Sizing sprinklers cross main

[scythe13]

Hi,
I have attached a sprinkler layout from NFPA 13. Can you tell me why the size of cross main after 10 branches has changed from 2-1/2" to 3"?
I think in hydraulic calculation, the criteria in sizing main branch is keeping the velocity and pressure in an appropriate range, so the number sprinklers does not matter. Am I right?

Also, is there any limit for the number of sprinklers on a branch? For example, can a branch consist of say 12 sprinklers?

Thanks

 

[cdafd]

One It is just an example

The reason for the change in size, because the 3 inch is actualy feeding the 2 1/2 inch

The riser is on the right side of the picture

 

[scythe13]

Thanks for your answer,
I want to know is it still correct to keep the size of cross main 2-1/2 and unchanged entire the cross main?

I also would appreciate it if you shed some light on the rest of the questions I asked

Merci!

 

[TravisMack]

You can size the pipe to anything you can prove. If the calcs work, it could be 2½" the entire way. The only way to know is to calc it.

There is nothing in NFPA 13 that limits the length of a branch line. You are only limited to what you can calc.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[cdafd]

If you look at how to pipe schedule a system that can answer some of your questions

But like stated above most systems are hyrdaulicly . calculated