Consider All Areas to Size Elevated Storage Tank?

12/19/2024

When sizing an elevated fire water storage tank for the minimum supply duration of a sprinkler system, should non-remote areas (areas closer to the tank) of the system be considered as well?

Since sprinklers closer to the tank would be under higher pressure, they would discharge more water than those in the remote area.

Thanks in advance.

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6 Comments

Glenn Berger

12/19/2024 08:14:53 am

Officially the hydraulically most demanding area needs to be considered when sizing the water supply system to serve the building's sprinkler system(s).

In areas, where more pressure is available, the sprinkler contractor can reduce pipe sizes to take advantage of the increase available pressure and this in turn can cause the area by the supply to not need more water due to lesser pressure available.

Anthony

12/19/2024 08:15:15 am

Any sprinkler system water supply will be evaluated based on the DEMAND of the various hazards. The calculation will determine the minimum supply pressures and volumes required. Any extra pressure is "bonus" and will only aid in the system performance (below 175 psi).

Franck

12/19/2024 08:37:55 am

Not with NFPA where you size your water supply (pump capacity and tank volume) on the most remote area of the most demanding area in terms of hazard (could be EH gr 2 or based onna storage iccupancy demand).
If it is fine for the most remote area, it will be fine with all other areas and you also don’t need to make a hydraulic calculation for a most favorable area.

Note that this is not the case with European standard EN12845 where you need to size your fire pump on the requirements from the most remote area (as per NFPA) and the most favorable area.

Jesse

12/19/2024 08:44:30 am

The tank is sized based upon the automatic sprinkler demand. The demand calc is from the hydraulically remote area which is not necessarily the most physically remote area.

Discharging more pressure doesn't necessarily equate to discharging more volume. In hydraulics we have to trade one for the other - we have to sacrifice pressure to get more volume and give up some volume to get more pressure. So, the size of the tank is sized to the overall volume demand of the system multiplied by the required duration.

David Kendrick

12/19/2024 09:48:19 am

I enjoy reading questions and posts from subscribers to Meyerfire.
There is a great satisfaction to see thinking minds consider the standards and perhaps finding issue with published recomendations.
With that keep in mind the considerable process that happens prior to publishing a standard. The recommendations from within or without the supervising committee of the particular standard(s).The knowledge and weight of the supervising committee members from their credintials as well as their sponsoring industries. The next steps of publishing changes are for comment then after several actions within the committee recommending to the NFPA members at convention for adoption.
These published standards are taken seriously. Potentially reviewed by hundreds of interested members of NFPA. Some have self promoting motivations but overall it they seem to have the best interests of all involved in safety. Safety both of lives and property.

This post is meant to applaud the question and not a criticism. This industry needs critical thinking minds to keep improving on safety.

Thanks for allowing the comment.

Jack G

12/19/2024 10:56:21 am

The tank is sized for the automatic sprinkler demand plus 5 % for the tank bottom ( sludge ) is how they are sized.
If the closer areas are not calculated as a " check area " ( as i call it ) using the increased pressure to reduce the sizes, then its possible with the increased pressure fewer sprinklers might open reducing the water usage.
On medium to larger systems i perform at least 2 check areas ( and a supply calc) .
I try to size the system so the piping controls the flow of water with the least amount of over discharge. I find this to be the most efficient way to size the system piping. ( size kinda like a deluge system-- where pipes control the flow)

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Consider All Areas to Size Elevated Storage Tank?

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