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How to determine the maximum fire water flow required by NFPA?

Question asked by minhyeong1.kim on Mar 22, 2019

Hi. I have a question about the maximum fire water flow in a particular case as below:

 

There is a turbine building in a power plant. Inside the turbine building, there are many fire protection systems, e.g. water spray systems, automatic sprinkler systems, indoor hydrants (class I), etc. As far as I understand the water supply should be considered following the below clause of NFPA 850:

6.2.1* The water supply for the permanent fire protection installation should be based on providing a 2-hour supply for all of the following:
(1) Either of the following, whichever is greater:
(a) The largest fixed fire suppression system demand
(b) Any fixed fire suppression system demands that could reasonably be expected to operate simultaneously
during a single event [e.g., turbine underfloor protection in conjunction with other fire protection system(s) in the turbine area, coal conveyor protection in conjunction with protection for related coal handling structures during a conveyor fire, adjacent transformers not adequately separated according to 5.1.4]
(2) The hose stream demand of not less than 500 gpm (1893 L/min)
(3) Incidental water usage for non–fire protection purposes

Based on the above the maximum fire water would be the sum of flows of fire protection systems:

 (1) automatic sprinkler system: 0.3 gpm/sf x 5000 sf = 1500 gpm (Sprinkler system is assumed to be the maximum flow fixed fire suppression system in this case.)

 (2) indoor hydrant hose stream: 500 gpm

Thus, the maximum water flow would be 2000 gpm.

 

However, the hose stream demand is much more larger as per NFPA 14. Clause 7.10.1.1.5 of NFPA 14 says:

7.10.1.1.5 The maximum flow rate shall be 1000 gpm (3785 L/min) for buildings that are sprinklered throughout, in accordance with NFPA 13, and 1250 gpm (4731 L/min) for buildings that are not sprinklered throughout, in accordance with NFPA 13.

Since the turbine building is quite large and multi-story building, more than 4 standpipe risers would be provided. Therefore, the hose stream should be 1000 gpm at the maximum as per NFPA 14. If it should be as per NFPA 14, the total fire water flow would be 1500 gpm of sprinkler system plus 1000 gpm of hose stream, that is, 2500 gpm.

I think that because NFPA 14 is a standard, whereas NFPA 850 is a recommended practice, the requirements of NFPA 14 are mandatory and have the priority over others.   

 

Someone who has similar experience, especially consulted by AHJ on a similar case, pleaes advise.

Thank you in advance.   

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