Is Phantom Flow Based on Larger Design Area?

8/10/2020

Would appreciate if you guys would weigh in on this disagreement I'm having.

Applicable Codes/Standards:
The project is subject to military code UFC 3-600-01 for Sprinkler Design Area and Design Density. Otherwise, NFPA 13-2016 applies to all other aspects of the system fire sprinkler design.

Description of System:
The system consists of a 1,000 sqft Ordinary Hazard Design area, surrounded by a larger (remainder of building) Light Hazard design area (these adjacent hazards are separated by full height walls that are capable of preventing a fire on one side from fusing sprinklers on the other side). Per UFC, the corresponding discharge densities should be 0.20 gpm/sqft over 2,500 sqft for the Ordinary Hazard design area, and 0.10 gpm/sqft over 1,500 sqft for the Light Hazard Design Area.

In accordance with the example provided in NFPA 13 Section A11.1.2, the size of the operating area for hydraulic calculations is determined by the Occupancy of the larger surrounding area. In this case, the hydraulic calculation design area is 1,500 sqft based on the larger surrounding light hazard area. A 0.20 gpm/sqft density would be provided for the 1,000 ft2 Ordinary Hazard design area, with a 0.10 gpm/sqft density for the remaining area.

Differing Opinions:
The difference in opinions for this situation is about how to apply NFPA 13-2016 section 23.4.4.2.5 to this situation.

NFPA 13-2016 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.

Opinion #1:
The required minimum design area as referenced in 23.4.4.2.5 should be the SAME design area that was utilized for the hydraulic calculations as determined by NFPA 13 Section 11.2.1. In this case the minimum design area would be 1,500 sqft.

Opinion #2:
Even though the remote area size was determined to be 1,500 sqft per section 11.2.1, the required minimum design area as referenced in 23.4.4.2.5 should be the minimum design area for the smaller, higher hazard area. In this case, the minimum design area should be 2,500 sqft based on the Ordinary Hazard design requirements. So if the flow from the 1,500 sqft hydraulic calculation is less than 500 gpm (2,500 sqft x 0.20 gpm/sqft), then an additional flow will need to be added to the remote area to bring the flow up to 500 gpm.

Which of the opinions do you feel is appropriate for this scenario?

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17 Comments

Dan Wilder

8/10/2020 09:31:28 am

Your example is very close to the A11.1.2 example and with the guidance detailed in A23.4.4.2.4.

The design area can be 1500 sq. ft. because there are barriers enclosing the higher density/area required room BUT you will still need to comply with the flow requirements as defined in 23.4.4.3.5 by adding the "missing" flow (actual design discharge - 500GPM) as calculated from A.23.4.4.2.4.

So the answer is "C"...smaller design area complying with the flow requirements for the larger density/area hazard. You eluded to it in Option #2 but your wording starts off like the larger design area is required but transitioned to using the "phantom flow" requirements.

COLIN LUSHER

8/10/2020 11:23:22 am

Thank you for your well thought out answer Dan. From an engineering perspective, it still doesn't make sense to me to add additional flow for a hazard that doesn't exist.

I guess the crux here is what is the "required minimum design area" in 23.4.4.2.5. My opinion is that with a mixed hazard, the "required minimum design area" would be the area that was determined in 11.1.2, which is 1500 sq.ft.

I would understand using the 2500 sq.ft. area (500 gpm min.) if it was NOT a mixed hazard, but I just can't wrap my head around using it here.

If your position is correct, then wouldn't a small 50sq.ft. Ordinary Hazard closet within a light hazard area trigger a minimum 500 gpm flow for the remote area?

8/10/2020 12:49:56 pm

I would then defer to 11.6.1(3) & 12.7.2(3) showing that the intent is not to reclassify larger, lesser hazard areas for ancillary single rooms that fall within the area.

I agree with the lesser area of 1500, but you would need to allow for the 500GPM vs actual discharge difference to apply your "phantom flow".

The intent of this section was to prevent the requirement of larger branch lines over an entire system due to the 2 options for calculations (Room Design or Density/Area) which either required the full remote area for smaller, higher hazard configurations spilling into lesser hazard areas like yours described or require the room to become rated.

This section was added to allow the designer to use a smaller design area while still meeting the intent of water discharge characteristics as if the larger area was being utilized BUT not requiring the branch lines to carry additional sprinklers (the reason the flow gets added at the main). If you calc the larger area, you should find that the system would be larger overall vs the use of the "phantom flow" method.

Colin

8/10/2020 01:23:27 pm

Thanks Dan!

Those sections you referenced are only for Hose allowance though, and do not apply to design area or design density. I guess you could use them to argue intent, but they technically don't apply to this scenario.

It just seems to me like the 11.1.2 states that the higher hazard does NOT need to flow into the surrounding light hazard area. And if that is the case, why are we adding extra flow for a hazard that doesn't exist?

Not sure I understand the branch-line sizing example...the phantom flow method would only help as you've described if your branch-lines were longer than the length of your remote area.

I really appreciate your thoughts on this!

8/11/2020 10:30:17 am

Correct as to the Hose Allowance and I would use those sections as intent that the higher hazard areas are secondary to the overall design when the room is small enough. This allows small, ancillary rooms to not drive up the remote area requirements.

As to the branch line comment. All things being equal as to sprinkler layout, assume the following (perfect square spacing, no over discharge):

-√1000 = 31.6'x31.6' (square room)
-1000 sq ft/130 sq ft (the OH area) = ~7.3 sprinkler - Round to 8
-1000/8 = 125 sq. ft (11.18'x11.18'). or 25GPM required per sprinkler
8x25=400GPM
-1500 sq.ft/225 sq ft (the remaining LH area) = ~6.7 - Round to 7
-1500/8 = ~214.3 sq. ft. (14.64'x14.64') or 21.43GPM required per sprinkler
7x21.43=150GPM

So to calculate that remote area, a minimum of 15 sprinklers requiring ~550GPM total and puts about 5 sprinklers on 3 branch lines - 1.2√2500=60'

-1000 sq ft/130 sq ft (the OH area) = ~7.3 sprinkler - Round to 8
-1000/8 = 125 sq. ft. or 25GPM required per sprinkler
8x25=400GPM
-500 sq.ft/225 sq ft (the remaining LH area) = ~2.2 - Round to 3
-500/3 = ~166.7 sq. ft. or 16.67GPM required per sprinkler
3x16.67=50GPM (the least amount of sprinklers that could cover the remaining area)
So to calculate that remote area, a minimum of 11 sprinklers requiring ~450GPM total and puts about 4 sprinklers on 2 branch lines and only 3 additional sprinklers on the 3rd branch line - 1.2√1500=46.5' (this is like .2' off from getting 4 sprinklers onto a branch line instead of 5 and the LH sprinklers are severely under spaced so I'm going to take a little liberty that the design can be better spaced). If these 3 sprinklers where closer to a max spacing like the first example, the additional GPM would be slightly less...closer to 28GPM instead of the 50GPM for "Phantom Flow" adjustment but again, this all assumes no over discharge.

So the remote area is now flowing 4 less sprinklers that require 50GPM less overall and that should bring your branch line sizing down (likely main sizing as well). This should provide for a smaller system as a whole but still complying with NFPA for the Density/Area calculation requirements (the main reasoning behind the rule addition).

8/11/2020 02:03:59 pm

Dan, you're awesome! thanks so much for the detailed explanation! I did get a ruling from NFSA that changes the way this situation is dealt with. See my comment on the main post below.

Brian Gerdwagen FPE

8/10/2020 11:33:16 am

Option #2. That is the minimum flow for Ordinary Hazard in the UFC. They could theoretically increase the hazard in the Light Hazard areas to be OH. But as this is governed by the UFC, the AHJ will have more to say about it than NFPA 13. Remember, just a few years ago, Light Hazard per the UFC was 0.10/3000, and OH was 0.20/3000.

They change the rules and AHJ's all the time, and getting buy-in to your decision is the most important thing. I would also question using NFPA 13 2016 edition and 3-600-01 and 1-200-01 say to use the most recent published editions of NFPA standards.

8/10/2020 11:59:30 am

Thanks Brian. So in this example, would a small 50 sq.ft. OH2 closet within a light hazard area still trigger the same requirement for a minimum 500 gpm flow?

Plans/specs specifically call for 2016 edition of NFPA-13....this jurisdiction (Navfac Hawaii) regularly deviates from UFC on the NFPA-13 edition.

Yes, AHJ buy-in what we're going for; we submitted to NFSA for an informal ruling and AHJ said they'd allow us to comply with what NFSA says. Just wanted an "FPE community" opinion on the matter as well.

Franck

8/10/2020 12:01:44 pm

By using 11.1.2 and A.11.1.2, this is how I would handle the issue with NFPA (and accept it as an Insurance Company, but I am not an AHJ in your juridiction).

I would use Figure 11.2.3.1.1 (Design/Area curves) with a common area of application for both occupancies.
If you take 1500 sq ft, this means 0.1/1500 for the light hazard and 0.2/1500 for OH Gr 2.
If you take 2500 sq ft, this would be 0.07/2500 for LH and 0.18/2500 for the OH Gr 2.
The 1500 sq ft is normally more favourable for a smaller flow demand at the pump.

In your example, because of the separation between the 2 areas (no need to extend the OH Gr 2 area beyond the limits), this would result in an hydraulic calculation with 0.2/1000 for the OH Gr 2 area and 0.1 over the remaining 500 sq ft.
And this will be very comfortable for the following reasons:
- with your separation, even though it is not a fire wall, there is little chance that the fire will go from your OH area to the LH area when the sprinkler are operating.
- at the beginning of the fire, you will have much more than 0.2 gpm/sq ft for the first operating heads
- statistic demonstrates that most fires are controlled with less than 5 sprinkler heads when the systems are adequately installed and maintained.

Using § 23.4.4.3.5 would be very conservative and not representative of your situation.

Again, personal opinion.

8/10/2020 12:13:26 pm

In addition to my above comments, I would consider § 23.4.4.3.4 & 5 only if I had a total building floor less than 1500 sq ft. Not for a mixed occupancy.

Please tell me if I'm wrong, as this is an interpretation from my side.

For example, if you had only 1000 sq ft as a floor area of your building occupied by an OH Gr 2 occupancy, I would make the hydraulic calculation for my 1000 sq ft with a 0.2 gpm/sq ft density and add the missing flow up to 500 gpm to determine my hydraulic demand to be supplied by the fire pump.
This is pretty conservative, but would ensure flexibility if you make an extension in the future (to avoid the choice of a too small fire pump, for example, just based on the calculation with 0.2/1000...).

Colin Lusher

8/10/2020 01:02:50 pm

Thanks Frank! We share the same exact opinion on how to apply 23.4.4.2.4 & 5. It seems some others above do not, but I guess that is why they're called opinions.

Yes, if the bldg was 1000 sq.ft., then I agree 100% that we would need to add the extra phantom flow. But in this situation, there is no lack of area, just a lack of OH2 hazard, so it doesn't make sense to me to add extra flow for a hazard that doesn't exist....seems super conservative.

Thanks so much for your thoughtful reply!

8/10/2020 06:01:27 pm

I found some direction in NFPA 13 - 2019 Handbook which seems to clarify. I've only change the code references below [xx] to match the corresponding section of the 2016 version:

"Paragraphs [23.4.4.2.4, A23.4.4.2.4 and 23.4.4.2.5], were added in 2013 to provide an alternative calculation approach for the system demand as opposed to having to calculate a full design area as generally provided by [11.1.2, A.11.1.2]. The approach was developed in response to concerns with calculation areas that crossed boundaries between hazard areas sometimes containing sprinklers having differing
orifice sizes. The inclusion of larger orifice sprinklers outside the portion of the design area required to operate at the higher pressure needed for smaller orifice sprinklers would cause an excessive overdischarge of water and higher overall demand. As a compromise, a concept was developed of utilizing an
alternative design that would ensure that the overall level of water demand was at least sufficient to meet the minimum discharge demand for the density/area application."

So according to this, it seems like it's an either OR situation? Either you calculate the full design area as required by 11.1.2 OR, you calculate only the higher hazard area sprinklers, but you must add additional demand per 22.4.4.2.4 & 5.

Does that seem reasonable?

8/11/2020 02:11:19 pm

Thank you everyone for your input! I submitted this to NFSA an got an informal ruling in the form of an Article titled "Phantom Flow" by Kenneth E. Isman, PE.

The relative point to this article was that the "Phantom Flow" options of 24.4.4.2.4, and 24.4.4.2.5 are OPTIONS, and can be ignored at the designers discretion. If they are ignored, then calculations are performed based on the "old rules", i.e., 11.1.2/A11.1.2.

I also learned that the main reason for adding 24.4.4.2.4, and 24.4.4.2.5 was to deal with situations similar to mine, but where there are but where there are no walls/barriers to prevent a fire in the higher hazard area from fusing sprinklers in the lower hazard area. In that type of situation, section 11.1.2 requires you to extend the higher hazard 15-ft into the lower hazard area, and to calculate the minimum design area for the higher hazard. In that case, you have the OPTION of using 24.4.4.2.4, and 24.4.4.2.5 while ignoring the minimum design area requirements of 11.1.2, and only calculate the higher hazard area (with 15-ft extension), with an additional "Phantom Flow" added at the main to make up for the smaller calculation area. This should reduce branchline size and be an overall advantage to the system cost.

In my case, because we DO have walls/partitions between the hazards, NFPA 13 section 11.1.2 does NOT require us to extend the higher hazard design area 15-ft into the lower hazard area. As this is the case, section 11.1.2/A11.1.2 allows us to calculate only 1500 ft2. In this specific case, it serves us best to IGNORE the options provided by 24.4.4.2.4, and 24.4.4.2.5, as it would actually increase our demand requirements, which is not the intent of these sections.

Again, thank you everyone for your time, and your valuable input. If you'd like a link to the "Phantom Flow" article, email me at [email protected].

8/13/2020 03:01:32 am

Many thanks for all the explanations.

It looked a bit contradictory between 24.4.4.2.4 and 11.1.2 and even more with 11.2.3.1.4 that we used in my company for years when having an area smaller than the minimum design area from the flow curves.

And to be honest, I never saw the use of the 24.4.4.2.4 rule in any of the project I have reviewed so far.

I am glad to see that opinions from different people go in the same direction (sense of judgement) and that some explanations were found for the story behind the rule.

Alan A.

2/18/2022 09:44:56 am

We are working on a large military project where the DOR is trying to push this "Phantom flow" rule on us. We've done hundreds of projects at this facility and it has never come up before. All the instances this DOR is trying to enforce this contradict what we would call common sense as well as the example given in the NFSA article you referenced.

Many thanks!

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

2/18/2022 11:21:19 am

Hi Alan, yes, it must have been part of an NAVFAC or USACOE seminar last year, because this was the first I'd heard of it as well, and the DOR seemed to be unclear as to the actual application of this rule. In my case, I had to concede to the DOR/AHJ even though their application of this rule was not correct.

ALAN A.

2/19/2022 12:21:26 pm

I believe the reason no one is familiar with it, is because of how unimportant it really is. The benefit is pretty minimal as the PE explains in the article.

We are not conceding this to the DOR. They had their own bogus explanation of how they interpreted this section of the standard. "NFPA added this section in case the area were ever expanded."... NO WAY. NFPA is not really in the business of governing the future, and our contract was not to plan for future renovations.

We are looking forward to another meeting explaining why we will not provide 750 gallons of water to a space protected by 6 SPRINKLERS! The building is 6 stories tall with multiple calc areas per floor. It spent months being coordinated with no space above ceiling. They are going to pay us if they want my 1,000' of 2" loop mains to become 1,000' of 4". on every level.

Is Phantom Flow Based on Larger Design Area?

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