Could Someone Please Explain "Phantom Flow"?

9/12/2023

NFPA 13 Density/Area Method Chapter 28.2.4.2.3 states that if the design area is smaller than the required minimum design area, you're allowed to include only the sprinklers in the available design area.

However, there's an additional provision. This provision details how to calculate the design discharge.

First, you subtract the flow from the available area from the required minimum design discharge. Then, you add this difference back into the equation. Essentially, this brings you back to the flow discharge of the required minimum design discharge.

Could someone explain this provision?

Is this calculation done to account for potentially higher flows in the main header?

What flow then determines the maximum flow for sizing a fire water tank?

I'm looking forward to your answers.

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4 Comments

Peter Howard

9/12/2023 06:45:55 am

Was a recent thing I picked up, so please take with a grain of salt and make sure to check other responses for the inevitable parts I missed:

Basically say you have a job where the entire modification to the sprinkler system is less than 1500 square feet. It's a light hazard space.

A light hazard space, without hose, has to be hitting a minimum of 150 gpm flow. (0.1 gpm/sq ft x 1500 sq ft).

Say you only modified a 750 square foot area and for whatever reason while you can trace the common piping back to the main, you cannot see adjacent spaces (different tenants, it's locked up, what have you). And apparently you have next to no friction loss affection your calc on this system so your total demand comes up to 85 gpm in a system that requires 150 gpm.

150 - 85 = 65. So you need 65 gpm to meet the nfpa minimum for the design criteria of the hazard.

So similar to adding 100 gpm of hose at the source node, you add 65 gpm of "phantom flow" to where the branch line feeding your 750 square foot space meets the common system piping.

And now you've calculated your required minimum.

An AHJ can still take issue with this if they feel your modification area is close enough to the remote area that they would like to see how your modification would affect the system as a whole and therefore require you to do the 1500 square foot calculation. But if you're nowhere near the most remote end of a system, this can work for you.

Dan Wilder

9/12/2023 07:15:43 am

Second part first - Water Tank Sizing

While I don't do a lot of these sizing exercises, my approach has typically been to not be minimally conservative. Use full remote areas (no reductions), maximum sprinkler spacing, and slightly higher but realistic over discharge.

Where you add the remaining "Phantom Flow" is more based on the applicable NFPA Edition but it was more to address smaller but higher density calculations within a larger, but less demanding system. This prevents what was (I'm guessing here) an issue with remodels and the undersizing of mains, risers, and underground pipe sizing.

Ken Isman did a great write up on this (a couple others as well, but my quick google search yielded more firewalls than useable links so...)

https://pdfcoffee.com/phantom-flow-pdf-free.html

This previous discussion as well:
https://www.meyerfire.com/daily/is-phantom-flow-based-on-largest-design-area

Casey Milhorn

9/12/2023 10:57:49 am

Great comments so far. I find it's rare that phantom flow is necessary, and I think a little common sense goes along way here. BUT, it should be used, in my opinion, when you have a situation like Peter described. It should not be used when you can use another method, like reduced area, or large room design method, etc. Basically it's there to make sure you aren't cheating the system.

Colin Lusher

9/13/2023 12:17:43 pm

I took a deep dive into this on a military project where the AHJ was requiring phantom flow methods be used for a 1000 sq.ft. Ordinary Hazard Occupancy that was within a larger light hazard occupancy.

Here is my previous post on Meyer Fire:
https://www.meyerfire.com/daily/is-phantom-flow-based-on-largest-design-area

And here is the BEST technical resource and explanation for it available, IMO:
https://www.dropbox.com/scl/fi/lcacwddlnjz8rbbjzqxsl/NFSA-Phantom-Flow.pdf?rlkey=0iqc7kavb10y7wa9843tphr0r&dl=0

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Could Someone Please Explain "Phantom Flow"?

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