I too have read many similar inquiries both here and on a sports car racing forum in which I participate. In every case, someone warns of the dangers of PVC bursting and shattering, but no one knows of any actual cases of that happening. On the other hand, I've been using PVC for that purpose for more than 30 years, the latest in a shop that I've had for almost 15 years, with no problems. The only thing I try to avoid is exposure to the South Florida sun. I have bumped my airlines with equipment a couple of times and broken it. In each case, it broke and the air leaked out quickly, but it never shattered or exploded, and I'm not sure I understand how it could -- even if the pressure discovered a weak spot, once that weak spot is breached, the air leaks out, and no further pressure exists.
In any case, all I'm doing is relating my personal experience, so if you do what I did and it explodes, don't blame me. /forums/images/graemlins/grin.gif
In my experience, Junkman is exactly correct in his recommendations for size and configuration. I used PVC ball valves at the bottom of each vertical drop for drainage. Just remember to keep your overhead lines at a slight slope leading to the vertical drops. Periodically let all the pressure out of the tank so any moisture in your first rising pipe/hose drains back in the tank, and keep the tank drained. I also used a pressure regulator that was combined with a water filter. If you use the air for painting, don't use a line oiler for the air tools -- the oil will get in the paint. Instead, oil each tool manually or install an oiler on the lines that are not used for painting.