Must Water Storage Tank Include Site Fire Flow?

2/4/2019

A remote project site does not have any water utility available.

Under the latest edition of NFPA 22 (2018 Edition of Water Tanks for Private Fire Protection, Section 4.1.1), the "size... of tanks shall be determined by the required fire flow and duration for the attached fire protection system(s) and the pressures required."

Is this implying that the site fire flow for firefighting operation should be included as part of the storage volume for the water storage tank?

For this project the sprinkler system's required water supply would be about 40,000 gallons, while water storage for site fire flow would be well over 240,000 gallons under IFC fire flow or 140,000 gallons under NFPA 1142. Prior versions of NFPA 22 required "the size... of the tank shall be determined by conditions at each individual property after due consideration of all factors involved."

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10 Comments

2/4/2019 12:54:13 pm

I have used the following to figure tank size. System Demand plus Hose Demand times Duration = Gallons (Tank Size).
NFPA13-2016 page 143

James Phifer

2/4/2019 01:08:08 pm

The outside hose demand can be ignored unless the hoses come directly off the tank piping. see appendix below from NFPA 13.

A.11.1.5.2 Where tanks serve sprinklers only, they can be
sized to provide the duration required for the sprinkler system,
ignoring any hose stream demands. Where tanks serve
some combination of sprinklers, inside hose stations, outside
hose stations, or domestic/process use, the tank needs to be
capable of providing the duration for the equipment that is
fed from the tank, but the demands of equipment not connected
to the tank can be ignored.

David C

2/4/2019 04:51:11 pm

RJ & James are correct, one other point is it depends on what the tank is for.

If the tank is only supplying the Fire flow for sprinklers then that's how you would size it.

If the tank is required to accommodate the required fire flow for fire fighting operations then you would need to add that value and duration.

I have in the past had to add both of those together and ended up providing 1.8 Million gallon water tank for site fire flow and sprinkler water demand (Airport)

2/5/2019 09:25:02 am

If your tank serves fire hydrants then you need to consider site fire flow demand for tank capacity. I would explore NFPA 1142 Standard On Water Supplies For Suburban And Rural Fire Fighting. Depending on your site you may be able to propose a reduced site fire flow per NFPA 1142, if approved by the AHJ.

PETE

2/5/2019 12:13:43 pm

IFC and NFPA 1 both allow for up to a 75% reduced fire flow requirement for "fully sprinklered" buildings, as approved by the AHJ. Also, NFPA 1 §18.4.3.1 allows for decreases by the AHJ for rural areas where the development of full fire flow requirements is impractical. This provision is also in the IFC §B103.1. Also, In NPFA 1 (2012)§18.4.5.3 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 individual demands. * On a personal note * The terms "fire flow" and "sprinkler demand" should not be confused or used interchangeably. Everything I've read in the codes makes distinctions between the two.

Franck Orset

2/20/2019 11:20:38 am

Water demand for hydrant should be added to the water flow for the largest sprinkler system demand if the water supply is used for both the sprinkler systems and the hydrants.
All insurance companies will recommend that as a minimum.
It is the same for sizing the fire pump (which should be able to supply the hydrants + the sprinkler flows).

In case of fire, the sprinkler system "control the fire" but you will need manual firefighting (i.e. hoses and hydrants) to extinguish it. Even if it is true that many fires are not just controlled, but extinguished by the sprinkler system only.
This is the reason why, with ESFR (for storage occupancies) the outside water demand is lower (250 gpm instead of 500 gpm) and the duration is lower (1 h instead of 1.5-2 hours for storage occupancies).

If, as indicated by James, the hydrant are fed from an independent supply, then the water tank + fire pump should be designed for the sprinkler demand only.

Note that in some cases, the water supply is not sufficient to supply the full demand for manual firefighting (hydrants). The complement can be asked from the private water supply (tank + pump) with some additional private hydrants located on this system.

Chris Pederson link

4/15/2021 05:31:39 pm

I agree that the required fire flow should determine the size of the tank. Too small and you run into a problem. Too big and you'll pay too much money to fill it up.

Tom Kennedy

10/8/2021 08:10:45 am

If the underground tank is the only supply for the sprinkler system and there is a dry hydrant connected to the tank, would a hose demand need to be included in the sprinkler calculation?

10/11/2021 09:04:28 am

yes, I believe so per Annex A from the 2016 edition.

A.11.1.5.2 Where tanks serve sprinklers only, they can be
sized to provide the duration required for the sprinkler system,
ignoring any hose stream demands. Where tanks serve
some combination of sprinklers, inside hose stations, outside
hose stations, or domestic/process use, the tank needs to be
capable of providing the duration for the equipment that is
fed from the tank, but the demands of equipment not connected
to the tank can be ignored

10/11/2021 10:31:34 am

Thank you, James. A.11.1.5.2 is referring to sizing the tank (volume). My question is referring more to the sprinkler system hydraulic calculation, wondering if a 250gpm outside hose flow would need to be added to an ord. hazard sprinkler system hydraulic calculation.

Must Water Storage Tank Include Site Fire Flow?

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