Wednesday, February 9, 2011
Fire sprinkler system
A fire sprinkler system is an active fire protection measure, consisting of a water supply, providing adequate pressure and flowrate to a water distribution piping system, onto which fire sprinklers are connected. Although historically only used in factories and large commercial buildings, home and small building systems are now available at a cost-effective price.
Sprinklers may be required to be installed by building codes, or may be recommended by insurance companies to reduce potential property losses or business interruption. Building codes in the United States for places of assembly, generally over 100 persons, and places with overnight sleeping accommodation such as hotels, nursing homes, dormitories, and hospitals usually require sprinklers either under local building codes, as a condition of receiving State and Federal funding or as a requirement to obtain certification (essential for institutions who wish to train medical staff).
If building codes do not explicitly mandate the use of fire sprinklers, the code often makes it highly advantageous to install them as an optional system. Most US building codes allow for less expensive construction materials, larger floor area limitations, longer egress paths, and fewer requirements for fire rated construction in structures protected by fire sprinklers. Consequently, the total building cost it is often less by installing a sprinkler system and savings money in the other aspects of the project, as compared to building a non-sprinklered structure. In the UK, since the 1990′s sprinklers have gained recognition within the Building Regulations (England and Wales) and Scottish Building Standards and under certain circumstances, the presence of sprinkler systems is deemed to provide a form of alternative compliance to some parts of the codes. For example, the presence of a sprinkler system will usually permit doubling of compartment sizes and increases in travel distances (to fire exits) as well as allowing a reduction in the fire rating of internal compartment walls.
Renewed interest in and support for sprinkler systems in the UK, largely as a result of effective lobbying by the National Fire Sprinkler Network, the European Fire Sprinkler Network and the British Automatic Fire Sprinkler Association, has resulted in sprinkler systems being more widely installed. In schools, for example, the Department of Children, Families and Schools has issued strong recommendations that most new schools should be constructed with sprinkler protection. In Scotland, all new care homes are sprinklered as are sheltered housing and high rise flats. Most local authorities in Scotland have a policy of fitting sprinklers to new schools and to most of their own new buildings.
Each closed-head sprinkler is held closed by either a heat-sensitive glass bulb (see below) or a two-part metal link held together with fusible alloy. The glass bulb or link applies pressure to a pip cap which acts as a plug which prevents water from flowing until the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler head. Because each sprinkler activates independently when the predetermined heat level is reached, the number of sprinklers that operate is limited to only those near the fire, thereby maximizing the available water pressure over the point of fire origin.
A sprinkler activation will do less damage than a fire department hose stream, which provide approximately 900 liters/min (250 US gallons/min). A typical sprinkler used for industrial manufacturing occupancies discharge about 75-150 litres/min (20-40 US gallons/min). However, a typical Early Suppression Fast Response (ESFR) sprinkler at a pressure of 50 psi (345 kPa) will discharge approximately 100 US gallons per minute, (380 litres per minute). In addition, a sprinkler will usually activate between one and four minutes, whereas the fire department typically takes at least five minutes to arrive at the fire site after receiving an alarm, and an additional ten minutes to set up equipment and apply hose streams to the fire. This additional time can result in a much larger fire, requiring much more water to achieve extinguishment.
Sprinkler systems are intended to either control the fire or to suppress the fire. Control mode sprinklers are intended to control the heat release rate of the fire to prevent building structure collapse, and pre-wet the surrounding combustibles to prevent fire spread. The fire is not extinguished until the burning combustibles are exhausted or manual extinguishment is effected by firefighters. Suppression mode sprinklers (formerly known as Early Suppression Fast Response (ESFR) sprinklers) are intended to result in a severe sudden reduction of the heat release rate of the fire, followed quickly by complete extinguishment, prior to manual intervention.