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I have an empty 100lb propane tank that I want to cut the bottom off of to make a large bell. How can I do this safely? I am thinking, remove the valve assembly, fill with water, and then cut the base off. Hopefully with something faster than a hacksaw, maybe an angle grinder with a cut off wheel,
Does anyone have any thoughts or experience with this? How thick are these tanks?

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The local gas supplier here said to remove valve, flush with nitrogen, then cut.

I had a couple that had the valves out for months before I got around to cutting them up and had no problems, though there was still a faint smell from the aromatic they use in the gas here. Filling with water should work OK.

The tank walls in Australia are around 4 - 5mm thick and chewed up a few cutting blades in a 5" angle grinder getting around them. Watch out for the double layer where the ends of the tank are welded in or you may end up cutting double.

I used one to make a wood heater, cut the bottom off at the weld ring, welded a grate in the tube, cut a door in the side and hinged it, cut a hole in the top for a 5" flange and flue pipe, three scrolled legs out of 50 x 10 and a shelf to sit the bottom on to catch the ash. Works a treat once you get it red hot and burn all the galv off. It got named "Mr Squiggle" after a piece of 1" hex sharpened to a point was welded on the door, a friend said it looked just like his rocket so I had to!

Regards
Andrew

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Pretty much what everyone else has said, pull the valve and flush out the tank a few times. I haven't tried it with nitrogen when I've done it, that might have done a better job of washing out the aromatic. What I've done that I found works well for me is to drill a hole in the tank at the around the line I want to cut, mark my ring and cut the tank with a jigsaw. I get a nice clean cut, minimal sparks and as long as my blade is sharp I like to think it takes less time than if I had used a grinder. YMMV That said, I haven't worked much with the medium tanks, just the smaller tanks so I can't speak to wall thickness.

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water is messy, instead of nitrogen you can run a pipe into it from the exhaust of an engine (car) and after it has filled the space cut away. engine exhaust has no oxygen in it so it keeps things from burning just like nitrogen or carbon dioxide etc will. drill a hole near the end opposite the valve hole and when the exhaust coming out your vent hole will not support flame you are ready. (give it 20-30 seconds first) Test with a burning piece of cardboard held to the hole.

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water is messy, instead of nitrogen you can run a pipe into it from the exhaust of an engine (car) and after it has filled the space cut away. engine exhaust has no oxygen in it so it keeps things from burning just like nitrogen or carbon dioxide etc will. drill a hole near the end opposite the valve hole and when the exhaust coming out your vent hole will not support flame you are ready. (give it 20-30 seconds first) Test with a burning piece of cardboard held to the hole.


Hmmm. Something about your test meathod seems risky. Like holding a match to a leaking joint to if there is a gas leak......just thinking out loud...
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Like holding a match to a leaking joint to if there is a gas leak........


If you think there will be an explosion you're wrong. If this were so I'd be dead 50 times over. The leaking fuel will burn for sure but I've caught hoses and even guages on fire many times and the big bang just don't happen that way. If you don't know there's a leak and the room fills with fuel then you've got the recipe for disaster cause the fuel will mix with air and that's a bomb waiting for a spark....
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r smith that is an edited quote! If you had quoted "Like holding a match to a leaking joint" you would have been technically correct but somewhere along the line the THC "allegedly" in your brain has modified it to suit. Sorta like Homer and Doughnuts. I have several thought lines about what a leaking joint could look like but these include several diferent sorts of hydraulical, pneumatical, geological, prismatic, electrical, physiological, genetic, and probably a couple of others that waving a match at them could mean a multitude of things.

Regards
Andrew

PS: and thats after only a couple of glasses of red

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The *big* problem in cutting tanks in general is when you use a cutting torch which can provide it's own unburnt fuel-air mixture building up in the inside until you get the unexpected boom, (especially if you are cutting a tank that *never* held a flammable substance like a water tank!) If you are not using a cutting torch the possible issues go WAY down and simple removal of any leftover "fuel" followed by cutting method of your choice may be tedious but works.

Water flush is the easiest though some people throw in some degreaser to help remove the odorant when tends to be an oily liquid that can condense out on the walls of the tank.

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r smith that is an edited quote! If you had quoted "Like holding a match to a leaking joint" you would have been technically correct but somewhere along the line the THC "allegedly" in your brain has modified it to suit. Sorta like Homer and Doughnuts.
Regards
Andrew

PS: and thats after only a couple of glasses of red

I know it is edited and I saw an opportunity to poke fun and..... Mr Fe Wood is from a place called Grass Valley in Northern California so I could not help it.
The test method I mentioned is a good one to double check that no oxygen is in a vessel, AFTER it has been purged a while, NOT to check to see if flammables are in it, in the first place, but as Mr MacBruce stated in post #9 not a recipe for disaster.
smith out
P.S.
Sorry if I offended anyone in the spirit of fun.
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It seems that we have this discussion about every year or so. I would like to make a few points, first Repetition don't make it right or Safe. Just because you have done something incredibly stupid several times and got away with it don't mean it won't blow up and kill you the next time. Second exhaust fumes contain unburned combustable components like Carbon Monoxide introducing an atmosphere that is too rich to burn into a tank does not make it safe to use spark producing or flame producing tools to cut it. All that needs to happen for an explosion is for more air to be introduced into the container. A cutoff wheel will do this because the wheel acts as a fan pulling air in as it cuts. Third as stated before, oxy/acetylene torches will introduce unburned gasses into the container that can cause an explosion. Forth, wash the container thouroughly by filling it with warm soapy water. Rinse the container by filling it with clean water until there are no more bubbles, at least twice. Remember that dish washing and other detergents contain alcohol. Dawn Dish Washing Liquid Concentrate when shipped by tank truck carries a Flammable Placard. Then cut the container with a sabre saw or better yet, sell the Propane tank and buy a Bell.,

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a friend and I cut two home sized ones open just last week with a plasma cutter while they were full of water. the walls were nearly 3/8ths thick. jigsaw or sawzall cutting the 20 total feet of cut was not appealling to my friend after wasting three blades. these are to be bbq pits in their next life. I was more afraid of electrocution than explosion personally, since the tanks were filled to the top with water.

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**If the gas bottle is empty,
and the valve works, Eg; will let air in and out,
open the valve and remove it.
**Flush out with air or water.
If cutting with gas flame, face the opening away from you for your first cut.

If the valve does not open, or you can not safely check if empty and flush.... Do not do anything with it bar get rid of it.
DO NOT TRY TO CUT IT.
If you do not understand any part of this, get a friend to help you.
If you both don't understand, DO NOT CUT IT.
If in doubt, Don't cut it.

Some gas in the bottle can make for a big bang,
I've had the odd metho drum go pop, but gas cylinders are a welded, hard steel.

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I was rather nervous the first time I cut into a 20lb cylinder, I did it almost exactly how you described. I had a friend around in case things went south and to help out. I vented the tank for a few hours, slowly cut the valve first and then filled with water and flushed before cutting the bottle with an angle grinder. Brass doesn't noticeably spark but can heat up from friction, in retrospect it was probably an unnecessary risk. But I'm here to tell the tale and the job is over so it can be done.
If I remember correctly the 20lb'ers are only about an 1/8" thick I wouldn't expect too much more if any from the larger tanks.

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Be sure tank is empty! I just hooked up my regulator and tried to light the forge......no flow......Tank is empty right. Now I'm confident it's empty right????....Nope just confident there is no more pressure. If I lower atmospheric pressure out side the tank more flammable gas WILL come out!!. This is a fact! If you're satisfied the pressure is equal on both sides of that tank you can safely remove the valve. Fill with water and rinse as mentioned above. Drain completely and leave upside down. Hole pointing down. Propane is heavier than air. Wait till tomorrow and refill and re-rinse. Once filled again I drilled a series of holes near to the size I wanted to make my cut. This way the tank is full of water and the drill bits stay cool enough to not cause sparking. I then drill larger holes so that I may fit a hack saw blade through and patiently cut until I know there can be no fuel trapped in there. If there is a way to do this without producing heat on that tank until it's open that's the safest route. Torches and grinders make heat. No matter how you spell it out it's dangerous. Please be careful whatever method you choose. One pressure vessel mishap can really ruin your day!

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If in doubt scroll back up and read Glenn's post.........."Safely is the operative word. Have a company experienced in this type work do the job for you." Glenn.........Has anyone consulted Darwin on this issue?

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If you want to purge with CO2, once the valve is out and the tank has been thoroughly washed with soap and water and rinsed with clear water, put dry ice into the tank, a bunch of it, it will continue to sublime producing CO2 gas for a long time. Then cut with a sabre saw and you should have no problem with explosion. Be careful with liquid soaps, some if not all have an alcohol base. The fumes from Dawn Dishwashing Detergent will set off a flammable gas meter and in concentrated form it is transported as a flammable liquid.

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