Natural Smoke Vent

[David Rooney]

ADB and all the guides I can find all say the same thing ....

"On detection of smoke in the common corridor/lobby the vents on the fire floor, the vent at the top of the smoke shaft and to the stairway should all open simultaneously. The vents on all other storeys should remain closed."

.... I've always understood this to mean that all other vents (aka smoke shaft doors) on all other floors than the fire floor should be "locked out" so that even if smoke drifted to another floor the vents should not open all the time the original fire floor vent was open.

Was talking to a Smoke Vent manufacturer today whose system will open up any vent on any floor that a fire is automatically detected. So you could end up with multiple smoke shaft doors open on multiple floors (as well as the smoke shaft and stair vents).

I can't find any other guidance in 7346 / ADB / FETA guide of Flats Guide ...... any ideas please  

[kurnal]

A few thoughts Dave and I would invite any others to correct my comments and / or add to them.

It has always been assumed that there will be only a single fire in the building at any time.

It has always been assumed that the fire will be in a flat and that the compartmentation and fire doors will play their role in containing the fire and any smoke leakage into the lobby will be cool and not very buoyant.

It has always been assumed that fire compartmentation is in accordance with the building design strategy and that therefore every floor is a compartment floor and all services are in protected shafts, thus minimising the risk of leakage of smoke between floors.

The BRE shaft approach was never proved scientifically to my knowledge, there are many newer approaches that have been fully engineered,though many of the parameters against which they are measured are considered by many to be iffy.

If intermediate doors into the smoke shaft were opened then this may overcome the weak convective forces and prevent the system working, there should be no risk of intermediate levels being smokelogged if everything else is as it should be BUT can we rely on this with poor standards of maintenance, widespread use of uPVC cladding etc.

The staircase vent provides make up air for the natural vents in the smoke shaft.

The smoke vents are widely used for ambient temperature control nowadays creating C&E  challenges for fire alarm and heating engineers, wish they were all as diligent as you!

Sorry not to have any definitive answers for you, I have always seen the BRE smoke shaft solution as you have and believe it is intended to work with a single door open.  If this is deemed inadequate then an engineered solution should be devised. After all the BRE shaft is huge and much more compact systems can be provided nowadays especially using fans.

I hope others will now contribute and if appropriate correct the topic. Have you checked with CP3 and it's later versions BS5588-1 and BS9991? I am on my travels at present so can't remind myself if they are more helpful.

[jokar]

http://www.feta.co.uk/associations/hevac/specialist-groups/smoke-control-association

[David Rooney]

Thanks Kurnal

What you say all makes sense .... if as you say, everything is perfect and works as it should!

The more I look into this the more I am amazed how vague some of the guidance is.

Thanks Jokar, I've read that and it just repeats the ADB line .... 

On detection of smoke in the common corridor/lobby, the vent(s) on the fire floor, the
vent at the top of the smoke shaft and to the stairway should all open simultaneously.
The vents from the corridors/lobbies on all other storeys should remain closed.

What no guidance goes on to say is what happens if smoke is detected on another floor or indeed if there are manual overrides on each floor as I've seen in some places if these should be locked out after detection of the first fire or if they should be available to be opened - potentially by the public - compromising compartmentation between floors.

[wee brian]

From the SCA Guide

6.2.7.1 Minimum control requirements
The minimum control requirements for natural ventilation are set in ADB:
Design should be based on a single floor level being affected by the fire and therefore only the smoke vents on the floor of fire origin and any other design critical vents (such as the head of the smoke shafts and staircase) are required to open. System designers should avoid opening ventilators on multiple floor levels, especially where connected by a smoke shaft, to avoid smoke spread to otherwise unaffected parts of the building, and/or reduction of ventilation rate from the floor of fire origin.

[wee brian]

The BRE shaft approach was never proved scientifically to my knowledge, there are many newer approaches that have been fully engineered,though many of the parameters against which they are measured are considered by many to be iffy.

Two fairly detailed studies using physical scale models and CFD. Not sure any of the other approaches can claim the same.

https://www.bre.co.uk/filelibrary/pdf/rpts/partb/Smoke_Ventilation.pdf

http://www.brebookshop.com/details.jsp?id=140333

[kurnal]

Thanks for the links Wee B - now I recall you have posted them before and it gives technical credibility to the BRE shaft solution and emphasises that the purpose of the lobby ventilation is to protect and keep the staircase clear of smoke. Sorry for forgetting your previous contributions.

As the whole purpose is to protect the staircase, and the tests indicate that tenable conditions will not be maintained in the lobby do we have a definitive answer to Daves question? All the tests were carried out with a single door open, there is no test data to show the effects of opening multiple doors if we are still assuming a single fire.

The system is designed to operate with a single door open into the shaft on the fire floor and any other scenarios should be supported by a fully engineered design. Such scenarios may be prompted by a misunderstanding that the system is intended to keep conditions  tenable in the lobby.

Anyone disagree?

Worrying to see the abuse of buildings designed to this standard though.....the BBC and LFB photos in support of the current tumble dryer debate seem to show some worrying external fire spread?

[wee brian]

I'm pretty sure the extract from the SCA guide answers Dave's question.

[Tom Sutton]

I think the Smoke Vent manufacturer has produced a system that does not conform to the accepted guidance, therefore is in the wrong, you should ask him what guidance he has used.

Or are you saying the accepted guidance is wrong, then this is a different question?

[Phoenix]

Why would smoke get into another lobby?  Some sort of failure, I guess.  Well, if smoke does get into another lobby and the vent to the shaft does not open in that lobby then the situation will not be great and further failures can be expected.  If a second vent does open in a second lobby then there's a reasonable chance that the smoke will still be drawn into the vent and up to the top of the shaft from both affected lobbies.  There's always the venturi effect at the top of the shaft and the general stack effect working to shift gases up the shaft.  It's not just reliant on the buoyancy of the smoke.

With two vents open to a shaft, the upwards velocity of gases within the shaft will not be reduced but the draw at each vent opening will be approximately halved. 

It seems to me, admittedly after very brief consideration, that if a vent opens in a second lobby then there is a chance it will keep the staircase clear, but if the vent in the second lobby stays shut then it is likely that some smoke will find its way into the staircase.

As with most things in fire safety, there are many other variables involved in any specific case and what works in one set of circumstances may fail in another.  I wouldn't worry too much about it.  Let both systems exist and one day someone might take it upon themselves to collect data on the performance of the different designs.

The more I look into this the more I am amazed how vague some of the guidance is.

Are you really amazed??
 

[David Rooney]

Thanks men .....

I'd not really taken that paragraph in Wee Brian so thanks again .... which would imply that Tom is right in that the "system" is wrong ..... except Phoenix makes a very good point for allowing other vents to open making it ok.

No wonder there are so many interpretations around!!

So the ultimate question is does this design meet 7346 and would you all be happy to accept a certificate to that fact?

[wee brian]

Why would smoke get into another lobby? 

Trouble with smoke, is that it gets everywhere. you don't need much to trigger the detectors.

No I wouldn't accept it unless it followed the SCA guide.

[col10]

TDs may have been increased due to AOV, in which case, "locked out" seems wrong.

[David Rooney]

Personally it makes sense to me that only the fire floor vent should open to the shaft and that all other events to the shaft remain locked out and closed.

If somehow smoke does manage to penetrate another lobby further up and open that vent to the shaft as well surely the problem of the fire potentially using the shaft to jump between floors is a far greater issue than some smoke in the higher lobby ?

 

[Jim Scott]

Personally it makes sense to me that only the fire floor vent should open to the shaft and that all other events to the shaft remain locked out and closed.

If somehow smoke does manage to penetrate another lobby further up and open that vent to the shaft as well surely the problem of the fire potentially using the shaft to jump between floors is a far greater issue than some smoke in the higher lobby ?

 

Exactly this happened at a block I performed an FRA on years ago.  It was due to vandalism in this case, but resulted in rescues.  If these rescues were actually needed is another story, although it proved the point.