Fire-Protection Water Tank Sizing

The tank has to carry the sprinkler demand AND the hose stream allowance for the full required duration. The hose allowance is added to the sprinkler demand at the point of connection, and it is frequently left out of a tank calculation because it does not appear in the hydraulic calculation of the sprinkler system itself -- on a light-hazard system it can be a quarter of the total, and leaving it out undersizes the tank by that much. The gross-versus-net distinction is the second thing that gets missed: the requirement is on NET USABLE capacity, so water below the outlet, the vortex-plate allowance, and any dead volume at the bottom do not count toward it, and a tank ordered at the net figure is short by whatever that fraction is. The refill line is the operational answer, because after a fire the tank has to be restored within a maximum time, so a tank fed by a small well can be the right volume and still be unacceptable. An ordinary-hazard system demanding 750 gpm at the riser with a 250 gpm hose allowance over 60 minutes needs 60,000 gal net, which at 8% unusable is 65,217 gal gross -- a 70,000 gal tank -- and refills at 60 gpm in 18.1 hours. Without the hose allowance the net would have been 45,000 gal and a 50,000 gal tank would have looked adequate. A sizing screen, never a stamp; NFPA 22 and NFPA 13 in full, the stamped hydraulic calculation, and the AHJ govern.

total_demand_gpm = sprinkler_demand_gpm + hose_allowance_gpm; net_volume_gal = total_demand_gpm x duration_min; gross_volume_gal = net_volume_gal / (1 - unusable_fraction); refill_hours = gross_volume_gal / refill_gpm / 60.

Fire-protection water tank sizing per NFPA 22 (net usable capacity, and a maximum restoration time after use) with the hose stream allowance added to the sprinkler demand at the point of connection per NFPA 13, both cited by name and not reproduced. A sizing screen, never a stamp; the stamped hydraulic calculation and the AHJ govern.

The volume arithmetic is multiplication. The sprinkler demand comes from the project's own hydraulic calculation and the hose allowance and duration from the applicable standard, entered rather than bundled.

Estimate. AHJ and licensed professional govern.

Field names used by the API: sprinkler_demand_gpm, hose_allowance_gpm, duration_min, unusable_fraction, refill_gpm, total_demand_gpm, net_volume_gal, gross_volume_gal, refill_hours

• Hose allowance added to the sprinkler demand at the point of connection; it does not appear in the sprinkler hydraulic calculationNFPA 13
• Net usable water below the outlet, vortex-plate allowance, and bottom dead volume do not countNFPA 22
• Restoration time the standard caps how long refilling may take, so the supply rate can govern the designNFPA 22