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Inert gas fire extinguishment: excessive high-level vents
INERT GAS
FIRE
EXTINGUISHMENT: EXCESSIVE HIGH -LEVEL VENTSSUMMARY
A previous publication specified critical a r e a s of high-level vent which, i f exceeded, would not allow reduction of the oxygen concentration in buildings t o zero. In fact, many building f i r e s will be satisfactorily extinguished if the oxygen concentration i s only reduced t o about 9 p e r cent t o 15 p e r cent. En this note i t is shown that vent a r e a s may exceed the c r i t i c a l values by a factor of 2 or even 4 under favourable circumstances.
A recent publication (1) discussed the l a r g e - s c a l e u s e of i n e r t gas for the extinguishment of building fires. It was shown that the limitation of this technique will usually be concerned with l o s s of the injected inert gas from high-level openings. An expression was given ( s e e Appendix I) for the maximum permissible a r e a of
high-level openings that will ensure that a l l a i r i s excluded from the building.
Where high-level openings exceed the c r i t i c a l value, a i r will enter the compartment through low-level openings (which a r e assumed t o be prevalent) and will mix with the inert gas t o give a virtually homogeneous mixture throughout. The object of this note i s t o extend the previous analysis t o cover this condition.
The analysis i s made in Appendix I, and the r e s u l t s a r e presented in Figure 1. Two types of gas and t e m p e r a t u r e condition a r e considered. In addition to the "N. R. C. gas" (i. e. one consisting of products of combustion and a l a r g e proportion (>60 p e r cent of water), a hypothetical inert gas with a molecular weight equal t o the
m e a n value for a i r is considered. It constitutes a limiting c a s e , giving t h e c u r v e on t h e e x t r e m e left of F i g u r e 1. C u r v e s for a l l other g a s e s and t e m p e r a t u r e conditions would l i e to t h e right of t h i s curve.
T h e f i r s t of t h e two t e m p e r a t u r e conditions c o n s i d e r e d i s
constant t e m p e r a t u r e which m o r e a p p r o p r i a t e l y r e p r e s e n t s t h e i n i t i a l s t a g e s of injection into a building heavily involved in f i r e and a l s o t h e u s e of a g a s including a substantial proportion of w a t e r vapour a t a t e m p e r a t u r e n e a r 100" C. In t h e l a t t e r c a s e , h e a t t r a n s f e r t o diluent a i r r e s u l t s not s o much f r o m cooling of t h e i n e r t g a s as f r o m condensation of a s m a l l p r o p o r t i o n of the water vapour. T h e second t e m p e r a t u r e condition a s s u m e s that t h e t e m p e r a t u r e of the g a s i n the f i r e e n c l o s u r e is t h e a r i t h m e t i c m e a n of the i n e r t g a s and diluent a i r t e m p e r a t u r e s . T h i s assumption l i b e r a l i z e s vent r e q u i r e m e n t s . In fact, t h e e x t r e m e right -hand c u r v e of F i g u r e 1 is not valid f o r i t d i s r e g a r d s t h e heat contribution given by t h e condensation of w a t e r vapour
.
A s both the e x t r e m e c u r v e s of F i g u r e 1 a r e only of somewhat a c a d e m i c i n t e r e s t , and as g r e a t a c c u r a c y is not c a l l e d for, it is r e a s o n a b l e t o m a k e a generalization f r o m the figure. T h e oxygen content of a building need v i r t u a l l y n e v e r be lowered below 9 p e r c e n t even t o s u p p r e s s smouldering combustion, and a t m o s p h e r e s with 12 t o 15 p e r cent 0 will usually inhibit f l a m i n g combustion. It c a n t h e r e f o r e b e conclu
f
ed that high-level vent a r e a s m a y b e allowed to exceed t h e c r i t i c a l value by a f a c t o r of between 2 and 4, depending on whether smouldering combustion is involved.A s c r i t i c a l high -level vent a r e a is l i n e a r l y r e l a t e d t o volume r a t e of injection of i n e r t gas, t h e above s t a t e m e n t might b e m o r e a p p r o p r i a t e l y o r i e n t e d a s follows. T h e capacity of a n i n e r t g a s g e n e r a t o r need only b e 1/2 t o 1/4 that specified by t h e c r i t e r i o n concerning high -level vent.
REFERENCES
1 . McGuire, J. H. L a r g e s c a l e u s e of i n e r t g a s to extinguish building f i r e s . T h e Engineering Journal, Engineering Institute of Canada, Vol. 48, No. 3, M a r c h 1965, p. 2 9 - 3 3 .
APPENDIX I
-
EXCESSIVE HIGH -LEVEL V E N T S
It has been shown that the relation between
high-level l o s s e s f r o m a building and high-level vent a r e a i s :
A
=
.
.
.
(1)T
0
The symbols used in this Appendix a r e defined in Appendix 11. The upper limit of a r e a which will still ensure that a l l a i r excluded fr o m the building i s :
T o proceed further, expressions must be derived for v, m and
T1.
In comparing A and Ac it will be assumed that the s a m egas generator i s involved s o that the r a t e of injection of i n e r t gas i s the same. Since v and v a r e quantities normalized to the s a m e
G
temperature, i t ther efor,e follows that
Use of the factor n i s v e r y convenient since i t follows f r o m i t s definition that the oxygen concentration of the atmosphere in the f i r e enclosure will be 21n p e r cent. It i s a knowledge of this quantity in t e r m s of A/A which i s required.
C
Two conditions of temperature will be of interest, one being where dilution of the atmosphere in t h e f i r e enclosure by a i r has little influence on t e m p e r a t u r e or T t o 'F. Substituting t h i s a n d expressions ( 4 ) and ( 5 ) in expression ( 3 ) gives:
If i t is assumed that the diluent a i r cools the atmosphere in the f i r e enclosure, then in evaluating T * it is convenient t o a s s u m e that different gases, at constant p r e s s u r e , have the s a m e molar heat. The gases of interest in the present context will range from diatomic with 5 degrees of freedom t o polyatsmic, with vibrating atoms, with 8 degrees of freedom, which will have a molar heat (at constant p r e s s u r e ) some 40 p e r cent higher. T h e most valuable justification f o r the assumption, however, follows f r s m ' t h e end r e s u l t of the numerical calculations, when it will. be seen that (a ) the assumption of diluent cooling does not have a great effect on values of A/A and
(b ) this latter effect p e r m i t s greater r a t i o s of A/A and in o r i e r to c r
ensure f i r e extinguishment it i s best assumed not to occur. As surning invariant molar heat,
T h e assumlption of diluent cooling implies that the gas t e m - p e r a t u r e i s little affected by heat transfer from the walls of the
enclosure. This assumption i s implicit in expression (7 ) and will
be taken t o apply t o expression ( 2 ) s o that in ( 2 ) and (3) T
=
T~
'
Using this information and expresaiotxs ( 4 ) and ( 5 ) in expression ( 3 ) gives
APPENDIX I1
LIST O F
SYMBOLS
T
=
Absolute ambient temperature of external atmosphere.0
T~ r Absolute inlet t e m p e r a t u r e of i n e r t gas.
T r Absolute t e m p e r a t u r e within f i r e enclosure ( c r i t i c a l c a s e ) .
T' r Absolute temperature within f i r e enclosure when dilution with
a i r i s occurring.
m z Mean molecular weight of air.
0
mG
=
Molecular weight of inert gas.m
=
Mean molecular weight of gas in f i r e enclosure.n x Proportion (by volume) of a i r in f i r e enclosure :(OC n< I ) .
A
=
C r i t i c a l a r e a of high-level vent.C
A
=
Area of high-level vent.k
=
Constant, itself dependent on height of building.r Volume r a t e of l o s s of i n e r t gas, normalized to a temperature
VG
T
0'
v
