Reference

Sprinkler Riser Wet vs Dry

Sprinkler systems are the most effective manner to suppress a fire.  The most simple sprinkler system has a pipe full of water and sprinkler heads.  When heat triggers a sprinkler head, water is immediately discharged. This is called a WET SPRINKLER SYSTEM  because the pipes are always full of water.

Wet sprinkler riser with water flowing