Fire Pressure 2

[StickyStick]

Question!:

I am looking at running a water line to a site that contains two buildings which are split into eight units. Each unit requires a seprate water and sewer service. So, there is a 6" DIP "main" line coming into the site, with 2" (for now) water service lines into each unit.

The question arose that IF they decide to sprinkle one of the buildings, the service sizes would change from 2" to something larger. THEN, would a 6" main line be able to support the increased fire flow?

I learned from the City that the static pressure of the main, just outside the property, is 60 psi. The total length of the 6" line is about 300'.

Any opinions/advice on this would be much appreciated!

 

[TravisMack]

If the buildings are apartments, which is what I gather by the description, then you would likely fall under the design criteria in NFPA 13R. This would likely require a maximum fire sprinkler demand of 80 gpm (4 sprinklers at 20 psi). You would likely want your own 2" line for the fire sprinkler system, instead of combining both domestic and fire. The 6" site fire main should be adequate to handle sprinkler demands. I assume it is adequate to handle the site fire flow requirements (likely 1500 gpm @ 20 psi available at any fire hydrant).

If you are on a relatively flat site, 60 psi at the property line should be adequate. You will probably figure in around 100 gpm domestic allowance in the fire line to be added to the sprinkler system. Therefore, you are only going to need about 200 gpm (100 gpm domestic allowance + 80 gpm for sprinklers + 20 gpm for over-discharge). The greatest pressure loss is when you size the incoming fire line supply to the building too small. I often see engineers trying to bring in a 1.25" or smaller line for the fire demand. If you flow 80 gpm through that, you eat up a lot of pressure. Do it with a 2" line and you are likely OK.

If you are required to provide protection per NFPA 13, then you would want a minimum 4" line to come into the building. However, the 6" site main should still be adequate.

Good luck!!

 

[StickyStick]

Thanks for the info. I will look into NFPA 13. This is actually to be office space (platted as condominuims, so each could own their own space). However, it is hard to say what it could be converted into in the future (residential, restaurant...).

Thanks again!

 

[TravisMack]

NFPA 13 will force you to protect the concealed combustible spaces. I am assuming something of this size is wood construction. You could be looking at minimal demands of 100 gpm and up for the sprinkler system + an additional 100 gpm hose allowance at the point of connection to the 6" piping, assuming it is feeding fire hydrants, otherwise you would add the 100 gpm hose allowance at the connection to the city supply.