The most commonly used steel pipe for sprinkler systems is black steel pipe. Copper tubing is another piping material listed for sprinklers in NFPA 13. Also, plastic pipe (namely, CPVC and PEX tubing) is permitted for certain types of sprinkler systems.
John Siegenthaler's been designing hydronic heating systems for 25 years, and he still comes across little nuances that when ignored can bring a system that is 99.9% correctly designed and installed to a virtual standstill.
