How can I choose the K-factor for sprinkler 1

[yassinemzili]

hello every body, I am new in the field of automatic sprinkler system and I wont to know if there are rules to follow in choosing the K factor depending on type of hazard
Thak's

 

[pipesnpumps]

Hello Yassin, and welcome to the forum.

Generally the NFPA 13 "K factor" for sprinklers is K=5.6. Generally sprinklers are 1/2" (~DN15) connection size.
The K factor is determined experimentally, but NFPA 13 currently mandates the use of a nominal K factor (e.g. 5.6) rather than the actual K factor. Makes things less confusing I think.

It is used in the following formula:
Q = K * (P)^0.5

Q=gpm through the sprinkler
P=pressure across the sprinkler

Note: If you are already familiar with K factors, the NFPA 13 "K factor" is not to be confused with K factors used as loss coefficients in Darcy Weisbach based hydraulic calculations (fL/D). This one is wholly unique to sprinkler design.

You can use sprinklers with K factors smaller than 5.6 but only in certain restricted instances (see NFPA 13). You can use K factors larger than 5.6 also, for example K=8.0 sprinklers are often used when a water supply is poor.

I act as both the owner's agent and the AHJ, and often have sprinklers contractors say "please issue a change order for a pump" when I already accepted the engineering design with a documented flow test, and they must think I am an idiot or a pushover one to let them get a fat change order. I simply tell them to increase their pipe size and/or use K=8.0 heads. Problem solved.

As for larger K factors, use them as necessary for the hazard involved, and install them within the limitations of their listing. K factors larger than 8.0 are only for specialized use like extended coverage (EC), CMSA, storage, and similar uses.




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.

 

[b1ueshift]

In most cases you will use the 1/2" (K=5.6) heads. This will do for most commercial and public buildings. You use smaller orifices for residential work and bigger sprinklers for storage (warehouses).

If you decide to use extended coverage sprinklers to save some labour, the k factor will be different also.

 

[yassinemzili]

ok thank's for the clarifications

 

[TravisMack]

You can't use less than 5.6k for Ordinary hazard. In light hazard, you can go down to 2.8k before you need to put a strainer on the line.

In storage, you are typically going to start with a 8k sprinkler. If the density is >0.34, then you need a 11.2k sprinkler as a minimum.

I use a lot of 4.2k sprinklers in wet attics. A max spacing of 130 sq ft in most attics. More realistic to be around 100 sq ft. A 5.6k sprinkler will discharge about 15 gpm at 7 psi. A 4.2 is about 11.1 gpm at 7 psi. I can often save a pipe size or 2 with 4.2k sprinklers in the attics.

It all comes from experience with calculations.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com