Pipe Size for Sprinklers Above & Below Ductwork?

10/18/2024

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.

So, whatever pipe size has been calculated to supply two sprinklers on the overhead system should be used to supply two sprinklers below obstructions. If you have one pipe supplying four sprinklers and two are overhead with two below obstructions, it should be sized as if it were serving just the two overhead sprinklers.

1" may work for your project, but that shouldn't be used as a rule of thumb for every project. Sizing should always be based off hydraulic calculations unless you're altering a pipe schedule system. Even then, it would serve the public better to hydraulically calculate the system, but that's an entirely different topic.

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):
28.2.4.7.3.1
Sprinklers under the obstruction shall not be required to be included in the hydraulic calculation of the ceiling sprinklers.

28.2.4.7.3.2
Where the piping to sprinklers under obstructions follows the same sizing pattern as the branch lines, no additional hydraulic calculations shall be required for sprinklers under obstructions.

We often utilize the second item for storage etc systems, especially when we're not in the remote area so we don't have to do tons of calculations. If we have a line of sprinklers under an obstruction we drop once and run along it with pipe matching the overhead sizing.

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.

10/18/2024 08:41:44 am

NFPA 13, 2025 added guidance to specifically address this situation for the first time:

§28.2.4.7.4.1 Supplemental sprinklers under the obstruction shall not be required to be included in the hydraulic calculation of the ceiling sprinklers.

§28.2.4.7.4.2 Where the piping to sprinklers under obstructions follows the same sizing pattern as the branch lines, no additional hydraulic calculations shall be required for sprinklers under obstructions.

§28.2.4.7.4.3 Where the requirements of 28.2.4.7.4.2 are not met, a level of supplemental sprinklers shall be calculated to verify pipe sizes.

(A) * The number of supplemental sprinklers shall include up to a maximum of four adjacent sprinklers attached to a branch line.

(B) For density/area applications, the area of coverage of each sprinkler shall only include the footprint of the obstruction it is protecting.

(C) For density/area applications, the design criteria shall be in accordance with Section 19.5.

(D) For CMSA and ESFR sprinklers, the discharge pressure shall be in accordance with 20.16.4.

(E) The level of supplemental sprinklers shall not be required to be balanced with the overhead system.

§19.5 Design Approaches for Supplemental Sprinklers.
When required to be included in the hydraulic calculations in accordance with 28.2.4.7.4.3, the design approach for supplemental sprinklers shall be permitted to be based on the hazard located directly below the obstruction.

Dan Wilder

10/18/2024 08:45:12 am

Depending on the applicable year of the adopted standard and what has been proven hydraulically.

It's a little grey for previous editions as to what you would calc the sprinklers beneath the obstruction to...If the overhead is calced a 130ft² (full floor coverage) but the sprinklers under the obstruction only need to provide coverage for 96 ft², does the lesser demand offset the loss through the extra pipe and fittings? Does calcing a line of sprinklers (individual feeds) beneath an obstruction (as that obstruction is a lesser sq ft in area) within the larger remote area mean the remote area needs to grow because you're not providing the correct density over the entire area as the uprights would or are up punished to provide the larger individual sprinkler coverage on sprinklers that do are covering lesser areas? How about ESFR sprinklers beneath duct as I often see a smaller pipe size to those sprinklers vs the overhead branch lines? Lots of examples to argue...

Run the calc for the branch line with the uprights and size accordingly, ignoring the sprinklers beneath the obstruction then apply the below sections.

If the hydraulic calcs show you can feed 1 sprinkler from 1", then then each pipe to a single sprinkler under the obstruction can be 1" as you do not need to calculate sprinklers under the obstruction per the below 13-2019 & 2022 (Chapter 28)
27.2.4.7.3
27.2.4.7.3.1
27.2.4.7.3.2 - Piping to sprinklers under obstructions specific.

If the hydraulics have say 1¼" sprigs to each sprinkler on the line or, unfortunately, when we run a grooved line with the same pipe size to the end, then (technically without any further calc support) the pipe sizing to any drops would also need to follow the same sizing of 1¼" UNLESS you can prove hydraulically a smaller pipe size (strictly following the standard).

The 13-2025 edition changes the requirements as these are now referred to as "Supplemental Sprinklers" in 13-2025 28.2.4.7.4 and now provides a means to run a calc to prove a smaller pipe size to sprinklers beneath obstructions via an "up to 4 sprinkler calc along a branch line" per 28.2.4.7.4.3.

And yes, in the past for a majority of my non-storge designs I have just run 1" to under obstruction sprinklers without any further proof.

Anthony

10/18/2024 09:03:24 am

New York is setback quite a bit with the NFPA year that is followed.

Some Jobs are 2007,2013 or 2016.

Typically, I had to use better judgement when sizing Branchlines.

The Layout of the heads underneath play a critical part in the sizing as well.

Lets say I have 2 Heads above and 3 Heads below of the same branch. Regular F1FR56 Uprights, 2 of the Heads that are underneath aren't close to the heads above.

I would Size the Branchline 1-1/2" to the 1st 3 Heads and then reduce down to 1" where applicable. Not sure if Pipe Scheduling would be easier than running a Hydraulic Calc to see if it ok with all 1inch.

A sprinkler fitter questioned me on it and honestly never really got called out on it.

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.
For standard or qr sprinklers.

Pipe Size for Sprinklers Above & Below Ductwork?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES