NFPA 850 - 6.2 Water Supply

[hssmiguel]

Hi everybody,

I'd like to ask you about your understanding of NFPA 850 point 6.2.2, related to the water supply. Do you interpret from this point that more than one tank (source?) for fire protection purposes is needed (recommended) in a power plant? I usually consider only one tank as source for fire fighting system demands.

For instance, when I estimate a flow for transformers deluge system, I add to it the hose stream demand of 500 gpm (1890 L/min) and I get a capacity multypling it by 2 hours, which I consider as the total water volume to be storaged. Would it be right, or should I consider two sources with this volume each?

Thank you in advance!

 

[DavidCR]

What I understand form this is that the NFPA850 is always using "should" since it is only recommendations. And each case shall be analyzed.

For example, if you use a penstock as direct source for the fire system, and the penstock could be empty, it is inferred from the NFPA850 that an additional source shall be included. But if you have a fire water tank in a safe place with an extra capacity and it is well installed and maintained following NFPA22-NFPA25, and the filling source is relaible, I think with only one source is enough.

Something similar you may interpret from the "two independent connections" requirement. It has no sense to install two connections in a NFPA22 tank, but it may be a "shall" if you are tanking water from a penstock or lake intake.

Note. I understand the "hose" 500gpm is for 2hrs, but the 2hrs dose not neccesarily apply to the deluge system (FM requires and hour).

 

[hssmiguel]

Hi David,

your interpretation sounds sensitive and logical to me. My understanding is also related to the "reliability" of the source; I assume that a tank with a fixed minimum (always available) water quantity reserved for fire fighting is reliable enough. At the same time, if the source from which this tank is replenished is reliable in the sense of availabilty, it shouldn't be necessary the consideration of any other source.

What always worried me is when I work in some bidding processes, and the tender specifications mention "2 tanks for fire fighting purposes". I think that this is inherited from older specifications, kind of copy & paste issues. In these cases, I need to justify the use of one tank instead of two, and if NFPA recommendations are asked to be fulfilled, it may be ambiguous.

Lastly, regarding the 2 hours, I only assume NFPA 850 recommendation of 2 hours for both most unfavourable water demand + hose. I'm aware that other times are considered for example in NFPA 15, when talking about transformers protection (1 hour and a 250 gpm hose). Unfortunately, I'm not used to FM, since what I do is a overall predesign of the fire fighting system based most of the times with NFPA, but then I need to request specialized contractors for quotation.

Thank you for your prompt answer!

 

[DavidCR]

My interpretation reading the 850: the fixed deluge system is not included in the 2hrs, only the 500gpm hose demand.
It is a logical deduction, since if your fixed system was a NFPA13 wet sprinkler system, or a NFPA11 foam system, it has no sense to asume 2hrs for these. So the specific code (in this case the NFPA15) should be followed. And con´pt forget it is a minimum, so you are wrigth to inlude 2hrs.