Use QR Instead of 30% Remote Area Increase?

6/20/2022

NFPA 13 Chapter 19 indicates that for sloped ceilings over 16.7 percent or 9.5 degrees, the system area shall be increased by 30 percent.

My understanding is that the test facility used had this limitation.

We further do recognize that highly sloped ceilings will cause heat to move fast towards the apex and will result in a delayed response of sprinklers, or worse, heads operate over areas not affected by fire.

With a ceiling of 10 degrees which is 0.5 degrees over the limit, will the affect on sprinkler operation really be affected to the extent where the 30% corrective is considered essential?

Without running a full CFD model, the three means of calculating response times of sprinklers we have do not consider ceiling pitch adequately. Looking at calculated results it simply would be more effective to just have lower sprinkler temperature ratings and use quick-response.

Would use of quick response sprinklers be enough to mitigate the concern in CMSA design instead of high temperature sprinklers (K11.2 or more)?

Thinking a little outside the box here, thanks in advance for the advice/input.

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5 Comments

Alex

6/20/2022 06:16:38 am

Hi,

I believe you are right when saying that the test facility had limitations. This opens the floor for performance based design. Sadly, I haven't looked into this issue and cant really advise on design alternatives.

At some point, the committee needs to make a final decision to add items into the standard. For example, with rack storage there are countless ways to store products on racks, with countless rack configurations. This causes open ends within the standard for engineers to work through.

Sorry I'm no help this morning!
Alex

Glenn Berger

6/20/2022 08:08:06 am

Lots of items contained within this question:

1) NFPA 13 does not indicate an allowance to use QR in lieu of the 30% rule.

2) The use of CFD modelling to prove alternative design is a risky adventure as you will still need authorities buy-in.

3) K11.2 refers to the orifice size and not to temperature rating.

Dan Wilder

6/20/2022 08:19:16 am

If you can provide and substantiate alternative means and methods proving either equivalency/superior performance AND the AHJ accepts the alternative, then there is no issue. This is one of the first things in NFPA 13 described - "Section 1.5 Equivalency".

Not so much a limitation on the testing (other than time and money), just a common construction angle (2:12 vs 2⅛:12) and corresponding test to prove a design approach and simulations. Providing several calculations showing similar results may be enough to allow that ½° of slope additional to what NFPA says and be enough to validate your approach, or the AHJ may reject it.

Using QR and a lower temp may put water on the fire earlier proving your hypothesis, or it may cause more sprinklers to open, especially if those sprinklers "skip" causing a larger demand on your system and put water in areas that the fire isn't....

Jesse

6/20/2022 09:31:32 am

I can see some risk in using QR sprinklers in this scenario, and doing so won't absolve you of the need to modify the design area due to slope. QR reductions aren't permitted for anything beyond OH2. And of course, not all K=11.2 sprinklers are created equal. Many aren't listed for storage protection at all.

Franck

6/20/2022 11:06:10 am

Back to the basic...
All initial tests with sprinkler systems were done with no slope at the ceiling level.
Then somebody realized that it could be problematic as with a slope, sprinklers may open at the edged, quite fire away from the fire. And they were right. They made some tests and they activated much more sprinklers.
So they have to set up a slope when some penalty need to be provided. And it ended up with 2:12. Why 2:12 and not 2:10? Because you're in the US and you like the 12, rather than the 10 (12 in. for one foot, for example). If it was in Europe, it would have probably be a 15% or 20% slope (not a 16.7% slope).
And the penalty was set up at 30%. Again, why 30% and not 25% or 50% ? The answer is... why not ? It is conservative enough and not too demanding... And it was already 30% for dry systems...
And there was a time when this 30% slope rule was also applicable for storage occupancies. Until they realized that it was really problematic for some commodities and with very technical sprinklers being developed at that time on the market (large drop, now designated as CMSA, and ESFR)
So it ended up with: if slope at ceiling (more than 2:12), you can't protect storage ! Sic ! Hopefully there are some possible acceptable alternative, such as providing in-rack sprinklers up to the last level. Or not considering a sloped ceiling with sawtooth type ceilings as the length for the slope is quite limited and the number of operating sprinklers along the slope would remain limited... This is clearly stated in NFPA handbook.
But this is a case by case analysis and need to be discussed and agreed with the AHJ.

Now, to come back to the question, the reduction with QR only applies for Light and Ordinary hazards only, and for building heights below 20 ft (reduction% varies from 25 to 40% depending on the height).
But this is covered with "multiple adjustment" section (11.2.3.2.7 in 2019 edition) where the 30% increase due to slope ceiling may be compensated by the 30% decrease with the use of QR sprinklers.
But be careful, when you add 30% and then remove 30%, the final result is not the same design area as the initial one !
Take the example of 2000 sq ft
2000 + 30% = 2600 sq ft
2600 sq ft - 30% = 1820 sq ft
Note that it doesn't matter if you add first and remove, or remove first and add after.
2000 - 30% = 1400 sq ft
1400 + 30% = 1820 sq ft

On the other hand, the change in temperature rating could be more problematic. Lower temperature sprinklers would activate far from the fire with a slope as they will activate with a lower temperature and you will activate more sprinklers away...
It is for the same reason that high temperature sprinklers are more effective for Extra Hazard and storage occupancies (to avoid activating more sprinklers away from the fire at the initial stage).
Thus a reduction proposed for Extra Hazard and storage occupancies when using high temperature sprinklers.

In your question, you also mentioned the use of CMSA. In that case the asnwer is simple: CMSA are not allowed if the slope is greater than 2:12.

Use QR Instead of 30% Remote Area Increase?

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