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R. L. TUVE and H. 5. PETERSON Chemistry Division, U. S. Naval Research Laboratory, Washington 25, D. C.
Characterization of Foams for Fire Extinguishment From an evaluation of three methods of application . . . Surface flowing Subsurface injection Particulate showering
THE
wide variation in conditions surrounding accidental or unwanted combustion generally discourages attempts at standardization of control methods. Nevertheless, the huge petroleum fuel handling program of the Armed Forces requires continuous, efficient standardized protection from fire if the needs of a peacetime (or a wartime) mobile military force are to be met with any measure of economy. Water is the most economical and least toxic of all fire-fighting chemicals for liquid petroleum fuel fire protection. However, the specific gravity of water must be changed by the addition of a suitable surfactant in intimate association with gases to form comparatively
stable foam. This extinguishing agent, although demonstrating many caprices, is capable of analysis of variables in the extinguishment system if its method of application to the burning fuel is considered separately from other conditions. Of the two principal types of foams used in fuel fire extinguishment a t the present time, the earlier developed chemical foam, produced by an effervescent type chemical reaction with bubblestabilizing substances present, has given way to mechanically formed foams which have many advantages in practical usage. Mechanical foams are nearly independent of temperature and reaction time requirements and thus become more flexible of application. They re-
quire much less volume and weight of material per unit of fire-fighting agent. and the employment of the liquid concentrates necessary to form mechanical air foams is performed more simply and accurately than is the proportioning of dry powdered chemicals into water. Since 1941 the fire protection of petroleum fuel hazards in the U. S. Armed Services has depended almost entirely on the use of mechanically formed air-water foams. Discussed here are the methods that have been devised to test and relate the physicochemical properties of air-water foam emulsions of the hydrolyzed protein type, so that optimum fire extinguishment performance on gasoline fires may be specified.
Early Investigations of Foam Testing Methods Amsel (7) and Brunswig ( 3 ) in Germany devised a series of ingenious laboratory test procedures for fire extinguishing foams, which resulted in empirical values for the heat stability of foams and their fluidity and specific gravity. The objecrive of this work was to obtain a foam index value or superiority classification for foams generated by existing devices. By correlating the results of testing of foams on small-scale and large-scale fires in fuel tanks, Amsel found the following relationship for evaluating a foam using surface-flowing addition of foam.
PRE M I X F 0 A M
where h = heat stability of foam s = foam specific gravity z = fluidity or viscosity of foam
Figure 1.
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Laboratory mechanical foam generator
INDUSTRIAL AND ENGINEERING CHEMISTRY
Clark (4, by performing an exhaustive series of experiments and using carefully calibrated laboratory tests, greatly enlarged our knowledge of the role contributed by foam fluidity, fuel resistance, stability in the presence of heat, foaming ability, and surface film characteristics
AQUEOU5 F O A M 5
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of various fire-fighting, foam-forming materials. In Clark’s work, as in that of Amsel, Brunswig, and Friedrich (6), the net objective was the comparative evaluation of fire-fighting ability of a foam generated in a foam-making device in general use. The figure of merit test worked out by Clark is still used with some modifications ( 8 ) as a government evaluation test for candidate foam materials in England. During the same time period from 1941 to 1945, a procurement specification for foam-forming materials was independently developed in the United States (TI) by U. S. Navy authorities. This empirical test specification was of necessity a “pass or fail” requirementtesting procedure based on fire tests essentially the same as found in field practice but smaller in scope.
ing a wooden ball at a constant rate of l foot per second through the foam sample contained in a standard 1-liter glass graduate. Readings were taken a t equilibrium movement resistance as shown on the Jolly balance spring pointer calibrated in grams u p to 600 grams a t its maximum extension. The values obtained by this device are comparative and indicate a measure of apparent viscosity or shear resistance. The characteristic of foam most general in consideration and typical to foam systems is that of rate of liquid drainage. The literature is replete with references for using this property to judge foam sta-
bility, foam syneresis, and the quality of foaming substances such as beer, wine, and egg white. Bikerman (2) gives an excellent treatment of this attribute of foam. Since any test for this property is an empirical one to show degree of mixing, the property being measured combines factors of stability of emulsion, viscosity, and volume of water coming into contact with a dissimilar liquid such as hot gasoline. The equipment devised consisted of a shallow aluminum dish, exactly 2 inches deep by 7 3 / ~inches in inside diameter used as a foam sample container. I t was placed on a slightly
Testing Equipment
Since the objective of this work involved the determination of basic physical requirements for foams used in fire extinguishment, it was necessary to divorce completely the factors of foam generating methods from the foams to be tested. The experimental generator design adopted to produce mechanical foams over exceptionally wide ranges of viscosity, rate of water dropout, and ratio of air to solution is shown in Figure 1. I t is similar to that used at a later date by Fry and French (7), and consists of a small liquid pump to achieve pressures up to 300 pounds per square inch gage through a flow-rate measuring device into an air mixing chamber. Compressed air is also measured and delivered to the mixing chamber where two types of mixing are used: a variation of inlet and outlet orifices to achieve variable foam qualities, and a variation in downstream column packing to give greater mixing. The expansion value or expanded volume of foam produced from an original foam-forming solution volume was measured by weighing a container of known volume and using the reciprocal of the density value obtained. Methods of determining the viscosity or fluidity of foam are not easily defined. Ordinary viscometers used in singlephase systems are not applicable to the measurement of the fluidity of constantly changing, pressure-sensitive, two-phase foam systems varying from watery, freeflowing liquids to plastic semisolids. The resistance of exposed bodies to movement when surrounded by foam, a principle used by Grove and coworkers ( g ) , is the most convenient test procedure. The viscometer used (Figure 2) was a refinement of Amsel’s method ( I ) . A constant speed motor was used for draw-
SPRING BALANCE
B
SYNCHRONOUS DRIVE MOTOR
WOODEN BALL
I- I . GRADUATE, CONTAINING SAMPLE
Figure 2.
Sphere viscometer VOL. 48,
NO. 11
NOVEMBER 1956
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Early in this work, the conditions of mechanical application of the foam to the burning surface were separated into three principal methods:
A M PATTERN
FUEL LEVEL R I S E R FOR
DR AW OFF
PIPE SIZES FOR VARYING I N L E T VELOCITI IN S U B S U R F A C E A PPL IC A T ION
FOAM BYPASS,
a VASAMPLING LVE Figure 3.
FUEL D E P T H 2 5 '
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INLET
Apparatus for small-scale drum fire test
Surface flowing, where the foam is gently applied a t one or several points on the periphery of a burning liquid, similar to the technique used by large petroleum tank fire fighters employing pourers or distributing pipes stationed at the edge of the tank for foam application. This technique is also employed by shipboard fire fighting teams, where the foam stream is caused to strike tangentially a solid surface inside the fire area and gently flow down to the liquid, progressively extinguishing the fire regardless of obstacles. Subsurface injection, where the foam is applied beneath the burning surface of the bulk fuel by a system of one or several subsurface injection points or submerged outlets connected to the foam generating device. This is used only in predesigned fuel storage tank fire fighting practice. Particulate showering, where the foam is applied in a solid cone or shower head pattern. This method is of greatest importance in the case of the rapid fire extinguishment of large areas of fuel, such as in accidents involving aircraft on the ground. Personnel exposure is most likely in these fires and the instantaneous control of the fire area is vitally important for the preservation of life.
Fire Test Methods The conducting of a multiplicity of fire tests with close variable control was made possible to a wide degree by using the small open-end drum fire test shown in Figure 3. The tank was filled to within 4
slanting holder with a drainage tube installed at one end to draw off the liquid a t intervals. Knowing the amount of liquid originally in the sample (by its expansion value), the time a t which 25% of its liquid has drained out is taken as the measured value. This test is also suggested by the National Fire Protection Association for foam evaluation (70). The foam-forming solutions used in this study were of the degraded or hydrolyzed protein type and had previously been screened for their practical fire-fighting ability by passing the roughly empirical fire test requirements of the standard government specification (77) for foaming agents. In part, this assured that the liquids used were comparable in their resistance to fuel and flame exposure. As far as possible, protein concentrates from the same source and method of treatment were used in the succeding fire tests.
RECORDER FOAM PATTERN
Method of Foam Application The beneficial property of mechanical foam which permits its application to many different methods of fire extinguishment is its wide flexibility. I n a study of methods of characterizing foams, this variable must be carefully considered.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
20FT. x 20FT. SHALLOW STEEL PAN WITH MUD LAYER UNDER V C I N C H GASOLINE LAYER Figure 4.
Field test arrangement for particulate shower foam fire tests
AQUEOUS FOAMS inches of the top with premium, summer grade gasoline motor fuel. The test procedure included a 30-second freeburning period before foam was added to the fire by the method of application desired. All recording of extinguishment times was based on complete cessation of flames. The entire assembly was mounted in a freely vented room where fluctuation of the weather conditions could not influence the results. In part of this work it was decided that some effort should be made to enlarge the size of the particulate-showing foam application fire tests to a semi-full-scale scope, thus substantiating more fully the results obtained in the small-scale dcum tests. Accordingly, a steel pan, 20 X 20 feet in area with sides 4 inches high, was placed in an open field where repeated fire tests could be made. Figure 4 shows the layout for this field test. A mud layer was used in the shallow steel pan on which 75 gallons of gasoline were placed, giving a '/d-inch-thick layer of fuel. Two total radiation receiving devices of the multiple thermopile type were placed 175 feet from the fire pan a t 90' to each other. These were connected in parallel to a recording potentiometer to give a curve of the progress of extinguishment. The target of each radiometer consisted of two circles at right angles to each other, 20 feet in diameter centered 10 feet above the burning surface. A foam generator of much enlarged size but similar to the one described previously was used to give foams of variable properties for distribution in this fire test.
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150 200 250 300 EXTINGUISHING TIME ( SEC.)
350
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Drum fire extinguishment tests
Surface application, variable apparent viscosity
Results Surface-Flowing Application. In expressing the results obtained using the drum fire test, it is convenient to combine in one figure the curves for time of extinguishment obtained at each of three rates of water-in-foam application, holding the expansion value of the foam constant and varying the apparent viscosity or the drainage time. Figure 5 gives the graphic results of the fire testing where the apparent viscosity is varied for each different expansion value and separate families of curves are obtained at each rate of foam addition (in t e r m of its water-in-foam application rate). Figure G shows the result of plotting the same fire tests but using the drainage time value as a basic variable. Subsurface Injection. If now the same foams are injected a t the base of the fuel contained in the small drum fire test equipment, the fire extinguishing tests show the results given in Figure 7 when plotted using variable apparent viscosity against time of extinguishment.
Figure 6.
Drum fire extinguishment tests
Surface application with foams of variable drainage times
VOL. 48, NO. 11
NOVEMBER 1956
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efficiency in this test (expansion 10 to 12) do not show wide variations in these characteristics.
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350
450
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Drum fire extinguishment tests
Subsurface injection, variable apparent viscosity
Similarly, Figure 8 shows the families of curves obtained when the drainage time values are plotted as the variable. Particulate Showering. By employing the same drum fire test but adding the foam to the surface by a shower head arrangement, the results shown in Figure 9 were obtained for the drainage time variable. These curves are at only one rate of foam application. The results shown in this figure indicated such highly different efficiencies from higher expansions of foams that the larger field fire test referred to above was utilized to explore this variable more extensively.
Figure 10 gives the result of extinguishing the field test fire with variable expansion value foams a t only the one rate of application. This is plotted against the rate of extinguishment of the fire. Very little significance can be attached to the variable of drainage time values and viscosities of foams applied in this manner in the field fire test. Difficulties of control of the variables of atmospheric conditions and mechanical difficulties involved in so large a test fire yielded inconclusive data concerning the drainage time and viscosity variables. Furthermore, foams showing greater
NOTE: NUMERALS AT THE FOOT OF EACH CURVE DENOTE EXPANSION VALUE AT THE GIVEN APPLICATION RATE. R A K E - H A T C H E D SYMBOLSDENOTE L A C K OF EXTINGUISHMENT WHEN HIGHER VALUES OF DRAINAGE T I M E U S E D
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Surface Flowing. Both the method of determining apparent viscosity of foams and the method of judging the rate of drainage of the foams used in these studies \vere empirically devised from a consideration of the problem of easily defining physical characteristics of fire-fighting foams in the laboratory. Detailed comparison of the extinguishment curves of foams using either apparent viscosity or drainage times as variables indicates the distinctly parallel relationship of these two tests. An interesting phenomenon of foams can be deduced from the consideration of the test methods. I t would be generally conceded that in the application of foams by surface spreading, as foams infinitely decrease in their viscosity, their speed of fire extinguishment should increase. This is shown not to be the case by Figures 5 and 7. A minimum time of extinguishment occurs a t a specific apparent viscosity, below which the time of extinguishment increases again. This is postulated to be due to the increase in water drainage rate of lower viscosity foams. When this occurs, the foam becomes more vulnerable to breakdown per volume of advancing foam front and a longer extinguishment time is found. In order to relate the function of expansion value of foams to their extinguishment times, the minimal times for each expansion are plotted for each rate of application (as shown in Figure 5) and Figure 11 is obtained. At each rate of addition of foam, the fire extinguishment time is nearly constant until a critical minimum point in expansion is reached. If the constant values for extinguishment times a t each rate are used for determining the function of water application rate to total volume of water-infoam required to extinguish the fire, the curve of Figure 12 is obtained. Obviously, water-in-foam rates lower than about 0.09 gallon per minute per square foot require increasingly greater amounts to extinguish equivalent areas of gasoline fire. Further, if the curve of Figure 12 is extrapolated, the equilibrium rate at which foam is being exhausted as rapidly as it is delivered can be seen to be something below 0.035 gallon per minute per square foot. Clark and his associates obtained a figure of 0.030 gallon per minute per square foot in similar experiments ( 5 ) . Subsurface Injection. When data obtained for extinguishing times using subsurface injected foam are similarly employed, the curve of Figure 13 is obtained, which shows the restricted ability of this method of extinguishment
AQUEOUS F O A M S where variation in expansion value of foam is to be expected. The sudden cutoff of the extinguishment time curves a t 4.5 and 5.0 expansion is evidence of gasoline occlusion in the foam, which becomes so severe above these expansion limits that the foam burns. Particulate Showering. When foam is applied to a burning gasoline surface by particulate showering, the rate of extinguishment was recorded in order to align with the practical considerations involved in such fire fighting procedures. The problem is to quell the radiation over the entire area as rapidly as is consistent with some economy of materials. The minimum rate of 0.10 gallon of water in foam per square foot, which was used in the field test plotted in Figure 10, was selected after considering the data obtained using surface-flowing addition of foam. Unfortunately, the expense of conducting duplicate tests (of which sOme 7 3 were made in order to obtain the data of Figure 10) a t other rates of addition of foam militated against further work in this direction. The erratic results obtained where an attempt was made to correlate extinguishing efficiency with drainage time (and thus with viscosity) indicated that the factor of density of the foam is primarily responsible for its efficiency. This is probably due to the difference in amount of submergence or the degree of “splashing” of the foam in the gasoline when falling freely to the surface. Foam of expansion 12 with specific gravity of 0.083 falls more gently than foams of expansion 4 with a specific gravity of 0.25. When little or no splashing or submergence in the fuel occurs, there is no coating of the foam with fuel which must subsequently burn off or volatilize, thus delaying the extinguishment time.
V
40
80 I20 160 200 EXTINGUISHING TIME (SEC.)
Figure
9.
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Drum fire extinguishment test
Particulate shower with foams of variable expansion and drainage time values Rate,
0.09 gallon per minute/square
foot
Summary The results of small-scale fire testing of mechanically produced foams generated from hydrolyzed protein type foamforming concentrates in general can be summarized as follows. When foam is gently applied from a single point of flow over a burning gasoline surface, the minimum economical rate of addition is found a t 0.09 gallon per minute per square foot of surface. Rates above this extinguish the fire proportionately faster. At the minimum surface-flowing rate, foams ranging in expansion value from 4 to 12 in this study performed equally well, provided the drainage time test values (related to the apparent foam viscosity) were within 3 to 5 minutes. Above the minimum rate, these characteristics become less critical. When foam is injected below the burning surface of gasoline by submerged
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80 100 120 140 FOAM APPLICATION TIME (SEC)
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Gasoline fire extinguishment measured by fire radiation Average values of varied foams
VOL. 48,
NO. 11
NOVEMBER 1956
2029
Keeping in mind the method of application under which the foam is to be used, test methods consisting of expansion and drainage time measurements should evaluate the desirability of a given foam for a hazard necessary of protection. The foregoing standards for foams have been substantiated in a degree by field experience in the Department of Defense and other installations.
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Literature Cited
(1) Arnsel, O., Oel u. Kohle 38, 293-310 (1942). ( 2 ) Bikerman, J. J., “Foams: Theory and Industrial Applications,” with J. M. Perri, R. B. Booth, C. C. Currie, p. 98, Reinhold, New York, 1953. (3) Brunswi H., Feuerschutztech. 22, 30 (1942y: ( 4 ) Clark, N. O., Dept. Sci. Ind. Research, Special Rept. No. 6 , London, 1947. (5) Clark, N. O., Thornton, E., Lewis, J. A,. J . Inst. Petroleum 33, 192-6 (1947). ( 6 ) Friedrich, K., Foam Conference Report, Apolda, Germany, 1943. (7) Fry, J. F., French, R. J., J . ApPl. Chem. 1, 425 (1951). (8) Zbid., 2, 60 (1952). f 9 ) Grove. C. S.. Jr.. Wise. G. E., Jr.. hlaich, W.’ C.,’ Gray,‘ J. B., IND. ENG.CHEM.43, 1120 (1951). (10) National Fire Protection Assoc., Bo+ ton, Mass., Standards for Foam Extinguishing Systems, No. 11, 1950. (1 1) U. S. Joint Army-Savy Specification JAN C-266,1945.
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Figure 1 1. Relation of expansion to extinguishing time at various constant water rates in surface application
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burning surface, the density of the foam is of greater importance than the drainage time value or the apparent viscosity, optimum fire extinguishment rates being obtained at expansion values of 10 to 12. Drainage time values between 24 and 36 minutes were used to achieve the optimum results in these tests.
inlets, the expansion value of the foam must be held between the limits of 2.0 and 4.5, with drainage time values of 2.5 to 3.6 minutes in order to obtain complete extinguishment. When foam is applied by particulate showering at the minimum rate of 0.1 gallon per minute per square foot of
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RECEIVED for review Xovember 25, 1955 ACCEPTED May 19,1956
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Figure 12. Water volume required as function o f a p plication rate in surface application
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FOAM EXPANSION VALUE Figure 13. Relation o f expansion to extinguishing time at various constant water rates in subsurface injection
