Water Storage Tank Size for Fire Flow with Refill?

5/20/2024

How do you size a water storage tank for fire flow when the city supply is significant, but not sufficient for Fire Flow?

For instance, if I have 1,500 gpm available from the city at 20 psi, but I need 2,000 gpm for Fire Flow, is the water storage tank sized for 500gpm for the required duration?

Or do I need to size the tank for the entire 2,000 gpm minus reliable refill?

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9 Comments

Jose Pinies

5/20/2024 08:09:07 am

Por favor, notificarme por correo los nuevos comentarios de esta publicación

5/20/2024 08:29:55 am

Unless local requirements or AHJ say otherwise, you can size for the refill. So in your example, if it’s a 60 minute duration, you need a minimum 30,000 gallon tank (500 gpm x 60 min).

One example of a local requirement saying otherwise would be if this is a high rise in a seismic zone requiring a secondary water supply. In this situation, you would want to size as if the city water got ‘cut off’.

5/20/2024 08:52:05 am

If the water storage tank is being used to support the fire hydrants I would size the tank for the fire flow.

Glenn Berger

5/20/2024 09:13:12 am

Need to obtain approvals from the AHJ and other approval authorities before proceeding with this method.

I have followed the latter example in your question - Sizing for the required flow rate minus the proven refill rate.

5/20/2024 09:37:19 am

After re-reading the question and your response I am a little confused. Would the flow tested 1500gpm at 20 psi not be the ‘reliable’ or ‘proven’ flow rate?

Dan Wilder

5/20/2024 09:30:00 am

NFPA 22'-23' Ed.

Section 14.5 on Break Tanks
Tank will need to be sized for pump operation at 150% minimum, then start looking at the deficiency.

Pump = 2000GPM @ X PSI
System Demand = 2000 GPM for 60 min
Supply =1500 GPM for 60 min(assumed)

Minimum tank size = 3000 GPM (150% of pump) for 15 min = 45,000 Gallons
Alternate tank sizing = 120,000 gallons (Fire Flow = 2000 GPM x 60 min) - 90,000 gallons (Refill = 1500 GPM x 60 min) = 30,000 gallons deficient
45K should be the minimum tank size

*Tank requirements are "Net" (Useable water - inlet of overflow to the discharge outlet per 4.1.6)

Steve Cooper

5/20/2024 09:54:44 am

The tank is sized per fire flow. They want the building to have water even if city supply cuts off. It's a safety factor.

5/21/2024 08:55:43 am

Reference Chapter 4 of NFPA 1142, 2017 Ed. for your minimum tank size. Run it by the AHJ.

Jack G

5/21/2024 05:21:19 pm

Your opening paragraph indicates it’s a water “ storage “ tank, for fire flow. ( not a break tank)
So take the fire flow times duration - add the dead water to the capacity and you get tank size.

The city water, obviously , with a 500 gpm pump will or should be sufficient for the sprinkler system.

Is the electric grid “ reliable “? Are Diesels “ reliable” ?
You don’t mention hydrants.
Insurance premiums take a 25% hit ( addionak) for no secondary water supply.
So why not propose 2 diesels and the reduction in premium pays for the extra pump in 4-7 years.

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Water Storage Tank Size for Fire Flow with Refill?

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