Water allowance for private hydrant

[doingwhat]

I have a question regarding water allowance for private hydrant.

According to NFPA 13, 11.1.6.2
Water allowance for outside hose shall be added to the sprinkler requirement at the city main or a private hydrant, whichever is closer to the system riser.

Project info.
5 stories office building.
A total of 4 private hydrant supplied by the same fire service.

so how much allowance i have to add to my overhead sprinkler calculation? 1250+100 gpm?

 

[MatthewJWillis]

No. If I understand your question, 100 gpm at the closet hydrant.

Out of curiosity, how where did the 1250 come from?

R/
Matt

 

[doingwhat]

All on-site fire hydrants shall flow a minimum of 1,250 gallons per minute at 20 psi for a duration of two hours. If more than one on-site fire hydrant is required, the on-site fire flow shall be at least 2,500 gallons per minute at 20 psi, flowing from two hydrants simultaneously. On site flow may be greater depending upon the size of the structure and the distance from public hydrants.

 

[doingwhat]

so why 100 gpm?

 

[TravisMack]

You have to understand that site fire flow is independent of fire sprinkler demand + hose allowance.

It sounds like you are in CA with the 1250 requirement.

For the sprinkler system, you calculate the flow from the design area + the hose allowance based on hazard. Office is typical light hazard which is 100 gpm.

So total demand = sprinklers + 100. The 100 gpm is added at the hydrant nearest to the base of the riser.

With a five story building, you also likely have manual wet standpipes. You don't have hose or sprinkler demand to the standpipe systems.



Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[chicopee]

One hose stream (HS) is equivalent to 250 GPM at 50 psi nozzle pressure at an 1-1/8" tip or at 20 psi if a pumper is needed to boost proper nozzle discharge. I don't know how many HS your office building requires because I don't know the type of construction material used and the ground floor area, therefore that number could significantly vary. Typically, noncombustible construction of 5000 sq.ft or less requires 1 HS with a duration of water supply at 1-1/2hr; between 15,000 and 30,000 sq.ft, 4 HS would be needed at a duration of 2-1/2hr. Additionally I estimate that your office building classified with Group I occupancy, would require about 100-500 GPM at a top sprinkler line pressure of 15 psi unless it is hydraulically designed.
What's needed to answer your question are: classification of occupancy group, construction classification of the building, ground floor square footage and duration of water supply.