Tank Size for Simultaneous Sprinkler & Fire Flow?

Long question - in essence - is Fire Flow considered on top of a flowing sprinkler system?

Recently there was a great discussion on the forum here about whether a water storage tank size needs to include Fire Flow in addition to sprinkler demand. There were some great points made (www.meyerfire.com/daily/must-water-storage-tank-include-site-fire-flow).

I have just come upon a project where we do have a municipal water supply, but it tapers off very steeply. The flow test in the area was 52 psi static, 25 psi residual at only 650 gpm. This would put the available Fire Flow around 712 gpm at 20 psi for the site.

In order for the AHJ to reduce the required Fire Flow under the IFC, the building(s) nearby need to be sprinkler-protected. This would bring down the Fire Flow requirement to 1,000 gpm at 2-hours, for this setup. The sprinkler system isn't designed yet, but we'd estimate around a 550-650 gpm demand with a fire pump. This gets really interesting based on the prior discussion.


When the fire department shows up on site during a fire, and then starts to pull water, is this Fire Flow in addition to the flowing sprinkler system, or considered separate?

In other words, is the 1,000 gpm (for 2 hours) for Fire Flow on top of a 600 gpm demand for sprinkler (for 60 minutes)?


If the water storage tank needs to meet the demand of the sprinkler and Fire Flow separately, then the calculation comes down to ~300 gpm needed to fill the shortage on Fire Flow, multiplied by 2-hours. That's a tank in the ballpark of 35-40,000 gallons.

If the water storage tank needs to meet the Fire Flow as the sprinkler system is operating, that's a flow of around 1,600 gpm to fight a fire. That's as much as ~900 gpm x 2 hours to a ballpark of 100-115,000 gallons. Huge difference.


I can't find any relevant code that stipulates one way or another, only that the tank must supply the sources it is connected to. 

Am I looking at this correctly? I see sprinkler and Fire Flow needing to be met by a tank that they're connected to, but is the Fire Flow robbing the sprinkler system, or vice-versa?

Thanks so much for reading. This is obviously TLTHFR (too long, thank-you-for-reading).

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