Sprinkler Area Used in Hydraulic Calculations?

4/30/2019

If a light hazard room is 13 feet x 13 feet, and a k5.6 pendent sprinkler is located seven feet from two of the walls, what is the sprinkler area used for hydraulic calculations?

In short, to determine the minimum flow from a sprinkler in multiplying the area by the required density, is the actual area of the room used (13'-0" x 13'-0" = 169 sqft) or is it computed the same as the maximum area of coverage for a sprinkler by doubling the furthest distance from adjacent walls [(7'-0" x 2) x (7'-0" x 2) = 196 sqft)]?

For a room like this with a K5.6 sprinkler k-factor, it would make a difference between delivering 19.6 gpm to a room (0.1 gpm/sqft x 196 sqft) and 16.9 gpm to a room (0.1 gpm/sqft x 169 sqft).

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13 Comments

Chad

4/30/2019 08:52:34 am

I am a little confused on you distances. If the sprinkler is 7'-2" off one wall and 6'-6" off the other. Your area coverage for that head would be 14'-4" x 13'-0"= 186.33 so your gpm for that head would be 18.6

MODERATOR

4/30/2019 08:54:54 am

Sorry, I didn't do a good job of vetting that. I believe the intent was 7'-0" off both walls with a room that is 13'-0" x 13'-0". I'll update the question to match.

Chad

4/30/2019 09:40:15 am

If it's 7' off two walls, your area coverage would be 14' x 14' regardless if the room is 13' x 13'. You take the largest distance of spacing.

Erick

4/30/2019 10:21:29 am

8.6.2.1.2 The requirements of 8.6.2.1.1 shall not apply in a small room as defined in 3.3.17; the protection area of coverage for each sprinkler in the small room shall be the area of the room divided by the number of sprinklers in the room.

This is from NFPA 13 2010. I’m on vacation and don’t have access to the latest NFPA but it’s been in there as long as I can remember.

Chad

4/30/2019 10:43:48 am

Completely forgot about the small room rule. Good Stuff!

Patrick

4/30/2019 10:18:04 am

Because this is light hazard, you can apply NFPA 13 8.6.2.1.2, which states: the requirements of 8.6.2.1.1 do not apply to small rooms as defined by 3.3.22; and 8.6.2.1.2.1 states: the protection area of coverage for each sprinkler in a small room is the area of the room divided by the number of sprinklers protecting it.

Dan Wilder

4/30/2019 10:27:05 am

Is it considered a small room? Look at 13-16' - 8.6.2.1.2.1

PETE

4/30/2019 11:21:26 am

I think the small room rule is being misinterpreted here. It is intended to eliminate the necessity for a second sprinkler (in the single sprinkler example) for a room that is slightly larger than the maximum spacing requirement on a single dimension. Look at the appendix.

It can't be used to reduced the area of coverage for a hydraulic calculation without the sprinkler being required to be over-spaced off of one wall. Obviously, if the sprinkler has enough pressure to deliver the design density to one wall, it's going to have to deliver the same density in all directions.

Franck

4/30/2019 11:45:24 am

The small room rule could be considered, as indicated above.
But to be honest, does it really make a difference?
Normally any water supply is able to provide a flow of 20 gpm, and the difference of pressure required at the sprinkler head is 9 psi (0.6 bars) in one case and 12 psi (0.8 bars) in the other case, i.e. a total pressure demand difference of 3 psi (0.2 bars) if only one sprinkler head is considered.

I would personally be conservative and use 196 sq ft anyway (this will give more flexibility, depending on what could be stored inside this room) if I would make an hydraulic calculation for this situation.

And to be even more honest, for only one head, I would not even make an hydraulic calculation :) (use the pipe schedule method for sizing the pipes for very small systems).

Bruce Seiler

4/30/2019 02:46:32 pm

The small room could be used in this case to calculated the square footage for the sprinkler head as stated above, however I agree with Franck, this is not going to save you much in calculations and could open the final design up to scrutiny. I have had AHJ's question the square footage of sprinklers when calculating the actual square foot protected. Say we didn't have a sprinkler head in the design area that was over 168 sf, they would say no sprinkler head in the system could be spaced over 168 sf unless another calculation was completed to prove that area worked with the higher demand of that sprinkler. I don't run into this that often, but it is something that could come up. Also, if this sprinkler head isn't the most remote sprinkler, it could be moot point. In the end, it would come down to is it saving you a pipe size or not. Good question though. The theory behind what to figure in calculations is always a good debate within my staff.

WES

5/1/2019 08:11:39 am

Well said Bruce. I agree that tightly designing the system would cause restrictions for future modifications or potentially open up different remote areas depending upon how much area is attributed to each sprinkler.

There are some projects that have such poor water supplies that the difference in 3-4 psi can make a big difference. As Mike says below, the 3-4 psi bump in the first sprinkler bumps up the pressure in the second sprinkler such that there's higher overall flow and higher overall required pressure. Most of the time this isn't an issue, but occasionally for very tight projects the approach on how these rooms are laid out can have a big impact.

Mike

4/30/2019 07:31:54 pm

Having a large end head pressure is not advisable for 196 Sq ft. It will jack up the flow of the 2nd head especially using 1“ pipe on maximum spaced heads resulting in higher overflow. Even though this is a small room, it may create an alternative "hydraulically" most remote area unless it is fire resistive per NFPA for Room Design Method.

Herminigildo Sia

11/9/2021 09:48:17 pm

is it considerable for my designed area of 840 sq. meters and my choose of coverage area is 12 and the spacing is 12.4 meters

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Sprinkler Area Used in Hydraulic Calculations?

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