Aspirating System Primary and Secondary detection required ?

[enzomorales]

Hi,

Appreciate any feedback on the following:

I've been asked to look at design of an aspirating smoke detection system (vesda) in a high airflow cleanroom type environment and have been referring mainly to the BFPSA COP and BS6266. I've identified the best sampling position for early detection to be in front of  the return air intake grills which I understand to be a "primary" configuration of detection ie. it's sampling the return air and not positioned with a more standard sampling point spacing arrangement directly within the area to be protected.

There seems to be recommendation that this primary detection only be used to supplement a more standard "secondary" detection configuration in the area to be protected ie. where point detectors or sampling pipework are configured with a more standard spacing arrangement directly within the area to be protected.

Would it be acceptable NOT to implement a secondary detection configuration on the premise that the air conditioning is never switched off  and rely soley on the primary sampling configuration ?

Thanks.

[Ashley Wood]

Yes, that would be fine. However, if it is a very large area and the A/C should fail, it will take a considerable amount of time before the system is activated if you are relying on the natural movement of smoke. In a small area this is not really a problem. Being realistic about it, it is unlikely that the A/C will shut down and the area be totally 'powerless' due to UPS or generators. Also, these types of systems are very sensitive anyway, so even if there was a complete power failure the system should still respond faster than a point type detector. Just remember to put unions on the pipe that passes over the return air grills so that it can be removed for access to filter within the A/C!

[enzomorales]

Thanks very much for your reply, appreciate your time. The floor space of the area to be covered is approx. 300m2 but it's split across two floors with the return air coming through the first floor into the ground floor, then through a fan chamber and ducted back through the ceiling of the first floor.

With the primiary detection in front of the fan chamber on the ground floor, I guess if the fan stopped it would make signals from the first floor very difficult to pickup. However, given that the fan is never switched off it seems like considerable additional expense to implement a comprehensive secondary detection arrangement - I'm just concious of being complaint with the guidance documents I've read.

[Fishy]

Just a thought - if you're applying BS 6266 then the kit that you're protecting is regarded by the Client as pretty important.  Are they happy that the detection, suppression etc that they're paying for is all dependent on the reliability of a HVAC system?  

Is it never switched off for maintenance, for example?  If there are standby generators or UPS, is the HVAC on them?  If it is, and the power goes, does the HVAC automatically start up again, of does it latch off?

Personally, if they've decided to go to the trouble and expense of specifying fire protection to BS 6266, I don't readily understand why they'd be happy to compromise on compliance by saving a few quid on the secondary detection?  Of course, I know very little about the installation, so I'm ready to stand corrected...

[enzomorales]

Thanks for your feedback Fishy. I understand what your saying but costs are an issue - having
said that, if secondary detection is required then that's what'll be recommended.....They've not specified BS6266, they've just specified use of an aspirating system to meet insurance requirements. The HVAC equipment is on UPS and generator backup and is configured to autorestart after power loss. HVAC outage for maintenance would be, at the most, annually and only for a few hours.

[David Rooney]

Presumably there is no fixed extinguishing system and the room is 24 hour manned?

If I was the insurer I wouldn't accept primary detection alone. Computer rooms have a habit of changing, with new racks and equipment being brought in that can effect airflow and therefore detection.

Don't know how big the room is or how many A/C units there are, but often the A/C s are paired master/slaves and they alternate which one is running at any particular time - this might effect your air flow alarms.

Personally I would still recommend point detection to 6266 recommendations.