ESFR - HYDRAULIC DEMAND 1

[SystemDesigner]

Hi guys

I have a query regarding how to take the
number of sprinklers in operation.
The rules state:

15.4.3 The design area shall consist of
the most hydraulically demanding area
of 12 sprinklers, consisting of four
sprinklers on each of three branch lines.

If this is my arrangement, which will
be the most demanding?
consider all measurements as uniform:

x1----x2----x3----x4------|-----x5----x6----x7----x8
|
|
x9----x10----x11----x12---|---x13----x14----x15----x16
|
|
x17----x18----x19----x20--|--x21----x22----x23----x24
|
|
|
|

 

[chevy4x4trucks]

It would be 4 heads on 3 lines so in the scenario you have above assuming the main is centered inbetween the heads you could pick either side of the main for your remote area.

 

[SystemDesigner]

would x1,2,3,4,9,10,11,12,17,18,19,20 be correct?
would i not have to take a full range? e.g 1,2,3,4,5,6,7,8,9,10,11,12?

I'm confused

 

[skdesigner]

heads 1-12 are on one branch line. Your first idea is correct. Like chevy4x4trucks said though- if the main is centered, and all your heads are equally spaced, you can pick either side.

 

[skdesigner]

sorry, make that heads 1-8. mondays...

 

[b1ueshift]

You do not need to take a full range as you are not required to assume the fire is long and thin.

The assumed area of operation is a rectangle. There are rules for selecting the rectangle which say the dimension in line with the ranges should be not less than 1.2 times the square root of the remote area (if I remember correctly). This usually means 4 sprinklers from 3 different ranges arranged in the most remote way.

 

[SystemDesigner]

Thanks for all the replies guys.

the reason i ask is because i have a riser on the main.
Will the riser have to be sized to accomodate all 8 sprinklers or only four sprinklers per branchline.

the riser will obviously be bigger if i have the take all 8 into operation as it needs to handle a higher flow?

 

[b1ueshift]

A little knowledge is a dangerous thing and you seem to have a little knowledge.

Can you provide more information about how you are designing the system? Are you using a pipe scheduling method?

 

[SystemDesigner]

No. I'm using a hydraulic program to size the pipes.
Why do you ask?

 

[b1ueshift]

You seem to be new at this kind of calculation and there's a lot of little details in NFPA 13 that you should be aware of.

Which program are you using? Is is specifically for designing sprinkler systems in accordance with the procedure set out in NFPA 13?

Anyway to answer your latest question, I was curious if you were trying to design an ESFR sprinkler system by using 'pipe scheduling' as there is no assumed area of operation for 'pipe scheduling'. I don't think you can do ESFR sprinkler design using pipe scheduling though.

 

[pipesnpumps]

Interesting - all of the ESFR I have ever seen is gridded.. I suspect this is because it is more economical to have two smaller cross mains and smaller branches, than to have a tree type arrangement.

But its all been in large warehouses, and with a fire pump. If you don't mind me asking, why not gridded?

 

[skdesigner]

"I don't think you can do ESFR sprinkler design using pipe scheduling though."

Sure can't. You don't use the density/area method, either.

From NFPA 13 2010 22.4.4.3 ESFR Method:

For ESFR sprinklers, the design area shall consist of the most hydraulically demanding area of 12 sprinklers, consisting of 4 sprinklers on each of three branchlines, unless other specific numbers of design sprinklers are required in other sections of this standard.

 

[chicopee]

Sprinklers closest to the Inspector's test connection since it should be the most remote point in the sprinkler system. Your drawing does not show where that test connection is located.

 

[b1ueshift]

It seem the thinking behind the calculation procedures in NFPA 13 is that fires spread out in all directions, they generally don't form a long thin line. The correct interpretation (as designers seem to be using) seems to be that 4 sprinklers on three ranges, refers to ranges that are next to each other, not in line with each other.

I also agree with pumpsnpipes in that most ESFR systems are grids as this tends to reduce the number of mains and also the size of the ranges. The ranges can be arranged at 90 degrees to the perlins and cliped to the underside of the perlins.

These days, there is calculation software which can generate a grid automatically so a grid is just as easy to design as a tree.

 

[SystemDesigner]

Thanks for the replies guys.

There is some politics behind this design and i've been asked to stick to a tree system.

Just out of curiousity, what were the range sizes on a grid system that was done by anyone?

 

[skdesigner]

Does "range" mean branchline? Asking for branchline/main sizes won't help you, unless you happen to have an identical building and water supply as the person you are asking.

Ask your estimator what he bid the job for, grid or tree. With 8 heads per line, and depending on the water, I would absolutely try to tree it first. Easier for your fitters to sling a big main down the center, than to hang two smaller mains, make the swing joint at the roof peak, and hit the primary and secondary dead on with the pre-fab pieces your fancy sprinkler design program magicked together.

That being said, the ability to do cost benefit analysis is what sets great designers apart from the rest.

 

[SprinklerDesigner2]

Being equal the heads calculated would be 1 through 4, 9 through 12 and 17 through 20.

"Just out of curiousity, what were the range sizes on a grid system that was done by anyone?"

Smallest ESFR grid I have managed was 2" branch lines w/4" mains. In my experience being able to use 2" is extremely rare.

The largest was 4" branch lines, three (3) 6" mains and 8" riser.

 

[SystemDesigner]

"You do not need to take a full range as you are not required to assume the fire is long and thin."

What if the fire occurs along a rack? A rack fire would be thin and long?

 

[b1ueshift]

'What if the fire occurs along a rack? A rack fire would be thin and long?'

Yes, this is true, a fire could possibly be long and thin. I'm glad you asked this question. Unfortunately I get similar questions a lot and I'll try to give you a sensible answer.

It seems for people from other fields that the design process is tied up with their ego. They come up with their own 'brilliant' ideas and defend them with their own retoric as the design and their support for their ideas represent them. Sometimes these people can look at a designer faithfully following a standard and think this person must be lacking in tallent and confidence because they are not using their own ideas.

I would point out to my colleagues that designing life safety systems is different to designing other systems. We follow codes and standards because that's what works. We don't brain storm, and use our creativity to decide what the dimensions of the design fire are - other people have already done that for us.

NFPA 13 is one of the most thoroughly reviewed and researched documents in the world. The team that produced it is huge and varied. Not only does it work, but it's trusted. Governments and insurers know NFPA 13 and they trust it. They don't know me and they aren't about to trusting my ideas.

 

[b1ueshift]

Also, I'll go along with sprinklerdesigner2 for typical sizes.

From memory I designed one once with range pipes 2 1/2" riser and primary main was 8" and float main 6".