Doing a mechanical machine room, and I need to add sprinkler coverage above and below ductwork.
I was told that if you have 2 sprinklers (one covering above the ductwork and one covering under the ductwork), you can run 1-in pipe to each. So here is the example: I have 4 sprinklers coming off a branchline but 2 of those are under ductwork. The uprights that are above and below are covering the same area. What pipe size do I need to use for this branch line? What code justifies this one way or another? Thanks in advance. Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
7 Comments
Brett
10/18/2024 08:14:00 am
Overhead sprinklers and sprinklers under obstructions should be calculated independently. The reason for that is that both sets of sprinklers aren't expected to discharge at the same time. If the number of sprinklers under obstructions isn't substantial, like in the case you've presented, then the size of the piping under the ductwork should match the sizing of the overhead system.
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Mike Morey
10/18/2024 08:16:09 am
Arguably you have to calculate one or the other (worst case) of the sprinklers above or below the obstruction. (see the first section below) Often the lower elevation will cancel out the added loss in the drop pipe, but you should at least contemplate that before deciding how to proceed. IE if you're flowing 20 gpm per head, what's the loss per foot in the pipe vs what's the gain in elevation. We often rule of thumb this stuff, but prescriptively you need to prove it, unless you go with (from NFPA 13 2022):
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Glenn Berger
10/18/2024 08:18:26 am
The easy answer to your question is 1 inch pipe to the 2 sprinklers below the ductwork would not raise too many eyebrows.
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NK
10/18/2024 08:41:44 am
NFPA 13, 2025 added guidance to specifically address this situation for the first time:
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Dan Wilder
10/18/2024 08:45:12 am
Depending on the applicable year of the adopted standard and what has been proven hydraulically.
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Anthony
10/18/2024 09:03:24 am
New York is setback quite a bit with the NFPA year that is followed.
Reply
Jack G
10/18/2024 09:49:50 am
Include only the highest demand from either the above or below sprinklers per NFPA 13. If 1 inch branch, then the additional line pipe can be 1 inch.
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