When Does Phantom Flow Requirement Apply?

12/8/2023

NFPA 13, 2022 Edition, Sections 28.2.4.2.4 says to that a design area can be only the sprinklers within the available design area whenever the available floor area is less than the density/area size.

The next section, Section 28.2.4.2.5, says that when the total flow is less than the density x minimum design area, an additional flow (phantom flow) shall be added at the common point of connection.

When do you apply one versus the other?

If I am interpreting this correctly, you only apply the 28.2.4.2.5 when you have the available floor area, but not the minimum flow?

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13 Comments

Pete H

12/8/2023 07:20:23 am

I think I'm having this conversation elsewhere and it's honestly making me think this is a conflict specific to NFPA 13 (2022 edition).

Cause the corresponding section in 2016 goes like this:

23.4.4.2.4* Where the available floor area for a specific area/design criteria, including any extension of area as required by 11.1.2 and Section 12.3, is less than the required minimum design area, the design area shall be permitted to only include those sprinklers within the available design area.

23.4.4.2.5 Where the total design discharge from these operating sprinklers is less than the minimum required discharge determined by multiplying the required density times the minimum required design area, an additional flow shall be added at the point of connection FURTHEST [please the caps cause this is what this post is about] from the source to increase the overall demand, not including hose stream allowance, to the minimum required discharge.

A.23.4.4.2.4 outlines the procedure for this calculation.

---

Now in 2022 we have the section read as follows:

28.2.4.2.4* Where the available floor area for a specific area/density design criteria, including any extension of area as required by 19.1.2 and Section 20.13, is less than the required minimum design area, the design area shall be permitted to only include those sprinklers within the available design area.

[and the annex section details the procedure for doing this where you are still adding the additional flow at the point of connection FURTHEST from the source. A.28.2.4.2.4 (4) ]

28.2.4.2.5* Where the total design discharge from the operating sprinklers is less than the minimum required discharge determined by multiplying the required design density times the required minimum design area, an additional flow shall be added at the point of common connection CLOSEST to the source to increase the overall demand, not including hose stream allowance, to the minimum required discharge.

[ The Annex for this section refers you to a figure... which still shows a point to add this flow... that sadly works as closest to the remote area/furthest from the source... but also closest to the source, because it's not a very complex figure ].

Anyone else's insight is welcome.

But I'd stick with 2016's rules and the Annex of 28.2.4.2.4 and go with the connection point furthest from the source (or nearest to the remote area you're calculating) as where to add the required additional flow.

I think if you have some miracle 900 square foot design area reduction calc that comes up with less than 150 gpm of flow (not counting hose) in light hazard, since you're not technically in 28.2.4.2.4 you can use the "closest to source" point for the additional flow, but if you're in 28.2.4.2.4, you still should apply the more stringent demand, which will be adding that flow at the connection point furthest from source/closest to your remote area.

I hope I made this make some sense.

Dan Wilder

12/8/2023 08:18:32 am

Had to look up why the original change but here is the substantiation. I believe the interpretation was being applied to the far end of systems and not related to the actual tie-in point of the system being applied with the additional flow requirements (so off page to the right of Figure A referenced in 28.2.4.2.5) so it was proposed to change in the 1st draft of the 2022 edition, then discussed in E&S and other venues as to where it should be applied (on the end of the main feeding the area, the connection point of the feed main to another feed main, the remote end of the full system regardless of the actual tie in point for the area being calculated, I think even dividing the flow by the number of branch lines and adding that in at each point came up). Ultimately, the POC from the smaller system to the building system "proper" was passed in Public Comment No. 258-NFPA 13-2020 [Section No. 28.2.4.2.5] as to allow the system main feeding the smaller main the ability to support the larger area being calculated under the "Phantom Flow" addition.

Resolution: FR-1315-NFPA 13-2019
Statement: The current wording contradicts itself. The text states additional flow is being added at the point of connection of the branch line to the cross main. After that, it states to go the furthest from the source to add the additional flow. The point of connection would be closer to the source if it is the branch line connecting to the cross main, not further away.

Pete H

12/8/2023 08:33:51 am

Yeah.... I feel like a lot of headaches could have been avoided if they just changed the words to the connection point on the main closest to the calculated area.

But, this stuff is decided by smarter heads than mine.

Dan Wilder

12/8/2023 07:35:04 am

Rereading things again...the light came on this morning as this is an OR statement but, as always, feel free to correct me...it's still early in the day. I've been lucky and not had to utilize it much other than some canopies.

To answer your question, this isn't a choose one, these are complimentary.

NFPA requires the remote area to be sized per the hazard (1500 for Light/OH1/OH2, 2500 for EH1/EH2, 2000 or 2500 for storage...all dependent on allowances like QR, high temp, larger K-Factor. If the higher hazard area being covered is smaller than the minimum required, there are 2 choices.

1-Calulate the higher hazard area
1a*-With the 15' extension of the higher density if there are no barriers, draft curtains, or elevation changes AND the additional area of the lower density to meet the minimum.
1b*-Without the 15' extension but still providing the total required area with the addition of the lower density area.

2-Calculate the higher density area (with or without the extension based on the construction)
2a-Use Phantom Flow to identify the shortage in total water demand from required Density x Area vs. the actual water demand. Then add that shortfall as required.

*Allowance per 19.1.2(1)(2)(3) and 20.13.1 (1)(2)(3)

28.2.4.2.4 is providing guidance to calculate using an alternate calculation method to the full area by using "phantom flow". 28.2.4.2.5 is providing guidance on where to apply the previous section's shortfall (Appendix Section (4) below)

The appendix has some additional commentary:

The following steps outline the procedure for calculation in accordance with 28.2.4.2.4:
(1) Calculate the hydraulic design discharge including those sprinklers within the available floor area.

(2) Calculate the minimum required discharge by multiplying the required design density times the required minimum design area.

(3) Subtract the discharge calculated in Step 1 from the discharge calculate in Step 2.

(4) Where the discharge calculated in Step 3 is greater than 0, the hydraulic design discharge is recalculated including an additional flow equal to that calculated in Step 3. The additional flow is added at the point of connection of the branch line to the cross main furthest from the source.

(5) Where the discharge calculated in Step 3 is less than or equal to 0, the hydraulic design discharge is as calculated in Step 1.

A good write up below:
https://pdfcoffee.com/phantom-flow-pdf-free.html

Eric R

12/8/2023 09:22:41 am

Dan,

You've got this down pretty good here.

I'll just reiterate that the designer does have a choice here, and that phantom flow is not always required to be used if the designer follows the adjacent hazards remote area rules.

In some cases you'll get a more efficient pipe system using the phantom flow approach, but other times you can save significant waterflow overall by using the larger design area approach of Dan's option 1 and controlling your densities via tighter head spacing in the more demanding area to balance your individual head discharge pressures so the heads in the lower hazard area aren't over-discharging.

I'll caveat this by saying a strict following of the adjacent hazard rules can create some theoretically sketchy protection schemes, especially if you mix in QR area reduction to the lesser hazard, so a bit of good judgement should probably be applied.

I personally prefer to use the adjacent hazards with the full size remote area most of the time nowadays, simply because it is easier to present on drawings/calc where you can show the actual densities on a head by head basis and just note which ones are in the higher hazard portion(at least in AutoSprink).

sean

12/31/2023 11:12:25 pm

the adjacent hazard option only works when the resulting flow is equal to or greater than density * area

Dave

12/8/2023 11:35:31 am

Yep, and the 2019 Handbook commentary says these sections "were added in 2013 to provide an alternative...as opposed to having to calculate a full design area....in response to concerns with calculation areas that crossed boundaries between hazard areas sometimes containing sprinklers having different orifice size.

[NOT sprinkler heads ;-) ]

sean

12/31/2023 11:13:14 pm

i think this was added to add clarity as some were treating it two different ways

Alex

12/8/2023 08:46:31 am

This has happened multiple times now where I am going to be submitting a drawing and this forum makes a post that has me changing my design! Thank you for whoever posted the original comment and everyone's feedback. This has caused me to increase my main to a second floor awning.

Nick

12/8/2023 08:59:06 am

To be honest, I have yet to apply this rule but my understanding is that you would in way need to apply both rules and adjust based on the result of your calculations.

I believe that you could only ever apply 28.2.4.2.5 if you are following 28.2.4.2.4, since assuming your actual design area is meets the minimum area requirement, your actual flow would never be less than your "minimum required discharge" (if you were doing things correctly).

You would first run the calculation with the full floor area (in this case less than your minimum design area) and see what waterflow is required. Due to overdischarge and other inefficiencies, you might be flowing close to the "minimum required discharge" defined as the density multiplied by the "minimum design area". Any differences between the actual system flow and the minimum required discharge need to be made up through the phantom flow.

There could technically be a situation where actual design area is smaller than the minimum required area, but the actual flow is as much or more than the minimum required discharge (as defined in 28.2.4.2.5).

That is my interpretation of the two sections.

sean

12/31/2023 11:14:14 pm

my example that this comes up is spray booths

Dave

12/8/2023 11:24:20 am

In this Reliable webinar about changes to the 2022 edition of NFPA 13, Cary talks about the change / correction of the point o connection at about 1:18:30. The whole thing is worth a listen.
https://youtu.be/NniMHPokyMQ?si=yMkR_bMUPTFS0eGe

Thomas

2/19/2024 08:46:30 am

Would 2022 edition of NFPA 13; 28.2.4.2.5 apply to a very small outside loading dock?

I have a 1,437ft² loading dock with a dry system. The design area includes (12) heads and 130ft² coverage per head. Per UFC design requirements, my design perimeters are .2 over 3,500ft². Would I be required to provide a total design discharge of 700 GPM for this tiny system?

I don't think room design method can be applied since I doubt the overhead doors are considered automatic or self-closing. Plus, NFPA 13; 19.2.3.3.5 references "interior walls" and nothing on exterior.

When Does Phantom Flow Requirement Apply?

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