My current job is inspecting the replacing of gas lines in part of Baltimore. This is an old system. Some of the mains we are replacing were installed in 1893 and 1903.
Plastic doesn't rot/ rust like steel, or cast iron.
But gas flow through a plastic pipe will generate significant static electricity. We have to attach a bond wire and ground anything before we cut. Fire is always a danger.
The system I am working on runs at 7-9 INCHES of water column.
OTOH, I worked on a new transmission line last year that was operating at 720 PSI and we tested it a 1080 PSI--yes, it was steel.

Installation details vary with the system. Our minimum depth for a main is three feet, and two feet for services. We are not allowed to let the crew work until we see a valid MISS UTILITY ticket for the area they are working. Even then they have hit a few services that MISS UTILITY failed to mark. No fires yet!!!

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags