a question regarding VISIBILITY_FACTOR in FDS

[linpeng]

Dear ALL,

 

1) In the user guide, it is 3 for reflecting signs (default value used in FDS) and 8 for light emitting signs.

2) A visibility of 10m is generally adopted as one of the tenability criteria in FDS simulation.

3）Annex B of NFPA 130 says: "B.2.1.3 Smoke Obscuration Levels. Smoke obscuration levels should be continuously maintained below the point at which a sign internally Illuminated at 80 lx (7.5 ft-candles) is discernible at 30 m (100 ft) and doors and walls are discernible at 10 m (33 ft)."

 

My doubts are as follows:

1) If we use light emitting exit sign in a building, is it well reasonable to take 8 for VISIBILITY_FACTOR in FDS? As visibility depends on many factors, e.g. illumination in the room, how do you consider these factors in your FDS?

2) as 10m visibility is generally accepted, but If the exits sign is more than 10m, then it is still impossible to recognize the way of evacuation(EXIST SIGNS) even we keep a visibility of 10m in the enclose. What is your understanding for the 10m visibility? Is it to help evacuees to easily identify the surrounding obstacles during moving ahead, or to identify the way of evacuation (i.e. see the exits signs)? If the former, the background for calculating the visibility should be surrounding obstacles other than the exits sign? Is it correct?

3) What is your suggestion for general value for the factor?

 

Lin P.

Fire engineer

 

[Kevin]

Timo Korhonen of VTT, Finland, can comment on any "tenability
criteria" that may be applied in the FDS+Evac model, but as for FDS,
there is no such thing.

FDS predicts the mass fraction of soot from a fire whose soot yield is
specified by the user. That is all it does. All of the visibility-
related output comes right out of the SFPE Handbook. We did not
develop the factors of 3 and 8 that are used in the visibility output.
We include these parameters as a convenience to users. We leave it up
to you to choose and defend the values you use.

[Jason]

Linpeng:

Default C is 3, it is possible to input any credible value as you
want.

Kevin:

I got another question related to visibility influenced by user-
defined species.
As I know you linked the user defined "my smoke" to the visibility,
can I say: if I input an artificial soot concentration, visibility can
be calculated regardless of the fire-related soot production?

Following is copied from the Issue Tracker 356:

I added VISIBILITY as an output parameter. In the upcoming FDS 5.2,
you would say

&SLCF PBY=..., QUANTITY='VISIBILITY', SPEC_ID='my smoke' /

assuming that 'my smoke' is an explicitly specified species. This has
been committed
to the repository but not released. When the new release comes out,
can you test
this for me. Thanks.

Jason

[linpeng]

Thank Kevin and Jason for your responses!

So it is still an open question when we link this parameter with acceptable
criteria of 10m in visibility.

In accordance with Annex B of NFPA 130: doors and walls are discernible at
10m.
In my view, it implies that the visibility of 10m is to identify the
walls/doors (non-illumination objects) other than the exits, so default C of
3 should be taken for visibility of 10m regardless of the illumination
condition of exits.

Regards

Lin Peng

[Kevin]

We're now at 5.3. It has been released.

[Micael]

Linpeng:

If you have determined your tenability criteria to be 10m to doors and
walls I would change the default VISIBILITY_FACTOR to 2.3.

Regards
Micael

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