Number of Sprinklers to Include in Calculation?

11/12/2020

A couple recently submitted questions relate to the same topic:

(1) Extra hazard, grid pipe layout of approximately 1,800 sprinklers. What is the criteria that drives the specific number of sprinklers that will be operating together?

(2) What's the best way to determine the design area and number of sprinklers within a calculation for a parking garage?

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3 Comments

James

11/12/2020 07:29:21 am

If you're in an NFPA 13 system, NFPA 13 Chapter 23 annex can help with determining the remote area shape once you have your remote area size from the code itself. Some criteria that is not design/area driven will stipulate a specific number of sprinklers in a calculation.

If you're using a different design standard, such as FM Global, then there is typically also guidance on either a remote area size or number of sprinklers to include in the calculation.

If you're looking for a full-length explanation on how to determine remote area size and shape, I would recommend NFSA's book Design and Layout of Fire Sprinkler Systems, or Fire Protection Hydraulics and Water Supply Analysis by Pat Brock.

franck

11/12/2020 11:53:41 am

Once you have your design density/area of application, you take the number of sprinkler to cover this area.
As an example, if your design is over 2500 sq ft and your area covered by sprinkler is 100 sq ft, you need 25 heads.
The area of application is not related to the total number of sprinklers installed. It would be the same with a system with 40 sprinklers or with 200 sprinklers.

Note that with 1 800 sprinklers you may have several different sprinkler systems (there is a maximum floor area per system depending on the occupancy) and as many most remote area calculations. If different hazards on a same system, you may also have different calculations for each hazard.

As indicated above, NFPA 13 will give guidance to determine the correct shape for the calculation

In all cases, I hope the designer and/or the sprinkler contractor knows how to proceed

Franck

11/12/2020 12:40:18 pm

Regarding your question for parking structures, you can have a look to NFPA 88A Standard for parking structure.

Note that according to this standard, there is no need for sprinkler protection in open parking structures.
An open parking structure is when you have have uniformly distributed openings on two or more sides (see § 5.5 for more details on openings size and distribution)

The sprinkler protection, according to NFPA 13, is Ordinary Hazard group 1 for parking garages.

Unfortunately, this may be revised in the future as car composition has greatly evolves over the time and the behavior of cars involved in a fire is not the same nowadays compared to former times.
There are several reasons, among which:
- the extensive use of electronic equipment (more combustible load inside the car)
- the extensive use of plastic components where metal was used in the past (example, front and back side of the car - bumpers)
- the use of plastic tank for the fuel, where it was metal in older time. In case of fire, the flammable fuel will be released immediately and contribute to a spill fire)

Future design may be more Extra Hazard that OH.

See an excellent article in the NFPA magazine (March-April 2019):

https://www.nfpa.org/News-and-Research/Publications-and-media/NFPA-Journal/2019/March-April-2019/Features/Protecting-Parking-Garages#:~:text=NFPA%2013%20classifies%20a%20parking,not%20a%20very%20big%20one%2C%E2%80%9D

There is a recent history of a very bad event that happened in a parking structure in the UK (Liverpool on December 31, 2017) where hundreds of vehicles burnt (not a sprinklered garage) - see :

https://www.bbc.com/news/uk-england-merseyside-46290095

One good news with the design, you don't need to consider the 18 in. (450 mm) clearance between the sprinkler and the top of the car for vehicles in concrete parking structures.

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