Chris,
The simple reason is that the test for K25s was run at 50 psi because
the manufacturer wanted a success and there was no previous data. The
test could be re-run at a lower pressure anytime somebody wants to to
risk the money it would cost.
K25 applications are based on individual tests because there performance
isn't equal to K14s on a flow-to-flow basis. K17s are close enough to
K14s to allow extrapolations from K14 tests on a flow-to-flow basis in
most cases.
Anytime you treat NFPA 13 as a scientific, consistent engineering
document, you're asking for trouble! It's a large collection of loosely
connected fire protection rules that range from well-supported to
unsupported, depending on when and under what circumstances they were
adopted. That doesn't make it a bad standard to follow, but it does
make it impossible to read between the lines without risking faulty
conclusions on occasion.
The best examples are the curves for class I-IV commodities that are
read down to the third decimal place. The vast majority of the tests
behind the curves (all of which were done more than 35 years ago) were
done with class II commodities. The class III curve was based on two
tests of greeting cards in boxes and drawing a "curve" through these two
points parallel to the class II curve (with no safety factor applied).
The class IV curve was done the same way (two tests of a mixture of
plastic tape, sandpaper, etc), except that both of the tests would be
considered failures if repeated today. There were no class I tests. A
curve parallel to the Class II curve was drawn arbitrarily. So why
haven't we been burning down warehouses? The two biggest reasons are
that the test were run with K5.6 sprinklers and economics have dictated
K8 sprinklers almost from the beginning, and the tests had to be
controlled by sprinklers for 30 minutes in order to be a success when
Fire Departments arrive more quickly in most cases. (By the way, those
that believe that the reason real world sprinkler systems do better is
the high start pressure and declining density aren't on firm ground,
either. In testing, this has been proven to be wrong several times.)
Those designers who play the game of sliding up the curve to sneak by
with 130 sq.ft. spacing and/or K5.6 sprinklers to save a few pennies do
so at the cost of fire protection that is on the ragged edge, even
though 13 can be interpreted to say that any point on the curve with any
sprinkler and any spacing will provide the same protection.
Joe
Chris Cahill wrote:
Anyone have a theory why uncartoned unexpanded plastics doesn't follow the
pressure patterns of everything else. In all other cases outside this the
smaller the orifice the larger the pressure.
For example 25' storage ESFR 25 - 50 psi, ESFR 14 - 50 psi, ESFR 17 - 35
psi. Slight changes as the storage gets higher but still really not
consistent with the general pattern.
Also flies in the face of the general rule of the larger the orifice the
better the fire control. Not really important just something I noticed this
morning and made me curious.
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
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