This article is adapted from Make: Fire, available at the Maker Shed and fine bookstores.

This article is adapted from Make: Fire, available at the Maker Shed and fine bookstores.

Propane is a wonderfully common fuel source. Blacksmiths and metal casters use propane to power torches; artists and makers use it to create flame effects like flambeau torches, flame cannons, Rubens’ tubes, and other visual displays. It’s available almost everywhere, is nontoxic and, given simple precautions, very safe to handle.

But when I’m inspecting people’s propane projects as a licensed flame effect operator (FEO) at events, I frequently end up explaining to people why their project, device, or effect is unsafe. Most of the time, it’s because they have no basic understanding of what propane is or how it interacts with the world.

You don’t need extensive chemistry or engineering knowledge. My book Make: Fire will provide you with everything you need to know to safely make use of this amazingly useful fuel.

Here’s how to build a simple low-pressure propane source for all kinds of fun fire projects — along with an overview of the key equipment and some critical mistakes to avoid.

Photo by Hep Svadja

Photo by Hep Svadja

Properties of Propane

Chemically, propane is extremely safe — nontoxic and noncarcinogenic. It’s colorless and odorless. If you think you smell propane, you’re really smelling an additive like ethyl mercaptan, put there so you can detect leaks.

Mechanically, propane is extremely dangerous because of the pressures and temperatures involved. Propane accidents can cause explosions, burns, frostbite, and asphyxiation. But compared to other fuels, these accidents are relatively easy to avoid with safe practices.

Fuel-Air Mixture

Fig1-4 - scale

Figure A. Comparing unpressurized vapor to cylinder sizes. A 20 gal. of liquid expands to 721ft3 of pure vapor. A 5 gal. of liquid expands to 180ft3 of pure vapor. Illustrations by James Burke

At normal temperatures and pressure, propane liquid will expand 270 times to become vapor (Figure A). However, that vapor will mix with air (a combustible mix is around 5% propane and 95% air) so that the amount of combustible vapor is much larger than the propane vapor alone (Figure B).

Figure B. Mixed with air, the combustible mixture is 20 times larger than propane vapor alone. A 20 gal. of liquid propane mixed with 5% air creates 14,556ft3 of combustible vapor. A 5 gal. of liquid propane mixed with 5% air creates 3,639ft3 of combustible vapor.

Figure B. Mixed with air, the combustible mixture is 20 times larger than propane vapor alone. A 20 gal. of liquid propane mixed with 5% air creates 14,556ft3 of combustible vapor. A 5 gal. of liquid propane mixed with 5% air creates 3,639ft3 of combustible vapor.

Liquid and Vapor

A propane cylinder contains liquid and vapor. Illustrations by James Burke

A propane cylinder contains liquid and vapor.

At normal pressures, propane boils at –44°F (–42.2°C). So ordinarily you’ll only encounter unpressurized propane as vapor. Inside the cylinder at room temperature, the propane boils just until it fills the empty headspace with enough vapor to pressurize the system and keep the rest of the propane liquid.

Heavier Than Air

Propane vapor is 1.5 times heavier than air. This causes it to sink rather than rise. This is important. If an indoor propane system leaks, the propane will pool in the bottom of the room, potentially asphyxiating people or causing an explosive hazard.

Strong Solvent

Fig1-4 - sizeLiquid propane is an excellent solvent of petroleum fractions, vegetable oils and fats, natural rubber, and organic compounds of sulfur, oxygen, and nitrogen. Acetylene red welding hose and other natural rubber hose is not appropriate for propane use due to its composition, nor is any equipment with rubber O-rings or seals.

Propane does not corrode or dissolve metals, polyvinyl chloride (PVC), or polyethylene (PE) — but its pressure and temperature may cause these materials to fail (perhaps catastrophically).

Combustion

For most readers, the point of working with propane is to burn it!
» Propane requires a spark to ignite in air, unless the temperature is above 920°F (493°C).
» Propane will only burn in a specific propane-air percentage, generally between 2.1% and 10.1%.
» For propane to burn cleanly it must be 4.2% in proportion to air. This is complete combustion, producing only carbon dioxide and water as byproducts (or stoichiometric combustion for you chemists and word lovers).

Less propane results in a lean burn, where the flames lift from the burner and try to go out. This is an oxidizing flame that introduces extra O2 into the atmosphere.

More propane gives a rich burn that creates large, yellow flames. This is a reducing flame that will pull oxygen from the air, creating carbon monoxide (CO) and possibly soot (carbon).

Taping and Tightening Threaded Fittings

Taping Joints

Photo by Tim Deagan

Photo by Tim Deagan

I’m a taper, not a doper. Therefore, most of the joints in my book, other than the flare fittings, rely on yellow Teflon tape to become gas-tight. Taping a joint correctly is easy to do. Four wraps of tape clockwise around the fittings will do the job. Use your thumb to hold down the first wrap so it doesn’t slip. Wrap with enough tension to allow the threads to make a sharp crease (but don’t overdo it), and don’t let any tape hang over the inner passageway of the fitting. When the wrapping is completed, pull the tape until it breaks itself off at the back of the threads.

Photo by Tim Deagan

Photo by Tim Deagan

If you take apart a taped joint, use a wire brush to get all of the old tape out of the threads (on both the male and female sides of the joint). Never retape over old tape. Tape is relatively cheap. Pull a joint apart and retape if you need to; it’s better to use a little extra tape than to have an unsafe joint.

Tightening

How 
tight should you tighten threaded fittings? Unfortunately, the answer is, “Tight enough to stop 
the gas from leaking.” This isn’t typically described in terms of torque — it’s something you develop a feel for. I typically tighten a taped NPT fitting until it starts to feel like it won’t turn much more, and then I give it a turn or two more. That’s vague, because the difficulty varies tremendously between fittings (and people’s ideas of difficult). So tighten it and then leak-test under pressure to get a feel for it.

The important tip here is to always use a wrench to brace a part you don’t want to have move. Use the second wrench to tighten the next part into it.

CAUTION — DON’T USE:

  • WHITE TEFLON TAPE. It’s too thin, so it shreds and clogs your valves and fittings. Use yellow gas-rated Teflon tape.

    Photo by Tim Deagan

    Photo by Tim Deagan

  • CAST IRON BUSHINGS. They’re too brittle; they’ll crack under torque, then leak gas. Use brass bushings.

    Photo by Tim Deagan

    Photo by Tim Deagan

  • HOSE CLAMPS. They’re not rated for gas pressures. Use pre-fitted propane rated hose instead.
  • PLASTIC VALVES. Typically designed for water or other liquids at pressures far below gas ratings. Plastics may also be corroded by propane. Use brass valves.

    Photo by Tim Deagan

    Photo by Tim Deagan

  • COMPRESSED AIR FITTINGS. They’re unrated for gas pressures, and they frequently have rubber seals that will corrode under contact with propane. Use gas-rated fittings with appropriate seals.