QuietStove.com

Tuesday, March 27, 2012

Canister Refiller -- WARNING

I've been seeing a particular canister refilling device on eBay lately as a "MacCoupler Liquefied Gas Refill Adaptor Gas Charging Valve Inflatable converter" [sic].    This particular device is just downright dangerous and in general should be avoided.  I'll explain why I say "in general" in a minute.

A very dangerous canister refiller.  ** AVOID! **
OK, so what's wrong with this refiller?  Well, in case you don't recognize the fittings, this refiller will refill backpacking type gas canisters from a "bulk" propane tank.
A "bulk" type 100% propane tank
Hey, now doesn't that sound like a great idea?  Hey!  I can take cheap gas out of my bulk propane tank (like for a BBQ) and refill my expensive backpacking canister.  Pennies on the dollar!  Sweet!  What could possibly go wrong?

What could go wrong?  A whole lot, and it could be bad, very bad.  First, backpacking canisters do NOT contain 100% propane.   Standard backpacking type canisters in the US are rated at not more than 20% propane (mixed with 80% isobutane) or 30% propane (mixed with 70% butane).  Propane is a very high pressure gas.  Put 100% propane in a canister rated for only 20 to 30 percent propane and what do you get?  Quite possibly a very nasty explosion.

Oh!  I know!  If my canister is only rated for 20 to 30% propane, I'll just fill it up to 20% of capacity and leave the rest empty!  Duh!  Problem solved.

Uh, not exactly.  When combined with butane or isobutane, propane actually forms a blend.  The resultant blend has a lower vapor pressure than 100% propane.  Odd as it sounds, you'll actually have less vapor pressure in a canister with 20% propane and 80% isobutane than a canister 20% full with propane alone.  Besides, what good is a canister you can only fill to 20% of capacity anyway?

Basically, you should never fill a backpacking type canister with 100% propane.


But what if you filled a canister to  80% of capacity with butane first?  If you filled to 80% of capacity with butane first and were exactingly careful about not exceeding the overall rated capacity, then maybe this adapter would be safe.  I'm not saying that it is safe, I'm saying that there's the possibility that the adapter might be safe -- from the perspective of the mix of gasses.  The adapter might still have other problems in terms of materials, design, and construction.  The very fact that someone put a dangerous refiller like this together suggests that they really don't know about gasses.  I have NO confidence that such a person has the ability to build a refiller that is safe in other respects.  I'm certainly not tempted to buy one.

Putting it into perspective
Now, am I just a "Nervous Nellie?"  Maybe this is actually pretty safe, but you know this is modern society, and we have to protect people from themselves, don't we?

I submit to you that I am no Nervous Nellie.  I refill my backpacking canisters all the time (with butane!!).  I use a windscreen with my upright canister gas stove.  I sometimes use my stove inside a tent.  In other words, I do all sorts of things that manufacturers tell us not to do lest doom and disaster overtake us.  I do things like refill, use a windscreen, and cook inside my tent because I've studied the objective dangers and found them reasonable for a person who is careful and has an understanding of how to mitigate the dangers.  But I will NEVER fill a standard backpacking type canister with 100% propane.  It's just too dangerous.  Period.

This refill adapter is:  NOT recommended (in the strongest possible terms).

I thank you for joining me on another Adventure In Stoving.

HJ

22 comments:

  1. I have a propane tree that I can attach hoses to, to fuel what was once dedicated to the the small propane canisters. The hoses are very heavy duty rubber to handle the pressure coming out of the tank feeding to the different appliances.

    The hose on this thing is clear, which would be a red flag to me that the hose is not reinforced.

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  2. What do you mean that propane is a very high pressure gas? As far as I know there's no such thing -- even though propane is a large molecule the molecular size of butane is pretty similar and I would assume about as compressible at very high pressures.

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  3. @George, the size of the molecule isn't the relevant issue. Vapor pressure is in general determined by HOW FAR ABOVE THE BOILING POINT you are. Water for example boils at 210F/100C. A container of water at room temperature has very little vapor pressure. Butane boils at 31F/-0.5C. At room temperature, one of those little plastic Bic type lighters is all you need to contain the pressure. Propane on the other hand boils at MINUS 44F/-43C. At room temperature, you're WAY above the boiling point, and that liquified propane in your canister is trying to vaporize like mad, exerting a tremendous force on the walls of the canister. Portable 100% propane canisters are typically made out of heavy gauge steel. Take a look at one of those green Coleman brand propane canisters for a camp stove. Compare that Coleman propane canister to a Bic lighter. The difference in materials is due to the difference in vapor pressure.

    HJ

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  4. @Alan, yeah, I'm not convinced this thing is well made, even if the gasses it was meant for were safe -- which they are not.

    HJ

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  5. So..how do you refill your canisters with butane?

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  6. @David, see http://adventuresinstoving.blogspot.com/2012/01/gas-canister-refilling.html

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  7. To put this a different way, at 80F the pressure in a canister is as follows:

    - 100% butane = 23psig
    - 70% butane / 30% propane = 54psig
    - 100% propane = 128psig

    psig is the pressure that a gauge put on the container would measure - it's the pressure above atmospheric pressure (14.7psia). psia is absolute pressure - it's the gauge pressure plus atmospheric pressure.

    I don't know the construction of the canisters, but I SERIOUSLY doubt that 100% propane is remotely safe!

    http://www.engineeringtoolbox.com/propane-butane-mix-d_1043.html

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    1. @TXRam:

      The pressure with propane is more than double than with a 70/30 butane/propane mix. I'm sure canisters have some safety factor built in, 10%, 20%, maybe even 30%, but a more than 100% increase? I don't think so.

      I believe the canisters have to pass their ratings at 50C. I'll have to look up what the pressures are at 50C.

      HJ

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  8. 50C would be 122F, the link only provides info to 110F, but interpolating from the curves I would guess approx. 240psig for 100% propane vs. 110psig for 30% propane / 70% butane at 120F.

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  9. 240psig vs. 110psig is a BIG jump.

    Irrespective of the exact numbers, putting a higher vapor pressure gas into a canister rated for a lower pressure gas is a very bad idea.

    HJ

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  10. Used to work in the aerosol industry. There are DOT specs to the canisters that are used, and can be transported. Any can with the rolled lip and standard aerosol valve, which is what you find on the gas blends canisters, is NOT rated for the pressures propane exerts. That is why the 100% propane canisters are thicker stronger steel, with more robust valves and safety release valves. It is all about the vapor pressure. And to the poster that said that propane and butane are not that different in size, you are right, there is only one -CH2- unit difference, but in a molecule that small, that is a huge percent increase in moelcular weight (propane MW= 44, butane = 58, 31% increase in MW), thus the large drop in vapor pressure from propane to butane.
    Another hazard of this is over-filling. All liquified gas cansiters should have "head space" to allow for expansion of the gas if it is heated. The hazard being, if you overfill the canister, there is no room for the gas to form, you get hydrostatic pressurization and the cansister fails. It is industry standard to elave 15-20% volume of a canister empty for headspace.
    I would also say that this device looks incredible dangerous from the perspecive that the tubing used looks like simple PVC or tygon tubing, which IS NOT RATED for the pressures you are going to be transferring that gas at. You should only be using tubing rated for pressure, which will have a braided stainless sheath around the tubing core.
    Another issue to consider as well, the DOT specs for canisters that are considered refillable vs non-refillable. Non-refillabel canisters do not have to pass results for long term use and are not spec'd to be corrosion resistant. When refilling a canister you are potentially introducing tiny amounts of water vapor which can potentially corrode the can causing catastrophic failure. Refillable DOT canisters are made with this corrosion resistance in mind, and therefore are "over-built" to make sure that they can withstand some corrosion without failure, at least until the next testing period. ALL refillable DOT canisters have notices on them regarding occasional hydrostatic testing.

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    1. Hi, Jason,

      Thanks for your perspective.

      I also didn't like the look of that tubing. It did not strike me as being of appropriate type. I'm not sure what it is made of, but several people have speculated that it is tygon. It is certainly nothing like the tubing I see from reputable manufacturers.

      I agree that avoiding over filling is very important. That's also very interesting what you're saying about non-refillable canisters corroding from within if any water gets inside. I've been refilling canisters that were a propane-butane (or isobutane) mix with 100% butane which I consider relatively safe because if it's comparatively low vapor pressure (as compared to the original gasses). I have encountered no problems, but I'll have to keep an eye on that.

      HJ

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  11. So which tool out there will let me refill my backpacking canister with propane, yes i understood i must first fill 70% butane and then 20/30% propane, but where can i find the adapter to let me do that? i search google but found nothing !

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    1. Take a look at http://alva.ne.jp/. I plan to put up a blog post on the subject relatively soon.

      HJ

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  12. I just bought one of these and aware of the dangers, apparently the LPG in our country is not 100% propane, but about 60% propane/40% butane, i dont know how safe that much gas would be, but i would tend to try and keep it 30% propane, i worked out if i half fill my canisters using one of these here at:
    http://store.taiwancamping.net/home/field-kitchen/gas-refill-adapter
    with butane gas, i can safely fill the rest with the propane/butane mix in my propane tank, its also possible for me to use some of the gas in my propane tank in cold weather during winter, and have a higher concentration of butane left in my propane cylinder.
    I can say that the tubing is a high pressure pneumatic tubing made from polyurethane, not pvc.

    I did have trouble making it fit my cylinder valve though, it seems to be rather tight fitting, it screws in fine about half way, and then goes tight.
    Are some valves a tapered fit? Mine doesn't appear to be, but i cant make it seal unless i can get it to screw in firmly all the way.

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    1. Hmm. Very interesting. You'd have to be very careful with this whole arrangement of course. If the percentages in the LPG vary, you could get more propane and therefore higher vapor pressure than you expect -- which could have very negative consequences. You could ensure some safety by only using LPG from your propane tank in cold weather. After all, there's no need for propane if the weather is over 10C degrees. You can just refill with typically cheap 100% butane which will always have the lowest vapor pressure.

      In cold weather, if you fill with butane first as you say and then add in propane, you should be all right in terms of vapor pressure if you use a receiving canister that was originally rated for a 30/70 mix and you do not exceed the capacity (in grams)of the receiving canister. I periodically weigh my canisters as I refill them to ensure that I don't go over.

      Canisters can oxidize from within, so I don't refill any given canister over a set number of times.

      As for the taper of the threads, I don't know. I think the best person to consult would be whomever services your propane there. It seems odd that it would screw in part way and then stop. It could be poor machining. It could be a taper. It could be a defect in the threads. QUESTION: Do other appliances screw in properly to your propane tank? I assume that they do. The fault then is with the refilling apparatus. Not much help there I'm afraid, but it might give you some ideas as to what to look at. Sorry I can't be of more help.

      HJ

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    2. Im interested to know if any gas suppliers can fill my tank with 100% butane, or even any mix that i request, im unaware of any that could, as that would be an unusual request, since most people are after higher propane content.
      What scars me is that people out there have probably used this to fill their canisters with 100% propane, have any exploded? I dont know, the main danger is obviously when using the stove, if it burst with a flame going, the results would be disastrous.

      Im going to do a few tests and cut my old canisters up after a few refills and examine for any corrosion.

      As for my Propane tank, i was given this by my grandfather, i have never used it before, and has a different valve than the one i use on my BBQ/Gas heater. My mates told me that there are several different valves, and some are actually tapered, but are a larger connector to start with, im going to try fitting it on my friends tanks and see if there is one that fits it properly.

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    3. I'm not sure if you can order specified blends off propane and butane. I would think that LPG is sold in bulk and that whatever the bulk blend is, that would be all that would be available. You might be able to get 100% butane however. In many places in Europe, 100% butane is available for cooking in bulk tanks.

      I have not heard of explosions from people using this refiller. "LPG" varies from place to place. In some locations it may be nearly all propane. In other places it may contain a great deal of butane. Either way, I would not want to put a high pressure gas in a container not rated for the pressure. You can get away with more in winter when vapor pressures are lower, but one would have to be very very careful.

      Good luck on finding the right fittings.

      HJ

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    4. Ive been ringing some of our local gas suppliers, one said how their mix is a good 80% propane, others am unsure, i dont think i will find any supplier who has less than 55% propane, which i may get away with, i would love to be able to do some pressure tests to see what these canisters can take before exploding.
      I probably will end up half filling with butane from the bayonet canisters and then topping up with my propane mix, once i can establish the propane content

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    5. I know that canister specifications require that a canister not deform at a temperature of 50C. I suppose if one knew the mix a given canister was at 50C, then one could calculate the pressure that mix would have at 50C and arrive at a safe maximum pressure rating. Personally, though, I would not want to exceed the original percentage of propane in a mix.

      The other trick to this is that pressure does not depend on the percentages of each component fuel by weight but rather by molar fraction. In other words, one must calculate how many molecules correspond to a given weight of propane or butane. It is the percentage of the molecules that are of a given gas that determines the resultant vapor pressure. One cannot approximate by weight. The calculations are quite complex; I've seen them. Caution advised here unless you are familiar with the mathematics and quite confident in your calculations.

      HJ

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  13. well in the north where you super cold temps i could see this being used to adapt these cylinders for winter use esp in places like alaska and canada. due to the vaporization point below 31F(?) canisters don't work so hot but propane does.

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  14. I have an item similar to this. I use it to put a small quantity of propane into the canisters during the winter. That being said, most of my canisters are straight butane for summer use. Those are stored through the winter. I only fill one tank with a 25/75 mix during the winter as I never stay overnight and can refill the tank when I have about 35g of fuel left. I found that the performance of my stove drops off significantly when at about 30g of fuel remains. (This is for a 110g canister and my upright stove). In really cold temps, or when I'm lazy and haven't refilled the canister, I just bring the white gas stove.

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