Required Fire Flow for hydrant & building sprinkler

[canman]

Hello all,

I'm a civil engineer in the Southern California region that needs an answer to what most will probably regard as an easy question. I design a lot of fire protection systems from the point of connection at existing watermains in public streets up to private on-site hydrants and the base of fire risers at new buildings. I do not design the building sprinkler systems themselves, only outside the buildings. What does N.F.P.A. 13 or 24 say about how much flow needs to be calculated for when designing the required flow for both a private hydrant and fire riser at a building? For instance, let's say I was assigned a task to design a new fire system for a private hydrant that requires 1500 gpm, at a minimum residual pressure of 20 psi, in addition to the 500 gpm required at the base of a riser to feed a 4,500 s.f. light hazard occupancy type building. Do my overall calculations need to combine the 1500 gpm for the hydrant and the 500 gpm for the building? Obviously designing for the 2000 gpm would make a difference in the price of the pipe, valves, backflow equipment, etc. I deal with lots of different City & County fire authorities and some require the calcs to account for both while others only require calcs for the larger number of 1500 gpm, thus the confusion on my end. Thanks for your input in advance.

Regards,

Canman

 

[BobPE]

Canman:

The world of NFPA is a strange one. I design the distribution systems for the sprinkler and hose flow combined. I have been taken to task on several designs in court to support insurance claims and I have been quite successful defending my calcs when they come under lawyer scrutiny. Your hydrant should be on the upstream side of BF prevention and you sprinkler at design flow needs to meet its minimum residual while the hydrant flow is occuring. The big trick is if you are doing the calcs, they are your calcs and you will be surprised at the lack of codes enforcement in the court room when you are defending the system design. I cannot stress enough that we are the engineers, and only we will be taken to task for what we design. I always err on conservative with NFPA designs...conservative but yet cost effective. The codes serve as a guide to get you where you got to go. Only an engineer can finish the task. So you are right in questioning and always keep doing so.

let us know how you make out...

BobPE

 

[TravisMack]

site fire flow is typically independent of fire sprinkler design calculations...the site fire flow requirements are, according to the UFC, based on building construction, use and square footage.

The fire sprinkler system flow is based on a number of sprinklers operating during a fire event. For a light hazard building, assuming a wet system, you will be looking at a sprinkler calculated demand of somewhere around 150 - 250 gpm. This may be able to be reduced with the use of QR sprinklers. Then, you must include an additional 100 GPM of hose allowance for fire dept use. However, you will need more than 20 psi for the sprinkler system to work.

I hope this helps...feel free to email if you have any more questions.

 

[canman]

Mfpd -

I was trying to stay away from calculating the fire flow requirement inside a building although you touched a nerve when you mentioned one would need more than 20 psi for a sprinkler system to work. N.F.P.A 13, 1999 Edition, Section 7-2.2.1 states that for light hazard buildings less than 5000 s.f. in size needs a minimum residual pressure of 15 psi, what are your thoughts on that?

Going back to my original question, would I need to size my line serving both a private hydrant and sprinkled building using the combined requirements for both or just the private hydrant?

Thanks for your response!!

 

[SB63]

Canman,
I really hope California can get around to adopting the ICC codes. In appendix B of the International Fire Code, there is a statement which is related to your concerns.

2000 IFC B105.2 Buildings other than one-and two-family dwellings. The minimum fire flow and flow duration for buildings other than one- and two-family dwellings shall be as specified in Table B105.1. Exception: A reduction in required fire flow of up to 75 percent, as approved, is allowed when the building is provided with an approved automatic sprinkler system installed in accordance with Section 903.3.1.1 or 903.3.1.2 of the International Fire Code. The resulting fire flow shall not be less than 1,500 gallons per minute.

 

[canman]

SB63 - Actually the California Fire Code has the same allowance for a reduction in the required fire flow up to 75 percent for buildings that are sprinkled. I just found out today that the largest fire authority in Orange County, California which serves 1.3 million people and 22 cities has a policy which states the following: "OCFA does not require that the total required fire flow include the combined simultaneous demand of both the fire sprinkler system and the fire hydrant system as long as the fire sprinkler system demand does not exceed the minimum required fire flow from a hydrant." Unfortunately one of the cities not served by this authority does not adhere to this policy in which I'm currently involved with. I suppose that N.F.P.A. doesn't state either way and leaves it up to the individual fire authority to decide.

 

[TravisMack]

The 15 psi you refer to is when you have a pipe scheduled system. However, it was found that a lot of pipe schedule systems do not work when calculated.

For instance, in a light hazard occupancy, a maximum spacing is 225 sq ft. At 0.10 gpm / sq ft that equates to 22.5 gpm. Using a 5.6K sprinkler (required in pipe schedule systems), the pressure at the sprinkler shall be a minimum of 16.1432 psi [p=(Q/k)^2]. Assume an elevation of 12', you need another 5.2 psi. If a double check backflow preventer is required, that is another 5 psi. Typical friction loss in a system for a building this small is around 10-20 psi, depending on pipe sizing. Therefore, you should need around 36 psi at the base of the riser at the system demand flow rate. Additionally, you have to add the 100 GPM hose allowance at the point of connection to the underground supply.

Back to your original question, IMO, the hydrant line is sized independently of the sprinkler system. However, the line shall be sized to the more demanding requirement.