Wet Pipe Systems

By a wide margin, wet pipe sprinkler systems are installed more often than all other types of fire sprinkler systems. They also are the most reliable, because they are simple, with the only operating components being the automatic sprinklers and (commonly, but not always) the automatic alarm check valve. An automatic water supply provides water under pressure to the system piping. All of the piping is filled with water. Until sufficient heat is applied, causing one or more sprinklers to fuse (open), the automatic sprinklers prevent the water from being discharged. Operation - When an automatic sprinkler is exposed to sufficient heat, the heat sensitive element (glass bulb or fusible link) releases, allowing water to flow from that sprinkler. Sprinklers are manufactured to react to a specific range of temperatures. Only sprinklers subjected to a temperature at or above their specific temperature rating will operate.

 

Source: http://en.wikipedia.org/wiki/Fire_sprinkler_system