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Interpreting Design Density Curves

Interpreting Design Density Curves

Interpreting Design Density Curves

(OP)
I found a very interesting article on the historical forming of the NFPA 13 design area curves:

http://magazine.sfpe.org/sprinklers/nfpa-13-sprink...

I have crunched through it and found very interesting points that really put the curves into context, but wanted to have a discussion on a couple of points that I could not seem to wrap my head around.

Reference comment: Chester Schirmer's 1972 note states "In 1972, the NFPA 13 light hazard curve extended to an operating area of 5,000 ft2 (460 m2) with a design density of 0.075 GPM/ft2 (3 mm/min). The revision for the 1974 edition resulted in a reduction of the allowable operating area from 5,000 ft2 (460 m2) to 4,000 ft2 (370 m2), at which 0.05 GPM/ft2 (2 mm/min) was indicated as the required design density."

Question 1: What exactly does it imply to have a reduction of the "allowable" operating area (ie: what is "allowable" and why is there a maximum)? If I am at a job site and find a design area of 5,000 sq. ft. on the riser placard, does that mean that the sprinklers need to be recalculated using a 3,000 sq. ft. area, or can I (using Fig. 16.2.1.3.2(b) curve "D" for example) project the .27/5,000 design density/area found at the job site to .27/3,000 on the appropriate curve to validate whether or not the system is adequate enough to protect the commodities? I am interested to hear your thoughts on the implications of this decision of the NFPA to remove all points on figures 16.2.1.3.2(a)-(g) above 3,000 sq. ft. in the NFPA 13 16.2.1.3.2 figures.

Question 2: On that same note of "projecting" square footage, can we treat a single-point density the same way as I described above? That is, (using Fig. 16.2.1.3.2(b) Curve "I" for example), can I project a .42/2,500 design density/area found at the job site to the single-point .42/2,000 requirement at curve "I" to validate whether or not the system is adequate enough to protect the commodities? Also, what are the implications if when on a job site, both density AND area are different than the single point (ie: .49/2,500 found vs. curve "I")? Does the system need to be recalculated in this case?

I am curious about the next plan of action when confronted with these scenarios. Do existing systems designed above 3,000 sq. ft. area = recalculate for 3,000 sq. ft.?

Thanks!

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