L. E. RlVKlND and IRVING MYERSON The Mearl Corp., Roselle Park, N. J.
Foams for Industrial Fire Protection Mechanical foam
b
Combats fires by cooling and blanketing
b
Prevents fires in spilled liquids and exposed structures
b
As foam concrete, acts as permanent fire-resistant blanket
T H E combination of flammable liquids and their environment presents a diversity of hazards which may result in fires that are best controlled by firefighting foams. Essentially metastable dispersions of gases in aqueous solutions, foams may be classified as “chemical” or “mechanical,” depending on the method of generation. Because of greater flexibility in operation and reduced personnel requirements, almost all new installations are of mechanical systems. The foam-forming material is provided as an aqueous solution, commonly referred to as “foam liquid,” which for use is diluted with water by means of a proportioning device. As a rule, the foam is formed from a 3 to 6% solution of foam liquid in water by the mechanical entrainment of air. In the twofold function of the foam liquid, the first facilitates the formation of films by preferential adsorption a t the air-liquid interface, establishing a concentration differential of solute between the bulk liquid and the surface. The second stabilizes the foam films against the destructive conditions to which they will be exposed in fire fighting. Mechanisms of Fire Extinguishment b y Foam
For most fire fighting water is the first choice as an extinguishing agent. I t is widely available, can be easily transported, and has high specific heat and latent heat of vaporization. The major limitation to its use for fighting fires in flammable liquids is its high specific gravity. Conversion of water to a low density, fuel-immiscible, stable foam by incorporating air makes it possible to cool the surface of the burning liquid, reduce radiation to the burning liquid, and exclude oxygen by blanketing the surface, thereby achieving extinguishment. The relative importance of cooling and blanketing in the extinguishment process depends largely on the nature
of the fuel. The surface of liquids having flash points greater than 212’ F., such as lubricating oils, can be cooled below this temperature by vaporization of the water in the foam applied to the surface, destroying a portion of the foam in the cooling process. Indeed, burning liquids with high enough flash points can be extinguished with water alone, preferably in the form of fog. Liquids of low flash point, such as gasoline with a flash point below -45’ F., can never be cooled below their flash points, and this mechanism for extinguishment cannot be relied on. In these cases, the fire is extinguished primarily by interposing a physical barrier of foam between the burning liquid and air. Types of Foam liquid
Many surface-active substances, including various synthetic surfactants, have been used for the preparation of mechanical foam liquids, often supplemented by other chemicals which assist in some manner in foam formation, stabilization, etc. The chemical components have been reviewed by Bikerman ( 3 ) , Clark ( 6 ) , and Grove (9). However, the foams derived from hydrolyzed proteins have been found much superior to others, represent the bulk of those currently available, and are used most extensively both in the United States and abroad. Although foams of this type provide very effective protection against burning hydrocarbons or other nonpolar liquids, they are unsuitable for use on watermiscible or other highly polar liquids. Such substances penetrate the foam films, displacing or otherwise disturbing the orientation of the surface-active molecules that stabilize the foam. Polar liquid fires require the use of the more recently developed “all-purpose” or “alcohol-resistant” foam liquids. The currently available all-purpose foam liquids are generally based on hydrolyzed protein, but depend on the for-
mation of a precipitate within the foam bubble walls, which are thus made impervious to penetration by the organic liquid. These films are insoluble in water, while films of foam suitable only for nonpolar liquids are water-soluble. T o develop insoluble films from allpurpose foam liquids, air entrainment must occur immediately after dilution of the foam liquid with water; the foam bubbles are therefore already formed when the precipitate appears a few seconds later. A longer delay before the introduction of air would permit the precipitate to appear in the bulk liquid, with a sharp reduction in effectiveness. This consideration does not apply to foam liquid for use on nonpolar solvents. Such foam liquid may remain in the diluted condition for long or short periods without impairment of its stability, making possible the use of a wider variety of foam equipment. Types of Industrial Fire Hazards
The possibility of an undesired combustion of a fuel in an environment not intended to contain such a fire con-
Figure 1 , In-line eductor connected between high pressure hydrant and hose line VOL. 48, NO. 1 1
NOVEMBER 1956
2017
v
T
Foam Generating and Distributing Equipment
FOAM LIQUID:
M E T E R IN6
BALL
FOAM
VA
CHECK
VALVE VALVE
CHECK V A L
F R O‘ M
HYDRANT
DISCHARGE HOSE
----_-THROTTLING
VALVE
Figure 2.
Around-the-pump proportioner
stitutes a fire hazard. Such hazards can be classified on several bases, all of which will assist in providing guides for precautionary measures. The chemically diverse flammable liquids may be classified on the basis of certain intrinsic properties. For example, flash point (75) and burning characteristics (4)will determine whether cooling or blanketing is more important in extinguishment. Another classification (74) is based on water solubility or polarity. This will largely determine the chemical nature of the foam-forming stabilizing materials that may be used effectively. The variety of equipment associated with flammable liquids makes necessary the use of different foam-generating devices, capable of producing a wide range of foam properties. Fuel storage tanks, oil quench tanks, dip tanks and drain boards, pump houses, loading racks, chemical reaction vessels, distillation equipment, and airport hangars and runways are some of the structures associated with flammable liquids. The obvious difference between an open liquid surface such as might be present in a fuel storage tank, and the complex maze of obstacles which might be present on the surface of a fuel spill a t the base of a distilling column, is reflected in the justified practice of using foam of different properties for different environments. Foam, applied to the surface to be protected, is the product of the operation of a foam system which includes the foam liquid and the foam-generating and distributing equipment.
such temperature, hardness, and alkalinity, including sea water, as may be encountered in practice. I t should be stable in storage even in vented containers, for extended periods of time over a wide temperature range. If frozen and thawed, its performance should not be impaired, nor should it show any sludge formation or phase separation. I t should not exhibit temperaturedependent viscosity changes large enough to interfere with the proper operation of the proportioning equipment. I t should require a minimum of mechanical energy for foam generation and be effective in all types of foamgenerating devices. I t should be substantially noncorrosive to fire-fighting equipment, and nontoxic on contact \\ith personnel. It should be of such composition that salvage of the fuels and equipment after extinguishment is economically feasible. PROPORTIONED F O A M SOLUTION
PROPORTIONING CONTROLLER FOAM
X%
Foam Liquid Properties
T o be useful in fire fighting, the foam liquid or stabilizer should meet the following requirements. I t should be effective with water a t
20 1 8
LIQUID
F O A M LIQUID STORAGE TANK
OF TOTAL FLOW
‘”’I
4
WATER Courtesy Rockwood Sprinkler Co.
Figure 3. Balanced pressure proportioner using flexible diaphragm pressure tank
INDUSTRIAL AND ENGINEERING CHEMISTRY
The properties of foam depend on both the solutions from which they are generated and the equipment used for this purpose. Three distinct functions are performed by the mechanical equipment -liquid proportioning, aeration, and foam distribution. They may take place almost simultaneously or with considerable time intervals intervening; in a single unit or in physically discrete units. Liquid Proportioning Devices, The commonly used devices are modifications of either eductors or pressure injectors equipped with orifice plates, variable metering valves, or flowmeters. The eductors depend on discharging pressurized water through a Venturi tube, inducting foam liquid to produce a solution of predetermined concentration. Figures 1 and 2 illustrate typical proportioners of this type, In pressure-injection proportioners, both water and foam liquid are fed into a common pipeline under pressure, forming the foam solution. The various types differ primarily in whether foam liquid is pressurized by water from the main supply, by a separate pump, or by gravity. The balanced pressure diaphragm tank (Figure 3) is a particularly effective proportioning system which lends itself readily to automatic operation. Aeration Devices. The most common commercially used equipment utilizes aspirated air. High velocity foam solution streams, impinging on each other and on the walls of the chamber into which they are discharged. or against screens, entrain air from the atmosphere. In these devices, the amount of air entrained is a function of the hydraulics of the system and the physicochemical nature of the foam-stabilizing liquid. Air may also be pressure-injected simultaneously with foam liquid and water into a common chamber by the use of either sliding vane pumps or three-fluid pump generators. I n this type of foam generator, the amount of air incorporated is directly controlled by the equipment. I t is in the aeration stage that the considerable differences between the various foams are developed. They may range from a very low expansion fluid “soup” to high expansion, rigid “whipped cream.’’ Foam-Distributing Devices. These vary in accord with anticipated use. Fixed units are installed to care for specific hazards where protection may be engineered in advance; mobile distributors are brought to the scene of a fire. Among the fixed distributors, the following are typical : TANK-MOUNTED FOAMCHAMBERS from which foam is directed against the side wall to reduce velocity of flow and facili-
AQUEOUS F O A M S tate gentle application to the fuel surface. SUBSURFACE FOAMINJECTORS, located near the base of a tank and feeding directly into the product line. The foam is discharged with sufficient pressure to overcome the pressure head of the product and rises freely through the entire height of the stored fuel. FOAMSPRINKLERor fog foam heads which discharge a dispersed pattern of foam over wide areas. They are fed from a manually or automatically actuated deluge system sypplying foam solution. Aeration also occurs a t these heads (Figure 4). Among the portable devices, the following are among the most common: STRAIGHT-STREAM NOZZLES. In the smaller sizes these incorporate all three functions of proportioning, aeration, and distribution. Larger sizes are mounted on fire trucks and operated with direct or remote controls. The discharge pattern is similar to that of the conventional hose stream from smooth-bore water nozzles. DISPERSED-STREAM NOZZLES. Also known as fog foam nozzles, they discharge foam over a wider area and at closer range than straight-stream nozzles. Velocity of impingement of foam on the fuel is reduced, making this type of discharge particularly effective for spill fires. When used inside pump houses, airplane hangars, and other structures, fog foam also results in a substantial amount of space cooling in addition to blanketing the area. VARIABLEPATTERNNOZZLES.This type of foam nozzle enables the operator to change from straight-stream to fog foam during operation and is available
in both portable handline nozzles and truck-mounted turret nozzles (Figure 5). FOAMTOWERS. These are portable foam applicators which are placed adjacent to a tank of burning fuel and located to discharge foam down the inside tank wall. Foam generated a t ground level, usually at a point somewhat remote from the tank, is then pumped u p the tower. Additional devices have been described (7 7, 72). Tacfical Properties of Foams
The foam properties which must be suited to the physical environment rather than the fuel are fluidity and expansion. They may be called the tactical properties, because they determine effective placement or application of foam to the burning fuel. The importance of these factors to successful fire fighting is evident from consideration of a few simple firefighting problems. In fighting tank fires by single point injection of foam below the burning surface of a fuel, only a relatively low expansion foam has been found successful (8, 76). For surface application to similar tanks, considerably higher expansion foams are preferable. If a foam is to flow around geometrically complex structures, or over rough terrain, it must be more fluid than if it need only flow across an unobstructed liquid fuel surface. The velocity of impingemen; of foam on the fuel surface and the foam distribution pattern are further modifying factors in influencing optimum values for expansion and fluidity; thus with a foam chamber, a more fluid foam may be
Figure 5. Portable handline nozzle with selector valve for discharging foam in straight stream (right) or disperse pattern (left)
tolerated than with a straight-stream nozzle. Fire-Fighting Foam Stability
I n extinguishing a burning liquid of low flash point, the prime requisite is that the foam be sufficiently stable under fire-fighting conditions to separate the fuel physically from the air and surrounding structures by a continuous blanket, until there is no longer danger of reignition. Four different kinds of stability are involved. Resistance Resistance Resistance Resistance
Courtesy Rackwaod Sprinkler Co.
Figure 4.
Fixed fog foam heads discharging disperse pattern foam in pumphouse
to spontaneous collapse to thermal attack to chemical attack to mechanical rupture
Resistance to spontaneous collapse or reversion to air and aqueous solution is of major importance. I t may be measured by various empirically developed test methods (7, 2, 78), in terms of the time for 25 or 50% of the original liquid content to drain out of the foam. I t will be a measure of the length of time the foam may be expected to persist after application to the extinguished fuel surface. In general, other factors being equal, slow draining foams are to be preferred. In practice there are both an upper and a lower limit to drainage rate: Foams that drain too rapidly are unstable, while those that drain too slowly are likely to be too rigid. Resistance to thermal attack is the VOL. 48, NO. 11
NOVEMBER 1956
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ability to withstand exposure to radiant heat from flames and hot structures for a sufficient time to effect substantially complete blanketing. Foams. based on synthetic surfactants which exhibit marked resistance to spontaneous collapse a t room temperatures, rapidly collapse if exposed to the elevated temperatures encountered in fires, resulting in flashbacks and complete loss of fire control. Thermal stability is a function of the variation with temperature of the bulk liquid viscosity, the extent of thinning of bubble wall, and the surface viscosity. Resistance of fire-fighting foams to chemical or solvent attack varies with the fuel as well as the foam liquid formulation. The attack mechanism has not been well established. However, it is known that displacement of one material from the film surface by another surfactant is generally destructive to foams (70). The presence of magnesium stearate, for example, in gasoline solution, will result in substantially greater breakdown of foam, particularly under the influence of heat. Similarly, the penetration of the foam film by watersoluble solvents such as ethyl alcohol and acetone exerts such destructive action that conventional foams useful for fires in hydrophobic solvents are entirely unsuitable for fire in these materials, which require foam derived from the all-purpose foam liquids discussed above. Obviously, in practical fire fighting, the foams must be resistant to combined thermal and chemical attack. Hot fuel residues even of the hydrocarbon type are more destructive than cold unburned fuels, both because of temperature effects and because some of the combustion products may be more surface-active than the unburned fuel, with a consequent disruptive effect on the foam films. Resistance to mechanical rupture of the blanket by falling debris and ability of the foam to flow around obstructions and into recesses are aspects of a single problem: establishing and maintaining a continuous blanket. The initially applied foam must be sufficiently mobile to penetrate crevices quickly, but must not be so fluid as to spread into a very thin blanket which will be destroyed as rapidly as applied. The ability of the blanket to reseal after mechanical disruption is a function of the fluidity of the aged foam. The superior foam fluidity on aging is one of the characteristics which dictated the choice of hydrolyzed protein as the base of modern foam liquids in place of synthetic surfactants. The stability and other properties of a foam are established by the interaction of the foam liquid and the foam-generating equipment. The latter, for example, largely controls bubble size and bubble size homogeneity, on which in turn such factors as fluidity and stability depend.
2020
Engineering Foam Protection
I n considering industrial hazards, selection of the type of foam installation will require a decision with respect to a number of factors.
What type of foam-generating and distributing system should be used? What type of foam liquid should be used? Should the equipment be fixed or portable? What should be the water-in-foam application rate? The most satisfactory foam system, comprising both equipment and foam liquid, will depend on the physical and chemical characteristics of the particular hazards. The choice should be made with the assistance of experienced fire protection engineers, familiar with foam installations. Economic factors will play a major role in the decision. A single mobile truck unit may cost much more than a fixed installation of similar capacity but may be more economical for tank farms requiring protection a t many points. Economic considerations must be tempered, however, by the limited capacity and time delays in bringing up mobile equipment, as opposed to potential disruption of fixed installations by exposure to heat and explosion. The system should be simple to operate and maintain under adverse conditions. Standards recommended by the National Fire Protection Association require minimum water-in-foam application rates of 0.1 gallon per minute per square foot of fuel surface (73). This is about three times (7, 77) the critical application rate for total extinguishment, so that a reasonable safety factor is provided. However, the rate of fire control varies with the water application rate (5, 7 , 77), so that from the point of view of keeping material destruction to a minimum, it is desirable to use as high a rate as possible. Here again, economic factors enter the picture, as pumping equipment and piping costs rise sharply with capacity. Miscellaneous Applications of Foam
Several applications of foams to fire protection rather than strictly to fire fighting deserve mention. Foam has been used as “fluid insulation” to protect tanks from radiation due to fires. I t has been used on airport runways for lubrication in anticipation of an emergency landing with damaged landing gear. The fire hazard due to possible gasoline spill was simultaneously minimized. I t has become standard practice at military and civil airports to cover all fuel spills with foam, for the same precautionary reasons. This practice could well be carried over into the general industrial field. Foam concrete is a recent development. This product is made by blend-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ing a water slurry of portland cement or other hydraulic binder combinations, such as lime and silica, with a mechanically generated foam. The hydration of the cement or other binders within the bubble wall stabilizes the structure in a true rigid foam with discrete noninterconnecting cells, as distinguished from sponges. Foam or cellular concrete has been the subject of considerable study a t the National Bureau of Standards ( 7 9 ) and industrial laboratories, including that of The Mearl Corp. I t has been used extensively abroad, and its domestic use is rapidly expanding. Foam concrete shows great promise for combined thermal insulation and fire protection for use on fuel storage tanks and pipelines in chemical plants and refineries, as 1x11 as in building construction, as it lends itself to field manufacture as well as prefabrication in standard units. Literature Cited (1) Amsel, O . , Oel und Kohle 98, 293 (1942). .-,( 2 ) Arbuzov, K. N., Grebenshchikov, B. N., J . Phys. Chem. U.S.S.R. 10, \ - -
32 (1937).
(3) Bikerman, J. J., “Foams,” pp. 240-2,
Ref. 86-192. Reinhold. New York, 1953. ( 4 ) Burgoyne, J. H., Katan, L. L., Richardson, J. F., J . Znst. Petroleum 35, 803 (1949). ( 5 ) Zbid., pp. 808, 810. ( 6 ) Clark, N. O., “MechanicaIly Produced Foams for Combating Petrol Fires.” Chem. Research Soec. Rep: 6, H.M. Stationery Office, London, 1947. ( 7 ) Clark, N. O., Thornton, E., Lewis, J. A., J . Znst. Petroleum 33, 195 (1947). ( 8 ) Dept. Sci. Ind. Research, “Fire Research,” H. M. Stationery Office, London, 1953. ( 9 ) Grove, C. S., Golub, L. L., “Fire Fighting Foams. Their Characteristics and Physical Properties,” pp. 35-41, Syracuse University, Syracuse. N. Y . . 1954. Michaelis. L.: Rona. P.. Bioshem. 2. 15, 196’(1909). Natl. Fire Protection Assoc., Boston, hlass., “N.F.P.A. Handbook of Fire Protection,” chap. 61, 1954. Natl. Fire Protection Assoc., “Standards for Foam Extinguishing Systems,” NFPA Bull. 11, Appendix (19 54). (13) Ibid., Sect. 31-13. (14) Zbid., Sect. 32-20. (15) Natl. Fire Protection Assoc., “Storage, Handling and Use of Flammable Liquids,” NFPA Bull. 30-L, Sect. 104.12 (1953). (16) Tuve, R. L., Peterson, H. B., “Study of Some Mechanical Foams and Their Use for Extinguishing Tank Fires,” NRL Rept. 3725, Appendix A, Naval Research Lab., Washington, D. C., 1950. (17) Zbid., Figs. 25-27. (18) Zbid., Fig. 26. (19) Valore, R. C., J . A m . Concrete Znst. 25, 773-836 (1954). I
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RECEIVED for review ii’ovember 25, 1955 ACCEPTEDAugust 15, 1956
