Sizing fire protection water storage tank 1

[Oremus]

When sizing a water storage tank for exclusive fire protection use, and fed by a fire pump, should the tank be sized on 150% of pumps rated capacity (at the req'd duration)or just the fire sprinkler demand + inside hose?

 

[LCREP]

Fire sprinkler demand + inside hose x duration= tank capacity. If u have any dry pipe or preaction system then as per NFPA 13 u have to add the gpm required to fill the systems for your total gpm for the tank.

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[LCREP]

Also if u have any hydrants connected to the tank u need to add this demand too.

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[Oremus]

Thanks LCREP!

 

[PEDARRIN2]

If you have standpipes per NFPA 14, you have to consider the flow/duration for those - if it exceeds what the sprinklers require.

 

[TravisMack]

It is my opinion that the water tank is to be sized for the greatest demand. If you have hydrants, that is often your greatest demand. Second is usually standpipes, followed by sprinklers. If you have a pump taking suction from the tank, you want to be sure you have enough water in the tank to be able to complete the pump test.



Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Oremus]

Travis, so what do you think in this case - make the tank able to supply 30 minutes at 150% capacity + 30 minutes at 100% capacity and compare that with the sprinkler + hose demand at 60 minutes and see which is greater?

1500 gpm pump @ 150% = 2250 gallons X 30 minutes = 67,500 gallons

+

1500 gpm pump @ 100% x 30 minutes = 45,000 gallons
---------------------------------
= 112,500 gallons



compared to sprinkler demand (+ hose stations inside) = 1,642 gpm X 60 minutes = 98,520 gallons


Of course the pump test durations were just arbitrary - not that anyone is going to need the pump running at 150% for 30 minutes to test it... but it would be embarassing to run out of water for the pump test and the above seems to dictate at least a 100,000 gallon tank.

What do you think?

 

[stookeyfpe]

I'm late to the party but I have a question: What is the fire flow? Travis stated this, but sprinkler demand is not the same as firefighting water demand, and that's the question in my arena.

Travis answered it, and the original poster neglected to even raise the question.

Without all the facts this is not a simple question. We don't know if this a community fire pump, a building fire pump, a campus fire pump or what the heck you are trying to protect. Tell me what is "exclusive" as you originally asked.

 

[Oremus]

Stookey, pump & tank are for fire sprinklers and inside hose stations- exclusively. No hydrants, no domestic, no nothing - but fire sprinklers.

 

[TravisMack]

I don't see why you would need to run the fire pump for 30 minutes at 150% and 30 minutes at 100%. It looks like your sprinkler demand is going to be the driving force.

Does this tank have an automatic float for refill. You can even use that to reduce the size of the tank if needed.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[stookeyfpe]

Oremus,

I stand corrected. I should have read the question more closely.

 

[DavidCR]

NFPA 851/850 (protection for power plants) states that the design water capacity of the system should able to be refilled in 8hrs or less, is there a similar requirement for just sprinkler-internal hose systems water tanks as per Oremus case?

 

[chevy4x4trucks]

When you run your pump test can't you run your hoses back to the tank so you don't have to refill the tank after your annual pump test? I've done this on a few projects and doing this combined with using a flow meter makes the pump test easy.

 

[LCREP]

David

NFPA 22, 08

14.4.2 The means to fill the tank shall be sized to fill the tank in a maximum time of 8 hours.

Chevy

Why bother with hoses, use a flow meter into the tank and make it easy it is permitted by NFPA 25,08 see below.

8.3.3 Annual Tests.

8.3.3.1.2.2 Use of the Pump Discharge via the Bypass Flowmeter to Drain or Suction Reservoir. Pump suction and discharge pressures and the flowmeter measurements shall determine the total pump output.

8.3.3.1.3 Where the annual test is conducted periodically in accordance with 8.3.3.1.2.3, a test shall be conducted every 3 years in accordance with 8.3.3.1.2.1 or 8.3.3.1.2.2 in lieu of the method described in 8.3.3.1.2.3.



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Fire Sprinklers Save Firefighters’ Lives Too!

 

[MJCronin]

How about a solved example that everyone agrees with ?

 

[SprinklerDesigner2]

If fire flows are an issue I could calculate it for my own purposes but would want verification by a PE or FPE before proceeding. After reading everything about it I could find I concluded ISO fire flows are an area sprinkler layout technicians aren't qualified to navigate. What the heck, a set of plans with $300 or so on a $10 million project could save me a lot of headache down the road so why take the risk?

Why the 1500 gpm pump on a project that requires but a 30 minute supply? Sounds light hazard to me unless there are ISO fire flows involved.

From NFPA #20 appendix A-2-3

A stationary pump for fire protection should be selected in the range of operation from 90 percent to 150 percent of its rated capacity. The performance of the pump when applied at capacities over 140 percent of rated capacity can be adversely affected by the suction conditions. Application of the pump at capacities less than 90 percent of the rated capacity is not recommended.

Seems a 500 gpm pump would handle anything up to OH2.

My understanding the correct way is to calculate total sprinkler demand plus hose stream allowance for whatever time requirement there was. 400 gpm sprinkler + 250 hose = 650 gpm which would call for a 39,000 gallon water storage regardless of the pump size.

Personally I like to size the tank for the pump at 150% for whatever time is required. In the case of a 500 gpm pump I prefer to size the tank at 750 gpm for 60 minutes and provide a 45,000 gallon tank. Come on, the cost difference between a 39,000 gallon and 45,000 is very minimal. What are we talking about, $6,000 more on a project that might be sellign for $10 million?

I've always been able to offer it to the owner as an alternate which they always accept when you explain the larger tank would take care of any additions or a density increase should something change down the road.

But the answer to the question (not addressing ISO fire flows) is sprinkler demand plus inside and outside hose stream allowance if any.