NFPA 13 '96 Sprink storage tank size? 1

[MarauderX]

The initial comment passed around:
"Site does not appear to have available city water and we will have to use the existing well for Domestic Water. As far as Fire Protection is concerned, there will be a need for a Fire Pump as well as a water Storage tank.

The storage tank needed for this building will have to be quite large and located somewhere near to the Fire Pump building. Initially, the storage tank should be filled via outside water source with a water feed from the domestic line controlled by a float switch will add water as necessary to maintain a minum level for fire protection.

I will need to know the hazard level of the building to estimate the storage tank size. If light hazard, the water demand will be Hose Stream Demand (100 gallons per minute for 30 minutes per NFPA 13 Table 5-2.3) plus the water supply for sprinklers ( .10 gallons per minute/square foot for 30 minutes). If a higher Hazard is considered, the water demand for the Fire Sprinkler System can be much higher than this.

From some quick calculations that I have come up with, we could be looking @ a 70,000-140,000 gallon tank depending on the Hazard level of the spaces. Remember, the tank has to hold enough water to allow the water flow @ a minimum gpm for 30-60 minutes depending on the Hazard level."


Reply to the above:
"The assessment indicated below is correct to the extent that a fire pump and storage tank are required if the building is sprinklered and the city water supply is not adequate to meet the demand. However, I question the tank size indicated in this email. I am assuming this is a car dealership with a service facility. The showroom would fall into an Ordinary Hazard, Group 1 occupancy while the repair shop would be Ordinary Hazard, Group 2. The hose stream demand and required duration of supply is the same for both classifications. For Ordinary, 2, the required density and area are .2 gpm/sq ft over a design area of 1,500 sq ft. This will require a flow rate of 300 gpm. The hose stream requirement is 250 gpm. The required duration for this occupancy ranges from 60 to 90 minutes. Taking worst case scenario you will be flowing 300+ 250 = 550 gpm for 90 minutes. Throw in 15% additional flow for safety and this would be 630 gpm. Flowing 630 gpm for 90 minutes requires a total of 56,700 gallons.

I think the authority having jurisdiction may allow a smaller tank. The clients insurance company may also have a say in this as well, therefore the AHJ and insurance carrier should be consulted."

Reading the NFPA 13 is confusing on this issue... we are lacking experience with this as we rarely do fire protection design.
Thanks in advance -
~MX

 

[TravisMack]

Let's address this in a couple steps:
1 - Will the tank/pump also supply site fire hydrants? If not, then the 250 GPM demand is not to be included in the requirements.

2 - Is there a remote station or central waterflow alarm station? If so, then the smaller duration will be allowed...so you could use 60 minutes.

3 - Since this is an ordinary hazard occupancy, if the ceiling (or deck if exposed construction) is less than 20', then the design area may be reduced per NFPA 13 allowance for QR sprinklers. If your deck is 20' for example, then the design area may be reduced to 1125 sq ft.

So, putting this all together, assuming a 20' ceiling/deck, the design area would be 0.20 gpm / sq ft over the most 1125 sq ft. This is 225 gpm. Add 15% for overdischarge/inefficiency...you get approx 260 gpm.

If the 60 minute duration is allowed as noted above, then you have a tank size of 15,600 gallons.

HOWEVER, if the tank is to supply the ENTIRE fire demand, you could be looking at some very heavy water demand issues. If you are in UFC land, there is a table in the appendix (I think) that gives you site fire hydrant flow requirements. This could be as high as 3000 gpm. Then the tank would be 180,000 gallons.


Feel free to email me if you need further clarification or assistance.

 

[MarauderX]

Thanks for the reply, it clears up some of the NFPA 13 confusion. To answer your questions:

1 - The tank will need to supply the fire hydrants, so the 250 GPM demand will need to be included.

2 - We will be installing a remote waterflow station, so the 60 minutes could be used.

3 - The ceiling (upper level) and the deck (lower level) are both less than 20'.

Also I noted under 5-2.3.1.3-a that has an area less than 1500 sqft, 1500 sqft should be used. From this I think the calc for the minimum is:
300+250= 550 GPM
550 + 15% = 633 GPM
Bare minimum (no safety): 550 GPM * 60 minutes = 33,000
With safety: 633 GPM * 60 minutes = 37980

Does this make sense?

Also, if we do not need to put in sprinklers per code but we do decide to for the owner's insurance company, would we still need the storage tank sized to comply with NFPA 13?

Thanks again

 

[TravisMack]

The insurance company will likely require that the system be installed per nfpa.

As far as the tank supplying the hydrants, 250 GPM is the allowance required to be considered with a sprinklered building. However, you may need to consider the site fire flow requirements. For example, many AHJs require a minimum availability of 1500 GPM @ 20 psi at any hydrant. This would greatly affect the size of the tank. That issue alone needs to be clarified by the AHJ, as they are seperate and distinct.

If you have a reliable water source underground, you could do a vertical turbine pump to supply the hydrants and sprinkler system. But, there are several other issues to consider when taking water from a "raw" source. We just recently supplied a rack sprinkler system from a vertical turbine pump. The underground water table was about 15' deep and was basically fed by the Mississippi river, so we had a great supply.