Water Storage Tank Size Related to Fire Pump?

1/20/2023

For determining the capacity of a water storage tank, is the tank capacity calculated by the max flow extension of a pump curve or by 140% rated capacity? Or could it even be 120% of the pump rated capacity?

Looking for guidance on how the pump size and water storage tank size would be directly related.

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10 Comments

Dan Wilder

1/20/2023 07:17:39 am

My approach has been to provide 150% of the pump capacity (140% refers to rated head I believe) multiplied by the duration needed as the "Useable" volume. Find the tank size that provides at least that minimum amount.

This assumes that the usage is based on the most flow demanding of the site fire flow, standpipes, and sprinklers for the durations needed based on what the fire pump is suppling (Fire Flow may have a different duration than the sprinkler/standpipe requirements). If the fire pump is not supplying fire hydrants, I have omitted hose stream requirements which greatly reduces the tank size.

The size of the tank can also be reduced with a reliable refill source offsetting the GPM usage by the available GPM refill.

Alex

1/20/2023 08:03:23 am

To add off of Dan, make sure you account for unusable water within the tank and ensure you have enough head to get to the suction side of the pump.

Dan Wilder

1/20/2023 09:14:25 pm

This is what happens when I start a paragraph and get distracted.

2nd paragraph is what I use , the 1st paragraph was to reference the 140% vs 150% as it relates to testing of the fire pump which is typically piped back into the tank anyway so it's not really being "used" per se.

but in whatever calc gets you there, make sure that the tank picked is based on usable gallons. The intake plate that sites 1' above the floor of the tank can easily lose up to 10% of the tank capacity.

RayJ link

1/20/2023 08:35:18 am

I have always used these formulas taken from NFPA 13.
System Demand + Hose Demand X Duration equals gallons.
(Approx.Tank size).
Density Area X Hazard + Hose Demand = Approx. Pump size (in gpm)

These figures can be then enlarged or reduced as needed.

COLIN LUSHER

1/20/2023 08:38:02 am

The only ACTUAL requirement from NFPA 22 that relates tank size to fire pump capacity is for a "break tank". In this case, the minimum size for a break tank is 150% of pump capacity for 15-minutes. For anything OTHER than a break tank, the demand is based solely on the demand and duration of the fire suppression system (standpipe or fire sprinkler, whichever is most demanding), independent of the fire pump capacity. While many, like Dan mentioned above, may consider the fire pump size when determining tank size, it is NOT a requirement of NFPA 22 to the best of my knowledge.

Casey Milhorn

1/20/2023 08:51:51 am

It's also a good idea to make sure the tank isn't expected to provide fire flow demand as well, which could be more GPM and duration than the fire sprinkler demand. I have had an AHJ take a tank from something like 15k gallons to 80k gallons on a small light hazard building all because of fire flow. That's assuming the tank is feeding hydrants or will have a draft hydrant connected to it.

Pete D.

1/20/2023 10:32:34 am

^ This.
I've always used fire flow, which is loosely defined as the greater of the overhead sprinkler demand or the fire flow tabulated value (with any reductions) where AHJ approved.

Then you can take a deduction for the assumed tank re-fill flow rate (giving you in essence a net tank drain rate) x fire flow duration.

And this is how industrial sites end up with 500kgal tanks.

Glenn Berger

1/20/2023 09:57:13 am

You need to consider the location of the pump and water tank with respect to hydrants. Sometimes hydrants are not located downstream of the pump/tank assembly. Also you need to consider the max flow of the sprinkler system and the max flow when testing the fire pumps.

Sean

1/20/2023 10:49:42 am

tank size is based off the dystem demand times the duration.

Franck

1/20/2023 11:18:30 am

For an existing system, take the flow demand of the most demanding area from hydraulic calculations, add the water supply requirement for hoses and multiply by the required duration.

Most of the time, as the nominal flow of the pump should cover the most demanding area, you can simply do the nominal flow (100%) times the duration.

The specific case of a break tank is different because it has a limited capacity and if the break tank is empty after 20 minutes and you need 1 h duration, then you have a big problem ! This is why 150% nominal flow of the pump is indicated (safety margin).

For a new project, when no calculation has been done, if you want a rough idea of the needed tank capacity, you can simply make the following calculation:
Most demanding (design density x area of application) x 1.15 (to consider friction losses, as a conservative approach).
Add the demand for hoses
Multiply the total by the duration.

For example, if most demanding area will be 0.3 over 2500 for 1h30 minutes, plus 500 gpm for hoses, the end result will be:
[(0.3 x 2500 x 1.15) + 500 ] x 90 = 122,625 gal
And if you want to be a bit more conservative, assuming that you will install a 1500 gpm pump (for the demand of sprinkler + hoses), you can design the water tank to 135,000 gal (1500 x 90). This will give you some margin for possible future project at this location !

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Water Storage Tank Size Related to Fire Pump?

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