Appropriate Density/Area for EH1 Aircraft Hangar?

7/14/2022

NFPA 13 required Extra Hazard Group I sprinkler protection for Aircraft Hangars.

What is the appropriate density/area be under CMDA design?

NFPA 409 has a minimum density of 0.17 gpm/sqft (6.9 Lpm) and uses k5.6 to k8.0 quick response sprinklers, however the design area is 15,000 sqft (1,394 sqm).

The hanger is only 9,680 sqft (900 sqm) and three light aircraft are to be stored in the hangar.

Would 0.30 gpm/sqft (12.2mm) density and an area of 2,500 sqft (230 sqm) with quick response not be more affective than 0.17 gpm/sqft (6.8mm) density and a 9,690 sqft (900 sqm) area?

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8 Comments

M. Newell

7/14/2022 08:11:15 am

If you look at it solely from a discharge perspective.

.3 x 2500 = 750 + 500 hose = 1250gpn

.17 x 9690 = 1647.3 + 500 hose = 2147.3

The discharge from 409 is 900 gallons more water discharged.

Also, if 409 references NFPA 13 for the discharge and not to take hangar area then you would have to add the additional discharge of .17 x 5303 = 901.51 gallons into your calc also.

See NFPA 13 2019 27.2.4.2.4-27.2.4.2.5 - for the additional flow to be added.

franck

7/14/2022 11:53:31 am

Note that this calculation only applies when different adjacent occupancies with different required densities (or different type of k-factor for the sprinklers) are present : i.e. you want to meet .3 over 2500 over a 2500 sq ft area only, but require .17 over 15 000 sq ft for the remaining area.
Which could be the case if you protect a EH gr 1 hazard on a given 2500 sq ft area and want to protect the remaining of the building with .17
This not seems to be the case in this situation, but rather a choice between.3/2500 for the entire building or .17/15000 for the entire building

Jesse

7/14/2022 08:25:41 am

I'm confused. Are you protecting the hangar under 13, or are you protecting the hangar under 409?

What Group is the Hangar?

Anthony

7/14/2022 08:27:11 am

I believe the AHJ/building code would tell you which standard takes precedent and thus which curve to use.

However do note the .17 over 9680 would require more total volume of water thus driving up the size of your riser and mains. The .3 over 2500 would require larger branch lines and heads but limit the size of your riser. So the first approach would best be best if the fire was considered to be a hazard for the whole of the hanger. The 2nd option if the fire was considered to be more likely to be contained to a smaller portion of the hanger and suppression was likely prior to spreading to other areas/aircraft.

.3 gpm/sqft x 2500sqft = 750 gpm
.17 gpm/sqft x 9680sqft = 1646 gpm

Might be worth your time to run a supply calc and see what you end up getting with an extra hazard design over the whole building.

Dan Wilder

7/14/2022 08:41:18 am

Need to identify the type of hangar first before deciding on a design approach.

Due to things like horizontal fire spread and wing shielding, the larger remote area may be the better choice vs the smaller design and the system being overwhelmed by a spreading fire.

Eric

7/14/2022 10:33:36 am

Dan's point about horizontal fire spread is really important here.

NFPA 13 densities for both occupancy hazards and storage hazards are generally based around standard combustibles and minimal obstructions.

Unique hazards like flammable liquids, aerosols, aircraft hangers and such have their own codes/standards because they present unique challenges that don't fit within the basic guidelines of NFPA 13. Making assumptions on which fire control approach is "better" is really risky and is the reason why full scale burn tests are performed by FM or the FRR.

Imagine a fire utilizing your proposed EH1 design that starts as a fuel spill underneath an airplane. Perhaps it even catches the bottom of the plane on fire. All this heat release starts activating the QR sprinklers, but hardly any of the water is reaching the fire. I could see this situation quickly overwhelming the system design as more QR heads activate away from the fire center until your design fails.

The alternate design with a lower density over a large area might not be as likely to control/suppress an early stage fire, but will however provide cooling and wetting over the entire area without failing by to many heads opening. This lower level of control may provide enough time for manual fire fighting to begin before things get completely out of control.

I'm not saying your design choice is wrong, but it is certainly something that should be thought deeply on as it is probably the hardest part of industry.

franck

7/14/2022 11:44:36 am

EH gr 1 only applies to hangars that are not under 409 requirements
If your hangar falls under 409, you need to use .17 over 9680
If not, you can use .3 over 2500

The main reason for a larger area is indicated by Dan. A higher density with a smaller area is not adequate when you expect to open many sprinklers in all cases.

Same approach for turbine buildings (power generation) where the design is .3 over 5000 because of a possible oil poil fire and 3 dimensional fire in the building. It even used to be defined with 2 design that had to be met simultaneously in the past : .2 over 10 000 AND .3 over 3 000, replaced in the late 80s by .3 over 5000 (one demand).

Tim

7/15/2022 04:54:16 am

Now I see the role of the automatic water sprinklers is clearly to cool other risks in the space. Fire suppression has to be low level in form of LX or MX foam. High density application from the roof will possibly have negative impact on the foam blanket. Its clear now why the lower density 0.17 over 5000 sqft for Type 2 (000) construction is recommended by NFPA 409. Design and hydraulics NFPA 13 the density and area NFPA 409. Low level suppression is a must do if you want to have any hope in successful automatic fire suppression or intervention.

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Appropriate Density/Area for EH1 Aircraft Hangar?

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