What is Effective Capacity of Water Storage Tank?

5/19/2025

How do you determine the effective capacity of a water storage tank for fire protection purposes?

Where are the unusable limits set for the bottom of the tank suction and the top air gap?

Is there a mandate for either of these in code or standards?

Thanks in advance.

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5 Comments

Dan Wilder

5/19/2025 08:09:48 am

NFPA 22 - 23' - 4.1.4 & 4.1.5

Basically, the discharge outlet/vortex plate to the overflow. Some AHJ's & Insurance agencies like to keep a little buffer in there and say "x" inches above the discharge outlet/vortex plate.

4.1.4*
For suction tanks, the net capacity shall be the number of U.S. gallons (cubic meters) between the inlet of the overflow and the level of the vortex plate.

4.1.5

For all tanks other than suction tanks, the net capacity shall be the number of U.S gallons (cubic meters) between the inlet of the overflow and the discharge outlet.

Todd E Wyatt

5/19/2025 08:27:40 am

See https://www.meyerfire.com/daily/calculations-for-water-storage-tank-in-nfpa-1142 ...
Calculations for Water Storage Tank in NFPA 1142

Glenn Berger

5/19/2025 08:51:32 am

I agree with Dan's comment, but I include a distance below (depends on the size of the tank) the overflow to properly have the high level alarms and flow control valve operation points included.

Jack G

5/19/2025 10:07:28 am

Dan has it. ( for suction tanks and other than suctiom) However Nfpa 22, 4.1.4 and 4.1.5 ( see appendix ) implies a 12 inch minimum for the water sensor ( evaporation and fluctuation and closure time for the fill valve) and maintaining a reasonable amount of water in the bottom of suction tanks to prevent cavitation of the fire pump should be factored into the overall tank size. For instance i sdd the demand flow ( system demand timed duration ) plus the 1 foot minimum( see abobe- fluctuations and evaporation . ( 5% of tank height gallons,)
At say 1000 gpm demand for say 30 minutes in a 20 ft elevation tank this would add about 6300 gallons to the tank size ( including the 1 ft gallons and the cavatation gallonsv) i do want the pump to survive).
Another thing to consider is a secondary water supply to eliminate 25 percent of insurancecharge for not having a secondary supply. If the dollars spent for that ( yearly savings times 15) is an amount that would cover those costs .

Jack G

5/19/2025 10:25:50 am

Also please check the MeyerFire archives. I believe this was discussed around March of 2024 ( if memory serves me right)
And had a good group of responses.

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What is Effective Capacity of Water Storage Tank?

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