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.
Around the globe, scientists are discovering our water supply may seem abundant, but our drinkable water supply is in high demand and short supply. That is why the plumbing industry has dedicated considerable efforts to protecting what drinkable water we have through devices and technologies that have evolved over the last century.