I'm not sure how the wording got revised over the years, but back-in-the day
the standard was a lot clearer that the residual pressure was measured at the
base of the riser. Here is what the old note used to say under the table for
Water Supply Requirements for Pipe Schedule Sprinkler Systems:
NOTES:
1. The pressure required at the base of the sprinkler riser(s) shall be
the residual pressure required under the roof plus the pressure required to
reach this elevation.
Whenever I've done such evaluations, the analysis would take into account the
losses due to elevation and friction losses in the underground piping, between
the water supply at the street and the base of the riser, based on flowing the
Acceptable Flow at Base of Riser (Including Hose Stream Allowance) from the
table.
Then I would factor in the loss through the backflow preventer and the pressure
due to the elevation to the highest sprinkler. Thus determining the residual
pressure under the roof.
Since the evaluation is for existing systems, the Minimum Residual Pressure
Required only needs to be the 15 psi for light hazard or the 20 psi for
ordinary hazard. The 50 psi value is only for new systems thar are greater
than 5000 sq.ft. For the new system, I would usually opt to calculate it
hydraulically.
It should be recognized that the above is my personal opinion and not that as a
member of the NFPA and should therefore not be considered, nor relied upon, as
the official position of the the NFPA, nor any of their technical committees.
Best regards,
Larry Keeping. P.Eng.
Senior Technical Specialist
PLC FIRE SAFETY SOLUTIONS
3413 Wolfedale Road, Suite 7
Mississauga, ON L5C 1V8
Phone: 905-949-2755 Ext 204
Fax: 905-949-1752
Email: [email protected]
Website: www.plcfire.com
Toll Free: 1-800-675-2755
-----Original Message-----
From: Sprinklerforum <[email protected]> On Behalf
Of Jamie Seidl via Sprinklerforum
Sent: June 4, 2021 4:14 PM
To: Sprinklerforum <[email protected]>
Cc: Jamie Seidl <[email protected]>
Subject: pipe schedule/residual pressure
Warning! This email came from outside your company.
I am looking at a project where we are renovating an existing building with a
new system. There are other buildings on the site (1930's vintage) that share
the underground lines. To bring the site into compliance with current codes,
RPZ backflows are being added at the entrance to the site.
The existing buildings are occupied, and the fire department is looking for
some assurance that the systems will work with the addition of the backflow
preventer. I remembered (though have never applied it) section 11.2.2.8 allows
for the calculation of the residual pressure at the highest sprinkler, and
deducting elevation, BFP losses ect... The buildings in question are over 5000
sqft, so the minimum 50 psi will apply.
So my question is, do I just utilize the residual pressure from the closest
hydrant, deduct the elevation and backflow losses from the residual and call it
a day? Or do I need to account for additional friction through the system
piping? Both 13 and the handbook seem to be mute on this.
Thanks again, Forumites.
Jamie Seidl
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