It depends where the firewater piping is ..... typical refinery type buried pipe might by FRP or HDPE. Could also be a number of metals such as cupro-nickel or iron.
Offshore commonly cupro-nickel, could be duplex or super-duplex stainless, may be FRP.
I believe virtually all different types of pipes have probably been used for fire water piping at one time or another. A well-known standard however relating to underground fire protection piping, now in its 2007 edition, is NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances. This standard now specifically includes cement mortar lined ductile iron piping to AWWA standards, steel and concrete pipe to AWWA standards, polyethylene pipe to AWWA C906 standard, pvc pipe to AWWA C900 standard, as well as copper pipe. With regard to steel piping, the standard now includes the specific statement, “10.1.2 Steel Pipe. Steel piping shall not be used for general underground service unless specifically listed for such service.”
Fire fighting lines, or dual purpose water supply/fire fighting lines, can at times and for various reasons represent quite rigorous service (e.g. due to effects of very high flow velocities, pumps and valve openings/closures, and sometimes in “emergency” circumstances etc.) I think it is for such reasons insurance-related entities such as Factory Mutual require quite high pressure (e.g. 4X the rated pressure) proof-of-design testing of pipes, fittings, and thrust restraints etc. involved as a condition of listing. Cement mortar lined ductile iron piping systems are widely used and offer substantial robustness in this regard for underground fire protection systems, with some manufacturers even offering piping and joint restraint products that carry a 350 psi (nearly 25 bars) Factory Mutual/Underwriter’s Laboratories pressure rating.
