Fuel Transfer Valves for Backpacking: Fuel Physics, Myths, Risks, and Real-World Performance

[Ryan Jordan]

Companion forum thread to: Fuel Transfer Valves for Backpacking: Fuel Physics, Myths, Risks, and Real-World Performance

This article examines canister fuel transfer devices from a thermodynamics and engineering perspective, focusing on pressure-driven flow, headspace constraints, fractional distillation of LPG mixtures, and measured transfer rates. It evaluates Alpenflow, FlipFuel, and generic EN 417 adapters and outlines conservative operating practices that respect established EN 417 design safety margins.

[Dustin V]

I wish I’d seen the AlpenFlow before I grabbed a FlipFuel. The AlpenFlow has a major advantage in that you can pop the receiver canister onto a scale very quickly to check the fill level and then pop the sender canister back on if needed.

[David Thomas]

It’s great to put all this info in one place and to promote best practices when we’re playing with things that can go boom.

I’ll print out the article and review it carefully at 35,000 feet this weekend.

I thought I could just post a quick correction to the thought that there is (could possibly be) liquid going down and vapor going up at the same time, but then I got to two instances of “The recipient canister, immediately after venting and sealing, contains only air at roughly atmospheric pressure”.

Nope, nope, noppity, nope.  1) a functionally empty canister still has fuel vapor inside of it.  2) It has to because there was no driving force through the (depressed) Lindel valve between the atmospheric pressure fuel vapor still inside and the atmospheric-pressure air outside. and 3) if there was air filling the canister, you’d not be able to refill it very completely because once the receiving canister has taken half its volume of liquid fuel, the air would be compressed 2-fold (and there’d be fuel vapor pressure as well).    You’d need to vent/burn most of that air-fuel mixture in the headspace and then top off the canister with more liquid fuel.

And I’m only 1/4 of the way through the article and need to get back to work when my stupid OS completes updating, so poke me in a week if I haven’t gotten back to this by then.

[Roger Caffin]

The recipient canister, immediately after venting and sealing, contains only air at roughly atmospheric pressure
I agree with DT: this utterly WRONG. In most cases the canister will still contain fuel vapour.

Air in an ’empty’ fuel canister being refilled? VERY dangerous. That means mixing high pressure oxygen with propane vapour. Could be interesting, if viewed from about 50 m away.

The idea that fuel vapour will go up the (generally) very small passage in the connector from the recipient to the donor is a bit of a myth. The small passage will be full of liquid fuel. The chances of bubbles of vapour pushing past this capillary flow of liquid to any meaningful extent is very minimal.

The chance of gravity driving the flow downwards is small. Oh, it might do so briefly, but the gravity force is very small. You may safely ignore it. What does drive the flow is temperature difference, and only temperature. Put the receiver in the freezer for 10 minutes and the donor in the sun, then connect the two (OUTDOORS!). When the receiver warms up and the donor cools down, disconnect and repeat. BUT weigh the receiver frequently, and (as DT warns) NEVER fill beyond the original nominal weight. There MUST be clearance above the liquid fuel in the receiver at ALL times. Indeed, if you find you have over-filled, vent the excess (outdoors) to get down to the nominal canister weight.

Finally, there seems to be another myth that walkers have these huge piles of half-empty canisters at home. Why? Surely one could simply use the old canister until the flame dies, and then replace it with a new one? When you get home with what seems to be an empty canister, take it OUTSIDE, sit it down on something solid, stick a solid nail in the valve, and hit the nail with a hammer, to break the guts of the valve. Hear it rattle inside. Then leave it outside to sort itself out. That would incidentally be safer all around.

Refilling can be done if you follow all the rules — ALL the rules. But compare the cost of a canister with the cost of fuel for your car: is it worth while?

Cheers

[Dan]

It’s important to exercise proper skepticism about technical information one finds on the internet, unless one has firsthand knowledge of the scientific credentials of the person providing that information. Especially when safety is at stake. When safety information is written and published in reliable sources, it is drafted by highly trained and credentialed experts, and then reviewed by other independent experts, prior to publication.

[Roger Caffin]

It’s important to exercise proper skepticism about technical information one finds on the internet, unless one has firsthand knowledge of the scientific credentials of the person providing that information.
Chuckle.
May I paraphrase?
It’s important to exercise skepticism about any information one finds on the internet.

Cheers

[Jon Fong / Flat Cat Gear]

It seems like this article should have been vetted prior to publication.

[Terran Terran]

My one pound refillable bottles have a separate release valve. Perhaps more of a hiss than a boom when canisters fail. Unless I’m missing the stories, anecdotal evidence isn’t supporting the skepticism. Not yet anyway. I don’t hear of old propane catching fire either, but I’ve seen it.

It’s important to exercise skepticism about any information one finds on the internet.

Maybe yes. Maybe no. Skepticism in itself deserves skepticism. I’m still in the undecided category. I’d rather not explain how the fire started. I don’t feel like it’s that important to use or to promote. Those that don’t err on the side of caution should maybe stay away from the tinder.

[Dan]

Chuckle.
May I paraphrase?
It’s important to exercise skepticism about any information one finds on the internet.

Skepticism is appropriate, but I wouldn’t want to suggest that all information on the internet has the same level of (un)reliability. One can find thoroughly vetted, peer-reviewed information on the internet as well.

Part of the problem we are having in the US currently (e.g. with regards to the safety of vaccines and other medications) is directly related to people’s inability to distinguish between real and fake science. So let’s not sweep everything under the bus … that is how it started.

But yes … if you’re referring to social media posts, I agree.

[David D]

A major youtube backpacking influencer promoted a type of plastic for cooking that wasn’t rated for boiling water, and refused to retract it or even engage when it was pointed out

Another typically sound youtuber recently posted dangerous and false info regarding bear safety.  To his credit he at least engaged in the conversation unlike the first, but unfortunately didn’t change anything either

When a financial incentive is attached, be especially cautious as saving face seems to prevail over the safety of the audience

[Jerry Adams]

Yeah, the air remaining in the canister is butane gas.

I wonder how they fill canisters?  When there is nitrogen and oxygen and other gases in the canister.

When you fill a canister that has butane gas, that has to condense into the liquid.  You wouldn’t have to vent it.

When I refill, I’ll put as much as 8 ounces into it.  Maybe there’s another volume of headroom that would hold another 2 ounces of liquid butane.

So, the initial volume of butane gas would be compressed 5X.  5 atmospheres of pressure.

I don’t think there is 5 atmospheres of pressure when I refill, so some of that must condense into the liquid.

When they fill the canister they must vent that air?

When they refill my large propane tanks, there’s a bleed valve they open.  As they pump propane into the tank, gaseous propane vents.  When it’s full (with just the right amount of headroom) liquid propane starts coming out so they turn off the pump.  That’s how they avoid overfilling.

If you put butane into a canister with oxygen in it, I don’t think it would be a problem, no ignition.  Just don’t put a spark in there.  Don’t light your cig there :)

I wonder if nitrogen/oxygen in the head space would affect the physics of performance in the cold?

[Roger Caffin]

I wonder how they fill canisters? When there is nitrogen and oxygen and other gases in the canister.
Think of a very simple method.
Like maybe cooling it all down to -30 C, where the standard butane/propane mix is liquid and stays liquid.
Then take a canister withOUT the Lindal valve, pour a controlled amount of liquid fuel into it, then add Lindal valve and crimp it on. Extract canister from chamber and package.

What happens to the small volume of air that was in the canister? It probably gets flushed out with butane vapour first. Cool the exhaust gas to -5 C and collect the liquified butane for recycling. Let the rest, which is air, vent off 0 ah, through a rather high tower maybe.

I am not saying this is the only way to do it, but when you are making several thousand canisters per day (or more), it does seem simple and fast. Much faster than trying to get the fuel in through the Lindal valve!

Cheers

[JCH]

When a financial incentive is attached, be especially cautious as saving face seems to prevail over the safety of the audience

“It is difficult to get a man to understand something, when his salary depends on his not understanding it.”

― Upton Sinclair.  From “I, Candidate for Governor, and How I Got Licked,” published in 1935.

Universal truth and all that.

[Roger Caffin]

Update:
“It is difficult to get a man to understand something, when his SHARES depends on his not understanding it.”

Cheers

[Casey Bowden]

David and Roger, thank you very much for your insights. Happy Holidays!

[Ryan Jordan]

David – good catch on the “air” comment in the empty canister, that was a missed edit on our part. The correction disappeared in a late draft during a document merge from our two peer reviewers. This has been corrected.

Roger – you’re right – vapor making its way back up into the donor canister isn’t going to happen in the early/mid stages of a transfer when liquid fuel velocity is spurting downward at several m/s. I’ve clarified this in the text as well.

These two corrections have been documented in the Updates/Corrections log at the end of the article as well. Thank you both for pointing them out.

And, just to emphasize the point in case it wasn’t obvious from the text – hydrostatic head isn’t going to have a meaningful effect until very late stages of a fuel transfer “run”.

[Don Montierth]

Speaking as a practicing chemical engineer, and very familiar with the physical and thermodynamic processes involved, although I don’t agree with every single last word in the article (or in the comments), it is very well written. It covers the bases quite well.

For me, a cardinal rule is to weigh the canister when purchased, when it is full. Write the weight on the underside of the canister with a permanent marker. And never, ever fill above that weight. Different fuel mixtures may have slightly different densities, but they are all within the margin of error of the weight you measure. It is not just possible to create a very high pressure in the vapor space of an overfilled canister, if it warms up enough and it has been filled too much with liquid, the liquid can “go solid” (no vapor space) inside the canister. Warming beyond that point results in a catastrophic failure due to the expansion of liquid, which is basically incompressible.  I have seen that happen with overfilled ammonia cylinders at a previous place of employment. They were left out in the sun in the Las Vegas area after being overfilled.

One myth that needs to be debunked is something I’ve seen in several so-called expert YT channels. They speak of needing to vent air from the canister during the filling process. They then demonstrate by venting fuel. There is no air in the canister.  All they are doing is cooling the recipient canister by venting (vaporizing) fuel.  That allows them to continue filling the receptor canister. All they are doing then is replacing what they have just vented.

[Jerry Adams]

” I have seen that happen with overfilled ammonia cylinders at a previous place of employment. They were left out in the sun in the Las Vegas area after being overfilled.”

video needed : )

I like to write the weight of the empty canister on it.  Which you can get by subtracting the capacity from the starting weight.

I use 1 ounce per day so I put in 1 ounce for each day plus one extra ounce for margin.

Same difference though – you want to never put in more weight than when it was full.

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