Beam detectors and heat plumes

[kurnal]

I am currently working on a new warehouse project and as part of the fire strategy we have to calculate the Available Safe Evacuation Time.

We are also advising on the detection system and two of the manufacturers  of the beam systems are making statements to the effect that the heat plume of a fire passing across the beam will make it more sensitive but are unable to quantify this.

Does anybody know if it is likely to be a significant effect and will it make any impact on time to detection of the fire?

Although sprinklers are to be installed the Fire Officer is  requesting calcs based on an ultra fast fire to cater for sprinkler down time.

The  default settings for the beam system will be 35% obscuration.

[wee brian]

Sounds like so much bull to me.

The usual approach to these assessments is to do a simple time to fill calc. It misses the point that there will be dirty great big flames whizzing about.  If its High rack then ultra fast isn't unreasonable.

Of course you may be demonstratingthat you don't need the sprinklers!

[kurnal]

Thanks Wee B

Yes it is a bit critical tho because there is an occupied mezz  floor height  9m- and the roof is only 13-14metres.

We will need sprinklers anyway due to size (new ADB) and are installing ESFR roof and in rack. The key in this case I think is a management strategy that ensures sprinkler maintenance and down time is controlled.
But will still work out tenability to keep FSO sweet (I hope)

[Tom Sutton]

Kurnal, sorry to wander off the thread and pose a question not an answer but in very basic terms how do you calculate the Available Safe Evacuation Time. Do you use fire modeling, formulas with available data or are there other methods.

[John Webb]

Kurnal,
The hot gases passing across the beam cause it to deflect slightly due to the differing refractive index - the 'twinkle' effect you might call it. Some beams will detect this through their automatic alignment or stabilization systems and could use this to alter the sensitivity to obscuration of the beam. It should be quantifiable to some degree, but the size of the fire, ceiling height, distance of plume from the beam would all have to come into it, which why the manufacturers may be very reluctant to try and apply it to your situation. It should speed up detection, but it will depend what is in the warehouse to burn. There would be a significant difference in heat output and smoke density between, say, cardboard boxes and motor car tyres.

[kurnal]

TW
Whilst there are formulae to work out the time it will take for conditions to become untenable there are too many squiggles and cubes for me to work it out even with a scienific calculator. And I slept ever so well during those sessions at moreton. So yes we use modelling- FDS and CFD-  least wise in these cases I usually pass the bulk of my paltry fee to a friend with some letters after his name and a better car than me to work it out on my behalf.

John

Thanks once again for hitting the nail on the head so concisely. I can see the logic in your explanation.

Another engineer has told me that as many systems use infra red,  hot gases would also affect the level of received signal. I spent many years looking at fires through an infra red thermal imaging camera and never did see any hot gases radiating infra red. Thats why we used them - to see through smoke.  And in any case as the beams are so focussed I would have thought a plume of hot gas in the path of the focussed beam would have little effect on the infra red beam.

[John Webb]

Kurnal,
I don't know if you recall the 1970s Laser Beam fire detector developed by the Fire Research Station. That worked predominately on the 'twinkle' effect rather than simple obscuration. The frequency of signals given off when hot gases from a fire were about differed significantly from the smaller and much lower frequency of signals resulting from physical movement of the building.
I'm not certain about clear hot gases having an effect on signal level; what may happen that water vapour and other chemicals present in fire gases do have a higher absorbtion of infra-red at certain wavelenths, but it's a long time since I looked at absorbtion spectra and don't have any reference books to hand on that subject.
Re TICs - I think they need to see at least heated particulate matter if not larger heated objects. I have seen hot gases but only with a very sensitive Aga camera that Building Research had and which could actually render in colour the small thermal differences present in heated buildings, for example. Not the sort of kit easily carried on a fire appliance!

[Tom Sutton]

Thanks Kurnal, unfortunately I did not get a chance to sleep during those sessions at moreton I had traveled into the sunset well before this brave new world of fire risk assessment.