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.
There are several things to consider not readily covered in NFPA 13D and 13R: water supply, backflow prevention, alarms/accessories and fire department connections.