Heads on a branch line

[jkampana]

I am calculating a NFPA 13 system in a 4 story apartment building. I used the largest room method which includes 3 heads in the largest room , all fed off one branch line and 2 heads in the communicating space nearest the room. In order for the calcs. to work I had to make the pipe from the Main line to the first head on the line 1 1/2" with the last two connected by a 1" line. The rest of the branch lines in the building are feeding multiple rooms that only contain 1 head each and in few cases 2 each. All being serviced by a 1" line.

My question is: Would I still have to mimic the process used in the calcs. throughout the rest of the building? Or am I able to use 1" lines since each is protection a different room?

 

[b1ueshift]

NFPA 13 isn't going to give you a simple yes/no answer to that question.

The most common approach that designers use (that I'm aware of) is to size all branches using a consistant scheme (ie in your case, 2 heads fed by 1" branch and 3 heads fed by 1 1/2" branch). You then identify the remote area by inspecting the drawing and using your judgement (which is the large room in your case).

In theory you could do something complicated and say 3 heads will be fed by 1 1/2" except where they are in different rooms, but this reduces the confidence that you have selected the correct remote area and it also makes it harder to demonstrate to a third party that your calculations are correct.

 

[TravisMack]

If you want to have different schedules, then calc multiple areas to prove what you are doing. There is nothing that states you only calculate one area.

Are you using residential heads? If so, you have to calculate 4 heads as a minimum. I don't believe you can use the room design method with residential heads, but I could be wrong on that one.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[jkampana]

I went ahead and calced multiple areas to prove a different schedule.

@TravisMack - yes I am using a residential head, The guidelines in 11.2.3.3 don't say anything about having a minimum of 4 heads nor not being able to use the room design method at all. If there is a place that it states this that I'm not aware of I would like to see it. I have done The room design method numerous times in this fashion and haven't had a problem getting it passed through the cities.

 

[skdesigner]

NFPA 13 2010 11.3.1 Residential Sprinklers

11.3.1.1 The design area shall be the area that includes the four adjacent sprinklers that produce the greatest hydraulic demand.

 

[Arsnman4]

I understood it as Travis does also. When using residential heads in a 13 design one needs to follow 11.3 but I could be wrong also.

"Fire suppression is a failure in prevention"

 

[pipesnpumps]

Agree. Seems to me if you use residential heads you are into 11.3 requirements (4 head method).

You are using the room design method and say the largest room has 3 heads.

Let me ask you, does this room have a UL listed or other NRTL 30 minute fire resistance rated fire barrier assembly for it's walls? In other words was the sheetrock one of the acceptable manufacturers and type and was it installed in accordance with its listing? That is necessary to use room design method.

Secondly, if the three heads are in the room, have you accounted for the additional two sprinklers in the adjacent corridor? Or do you have self-closing doors to this room (not likely).

It sounds to me like you need to re-think the design, and use the 4-head method with those residential heads, instead of misapplying the room design method..







Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[pipesnpumps]

I just re-read the post and overlooked that you accounted for 3 heads in the romo plus 2 in communicating space. Disregard what I said about that then..

Everything is enough that I would still consider revising the approach.

Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[jkampana]

Well it looks like I need to read a little closer than I have been. After reading 11.3.1. I was incorrect. Thanks for the help. Looks like I will be using more 1 1/2" than I thought.

 

[jkampana]

In regards to the 4 head calc. I no longer need to include 2 heads in the communicating space correct? As that only applies when using the room design method. Is this correct?

 

[firepe]

jkampana said:

"I have done the room design method numerous times in this fashion and haven't had a problem getting it passed through the cities."

Just because you got it past the reviewing authority does not make it right. Many authorities do not have the expertise needed to catch this.

In fact it is my belief that a mojority of designers do not go the extra step to find less obvious areas. Designers like Travis, that do put in the extra effort, are the exception, rather than the rule.

NFPA 13 2010 A.22.4.4.6 addresses this issue. "When it is not obvious by comparison that the design selected is the hydraulically most remote, additional calculations should be submitted. The most distant area is not necessarily the hydraulically most remote."

Based on this it would follow that you would have to prove less obvious, differing piping/sizing/routing that did not match or is not based on what was previously calculated.

This is neglected more often than not.

Fire Sprinklers Save Lives.
Can You Live Without Them?

 

[jkampana]

@firepe - I agree with everything you have stated. I should have prefaced my OP to say that I am fairly new in the Fire protection design industry. So the help is greatly appreciated since I still have a lot to learn.

I'm glad I came here and asked the question as now I can fix my design so that it is correct and use the correct methods in the future.

 

[TravisMack]

Don't confuse 13 and 13R. In a 13 system, you calculate FOUR heads. It doesn't matter if they are in 1 compartment or 4 compartments. 13R is all heads in a compartment to a maximum of 4.


Travis Mack
MFP Design, LLC

http://www.mfpdesign.com