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Hydraulic Calculation Clarification for Pump Sizing NFPA 13 & NFPA 14

Question asked by cruzsherr on Apr 8, 2019

I am new to Firefighting Hydraulic Calculation and I need your help to have your inputs as clarifications or confirmation to what I interpreted for the below statements:


NFPA 14 - 2010 Edition
" Where the sprinkler system water supply requirement,including the hose stream allowance as determined in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems, exceeds the system demand established by Section 7.7 and 7.10.1, the larger of the two values shall be provided."

NFPA 13 - 2013 Edition
" Where the combined sprinkler system demand and hose stream allowance of Table exceeds the requirements of NFPA 14, this higher demand shall be used."


Typically the intention of NFPA is to pick whichever yields a Higher Flow, but for the Pressure Requirement it is obvious that Landing Valves requires even more than the sprinklers. Example: if I have 650gpm @ 85 psi Pump Inflow sprk + hose stream demand for a certain building, and 500 gpm @ 140 psi Pump Inflow for the Standpipe System 2 most remote Landing Valves (1 LV per floor). In my interpretation, these two parameters are needed to be met by the Fire Pump Operation with reference to Fire Pump Curve.


If I am going to size this, I will pick 500gpm @ 140 psi Pump operated at 130% of Rated Capacity (650gpm) meeting the demand. Typically a UL Listed Pump will have a min pressure of 65% (90 psi) at 150% of Rated Capacity which definitely meets 85 psi requirement for sprinklers. Say the Operation of Pump as per Sprinkler Demand is 650 gpm 110.6 psi interpolated, this capacity will serve the Sprinkler Demand, just the Tank should be sized based on the Demand and Duration.


Another Example: I have a 320 gpm @ 45 psi demand for sprk+hose stream and a 100gpm 75psi demand for Class II System. The hose stream allowance is usually intended for the reliability of Sprinkler System in spite of simultaneous Fire Hose Reel operations for the initial respond, now how can I size the pump where both demand (Sprk & Hose Reel) will be met? Shall I consider calculation for both simultaneously?


Another thing that I am unsure is that, if inflow pressure of pump will increase, the Flow of Sprinklers will follow. This is because the K-factor is constant all throughout the operation and an increase in pressure is also an increase in flow. I'm not sure though if I have to disregard this. If there are things that I said wrong, feel free to correct me. Thank you so much.