Determining maximum number of sprinklers on a main - hydraulic method

[RealKayfactor]

Hi Guys,
This has been bothering me a lot recently. It's either I am just missing a very simple method or its much more complex.
NFPA13 pipework schedule for OH shows 275 sprinklers on a 6in pipe is the maximum otherwise use hydraulic calculation to size pipework.
I definitely know how to do the calc for the area of operation, using the density curve the move sprinklers you could have flowing in the area of operation is 50, so technically you would only ever need to satisfy the pipe sizes can serve your area of operation.
But why is there then a limitation on how many sprinklers a DN150 pipe can serve? As long as you can prove the main pipe can serve your area of operation, why does it mater if there are 275 sprinklers on the DN150 main or 500 ?

Second question - if it does matter (obviously it does) everywhere it says show by hydraulic calc, what is this calc? What do I need to calculate exactly ?

Please and thank you

 

[UFT12]

It seems you are mixing two different design philosophies for pipe sizing. Pipe schedule is an older approach to size your pipes and today practically obsolete. Nowadays every designer should always hydraulically calculate the system. When you do analytical hydraulic calculations you are not limited to the number of sprinklers nor to any pipe size (within reasonable limits) and you do not need to follow the provisions of the pipe schedule method.

Back in the day where there where no computers and was hard to perform calculations or even when no analytical mathematical model existed, there had to be a way to size the system so the pipe schedule came up. By mostly experience, sizes where determined in an analogy to the overall size of the system. So that means that a system of 275 sprinklers would cover a larger area than one with 160 which means it will have to travel farther to reach the 50 or less sprinklers you mention and hence you need 6" instead of 4". But again, only if you size pipes per pipe schedule, aka no calculations.

 

[TravisMack]

As stated above, it seems you are referencing the pipe schedule method. If so, you simply follow the method for number of sprinklers per size.

If you calculate the system, you are creating your job specific pipe schedule to follow. There is no limit on number of sprinklers. If you have great pressure, you may be able to have mains of 3" only on a system of 400 sprinklers. You are only calculating the 1500 sq ft (with appropriate area modifications as required / allowed).

You may want to see if you can find a local mentor to help you get a better understanding of the NFPA standards as it relates to this topic. Best wishes to you in this industry.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC

http://www.mfpdesign.com

 

[RealKayfactor]

Thanks all.
In such a case limiting factor to determine appropriate size through the hydraulic calculation will be velocity ? Capped at 20 - 30 ft/s? and Yes, the pressure.

 

[TravisMack]

Why are you limiting velocity? Where in the NFPA 13 standard does that come in to play? Velocity is a self limiting issue.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC

http://www.mfpdesign.com

 

[DavidCR]

If you have a hydrant or sprinkler grid where noise, vibrations or pressure loses where a high Reynolds number would be an issue or if you want a "balanced" system, it is a desirable option to consider velocity limits, but not a code requirement. For example, a well code design may worry ocupants if there are strange noises during a flush or fire brigade practice, or some dry walls may be cracked, so if posible perform a velocity check. (Consider also the possibility of a loose hanger or support, or loose pipe where seismic restraints are not required).