HOW FIRE SPRINKLERS OPERATE

The easiest way to understand the operation of a fire sprinkler is to think of a fire sprinklers as a "heat-sensitive" plug.

Contrary to what is commonly portrayed in movies and on television, normal fire sprinklers operate only when heated to a designated temperature. This means that if you have a fire in a room, only the head(s) that get hot enough to open will open and spray water (statistically, that number is between 1 & 2). Smoke (and attractive people) do not actually cause regular fire sprinklers to operate.

The typical options for Fire Sprinkler System types are:

1. Wet-pipe system (Wet System) - Wet systems are the most common type of fire sprinkler system and are the simplest. Wet systems have water-filled pipe and release water as soon as sprinkler heads operate. (Wet systems are restricted to use only in areas where the temperature is maintained above 40^o F.)

2. Dry-pipe system (Dry System) - Dry systems are used where the area protected by the sprinkler system is subject to freezing. Dry systems have a system control valve that is held closed by compressed air within the system piping. When a head breaks, the air is released, and the valve opens and allows water to enter the piping and discharge onto the fire. Dry systems understandably take longer to respond to a fire than a wet system, so different design criteria are used for dry systems to account for the delayed response.

3. Pre-Action system (Preaction System) - Like dry systems, preaction systems normally have only pressurized air in the pipe, but unlike a dry system, merely releasing the air pressure will not allow water into the pipe. Preaction systems use an electric solenoid-operated "preaction valve" to hold back water. The preaction valve is controlled by a special fire alarm panel, called a releasing panel, which can be configured to release water after receiving either one or multiple signals (whether or not sprinkler heads are broken). Common methods of fire detection used for releasing a preaction valve are: monitoring system piping for air pressure loss (supervisory air), Fire Detectors (Spot-type Heat; Linear Heat; Smoke; and Flame), and pilot sprinklers. Some examples of areas where preaction systems make sense are: Rooms where the system piping is likely to be subjected to mechanical damage (i.e.: palletized storage racking), or where the contents are extremely valuable and fire fighting with water is a last resort (i.e.: server rooms, clean rooms, telecom switch rooms), or rooms where a dry system would be appropriate because of low temperature, but a delayed response (from exhausting air) is unacceptable (i.e.: warehouses with contents that would develop high-challenge fires).

4. Deluge systems - Deluge systems are systems with open nozzles that are intended to spray every part of a structure in the event of a release. Deluge systems are released by means of a releasing panel as a preaction system is (but supervisory air is not an option due to the open nozzles). Deluge systems are used to protect buildings or structures that will develop fast, high-challenge fires, such as Petroleum Storage Tanks, Fuel Tanker Loading Racks, Cooling Towers, etc.

5. Anti-freeze system (Antifreeze system) - Antifreeze systems are wet systems with special control valves installed that permit filling the piping with a non-freezing solution. Antifreeze systems are frequently used to protect small areas which are subject to freezing, within buildings that are otherwise protected with wet systems (such as attics, garages, or exterior canopies). Sometimes large antifreeze systems are installed to protect commercial cold storage buildings, but such installations are expensive and maintenance intensive.

Systems for Residences are usually designed and installed per NFPA 13d or NFPA 13r (occasionally per NFPA 13), and are usually wet systems (if attic protection is required it is often protected by a dry system, occasionally by an anti-freeze system.

Systems for Hotels and Motels are usually designed and installed per NFPA 13r (4-story and less) or NFPA 13, and are usually wet systems in the main building, and dry systems in the attics (where required) and other non-heated combustible spaces.

Systems for typical Commercial buildings are usually designed and installed per NFPA 13. Commercial systems are often wet systems or dry systems (depending on the type of building construction). Many commercial buildings also have anti-freeze systems and preaction systems protecting portions of the building. Some high-rise buildings even have sprinklers on the exterior (or at interior window openings) to provide protection if a neighboring building catches fire.

Systems for Industrial buildings and structures are often designed per NFPA 13, but frequently have another governing standard as well. Industrial buildings frequently have combinations of wet, dry, preaction, anti-freeze and even deluge sprinkler systems, depending upon the use of the structure, and the type of fire likely to occur inside it.

Many Commercial and Industrial buildings also have Standpipes. Standpipes are large, strategically located pipes with fire hose valves attached which firefighters can use for fire hose supplies while fighting fires within or around or on top of buildings (hose valve locations vary depending on building type and local requirements) Standpipe design and installation is governed mainly by the requirements of NFPA 14, but other standards also apply depending on the building or structure's intended use.