The Shop > Metal Stuff
Foundry Furnace for the Tiny Shop
vtsteam:
I've debated whether to explain my procedure for cutting up a propane tank. I'll put it down here, but this isn't a recommendation to anyone, just an explanation of what I did, and why I did it that way.
I used to build pulse jet engines, and some of the principles apply, to my way of thinking.
The first principle in avoiding a detonation in a container is to remove all fuel. No fuel, no detonation.
The second is to reduce the volume of potentially combustible mix as much as possible. Reduced volume, reduced energy available.
The third is to provide a relatively large opening in the container. The less containment/restriction the more likely conflagration (rapid combustion) vs detonation (explosion).
Taken all together:
1.) I opened the gas valve on this propane tank and left it outside for 3 years. That still doesn't guarantee no "fuel" since these tanks typically have a tarry residue internally, which if heated will boil and offgas under torch cutting heat and even though it isn't propane itself, it is flammable (and explosive if contained).
2.) Removed the valve entirely with a pipe wrench. This increases the size of the orifice greatly compared to leaving the valve on. This alone reduces the chance of a detonation.
3.) Washed the tank out with hot soap and water. This won't remove most of the tarry residue. So it would be foolish to depend on it. It will remove lighter oils, to some degree.
4.) Filled the tank with water to an inch under the top. This drastically reduces the volume of potential air and fuel mix. The ratio of the current orifice size (about 3/4" diameter) to the contained gas volume (about a pint) has now been increased maybe 1000 times what it was with the open valve attached and the tank empty. There is little volume to burn, and an easy way for hot gasses to exhaust if it does.
5.) With the tank full of water I lit a match and held it over the orifice. No reaction. no pop. Nothing. (Again, I'm not reccomending this to anyone, I'm just repeating what I did and why I did it this way. What I did applies only to this tank, this type of fuel, and this lead-up procedure, not every situation. Make no assumptions about different situations based on this.)
6.) I drained out about a quart of water and repeated. No ignition.
7.) I drained out enough water to lie a little below my cutting line, and repeated. No ignition.
8.) I used a 4-1/2" grinder with a cutting disk in it to score around the tank, still filled with water, and then cut through. The water level was low enough not to wet the disk or grinder, and I wore rubber gloves.
I did NOT use a cutting torch. A cutting torch raises the temperature of the surrounding metal tremendously, and I did not want to vaporize any possible tarry residue in the tank. A torch is in my opinion a bad tool for this kind of thing, and procedures which use inert gasses in the tank are less safe than using water. If the gas isn't right, you can't tell, and the contained volume is way too big for safety. Leaving the valve on the tank really increases the danger.
That's my personal opinion. Many tanks have been cut with torches, and many swear by inert gas fills to do it with. I prefer my way. It makes sense to me, and results are apparent at every step.
Sorry to go on at length here, but I haven't ever seen this explained this way before and thought it might be useful. Nothing in this guarantees safety, and every person must assume personal responsibility if they try to use a propane tank. Other substances and tanks can behave completely differently.
RobWilson:
:clap: :clap: Nice one Steve , I do like a good foundry/furnace build :thumbup:
Rob
PS ,That white stuff looks cold :(
vtsteam:
Thanks Rob, Andrew, Don! :beer:
Maybe also a word on cutting the exhaust hole in the tank top:
I took a piece of hardwood and whittled it into a tapered plug, then hammered it into the threaded valve hole. Then sawed it off flush. Marked the center, punched it. Then used that as a guide for the pilot of a 3" bi-metal hole saw in an electric hand drill. Low speed and light pressure cut a perfectly clean hole, centered on the top. I did this before filling with the plaster mix.
The tuyere hole was also cut with a hole saw sized to fit U.S. 1-1/4" steel water pipe (1-38" ID actual, and about 1-5/8" OD) started square radial but after cutting through the outer metal and into the plaster angled to be more or less tangential to the furnace chamber. The bottom of the tuyere is about 1" above the furnace chamber bottom.
The tank and top were wire-brushed to remove the tarry residue in the tank before pouring with plaster, as well.
vtsteam:
It's started to snow, and get dark out and the furnace has been cooking for 6 hours now, so I added more charcoal and about a minute of soft blast to get the coals all lit. They're carrying on well, now and the heat is definitely up a notch:
mattinker:
Steve,
I see where you're going, plaster does have good refractory characteristics. My only fear is that it may not stand up to repeated heat cycles. It is relatively soft. It is used to make moulds for lost way casting which are only used once and are then broken open. I've never seen any references to it in kiln or foundry construction. I would think that something like ITC100 painted on as a surface protection might be useful to increase usable life. This is an interesting experiment!
All the best, Matthew
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