Vertical & Lateral Sprinkler Spacing Near Ducts?

4/23/2024

Hi All, I have a two part question regarding sprinklers below ductwork exceeding 48" wide. Both areas below (in different locations) are Ordinary Hazard, 130 sqft maximum spacing.

When protecting below the ductwork in the elevation sketch below, is the 6'-11" dimension doubled to be 13'-10" the same as when spacing to a wall (SxL) ?

Or can the distance to the next sprinkler be 15-ft (15 x 6-11 = 103.75 sqft)?

In the plan view below, are sprinklers required below the 60" wide duct ABOVE the AHU (shown in green), if the vertical distance between the AHU and bottom of duct is greater than 24"?

This would be under NFPA 13-2019, Section 9.5.5.3.1.5.

Many thanks!

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9 Comments

Glenn Berger

4/23/2024 08:13:42 am

Sprinkler below duct are there to protect the space below the obstruction only. The distance to the end wall is not applicable IN THIS CASE, since there is a significant distance from the wall.

The next sprinkler into the page can be a maximum of 15 feet away from the current sprinkler.

Jesse

4/23/2024 08:36:11 am

Pretty much what Glenn said. In OH1 and OH2 you can have 15' sprinkler spacing as long as sprinkler coverage doesn't exceed 130- sq. ft.

In the elevation view, I don't see how the sprinkler is going to collect heat... ?

Justin

4/23/2024 09:47:16 am

Thats what I thought too but NFPA 13 allows it as long as the head is no further than 3inches from the outside edge and the sprinkler has a water guard to protect from activated sprinklers from above. Specifically Code requires an intermediate rack type which is the same as a multi level sprinkler noted in the elevation view. However it would be nice to see some test data back up the 3 in allowance claim.

NFPA 13 code:
9.5.5.3.1.2 *
Sprinklers located under obstructions shall comply with one of the following:

(1) Installed below the obstruction
(2) Installed adjacent to the obstruction not more than 3 in. (75 mm) from the outside edge of the obstruction

9.5.5.3.1.3
Where sprinklers are located adjacent to the obstruction, they shall be of the intermediate level rack type.

Handbook note: Sprinklers are required to be of the intermediate level rack type, which are provided with a water shield, to prevent the discharge of a sprinkler located at the ceiling level from cooling the thermal response element of the sprinkler located adjacent to an obstruction below. If the thermal response element is cooled by the sprinkler overhead it may delay or prevent the sprinkler from operating.

Pete H

4/23/2024 08:50:24 am

Echoing Glenn and Jesse, you can go up to 15'-0" apart. The double to the wall measurement doesn't apply because you have protection on either side of the obstruction and there isn't actually a wall stopping spray. You are just protecting underneath the obstruction with this sprinkler.

Further echoing Jesse, I really think you want the sprinkler to actually be underneath the duct so it can collect heat.

4/23/2024 09:27:46 am

As a follow up to my original post - I have always failed to see how a sprinkler head below a duct will "collect" heat without a draft stop but have always designed that way until recently - that said - I have started using NFPA #13 (2019) section 9.5.5.3.1.2 (see the figure) - This allow sprinklers to be adjacent to the duct rather than beneath it. As to the 2nd part of my question - I believe that sprinklers will be required either adjacent to or below the duct if the vertical distance to the top of the AHU to bottom of duct exceeds 24"
Thanks - I Always appreciate the valuable insight.

Paul

4/23/2024 09:32:09 am

I am not in total agreement with the above responses, the edge of your duct work would be treated as a wall. You are doubling that distance as the sprinkler is going to discharge to cover that area and in other directions (e.g. 6.917 ft. x2=13.834 ft. resulting in a max spacing of 9.397 ft. between sprinklers for a maximum 130 sq. ft. coverage area and a water delivery of 19.5 gpm minimum)

While the code is not going to give you step by step guidance on this you are limited to the 130 sq. ft. of protection area and you treat the edge of the duct as a wall for determining that spacing. This is the same approach as if you had a vertical elevation change of your ceiling in a building. I only have the 2016 Edition of NFPA 13 available but take a look at Figure 8.6.4.1.1.3(A) which explains this. If you have an NFPA 13 Handbook handy it speaks more on this.

Other solutions to get larger spacing between sprinklers is to armover under the duct up to the maximum permitted for unsupported armover lengths shrinking you 6'8" to edge of duct.

Dan Wilder

4/23/2024 10:00:14 am

Ductwork, that is the half distance between the sprinkler and the edge of duct (as if there was a wall on the far side) so that distance x2 so the max distance along the duct should not exceed ~9'-4" on center (130ft² / (6'-11" x 2)) = 9'-4". The sprinkler is expected to spray that far so under any other circumstance, it would be the half distance.

As to the clear distance above, yes, I would consider that an area that would require sprinklers. The bottom of the obstruction exceeds 24" above the gear. Same approach we use for conveyors.

Casey Milhorn

4/23/2024 10:17:18 am

I might be in the minority here, but I think this is a case of overthinking it. The fact that you have a head installed there at all is 90% of the battle. Worrying about discharge, calculations, etc... for an obstruction level head is majoring on the minors as one of my mentors would say. If you are worried about it, use a vertical sidewall which I know would be limited to 10x10 in this scenario, but would make a lot more sense in a practical way. At the end of the day, IF this head were to active first, which it probably won't, then great job, it is helping control a fire in it's early stages. If it doesn't active first, it probably won't active at all due to heat collection and cooling/suppression by ceiling level heads.
Obstruction level heads are not part of the calculation and are definitely secondary to the main level of sprinklers.
I'm not downing the question and maybe I've become the old designers that I always made fun of, but as an industry I think we can get caught up in majoring on the minors, and minoring on the majors. I spend a lot of my time now worrying about the warehouses in my town that have OH1 systems with 30' tall storage, the underground gate valves that are shut off, or mostly closed without anyone knowing about it, or all of the unsprinklered hi-rises/apartment buildings we still have in this country. The question is a good one and is worth answering, but I think it's important that we not only teach new designers the finite details, but also a practical perspective that only comes with being in the industry for an extended amount of time.

Jack G

4/23/2024 03:59:51 pm

If there is protection on both sides of the obstruction ( 1/2 x on each side) then protection beneath the duct is for under the duct.
I would use a drop from above to a 1 inch x 1-0 arm to get the head under the duct ( most owners like that as opposed to a lower branch line) my drops would have drain plugs.

Vertical & Lateral Sprinkler Spacing Near Ducts?

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