Sprinklers Below 2.3-ft Diameter Duct under FM?

4/14/2025

We are currently discussing with the authorities regarding obstructions and their implications subject to FM Global design requirements (FM 2-0, 2021-10).

Specifically, the scenario involves a round duct (2.3 ft / 700 mm in diameter) positioned 12 inches (300 mm) below the sprinkler deflector in a non-storage application.

Is it necessary to install sprinklers below the duct in this case?

Below is a relevant excerpt from FM 2.0 (2021-10):

2.5.2.5.4.1 Any object up to 4 ft (1.2 m) wide, as measured in the object’s least dimension and in a plane that is parallel to the floor, does not qualify as an obstruction to the sprinkler’s discharge pattern.

I would like to clarify whether any object up to 4 feet wide is considered not an obstruction, regardless of the vertical distance from the deflector (or thermal element).

Alternatively, must we adhere to the 3 feet (0.9 m) clearance requirement?

Your expertise and clarification on this would be greatly appreciated.

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4 Comments

Pete H

4/14/2025 06:39:36 am

What type of head are you using? You say it's not a storage application but bring up a 3 foot clearance requirement. Are these ESFR or Extended Coverage heads?

Where are you getting the 3 foot clearance requirement from in FM and does it apply to your situation?

Brett

4/14/2025 08:34:07 am

There's not enough information here to be able to confidently answer the question as Pete indicates above. Most likely you want to look in sections 2.5.2.5.1 through 2.5.2.5.3.3 of FMDS 2-0 to answer your question about the duct.

More generally, if an obstruction is close to a standard spray sprinkler (within 18") then typically you're looking at potentially placing additional sprinklers on either side of the obstruction and not below it. There's a difference between sprinkler discharge pattern development (obstructions close to the sprinkler) and getting the water to reach the hazard (large obstructions below the sprinkler).

Jack G

4/14/2025 10:22:07 am

As indicated the head type would be nice.
If ESFR NFPA 13, chapt. 14 ( 2019) I think 14.2.1.2 requires sprinklers under obstructions no matter their vertical distance if it exceeds 2 ft diameter.
They are different animals and have specific rules.
If you’ve ever installed a Walmart superstore, all the units ( are greater than 2 ft diameter) have sprinklers beneath them.
What you include in the hydraulic calcs —- review. I think it’s 2 beneath ( NFPA 13—— 1 for fm )

Ricardo Gonzales Jr

4/14/2025 11:56:37 am

Don't get confused. Codes are enforceable by the AHJ, FM requirements are not. If the FM information is adopted as an Amendment to the Code and not called an FM document. However, if the owner is FM insured and the insurance wants the FM requirements to insure the site and is more stringent than the Code, the AHJ can't stop this enforcement. In fact, it should be seen by the AHJ as getting more than what they need for their minimum approved system. A win without the need or fight.

Sprinklers Below 2.3-ft Diameter Duct under FM?

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