Stratification

[Wiz]

Assistance required please gentlemen.
All I know about stratification (as in that likely to affect smoke from a fire) is that mentioned in clause 22.1 of BS5839-1:2002+A2:2008. I have been given the task to answer all questions raised by the following comments made by someone in respect of an installed fire detection system

There are 3 high level smoke detectors, their actuation may be delayed by smoke stratification. Calculations use 5 seconds as the time to AFD actuation, which may not be realistic at the best of times.
Smoke temp is said to reach 31 degrees C at 121 seconds, this is the time an evacuation should be complete.
But - smoke at 31 degrees C may not reach detectors for some time on a hot day, as ceiling temp could be around 31 degrees C

Can anyone throw some light on how the figures of 5 seconds, 31 degrees C and 121 seconds have been calculated and also any data on when and how it is ascertained that  smoke stratification is likely to occur in any given size of area or construction and/or use of that area. Or point me in the direction of where I might find this information.


Finally, the above mentioned BS clause (22.1) which mentions 'stratification' is only a 'Commentary' clause, and the actual 'recommendation' clauses following the commentary, do not, as far as I can tell, contain any specific recommendations created to deal with the problem of potential stratification. The commentary does suggest that additional detectors might be required to be sited at a lower level if stratification might occur; How might this be achieved in an area with 4 vertical walls and a horizontal ceiling (other than with beam detectors - which might be impractical)?

[kurnal]

Dr Wiz
It appears on first reading  that fire engineering calculations have been carried out to justify a particular design, probably a fire engineered solution, and one of the enforcement agencies has aked questions about the parameters used.

It is impossible to answer the detail of these questions. Whoever did the calculations appears to have assumed the time from initiation of the fire to the detection of smoke would be 5 seconds- surprisingly quick, and that the smoke temperature in the modelled scenario would only reach 31 degrees before everyone was outside the building.  The enforcement agency quite rightly has pointed out that smoke at this temperature may not even reach the ceiling on a hot day as ambient temperatures will be of this order. So its not a problem for the fire alarm designer or the installer to resolve- the fire engineer needs to review the whole basis for their calculations from the beginning.

If you go down to the library CIBSE guide E is a good starting point to read up on this but it really should not be your or your clients problem.

[Wiz]

Cheers for that answer Prof. K.

Out of interest, who would you guess has originally calculated the modelling, and is 31 degrees C a typical figure for the heat from a typical fire at ceiling level in a space (exhibition display area) with a ceiling height of 6 metres?

Has anyone else got anything to add to the Prof.'s comments (not that I don't believe him - he is my hero )

[kurnal]

It all depends on what is burning and how long it  has been burning for. What someone is  suggesting is that 121 secs into the fire they have modelled there will be no significant increase in temperature at ceiling level. Tha might be true for a very small fire indeed or one which is growing very slowly but we cannot comment without knowing what there is to burn and what ignition sources there are. To suggest such rapid detection will occur together with such a slow rate of fire growth appears on the face of it to be contradictory.

I believe the comments on stratification were a signpost to say go away and reconsider the problem not to try and design a fire detection solution based around the scenario.

[John Webb]

I am much inclined to the Prof's lines of thinking. In an exhibition area with 6m ceiling height I would have though detection times of tens of seconds would be more realistic. Faster-growing fires will presumeably be the more hazardous but on the other hand the plume temperature of such a fire will increase much quicker as well and overcome any stratification sooner. (On that score I have seen temperatures under a 12m flat ceiling/roof construction reach between 35/40 degC due to a hot sunny day outside the building.)

[SamFIRT]

All of the above is true. But the actual process of stratification is not just dependant on the convection heat from a fire but also on the phenomena of inversion. Colder more dense air at height in a building sometimes acts as a ceiling itself and prevents the less dense hot air from rising through it. This effect is enhanced if there is a trap in the ceiling shape creating an inverted pool where there is no ventilation to allow air movement and is therefore dependant on the shape of the building. You can for example have a situation where hot smoky gasses on the ground or first floor are unable to rise to the third floor due to this effect. This is sometimes believed to be caused by the cooling of the plume but it is not always the case.

What is true………. is that fire dynamics are very complicated……

[Wiz]

Thanks for all your answers guys, which I am finding most informative and enlightening.

There appears to be so many 'variables' and 'what ifs' that it is obvious that there must be no simple answer to the question of how 'stratification' might occur in any instance. Indeeed the commentary in BS5839 states; It is usually difficult to predict with any degree of certainty the level at which stratification occurs....

I wonder, therefore, how BS can provide a whole load of recommendations for the siting of detection and including recommendations in areas with mounting heights of upto 18m for the most common, i.e. point, detection on the basis that stratification won't occur when it now seems obvious that some people see stratification as a major issue in preventing the detection from working and also that it is difficult to predict when it will or won't occur.

It seems to me that either BS recommendations should be valid for mounting heights of only up to such height that is determined to be that which stratification is very unlikely to occur from the type of fires most likely to occur, or there should be more recommendations in BS confirming how the problem can be overcome rather than the very basic information contained in the Commentary; supplementary detection may be provided at lower levels in the hope of detecting the stratified layer .

Alternatively, is it possible that the 'what if' scenarios of stratification are so unlikely, or the modelling requirements for calculation so onerous, that the likelihood of it being a real problem in a system designed to all the current BS recommendations, is so small that it should be ignored for most circumstances?

[Golden]

Could I ask whether this system is for an engineered solution and if it is protecting some kind of special risk or as a compensatory factor? When has a 6m ceiling been 'high level' - the CIBSE guide talks about 10.5m as a limiting ceiling height.

There seems to be a whole lot of figures being bandied about in this query however I do agree that ceiling temperature (dependent on ceiling construction/air handling etc.) could be above 31^o - I would also be interested to know what design fire is being used to get the figures of 31^oC after 121 seconds. An interesting post!

[Wiz]

Golden, the smoke detectors in this area are part of an L1 system and have all been sited as per the recommendations of BS5839-1. As far as I know they are not installed specifically or in a particular manner as protection of some kind of special risk or as a compensatory factor.

I'm interested to hear your comments about the mounting height and the CIBSE guide. Are you saying that the CIBSE guide infers that the likelihood of smoke stratification is only at 10.5m and above? If so, can you provide more details?

The details in my original post in italics are as written by a Fire Officer. The document was handed to me for my opinion by a fire system installer who had not even heard of the term 'smoke stratification'. I have told him about the basics of stratification of smoke but could not answer anything about the facts and figures quoted in the document. Prof. Kurnal has given me the impression that, at least some of, the facts and figures in the document have obviously been calculated by an unkown third party and the Fire Officer is using them to warn of the likelihood of smoke stratification. I have had no input into the design or installation of the fire detection system.

[kurnal]

No Dr Wiz to be clear I am saying the fire officers comments have been misunderstood. I am sure the fire officer was NOT saying go away and install a fire alarm system that takes stratification into account, he was saying go away and tell the fire engineer who came up with these figures to *$%^  &$$ undertake a formal review of his objectives methodology and parameters to determine whether they are appropriate.

Stratification may be a total red herring here. What we need to know is this- for the likely fire scenario and growth how long will it be before the fire is detected. That is the job of the fire engineer not the fire alarm engineer.

In simple terms as I read it the Fire Officers view was this :
 "The fire engineer has presented a report that says we needent worry about what happens in the bui,llding in a fire because the alarm will be raised early and people will have enough time to leave before it gets hot and smokey. The trouble is that if we were to believe the report  presented by the fire engineer and the numbers he chose to make his case, it is actually likely that on a warm day the smoke will never ever reach the detector so the alarm will never sound and the chance therefore of anyone hearing the alarm  within 5 seconds, responding and leaving the building within the 121 second evacuation time that you claim is cloud cuckoo land sunny jim"

[Wiz]

Thanks prof. K.. Actually, my earlier understanding of the comments was basically how you have subsequently described it, and not anything else.

Obviously someone came up with the figures presented to the Fire Officer and I have no clue who that was. I certainly don't think it was the fire aalrm system installer (who had never heard of smoke stratification anyway!)

[Phoenix]

I think kurnal has got to the root of this problem quite succinctly (as usual) but here is some peripheral information. 

There will be no stratification in a room 6m high though, of course, it will take longer (as said above) for smoke to reach a detector at the ceiling.  A smouldering fire might smoke log the space without the detector being reached but that is not strictly the same as stratification. 

There is a model for stratification above an axi-symmetric fire that is quite simple to apply but even with with a very steep temperature gradient across the height of the space (i.e. where ambient air is much warmer at the top of the space compared with the base) typical values for the height that smoke will rise to are well in excess of the 10.5m mentioned above.  For example, for a small 50kW fire in a 10m high room where ambient air is 20 degrees hotter at the ceiling than at the base (which is, frankly, much worse than would realistically be expected), the model predicts that smoke will rise to the full height of the room.  What the model doesn't tell you is how quickly it will get there.  For that, I would say you would need CFD (or a full scale test!).

There would have to be unusual air movements around the top of the space to maintain the inversion mentioned above.  An inversion of cold air above warm air in the still air inside a building is as likely as a brick floating.  Having said that, HVAC systems could maintain the phenomenon.

Out of interest, I tried many fire growth scenarios using different fire growth rates, radiative heat loss coefficients, air entrainment coefficients, etc and I could get close to 31 degrees at 121 seconds but not spot on.  It would seem about right for a slow to medium fire growth rate in such a space.

Coming back to practicalities and solutions, I'm surprised no one has suggested the use of beam detection angled diagonally down through the space to cover the maximum volume possible - possibly the use of two - to give the earliest detection and warning.  Or an aspirating system.  Point detection is problematic anyway on high ceilings, how do you get to the things to test and maintain them?  I've seen fun with cherry pickers, cracking marble floors and breaking glass channelling screens.  I've even come across people dangling out of loft access hatches to reach the things.

Stu

[CivvyFSO]

Just to back up Stu's version:

Using FPETOOL, a slow fire gets to just less than 31C in 121s and a medium fire gets to just over 31C in the same time. Different methods get different results regarding the temp and velocity of the plume, i.e. Mowrer and Alpert, and I don't know which method FPETOOL uses but it would probably explain any such difference.

The person stating that if the smoke is the same temp of the room that it would stratify is almost right, but no account there has been taken of the temperature being an increasing value which would quickly pass the 31C mark.

I personally think that you have two problems here: the fire engineer not doing his job right, and the approving body possibly not having a proper grasp of what is going on.

[Wiz]

Thanks Pheonix and Civvy FSO for the additional interesting and informative comments.

To pick up on a couple of things that Stu mentioned;

(1) Using 'angled up/down beam detection' - is this a common solution to the problem of providing additional low-level protection in the UK? I personally have never seen it, but I did find it mentioned in a document of USA origin:

http://www.clubs.psu.edu/up/ashrae/Archives/Events2003-2004/Klote-PSU-Talk.pdf

I have never seen the possibility of using angled up/down beams mentioned in manufacturer's literature or BS5839-1 even though I can see that it could work and could be a good solution to situations where smoke stratification is likely.


(2) The difficulties of reaching point detectors mounted at height are well known. However, if we are talking about the most likely scenario, after installation, of just removing and replacing detectors, this is not normally too much of a problem at 6m. Obviously using beam detection can be a good option, but it is expensive. This is especially true when supplementary beam detection is also required at lower levels since the spacing between beams needs to be less. It is quite easy to end up with four beam detectors in a space where one might do the job before smoke stratification comes into the calculations

[Wiz]

No Dr Wiz to be clear I am saying the fire officers comments have been misunderstood. I am sure the fire officer was NOT saying go away and install a fire alarm system that takes stratification into account, he was saying go away and tell the fire engineer who came up with these figures to *$%^  &$$ undertake a formal review of his objectives methodology and parameters to determine whether they are appropriate.................

Upon reading your post again, Prof. K, I would now add something that I hadn't previously mentioned. The author of the comments had preceded them with the comment;  We were told this would have BS5839 L1 cover, this is not the case .

Do you think it is right that the fire alarm system designer is being accused of totally failing to meet BS5839-1 on the basis of the possibility of smoke stratification not being taken into account by the fire alarm system designer?