There seems to be some confusion about which type of pipe threads need to have some sort of sealant on them, and I hope this article will help clear that up as well as a few other issues about fittings and running pipe for your oxygen and fuel gas.
First of all, let’s define thread sealant – there are two different types of acceptable pipe thread sealants: teflon tape and thread “paste”. Thread “paste” is a thick liquid, thicker than molasses that comes in a jar with a brush cap. You simply “paint” it on the threads of the pipe you want to seal. There are two main varieties of “paste” – standard and fuel-gas rated. Fuel-gas rated “paste” should be used on any pipe thread that will be exposed to Natural Gas, propane, or any other fuel gas (it can also be used on non-fuel gas pipe threads). Standard “paste” can be used on any pipe thread that is not exposed to a fuel gas.
There are many different types of pipe – stainless, galvanized, black (actually iron), copper, brass, etc., but the key is the actual TYPE of thread that the fitting (or pipe) has. If the fitting/pipe has an NPT (National Pipe Thread) thread on it, you absolutely MUST use a thread sealant on it. I have seen some mis-information spread around saying that brass threads do not need sealant. This is patently false. If the brass fitting/pipe has NPT threads, then it MUST have a sealant applied to the thread or you will have a leak.
NPT threads come in fractional inch sizes. For example: 1/8, 1/4, 3/8, 1/2, etc.
What confuses some people is the type of fittings that are available for use with these different pipe sizes and making the connection to standard welding fittings.
There are several different types of fittings that are acceptable for use, and one that should never be used under any circumstances (I’m talking strictly about plumbing for oxygen and fuel gas here, mind you).
NPT threaded fittings
Flare type fittings
“B” (or welding connector) fittings
Compression type fittings
NPT and “B” fittings are the most common that glassworkers will use when connecting their oxygen and fuel gas lines to their torches.
Flare fittings must be used when you use soft copper tubing as your main supply line for fuel gas. Soft copper has many positive attributes to consider if you are planning out your fuel gas or oxygen run. It can be run through exterior walls. It can easily be bent (with a bending spring) to conform to corners and odd shapes. It can easily be flared. It is much easier to use (and much less expensive) than rigid pipe.
However, keep in mind that a flare fitting, even though it is a metal-to-metal sealing fitting, is not the same as a “B” fitting. “B” fittings are only found on welding equipment like regulators and hoses. “B” fittings are also metal-to-metal seals but will not work with a flare fitting.
“B” fittings come in right-hand and left-hand threads. The fuel gas line (red) is always left handed. The oxygen line (green) is always right handed. And here’s the crazy thing that really confuses a lot of folks: if you are running a hard line through your wall for your torch, you DO NOT need to make all of the threaded fittings left handed. Keep it simple, keep it inexpensive, and use standard right hand threads. Even the existing lines inside your house right now are right handed up until they actually hook up to your appliance. The appliance thread is the only left handed thread in the entire installation.
And one last point: regardless of where your oxygen/fuel gas pipes run, be sure that there is a sign on the pipe indicating what gas is flowing through the pipe and in what direction. The shut off valves need to be marked clearly as to which gas they are shutting off and in which direction the handle must be turned to shut off the flow.