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Mcgyver
01-31-2009, 11:21 AM
always a good idea to avoid blowing oneself up......there was a thread recently on how to purge larger propane tanks so they can be machined or otherwise worked....anyone know how to safely set up the small hand held bottles for machining etc? basic instructions sought!

thanks

barts
01-31-2009, 12:28 PM
Propane flammability limits are narrow (2 - 10% approx), so the easiest thing to do is to drain tank completely w/ propane torch or equivalent , and using an adapter fitting, repeatedly fill cylinder w/ compressed air and then discharge completely in open air,

After 3 or so cycles, you're completely good to go. You can weld, machine, plasma cut, etc. If you're nervous, try igniting the air discharging from the empty cylinder.

It's much easier/safer than purging a gasoline tank, since the liquid propane all turns to vapor when you empty the tank.

- Bart

Evan
01-31-2009, 01:04 PM
Or you can put in a bit of water and boil it for a few minutes. This will purge the propane and also any oxygen by replacing it with water vapor until it cools down.

Or, dump in a few tablespoons of bicarbonate and a 1/4 cup of vinegar and it will fill with CO2. Pour out the slurry and plug the hole to keep the CO2 inside. No bang.

Or, leave the tank open (drill hole?) and let sit for a year or two.

Or, fill it with some ethanol and slosh it around. Pour it out and rinse with water. Propane dissolves in ethanol and will be rinsed out along with the ethanol.

barts
01-31-2009, 01:53 PM
Or you can put in a bit of water and boil it for a few minutes. This will purge the propane and also any oxygen by replacing it with water vapor until it cools down.


This reminds me of one of my favorite demonstrations for kids; take an empty aluminum soda (pop) can and put a bit of water (a couple of tablespoons) in it... now heat the bottom of the can (a gas stove or propane torch works well) until the water is boiling... using a pair of hot dog tongs, in one smooth motion invert the can into a open container of water. It will instantly collapse as the steam from the boiler water (which displaced all the air) condenses back into water, creating a vacuum. Air pressure then does the rest; the opening at the top of the can is too small to allow the water to be drawn in fast enough to prevent the can from collapsing...

Your Old Dog
01-31-2009, 04:01 PM
If you're nervous, try igniting the air discharging from the empty cylinder.

ROFLMAO ! Kowalski is that you? :D

Evan
01-31-2009, 04:50 PM
This reminds me of one of my favorite demonstrations for kids;. . .


Yeah. It works well. It's amazing how much air really weighs.

http://metalshopborealis.ca/pics/tankcar.jpg

pressurerelief
01-31-2009, 05:04 PM
You should not purge anything flammable with air since you will have a flammable mixture inside the vessel at some point. This is actually prohibited by DOT, the agency having jurisidction. Inert gas or CO2 is ideal and keep the cylinder with the valve down while purging a gas with a vapor density greater than one. You can actually layer gases in a cylinder via vapor density. It does not just mix as alot of people assume.

I work on gas cylinders and have a new customer that use to purge with an air compressor. He blew the side out of a cylinder via a flammable mixture and static electricity. I do all of their cylinders now that he has his shorts cleaned out.

The rail car picture is from steam cleaning and the last one off the top closed the manway and let it cool. OOPS

P/R

tony ennis
01-31-2009, 05:24 PM
What surprised me was how quickly it can happen (http://www.youtube.com/watch?v=lhMPBCeQKrw&feature=related).

I figured it would be slow, even for an aluminum can. Nope!

55 gal drum... (http://www.youtube.com/watch?v=P4LRz-1LaMY&feature=related)

Another 55 gal drum (http://www.youtube.com/watch?v=Uy-SN5j1ogk&NR=1)

And for completeness, here's the train car pictured above (http://www.youtube.com/watch?v=2WJVHtF8GwI&NR=1) getting crushed!

Alistair Hosie
01-31-2009, 05:24 PM
Fill the tank with malt whiskey twelve years old and send immediately to his lordship.:D Alistair

SGW
01-31-2009, 05:27 PM
I made a tool to unscrew the valve (it's like a tire valve). After unscrewing the valve, I've let the cylinder sit around going through temperature changes for a while (a couple of weeks) so it can purge itself.

Mcgyver
01-31-2009, 08:50 PM
I made a tool to unscrew the valve (it's like a tire valve). After unscrewing the valve, I've let the cylinder sit around going through temperature changes for a while (a couple of weeks) so it can purge itself.

as always, great input here.....SGW any details on what the tool or valve looks like? If I could get it open, wouldn't filling with water be an effective purge?

SGW
01-31-2009, 09:09 PM
You've never seen the little gizzies used to unscrew tire valve cores? You used to be able to get valve caps that would double as valve core wrenches. You could probably see one at any tire repair shop.

Look down in the hole in the cylinder that holds the valve. You'll see a small rod in the center, like a tire valve. If you push down on the rod, you open the valve. The rod goes through the center of what looks like a crossbar in the valve hole.

Get a rod that just fits down the hole, drill a clearance hole for the valve rod axially in one end, and cross-slot the end of the rod to fit over the crossbar that you see. Then just turn it to unscrew. As I recall, it needs a bit of persuasion to get started, but there's nothing weird about it.

I don't know if a standard tire valve wrench would fit -- it might.

Mcgyver
01-31-2009, 10:24 PM
perfect, thanks. needed some hand holding :)

GKman
02-01-2009, 07:40 AM
I just punctured a few "Emptys" so the scrapper would take them and discovered a new way to get injured. FROSTBITE.

pressurerelief
02-01-2009, 10:30 AM
Another thing you need to watch out for is that all of the liquid does not necassarily turn to vapor when the valve is opened on a cylinder. If you evacuate the cylinder by vapor, the liquid may cool below it's boiling point. You can end up with a cylinder that has liquid propane in it with no vapor pressure and appears to be empty. The term for this is auto-refrigeration. Cut the vessel and the heat from the saw will heat up the propane and guess what happens!

As with any cylinder "Empty" is a relative term. Once all of the pressure is relieved the vessel is not empty. It will still contain one volume of the vessel at atmospheric pressure. When dealing with railcars or fixed vessels this can still be one heck of alot of product.

Simply removing the valve and letting the vessel sit can be dicey. If you have a gas that is heavier than air (as more than 95% are), gravity will keep it in the vessel unless acted on my other outside forces. A water flush will do a couple of things for you. If any cooled liquified gas is present, it will be heated and volatalize. The remaining atmosphere of gas will be displaced by the water and replaced with atmospheric air when the water is removed.

P/R