Figure 1.
Aer-0-Foam providing exposure protection
JOSEPH M. PERRI and CHARLES CONWAY National Foam System,
Inc., West Chester, Pa.
Foam as a Fire Exposure Protection MediumEvaluating Effectiveness of Wetting and Protein Agents Tests for insulating abilities of mechanical foams show that
b
alkyl aryl sulfonates are the best wetting agents protein foams show less deterioration under fire test conditions
h
protein foams appear more effective at lower concentrations than those recommended for fire-fighting service
T H E use of foam as a means for protecting objects or areas exposed directly to fire as contrasted to its use as an extinguishing agent has been a recognized practice almost from the inception of the development of fire-fighting foams. One manufacturer’s catalog (6) states I n addition to the use of foam for oil fires, it may be used either as a means of direct extinguishment or to prevent the ignition of structures in the vicinity. Foam not only makes a good insulating . cover.
..
The National Fire Protection Association pamphlet ( 5 )haslongcarried thenote that Foam has the property of adhering to surfaces, combining a blanketing effect and a cooling effect for fire extinction and protection against adjacent fires. Specific examples may be cited in this connection wherein foam is applied by
hand lines or turret-operated devices to coat exposed objects. The reference is to exposed adjacent tanks in the case of tank farm fires and to the fuselage of the airplane in crash fires. The development and establishment of the use of this principle on a sound engineering basis has lagged until comparatively recently. The approach has been in two directions : 1. The application of foam a t a rate which had been predetermined as sufficient to effect extinguishment, as per the usual practice in the use of fire fighting foams for flammable liquid hazards and accomplishing protection of exposed surfaces in the process of so doing. 2. The application of foam to the exposed surface a t a rate which will control or limit the rate of heat transfer to such exposed surface. For example, a tank containing a flammable liquid may not itself be afire; however, it may be di-
rectly exposed to a continuing fire which is beyond the ordinary means of control or extinguishment. By the use of a running blanket of foam covering the tank completely, the heat input to the tank would be limited so that the extent of vaporization would not exceed the venting capacity of the tank. A notable example of the first-mentioned type of protection is given in a publication of the Factory Insurance Association ( 3 ) . The hazard involved oils, thinners, and other combustible liquids which were stored in vertical and horizontal tanks along a 450-foot private railroad siding upon which tank cars stand waiting to be unloaded. A foamdistributing system (Figure 1) was provided which produced curtains of foam for adjacent building protection and tank coverage in the process of building a foam blanket on the ground a t the rate of a 3-inch depth per minute. VOL. 48, NO. 11
NOVEMBER 1956
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A.
B.
Figure 2.
Preburn period
Fire extinguished
The second aspect of exposure protection, wherein the fire cannot be controlled or should not be extinguished for the well-known reasons, had initially been investigated by Duggan (2). Tests were conducted using a synthetic surfactant as a foaming agent. The solution was converted into foam by use of a conventional device mounted atop a tank measuring 20 feet in diameter and 24 feet high. The tank was located centrally in a diked area which served to contain the flammable liquid fire. The foam was applied at a solution rate of 0.1 gallon per minute per square foot of exposed tank surface. Inasmuch as it was of interest to determine the extent of reduction of heat input attributable to foam, the foam was channeled away from the tank at its base so as not to interfere with the necessary continuity of the test fire. Tests conducted on the unprotected tank yielded a heat input of 28,000 B.t.u. per square feet per hour. A value of 5500 to 5700, a reduction to 2070, was obtained for the foam-protected tank at the solution rate of 0.1 gallon per minute per square foot.
A.
Hydrolyzed protein film
E. Alkyl Figure
Foam water sprinkler test
aryl sulfonate foam
3. Exposure protection test
Experimental
A more recent installation based upon the use of foam-water sprinklers was described by Schmidt (7). The hazard involves probable spillage of aviation fuel to the extent of 32,000 gallons from some of the aircraft stored in the hanger. T h e building has a floor area of 90,000 square feet and is 104 feet to the top of the arch. The fire-protection system includes 1252 foam-water sprinklers which would deliver approximately 135,000 gallons of foam per minute from near-ceiling height providing a veritable snowstorm of foam when in full operation. The foam used is of proteinbase type described in a specification (8). Some of the preliminary test work done in the evaluation of the effectiveness of the foam-water sprinkler was conducted using the test setup shown in the photographs. Figure 2A shows a gasoline fire (wind velocity of 20 miles per hour) contained in a tank measuring 10 X 10 X 3 feet. The wooden roof is at an elevation of 8.5 feet and four foam sprinklers are mounted on the pipe frame on 8.5-foot centers at a distance of 6 inches from the roof. Each sprinkler has a solution capacity of 12 gallons per minute a t the operating pressure of 20 pounds per square inch. Figure 2B shows the condition of the tank after approximately 3.5 minutes of operation. A further performance requirement is that the blanket formed from a 10-minute application of foam shall be capable of withstanding a 30-minute application of water a t the same rate, without deteriorating to such an extent that the flammable liquid surface would become exposed.
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stant head. The rate of flow was slightly in excess of the burning rate under conditions of test. The foam was applied through a piping arrangement having a 2-inch outlet. The foam maker used is the test nozzle described (8) and was operated at a pressure of 65 pounds per square inch delivering 5 gallons per minute of solution or water to give a rate of 0.1 gallon per minute per square foot of total exposed surface. The sequence of operations was as follows: Approximately 0.25 inch of kerosine was layered onto a 3-inch depth of water contained in the fire pan. Five gallons of gasoline were used to prime the fire. The foam was started immediately prior to ignition and kerosine flow was started. In most instances the fire was permitted to burn for 10 minutes. Extinguishment was effected instantaneously at the end of a run by a blast from oversized water spray devices. Data were considered valid only for those tested in which 90% or more fire coverage was obtained. After fire extinguishment, the lid was removed from
Since regular fire-fighting foam, the hydrolyzed protein type, had been successfully employed (3, 4, 7), it was desirable to investigate such by a similar technique, however, on a much smaller scale. Figure 3 depicts the apparatus. The central object is a tank of approximately 225-gallon capacity having dimensions of 36 X 54 inches. The water content of the tank provided for a wetted area of 40.0 square feet. The tank is supported over a pit with a connecting discharge channel for easy removal of the foam from the test area. The fire pan is constructed of steel plate and has over-all dimensions of 7.5 X 7.5 feet X G inches. An opening is located centrally having a diameter of 3 feet 8 inches to accommodate the placement of the tank, leaving a 4-inch clearance. Two obstructions in the form of welded pipe nipples were located on the removable tank lid. The fuel, kerosine, flowed into the fire pan a t a rate of 4 gallons per minute directly from tandem 55-gallon containers maintained a t con-
Relative Heat Transfer Application rate, 0.1 g.p.m./sq. ft.
Condition Water Sodium alkyl sulfate Hydrolyzed protein Alkyl aryl polyether alcohol Hydrolyzed protein-wetting agent mixture Sodium alkyl aryl sulfonate
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Concn., Active Ingredient, %
...
Heat I n p u t , Protected X 100 Heat i n p u t , unprotected (A'o protection = 100)
0.25;0.50 0.4-1.2 0.93; 1.90
36 18;21 18-20 11; 13
0.4-1.2 0.24; 0.48; 0.72
8 15; 7; 8
AQUEOUS F O A M S the tank and contents agitated to yield uniform temperature readings. Discussion of Results The foaming agents investigated were selected as representative of the following classes-sodium alkyl aryl sulfonates, sodium alkyl sulfates, alkyl phenyl polyethylene glycol ether, alkyl aryl polyether alcohol, hydrolyzed protein solution, and a mixture comprised of hydrolyzed protein and a wetting agent. T h e data obtained were converted into relative heat transfer values and are given in the table. The increased effectivenessof the foaming agents in reducing the rate of heat input over that of water alone was noted. The relative effectiveness of the various agents can be explained on the basis of the following requirements, disregarding foam expansions for the moment:
1. Resistance of the foam to thermal shock 2. Spread ability or wetting action 3. Foam fluidity The relative heat transfer value of 36 obtained for water would have been considerably reduced, approaching that of wetting agents if it were not for the obstructions. The obtaining of a continuous film of water is thought to be characteristic of this particular test setup wherein a very limited tank surface area is involved. The value of 20 obtained for the first wetting agent listed approximates that which would be obtained by complete wetting with water.
A.
Alkyl aryl sulfonate foam
The appearance of this particular wetting agent foam under fire conditions indicated that its contribution to insulation was a t a minimum. Figure 3A depicts the condition obtained using hydrolyzed protein foams. Values approximating that of sodium alkyl aryl sulfonates were obtained. The coincidence of values, however, was achieved by a different mechanism. Tank exposure beneath the obstructions is evident; however, the good insulating quality of the foam counterbalanced this negative effect. An intercomparison of the three concentrations of protein foam shows that the lower concentrations produce more fluid foams, hence greater covering ability and reduced fire resistance and insulating effect. The alkyl aryl sulfonate produced a foam of good volume and structure, Fjgure 3B. The relative heat transfer was around 8 for the higher concentrations. I n this instance, complete coverage was obtained due to good wetting action and fluidity. T h e primary difference in performance between this agent and the alkyl sulfate is that the alkyl aryl sulfonate possesses a greater ability to withstand high temperatures. This was demonstrated independently in a series of fire tests to determine the minimum rates of application necessary to accomplish extinguishment. The value obtained for the alkyl aryl sulfonate foam was 0.05 gallon per minute of solution per minute as against 0.1 for the alkyl sulfate. By the same test procedure, incidentally, the value for protein hydrolyzate foam is 0.035 (7). Figures 4A and B are of interest inasmuch as they illustrate the difference in foam blanket stability between wetting agent and protein hydrolyzate foams. The photographs were taken in each case 5 minutes after fire extinguishment. A third type of foam, the hydrolyzed protein-wetting agent mixture, possessed wetting ability in addition to the characteristics of the protein-type foam. A relative heat transfer value of 8 was obtained. Figure 5A illustrates its general appearance on the test tank. This foam yielding a heat transfer value of 8 was equal to the best obtained in spite of its relatively poor appearance when compared to the fine cell structure of the alkyl aryl sulfonate. Figure 5B shows however the condition of the alkyl aryl sulfonate foam when subjected to a limited fire. Considerable deterioration has occurred. Such was not evident in the case of the more fire-resistant protein-wetting agent mixture.
Wetting agent foams differ in their capacities to provide protection. The order of decreasing effectiveness for the agents tested was found to be alkyl aryl sulfonates, alkyl aryl polyether alcohol, and alkyl sulfate. The physical characteristics of wetting agent foams are appreciably more affected under fire test conditions than those of protein foams. The protein-type foams appear to be more effective at concentrations lower than those ordinarily recommended for fire-fighting service. Literature Cited (1) Bikerman, J. J., “Foams,” p. 234,
Reinhold, New York, 1953.
( 2 ) Duggan,. J. J., Carbide and Carbon
(3) (4) (5)
(6) (7)
(8)
Chemical Go., Inc., New York, Fire Research Lab. Rept. No. FRL-62, December 1954. Factory Insurance Association, Hartford, Conn., Sentinel 5 , No. 12, 2 (December 1949). Jones, C. L., N a t l . Fzre Protect. Assoc. Quart. 42 (April 1949). National Fire Protection Association, Boston, Mass., Standards for Foam Extinguishing Systems, pamphlet No. 11, July 1954. National Foam System, Inc., Philadelphia, Pa., National Fomon Gatalogue, 1943. Schmidt, Joseph K., Natl. Fzre Protect. Assoc. Quart. 48, No. 3, 327 (January 1955). U. S. Dept. of Navy, Bureau of Supplies and Accounts, Washington 25, D. C., Joint Army-Navy Specification Jan-C-266, December 4, 1945.
RECEIVED for review November 25, 1955 ACCEPTEDJuly 5, 1956
A.
Protein wetting agent foam
Conclusions 6. Hydrolyzed protein foam
Figure 4.
Aged foam blanket
The test procedure apparently is satisfactory for obtaining relative rates of heat transfer.
6. Alkyl aryl sulfonate foam under limited fire condition
Figure 5. VOL. 48, NO. 1 1
Exposure protection test NOVEMBER 1956
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