Tank Sizing: Where to Add Hose Allowance?

7/24/2024

If the system demand is 321.5 gpm for an Ordinary Hazard Group 2 system, do you add the 250 gpm hose allowance before or after calculating the 90-minute supply?

For example, 321.5 gpm + 250 gpm = 571.5 gpm x 90 minutes = 51,435 gallons, or, 321.5 gpm x 90 min + 250 gal = 29,185 gallons.

Looking for input to order a water storage tank and the owner is disputing the water storage tank size. We told them we would need a 52,000 gallon tank.

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7 Comments

Daniel

7/24/2024 08:05:10 am

Check NFPA 13 A.19.1.5.2 (2022). There is some nuance in answering this without all of the information.

Daniel

7/24/2024 08:06:09 am

"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. Where a tank is used for both domestic/process water and fire protection, the entire duration demand of the domestic/process water does not need to be included in the tank if provisions are made to segregate the tank so that adequate fire protection water is always present or if provisions are made to automatically cut off the simultaneous use in the event of fire."

Pete H

7/24/2024 08:06:24 am

From NFPA 13 (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 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 the equipment not connected to the tank can be ignored. Where a tank is used for both domestic/process water and fire protection, the entire duration demand of the domestic/process water does not need to be included in the tank if provisions are made to segregate the tank so that adequate fire protection water is always present or if provisions are made to automatically cut off the simultaneous use in the event of fire.

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So my immediate question: is this tank only providing for a sprinkler system or other attachments such as hose stations?

Jesse

7/24/2024 08:09:00 am

You're correct. You'll need to the higher amount, but that presumes that the tank is supplying the hydrants - and it presumes the system is unsupervised.

If the automatic sprinkler system is supervised and the tank is only supplying the AS system, you can extrapolate your system demand for 60-minutes to get your total capacity.

Glenn Berger

7/24/2024 08:10:35 am

As pointed out by the previous responders, if the water tank supports the hoses then the water storage tanks needs to account for the entire duration required by the hoses and sprinklers.

Franck

7/24/2024 08:14:03 am

I do agree with Jesse, the first calculation is the right one. You need the water supply for manual firefighting for the same duration as the sprinkler system (it would be weird to leave the sprinkler system works for 30 more minutes while stopping manual firefighting actions).

Jack G

7/24/2024 10:31:25 am

All has hit it on the head. 150% of fire pump, system capacity x duration plus hose water. Agree.
The dead water in the tank/ water not being able to be flowed by the pump, usually the elevation to the top of the suction outlet is considered dead water.

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Tank Sizing: Where to Add Hose Allowance?

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