Draw Remote Area on Inside or Outside of Walls?

11/16/2020

Where exactly is a remote area for a fire sprinkler hydraulic calculation drawn with respect to interior and exterior walls?

Is the remote area boundary along the inside, centerline, or outside edge of a wall?

Also, are small wall cavities or unsprinklered shafts right in the middle of a remote area included in that remote area square-footage?

I recognize that as much as I can fine-tooth a layout this really doesn't matter much in the overall scheme - and I typically would add a sprinkler or two to a calculation if the exact location of the remote area boundary would make or break an area threshold - but I've been curious about this as I'd like to be consistent with code and what should be done.

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4 Comments

Dan Wilder

11/16/2020 08:13:40 am

I have approached this by the floor area covered (the same approach for sprinkler spacing which defines actual minimum density required per sprinkler in most scenarios). This would provide me with floor layouts to the interior face of walls however my layout will include the full width of any walls or shafts that falls within the hydraulically most remote area (HMRA).

Depending on the size of the unsprinklered areas within a remote area, I adjust accordingly (kitchen hoods tend to be the biggest thing I run into as to large areas not included in my sq. ft. layouts and make a remote area look like a horseshoe or donut). If there is a column block-out or a duct shaft that falls within an area, I don't make the adjustment unless it constitutes a large area (what that is I can't define, but it's truly the exception, not the rule in my experience).

As to a specific section of NFPA I can quote...I am at a loss. I even went to grab my NFSA Layout, Detail, & Calculation book which also does not go into this level of specifics but it does have this wording:

"The minimum design area has to cover actual floor area"

I will throw in the technically not wrong alternate approach (if you ever take a layout course by Cecil Bilbo or see a seminar taught by him, I highly encourage you to enroll), there is also the approach that a standard Density/Area calculation does not take into account walls (rated or not) for the HMRA, why should the sprinkler discharge area be limited to those same constraints. For example, if an interior wall is 3' from a sprinkler but that sprinkler is designed to discharge over an area that allows 5' of spray, utilizing that additional 2' over whatever length falls within the HMRA, is still allowable per NFPA 13.

I look forward to the responses from others on this....

Franck

11/16/2020 11:15:29 am

As Dan indicated (always good comments :), you have to determine first your floor area (with the 1.2 rule to determine the length and the width of your area).

Note that this is the floor area, not the ceiling area (sprinklers are spaced at the ceiling along the branch lines, but the area they cover is on the floor).

By doing this, you end up with a certain number of sprinklers falling into your area.And you make your calculation for all these sprinklers.
All sprinklers within this area have to be considered.

And to calculate the pressure at the end head sprinkler, you calculate it based on the "virtual area" covered by this sprinkler (not the actual area on the floor, i.e, if the sprinkler is in an angle at 3 ft from 2 walls, but located 12 ft (S) from the next sprinklers, the area to consider is not 9 x 9 (S/2 + 3) - 81 sq ft - but 12 x 12 (S/2 + S/2) - 144 sq ft).
Because if you want to have an adequate density on the direction of the next sprinkler, you need the same density on the other side (even though half of the water will splash on the wall).

This means that at the end, you will be a bit more conservative as your sprinklers will cover a "virtual" area which is greater than your actual area.
But this is the way to calculate, as all sprinklers may not cover the same area (sometimes the spacing between sprinklers varies from one branch line to the other, possibly because of obstructions, or provision of separation walls...), and this will cope with the water thrown on the walls if sprinklers are located at less than S/2 from the walls.

Franck

11/16/2020 11:23:04 am

Regarding the small unsprinklered areas within your sprinkler area, this is not a concern as long as they are limited in area.

Your fire will not develop in these areas.

Remember that when you determine your remote area, this is the area where you will control the fire starting at one point (generally in the middle of your area) and spreading in all directions.
If there is a shaft in the middle, it is anyway part of your remote area as your fire spread will be the same.

A possible exception would be a large unsprinklered area in the middle of your remote area...
In that case, it would mean that this is not the correct "remote area" (as an adjacent area not covering this unsprinklered space would require more sprinklers, thus more flow and more pressure).
The "donut or "horseshoe" configuration proposed by Dan would be a conservative approach in that case.
But nobody would blame you for being a bit conservative (especially not the AHJ).

I'm at work

11/16/2020 04:59:34 pm

I always value you guys' comments. I am also of the school that if it is Density/Area, not Room Design Method, then you ignore the walls. While I wish NFPA #13 would come out and say that, if it wanted you to follow walls, it would tell you so. I have seen support of this through industry informal interpretations and articles, including those written by Ken Isman and Victoria Valentine.

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