I'm confused. Why are you averaging? You need 46.5' along the BL, right?
There is no requirement for any number of heads (except for ESFR and other
head count designs). Other than saying you have to work in whole sprinkler
heads the code process of design area selection never mentions head counts,
right? The introduction of S to create a head count related to the 46.5'
comes from the annex and is an EXAMPLE for ONE particular arrangement. It
certainly isn't there as an example for any other situation.
If I have varying lengths between the heads I check, starting at the wall,
the number I need to get to 46.5' with a RULER. This establishes the
minimum number of heads I will calculate, call it calc #1. Then I work my
way down the line with my RULER and check the next series of heads minus the
last head on the line. If I get a head count higher than the first that
becomes a second calc I HAVE to do because I don't readily know if the
length or the extra head is more of a hit in the calcs. I check all the
possible continuous heads for the 46.5'. Seems easier to me to than doing a
bunch of math which if right is by coincidence or if wrong has no
correlation to code. 46.5' is the only right answer.
And note you absolutely have the ability to shift the heads on the line if
you come up real close and have some area left in the sq.ft./head limits.
Measure 46.4' to the center of 2 heads. Shift one an inch and save yourself
a head on that calc. *****BUT***** that inch needs to be checked closer to
the main as you probably will get caught someplace with an extra head.
Hydraulically, it usually is better to take the hit for the extra head over
the main than at the end of the BL.
Chris Cahill, P.E.
Fire Protection Engineer
Sentry Fire Protection, Inc.
763-658-4483
763-658-4921 fax
Email: [email protected]
Mail: P.O. Box 69
Waverly, MN 55390
Location: 4439 Hwy 12 SW
Waverly, MN 55390
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Bob Knight
Sent: Tuesday, April 20, 2010 1:30 PM
To: [email protected]
Subject: RE: Calc Area
George,
The way I do this is to average the lengths between heads. For example, you
have a branch line with 6 heads. They are spaced 8', 12', 9', 11', 7', &
10' apart. The average will be 9.5' applying the 1.2 rule (assuming 1500
sf) you will have 4.89 (round up to 5) heads per line. If you use the
greatest distance of 12' you end up with only 3.87 (round up to 4) heads per
line. If you chose to use the least dimension of 7' you will end up with
6.63 (round up to 7) heads per line. 7 seems like to many and 4 looks like
not enough. This is why I average the distance between heads in a case such
as yours.
Bob Knight, CET
1660 Hill Rd
Melba, Idaho 83641
(208) 318-3057
(208) 495-2057 fax
[email protected]
www.firebyknight.com
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of George Medina
Jr
Sent: Tuesday, April 20, 2010 12:41 AM
To: [email protected]
Subject: Calc Area
If I am calculating a tree type system using the area/design method, and
have several different lengths between sprinklers on a branch line. Would I
use the longest length to determine the amount of
sprinklers on a branch line. I have always used an average (which I've
probably been wrong all these years).
1.2vdesign area
S
S= Distance between sprinklers on branch line
1.2vdesign area
S
S= Distance between sprinklers on branch line
George Medina Jr.
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