Fire Sprinkler System - Design

Design

This chart from the
New Zealand fire
safety standards
indicates the colour
of the bulb and the
respective operating
temperature.

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.

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 1,500 square feet (140 m2) and the design density would be 0.1 US gallons per minute (6.3×10−6 m3/s) per 1 square foot (0.093 m2) or a minimum of 150 US gallons per minute (0.0095 m3/s) applied over the 1,500-square-foot (140 m2) design area. Another example would be a manufacturing facility classified as ordinary hazard group 2 where a typical design area would be 1,500 square feet (140 m2) and the design density would be 0.2 US gallons per minute (1.3×10−5 m3/s) per 1 square foot (0.093 m2) or a minimum of 300 US gallons per minute (0.019 m3/s) applied over the 1,500-square-foot (140 m2) 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. Typically these calculations are performed using computer software but before the advent of computer systems these sometimes complicated calculations were performed by hand. This skill of calculating sprinkler systems by hand is still required training for a sprinkler system design technologist who seeks senior level certification from engineering certification organizations such as the National Institute for Certification in Engineering Technologies (NICET).

Sprinkler systems in residential structures are becoming more common as the cost of such systems becomes more practical and the benefits become more obvious. Residential sprinkler systems usually fall under a residential classification separate from the commercial classifications mentioned above. A commercial sprinkler system is designed to protect the structure and the occupants from a fire. Most residential sprinkler systems are primarily designed to suppress a fire in such a way to allow for the safe escape of the building occupants. While these systems will often also protect the structure from major fire damage, this is a secondary consideration. In residential structures sprinklers are often omitted from closets, bathrooms, balconies, garages and attics because a fire in these areas would not usually impact the occupant's escape route.

If water damage or water volume is of particular concern, a technique called Water Mist Fire Suppression may be an alternative. This technology has been under development for over 50 years. It hasn't entered general use, but is gaining some acceptance on ships and in a few residential applications. Mist suppression systems work by using the heat of the fire to 'flash' the water mist cloud to steam. This then smothers the fire. As such, mist systems tend to be highly effective where there is likely to be a free-burning hot fire. Where there is insufficient heat (as in a deep seated fire such as will be found in stored paper, no steam will be generated and the mist system will not extinguish the fire. Some tests have shown that the volume of water needed to extinguish a fire with such a system installed can be dramatically less than with a conventional sprinkler system.