Eric,
Here's the way I have used this. For OH occupancies, 850 gpm at 20
psi is required at the highest sprinkler. (assumption: 7-2.2.2 is
met) To that, you would add the elevation loss. If the house is 30
feet high, you would add 13 psi to the 20 for 33 psi at the riser (at
850 gpm). I also add in a backflow preventer loss and an underground
main loss to get my demand in the street. If the main and BFP losses
are 5 psi each, then 10 psi is added to the 33 for a total
requirement at the street of 850 gpm at 43 psi. (whereas the UG loss
is not required, it gives an extra safety factor on a potentially
sketchy situation)
From what I have seen, the symmetrical pipe schedule systems (such
as what you would see in large, open areas) can calc out OK. However,
all bets are off when you have irregular shaped systems (like in a
house), long runs of main, lots of extra heads due to configuration,
or other things that would add more equivalent length or sprinklers
to the system. Also, pipe schedule never differentiated between wet
and dry systems, so the increased area and decreased C factor for
dry, will make things worse. Your results, while possibly extreme,
are not necessarily unusual.
At 03:10 PM 5/18/2007, you wrote:
I am evaluating an existing pipe schedule system that was installed in
1961 in an old house. It is now used as a business-type occupancy and
the AHJ has asked that the system be evaluated to the current (1999)
edition of NFPA 13.
In addition to evaluating the available flow and pressure against the
requirements of Table 7-2.2.1 for pipe schedule systems, I set up a
hydraulic calc for the upper floor and found that while Table 7-2.2.1
was met, the system fell 132 psi above the water supply curve. Ouch!
And I didn't even have enough floor area to get to 1950 square feet for
this dry-pipe system, even assuming the combustible concealed spaces
meet the limits that would make me go to a 3000 sq. ft. area.
Section 7-2.2 says to take the residual pressure at the flow in Table
7-2.2.1 and subtract the elevation head of the highest sprinkler. Is my
interpretation correct?
Is this unusual for a pipe schedule system? I see that in light hazard
up to 30 heads may be fed from a 2.5" pipe, but at 7 psi each that's
over 450 gpm. In my building there are a lot of extra elbows and fairly
long runs of 2.5", but I'm still taken aback by the results of my calc.
Thanks in advance,
Eric J. Shelton, PE
Senior Fire Protection Engineer
Hankins and Anderson
Consulting Engineers
4880 Sadler Road Suite 300
Glen Allen, VA 23060
v: (804) 285.4171 f: (804) 217.8520
http://www.haengineers.com <http://www.haengineers.com/>
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Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
860-535-2080
www.fpdc.com
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