When PVC fractures, it shatters and can be like schrapnel flying around!  There are some modified ABS resins and HDPE resins being made into non-metallic compressed air piping systems.

Thanks, jdarco.  I thought it might have something to do with failure.

---KenRad

There is a possibility of static electricity buildup when PVC pipes are used for compressed air/vacuum systems.If there are flammable gases or substances present in the vicinity,it could be risky.

The shrapnel aspect is the only specific reason I know of, but I've always looked at PVC in this service the same way as soft solded copper pipe - what happens in the event of a fire in the plant? With any "luck" at all, you can compound the excitement of a plant fire with a pressure piping system that's failing at the exact same time, in the exact same location.

PVC has a low fracture toughness. Its failure mode is to shatter. Below 5C the fracture toughness of PVC decreases drammatically.

For an experiment put a PVC fitting in a domestic refrigerator (temp 4C). When cool hit with a hammer.

PVC is not real good with fatigue either.

If you break a water pipe, the water expands very little so it doesn't propel fragments in every direction at high velocity.  When a metal compressed air pipe ruptures under pressure, it typically fails either at a joint (leak) or longitudinally along the pipe (seam).  Typically, this results in a partial break without separation and with few fragments.  This is more like a tear than a fracture.  Once pressure is sufficiently relieved, the tearing stops even though air continues to escape.  When a PVC water pipe breaks, it too may break at a joint or longitudinally, but can also break transversely to the axis of the pipe. Yet even this is little more of a people hazard than metal pipe or reinforced hose. However, when PVC pipe ruptures from gas pressure, it fragments with many small shards propelled at high velocity, and remaining pipe may be whipped like a hose with continued fracturing until the remaining pipe is at a point that is well constrained.

PVC is also subject to greater abuse from being struck or strained.  Consider bumping hard against or stepping on a PVC line carrying 80 PSIG water versus a PVC line carrying 80 PSIG compressed air.  When the former is bumped it may well leak, spraying the person and possibly injuring him, but the compressed air line, if cracked, will shatter explosively and will certainly cause severe injury.  

For these reasons it is good practice to avoid using PVC to convey high pressure gases.  Code enforcement has gone further.  In 1988, OSHA ruled PVC pipe is no longer to be used for other than buried compressed gas transmissions such as compressed air.  Many local regulatory agencies have similaryly outlawed PVC for above ground compressed gas applications.  So, it is no longer simply a matter of bad practice, it's illegal.

Bob S

I concur with those who point out the physical failings of PVC.  Although I think the reason it was outlawed in buildings was because of the hazard PVC fumes posed to fire fighters during building fires.  According to the attached msds  “Burning (PVC) in an open flame may yield methane, ethylene, ethane, propene, benzene, toluene, xylene, ethylbenzene, naphthalene, carbon, hydrogen chloride, metal oxides, carbon monoxide and carbon dioxide.”

Note that PE pipe is also outlawed in buildings in conjunction with combustible gases because during a building fire, the melting plastic pipe will fail and allow combustible gas to enter the building and burn.  
  

www.jm.com/msds/EN3202.pdf

RGasEng,
I was a little surprised not to see hydrogen chloride and carbon monoxide (that I thought were once some potential combustion by-products at least of pvc pipe) on your sort of bad actor list!  While I can’t necessarily vouch for the following references might also need to be quite careful with at least some kind of welding (see case account at http://www.aldewitt.com/soc.htm), and best don’t fabricate any kind of kind (smoking) pipes out of small pvc (pipe etc.-- see “Claudia Morgan"’s letter at http://www.motherearthnews.com/library/1991_August_September/Country_Lore)!

They’re there.

Interesting anecdotes, I’m sure a Google search would turn up dozens.  PVC stories have probably achieved the status of urban legend by now, but I'm pretty sure I recall the NYC DOB banning the use of PVC in new construction because of the hazardous fumes.

I just found the attached webpage that gives more detail on PVC.  You can probably find more recent info.

http://www.mindfully.org/Plastic/Polyvinylchloride/PVC-Health-HazardPWG25oct01.htm

oops,  RGasEng (sorry, I read your post too fast and carelessly! -- HCl etc. were what I thought I had remembered reading in the New York etc. firefighting discussions).

I'm pretty sure the prohibition against rigid PVC piping for compressed air has to do with both the likelihood of failure due to embrittlement by exposure to compressor oil, and the likelihood that PVC will produce damaging schrapnel when it does fail.  These are both realistic possibilities.  PE and nylon tubing for compressed air user drops aren't subject to either of these concerns.

If there's a prohibition against PVC due to fumes for firefighting, it doesn't apply to the thousands of other PVC articles which abound in every building, so that doesn't make much sense.  Any firefighter entering a burning structure has to be prepared to face a myriad of nasty pyrolysis products from modern materials of construction.

As to the fire failure safety of compressed air systems, I'd rather have my compressed air system leak during a fire than have the main storage vessel rupture due to fire exposure.  Most of the small compressed air systems I've seen don't have fusible plugs in them to protect against fire, only a pressure relief valve which isn't enough.

It looks like Governor Pataki vetoed the extension of the ban on PVC pipe.  So I guess it's OK now.  See attached article.

http://www.iaff.org/across/news/Archive2004/122104pvc.html

I'd rather have my compressed air system remain intact during a fire, and have the receiver properly protected, as required.

I've seen a hot condensate leak result in the failure of a plastic air line. While the condensate line shouldn't have leaked, that kind of thing does happen in plants, along with cowboys on lift trucks, people using horizontal pipe for ladder rungs, hanging chainfalls, etc. Unless a fluid is being handled that really requires the characteristics of plastic pipe, it's - at best - problem just waiting to happen. At worst, it's a disaster waiting to happen.

I happened to notice some interesting European research today with regard to the issues of various pipes in fires at http://www.saint-gobain-pipelines.co.uk/soildrain/news2.cfm?newsID=139&latest=1 (I don't know if any of the parties that have debated this subject in various U.S. states etc. had actual research like this or not).