Most
sprinkler systems installed today are designed using an area
and density approach. First the building use and building contents
are analyzed to determine the level of fire hazard. Usually
buildings are classified as light hazard, ordinary hazard group
1, ordinary hazard group 2, extra hazard group 1, or extra hazard
group 2.
After
determining the hazard classification, a design area and density
can be determined by referencing tables in the National Fire
Protection Association (NFPA) standards. The design area is
a theoretical area of the building representing the worst case
area where a fire could burn. The design density is a measurement
of how much water per square foot of floor area should be applied
to the design area.
For example, in an office building classified as light hazard,
a typical design area would be 1500 square feet (139.35 square
metres) and the design density would be 0.1 gallons (0.38 litres)
per minute per square foot or a minimum of 150 gallons (567.8
litres) per minute applied over the 1500 square foot design
area. Another example would be a manufacturing facility classified
as ordinary hazard group 2 where a typical design area would
be 1500 square feet and the design density would be 0.2 gallons
(0.76 litres) per minute per square foot or a minimum of 300
gallons (1135.6 litres) per minute applied over the 1500 square
foot design area.
After the design area and density have been determined, calculations
are performed to prove that the system can deliver the required
amount of water over the required design area. These calculations
account for all of the pressure that is lost or gained between
the water supply source and the sprinklers that would operate
in the design area. This includes pressure losses due to friction
inside the piping and losses or gains due to elevational differences
between the source and the discharging sprinklers. Sometimes
momentum pressure from water velocity inside the piping is also
calculated.
