Different ESFR K-Factors at Offset Roof Heights?

7/10/2024

Can ESFR K-Factors be different within the same area, just at different roof elevations?

The roof elevations here are dictating different design requirements.

We have multiple ESFR sprinkler systems throughout a new building. The building has two different roof elevations, 38'-4" for the majority of the building, 43'-4" on remainder. We're under NFPA 13-2019 Edition.

The Contractor is wanting to use K-16.8 pendents at 52 psi at the low roof. K-25.2 at 40 psi at the high roof.

One sprinkler system will cover the area where the roof steps, with two design areas (high/low roof). The outer thirds of the building separating the roof elevations will have full height concrete tilt-up walls. The center third only has a large open girder truss, with nothing stopping heat from traveling between the low and high roof.

Section 7.2.2.6 says ESFR K-factor shall be selected for appropriate hazard. Sections 20.6.2.6 & 23.6.2 indicate ESFR shall be based on building height (amongst other things). Section 23.1.2 for Draft curtains, only reference the adjacent system with standard response sprinklers, not ESFR with different K factors.

Section 27.2.4.9.2 says K-factors can't be mixed "for the purpose of minimizing sprinkler over discharge" (consolidated), which is not the case in this scenario. The Roof elevations are dictating the different design requirements.

The K-25.2 sprinklers could be used throughout, but the low roof would only require 25 psi and the high roof 40 psi. However, there is a large cost savings going with the K-16.8 sprinkler for the majority of the building, along with additional storage allowance of exposed nonexpanded plastics.

My thinking says that there should be some kind of draft stop at the girder truss/roof step to collect the heat, separating the two K-factor/roof elevations, but I am not finding a code reference that dictates it. Any input will help.

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10 Comments

Anthony

7/10/2024 07:42:20 am

See: NFPA 13-2019: 20.10 (1) , (2) and (3).
In short the ceiling height change is over 2'-0 thus not requiring draft stops. Both areas can be considered separated and use different K factors.

I, however, would go with the K25 sprinkler head throughout the area. That makes maintenance way easier in the future as you don't have to go looking for the right head for replacement or system modifications. I would also suggest looking at what a K25 can get you at the lower roof for a higher hazard. That will offer different options in storage in the future without a sprinkler retrofit.

Casey Milhorn

7/10/2024 08:13:42 am

I would agree with Anthony on not needing a draft stop. From a practical perspective I think it's fine. You have a 3/4 outlet for one and 1 inch outlet on the other. That all but alleviates the chance of putting the wrong head in. It doesn't clarify if these are on two different systems, but assuming they are, if a fire were to occur at the elevation change, discharge should be way overkill if heads activate on both systems, effectively splitting up the waterflow and reducing friction loss. I wouldn't lose any sleep over this one. Overall I'm of the opinion bigger isn't always better in fire sprinkler hydraulics.

Glenn Berger

7/10/2024 08:19:08 am

The 2022 edition of NFPA 13 had significant changes to the storage protection chapters (again).

With that said - I agree with Anthony's 2nd paragraph in his response above. It is never recommended to mix sprinklers within the same general area of a building.

Dan Wilder

7/10/2024 08:19:50 am

Also check out 19.2.2 for Adjacent Design Methods, Section 3 - "The requirements of 19.2.2(1) (The extension of the higher hazard 15' into the lower hazard area) shall not apply to the extension of more demanding criteria from an upper ceiling level to beneath a lower ceiling level where the difference in height between the ceiling levels is at least 2 ft, located above an aisle, where the aisle has a minimum 2ft horizontal separation form the adjacent hazard on each side. April 2019 #9 NFSA Tech Notes

The issue may be that while the sprinklers are allowed to change K-Factors with the elevation change, the design criteria from the higher hazard may not be able to change (within the 15' extension) without the aisle and clearances also being provided. The above design allowance doesn't provide an "and/or" option....it needs all the criteria taken care of.

This were looked at with the EOD answers from the NFSA EOD books - https://nfsa.org/eodhandbook/

Jesse

7/10/2024 08:25:22 am

Agree that you can use different K-factor ESFR sprinklers and a draft stop won't be needed. I'd also reiterate Dan's comment about the higher hazard.

I'd probably go K=25 throughout.

Pete H

7/10/2024 10:06:04 am

Agree with the consensus as Anthony initially outlined: K25 throughout is the better practice and overall install, especially where maintenance is concerned.

But I don't see a code reason you can't do the split.

Jack G

7/10/2024 10:46:38 am

I would design using 1 - k factor. I also agree that a draft stop is not required for the 1– or 2 k factor sprinklers.
I would also make the owner draft stop that portion of the building no matter, to make the building “ uniform” and the system reaction to a fire in this area the same.
The two systems were not tested flowing heads of 2 different k factors so the effects of the higher pressure 52 psi and 40 psi, are not demonstrated mathematically, so if the areas were off the same system the lower psi sprinklers in the calculation would balance at a higher pressure in the program ( and would affect the overall system flow and water supply) . Another reason to use the same k factor.
If off 2 separate systems ( 2 high demand storage sprinkler systems) with no separation and different k factors ) would the opening of the high pressure sprinklers affect the flow and pressure of the 40 psi sprinklers ? ( say if you split the operating sprinklers?)
I think they would balance themselves also but would still affect performance. ( are the systems themselves balanced? Would a demand calc yield the same result as a supply calc… if not there is overflow to deal with, and safety factors)
So I d go with a draft stop no matter.,

7/10/2024 11:26:08 am

In addition, if the 16.8 k factor solves the problem with higher benefits, then that’s the route.
Safety factors, I’ve not been a big fan of ( worked for 10 years special hazards with open head deluge systems—- where you actually see performance )
The sprinkler system is “ dumb”, so if you calculate using a safety factor say 10 psi below the curve,the resultant would not be the same as calculating adding 10 psi to the system sprinklers . ( deluge systems usually are balanced better , to work properly ). So you may even experience a deficiency with the safety factor ( if your systems is not balanced — pipes sized to flow x amount of water when opened) —- so as a check, try a supply calc on a system ( demand calculated to 10 psi below the curve — the safety factor ( — check your results on a system you’ve performed a demand calc per NFPA 13, and a safety factor 10 psi below the curve, you will not like the results of the supply calculation, as the pipes are not large enough in most cases for the 10 psi.
This is a pet peeve I have with NFPA . Demand calcs are required, Engineers require safety factors —- so say 10 psi below the curve or at the end head , affecting the sizing ( and cost ); of the system?
Maybe Joe Meyer can answer this. NFPA has given be double talk thru the years, based on what is written in their text, and not on physical performance.
So I m lobbying for safety factors not being permitted.

Dwight Havens

7/10/2024 12:46:24 pm

The topic of safety factors was introduced in this discussion. This is a topic for another discussion, but I would add here that safety factors are often required by an AHJ, to offset the lack of information on the variability of the available water supply. Designs are often based on a single water test, despite the code requiring that the variability of the water supply be taken into account in the design.

7/11/2024 11:56:27 am

All, Thanks for your input. I've done more research on this since this question was sent in.

The over 24" elevation change is sufficient per NFPA 13 2019 20.10 (1), (2) (3). No storage below this area, both sides of the roof change are currently OH2 and the entire section below the roof change is used for large product transfer from one production area to another, so the 24" aisles on each side are not an issue.
A 2' draft curtain is an easy add, if this area is converted to storage in the future.

NFPA 13 2022 new text deals with the roof elevation change more in 20.9.2.5, 20.9.2.5.1 & A20.9.2.5. (This text is good in a way but could really cause confusion if you change designs along the slope of the roof. See Figure A20.9.2.5. Probably more for a future discussion.)

Although future maintenance is always in the back of my mind, the main driving design factor is always getting the owner the most out of their system, current and if possible for future. NFPA 13 2019 Table 23.6.1, 16.8K @ 52 psi allows the "exposed nonexpanded group A plastic" storage in the 40' high portion of the building (that option looks to be removed in the new 2022 ESFR table 23.3.1, which sucks. That is on the list for a NFPA 13 Technical Question, for why this was removed).
The 45' roof portion does not have "exposed nonexpanded" option and requires the 25.2K.

All this being said, mixing K-factors usually is not an option, but in this case it is a good option.

Different ESFR K-Factors at Offset Roof Heights?

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