Sprinklers Required Below Group of Pipes?

1/27/2020

I have an obstruction question regarding what I feel is a gray area of NFPA 13. I'm looking at obstructions greater than 18 inches below sprinklers.

NFPA 13 states that any obstruction over 4 ft wide requires sprinkler protection under the obstruction. What if you have multiple parallel obstructions that are less than 4 ft wide with gaps in between?

For example, multiple 36-inch diameter parallel pipes with 6 inches of space between each pipe.

I feel that even if these obstructions do not require sprinkler protection below per code, they would still significantly alter the listed spray pattern of the sprinkler. I have seen some info regarding a 50% rule, but I can only find that in code as it relates to ceiling fans.

I am curious what other opinions are. Thanks!

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8 Comments

Randy

1/27/2020 09:59:34 am

I would agree that the code does not require additional sprinklers beneath these particular pipes. You use the word "significant" - but 4' is inherently defined as the width threshold for being "significant"

If you are the designer, nothing prevents you from applying your best judgment and exceeding the code

Alex

1/27/2020 10:43:01 am

Great question. One thing to consider is the ability for the obstruction to form a layer of hot gas beneath it in a sufficient manner for a head to operate. A bank of large diameter piping can probably cause turbulence in the buoyant gases, but is not likely to "hold down" a layer of hot gas like a continuous 4 ft obstruction would.

I think the best design practice would be either to locate a row of heads directly above the obstruction or to place a row of heads on either side, with the obstruction splitting the distance.

Franck

1/27/2020 02:17:30 pm

As you said, this is the grey area. Each pipe individually is not an obstruction, but all together doesn’t make you feel comfortable
If there are too many (sense of judgement, arbitrary decision, whatever you call it), I would consider it as an obstruction and put sprinklers on either side (if possible) or above and below the « obstruction »

If your « gut feeling » says it is not good, then it is not good...

Franck

1/27/2020 02:22:35 pm

The 50% rule (sometimes it is 70% or even 80%) is often a reference for shelves in racks (when do we consider open shelves or solid shelves)

There are also some criteria in nfpa 13 regarding the position of sprinklers above (and below) open steel gratings at the ceiling level. As they can also affect the sprinkler distribution pattern.

Wayne Ammons

1/27/2020 03:25:51 pm

If there are no clear guidelines outlined in chapter 8 of NFPA 13 (prior to the 2019 edition) on how to address the continuous and/or noncontinuous obstructions in a specific scenario, interpretation can tricky and may require discussion with the engineer of record and AHJ. You could try getting a formal interpretation from NFPA or utilizing NFSA's Expert of the Day service to help determine an appropriate protection scheme.

FM Data Sheets have been developed through fire testing and could also be a useful resource when criteria is unclear or absent from NFPA standards. Below is the definition of an "individual object" that I keep in mind when evaluating closely spaced runs of pipe, conduit, duct, etc.:

Individual Object (Obstruction): An object can be considered an individual object for obstruction purposes when the horizontal distance between it and the nearest object is more than 3 times the objects least dimension. Otherwise the width of the object, for obstruction evaluation purposes, would be based on the width of both objects least dimensions plus the horizontal distance between them. An example of an individual object would be a 3 in. (75 mm) wide service pipe that is located at least 9 in. (225 mm) horizontally away from any other object.

In your example, just two runs of 36 inch diameter pipe with a 6 inch separation would create an obstruction 78 inches wide. If I were designing for that particular scenario, I would provide protection below. I seem to recall NFPA addressing this somewhere, but I am having a hard time putting my finger on it...

Matthew King

1/28/2020 11:39:00 am

An obstruction is anything that changes the discharge of a sprinkler head.

The width of the obstruction include the gaps between piping, outer edge to outer edge (e.g. is not the sum of pipe ODs alone).

4ft or more wide and 2 feet or more above ground qualify as a not permitted obstruction.

I do not the grey, you have a permitted obstruction by letter of the code, as long as the owner does not specify otherwise.

As the owner, I have recently specified this in a project in design where I work.

The intent of code is fire safety

1/29/2020 01:11:50 pm

Just came across this very same issue on one of my jobs. The AHJ suggested I install heads above and below to remedy the situation.

Matthew King

2/3/2020 11:50:20 am

I agree. On the job I reference above, they are adding pipe runs. Our overhead sprinklers were obstructed, they are add heads below. We have had several comparable situations when making modifications to duct runs.

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