underground and aboveground for power plant firefighting system piping network connected to fire pump and supply the hydrants and building fire systems.
Underground: Pipe has to be UL listed or comply with the applicable AWWA standard (see section 10 of NFPA 24). It can be any number of materials, but HDPE seems to be the preferred choice at the utilities I previously worked at on new builds. Older units either had coated CS or asbestos cement (it really old).
Above ground: Can be a number of metallic materials (no plastics since it can melt in a fire). Coated (painted) CS or galvanized is the most common that I have seen. Find a copy of NFPA 13 for more details.
I would definitely go HDPE underground and galvanized above ground then. I just left one of the largest utilities in the US last fall. I developed and/or worked on construction for 6 combined cycle and 2 simple cycle plants in the last 3 years and that is what we used on all of them.
HDPE underground may be used, but it is more common to use PVC (AWWA C900 or C905) or Ductile Iron (C151) for underground piping and galvanized pipe for the sprinklered areas.
PVC is more common for the fire main? Not sure where you have seen PVC used for a main. The main from the fire pump will be 8-12" line size depending on fire system hydraulics. The lines to the hydrants will be 4 or 6". I'm certain none of that would be PVC. My guess is the main will come into a valve house of some sort and then be distributed to the various fire areas in 2" lines. If going above ground, galvanized is the way to go. If it goes back underground maybe somebody might use PVC there, but I have always seen metallic or HDPE used.
