Fire Main Pipe Sizing 1

[fayazdin1]

I am working on a 6000 sq. m strip mall with six 2-story stores that require total of 510 sprinklers.
Considering 1500 sq. ft for the most demanding fire area, I need 4” pipe all the way to the source.
The question is …………..
Why do we increase pipe sizes above 4”
1) Because of pressure drop calculations? (Which is understandable)
2) Because additional sprinkler and branches connected to the main (which confuses me)

Area density method considers 1500 sq. ft one-fire instance, then why do I have to include more sprinklers and branches to size the main?

Please help me understand
Fayaz

 

[Oremus]

Hydraulic calculations and your available water supply determine the pipe sizes. 4" is pretty typical for your ordinary / light hazard project of medium size with a decent water supply. You typically see main sizes larger than 4" in higher risk hazards such as storage occupancies or extra hazard - then you can expect to see 6", 8", and larger if necessary.

You do not increase a main size because of additional branchlines strictly speaking. You may be confusing hydraulic calculations with pipe schedules; they are two different ways of sizing your pipe. Generally hydraulic calculations result in a more economical design with smaller pipe sizes.

Your last question was :

"Area density method considers 1500 sq. ft one-fire instance, then why do I have to include more sprinklers and branches to size the main?"

You don't. If it's determined that you need to calculate 1500 sq. ft. (and you don't have any increases or reductions) then that is all you calculate at the worse case scenario (ie. the hydraulically most demanding 1,500 sq. ft.). Most of the time you will be calculating a little over 1500 sq. ft. since your sprinkler spacing will not divide into 1500 evenly. For example at 130 sq. ft. spacing you need to calculate 12 sprinklers since 130 does not divide into 1500 evenly - in this case (if you have typical 130 sq. ft. spacing) it would come out to 1,560 sq. ft.

12 x 130 = 1560.

 

[Oremus]

You said you have 510 sprinklers for 6,000 sq. ft. - I take it you meant 60,000?

 

[skdesigner]

6000 sq. m = 64,583 sq. ft

 

[Oremus]

Ooops, didn't catch the 'm'

 

[chicopee]

Oremus is correct,hydraulic calculations will determine pipe sizes.

 

[stookeyfpe]

Or he can use pipe schedule.