Fire alarm detection for high bay space

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Leo Li

Member
Location
China
Occupation
Engineer
Hi Friends, Is there any requirements of two fire alarm detection systems should required for high space(space>12m)in MA and NJ? Thanks!
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Hi Friends, Is there any requirements of two fire alarm detection systems should required for high space(space>12m)in MA and NJ? Thanks!
In a large open area such as a warehouse or multi-story atrium, the highest practical height for smoke detectors is about 20'. Some folks will say 15'. There are two reasons for this. First, maintenance. Smoke detectors are tested once a year; how would you reach a detector at 30' above the floor? Most inspection companies don't own a lift, and no customer wants to pay the inspection company to rent one. Second, stratification. In a fire, the smoke and heat rise in a plume. This plume cools as it rises. Eventually it reaches the same temperature as the surrounding air and stops rising. If this happens at 20' (which is typical) and your detectors are at 30', they will never detect the fire. So, for a ceiling of 12 meters or a little over 39 feet, it doesn't matter how many detectors you put up there, they won't go off.

What you need to do is install beam detectors at the expected height the smoke stops rising to provide proper detection coverage.
 

Leo Li

Member
Location
China
Occupation
Engineer
In a large open area such as a warehouse or multi-story atrium, the highest practical height for smoke detectors is about 20'. Some folks will say 15'. There are two reasons for this. First, maintenance. Smoke detectors are tested once a year; how would you reach a detector at 30' above the floor? Most inspection companies don't own a lift, and no customer wants to pay the inspection company to rent one. Second, stratification. In a fire, the smoke and heat rise in a plume. This plume cools as it rises. Eventually it reaches the same temperature as the surrounding air and stops rising. If this happens at 20' (which is typical) and your detectors are at 30', they will never detect the fire. So, for a ceiling of 12 meters or a little over 39 feet, it doesn't matter how many detectors you put up there, they won't go off.

What you need to do is install beam detectors at the expected height the smoke stops rising to provide proper detection coverage.
Thanks!
 

MichaelGP3

Senior Member
Location
San Francisco bay area
Occupation
Fire Alarm Technician
So, for a ceiling of 12 meters or a little over 39 feet, it doesn't matter how many detectors you put up there, they won't go off.

Not exactly true, as I've seen installations where spot type smoke detectors were installed at 3 different heights in a space to cover the scenario you describe.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Or a VESDA system.
A VESDA won't suck up the smoke from the stagnant layer at any appreciable distance. It might increase the detection height a couple of feet, but that's about it. If you are on the hairy edge of detection it might push you home.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Not exactly true, as I've seen installations where spot type smoke detectors were installed at 3 different heights in a space to cover the scenario you describe.
What kind of space? Mounted on the sidewall of a balcony in a narrow atrium?
 

MichaelGP3

Senior Member
Location
San Francisco bay area
Occupation
Fire Alarm Technician
A VESDA won't suck up the smoke from the stagnant layer at any appreciable distance. It might increase the detection height a couple of feet, but that's about it. If you are on the hairy edge of detection it might push you home.
There are various ways for a VESDA system to protect this kind of space - some of their systems employ 4 sampling tubes. There is still a way to accomplish this with a single sampling tube system. I could explain it, but you might want to give their website a peek before you shoot the idea down.
 

MichaelGP3

Senior Member
Location
San Francisco bay area
Occupation
Fire Alarm Technician
What kind of space? Mounted on the sidewall of a balcony in a narrow atrium?
45' high bay, 4" square boxes mounted to the ceiling with cover plates incorporating a KO, conduit attached to this KO and dropping vertically to one of 3 heights the fire protection engineer determined to give optimal coverage. I'm recalling that it was an ADT Unimode conventional fire alarm. I don't think that there were any detectors mounted to the ceiling.

Edit: This was in a R & D facility in California, and that facility owned their own scissor lift.
 
Last edited:

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
45' high bay, 4" square boxes mounted to the ceiling with cover plates incorporating a KO, conduit attached to this KO and dropping vertically to one of 3 heights the fire protection engineer determined to give optimal coverage. I'm recalling that it was an ADT Unimode conventional fire alarm. I don't think that there were any detectors mounted to the ceiling.

Edit: This was in a R & D facility in California, and that facility owned their own scissor lift.
OK, that works. Incredibly expensive and kludgey and a PITA for maintenance, but it works.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
There are various ways for a VESDA system to protect this kind of space - some of their systems employ 4 sampling tubes. There is still a way to accomplish this with a single sampling tube system. I could explain it, but you might want to give their website a peek before you shoot the idea down.
I am very familiar with the VESDA line, and for this application would very much prefer their OSID product.

Sure, you could dangle a sampling nozzle on a piece of tubing off a tee in the main sampling line, but that's not how it's intended to be used.
None of the conversation so far addresses the OP's concern, which would need the input of the local AHJs.
OK, speaking for NJ, the answer is "no".
 

ron

Senior Member
In a large open area such as a warehouse or multi-story atrium, the highest practical height for smoke detectors is about 20'. Some folks will say 15'. There are two reasons for this. First, maintenance. Smoke detectors are tested once a year; how would you reach a detector at 30' above the floor? Most inspection companies don't own a lift, and no customer wants to pay the inspection company to rent one. Second, stratification. In a fire, the smoke and heat rise in a plume. This plume cools as it rises. Eventually it reaches the same temperature as the surrounding air and stops rising. If this happens at 20' (which is typical) and your detectors are at 30', they will never detect the fire. So, for a ceiling of 12 meters or a little over 39 feet, it doesn't matter how many detectors you put up there, they won't go off.

What you need to do is install beam detectors at the expected height the smoke stops rising to provide proper detection coverage.
I completely agree that stratification needs to be considered, but I believe you would be required to have a layer of detectors within 12" of the underside of slab above and then as a performance design, to determine a 2nd layer at the lower stratification layer.

You might be able to convince an AHJ to avoid the top layer of detection if there were zero combustibles between the lower stratification level and the underside of the structure, but I've not had that luck, due to Mech piping insulation and other combustibles that are up there.

I typically use Vesda (or a competitor) with the end port brought down to a reasonable height for annual service, due to accessibility.
 
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