I
H. B. PETERSON, R. R. NEILL,
and E. J. JABLONSKI
Chemistry Division, Naval Research Laboratory, Washington 25,
D. C.
Research Studies in Foam-Generating Equipment Foam pumps offer a positive method of generating fire-fighting foams of widely different characteristics under varied conditions and for this reason are preferred to the lower cost, but inflexible, aspirating nozzle
R E S E A R C H in mechanical foam generation at the Naval Research Laboratory began in 1942. A need arose a t that time for a fire-fighting foam with specialized properties radically different from those then normally in use. It was needed for subsurface application, where foam is applied at the bottom of a tank and allowed to bubble up and spread over the burning fuel surface. These new requirements were for a foam of very low expansion-Le., 3 to 4produced under sufficient pressure to overcome the hydraulic head of the tank product plus the friction losses in the piping. In Britain, a “forcing-type branch pipe” was used. This is a common aspirating device modified by the insertion of a Venturi-shaped section. Such units were successful in extinguishing a test fire conducted by the Naval Research Laboratory in a tank, 55-feet in diameter, containing heavy fuel oil. Laboratory scale fire testing established some of the optimum foam properties for extinguishing fires in lighter hydrocarbon fuels such as Diesel oil and gasoline. At present, two criteria are used for judging fire-fighting foams. The first value is expansion, the reciprocal of the foam’s specific gravity as determined by weighing a foam sample of known volume. The second value expresses the degree of dispersion of the air in the
foaming solution which is determined by measuring the rate a t which 25% of the liquid originally in the foam drains out from the foam body. This value is expressed as the drainage time. Since this measurement will vary with the relationship of sample cross-sectional area to depth, a specified apparatus (7) must be used to duplicate NRL values.
FOAM EXPANSION Figure 1 . Maximum drainage times of foams according to expansion
All foams described in this article were made with protein hydrolyzates, as stabilizing agents, in the conventional bOj, aqueous solution. The agents, made under specification ( 2 ) ,are widely used both commercially and in the military services for fire-fighting purposes. Foams made with these protein-base stabilizers ordinarily do not coalesce or collapse during decay; the solution slowly drains out, leaving a skeleton form of the original bubbles. Thus, methods suitable for working with soap bubbles are not applicable for proteinfoam testing. The water retention ability of foams is closely related to foam expansion. Therefore, foams cannot be directly compared only on a basis of drainage time. Low-expansion foams are extremely difficult to stabilize and cannot be made to approach the stability of high-expansion foams. Figure 1 shows the maximum drainage times for producing various expansions with compressed air apparatus. Unless excess quantities of air are used, the maximum expansion producible appears to be limited to approximately 20. Aspirating Generators
Considerable experimental work was required for modifying the aspirating VOL. 48, NO. 11
NOVEMBER 1956
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IO 15 20 25 30 DISCHARGE PRESSURE 'b.'sq,in, Performance characteristics of Ven-
Figure 2 . turi aspirating generator
Nozzle pressure 100 pounds per square inch. area to throat area i s 0.50.
Venturi nozzle. By varying the proportions and profiles of the nozzle, throat, and diffusing sections, equipment was developed with a limited inlet pressure to produce the desired foam under varied discharge pressures. Figure 2 illustrates the performance of a typical Venturi unit. The top curve is the variation of expansion with increasing back pressure and the bottom curve is the variation of drainage time with increasing back pressure. With a multijet driving nozzle operating a t 100 pounds per square inch it was possible to make an acceptable foam up to a discharge pressure of about 20 pounds
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0' Figure 3.
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32 40 48 DRAINAGE TIME (MINI
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2
Comparative energy requirements for generating foams
Ratio of let
per square inch using a jet area to throat area ratio of 0.5. Under certain conditions higher foam pressures can be achieved by feeding the discharge from a n aspirating nozzle into the suction of a centrifugal pump. Triple Pump Generator
hlrhough these aspirating units extinguish fires under many conditions, a foam generator of more positive action and possessing greater flexibility is desirable. A triple-pump generator \vas developed utilizing three rotary
FOAM OUTLET
positive-displacement pumps (one each for water, air, and foam concentrate), so arranged as to be driven from a single shaft. By this method, the proportioning is constant regardless of discharge pressure, and the rate of output can be varied by regulating the engine speed. The foam flow rate is set according to the tank surface area in order to mainzain the proper inlet velocity of the foam as it enters the product a t the base of the tank. The final triple pump generator model evolved is now in use in Naval fuel depots in conjunction with subsurface installations. By using pumps having close tolerances, it is possible to obtain a foam with uniform characteristics at 2000 gallons per minute up to a maximum pressure of 70 pounds per square inch.
t Compressed Air Generators
REENS
END PLATE
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SOLUTION INLET
Figure 4.
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VANES Interior of a foam pump
INDUSTRIAL AND ENGINEERING CHEMISTRY
During the \var two vehicles were constructed using reciprocating air compressor-centrifugal pump combinations. However, the problems encountered in the speed relationships and flow control made operations too complex and they were dropped from further consideration. For experimental foam studies the advantages of the compressed air-type generator are obvious. Using flow rate meters for the solution and for the air, both ratios and rates can be readily varied and accurately controlled. Two such pieces of apparatus in regular use at the laboratory provide foams with a capacity range from 1 to 700 gallons per minute. A study of the operation of laboratory compressed air generators gave further knowledge of the mechanisms involved in making foams. Either of two types of dispersing devices is used. The first is a packed column.
AQUEOUS FOAM5 cannot provide sufficient energy for distributing it over large fire areas. A pump-type generator was indicated ; however, because of weight and size restrictions, the triple-pump design was undesirable. For many years the Royal Air Force had used a flying vane, or what in this country is called a sliding vane, compressor of light-weight design on their aircraft fire-fighting vehicles. One of these pumps was obtained and after a successful test period, domestic sources of similar models were established to provide foam pumps for Navy aircraft fire-fighting and rescue vehicles. Figure 4 is an interior view of the working parts of an American foam pump with a capacity of 3000 gallons per minute. The available engineering information on the operation of such a pump has been meager but attention is now directed to this phase of the problem. For testing purposes, the pump is mounted on a dynamometer stand for taking horsepower measurements and equipped with a Venturi for air intake rates, a meter for water feed rates, manometers and gages for pressure readings, taps for taking foam samples, and a pressure transducer for compression cycle observations. I n operation the pump must serve as a pump-compressor and an emulsifying device. A number of special runs were conducted on the pump to determine the effect on operation of such factors as foam solution feed rate, back pressure,
Here, the gas and liquid are allowed to expand rather slowly in intimate contact as they pass through a long column packed with metal turnings. I n the second system, the liquid and gas undergo a sudden expansion or flashing in passing through a sharp-edged orifice. I n both methods the degree of dispersion achieved varies directly with the pressure differential across the restriction. Analyses have been made on the relative energy requirements for producing foam from 1 gallon of water in the laboratory compressed air generators. Figure 3 is a plot of these results, illustrating the relative ease with which stable foams of high expansion can be made. The data serve as a basis for judging the efficiency of foam generators of all types. These values are based on delivery of the air and foam solution to the mixing device under pressure, and, do not reflect the efficiencies of a pump or air compressor.
Foam Pump Generators
The need for the higher expansion foams for aircraft fire fighting introduced the problem of generating an entirely different type of foam in large quantities. Despite the fact that little energy is required for good dispersion a t these expansions, it was difficult to find suitable equipment. Aspirating nozzles can generate acceptable foams but they
and pump speed. Another objective was to establish the source of air losses evidenced by the difference in pump displacement volume and foam output volume. Air displacement of the pump was determined by subtracting the foam solution feed rate from the calculated pump displacement. The difference between the air displacement rate and the actual air intake rate as determined by the Venturi meter was attributed to pump inlet losses. The difference between the actual air intake rate and the air rate in the foam as determined by foam samples was attributed to mixing losses. The curves in Figure 5 show this distribution of air flow in the system as a function of foam solution feed rate. The top curve, representing the air displacement of the pump, drops slightly as the volume of foam solution entering the pump increases. The middle curve is the volume of air drawn into the pump as measured by the Venturi meter; it drops off a t a greater rate than the upper curve, indicating increasing losses a t the inlet port with increasing foam solution rates. The lower curve is the volume of air which is actually discharged in the form of usable foam. At a feed rate point of approximately 300 gallons per minute, all of the air taken into the pump is completely dispersed and discharged as foam. At this point the air-water ratio is such as to make an expansion 10 foam. How-
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Figure 5. Distribution of air losses in foam pump with varying feed rate Constant speed of 1200 r.p.m.
Figure 6. Foam pump cross section showing port and pressure tap locations VOL. 48, NO. 11
NOVEMBER 1956
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Trace B, tap No. 2. Discharge pressure 10 Ib./sq. in.
Trace A, fap No. 1. Discharge pressure 10 Ib./sq. in.
Figure 7.
ever, as the feed rate is decreased, the air-water ratios result in foams of expansion greater than 10 but it is more difficult to entrain all the air, and increasing amounts of air pass on through the system in the form of slugs. The pump is inherently a poor foam-maker for, even with sufficient air passing through it to generate an expansion of 70, it produces only an expansion of 9. I n fact, a t all feed rates the mixture as it left the pump consisted of expansion 3 foam plus varying amounts of free air. Internal pressure recordings were made utilizing transducers and an oscillograph to study the compression cycle a t a rate of 100 cycles per second within the pump. Figure 6 is the pump cross section showing the pressure tap locations, the angular location, and length of the ports. I n Figure 7 pressure traces A, B, and C represent readings taken 1200 during normal operation-Le., r.p.m., feed rate of 250 gallons per minute, and discharge pressure of 10 pounds per square inch. Trace A was obtained with a slower sweep speed. All other
Trace C, tap No. 3. Discharge pressure 10 Ib./sq. in.
Pump internal pressure values
traces were recorded at such a speed as to isolate pressure relationships during the cycle of each sector. Each complete wave represents the passage of one sector or one fifth of a revolution. Two significant points were observed in these data:
ating and pumping foams of expansions 10 to 12. The reason for considering a centrifugal pump is to provide a generator, simpler in operation with consequent reduced wear and maintenance.
1. The initial pressure was considerably below atmospheric, indicating inadequate inlet porting. 2. The pressure increase was very abrupt, indicating no internal compression due to the very early uncovering of the discharge port.
From the studies made, it is believed that the specification of foam expansion and drainage time provide sufficient information to form a criterion for evaluating foam-generating systems. Additional information on the efficacy of a system may be had by computing the total horsepower requirements per gallon of water used and comparing this value to the appropriate one in Figure 3. The Venturi unit operates a t an efficiency of 43y0 on this basis and the foam pump a t 53y0. More research is needed on the process of dispersing air into the foam solution. The goal is to make more stable foams using less energy, thereby lightening the weight of the generator and its prime mover. In conclusion, foam generating nozzles of the aspirating type are simple in operation and low in initial cost but the flexibility of operation is also extremely limited. Until radically new designs are forthcoming, their fields of usefulness will be restricted to applications where costs are considered more important than performance. As research in foam fire-fighting continues to provide clearer definitions of optimum foam properties for various hazards, the compressor-type foam generators will be found capable of meeting the new requirements because of their high degree of adaptability.
If the discharge port were shortened to allow compression along an isothermal curve, PV = constant; the energy input could be reduced. The following equation is suggested for representing the energy requirements of the present pump :
14.25 (P2 - 14.7) Vi 33,000
(1)
where
HP = total energy, horsepower Q = water rate, gallons per minute
PZ
= pump pressure, lb./sq. in. abs.
VI
(this pressure is 3 lb./sq. in. above the observed discharge pressure) = air rate, gallons per minute
Figure 8 plots the relative energy requirements calculated by Equation 1 as well as on an isothermal compression basis, and observed horsepower determined from dynamometer measurements. Centrifugal Foam Pump
Discussion
Literature Cited (1 Tuve. R. L.. Peterson. H. B.. Naval Research Laboratory,' Rept.' No. 3725, ,4ug. 23. 1950. ( 2 ) U. S. Navy, Bureau of Supplies and \
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20 PRESSURE
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Figure 8. Comparative energy requirements for foam pump
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Trace D, tap No. 3. Discharge pressure 18 Ib./sq. in.
Work is progressing on the development of a centrifugal foam pump. Although standard water centrifugal pumps have been used for pumping lowexpansion, partially formed foams for experimental purposes, no centrifugal compressor has been designed for gener-
INDUSTRIAL AND ENGINEERING CHEMISTRY
,
-Accounts, Washington 25, D. C., Joint Army-Savy Spec. JAN-C-266, Dec. 4, 1945.
RECEIVED for review November 25, 1955 ACCEPTEDJuly 24,1956
