar, melting them together in such a bottle. If you can't immediately see why that would be a bad idea, then you may want to hit the books a little more.

unionised - 25-3-2017 at 13:10

15PSI is probably the top end of what you can use it for, but did you consider the good old mercury lute?

Melgar - 25-3-2017 at 14:35

Quote: Originally posted by entheologist  
Theres a documentary on youtube where they show a couple of people performing the reaction. They didn't use diethyl ether, they used lighter fluid (C6-C10 I'm guessing), from what I could see they filled a plastic bottle with lighter fluid, added ammonium nitrate, NaOH and crushed tablets. Then added lithium strips. And then the part that boggles my mind. They added a cap full of water and started shaking the bottle violently, and somehow they don't blow themselves up. I'm guessing if they used sodium it'd be a flaming inferno the moment they add that cap of water.

Ohhhh.... I get it now. First off, are you sure they weren't using starting fluid? Starting fluid is about 20-30% diethyl ether. I thought that part was necessary, but I could be wrong. The reason they use lithium is that it floats in ether/hydrocarbons. Everything else would sink to the bottom. The NaOH would both release ammonia and amine alike from their salts, but it couldn't until there was some water to allow it to go into solution. The aqueous layer would be so saturated with NaOH and salt, that lithium would probably barely even react with it, and of course, they'd use a huge excess of lithium in any case.

entheologist - 29-3-2017 at 08:27

WTF is this:


I opened up an old ABC (its made of aluminum so unsuitable) and about a kg of this blue/green powder came out
https://sc02.alicdn.com/kf/HTB1k4GSJVXXXXaqXXXXq6xXFXXX4/50-...
https://www.ylea.co.uk/userfiles/image/produit/grd/-hqtq.JPG

I'm disturbed as I read ABC fire extinguishers are usually a mix of monoammonium phosphate and ammonium sulfate. Neither of these chemicals look like this. I haven't bothered testing it in any way but I will soon because I have to know.

JJay - 29-3-2017 at 14:46

Hmm... this looks suspiciously like a photo from Alibaba....

Melgar - 29-3-2017 at 15:04

Quote: Originally posted by entheologist  
WTF is this:
...

I opened up an old ABC (its made of aluminum so unsuitable) and about a kg of this blue/green powder came out
https://sc02.alicdn.com/kf/HTB1k4GSJVXXXXaqXXXXq6xXFXXX4/50-...
https://www.ylea.co.uk/userfiles/image/produit/grd/-hqtq.JPG

I'm disturbed as I read ABC fire extinguishers are usually a mix of monoammonium phosphate and ammonium sulfate. Neither of these chemicals look like this. I haven't bothered testing it in any way but I will soon because I have to know.

Does it look like this?

https://www.google.com/search?tbm=isch&q=ammonium+iron+s...

If so, it could just be from a slow reaction whereby diammonium phosphate is formed, alongside ammonium iron sulfate, by reacting with the iron in the fire extinguisher.

[Edited on 3/29/17 by Melgar]