Design / Density Curve - NFPA 13 3

[sl3656]

Referring to the design / density curve from NFPA 13, although the selection point along each curve (e.g. LH, OH-1, OH-2, EH-1 and EH-2) is at the discretion of the fire protection designer, I have seen most designers select the lowest area to achieve the lowest mass flow rate upon activation, and therefore the most cost-effective solution.

What rationale would a designer have for moving 'up the curve' to a higher sprinkler operational area? I am in a situation with a client, where he is requesting a specific higher sprinkler activation area, and wants us (the sprinkler designer) to provide engineering analysis to substantiate the higher operational area. I want to avoid a performance-based approach with engineering analysis - is there anything in the various NFPA codes that I could used to counter the client's proposed approach? I'm of the opinion that if NFPA advocated a performance-based approach with sprinkler design, us designers would be charging more hours for each and every design we complete!

 

[LCREP]

Before retirement from the insurance engineering department we always asked/recommend/mandated a larger design area.....why...if the water supply decreases over the life of the building you MAY be able to meet a new design area ie 2000 vs 3000 or perhaps a slightly higher density if occupancy/storage changes. We would also recommend a 10 psi or 10% safety cushion whichever was higher again to give us a chance of having an adequate design over the life of the building. FM and IRI always had 3000 sq ft design areas.

 

[TravisMack]

From a design side, there are some benefits of "going up the curve."

The biggest advantage comes if you can get the density below 0.25 gpm / sq ft. That is the break point on spacing. At <0.25 gpm / sq ft, I can space sprinklers at 130 sq ft and have a max spacing of 15' between sprinklers. However, at 0.25 or greater, then I am limited to 100 sq ft and a max spacing of 12' between sprinklers.



Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC

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[UFT12]

Sliding up the curve could be beneficial too for cases where the application area (AA) is mandated to be increased. An example would be the case of combustible conceal spaces where an AA of 3000 ft2 is required. The designer could just pick the corresponding density for the given AA size.

 

[TravisMack]

Yes. We do that frequently. When you have to go to 3000 sq ft, we do the light hazard areas at 0.07 gpm / sq ft and utilize 4.2k sprinklers to cut down on the over discharge. It works quite well in those situations.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC

http://www.mfpdesign.com

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[sl3656]

Thanks for the replies and sorry for not responding back sooner.

Is there somewhere in NFPA-13 (or a related NFPA code) that stipulates a requirement to perform engineering analysis in determining the sprinkler design parameters (incl. operational area and density), if a specific fire size (in MW) is known?

 

[TravisMack]

No. NFPA 13 is a prescriptive standard. Follow the rules in 13 and you are set.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

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[sl3656]

Thanks for clarifying Travis.