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Tank Size for Simultaneous Sprinkler & Fire Flow?

8/29/2023

6 Comments

 
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).

​​​​​​​​​​​​​​​​​​​​​​​​Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
6 Comments
Chris
8/29/2023 08:08:55 am

NFPA 1
18.4.5.4* Required Fire Flow and Automatic Sprinkler System
Demand. For a building with an approved fire sprinkler
system, the fire flow demand and the fire sprinkler system
demand shall not be required to be added together. The water
supply shall be capable of delivering the larger of the individ‐
ual demands.

Reply
Dan Wilder
8/29/2023 08:31:09 am

IFC Section 507 & Appendix B

507 - Determining the fire flow via an approved method

B105.3 - Water supply for buildings equipped with an approved automatic sprinkler system, the water supply shall be capable of providing the greater of: 1. Automatic sprinkler system demand 2. Required Fire Flow

*Note that the word "Automatic Sprinkler System" is italicized for impportance.


NFPA Article comparing different fire flow methodologies:
https://www.nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Emergency-responders/RFEvaluationofFireFlowMethodologies.ashx?la=en

Reply
Sean Feeney
8/29/2023 08:50:21 am

As Dan said above, IFC annex B says greater of the two. Also note that within IFC jurisdictions, this Annex B method is usually not adopted by the code. An "approved method" must be used, and there are several methods available. The AHJ may have the authority to request that the supply be sized for both manual and automatic extinguishment, but the norm is to size for the larger of the two.

Reply
Casey Milhorn
8/29/2023 08:51:25 am

I've seen many projects that can't supply the IFC fire flow demand. If every rural church, factory, school, etc. were made to meet the fire flow demand and duration, the landscape would be littered with ground storage tanks. Not saying it's right or wrong, but the AHJ's give this a pass many times. To dive deeper, WHERE is the fire flow demand required to be met? What if you have a public hydrant that can meet the demand somewhere in the vicinity of the building, but your private hydrants on site can't meet the fire flow demand? Does that mean it's not up to code and should require a tank or other backup water supply?

Reply
SCHULMAN
8/29/2023 09:28:11 am

please check with your local AHJ. I have designed systems in areas where the total IS required for "safety factor" by Fire Departments who know full well they will pull at least a portion of the available water from the sprinkler system when they attach their pumper truck to the hydrant and fight with hoses.

Reply
Glenn Berger
8/29/2023 10:12:20 am

Fire flow in accordance with NFPA 1 (or similar) does not need to be added on to sprinkler flow requirement in accordance with NFPA 13. The greater of the two requirements is what the basis of the design water supply should be based upon. Also an allowance for other wate4r needs should be added above the fire protection requirements.

Reply



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