When Can Pipe Just be "Similar Size" to Existing?

12/15/2023

Does NFPA 13 address the ability to size sprinkler pipe simply by being "sized by similarity"?

For instance, I have a small room that has (4) 1/2" pendent sprinklers to be located in it. I cannot determine any pipe routing from above due to an old ceiling but can verify the lines are 1-1/2" schedule 40.

I've heard of this method being used but do not find it in NFPA 13 so I'm hesitant to use this method.

Thanks in advance.

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7 Comments

Dan Wilder

12/15/2023 07:58:22 am

Are you trying to not provide calculations for a project? Typically, when we provide TI services, the existing piping schedule, piping method, and sprinkler K-Factor gets followed or we provide calc's to prove something new (like adding flex hoses vs hard pipe).

If the rooms are like for like along with the sprinklers and piping methods are like for like, none of my AHJ's require calcs. If we need to double up sprinklers in an area or add a branch line to avoid loading up a branch line, depending on its location to the riser and what the calc plate indicates for calc area and hazard, it is a case by case basis.

Glenn Berger

12/15/2023 08:12:36 am

In small applications, the replace in kind method tends to be acceptable as long as the contents/room being protected is "Light Hazard."

I do feel your pain when trying to retrofitting small portions of old facilities.

Jesse

12/15/2023 08:50:14 am

I feel ya. Seems this is Light Hazard and I'm guessing by the age and Sch 40 pipe, this may be a Light Hazard Pipe Schedule. In that event, yep.

The requirement to prove hydraulic efficacy rests with the AHJ, and all of our respective AHJs differ. I do have one or two AHJs that would require calcs for this, but most of them are ok with it.

Casey Milhorn

12/15/2023 08:56:45 am

There are millions of different scenarios that NFPA 13 just cannot cover, especially when it comes to remodels, upfits, etc... Not that 13 does a bad job, it just isn't practical for it to cover everything. Many AHJs have their own rules on this. One of my favorites is not needing drawings for 10 heads or less being relocated, and not needed calculations for 10 or less new sprinklers (when matching existing pipe schedule). Of course even this can be abused, but it's a good starting point. But to answer your question, it's going to be AHJ specific. I've found most AHJs just want some type of common sense reasoning on why you are doing what you are doing, and if they trust you and know you are ethical and knowledgeable, they will typically show a lot of grace.
Now, that being said, there are some AHJs that really just don't understand or are mistrustful of everyone. If you look at the stats, specifically in light hazard occupancies, the vast majority of the time one sprinkler controls or extinguishes the fire. I've actually had a small restaurant under construction, with only test water on it, where the sprinkler extinguished the fire (some paint rags ignited in a plastic bucket). I think AHJs, and even us sprinkler designers/engineers, get carried away sometimes on being ultra obsessed with the details. Being in this industry for some time now, leaving a head out of a calc area, adding one too many sprinkler heads from 1", etc.... doesn't keep me up at night. Unsprinklered buildings, unknown valves in the yard being shut off, whole sections of buildings unsprinklered because of bootleg construction, 30' of group A plastic storage in an OH2 building, out of commission fire alarm systems, etc... is what keeps me up at night. Not that I think we can ignore the small details, but we as an industry can do better educating AHJs to the bigger issues. Major on the majors, minor on the minors.

Alex

12/15/2023 09:29:55 am

If the system is designed as pipe schedule, I can see it working but if its a hydraulically calculated deign, I don't see how you can justify simply matching pipe sizes since you will be adding fittings and additional sections of pipe.

Jack G

12/15/2023 12:02:41 pm

When we calculate a system it is “the hydraulically most remote area “.
We then size similar branch lines like the remote area. In essence we ‘ve created a “ hydraulic pipe schedule “ for the remainder of the system.
Areas that are not piped like the remote, but are closer to the source,need a “ check area calculation” in my opinion, which I include with my submitted.
So if your area is pretty similar to the remote, modifications I feel can be made per the systems “ hydraulic pipe schedule”.

sean

12/31/2023 06:01:48 pm

I am a reviewer so i take these factors into account.
1 - does the information presented show how the existing system was feed.
2 - does the existing feed seem rational
3 - how extensive is this work? is it 1 head or 100 heads

So if i have a system that is made up of a tree with 1-1/2" branch lines I would not have an issue with a small additional branch line installed based of the existing hydraulic basis. If you need to do this for multiple lines I become wary.

There is really nothing in the code to back it up more than you are just backing up the original design. Same way with a relocate how do we know the arm over and drop wont be an issue most of the time? experience.

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When Can Pipe Just be "Similar Size" to Existing?

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