Sprinkler Main Larger than Underground Size OK?

7/14/2020

Have a situation I've never crossed before: we have a very small building that's getting expanded. Originally the sprinkler contractor gave two options for the general contractor to price their underground fire sprinkler service - (1) is to provide a 2-1/2" main, the other (2) is to provide fire barriers in certain areas with automatic door closers to allow the room design method be used, and bring in a 2" underground.

I completely recognize that small underground pipe is not good practice and limits the future potential of the building. We recommended a 4-inch underground, but the increased cost for tap fees and underground pricing added $10-15,000 to the job which the owner adamantly refused.

We were given notice to proceed with 2-1/2" underground and thus provided a 2-1/2" riser and 2-1/2" mains.

The GC gave a different direction for the underground and pulled 2" underground and is providing fire barriers and automatic door closers.

We've recalculated using the room design method and everything still works with the 2" underground, but the 2-1/2" riser and 2-1/2" main is already ordered and on the way to the jobsite.

Does NFPA 13 have any restriction against having a larger riser or main size than the underground?

Because of the change in building construction, we only need a 2" riser and 2" main but will instead already have prefabricated 2-1/2" in both. My thought is to have a permanent placard clearly affixed to the riser showing the situation and clearly identify on as-builts the methodology used.

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15 Comments

Jesse

7/14/2020 10:09:50 am

You have a problem.

The lead-in can not be smaller than the riser. Typically, underground can not be smaller than 6-in, but 13 makes a provision for smaller if A) no hydrants are attached to it, and B) it is no smaller than the riser.

I wouldn't stamp that drawing. Sorry, I know that's not what you hoped to read

Dustin

7/14/2020 10:22:34 am

Please provide code reference.

Jonathan Sullivan

7/14/2020 10:28:34 am

Regarding provision "B)",13-16 24.1.3.2 (2) only adds the not smaller than the riser provision if the system is not hydraulically calculated.

TC Lambert

7/14/2020 10:32:06 am

Being curious, as I have come up against situations like this here and there, I looked at NFPA 13. This limitation on the UG pipe size being at least as large as the riser is only for pipe schedule systems. If it can be proved by hydraulic calcs that the system demands can be satisfied you should be fine. That being said, I don't like the small UG size either. Since the GC has stacked the deck against you, I would push for an ample safety factor to be included in those calcs.

SUBMITTER

7/14/2020 10:34:23 am

Section 24.1.3.2 (2) is saying that the underground is at least as large as the system riser. That's only for pipe schedule systems - this system is hydraulically calculated.

Also, there are no hydrants connected to it and it is a dedicated sprinkler feed (no domestic).

7/14/2020 10:41:10 am

I wouldn't sign/seal the drawing either - if it wasn't code allowed.

The question (at least in my mind) isn't whether it's good practice to have an underground that's smaller than a main. It's not good practice and it doesn't lend itself to future building changes.

The question to me is whether it's allowed by code, and if hydraulically it can suppress a fire under the NFPA 13 standard.

As far as I can tell, despite the pickle the GC has us in, is that it would be allowed by code and does work hydraulically.

Ironically, if the main and riser is all 2-inch instead of 2-1/2-inch, then this discussion kind of goes away and the whole thing starts to look more traditional. That CAN actually work in this situation hydraulically - it's just that we already have the pipe cut at the larger 2-1/2 inch size. It's a goofy arrangement, no doubt, but being unconventional here really is only helping the situation hydraulically.

With the Room Design Method, 2-inch underground, and 2-1/2 inch main, we're sitting at a 12 psi (19.6%) safety factor.

Rusty Scott

7/14/2020 10:15:49 am

I've seen this before with warehouse risers where 6" underground is provided but in order to make the calc's work, a 8" riser is used and then sometimes transitions back to 6" on the horizontal. This would be the difference in bumping up the branch lines over the gridded system which is $$$. So I believe it is allowed due to code not explicitly stating against it (at least I haven't seen a code against it, doesn't mean it's not there), I believe it's more or less just not considered best practice.

Sounds like you have a fun G.C.

7/14/2020 10:23:43 am

Your situation would be allowed, because of the 6" main. That's the minimum size allowed.

In the OP situation, the underground size is 2". This is only permissible if the system riser is 2" also and there are no hydrants on that lead in

7/14/2020 10:35:02 am

Correct - no hydrants on the lead-in, hydraulically calculated, no domestic flow either.

Colin

7/14/2020 01:03:51 pm

Jesse, this is a hydraulically calculated system with no hydrants. Please provide the code reference where it states that the riser must be the same size as the underground. I believe that's only for pipe schedule systems. While its not "best practice", I'm not aware of a code prohibiting what the OP is asking.

Nic

7/14/2020 10:27:15 am

NFPA 13 2019ed 5.1.3.2 says the incoming fire main can be less than 6" if the incoming main is at least as large as the system riser. So currently you have a problem if your riser is 2-1/2" and incoming main is 2"

But I could technically see a loop hole if you made your system riser 2" and overhead feed main 2-1/2". Not sure if your calcs would work with that.

7/14/2020 10:36:33 am

Even the 2019 section you've referenced, though, is only fir pipe schedule systems.

5.1.3.2(2) "Systems that are not hydraulically calculated shall have a main at least as large as the system riser."

This project is hydraulically calculated, and with the room design method in NFPA 13 it works with the 2-inch underground and 2-1/2" mains.

7/14/2020 10:50:23 am

Ah good point. Not sure how I missed that. I don't see where you it says you can't do what you have planned then.

7/14/2020 10:46:49 am

It's common around here to tap the city main with one size, and then to upsize the pipe after the water meter. For example, you could tap the city main with a 2" pipe and then after the meter, upsize the underground to 2½" pipe or 3" pipe (depending your system demand). This obviously has an effect on your hydraulics, but if you can figure out a way to make the calcs work, there technicially isnt any problem with this.

7/16/2020 07:34:29 am

KN - just out of curiosity - do the hydraulic calculations typically accurately reflect the loss through the water meter and smaller tap?

I rarely see water meter losses accounted for, then again around here its uncommon to have fire flow through the meter.

Sprinkler Main Larger than Underground Size OK?

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