Using the curve 1

[TravisMack]

I have a small building that is spec'd to have a deluge system with Ex Haz Grp I for the sprinkler density. The room is just under 5000 sq ft. Since this is going to be a deluge system, I have to flow every head in the area with a design area of 5000 sq ft. I want to "go up the curve" and have the density of 0.20 gpm / sq ft over the most demanding 5000 sq ft.

I don't see any reason why I can't do this, but just wanted to get some input as to whether it is a good idea or not? Some other things that come about with the lower density is that I can stretch the sprinklers to 130 sq ft instead of 100 sq ft and go up to 15' between sprinklers instead of 12'. This will allow saving a couple sprinklers and branch lines in this area.

So, good idea or bad? Anything I am missing?

Thanks in advance.

 

[stookeyfpe]

Travis

I'm confused and will preface by saying I am traveling and don't have NFPA 13 in front of me. The specified design density is 0.30 GPM/Sq.Ft. (EX Hazard Group 1) but you want to reduce the density to 0.20 GPM/Sq.Ft? I need some clarification.

Scott

 

[SprinklerDesigner2]

Just curious, why would an EH 1 be a deluge system?

 

[TravisMack]

There is a curve for Ex Hazard design. Same as there is one for Light, OH 1, OH2, EH1 & EH2.

For example, for OH2, the typical is 0.20 gpm / 1500 sq ft. However, for various reasons, I may also choose 0.18 gpm / 2500 sq ft. This is along the same curve for the OH2 criteria.

In this question above, the curve for EH1 allows a range from 0.20 gpm / sq ft / 5000 sq ft to 0.30 gpm / sq ft / 2500 (can be reduced to 2000 sq ft with high temp sprinklers). So, I figure that since I am using a deluge system and have to figure the entire 5000 sq ft flowing, why shouldn't I "go up the curve" and reduce my design area. The only thing that made me question it was the deluge system. If this were a closed head wet system, there would be no questions.

I also do this a lot in storage applications. Let's say I have a warehouse with 20' of solid piled class IV commodities. This is 0.295 gpm / sq ft over the most demanding 2000 sq ft using high temp sprinklers. But, I can go to 0.24 gpm / sq ft over the most demanding 2700 sq ft. Now, by doing this, I get to lower my end head pressure AND I can use 130 sq ft for my sprinkler area of protection. This can make a big difference in the layout and costs of a system. My demand goes from 590 gpm to 648 gpm, but I can potentially use ~30% fewer sprinklers and fewer lines since I can go to 15' between the lines. Picture a job with 30' bays. Most people would go with the density of 0.295 and put in 3 lines / bay. If I go with the .24/2700, then I can get 2 lines per bay. This can be a significant savings on a project. It also still meets the design curve that is allowed to be used.

Another place where this comes in to play is when you have to do 3000 sq ft because of non sprinklered concealed combustible spaces. Again, using the curve for light hazard, I can reduce the density to 0.07 gpm / sq ft. Now, if I have 14'x14' spacing for my sprinklers, I need 13.72 gpm / sprinkler. A K5.6 head is going to have 14.82 causing over discharge at every sprinkler. However, a 4.2k sprinkler would allow me to reduce my over discharge significantly. When you are doing systems like this, the reduced density by using the curve is of great help. I actually had this pointed out to me by an AHJ in these kind of applications.

Again, the only thing that made me think that it might not be right is that this system in question is a deluge system.

 

[TravisMack]

SD2:

This is strictly what the engineer of record for the project specified. He wants Ex Haz 1 for a deluge system. They are protecting acetylene tanks. I don't have the quantity of the tanks right now. We are stuck with "what this engineer wants goes" as the sprinkler contractor was directly informed. It is one of those jobs

Travis

 

[stookeyfpe]

Travis

I understand what you are attempting to do in your design. You want to apply a discharge density adjustment for a deluge system.

I just got back into the office and there is nothing in NFPA 13 that would not permit you from doing what is proposed. However, I would not accept it as an acceptable design because all of the sprinklers must operate at a given density. In other words, if 10 sprinklers must each discharge 30 GPM to meet the density, and the area of the room requires 10 or more sprinklers because a deluge system is required, then all of the sprinklers must flow the minimum volume of water.

On a second note that may benefit you and your engineer, NFPA 51A was revised to no longer require deluge sprinklers. NFPA 51A, Section 11.2.1.2 now accepts an EX1 discharge density over a minimum design area of 2,500 Sq.Ft. You may want an RFI from your engineer.

 

[TravisMack]

Stookey:

Again....you are da man!! thanks for the insight!!!!

Travis

 

[firepe]

Travis,

I agree with Stookey, this probably does not need to be a deluge design. That being said, i also agree with sliding along the curve, whatever you can do to your advantage. But in this case, I don't believe that deluge and EX should have been used in the same sentence. It is more appropriate to stipulate a density but not an area for a deluge system, and therefore , in my opinion, not appropriate to slide on the curve.

Just my opinion.

 

[TravisMack]

Firepe:

I am going to have them go back and clarify from the GC and engineer as to what this should be. I will use the section that Stookeyfpe pointed to above.

I had never had a deluge system where I was given "design to Ex Haz I." I had always been given something like: "the sprinklers shall discharge at a rate of 0.25 gpm / sq ft throughout the entire area protected by the deluge system." This seems to be more appropriate as to how the deluge system should be.

Thanks again for putting some clarity to this issue.

Travis

 

[stookeyfpe]

Travis:

Exactly. When you or I declare deluge it is all or nothing. It is interesting that the NFPA 13 committee has skipped this. I suspect it is a lack of experience.

I will say that I have been there and done it on some projects. Its not pretty nor a real pleasure.

Thanks Travis. I learned something new and intersting about NFPA 13 this week.