Sprinkler System Types And How They Operate
Industrial Fire Safety
There are many types of automatic sprinkler systems and methods of operation, such as wet, dry, deluge, pre-action, outside, recycling and packaged systems, plus others that will be outlined later. Each system has characteristics that make it more desirable than others for providing adequate fire protection in a specific application.
Therefore, before any system is installed, the purchaser should obtain recommendations for the installation of a system evaluated in terms of the hazards to be protected. These recommendations should be made by a qualified source, whether it be the authority having jurisdiction over the property or a fire protection engineer.
Wet-pipe systems are probably the ones most often installed. From the underground supply pipe to the most distant branch line, this system contains water under pressure. When a fire occurs, the heat fuses one or more sprinkler heads and water immediately flows through them. The ever-present water in the piping is the main reason why wet-pipe systems are preferred over drypipe systems. There also are maintenance advantages in that there are no compressed air leakage problems nor a need for a compressor. Wet-pipe systems, of course, can be installed only in buildings or areas where the temperature remains above freezing.
Systems combined: When special conditions require it, a small dry-pipe valve system can be connected to a wet system. In some areas exposed to freezing temperatures, wet system pipes can be drained beyond a secondary valve, but this is not desirable because of the need for the manual operation of this “closed in winter” valve for a fire emergency. Also, piping in areas subject to freezing temperatures can be filled with an antifreeze solution.
In the standard dry-pipe sprinkler system, all the piping above the sprinkler valve is filled with air maintained at a required pressure by a compressor. The air pressure keeps the clapper in the sprinkler valve closed until a sprinkler head fuses. When this happens, the air pressure drops as air flows through the sprinkler head and water pressure is able to open the clapper and flow to the sprinkler head, or heads.
These systems generally operate slower than wet systems, which is a drawback to prompt extinguishment of a fire. But this slowness can be partially overcome by the use of quick-opening devices, such as accelerators or exhausters. When properly maintained, these devices cause air to be expelled faster from the piping and allow a more rapid flow of water.
Dry-pipe systems are installed in unheated buildings or portions of buildings, such as concealed spaces and loading docks, where freezing temperatures can be expected.
Dry-pipe systems have some disadvantages. The maintenance and installation costs are higher, and under similar fire conditions, more heads may open in a dry than in a wet system.
Pre-action type: Like a dry-pipe system, a pre-action sprinkler system has no water in the piping. If a fire occurs, an automatic detection device releases the sprinkler valve, which lets water flow through the system before any sprinkler heads fuse from the heat of combustion. The fire detection device may operate on a rate of rise or fixed temperature principle, a combination of both, or even other principles.
The advantage over the ordinary dry-pipe system is that water is available at a sprinkler head when it fuses. Because of this, some underwriters consider a pre-action system to be equal to a wetpipe system. Also, there is likely to be less fire and water damage, and there is an advantage in an alarm being sounded before the discharge of water.
The combined dry-pipe and pre-action system is an approved way of supplying water through two dry-pipe valves connected in parallel to an automatic sprinkler system that is larger than permitted for a single standard dry-pipe valve. The combination is used mainly to supply systems that normally are subject to freezing temperatures and have long runs of piping.
This combination eliminates heating or insulating long runs of exposed piping. It also means fewer feet of compressed air piping and installation economy. The combined system has other advantages, such as quick operation of the detection system along with the sounding of a fire alarm, flooding of the piping prior to the fusing of heads and reliability of operation.
Maximum sprinkler operation: A deluge system has open sprinkler heads and when a detector releases the sprinkler valve, water floods the entire system and flows through all the sprinkler heads. Deluge systems are used to protect hazards that require vast amounts of water over the entire protected area. Installations are made in areas such as aircraft hangars, assembly plants and flammable liquid processing locations subject to flash fires. Deluge systems also protect outside hazards such as chemical and flammable liquid storage tanks.
The recycling sprinkler system is a refinement of the pre-action, dry-pipe type. The water is held in check at the sprinkler valve until a signal from a heat detector system releases water to the sprinkler heads. Through a system of heat detectors and solenoid valves, the water is automatically shut off when the fire is extinguished, but if the fire rekindles, water again is discharged. This cycling continues automatically as long as the fire continues to rekindle or if another fire occurs in the protected area.
This type of system is invaluable in minimizing water damage, especially in operations that are unattended or in areas without adequate watchman service or a central station automatic fire or water-flow alarm.
Outside installations: Also worthy of mention are outside installations that protect the interior and exterior of buildings from floor to floor with window and cornice sprinkler heads. Usually a drypipe system is used, and it is operated manually in an emergency.
Packaged sprinkler systems are available in a variety of forms. They use pressure tanks and standard sprinkler hardware. They are usually installed where there is no water supply, where spot hazards must be protected, or as a matter of economics. These systems can be acceptable to some authorities when special conditions exist.
We also must include among the packaged systems the all-aluminum system for providing spot protection for construction site hazards. This type was mentioned in our column last January.
As this is only a brief commentary on sprinkler systems, it is recommended that you get a copy of NFPA No. 13, “Standard for the Installation of Sprinkler Systems,” and obtain further information from an authority having jurisdiction over your property, whether it is a rating bureau, the Factory Insurance Association, Factory Mutual Engineering or a similar engineering supervisory organization.