Evaluating the Use of Firefighting Foams

Evaluating the Use of Firefighting Foams

EXTINGUISHING AGENTS

Photo by Greg Noil

The expanding use of polar solvents as gasoline additives has forced the petroleum industry to reevaluate a number of its operating practices relative to the storage and handling of gasoline products.

Polar solvents, which are alcohol based, water soluble, flammable liquids, are known to mix with conventional foams, particularly those blended with ethanol and methanol, negating foams’ air-tight quality and ability to smother a fire. Because of this, the American Petroleum Institute (API) sponsored a research project to evaluate the effectiveness of currently available firefighting foam concentrates in controlling and extinguishing fires involving gasoline blended with polar solvents. The actual research was conducted by the Mobil Research and Development Corporation, Paulsboro, NJ, under the direction of Louis R. DiMaio. Supervision was provided by the API Committee on Safety and Fire Protection, Foam Project Task Force.

No one fire test can best evaluate every generic foam. The selection of the foam test procedure was based upon the need for effective comparative evaluations with a good reference point (basic test/ fuels). A representative foam from four generic foam classes was used for all testing: protein, fluoroprotein, alcohol resistant concentrates, and aqueous film forming foam (AFFF).

The test tank consisted of a 50-square-foot circular steel pan with an 18-inch deep shell. The size, 50-square-foot, was chosen because experience has shown that meaningful fire data can be generated in this size tank. Because the scope of the project was primarily oriented towards storage tank fire protection, a round shape test tank was selected to simulate conventional tankage. In addition, the round test tank would provide more reproducible data. (See Table 1 for specific information on the foam test procedure.)

Thermocouples were installed on the tank shell to allow for temperature measurements of both the fuel and tank walls during pre-burn and extinguishment. During selected test fires, radiant and total heat flux measurements were also made.

The foam maker or discharge device was positioned on a fixed stand throughout the tests to simulate a foam chamber-type application. The foam was applied for a full five minutes, even though the fire may have been extinguished before the five minutes transpired. The five-minute parameter was established for consistency with the sealability and burnback tests.

To assess the extinguishing efficiency of each particular generic foam on alcohol/gasoline blends, four polar solvent additives were combined with regular unleaded gasoline:

• Ethanol, 10% by volume (10% ETOH);

TABLE 1

API Foam Test Procedure

NOTE 1—Gasoline blends are regular unleaded gasoline + 10% ethanol + 10% methanol + 7% tertiary butyl alcohol, or + 7% methyl tertiary butyl ether.

NOTE 2—A water bottom was present for screening tests with regular unleaded gasoline. However, a water bottom was excluded in the gasoline blend tests to prevent extracting the gasoline additive from the gasoline blend.

Sealability test procedure

PROTEIN AND FLUOROPROTEIN FOAM. Ten minutes after foam application ceased, a lighted torch was passed over the foam blanket without touching or penetrating the foam blanket. Fourteen minutes after foam application ceased, the foam blanket is touched with the torch at several points. The foam passed this test if there was no ignition or if any flickers self-extinguished in less than one minute.

AQUEOUS FILM FORMING FOAM AND ALCOHOL RESISTANT FOAM. Eight minutes after foam application ceased, a torch was passed over and touched to the foam blanket. The foam passed this test if there was no ignition or if any flickers self-extinguished in less than one minute.

Burnback test procedure

PROTEIN AND FLUOROPROTEIN FOAM. Fifteen minutes after foam application ceased, a six-inch square hole was made in the foam blanket and the exposed fuel was ignited. The fire was permitted to burn five minutes, during which time the fire spread was not to exceed two square feet.

AQUEOUS FILM FORMING FOAM AND ALCOHOL RESISTANT FOAM. Nine minutes after foam application ceased, a 12-inch diameter stovepipe was placed in the fuel and this area cleared of foam, ignited, and allowed to burn for one minute. The stovepipe was then carefully removed. If the burning area receded, self-extinguished, or enlarged to less than 20% of the liquid surface area in five minutes, the test was rated favorably.

• Methanol, 10% by volume (10% MEOH);
• Tertiary butyl alcohol, 7% by volume (7% TBA);
• Methyl tertiary butyl ether, 7% by volume (7% MTBE).

RESEARCH FINDINGS Protein foams

Highly volatile fuels such as regular unleaded gasoline, especially those containing various amounts of alcohol or ether, pose a “difficult fuel” for protein foams. A baseline critical application rate of 0.10 gpm/ft² was established for all protein foam tests.

• The conclusions drawn from the protein tests are as follows:
• Of the four alcohol/gasoline blends tested, a gasoline fire fueled with the ethanol blend was the most difficult for regular protein foam to extinguish.
• Protein foam is not stable on 10% ethanol/gasoline blends and the use of this firefighting foam on 10% and higher ethanol/gasoline blends may be ineffective if used from monitor nozzles or handlines, especially on in-depth storage.
• Increasing the application rate from 0.10 gpm/ft² to 0.16 gpm/ft² on the gasoline fire with the ethanol additive did not improve foam performance.
• Protein foam was effective on the blends with the other three polar solvents at the 0.10 gpm/ft² rate.

Fluoroprotein foams

Under the same test conditions, fluoroprotein foam exhibited better all-around foam stability than did the regular protein foam. One can assume that this is due to the presence of the fluorocarbon surfactant [additive to alter (reduce) the surface tension of water] in the fluoroprotein concentrate.

The following conclusions were drawn from the data:

• The 10% ethanol/gasoline blend presented the most difficult fuel for the fluoroprotein foam.
• Fluoroprotein is ineffective when applied by subsurface injection on a 10% ethanol/gasoline blend, and this application technique is not recommended.
• Fluoroprotein foam exhibits better all-around foam stability than regular protein foam when compared under the same test conditions.

Aqueous film forming foams

The results for the aqueous film forming foams were markedly inconsistent. Fire performance was characterized by ease of control and difficulty with final extinguishment.

Although the control and extinguishment with flicker times were similar to those of the alcohol resistant concentrates, there were distinct differences in the final extinguishment times. The flicker fires eroded the AFFF foam blanket and flashed around the rim of the tank until raw fuel became exposed, subsequently leading to sustained burning of the entire tank. In addition, increasing the application rate from 0.04 to 0.13 gpm/ft² did not improve overall fire performance.

It was the opinion of the researchers and of the API Foam Project Task Force that some of the following conditions may have influenced the results:

• The foam stream was turbulent and too diffuse.
• Submergence of the foam in the fuel on impact leads to sub-
• stantial fuel pickup by the foam bubble.
• As the application rate was increased, the increased velocity of the stream caused the foam to spread over a larger area, thereby deflecting the foam into the fuel surface. This condition was lessened, however, by varying the nozzle application point in every test.
• This particular fire test does not emphasize the strengths of aqueous film forming foam.
• A manual nozzle attack (as compared to the fixed nozzle position) was very effective even though it placed foam directly on the fuel surface and off the tank walls. In addition, this technique did not seem to entrap excessive amounts of fuel.
• A simulated foam chamber application test was very effective and passed all test criteria.
• Because of the erratic results, it is difficult to draw any firm conclusions. It is suggested that:
• Satisfactory fire performance may be dependent upon the mode of application (portable vs. fixed).
• Aqueous film forming foam tends to drain quickly and may entrain vapor and/or fuel, which can lead to flashover and progressive foam blanket erosion.

TABLE 2

Minimum Acceptable Foam Application Rates with Fresh Water (see Note 1)

NOTE 1—Actual field application rates may be higher to ensure effectiveness. Minimum application rates discussed throughout this report apply to the test procedure used under test conditions.

NOTE 2—Regular unleaded gasoline used as a control.

NOTE 3—Aqueous film forming foam provided inconsistent fire test performance.

Key: ETOH ethanol

MEOH methanol

MTBE methyl tertiary butyl ether

NR not recommended

RUG regular unleaded gasoline

TBA tertiary butyl alcohol

• In these tests, the AFFFs did not seal as effectively against the tank wall as did the protein-based foams.

Alcohol resistant foam

All of the alcohol resistant foam concentrates used in the testing contained some fluorocarbon surfactant. All of the foams tested were listed as 3% agents for hydrocarbons and as 6% agents for use on polar liquids. As gasoline is primarily hydrocarbon, the alcohol resistant concentrates were applied at the 3% concentrations.

Although foam quality was affected by applying the foam at the 3% concentration, the fluorocarbon activity did not diminish. There was no significant difference between the 3% alcohol resistant concentrate and the 3% aqueous film forming foam when comparing knockdown or control times. While the foam could knock down and even extinguish the fire at very low application rates (0.04 gpm/ft²), the resulting foam blanket was thin and weak and not resistant to reignition or burnback. As a result, an application rate of 0.06 gpm/ft² was established for use on regular unleaded gasoline. When the foam was used on polar additive blend fires, the application rate was boosted to 0.10 gpm/ft² to meet the test criteria.

The following conclusions can be drawn from the testing with the alcohol resistant concentrates:

• The use of alcohol resistant concentrates at a 3% concentration is satisfactory on gasoline fires with polar additive blends at the percentage levels used in this project.
• The control and extinguishment times are significantly shorter for the alcohol resistant concentrates than for either the regular protein or fluoroprotein foams.
• Increasing the application rates (which ranged from 0.04 to 0.16 gpm/ft²) did not necessarily improve fire performance in these tests.
• Overall foam stability, as demonstrated through the scalability and burnback studies, was consistently better than it was for the aqueous film forming foam.

GENERAL COMMENTS

The degree of difficulty to control, extinguish, and seal fires involving gasoline/polar solvent blends increases as follows:

Regular unleaded gasoline (RUG)

RUG + 7% methyl tertiary butyl ether (MTBE)

RUG + 7% tertiary butyl alcohol (TBA)

RUG + 10% methanol (MEOH)

RUG + 10% ethanol (ETOH)

Subsurface foam application tests were made to determine the feasibility of this technique for extinguishing storage tank fires involving a gasoline/polar additive blend. Although successful fire performance on a small scale is not indicative of success on a full-size tank, the results are indicative of polar additives’ foam destructive nature.

Fluoroprotein and alcohol resistant concentrates were also evaluated as to their ability to control and extinguish fires involving regular leaded gasoline. Fluoroprotein foam gave satisfactory fire test performance on both regular leaded and unleaded control fires when applied at 0.10 gpm/ft². However, it was unable to extinguish a 10% ethanol blend or a 7% tertiary butyl alcohol blend even when applied at 0.16 gpm/ft². The fluoroprotein foam did give satisfactory performance on a 7% MTBE fire when applied at 0.16 gpm/ft².

The alcohol resistant foam at a concentrate of 3% was unable to extinguish either the 10% ethanol blend or a 10% methanol blend at a 0.10 gpm/ft² application rate. When the application rate was increased to 0.16 gpm/ft², satisfactory performance was found with the 10% ethanol blend. Overall, better fire test performance was obtained when the alcohol resistant foam was used at a 6% concentration and a 0.10 gpm/ft² application rate.

• Theoretically, phase separation can be used as a fire suppression technique for these gasoline blends. Sweeping the burning gasoline surface of a spill fire with fog streams can cause phase separation, and then firefighters would be faced with a conventional gasoline fire that could be extinguished with firefighting foams. However, there is also the possibility that firefighters may actually increase the runoff problem and create a flowing spill fire. In addition, the water application would have to be stopped before foam application begins, or the foam blanket will be washed away.
• Personnel should approach a gasoline ethanol additive fire with caution. Ethanol fires may burn with a nearly invisible flame and may generate little or no smoke. Therefore, the existence and/or boundaries of ethanol fires are often difficult to determine.
• While the research project was not designed to answer all questions on gasoline polar additive blends of the future, it did indicate the status of currently available firefighting foams and their capability to offer sound fire protection for the present gasoline blends. Many questions were raised during the course of this project that were beyond its scope. Areas that are in need of future research include the following:
• Gasoline/polar additive blends that exceed the 10% level.
• A study of specific foams and
• their frequency of application to seal a gasoline/polar additive blend under no fire conditions (vapor suppression preventive measures).
• A study of the relative values of generic foams when evaluated by different test methods simulating field conditions.

Leaded and Unleaded Gasolines

A basic understanding of the differences between leaded and unleaded gasolines can help in learning about the application of polar solvents in unleaded gasoline and their effects on firefighting foams.

Octane ratings are a measure of the knocking properties of gasoline (the higher the rating, the fewer the knocks); traditionally, gasoline fuels have used tetraethyl lead as the primary additive to enhance octane ratings. In the early 1970s, however, concerns about its effect on air quality brought about significant changes. Unleaded gasoline became available and federal requirements reduced the use of lead as an octane booster. In March 1985, the Environmental Protection Agency (EPA) announced a two-stage phasedown of the legal amount of lead in gasoline. In January 1986, the EPA required a 91% reduction in the lead content of gasoline. Some communities in the United States, such as Cook County, IL, are totally restricting the sale of leaded gasoline.

In addition to being a poison and a potentially serious environmental pollutant, lead is destructive to the platinum catalyst used in emission control devices on late model vehicles. As a result, recent model combustion engine fuel tanks are designed to accept only gasoline from which tetraethyl lead and other related compounds have been omitted. Today, approximately 60% of all gasoline marketed in the United States is unleaded.

The changeover to unleaded gasoline has seen the petroleum industry gradually replace tetraethyl lead with other additives intended to act as octane boosters and volume extenders. Attention has focused on materials such as ethyl alcohol (ethanol), methyl alcohol (methanol), tertiary butyl alcohol (TBA), and methyl tertiary butyl ether (MTBE). These additives are commonly referred to as oxygenated hydrocarbon compounds because of their chemical structure.

Ethanol

The blending of ethyl alcohol (ethanol) with gasoline is commonly referred to as gasohol. Gasohol is normally considered as a mixture

resulting from a blend of 10% denatured anhydrous ethanol and 90% gasoline. The ethanol is 198 proof (99% pure) and is denatured with up to 5% methyl isobutyl ketone.

The water tolerance of the gasoline that’s been blended with both ethanol and methanol is critical. Critical water tolerance is defined as the maximum amount of water that can be dissolved in the finished, blended fuel beyond which phase separation (water-alcohol layer/gasoline layer) will occur. For example, the addition of as little as 2% water to gasohol can cause phase separation, and the gasoline will float above the now precipitated alcohol and water mixture.

Methanol

Although current federal regulations effectively prohibit the blending of methanol alone into unleaded gasoline, it can be used in conjunction with a butyl alcohol cosolvent such as TBA (tertiary butyl alcohol). There have been experiments using methanol-gasoline blends as high as 15% pure methanol and 85% gasoline; however, no gasoline is currently being marketed in the United States at this concentration.

Tertiary butyl alcohol (TBA)

TBA has been used as a gasoline blending agent for over 15 years. As an anti-knock additive, it is being added to gasoline at concentrations of 7% to 10%. In addition, it is used as a cosolvent for methanol and unleaded gasoline. As a cosolvent, TBA improves the water tolerance properties of the methanol-gasoline blend, acts to inhibit the corrosive potential of methanol, and reduces methanol’s impact on vapor pressure. Both the TBA and methanol are added at equal concentrations of 5% for a total additive concentration of 10%.

Methyl tertiary butyl ether (MTBE)

MTBE has been used for its octane blending value since 1977. It is commonly used at 7% to 10% concentrations. Unlike ethanol and methanol, MTBE provides a more stable blend with gasoline as it is not subject to phase separation.