NATIONAL BUREAU OF STANDARDS REPORT

5972

A METHOD FOR APPLICATION OF POWDERS TO TEST FIRES

By

A. F. Robertson

and

T. G. Lee

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U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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national bureau of standards report

MBS PROJECT NB8 REPORT

1002-11-^8 73 July l4, 1953 5972-

A METHOD FOR APPLICATION OF POWDERS TO TEST FIRES

by

A. F. Robertson

and

T. G. Lee

Report to

Bureau of Ships

Department of the Navy

Code 538

Index Number NS-183-001

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<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
. ' . 3 " ' - I * ; ' J

V ' ’ ; • •* • • «
A METHOD FOR APPLICATION OF POWDERS TO TEST FIRES

ABSTRACT

A device is described which permits controlled application

of dry powders to flammable liquid test fires* Measurements

have been made with its use in studying the relative extinction

efficiency of commercial dry chemical and potassium oxalate

mo no hydrate* It is shown that the relative quantities of these

powders required for extinguishment of 6-in. diameter n-heptane

fires are quite similar to those previously found necessary for

fires of about 1-in. diameter*

INTRODUCTION

A previous report, No* 5200, described measurements which

were made to determine the extinction efficiency of commercial

dry chemical (a treated sodium bicarbonate powder) and potassium

oxalate monohydrate for the extinction of heptane fires in cups

of about 1-in* diameter* It was shown that for a given powder

i. article size, the weight rate of application required for extinguishment was markedly less with potassium oxalate monohydrate

than with sodium bicarbonate* Because of this, it seemed likely

that differences in the physical chemical properties of the powders

might be responsible* This possibility encouraged the hope that

improvement could be made in the extinction efficiency of dry

chemical extinguishers. However, it was recognized that the results

obtained had been secured with the use of very small fires and it

was considered likely that similar effects might not be observed

with larger fires*

The work reported here was performed in an effort to secure

some information on the effect of changes in fire size on the

relative effectiveness of the two powders. The chief problem involved in performing this work was the development of a device

which would permit controlled application of powder to the fires.

The device desired must be of such a nature that; (l) the weight

rate of application could be precisely controlled in a continuous

fashion over a wide range, and (2) the end effects of the nonuniform portion of the discharge during starting and stopping must

be short in time. The second requirement is obviously desirable

to obtain a long period of uniform discharge characteristics*

2

APPARATUS AND PROCEDURE

After several unsuccessful attempts to construct a powder

dispenser, a device was built which showed good performance., This

device is shown diagramatically in Figure 1. The powder, held in

container A, was fed by means of a serrated rotor to a short

passage Co Air entering both ends of this passage was used to

strip the powder from the serrations and feed it through a passageway

to the nozzle Do Since the speed of the rotor could be changed, by

means of a variable speed drive, it was possible to regulate the

rate of flow of powdero Figure 2 illustrates the arrangement used

for studying the behavior of powders projected by means of this

uni to A clock-driven timer was used to control the starting and

stopping of the motor and thus the discharge of powdero In order

to further reduce the effects of transients during discharge of

this powder, a solenoid operated powder diverter through which the

powder was removed by means of a vacuum system, was used to limit the

duration of powder application..

Calibration of the dispenser operation was accomplished by the

use of a cellulose extraction thimble within the diverter heado

This thimble provided a means for collecting the powder discharged

during the time interval the diverter was positioned before the discharge nozzle o Discharge measurements were made under two different

air flow conditions involving supply pressures of about 2 lb/in. ^ and 7° 5 lb/in 0 2 gage, This resulted in low and high air flow rates

through the dispenser of respectively 350 CoCo/sec and 750 CoCo/sec

prior to initiating the powder discharge process. During the discharge these air flow rates were considerably reduced.. The amount

of reduction depended on the rate of powder additions.

Extinguishment studies were performed with a fire of 30 ml

n-heptane floated on water at room temperature contained in a 6-in,

inside diameter brass container having 1/8 in, walls. This container was 4-ino high and was fitted with a horizontal flange which

was 12 in, in diameter. During tests, a freeboard of 1/8 in, was

maintained above the water surface and a preburn period of h-5 sec

was . used. Powder application to the fire was limited to 1,8 sec.

Calibration checks we re performed immediately following each

extinguishment test.

The dry powders used for study were prepared from a commercial dry powder which was separated into various particle size ranges by means of an _elutriator . The potassium oxalate monohydrate powders

were ground in a ball mill with 2 percent, by weight, of zinc

stearate to promote free flowing behavior. These, again, were separated into various size ranges by sieving and elutriation. The particle sizes reported were determined with a Fisher Subsieve-Sizer based on air permeability principles.

- 3 -

RESULTS AND DISCUSSION

Figure 3 presents results obtained from calibration tests of

the dispenser unit. At low speeds the powder discharge rate was

found to be a linear function of rotor speed* At higher speeds

the air flow rate became critical and for low air flow rates powder

discharge became nearly independent of speed* At all speeds the

reproducibility of the data was not as good as would have been

desired* Five percent variations in discharge rate were common and

at low speeds greater variations were observed* In spite of these

difficulties, the dispenser was considered highly successful*

Experience gained with its use encourages belief that an improved

air supply system can materially Improve the efficiency and thereby

reduce the variations observed*

A limited number of tests have been performed to compare the

application rates required for extinction of fires with both

sodium bicarbonate and potassium oxalate monohydrate powders*

The results are shown in Figure Here the weight of powder discharge is plotted against particle size of the powder* All powder

applications which resulted in extinguishment have been encircled*

The unencircled data represent unsuccessful extinguishment attempts*

The dashed lines have been replotted from Report 5200* In doing

this, a scaling constant was used to permit plotting the two curves

In approximate agreement for powder particle size of 16 microns*

The difference in techniques used for powder application obviate

an exact comparison of application rates* What does appear of

interest is the fact that the relative quantities of the two powder

types required for extinguishment seem to remain fairly independent

of fire size In the range of 1- to 6-in* diameter* *

It is possible that the similar behavior of the extinguishing

agents on the two fire sizes provides some evidence that the

burning behavior of the fires are not significantly different* This

would, be surprising but may be possible* The fact that the dispensing unit tested has performed so well makes it seem quite practical to build a still larger unit which can be used for full size

fires outdoors*

SUMMARY

1* A dispensing device has been constructed for application of

powders to fires and can be used successfully at application rates

as high as 9 gm/sec*

2* During application of two different chemical powders to fir-p,r

. of 6-in* diameter , similar .difjTe prices in application rates were observed as had been previously obtained for much smaller fires*

C MM-NBS-DC

POWDER CONTAINER

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FIG. ! - DIAGRAMMATIC VIEW OF

DISPENSER CONSTRUCTION

il

FIG. 2-

GENERAL

ARRANGEMENT

FOR

EXTINGUISHMENT

TESTS



DIAMETER,

MICRONSBASED

ON

AIR

PERMEABILITY
u' /* .... • * , - r • - L
u. s. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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