Is Duct Within 18" of Sprinkler an Obstruction?

1/10/2020

I could use some help with sprinkler obstructions.

Generally, I’m looking at piping and ductwork, that is installed within 18” below the plane of the standard upright sprinkler deflector. Light or ordinary occupancies.

I called NFPA 13 and got an interpretation that these would be considered a sprinkler obstruction, and I would need sprinklers on either side, regardless of the size of the duct/pipe (no rule here about only being over 4’). This was per Section 10.2.7.2 in the 2019 edition.

I didn’t think to ask, but could I still apply the three times rule of 10.2.7.2.1.3, even though these aren’t structural members? As long as I was three times, or 24-inches away, it wouldn’t be an obstruction, right?

I’ve seen these close configurations all over the place in many different peoples’ designs, so I’m wondering if I’m missing some allowance.

Most recently, I observed a 14” wide (horizontally) x 6” deep duct (vertically), with a sprinkler installed directly above with the deflector 5” above the top of the duct. The next sprinkler head is approximately 12 feet away. Also observed in the same room, was a 5” diameter pipe about 8” diagonally down from a deflector, measured at a 45-degree angle. If I’m understanding NFPA 13 correctly, these would both be considered obstructed. I have a question into NFPA 13, but would appreciate others’ opinions as well.

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4 Comments

Wayne Ammons

1/10/2020 07:52:22 am

The annex material for section 10.2.7.2.1.4 clarifies that nonstructural members are exempt from the "three times rule" and that other obstruction rules such as the "beam rule" and "wide obstruction rule" still apply.

A.10.2.7.2.1.4 It is the intent of this section to exempt nonstructural elements in light and ordinary hazard occupancies from the obstruction criteria commonly called the “Three Times Rule.” However, the other obstruction rules, including the “Beam Rule” (see 10.2.7.1.2) and the “Wide Obstruction Rule” (see 10.2.7.3.2), still apply. If an obstruction is so close to a sprinkler that water cannot spray on both sides, it is effectively a continuous obstruction as far as the sprinkler is concerned and the Beam Rule should be applied.

Franck

1/10/2020 10:16:51 am

You have a good indication for the answer to your question in 10.2.7.2.1.8.
This is a rule that many contractors forget, but even the sprinkler piping can be an obstruction if the pipe size is 3 in. or larger.
In that case, the 3 times rule should apply.
If you consider this, it means than any pipe or duct that is strictly smaller than 3 in. is not to be considered as an obstruction, while any duct or pipe that is 3 in. or more should be :)

This also means that any upright sprinkler positioned on a branch line that is 3 in. or more should be elevated to avoid the obstruction (with a riser nipple).

It becomes more difficult when you have several pipes smaller than 3 in., since the total obstruction might become worst than a single pipe of 4 in.
In that case, common sense should be applied.
If it looks like an obstruction, there is high chance that it is an obstruction !

1/10/2020 10:19:22 am

In your 2 examples, they have to be considered as obstructions (more than 3 in. and sprinkler position less than 3 x the obstruction size).

Ron Pierce

1/23/2020 10:26:05 am

As our campus safety manager, I seem to know a little about a lot of things, but not a lot about the little things; so, not an expert by any stretch regarding sprinkler systems, NFPA, etc. I digress.

So, we have a new multi-level health sciences building brought online this past fall semester. We hired a local FS engineer to inspect the systems as construction progressed, and he noted pipe/ducting obstructions in mechanical and electrical rooms. I'm thinking, "What else do you expect to see in utility rooms?" You look up and virtually every square foot of ceiling space has some sort of conduit, duct, or other device. How else can you run FS heads if there is always a proximity issue?

Is Duct Within 18" of Sprinkler an Obstruction?

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