NFPA fire tank sizing. 3

[EBazigos]

I am currently trying to find a section in the NFPA codes that roughly states, that if your fire tank size is not sufficient to provide for 100% of the demand then you can have a reduced tank size but need to have an increased water inlet to the tank.

Could someone tell me exactly which NFPA and what section of that specific NFPA this is?

Thanks a lot.

 

[TravisMack]

I believe you are looking for what is typically called a break tank.

Let's say that your water supply can only deliver 500 gpm, and you have a 1000 gpm sprinkler demand. You can have a tank that will supply 500 gpm for your water supply duration. You will need to have an automatic float valve on the tank so that it is automatically being filled by the public water line. I have seen these in rural areas, and also in systems where we have a very high static and very low residual pressure.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Sdpaddler50]

I see these alot in high rise buildings as well. They can work pretty well, but as Travis notes, its important you have the proper amount of auto and manual fill lines that are adequately sized to keep the tank full, when pulling a large sprinkler/hose demand.

 

[fpst]

I'm not qualified in any way to answer your question, I just thought I would take a look at it.

Here is what I found using Travis Mack's suggested keyword 'break tank':

Standard for the Installation of Stationary Pumps for Fire Protection[/i], 2010 Edition]
3.3.6 Break Tank. A tank providing suction to a fire pump whose capacity is less than the fire protection demand (flow rate times flow duration).

To me, this suggests that the NFPA 20 standard doesn't explicitly state that break tanks can be used as water supply supplements to systems without a fire pump because it states by definition, break tanks provide suction to fire pumps, but that is a strict interpretation.

So then I went to:

Standard for the Installation of Stationary Pumps for Fire Protection[/i], 2010 Edition]

4.31.1 Application. Break tanks shall be used for one or more of the following reasons:
(1) As a backflow prevention device between the water supply and the fire pump suction pipe
(2) To eliminate fluctuations in the water supply pressure and provide a steady suction pressure to the fire pump
(3) To provide a quantity of stored water on site where the normal water supply will not provide the required quantity of water required by the fire protection system

My loose interpretation of this is that option (3) isn't constrained to only fire pump situations, in case your system isn't using one this would be vital.

My opinion is that you should take this information to the AHJ, along with the equivalency clause below, which you can use to argue that even though break tanks may be defined to be used for fire pumps only, it's the same concept without a fire pump:

Standard for the Installation of Sprinkler Systems[/i], 2010 Edition]
1.5 Equivalency. Nothing in this standard is intended to prevent the use of systems, methods, or devices of equivalent or superior quality, strength, fire resistance, effectiveness, durability, and safety over those prescribed by this standard.
1.5.1 Technical documentation shall be submitted to the authority having jurisdiction to demonstrate equivalency.
1.5.2 The system, method, or device shall be approved for the intended purpose by the authority having jurisdiction.

If all of that goes smoothly, then you should take a closer look at NFPA 22 Standard for Water Tanks for Private Fire Protection, 2008 Edition as well as these sections in NFPA 20 Standard for the Installation of Stationary Pumps for Fire Protection, 2010 Edition:

Standard for the Installation of Stationary Pumps for Fire Protection[/i], 2010 Edition]
4.31 Break Tanks.
4.31.3 Refill Mechanism. The refill mechanism shall be listed and arranged for automatic operation.
4.31.3.1 If the break tank capacity is less than the maximum system demand for 30 minutes, the refill mechanism shall meet the requirements in 4.31.3.1.1 through 4.31.3.1.5.
4.31.3.1.1 Dual automatic refill lines, each capable of refilling the tank at a minimum rate of 150 percent of the fire pump(s) capacity, shall be installed.
4.31.3.1.2 If available supplies do not permit refilling the tank at a minimum rate of 150 percent of the rated pump capacity, each refill line shall be capable of refilling the tank at a rate that meets or exceeds 110 percent of the maximum fire protection system design flow.
4.31.3.1.3 A manual tank fill bypass designed for and capable of refilling the tank at a minimum rate of 150 percent of the fire pump(s) capacity shall be provided.
4.31.3.1.4 If available supplies do not permit refilling the tank at a minimum rate of 150 percent of the rated pump capacity, the manual fill bypass shall be capable of refilling the tank at a rate that meets or exceeds 110 percent of the maximum fire protection system design flow.
4.31.3.1.5 A local visible and audible low liquid level signal shall be provided in the vicinity of the tank fill mechanism.
4.31.3.2 If the break tank is sized to provide a minimum duration of 30 minutes of the maximum system demand, the refill mechanism shall meet the requirements in 4.31.3.2.1 through 4.31.3.2.5.
4.31.3.2.1 The refill mechanism shall be designed for and capable of refilling the tank at 110 percent of the rate required to provide the total fire protection system demand [110% × (Total Demand −Tank Capacity) / Duration].
4.31.3.2.2 A manual tank fill bypass shall be designed for and capable of refilling the tank at 110 percent of the rate required to provide the total fire protection system demand [110% × (Total Demand −Tank Capacity) / Duration].
4.31.3.2.3 The pipe between the municipal connection and the automatic fill valve shall be installed in accordance with NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances.
4.31.3.2.4 The automatic filling mechanism shall be maintained at a minimum temperature of 40°F (4.4°C).
4.31.3.2.5 The automatic filling mechanism shall activate a maximum of 6 in. (152 mm) below the overflow level.
4.31.4 Installation Standard. The break tank shall be installed in accordance with NFPA 22, Standard for Water Tanks for Private Fire Protection.

 

[TravisMack]

I was assuming that you were going to have a fire pump. Typically, any time we have a tank on site, you are going to need a fire pump, unless you have a very elevated tank.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com