Sprinkler Head K-Factor Selection 1

[Dormer1975]

I'm trying to develop a cookie cutter criteria for dictating when to change the sprinkler head k-factor. Maybe, it's not possible to have a fixed plan for this, but I thought I'd throw this out to the community to see what you think.

I'm designing a system that is Ordinary Hazard Group II. I'm using Quick Response sprinkler heads and am able to reduce the remote area by 38% [NFPA 2007 11.2.3.2.3.1]. So, I'm dealing with 0.20 gpm/ft^2. The minimum pressure per head is 7 psi. My coverage per head is 127.5 ft^2. So, flow (Q) is 25.5 gpm (Q = δ * Area) out of the most remote head. Using a 5.6 k-factor head, this yields a pressure of 20.74 psi (Q = k * p ^(1/2)). Flowing 8 heads in this scenario. This system is fed from an existing pump (1500 gpm @ 100 psi) and I'm well under the curve (38.5 psi below the curve). I could use these heads and feel fully confident they would function to protect this space.

However, if I go up to an 8.0 k-factor head, the minimum end head pressure drops to 10.2 psi. This results in 44.7 psi below the curve. 8.0 k-factor heads are more expensive, though. So, what are the criteria that dictate when to raise k-factor. Is the thought process just "as long as I'm at least 10% under the curve, I'm fine"?

 

[MGXFP]

The K factor doesn't just affect the end head pressure at the sprinkler but also affects the friction losses through the system.

Your extra cushion of 44.7 psi under the curve could be used to justify smaller pipe and save some money.

Backflow preventer and other device loss curves are not always linear, a change in sprinkler orifice size can affect the losses through devices too.

Personally I look at every job as unique and design is an iterative process. I don't know that one can boil it down much more than that.

 

[TravisMack]

The only real thing you have to go by is that when you have storage with a density of 0.20 or greater, you must use 8k sprinklers. When the density is 0.34 or greater, you must use 11.2k or greater sprinklers. Also, you can't use less than 5.6k sprinklers in areas other than light hazard.

I have some general guidelines I follow in design:
Concealed combustible spaces with wet systems where structure is <36" on center, I typically use 4.2k sprinklers. It cuts down on the excess flow from 5.6k at 7 psi.

General go to is 5.6k sprinkler in most light and OH1 areas. When the discharge per sprinkler starts to exceed 22 gpm in OH2 areas, then I start to look at K8 sprinklers.

Extra Hazard I areas are going to start at K8 and EH2 is going to start at 11.2k with the thought to increase K factor as needed to help with pressures.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Oremus]

The main criteria is which k-factor is going to result in smaller pipe sizes saving the company money and making it easier for the fitters to install while maintaining a reasonable level of confidence that the water supply can supply the demand. Because in your scenario water supply is not an issue I would scrap the 127.5 sq. ft. of spacing and just calc it as 130 sq. ft. Chances are the fitters in the field aren't going to maintain exactly 127.5 sq. ft. and if there's no need to penny pinch on the hydraulics because of the extravagant water supply then don't.

Ultimately the determining k-factor is selected based upon the water supply. Yes the 8.0k heads require less pressure but they also discharge more water and so one is selected based upon what is available. Sometimes you'll have city water supplies with high pressures but low volume in which case a 5.6k or less (depending on occupancy)may be more suitable. 8.0K sprinklers are more well suited for water supplies with lower available pressures but more volume. I'm talking typical light and ordinary hazard occupancies. Storage scenarios are a different game.

But in your case, with that much cushion, I'd be looking at how extended coverage sprinklers would impact the profitability of the project.