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Fire water storage refilling time as per NFPA 850

Question asked by on Nov 19, 2020

I am working on a power plant project now. Since it is a power plant we follow requirements of NFPA 850 most of the time. The of NFPA 850 (2015 ed.) says that the 8-hour requirement of refilling time can be extended if initial storage exceeds minimum storage on a volume per time ratio basis. My question is how exactly I can calculate this. 

For example, if I need 1000 m3 storage as a minimum, but refilling rate is 100 m3/hr, then 100 x 8 = 800 m3 is the refilled volume in 8-hour duration so that I need initial 1200 m3 (additional 200 m3) water storage. Is it what the requirement intends? 

If it does so, hypothetically no refilling is required for a storage tank with 4-hour water supply, isn't it? 

Please share your thought.