There are a number of posts on various Internet forums with conflicting and dangerous information on LP (Low Pressure) air piping materials. This is not intended to be "the last word" on the issue, but here is what I learned.

PVC piping is prohibited by OSHA for compressed gas above ground because it is a shrapnel hazard on failure. Click here for info. OSHA often moves pages around so just search on PVC if the link becomes invalid.

It is very easy to accidentally bump a PVC air line and end up with a face full of plastic shards. The risk is much greater in cold climates. It is NOT an issue of pressure rating, only failure mode. I am told by a licensed plumber and a Ph.D./P.E mechanical engineer that it is also prohibited by most building codes for the same reason (definitely in Southern California).

Note that compressed gasses continue to expand, and propel particles, when pressure is released where liquid pressure drops virtually instantly making the risk of projectiles far less hazardous. Anyone who has built an air cannon knows how dangerous LP air can be.

I was told by these same people that Type L and K copper pipe is routinely accepted by OSHA and building inspectors in the US, but the thinner wall Type M is prohibited by all building codes for compressed gas above ground. I could find no specific reference at OSHA on copper pipe weight, though that doesn't mean there isn't one. Both men told me that black iron (steel pipe that is not plated) and galvanized pipe are accepted by building inspectors and OSHA and is preferred (sometimes required) where there is a high risk of fire. Copper LP air pipe will fail in a fire with the risk of feeding the fire with even more oxygen. Assessing the fire hazard (like a woodshop) is usually a judgment issue with the building inspector and fire marshal.

The ME, who designs laboratories all over North America always uses Type L copper pipe for LP air, medium vacuum, and non-corrosive compressed gasses. The main reason is the ease of installation, lack of corrosion (no paint needed outside and to avoid contamination inside), and far fewer leaks that develop over time with soldered joints than threaded NPT pipe.

I prefer 3/4" Type L Copper for shop air, but use galvanized steel for very long runs and large diameters (1-1/2" to 2"). I always add a filter and moisture separator before transitioning to copper for distribution and drops.

It is very easy to learn to solder Copper pipe with a Propane torch and many kits come with instructions. Just make sure joints are clean, use a compatible flux, and heat the fitting until solder melts on the pipe and is drawn in by capillary action. Use a good pipe dope of Teflon tape on all NPT (National Pipe Thread) connections. I prefer Teflon on Copper threads and a semi-hardening dope on large Steel threads.

Galvanic action is the biggest problem with copper. It is not a critical issue with heavy screwed pipe fittings but thin Copper pipe can be dangerously pitted by the wrong pipe clamps or hangers. Secure Copper pipe with plastic or Copper pipe brackets as steel clamps will react with the pipe.

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