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American Fire Sprinkler Association
Monday's Technical Update
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A weekly newsletter dealing with codes and other technical issues
December 22, 2014
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Assigned Area of Coverage
"We have an area that has a standard spray sprinkler spacing layout of 8 ft x 12
ft. The design density of this area is a 0.20 gpm/ sq. ft. So, with an 8 ft x
12 ft = 96 sq. ft. per sprinkler x 0.20 gpm/sq. ft. = 19.2 gpm per sprinkler,
with
a K-factor of 5.6 we end up with a head pressure of 11.76 psi. Our calculation
is based on a minimum of 19.2 gpm/sprinkler at a minimum pressure of 11.76 psi.
The mechanical engineer disagrees and is telling us with a spacing of 8 ft x 12
ft that we need to figure our minimum head pressure and flows as if the head
spacing
is 12 ft x12 ft which results in a minimum flow of 28.8 gpm at a pressure of
26.45
psi. She believes that the coverage area defined by the S x L method is
different
from the calculated area because the sprinkler can't discharge in an 8 ft x 12
ft
rectangle. It must be calculated as a 12 ft x 12 ft square. Can you provide
your
informal interpretation as to which way the design density should be
calculated?"
We have reviewed NFPA 13, 2013 edition as the applicable standard. Our informal
interpretation is that the coverage area defined by Section 8.5.2 (the S x L
method)
is used in the calculation for standard pendent/upright spray sprinklers.
I understand that there is concern that the sprinkler can't discharge in an 8 ft
x 12 ft rectangle because it is true. It is also true that it can't discharge
in a 12 ft x 12 ft square. In reality, the sprinkler discharge is a circle that
is 21 ft to 24 ft in diameter (depending on pressure and ceiling height). In
truth,
reality has little to do with the hydraulic calculation method of NFPA 13. Some
of the more significant facts in fire dynamics and sprinkler operation compared
to the design methodology are: the pattern of sprinkler activations would be a
fluke if it were a rectangular shape as assigned by NFPA 13 for the remote area
(1.2√A), the already mentioned discharge in a circle instead of a
rectangle
or square, the density hitting the floor is not 0.2 gpm/sq. ft. in your design
(considering
that the discharge is over a larger circle than the assigned box and even IF one
accounted for that aspect, the density is not uniform throughout the circle of
discharge), and the friction loss calculation is not academically pure. The
hydraulic
calculation method is nothing more than defined steps that allow different
parties
to create a similar layout that are conservative (that is it exceeds the true
minimum
demand necessary to control the fire as shown by over 100 years of experience).
As stated in Section 23.4.4.5.3 under the Calculation Procedures, the area of
coverage
for each sprinkler used in the hydraulic calculations is the measured distance
between
the sprinklers and the branch lines in accordance with Section 8.5.2 (the S x L
method). It goes without saying that extended coverage and residential
sprinklers
do not apply Section 8.5.2 but are driven by their listing.
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Technical Update is prepared by the Technical Services Dept. of the AFSA: Vice
President
of Engineering and Technical Services Roland Huggins, a PE registered in fire
protection
engineering; Phill Brown, a NICET IV certified automatic sprinkler technician
and
NFPA Certified Fire Protection Specialist (CFPS) and Tom Wellen, a PE registered
in fire protection engineering. This is provided with the understanding that
the
AFSA assumes no liability for this opinion or actions taken on it and they are
not
to be considered the official position of the NFPA or its technical committees.
Copyright 2014, American Fire Sprinkler Association.
All Rights Reserved
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Training Calendar
AFSA Beginning
Fire Sprinkler System
Planning School
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January 19 - 30, 2015
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