Phantom Flow for 4-Sprinkler Fire Pump Room?

10/16/2024

I am working on a project with a diesel fire pump room that is Extra Hazard 2 (0.4 gpm / 2500 SF). The room is only 300 SF, which is well below 2,500 SF.

The 2022 Edition, NFPA 13 28.2.4.2.4, indicates following a set of steps shown in A.28.2.4.2.4. This section indicates that if the flow is less than the minimum required flow, then the difference in flow must be added to a node on the system. A.28.2.4.2.5 shows a picture example of where to add the flow.

The current design has 4 sprinklers within the room, each discharging around 30 gpm ((0.4 gpm x 300 SF) / 4). When reviewing the shop drawings, 880 gpm was added to a node, increasing the system pressure to about 150 psi!

How is NFPA 13 28.2.4.2.4 supposed to be accurately calculated within the hydraulic calcs?

Do you create an imaginary system and, in this case, add 21 more sprinklers for a total of 25 (2500 / 100 SF per sprinkler)?

Adding the 880 gpm to the system shown on the shop drawings and getting a required pressure of about 150 psi doesn't seem right.

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10 Comments

Brett

10/16/2024 06:32:54 am

To answer your question, yes that is how it's supposed to be done as long as the flow is added at the appropriate point.

My question to you is why are you calculating just these four sprinklers? If you're calculating via the room design method then there is no need to account for phantom flow. If you're using the density/area method then the design area should be extended to the appropriate size.

Anthony

10/16/2024 07:59:29 am

See below for the relevant code (NFPA13-16). You are correctly calculating just the hazard area as you have partitions separating the hazards.The point of the 'ghost flow' is to ensure the 'main' is sized for the hazard.So in this case add it jsut bast you 2 branch lines. I'm not an expert in hydracad or sprinkcad, not sure which you're using but you could just add a fake sprinkler or several flowing hoses to size the main there.

You could also just do the math by hand. To size the main properly.
At 1000 gpm you lose:
.1965 PSI/ FT through 4'' sch 10
.028 PSI/FT through sch 10 6''
.0078664 PSI/FT through 8'' sch 10

23.4.4.2.4* Where the available floor area for a specific area/
density design criteria, including any extension of area as re-quired 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.

11.1.2 and 12.3 say you have to extend the design area if not separated. This case clearly has a separated hazard. It would be wrong to extend an extra hazard density outside the fire pump room in this case.

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 design density times the required minimum design area, an additional flow shall be
added at the point of connection of the branch line to the
cross main furthest from the source to increase the overall
demand, not including hose stream allowance, to the minimum required discharge

I think you'd be fine asking the AHJ if you can size the main just for those sprinklers in that area as you are right at the pump not some remote warehouse location.

Dan Wilder

10/16/2024 08:11:29 am

Brett nailed it but some food for thought.

The procedure is mostly correct, the application is incorrect. In this case, the fire pump room should be rated (NFPA 20 - 4.14.1.1.2), thus allowing for room design method (19.2.3.3.3 & 19.2.3.3.5) and avoiding the "Phantom Flow" requirements.

If "Phantom Flow" was being applied, I would also be using any available reductions (high temp sprinklers per 19.2.3.2.6, 11.2K sprinklers or greater per 19.2.3.2.7 for extra hazard area reduction of 25% but no lower than 2,000FT²). Then, .4 x 2000 = 800GPM (total required per 28.2.4.2.5). The 4-sprinkler discharge of ~120GPM would mean that the additional required GPM is only 680 GPM (800GPM - 120 GPM).

Some previous discussion for Phantom Flow

https://www.meyerfire.com/daily/could-someone-please-explain-phantom-flow

Mike Morey

10/16/2024 09:24:34 am

As usual Dan beat me to it, but this is the way to go. Room design method is not Density Area and thus not subject to phantom flow.

RayJ link

10/16/2024 09:42:53 am

Dan seems to beat a lot of people around here!

10/16/2024 09:49:53 am

The only caveat with the room design method is that the rating of the room has to be equal to or greater than the required water supply duration. If the room is EH2 (90 minute water supply required assuming it's electronically supervised) and it has 1 hour rated walls, then the room design method cannot be used.

Mike K

10/16/2024 03:18:29 pm

The room is 2 hr rated. Thanks for the input!

Jessica Lutz

10/17/2024 11:42:06 am

Wilder on point again. ....came here to comment on the Pump room 2-hr rating requirement.

Jack G

10/16/2024 09:03:47 am

MeyerFire 5/7/2020 indicates the design density should be. .25 for a diesel fire pump. I would try submitting that wit “ room design method”.
A .40 would be per the spec——- but room design method.
I m not a believer in phantom flows.
A sprinkler system does not have a switch controlling the flow. If one sprinkler goes off, it’s getting many times the density.
Try doing a supply calc ( best description of system flow ) . Especially with a 10 psi safety factor)
You will find the supply calc exceeds the water supply and your NFPA 13 demand calc.
Pipe sizing can control water flow.
Same with phantom flow.

Jesse

10/16/2024 09:08:14 am

I love these threads that make you think. Topics like these are ones I kind of forget about until we need to do it.

Agree with Dan and his statement about the application being wrong.

Phantom Flow for 4-Sprinkler Fire Pump Room?

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