This instructable presents my method of building a propane powered fire pit inspired by another fine instructable: http://www.instructables.com/id/Sand-Fire-Garden/
My method makes use of readily available copper tubing and fittings and requires soldering rather than welding. Also, my vessel is a simple ceramic flower pot rather than a stainless steel wok.
Before we get started, be aware that the construction and use of this fire pit involves fire, which when in the wrong hands can be extremely hazardous to life, limb, and property. Please observe all applicable safety precautions and local ordinances when building and using this device. Follow these instructions at your own risk.
Step 1: Bill of materials
(1x) 10' roll of 1/2" bendable copper tubing
(1x) 10' roll of 1/4" bendable copper tubing
(1x) 5' section of 3/4" rigid copper tubing
(3x) 3/4" copper pipe caps
(3x) 1/2" to 3/4" copper T's
(3x) 1/2" copper T's
(1x) 1/2" copper to 1/2" FPT adapter
(1x) 1/2" MPT pipe nipple 1.5" long
(1x) 1/2" FPT 90 pipe elbow
(1x) 1/2" MPT to 3/8" flare adapter
(2x) Large stainless fender washers
(2x) Large silicone O-rings (with greater than 3/4" inner diameter)
(1x) Adjustable high pressure propane regulator
(1x) 10' long high pressure propane hose
(1x) Ceramic flower pot
Play sand or similar to fill your flower pot
Tools and Equipment:
Lead free solder
Lead free soldering flux
Propane torch or equivalent
Wire brushes or emery paper for cleaning copper
Tubing cutter or hacksaw
Teflon tape for pipe fittings
Power drill and 1/16" bit for drilling holes in burner
Spring tubing bender or similar
Soapy water and spray bottle for leak checking
Steady hands and an iron will
Step 2: Build the stand
You'll also want to decide how tall you want the fire pit to be. With a 5' section of 3/4" pipe you can make each leg up to 20" long. I cut mine to about 13" to keep the fire pit rim about 16" off the ground. I didn't want it to be tall and top heavy. Use the tubing cutter or hacksaw to cut the pipe to length.
Once the three legs and arcs are cut you can start dry fitting the pieces together to test for proper fit. You may need to bend the arcs or shorten them so that the assembly grips the pot well. The increased size of the T fittings makes a snug fit all the way around difficult, but you can increase the number of contact points by straightening the arcs so that the arcs touch the pot at their midpoints. With the arcs, Ts and legs dry fit, make an estimate as to how big your lower circle will be. Take the 1/2" bendable tube and construct a circle by carefully bending it until it is at the size you want. You can have the lower circle inside or outside of the legs. I put mine inside since there wasn't enough tube for putting it outside and I didn't want anyone to step on the lower tube. This bendable tubing is not strong so don't try to use it as a foot rest.
Lastly, you should cut 4-6" sections of the 1/4" copper tubing that will end up being bent over the rim of the pot to serve as a retaining mechanism for the pot in the stand.
Step 3: Solder the stand
The basics of soldering copper fittings is that you first clean the outside of the end of the tube to be inserted into a fitting with a special wire brush or emery paper as well as the inside of the fitting. You are looking for nice clean shiny metal. Then, apply flux paste to the now clean tube end and fitting. Fit all your cleaned and fluxed pieces together and break out your torch. You want to heat the joint until you can place the solder rod at the seam and the solder easily melts and is sucked into the joint by capillary action. You'll need to sweep the torch over the fitting as well as the tube to get them both hot enough, and you want to remove the torch before applying the solder. If you want, you can buy those cast solder rings to place in the fittings. They are more expensive than regular solder but are pretty slick and give nicer joints since you don't end up applying too much solder which can drip everywhere. In the case of the rings, you just heat the joint as before until you see the solder wick out and a meniscus form at the seam. Nice.
Take care on the order of soldering, and it is easiest to solder the stand when it is all fluxed and assembled so that you can get the angles right on the legs. I used a high temperature kapton tape to hold some parts in place during the soldering. This tape will still be burnt by direct heat but won't melt from touching the hot copper tubes.
I soldered on the caps to the bottom of the legs first, and then the Ts to the tops of the legs. Then I fluxed and fit the arcs to the Ts, and taped the lower hoop in place to set the proper leg angle. With that all in the correct location I soldered the 1/2" tube arcs to the Ts. Then I used a pair of pliers to hold the 1/4" retainer spikes in place while I heated and soldered them in place. This is a bit tricky since you have to hold the pliers very still while the solder cools.
Lastly, I used an extra piece of 3/4" pipe to bang a dent in the 1/2" bendable tube loop to increase the surface area of contact between the hoop and the leg. After cleaning and fluxing these areas I soldered the lower loop in place while the stand was upside-down so that gravity help the loop in place. I matched up the seam in the loop at one of the legs to help it hide. You'll probably need to feed in a lot of solder to get a good joint here, it is kind of tough but it doesn't need to be perfect.
Take a look at some of the photos to get an idea of how the joints come together.
Step 4: Build and solder the burner
The burner is basically 2 arcs connected with a central spoke by 2 T fittings. The central spoke has a T fitting in its middle as well which points down to the hole in the bottom of the flower pot. See the photos for a clearer idea.
To start you should bend a circle out of your 1/2" tubing that fits inside your flower pot and is about 2" away from the side walls when placed about 2-3" below your envisioned sand level in the final fire pit. It is tough to bend the 1/2" tubing this tightly without kinking, so use a spring bender or similar if needed, or fill the tube with sand or other method. The circle doesn't need to be perfect. Cut the arcs to size so that they mate with the Ts properly. Cut some straight sections for the central spoke and T as well.
Depending on how tall your burner needs to sit in the flower pot you may need to add a length of 1/2" tube between the central T and the 1/2" FPT fitting. I needed the burner to sit as low as possible so I used a 1/2" FPT fitting that slid directly into the T. See below.
Once all the parts are ready, clean and flux them, and then fit them together for soldering. Solder it all together. Do not drill the holes yet.
Step 5: Assemble and leak test
See the photos to get an idea of the setup. Yours may vary. The accompanying sketch shows a cross section of my fitting setup.
Be sure and use teflon tape on all the pipe thread fittings, do not use teflon tape on the flare adapter to the gas hose. If you like a belt and suspenders approach, the safest way is to use the teflon tape and then put teflon paste over the tape prior to assembly.
Assemble all the fittings and the burner. Connect a pressure source to the setup and pressurize to a few psi, like 10. Spray the burner and all fittings with water mixed with liquid soap (like 1 tablespoon of soap to 1 pint of water) and look for blowing bubbles. If you don't see any bubbles then there aren't any leaks. The easiest way to pressurize the system is to hook up the gas hose to the flare fitting, hook up the 1/4" MPT end of the hose to the regulator (use teflon tape) and hook the regulator to a propane tank. Turn on the gas to pressurize the system. This can be dangerous if you have a leak though...
Step 6: Drill holes in burner
Step 7: Fill with sand and test
Connect the gas hose to the flare fitting on the bottom of your flowerpot, and then connect the hose to the regulator and the regulator to the propane tank.
Next, seat the pot firmly in the copper stand so you are satisfied and then bend the retaining spikes over the rim and into the sand so that they hold the pot firmly.
With everything connected, sand in place, pot retained, then turn on the gas valve on the propane tank. Slowly turn the pressure regulator knob to increase pressure until you can hear gas flowing. If you turn the knob too rapidly you may trip the safety valve in the propane tank which will sound like a click. You'll have to turn down the pressure knob until you hear it click open again to allow gas to flow again. Once you hear gas flowing, start trying to light the top of the sand with a match or your torch or other flame source. You may need to wait for the sand to fill with gas. Once you've got a fire on the sand, then you can adjust the flame level by slightly adjusting the pressure regulator higher or lower as indicated on the knob.
Step 8: Enjoy!
Sit back and enjoy!