Fire. Man's best friend, scourge of vampires, zombies, and other things that go bump in the night. Mastery of fire is what separates mankind from the animals. That and it is fun to watch.
This instructable presents my method of building a propane powered fire pit inspired by another fine instructable: https://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
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
The first step is to build the stand to hold the flower pot. Depending on the size and shape of your flower pot you may have to alter the presented design somewhat, but the following should serve as a decent guide. The design of my stand is based on a 3 legged stool configuration with a ring around the top and bottom for stability and strength. Using 4 legs is less desirable since it will most likely rock back and forth on uneven or even surfaces. I didn't take a lot of pictures during the build process so I've mocked up some CAD to let you see how the parts go together. The first and hardest parts will be making the 3 arcs that encompass the flower pot and mate with the 3/4" to 1/2" T's. Use the natural coil of the bendable 1/2" tubing and gently and slowly increase or decrease the curvature to match your flower pot. When you get a good match, use the tubing cutter to cut 3 equal arc-length segments that when mated into the T's encircle the pot snugly. If your pot has a rim or lip it might be a good idea to make the circle just big enough to allow the pot to rest its rim on the copper circle.
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
I'm not going to go into detail on how to solder copper fittings, but if you are not confident in soldering then please start with the stand so you can get better in time for the burner. The burner needs to be gas tight, so good joints are important there, while the stand just needs to be strong, not gas tight so is a good place to practice.
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
Now that you are a pro at soldering, it is time to tackle the burner assy. Now I'm sure some folks would be nervous about using soldered connections for a burner, and so am I. The fear is that if the burner gets hot enough, the solder joints will liquify and the gas pressure will blow them out and the gas will come out in an uncontrolled fashion. I haven't seen this in practice. The good news is that modern propane tanks have a valve that automatically closes off if the flow of gas increases too quickly indicating a sudden leak. Be safe.
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
The next step before we drill the holes in the burner is to assemble everything and do a leak test to ensure that gas is not leaking from any of your solder or pipe joints. Depending on how thick the bottom of your flower pot is you may need to use a shorter or longer pipe nipple to bridge the hole. The purpose of the washers and o-rings is to grip the pot tightly, prevent sand from falling out, and provide some cushion when tightening the pipe fittings. Getting the pipe fitting stack up to be tight and leak free exactly at the right distance to grip and seal to the flower pot while not crushing and cracking it is a challenge so the o-rings give you some leeway. If you need more make-up height than o-rings can provide, use a stack of additional washers, a thicker gasket material, or some other method. I used o-rings I had on hand. You can also adjust the amount of teflon tape you apply to slightly increase or decrease the number of turns required to get a tight leak free seal.
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
Now that you have ensured there are no leaks in your system, it is time to introduce leaks into your system. First off, you'll need to unscrew the burner assy from rest of the system, and wipe up or remove any teflon, ensuring none falls into the exposed lines. Then take a 1/16" drill bit and drill a hole in the bottom side of your burner every inch or two. Drill on the bottom to keep sand out as much as possible. See the pic for a better idea. Once the holes are all drilled, you'll want to reapply teflon and rescrew the burner into the flowerpot and pipe nipple.
Step 7: Fill With Sand and Test
Next, fill the flowerpot to the desired level with sand. You'll want 2-3" of sand on top of the burner for some reason that I can't think of.
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!
With all that work, now is the time to fire up the fire pit, crack open a cold one, and roast some marshmallows. Cooking on this fire pit is not advised but careful roasting of marshmallows seems to be ok. Care should be taken not to touch the sand with your marshmallows or with anything you don't want burnt. After an hour or so of running, the pot and sand can get pretty hot so don't touch it during operation and allow it to cool before handling. The copper stand does not get all that hot, which is good since it is soldered together.
Sit back and enjoy!