Fire Sprinkler Designs
- Sprinkler design is based on both the size of a building and the potential for fire. NFPA 13 categorizes buildings as ordinary hazard, extra hazard or special-occupancy hazard. Buildings that fit the ordinary category are a low-risk for fire and contain no combustible materials that would cause a fire to produce very high levels of heat. Extra-hazard buildings generally include industrial and manufacturing facilities, where the presence of chemicals or combustibles makes fire more probable. Based on these categories, NFPA suggests appropriate flow rates that should be released during a fire. These flow rates apply to each square foot of building space. For example, a lower hazard building may require .1 gallons of water per minute. If the building measured 5,000 square feet, the sprinkler system should be designed to release 500 gallons of water per minute. The layout of the system, available water pressure and specified materials are all adjusted to accommodate this flow rate.
- The most common system used in sprinkler design is the traditional wet pipe system. This system is quite simple when compared to other designs and features only piping, a water supply, pipe fittings, and sprinkler heads. The pipes are filled with water, which is only released in the event of a fire. The sprinkler heads contain a release element, such as a metal chain or filament. When exposed to high heat, this release element will break, allowing the sprinklers to operate. With a wet pipe system, only sprinkler heads exposed to a pre-determined level of heat will operate. This means that water is released only in the area around the fire, leaving other spaces dry to minimize property damage.
- Dry systems are used in areas where wet systems would be ineffective or could lead to frozen or burst pipes. This includes places like storage buildings and parking garages that may not be heated. With a dry sprinkler system, there is no water in the pipes. Instead, the pipes are filled with air, which holds the water back at its source and keeps it from filling the pipes. When the release element on the sprinkler head is broken due to heat, the air rushes from the head. This provides space for the water to fill the pipes, and then exit through the sprinkler heads. There is a slight delay when using a dry system design, as it takes time for the air to exit the pipes and then for water to reach the heads.
- Some sprinkler designs rely on foam or chemicals rather than water. Foam is typically used in areas exposed to severe fire hazards and helps to contain the fire more effectively than water. It can be used with either a wet or dry system, and in certain cases, water may be mixed with the foam to better fight the fire or increase pressure in the pipes. In areas where water damage is a primary concern, such as museums or computer rooms, a dry chemical design may be used. These systems use traditional sprinkler pipes and heads, but rely on carbon-based chemicals such as halon to suppress the fire. Halon works by absorbing the oxygen in the room, which keeps the fire from spreading. Chemical systems must be designed carefully due to environmental and health risks associated with halon and related products.
- Pre-action systems are an alternative to chemical sprinklers and are designed to prevent damage from accidental discharge. They are often used in museums or buildings that house expensive equipment. Pre-action systems start with dry pipes. If the system receives a signal from a fire alarm or smoke detector, a valve is released to change the dry system to a wet one. Before water is released from the pipes however, the sprinkler must receive a second signal, which may consist of a heated release element or an electronic signal from the building's fire alarm system. After this second signal is activated, water is released from the pipes in the traditional manner. A pre-action system provides the benefits of a dry system without the potential delay caused by air exiting the pipes.
