NATIONAL BUREAU OF STANDARDS REPORT

5200

A LABORATORY METHOD OF EVALUATING THE

EXTINCTION EFFICIENCY OF POWDERS ON SMALL SCALE

FIRES

by

C. S* McCamy and T. G. Lee

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002”30-If873 March 26, 1957 5200

A LABORATORY METHOD OF EVALUATING THE

EXTINCTION EFFICIENCY OF POWDERS ON SMALL SCALE

FIRES

by

Co So McCamy and T. G. Lee

for

Bureau of Ships

Department of the Navy

Code 538

Index Number NS -183-001

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A LABORATORY METHOD OF EVALUATING THE EXTINCTION EFFICIENCY

OF POWDERS ON SMALL SCALE FIRES

ABSTRACT

A method of evaluating the fire extinguishing

effectiveness of powders on a laboratory scale is described

and some experimental results are given. The method

involved dispensing powder in the air at a controlled rate

and slowly introducing a small heptane flame into the

falling powder stream under controlled conditions

Various rates of powder application were used to determine

the minimum effective rate, which is used as a measure

of effectiveness. The method has been used to study the

effect of particle size and chemical nature on effectiveness* Some data indicate that the results may not be

extrapolated to large fires.

1. INTRODUCTION

Various chemical and physical mechanisms have been

suggested to account for the extinguishment of fires by

the application of dry powders (1), Sodium bicarbonate,

in a specially prepared form. Is in common use and is

known among fire fighters as ,! dry chemical, M Apparently

sodium bicarbonate came Into use because it was believed

that upon thermal decomposition in the flame this powder

would release CO2 and smother the fire (2), This

explanation appears invalidated by quantitative experiments with sodium bicarbonate and experiments in which

fires were extinguished by materials to which the

explanation is inapplicable (3). As a result of more

recent work, the shielding of the fuel from the radiant

energy of the flame and the Interruption of combustion

chain reactions have come to be considered likely

mechanisms ( 4-) 0 Other studies have contributed data

related to the problem of determining the mechanism

of fire extinction by means of dry powders (5, 6,

7, 8, and 9) but the problem remains
- 2 -

unsolved. An understanding of this mechanism should

contribute not only to improvements in the technology

of dry powder but to a better understanding of fire

extinction in general.,

One method of attacking the problem is to measure

the effectiveness of various materials as extinguishing

agents and correlate the effectiveness with the chemical

and physical properties of the materials. Knowing the

properties of effective materials , one is better able

to formulate hypothetical mechanisms. This paper

describes a laboratory method of evaluating the

effectiveness of powders on a small scale and presents

some of the first results obtained. The method was

simple in principle. While powder is dispersed in the

air at a controlled rate, a small flame was slowly

introduced into the falling powder stream. In some

cases the flame was extinguished. A number of trials

were made, using different discharge rates, to determine

the minimum rate which would bring about extinction.

This minimum rate provided a basis for comparing the

effectiveness of various powders.

2. APPARATUS AND PROCEDURE

The essential features of the apparatus are shown

in Figure 1. The dispenser was a thin-walled brass

cylinder 5 cm in diameter and 7 cm in height, with a

fine stainless steel screen at mid-height, upon which

the powder was placed. The screen had 100 to 200 meshes

per inch, as necessitated by the particle size and flow

properties of the powder. The dispenser was connected

to an electromechanical vibrator of adjustable amplitude.

The rate of discharge of powder was governed by the

amplitude of vibration. To help maintain a uniform

powder load over the whole screen, a circular brass plate

1/4 inch thick, with a uniform pattern of 1/16 and 3/16

inch holes through it was placed on the screen. The

powder fell in a vertical stream 3=8 cm in diameter.

A brass cup having an inside diameter of 29 mm and

a depth of about 7 mm was used as a burner. Normal

heptane was chosen as a fuel because it is a fairly

representative constituent of gasoline, the most common

flammable liquid, but has a nearly constant burning

rate in a cup-type fire. At the beginning of each
-3

experimental procedure the cup contained 3*0 ml water and

1.0 ml heptane, leaving about 1 mm of the rim above the

fuel level

The burner was placed on a flat carriage which moved

horizontally at a uniform velocity of 4*8 cm/sec so that

the burner passed through the powder stream. After

leaving the powder stream, the burner passed beneath a flat metal plate which smothered the fire if it had not

been extinguished by the powder.

To measure the rate of discharge of powder, another

cup, identical to the burner cup, was placed on the

carriage two inches ahead of the burner cup and the

collected powder was weighed on an analytical balance.

To calculate the rate of powder application in gm/cm2 -sec

from the weight falling in the sampling cup, it was

necessary to take into account the size and shape of the

cross section of the powder stream, the size and shape

of the cup, and the motion of the cup through the stream.

The details of this calculation are given in Appendix I,

Previous work (4) had shown that the weight rate of

flow of powder in a free-falling stream was proportional

to the optical density of the stream at a fixed point.

To provide a continuous indication of rate of discharge,

the optical density was measured and recorded continuously,

using an incandescent lamp as a source, a simple lens

and slits to collimate a light beam 25 mm wide and 3 mm

in height, a photovoltaic cell, and a potentiometer

recorder. This system was calibrated periodically with

a series of light filters of known transmittance, mounted

on a revolving turret . The photometric system gave the

relative intensity of light entering the stream I Q and of

that transmitted by the stream I® By definition, optical

density D Is given by D = logyg The variations in

the rate of discharge were Indicated by the densitometer

record and an absolute measure of rate was obtained by

weighing the collected powder.

The entire apparatus was enclosed in a cabinet

equipped with a viewing window, exhaust blower, spark-gap

ignition source, air hose connections for humidity control,

a humidity sensing element, and controls for operating the

equipment without opening the cabinet. The humidity

could be controlled within a few percent at values in

the range from 10 percent to 90 percent.
A fixed procedure was established so that the conditions could be closely reproduced from trial to trial.

The procedure consisted of the following steps. The

dispenser was loaded, the burner was filled, the

densitometer recorder was started, and the burner was

ignited by an electric spark. The flame was allowed to

burn 30 sec before the powder discharge was started and

the carriage was set in motion. Since it took 5 sec

for the burner to reach the powder stream, the total

burning time before the trial was 35 sec. The operator

observed whether or not extinction occurred.

An effort was made to bracket the minimum effective

rate of application as closely as possible with a number

of trials at slightly higher rates and at slightly lower

rates. In some cases, all rates higher than a certain

value produced extinction and all rates below that value

failed. In most cases, however, there was an overlapping

of extinction and non-extinction points. The data were

analyzed by the statistical method given in Appendix II,

3, RESULTS ANT) DISCUSSION

The first experiments were done with sodium bicarbonate because it is the material ordinarily used for

fire extinguishment. The material was ground and sieved

to obtain samples of various average particle sizes.

Since the starting material was the commercially available

"dry-chemical

,

n the powders contained the additives

employed to improve the flow properties and resist caking.

The mean diameter of the particles in each sample was

determined from the measured air-permeability of the

sample. The variation of optical density of the powder

stream with rate of application of powder is shown in

Figure 2 for sodium bicarbonate having a mean diameter

of 26 microns. It may be seen that the optical density

was a linear function of the rate of application,

within experimental error. In this figure, the encircled

points indicate that extinction occurred, while the other

points indicate non-extinction. The existence of a minimum

effective rate of application is clearly indicated in this

figure. This corroborates the observation of such a

minimum in full-scale extinguishment tests under experimental conditions quite different from those used here (9)»

Much of the sprea.d of the data in Figure 2 is

attributed to variations in measured optical density
resulting from slight changes in the flow pattern when

the dispenser was reloaded or a new flow rate was set.

This source of error did not occur In the extinction

experiments because the weight-rate was measured byactual weighing for each trial. The slope of the line

in Figure 2 was used in correcting the measured rate

for any variation in rate between the time of the transit

of the sampling cup and that of the burner cup.

The data shown in Figure 3 demonstrate the effect

of two factors amenable to study by this method, particle

size and chemical nature® Each point indicates the

minimum effective rate determined from about 20 separate

trials, by the method of Appendix II, The results

show that, under the conditions of the experiments, the

effectiveness was greater for more finely powdered

materials than for coarse materials and potassium

oxalate was far more effective than sodium bicarbonate.

It is of some interest to convert the rate data from

a weight basis to an area basis, to investigate the

relationship between the effectiveness of a powder and

its surface area, but this Is not essential to the method

of measurement described here. With this transformation,

the curves of Figure 3 become nearly linear but the

minimum rate, on an area basis, does not appear to be

constant for these experiments®

It should be noted that the results of experiments

on the small scale employed here are not necessarily

applicable to much larger fires. In several large

scale fire tests, the differences in effectiveness

between various powders appeared to be considerably less

than that Indicated by small scale experiments o It

seems likely that extinction may Involve a number of

processes and that the process or processes mainly

responsible for bringing a fire under control may depend

upon the size of the fire® It Is believed that a full

understanding of the nature of these processes will

require experimentation with fires of various sizes.
6

APPENDIX I

Method of Calculating the Rate of Discharge of Powder from

the Weight Collected in a Cup

Let the cross section of the falling powder stream he

represented by a circle of radius a. Let the cross section

of the sampling cup be represented by a circle of radius bo

The sampling cup moves through the stream at a uniform

velocity v. The arrangement may be represented as in Fig 1„

If the powder is falling at a constant rate w,

expressible in gm/cm2 ~sec 9 the weight W falling on an area

A in time t is given by the equation

W = wAt.

In the present case, however, we do not have the

entire area of the cup receiving powder simultaneously

over a fixed time* The effective area and time must be

integrated. As an element of area of one quadrant of the

cup, we have

<ja = ./prrp

2

dy.

Any point in that element of area will be within one

quadrant of the circle representing the powder stream a

time t given by the equation

t = v

/7'2

ct 2

V
-7-

The weight would then be given by the integral

equation:

8w b

^(a^-y2

) (b 2 -y2

) dy,

the factor of 8 being introduced because each of the four

quadrants of the cup passes through two quadrants of the

stream* This is an elliptic integral and, therefore, can

not be evaluated in terms of elementary functions.

If we substitute bs for y, multiply and divide by a,

and multiply numerator and denominator by the integrand, we

obtain the following form, where k = b/a:

The first integral may be put into the canonical form

of the complete elliptic integral of the first kind by the

substitution s = sin 9. The value of this integral, denoted

by the symbol K, is tabulated as a function of k (see

reference 10)*

The other two integrals are known as the Jacobian

elliptic integrals of the second and fourth powers

respectively. They may be evaluated in terms of elliptic

integrals of the first and second kinds and the three

elliptic functions, all of which are tabulated (10).

In the present case, a = 1.90 cm, b = 1.45 cm,

and v = 4.75 cm/sec. Using these values we find W = 4„86w,

or to determine the rate of discharge w from the measured

weight W, we have:

w = 0o206W.
-8 -

APPENTIX II

Approximating the Best Value of the Minimum Effective Rate

Powder is applied to a specified flame at various

weight rates w expressed in gm/em^-seco The objective is

to determine the minimum weight-rate for extinction, wm .

The probability of extinction in one trial approaches

zero at rates much less than the minimum and approaches 1

at rates much greater than the minimum. The minimum

effective rate may be thought of as the rate at which the

probability of extinction is 0 * 5 ° The probability of

extinction E(w) and non-extinction l-E(w) may be

represented by curves such as those in Figure 1, although

the exact form of the curves is unknown..

Figure 1. Probability of Extinction E(w) and NonExtinction l-E(w) in a single trial.

In any group of trials covering the range about the

minimum effective rate, there is some rate at or below

which no extinctions occurred. If this rate were

determined for each of a large number of groups of trials

it would be expected that the values would fall in a

group about the most probable value, A. Similarly, there

would be some rate B representing the most probable value

of the rate above which no non-extinctions occurred.

Usually, no extinctions occur at rates below A and no

non-extinctions occur at rates above B, therefore for

the purpose of data analysis, we may make the simplifying

approximation that E(w) is 0 below A and 1 above B.

The foregoing theoretical representation is the basis

of the interpretation of a single group of observations.

The data for the group consists of a closely spaced series
9

-

of values of weight rate and an indication of whether or

not extinction occurred in each case.. Each extinction

point is evidence that the minimum rate is less than that

tried and may he indicated on the scale of weight-rates

by the “less -thann symbol < » Correspondingly, nonextinction may be indicated by the n greater-thann symbol > . A representation of one such group of trials is given in

Figure 2o

> > »><>—«-»—-«>< ><< « w

cl b

Figure 2„ Graphical Representation of One

Group of Trials

We find an actual rate "a” at or below which there

were no extinctions and a rate b at or above which there

were no non-extinctions » All evidence outside the interval

ab agrees that wm is in the interval ab; the data in the

interval admit wm in the interval but do not agree on the

location in the interval 0 If there were no points between

a and b we might assume a = A and b = B, but we have points

between a and b which may give some information about the

position of the interval ab with respect to the theoretical

interval ABo

The more the Interval ab is shifted toward high values

of weight-rate, the more probable are extinctions within

the interval o Therefore, in the data analysis, the more

extinctions there are among the trials in the Interval,

the more likely it appears that the Interval ab is shifted

toward high values and, correspondingly, the more it appears

that the best value of w m

Is toward the lower end of the

interval abo

It is not enough, however, to consider just the number

of extinctions or even the .ratio of the number of extinctions to the total number of trials within the interval

„

The position of the points is as important as their number

For trials at rates just slightly less than b there would be

a higher probability of extinction than there would be for

trials elsewhere in the interval., For this reason, greater

significance should be attached to extinctions occurring in

trials lower in the interval. 0 Likewise greater weight

should be given to non-extinctions appearing at higher

values in the Intervale Each point should be assigned a
-10 -

significance proportional to its probability of not

occurring, which is one minus the probability of its

occurrenceo

Not knowing the form of the probability function E(w),

we can not take into account the exact variation in

probability o For simplicity, we approximate E(w) by

a linear function which equals zero at A, and equals one

at Bo On this basis, the probability of extinction is

proportional to displacement from A in the interval AB„

Since A and B are the most probable values of a and

b, respectively, we take the interval ab to be the interval

AB, as a first approximation «, Then, by use of the frequency

of extinctions and non-extinctions, weighted on the basis

of approximate probabilities, we may interpolate wm in the

interval

o

Since we wish to take into account the number of

extinctions and their position on the w scale, and the

corresponding data on the non-extinctions, we form the

summations

:

j

x

n=l

(wn

-a) a < wn < b

and

k

y = s' (b-w ) e=l . e a < we < b

where wn j_ s a welght-rate at which non-extinction occurred,

j is the number of non-extinctions within the closed

interval ab, we is a weight -rate at which extinction

occurred, and k is the number of extinctions in the closed

interval abo The value of x is proportional to the number

of non-extinctions and their displacement on the w axis

away from a<, The value of y depends in a similar way on

the extinctions o With these factors accounted for linearly,

we combine the effects of x and y linearly so that nonextinctions tend to increase the value of wm

in the interval

ab and extinctions tend to decrease the value of wm , in the

following equation for the interpolation of wm between

a and b:

Wm. x

x*y a + (b-a)

o
"II

REFERENCES

1. Fryburg , George, Review of Literature Pertinent to

Fire -Extinguishing Agents and to Basic Mechanisms

Involved in Their Action* NACA Technical Note 2102

(1950).

2, NsFoPoA* Handbook of Fire Protection, Eleventh

Edition (195*0 j National Fire Protection Association,

Boston*

3* Lee, To G» and Shoub, H. , Extinguishment Efficiency of

Some Powders on Gasoline Fires „ NBS Report 2310,

March 2, 1953®

A. MaCamy, C, S* , Shoub 5 H. 5 and Lee, T* G* , Fire Extinguishment by Means of Dry Powder* Sixth Symposium

(International) on Combustion, Rheinhold Pub* Co.,

to be published.

5® Dufraisse, C,, and German, M. , Etude sur le mecanisme

de 1’ extinction seche par les poussieres. Compte

Rendus, 20Z? 1221 (1938).

6, Willbourn, A, H, 5 and Hinshelwood , C» N, , The Mechanism

of H2 -O2 Reaction, III* The Influence of Salts.

Proc* Roy* Soc* London Al85 q 376 (194-6),

7* Von Elbe, G* , and Lewis, B. , Mechanism of the

Thermal Reaction Between Hydrogen and Oxygen, J.

Chem* Phys, 10, 366 (194-2)*

8. Von Elbe, G* ? and. Lewis, B. 9 Combustion, Flames, and

Explosions* Academic Press Inc., New York, 1951®

9* Report of the Fire Research Board with the Report of

the Director of Fire Research for the year 1955? p* 23?

Her Majesty's Stationery Office, London, 1956.

Byrd, P. F, and Friedman, M. D., Handbook of Elliptic

Integrals for Engineers and Physicists; Lange,

Maxwell and Springer Ltd, , N, Y,

10 *
'
APPARATUS

FOR

EVALUATING

THE

FIRE

EXTINCTION

EFFICIENCY

OF

POWDERS

RATE

OF

APPLICATION

X

i0 3

gm/cm-sec

OPTICAL

DENSiT

FIG.? OPTICAL DENSITY OF POWDER STREAM VS RATE OF

APPLICATION FOR 26 MICRON NaHC0 3

O It) ?0 30 40 50

DIAMETER, MICRON S

MAM U ON AIR PF.RMfc ABILITY

FIG :*> MINIMUM RATE OTAPPI ICATION OF POWDER

I nil! XI INI i ION VS Ml AN 1)1 AMI IhR.RASF D

t u I All I'll 'Ml ANll I i i Ml A' .1 iitl ivii n r .

THE NATIONAL BUREAU OF STANDARDS

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4