My response to PVC pipe swelling and bursting is: The issue of using plastic pipe for plumbing compressed air has been covered on a number of pages on this website, though clearly, it bears repeating based on this recent request from a visitor.
PVC pipe is not normally pressure pipe as it pertains to use in compressed air systems.
The reason is that the pressure rating of plastic pipe is predicated on the temperature of the area in which the pipe is to be installed. A specific PVC pipe may have a burst rating of 175 PSI at 70 degrees F, for example.
However, the burst or fail pressure of plastic PVC pipe falls dramatically as the temperature rises. Ambient temperatures at the installation location greater than the rated 70 degrees F means that the pipe wall is weakened by the higher heat, and can no longer handle the pressure it was formerly rated for. That PVC pipe will burst, much like the PVC the visitor writes about.
Do your workers and your liability insurance folks a favor, and remove all plastic pressure pipe (unless the manufacturer says you can use it) completely! Fast!
I am not a supporter of using black metal pipe for plumbing air. Yet, since it is low cost though, that is the pipe of choice for many folks to use for their air mains.
I feel that using black pipe requires some expertise and special equipment to install it. Black pipe is quite heavy to use. In time, with the amount of water that a typical air compressor puts into the air mains, it will not be long before the black pipe is rusting, pitting and scaling. This means that you will need effective air preparation equipment installed downstream from the black pipe to remove all the crud that it will, eventually, put into the air stream.
There are some air main systems that use aluminum piping. I perceive these to be the highest cost air main material, though the systems lend themselves to a more easier system than that of black pipe.
There are a number of plastic tubes available that are built to handle typical compressed air plumbing; polyethylene and urethane are a couple. They typically are small in diameter, and are meant to plumb compressed air from a manifold or air main to the point of use; air tools, valves etc. They typically are sized up to 1/2" O.D., and as such, are not large enough for plant air mains.
Please see the site map page on this site, under plumbing, for more information about installing compressed air lines.