In a Nutshell
We often see sprinkler systems drenching a room in the movies with the application of a small amount of smoke or a lighter applied to a single sprinkler head. In reality, most sprinkler systems do not operate this way. Wet sprinkler systems will only spray water out of the sprinkler head that is exposed to a temperature exceeding the rating of its heat element, causing water to flow, which triggers an audio/visual fire alarm condition. A deluge sprinkler system has dry piping that only fills with water when smoke or heat triggers a detector in the room. This type of rarely used system could produce the effect we often see in the movies, but only if a local smoke alarm were to occur, not with the application of a lighter to the sprinkler head.
The Whole Bushel
You have probably seen movie scenes where a small amount of smoke or a lighter applied to a sprinkler head causes the fire alarm system to go off and water to flood the entire area out of every sprinkler head. Do sprinkler systems really operate this way as shown in the movies? The answer is yes and no. Standard wet sprinkler systems that are installed in most buildings do not operate in this manner. Wet sprinkler systems are called “wet” because the piping is normally filled with water at all times. Each sprinkler head has an individual fusible link or glass bulb that reacts to heat. When the head is exposed to hot gases or flames that reach a predetermined temperature, the heat sensitive element opens and allows water to flow out of that individual head only. Once water starts to flow, a water flow switch in the sprinkler line will detect the flow of water and cause the fire alarm system to go into alarm. So this type of system will not produce the effect we typically see in the movies where all of the sprinkler heads release water simultaneously.
However, there is a system called a deluge sprinkler system that is typically used for high hazard industrial and commercial applications, including power plants, chemical processing facilities and aircraft hangars where high volume, high-velocity fire suppression is required to prevent the spread of the fire to other areas within the facility. This system operates when one or more separate smoke, heat, ultraviolet (UV), or infrared (IR) detectors in the area go into alarm. The piping is dry, meaning it does not contain water under normal conditions and the sprinkler heads are actually open nozzles without heat-sensitive elements. When one or more of the sensors go into alarm, a signal is sent to a relay that automatically opens a deluge valve, allowing water to flow freely through all of the heads simultaneously. The fire alarm system also goes into alarm.
Therefore, a deluge system would produce the effect we normally see in the movies where the release of smoke in the area near a detector causes all of the sprinkler heads to spray water at the same time while the fire alarm system annunciates an alarm condition, as long as the system uses smoke detectors to operate. However, this would not happen with the application of a lighter to a head as we often see because the heads are just open nozzles that do not initiate the release of water. The application of a lighter to a head in a standard wet sprinkler system would only cause water to flow from that head, not all of the heads at once. Also, as noted above, deluge sprinkler systems are not commonly found in most buildings but are rather more likely to be seen in a building with high hazard areas. Therefore, as usual, you shouldn’t believe everything you see in the movies.