Used/expired 20lb propane tank for a furnace?

Discussion in 'Furnaces and their construction' started by Redwolf947, Mar 5, 2019.

  1. Redwolf947

    Redwolf947 Copper

    So, I have an old LP Tank sitting here that's past its prime. I see some have used Kegs and I though if I pulled off the valve and filled it with water before I cut it would it be a good small size for a furnace? I'd be using soft brick and a burner like Oldironfarmer has? Thoughts concerns?
     
  2. FishbonzWV

    FishbonzWV Silver Banner Member

    Last edited: Mar 5, 2019
  3. Tobho Mott

    Tobho Mott Gold Banner Member

    Plenty of nice little furnaces have been built inside old bbq propane tanks. Should work great, so long as you are suitably careful cutting it open. I've read where some people recommend filling it up with water more than once, to remove all traces of propane, but I haven't built a furnace this way in order to know how necessary that is; my furnaces are built in a section of round 14" duct tubing and an 18" diameter steel grease barrel. I'd say it sounds like you know what you're doing, but IMO it never hurts to err on the side of caution when getting blown up is on the table...

    Good luck! Looking forward to following along with this build.

    Jeff
     
  4. Petee716

    Petee716 Gold Banner Member

    It'll work fine but it also depends on how much metal you want to melt. The steel skin doesn't need to be heavy so you have a lot of options. One of my furnaces is made from a 5gallon paint bucket. I used solid refractory and it fits a #10 from Legend https://www.lmine.com/index.php?main_page=product_info&cPath=1_2_142_374&products_id=5722.
    It works with all fuels including charcoal. The lid is just a 3" thick solid refractory disk with a 4" hole in it and a pair of handles bolted to an aluminum outer ring. No complaints with this kit.
    But I found the size to be limiting for my purposes so I built one out of a keg with a 10" bore. That will take up to a #18 of the same crucible. Now I wish it was a 12" bore. See where this is going?
    My primary fuel is waste oil which requires more combustion volume than propane, but it's free so I don't mind putting a too-small crucible in a larger furnace. But with propane you really do need to pay more attention to matching your crucible size with your furnace bore and vice-versa.
    So consider what you think you will be casting but be sure to consider sprue and gating as well. Sometimes that equals or exceeds the weight of the casting. Short pours are the pits.

    Pete
     
    joe yard likes this.
  5. dtsh

    dtsh Silver

    My little furnace is made from a propane tank; a couple of layers of wool and a slathering of satanite and it's held up to a few firings already with no sign of problems. I can fit an A12 crucible in it for just shy of 2liters of aluminum capacity, but that's about the limit of what I think could be reasonably stuffed into a propane can.
     
  6. My thought is it's fast and easy and unless you are a very exceptional person it won't be the last furnace you build. We all seem to want to build furnaces about as much as we want to cast.:rolleyes: I'm a big proponent of getting your feet wet without extensive analysis.

    The key is make sure your crucible will fit inside with room above for flue gases and below for a plinth. Everything gets taller than you first imagine.

    If you plan to use firebrick you'll need to cut them all to get them to fit in a space that small. A larger firebox like a 33 gallon drum will work too. The outside shell does not need to be very heavy.

    If you plan to build a portable furnace an advantage of a propane tank or keg is they provide a good structural shell to build out from.

    Depending on the lid lift style you choose, you need to think about furnace stability too. A very small furnace with a swinging lid can get unstable as the lid is swung open.
     
  7. Redwolf947

    Redwolf947 Copper

    Wow, thank you all for your very in depth replys. I'm not sure what to go with yet. Per the safety factor at one point I worked at a salvage yard so I have a good idea about safety. I've heard about people blowing themselves up welding gas tanks and in the same token how to avoid doing that.
     
    Tobho Mott likes this.
  8. Jason

    Jason Gold

    I would skip the propane tank... Look for a junk air compressor and cut it shorter. Better would be a 30gallon oil drum. A 20lb propane tank is only 12 inches in diameter (I know, they look bigger) and the oil drum is 18". Now add 2" of wool and 1" of mizzou and now you have a 6" diameter hole with a propane tank.... Doesn't leave much room for lifting tools. And if you run an oil burner later, you be glad you have that extra breathing room in the furnace. With the drum, you'll have a comfy 12" bore with lots of room and the ability to shove a 20 if the need arises. You really don't want to build another furnace unless you wear yours out. This is one of those things that wont cost you any more money to do this right the first time instead of doing the entire thing all over again later. My furnace is in reality almost a FOOT too tall. No one spoke up and told me to cut it down. :mad: At least I got the diameter right (air compressor) You'll buy a box of kaowool, run 2" thick on the walls and she'll heat up plenty fast. A good solid inch of mizzou on top of the wool, and you'll have it for the rest of your life. Plus, you can always sell if for what you have in it.
    If you can't find a 30gallon tank, I can look around here. Tons of this stuff in west texas.
     
  9. DavidF

    DavidF Administrator Staff Member Banner Member

    Ive made several from propane cylinders. If you take and open the valve on top of the tank, then screw in the nipple from the regulator and place in the sun for a day. After that you can remove the valve from the top of the tank, fill it with water and either replace the valve and cut a 3" hole in the bottom with a hole saw, or not reinstall the valve and just cut the top off the tank with it full of water....Hope I make sense....
     
  10. Rocketman

    Rocketman Silver Banner Member

  11. PatJ

    PatJ Silver

    I filled a propane tank with water, but it was a bit tricky.
    I hooked a water hose too it, and filled it as much as it would take, and then had to remove the hose and let the residual gas/air come out of it, then hook the water line up again, fill it some more, and repeat.
    It took several cycles of this to get the cylinder completely full of water, but once it was full, I could drill/cut into it safely.

    I did all this outdoors of course, and I say this because there was a guy on another forum who said "I opened the valve on my propane cylinder and left it outside for two weeks. I brought it in the shop, and removed the valve, and a lot of propane shot out of it".
    He did not realize the tank had a safety valve that prevented release of gas unless the hose was screwed in.
    He was extraordinarily lucky to have not blown himself off the face of the earth, and it just goes to show some of the folks we run across.

    Another guy said "how do you make this oil burner work" and was jerking the controls all over the place (in a video).
    His next post said "My furnace just blew the lid up into the air and over the top of the house; what the @#%&*& is wrong with my burner !"
    Newsflash: It was not the burner.

    .
     
    Tobho Mott and oldironfarmer like this.
  12. Gippeto

    Gippeto Silver

  13. Generally propane cylinders are clean inside. But if you happen to get one with some sludge or rusty scale, you will find there can be enough hydrocarbons in the sludge and under the scale to cause an explosion. It is best to leave the water in when cutting. It's messy but safe. My experience comes from working on pressure vessels in refineries, and it does not take a lot of residual propane to make an explosive mixture. Again, if the tank is clean inside dumping the water works, but if it is not clean the water displaces the oxygen and absolutely prevent the opportunity for an explosion.
     
  14. Redwolf947

    Redwolf947 Copper

    All excellent safety tips!! I didn't know about the rusty or dirty insides, figured that it would be clean.. Thanks.

    I'm thinking tho I have a really old compressor with a smallish tank but probably a good 18 to 24" diameter got it free 20+ years ago. Not sure haven't looked at it in a few years.. huh, might be safer in the long run..
     
  15. Tobho Mott

    Tobho Mott Gold Banner Member

    I have no complaints about the size of my furnace, built in an 18" diameter steel drum with 2" of ceramic fiber and an inch of dense castable refractory, leaving a 12" bore that I'm still not done growing into. It makes my #12 crucible look tiny, but as a waste oil burner user I can live with burning a bit more (free) fuel than absolutely necessary if it saves me having to build another bigger furnace down the line. Although I would probably enjoy that too other than the expense; building furnaces is fun!

    View attachment 117

    Jeff
     
    Jason likes this.
  16. Jason

    Jason Gold

    It's funny how a 2" layer of wool and just 1" of refractory adds up when placed inside a circle. That's 6inches off the diameter right off the bat! A number 6 crucible is 5inches across. Public school math here>>>>> 6+5=11. That leaves half an inch on each side to wiggle tools in there. I think what happens is, guys with propane tanks have crushed the wool down or didn't start with a solid 2" of wool. I am totally guilty of this myself. My air tank is 14" and I don't have the thicknesses of wool or face that I planned for or wanted, but I've got decent space to wiggle tools in there. I would never go less than 14" and my next one will be 16" or 18". Any bigger than that and it will be a fuel pig for sure.
     
  17. FishbonzWV

    FishbonzWV Silver Banner Member

    Jason, that's old school math, the new math is.
    (2) 2 + 1/4 = 4.5 >>> 4.5 + 5 = 9.5

    Gives you plenty room for complete combustion.

    I predict...
    Just like bifilm theory is changing casting, light weight fuel efficient furnaces are going to be the norm.
     
  18. Petee716

    Petee716 Gold Banner Member

    I paid about $20 for a brand new empty at lowes around 5 years ago when I built my Van Torne muller. Yes it cost me 20 but the valve came off easy, and no anxiety.

    Pete
     
    Jason and oldironfarmer like this.
  19. Joe Avins

    Joe Avins Copper

    I wouldn't dare contradict this, yet I do need to add that one can fool oneself into being obstructively cautious. I knew a guy who'd taken the valve and float out of a large propane tank, inspected the inside which was clean, flushed it with water, left it in the sun for a week, etc. and was afraid to touch it because he still "smelled gas." You'll smell the mercaptan additive for a long, long time unless it's baked/burned out even when there's no danger.

    Be safe. Be very safe. Also be realistic.
     
  20. I agree with you, and I'm not overly cautious. But if you don't have an LEL meter (lower explosive limit) meter, the slam dunk method is to make cuts with as full of water as you can get it.

    For those casually reading this post:

    If you're a real player you can cut on something above the upper explosive limit. For propane the lower explosive limit is 2.1%, it will not burn or explode below that limit. The upper limit is 9.5%, above that limit it won't burn or explode. That's pretty low. You can cut into a freshly drained propane tank and not create an explosion, until enough air gets in. If you're wrong you're gone, and your insurance may not pay off for suicide. Here's a chart, note acetylene goes all the way to 100% upper explosive limit. https://www.mathesongas.com/pdfs/products/Lower-(LEL)-&-Upper-(UEL)-Explosive-Limits-.pdf

    Some of the lower explosive limits are so low you can't detect them without a meter. Closed spaces, even five gallon cans, can kill you.
     

Share This Page