Refilling propane bottles

[Johansson]

I have a 5 kg composite bottle for my jetcart that I want to install permanently, is it possible to refill it from a larger bottle that sits higher up and is turned upside down so the liquid propane can drain down the hose?

I know that I have to be aware that I must leave the composite bottle 3/4 filled so it can expand from sun heat without bursting the bottle.

[Mike Everman]

You'll need a vent valve at a high point so the liquid can go in. It's always seemed a wasteful way to fill. Definitely something I'd do out in open spaces.

[Johansson]

Bugger, I didn´t know that. When thinking about it it seems pretty straightforward, just connect the two bottles and pour the liquid from one bottle to the other but Í should have figured that it wasn´t that easy.

I guess we won´t be the most popular guys at the event if we dump a kilo of propane in the middle of race cars and bikes...

[ace_fedde]

You'll need a vent valve at a high point so the liquid can go in. It's always seemed a wasteful way to fill. Definitely something I'd do out in open spaces.

Mike,
I don't have the experience, but it seems to me that you're only partly right. The little bottle will have little pressure left. So the bigger bottle will push the liquid into the little bottle till the pressure is the same, from filling and from expanding.
After that the little bottle will (slowly) exchange gas (bubling up through the hose) for liquid (pouring down). The wider the hose, the better. But of course the hose must be able to withstand the full pressure of the bigger bottle!
As said, no experience, only my theory.
Fedde

[Mike Everman]

It's why all propane bottles have two valves. Even the camping variety. Watch the guy at the filling station; he has to open a little blow-off valve with a screwdriver, and gas hisses out the entire time it is being filled with liquid propane. What you say is true, the pressure will equalize between the two tanks instantly, but you must be able to let the gas out the top of the one being filled, so the liquid can flow down into it through the other open valve.

[GRIM]

Hi All ,
I have refilled 5 kg cylinders from 15 kg cylinders on several occasions ,
I used a HYDRAULIC hose and threaded fittings between the two cylinders , The hose chills down a LOT and standard propane hose and hose clips may not hold up ,

I put the bigger cylinder in the sun and the smaller cylinder in the shade to create a pressure differential , it takes several "shots" , the bigger cylinder chills down and looses pressure, so just shut both valves , let the temeratures stabilize , and go again ,

I Did not weigh the cylinders that I was filling, just went for about 50% full , I would strongly recommend weighing if going for 75% volume , to avoid overfilling ,

I believe, Most commercial filling devices have a bleed line or bleed valve to allow the liquid to displace the gas ,and I also believe the main reason for this to allow fast refilling, (less than one minute for a typical fork lift tank ) , However you can fill a cylinder if you can create the pressure differential and have some time on your hands , above all BE CAREFUL , NO FLAMES, SMOKING ETC ,

Joe

[ace_fedde]

It's why all propane bottles have two valves. Even the camping variety. Watch the guy at the filling station; he has to open a little blow-off valve with a screwdriver, and gas hisses out the entire time it is being filled with liquid propane.

And there we might have found another difference between standards in Europe (or just Holland?) and the US! A screwdriver won't help you much in Europe unless you manage to stab a hole in the bottle with it.

Fedde

[GRIM]

Heres a link , filling small cylinders, his dripping valve would make me very nervous ,
MY wife would not appreciate a 5kg propane cylinder in the fridge , but then again she wouldnt know if i done it when she went shopping right

http://www.youtube.com/watch?v=hKMmMmHB00o

[Rossco]

Guys, no need to "discuss" this back and foward on whether it is possible or not.
Very simple logic/physics here, and rather frustrating for the filler! Ive tried to trick it many ways. Physics is stubourn!

By ballancing pressure you are feeding some gas into the filling bottle, hence your hose getting cold, its boiling at the lower pressure and absorbing energy. As soon as the pressure balances, at the vapor pressure, it will stop transfering, so, you will have a "full" bottle of "gas", at a ratio of the volume of liquid density (not full). Doing this with a temp difference will help, but not enough to fill much at all. If you have your restriction on the small bottle, so it is boiling in there, it will go cold automaticaly, but, you are still only working with a small total pressure diff, and a large difference in density.

If you bleed off the gas as you fill, (yes, very wastefull and looks very dangerous to those around you) it will fill the bottle with liquid. This is probably the best way, and definitely quickest, as filling stations wouldnt loose that gas for fun.
Its a balance here, you are pushing in liquid (the doner bottle must be providing liquid!) and loosing a percentage of it in gas at the same volume but lower density.
Its funny to think on really. You are bleeding gas off, to drop the pressure for the liquid to come in, but in the lower pressure the liquid boils to gas that you have to bleed off, you are chasing this cycle, to eventualy catch up... there must be a better way. There is, pumping it in, just like the automotive gas fillers.

I have tried other methods, and your original idea is a very good one.
BUT, to just "pour" the liquid between two joined bottles would take a very big and direct connection.
What i did try is syphoning from one to the other. Although you need bottles with gas and liquid valves and fittings, like my AC gas bottles.
I think this worked, although my bottles were not full, and it was extreamly slow, as the pressure difference was tiny, and the connecting hoses had some very small restrictions. It was easier just to pump it.
Setup, one bottle high, one low. Liquid to liquid, and gas to gas. The liquid propane trickles down to the lower tank, but! you need to get it started, like a syphon hose.
No just sucking a mouthfull here tho! Tricky. As i said, i really dont know if it worked, as there is no indication unless you measure weights as you do it, which i didnt.

The other way is vacuum! this seems a sure fire way. Vac your recieving bottle down then hook up to your donor bottle on liquid. Suck, done. BUT, it will still not be full tho. Just as before, the gas flash boils and ballances the pressure quickly. You do get a larger fill, as the bottle cannot supply enough heat to boil it all and liquid transfers.

edit, this race is on ice... pumping would be the only way to lower the pressure of the recieving bottle, not putting it in the freezer, before anyone says its the same.

Rossco

[Johansson]

Great info, now I know that it isn´t worth the trouble.