Down Stands, Obstructions and AFD

[SidM]

I am trying to understand the rationale behind siting detectors away from obstructions and down stands.  In a room with down stands or light fittings how exactly would a detector sited too close to a light fitting/down stand obstruct the flow of smoke.

[CivvyFSO]

The detectors are given their response time (rti) based on a test where the velocity of smoke is a large contributing factor. A downstand/light fitting would clearly interfere with the velocity, meaning that the detector would not perform as designed. Downstands can also lead to small pockets of air that don't get disturbed, therefore the smoke doesn't penetrate effectively.

Instead of thinking of a volume of smoke simply moving up and hitting the detector, think about it as the smoke going up, hitting the ceiling, moving horizontally away from this point, THEN hitting a detector. Common sense dictates that obstructions in the way can't help the matter.

[SidM]

Hi Civvy - I don't get that.  Imagine a normal office with a down stand down the middle.  Two detectors are sited closed to the down stand on either side.  A fire starts.  Smoke goes up, mushrooms across and hits the detector which it would do regardless of whether there is a down stand in the way or not.  What exactly is the impediment caused by the down stand?

[nearlythere]

Hi Civvy - I don't get that.  Imagine a normal office with a down stand down the middle.  Two detectors are sited closed to the down stand on either side.  A fire starts.  Smoke goes up, mushrooms across and hits the detector which it would do regardless of whether there is a down stand in the way or not.  What exactly is the impediment caused by the down stand?

Downstands can create smoke reservoirs which could obstruct smoke travel along a ceiling towards a detector delaying detection.

[SidM]

Yes but the reservoir is created regardless of where the AFD is sited.  Therefore the situating of the detector is irrelevant.  The problem is the down stand itself.

[nearlythere]

Therefore the situating of the detector is irrelevant. 

It would be if the downstand split the room and created two reservoirs and there was a detector in only one of them.
If a fire started under the reservoir without the detector it would have to fill it before it spills over into the one with the detector. It will happen eventually but the detection time will be longer.

[SidM]

I understand that but the issue remains:

Imagine a normal office with a down stand down the middle.  Two detectors are sited close to the down stand on either side.  A fire starts.  Smoke goes up, mushrooms across and hits the detector which it would do regardless of whether there is a down stand in the way or not.  So what exactly is the problem of siting the detector close to the down stand?

[Mike Buckley]

The problem is smoke and hot gases from a fire do not act like that. They act as a body of gas on their own and as they move across the ceiling they push the air out of the way. When the smoke gets close to the downstand it pushes the air into the corner and is deflected down. If the detector is too close to the downstand it will remain in the "clean air" and its response will be delayed.

[kurnal]

Is the detector within the area of dead space very close to the junction of the ceiling and the downstand? Or is it the issue of distance from light fittings and similar obstructions that most interests you?

If I recall these issues were the subject of research by BRE, the findings of which were incorporated with a summary explanation inthe 1988 version of BS 5839 -1. You could try looking at their archives for more detailed information.

Heres a 1995 NIST research report on the subject

http://www.fire.nist.gov/bfrlpubs/fire95/PDF/f95031.pdf

[Clevelandfire 3]

Basically the downstand acts like a bunding and contains the smoke in a resevoir. The resevoir of smoke will eventually overfill but if the smoke cools it will drop and may not reach the nearest detector. The detector will eventually go off but no where near as fast as it would if the downstand wasnt there.

[Wiz]

I am trying to understand the rationale behind siting detectors away from obstructions and down stands.  In a room with down stands or light fittings how exactly would a detector sited too close to a light fitting/down stand obstruct the flow of smoke.

Imagine a large rock in the middle of a river. There will be a lower level of water immediately behind (direction of river flow) that rock . Relate this to smoke and you will see that there is less likelihood of smoke reaching a detector positioned immediately behind an obstruction. Obviously everything depends on the direction and strength of the stream of smoke being moved on the current of air, but detectors mounted away from obstructions are more likely to sense the smoke. The detector needs to be mounted away from the obstruction at a distance twice the depth (from the ceiling) of the obstruction. (up to a maximum of 500mm away from the obstruction i.e. an obstruction of 400mm in depth only requires the detector to be mounted 500mm away and not 800mm)

I also understand that the layer of hot gases at the highest level created in a fire (i.e those which require us to have the sensing element of a detector at least 25mm away from the ceiling) build up in the corners created by horizontal and vertical surfaces so that smoke cannot reach these areas. This is why detectors on ceilings should be located at least 500mm away from walls or ceiling obstructions greater than 250mm in depth.

The problem of a structural beam in a room creating a resevoir of trapped smoke and therefore the need for detection
either side of the beam is only required where the depth of that beam is more than 10% of the floor to ceiling height.

[CivvyFSO]

Simplified considerably, but this is what I believe to be what we are trying to say:

[nearlythere]

Clever boy.