Is Sprinkler Obstructed by Duct?

9/27/2017

An 18-inch deep x 3-foot wide duct is located with the top of the duct 20 inches below the upright sprinkler deflector. The sprinkler is centered above the duct. NFPA 13 (2016 Edition) has the 3x rule (8.6.5.2.1.3) which only applies to structural members in light and ordinary occupancies, and the duct is not over 4-feet wide which would require sprinklers by 8.6.5.3.3. Is this arrangement acceptable by code?

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2 Comments

MAP

9/27/2017 08:11:58 am

Intent of the code is met

Patrick Turner, P.E.

9/27/2017 08:49:48 am

First of all, what type of upright are we dealing with (standard spray, extended coverage, residential, CMSA, ESFR, etc.)?

Once we know what type of upright that is in question, we can then define "obstruction", as they are defined differently for standard spray and extended coverage sprinklers than for CMSA and ESFR sprinklers.

If the upright in question is a standard spray, extended coverage, or residential sprinkler, then NFPA 13, 2016 Edition Sections 8.5.5.2, 8.5.5.3.1, 8.6.5.2.1.1, 8.6.5.3.1, 8.10.6.3.1, and 8.6.10.3.2 would not define this duct as an "obstruction" as the top of the duct is more than 18 in. below the upright deflector, and the duct is less than 4 ft. wide.

If the upright in question is a CMSA sprinkler, NFPA 13, 2016 Edition Sections 8.11.5.2.1.1 and 8.11.5.3.2 would define the duct as an "obstruction" as the top of the duct is less than 36 in. below the upright deflector and the duct is more than 24 in. wide and is not positioned so that the minimum horizontal distance from the near side of the obstruction to the center of the sprinkler is not less than the value specified in Table 8.11.5.3.2 and Figure 8.11.5.3.2.

If the upright in question is an ESFR sprinkler, NFPA 13, 2016 Edition Section 8.12.5.3.1 would define this duct as an "obstruction" as the top of the duct is greater than 24 in in width (along with other obstruction rules that would apply even if the duct was less than 24 in in width).

Therefore, it depends on the type of upright in question. I am assuming this is a standard spray upright since you referenced the 3 times rule (which only applies to standard spray sprinklers when an "obstruction" is present; the 4 times rule would apply if it was a residential or extended coverage upright). The 3 (or 4) times rule would then be applied if an "obstruction" was present as the requirement for the distance away from the nearest edge of the "obstruction". There are different specific requirements for distance away from "obstructions" for CMSA and ESFR sprinklers.

Hope this helps.

-Patrick Turner, P.E.

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