Hydraulically Most Demanding Area

[Chrisele]

I am new to sprinkler systems designing process and i have a question on understanding the basic concepts of NFPA13 and especially about the most demanding area of operation.

NFPA13 2019 handbook has examples of the most demanding are in systems. Figures A.27.2.4.1(a),(b). In those examples i understand that "most demanding" refers to maximum needed head of the system operation.
After we apply a certain supply there will be areas of operation in the system where the total demand in flow will be much higher than the flow calculated in the Hydraulically most demanding area.

EN 12845 addresses this by demanding the calculation of the Hydraulically least demanding area in the system, then maximum flow demand and tank sizing shall be determined by the flow calculated for this least demanding area of operation.

So my question is this. When designing by NFPA shall the designer calculate the maximum flow demand just by the hydraulically most demanding area of operation or shall he calculate for the most demanding and the Least demanding areas in the system?

 

[UFT12]

NFPA 13 does not require to calculate any demand other that the most demanding. See NFPA 13 ed. 19, section 5.1.2 where the tank must be sized based on this demand. With that said, you will not get water for the durations of table 19.3.3.1.2, should the system is still flowing that long. As to why this is allowed, although not clearly stated, my personal belief is that you get more water (higher discharge) for the expected fire size but for a shorter duration which makes sense if you think of it like battery capacity, that is flow x time as current x time. The product remains the same which simplifies the whole story considerably and provides a more cost effective installation.