Characteristics of Foams And Use on Polar Solvents

Characteristics of Foams And Use on Polar Solvents

Comparison of characteristics of three types of foam.

Many products of the continually expanding chemical industry require flammable, polar liquids as intermediates or solvents. Therefore, the need for effective fire protection for these polar materials will be a steadily increasing problem. Burning polar solvents require a special type foam for extinguishment because these liquids destroy the widely available proteinbase foams and aqueous film-forming foams (AFFF).

In 1933, it was discovered that chemical foam could be modified to give an alcohol-resistant foam. Several years later, two variations of mechanical alcohol-resistant foam concentrates were produced commercially and they are still in limited use. These depend on the precipitation of an insoluble soap in the bubble wall, which gives the foam some protection from the hydrophilic (water absorbing) action of the solvent.

Gentle application needed

However, the major limitation is that these types of foams must be applied very gently. NFPA Standard No. 11 designates a Type I application, by apparatus which delivers foam gently to the fuel surface, such as by a foam chute or Moeller tube, which is used in most bulk storage installations.

Another limitation of these kinds of concentrates is premix or transit time. This is generally limited by the temperature of the water used for foam making and is influenced by the fuel to which the foam is applied. In some cases, it has been necessary to use dual piping in alcohol-resistant foam systems to allow injection of the foam liquid concentrate near the foam maker. Because of this, the engineering, installation, and maintenance costs of these systems have been relatively high.

In addition, the storage life record of these concentrates is not as good as that of regular foam liquids.

Some limitations overcome

During the early ‘60s, a new foam, National AER-O-FOAM 100, was introduced as the first agent to overcome some of the limitations of earlier products. This type of foam concentrate depends on an entirely different principle and it is still widely used to protect difficult polar-type hazards.

The foam concentrate contains a low molecular weight water-soluble polymer. When the concentrate is injected into a water stream with a catalyst, the polymer’s molecular weight increases and a semi-gelled foam solution is formed. This solution is then foamed through the usual tube mechanical foam makers, such as chambers and nozzles.

This new concentrate eliminated the need for very gentle application and it also removed the premix and transit time limitations. The more durable foam bubbles, because of the precipitated polymer’s stabilizing effect, made this a much more effective fire fighting agent.

Costs somewhat higher

However, because the agent is a two-part system, the engineering, installation, and maintenance costs have been somewhat higher and the system operation is more complicated. Whereas the older type alcohol-resistant foams are used at 6 percent, the AER-O-FOAM 100 has to be used at 20 percent with the addition of a 3 percent catalyst to mix with 77 percent fresh or sea water.

In general, any alcohol-type foams work well on thin spill, or skin, fires because fuel surface dilution aids extinguishment. With skill, many polar solvent skin spill fires can be controlled even by regular type foams, AFFFs, or detergent foams—also because of surface dilution effect. As fuel depths increase, as in tanks, the surface dilution advantage quickly disappears and the need for true foam stability on the fuel becomes apparent.

The various alcohol-type foams have varying degrees of stability on deep polar fuel fires and, in some cases only a marginal effectiveness on hydrocarbon fires.

New agent developed

Through years of research, a foam has been developed that overcomes these limitations and provides extra protection, extra confidence and better performance on a broader range of flammable liquids. Universal Foam is in one agent an alcohol-type foam, a regular-type foam and an aqueous film-forming foam with none of the disadvantages of any of these agents. In addition, it is dry chemical compatible and is as effective as fluoroprotein foam on petroleum products, both topside and subsurface.

The new Universal foam concentrate is technically classifiable as a pseudoplastic sol comprised of a multipolymeric/solvent system which gives it unique fire fighting characteristics. It is a one-part system which can generally be used at a 10 percent concentration for difficult polar hazards.

The polymeric system undergoes a process known as syneresis (the contraction of a gel with the separation of liquid). When the foam comes in contact with a hydrophilic liquid, such as isopropanol, the fuel competes with the polymer for the water with subsequent polymer coagulation. A thin but very durable, cohesive, polymeric membrane which is formed on the fuel surface protects the foam from further breakdown. Destruction of the polymeric membrane by agitation results in the production of more polymeric membrane. Therefore, the agent exhibits remarkable self-healing properties.

Conventional proportioners

The Universal agent can be readily used in most proportioning equipment for conventional air foam liquid concentrates. It is a highly stabilized system and therefore has a high water retention characteristic far superior to that of the AFFFs.

The unique surfactant system coupled with high water retention gives the agent tested subsurface capability on hydrocarbon fuels. Since the agent possesses a high spreading coefficient, it will act as an AFFF on hydrocarbon type and less water-miscible fuels. The fire is rapidly knocked down and extinguished by the action of the aqueous film which drains from the foam blanket.

In some cases, depending on the fuel, neither polymeric membrane nor aqueous film is formed and the foam simply performs as a true foam. Likewise, the corollary tells us that in other cases all of the mechanisms may be interacting to produce a synergistic effect. In all instances, foam is present to transport and hold water for cooling, to smother by exclusion of oxygen, and to suppress and separate flammable vapors from the atmosphere. In short, Universal Foam has the fluidity of AFFFs or syndet foams and the stability of fluoroprotein.

Concentrations used

It has demonstrated effective fire performance on water-miscible flammable liquids at a 10 percent concentration. Nonmiscible or hydrocarbon flammable liquids are easily handled with Universal at a 3 percent concentration.

When fire tests were conducted, fuels which were recognized as more difficult to handle were considered apart. For example, preliminary fire test work with Universal Foam on triethyl and dipropyl amines showed promise when the application rate was higher or the application was more gentle. Amines are notorious foam breakers and are often used for that purpose.

All Universal testing mentioned in this article used Type II application techniques and devices. NFPA Standard 11, “Foam Extinguishing Systems,” defines a Type II discharge outlet as one “which does not deliver foam gently onto the liquid surface but is designed to lessen submergence of foam and agitation of the surface.” Type II application does not imply direct plunging, which is not a recommended technique for any fuel.

Foam flows feely

Universal Foam is a fluid, fast-moving foam which possesses excellent water retention properties as measured by NFPA test methods. This allows foam to flow freely, seal around obstructions and self-heal when the blanket is physically disrupted.

A 100-square-foot methanol fire was extinguished within 2 minutes with a 10 percent Universal Foam discharged from foam spray deflectors 23 feet above the fuel surface while in an identical test regular foam/continuous water deluge took more than 21 minutes. The former produced a 1 1/4-inch foam blanket and fuel dilution of less than 12 percent; the latter produced no foam blanket and a fuel dilution of 79 percent. The Universal polymeric membrane established itself early in the application, allowing a stable foam blanket to build and extinguish the fire.

An added bonus is Universal’s superior resistance to burnback in comparison to the earlier soap-based alcoholtype foams. Early conventional alcohol-resistant foams did not possess the heat-resistant properties or the capability to self-heal mechanically damaged foam surfaces.

Tested in conventional devices

Designed for use in all conventional foam making devices, Universal Foam has been tested and evaluated through portable and stationary (oscillating) nozzles, spray deflectors, and fixed discharge outlets, such as foam chambers and deck (dike) nozzles. Proportioning can best be accomplished by using positive displacement (rotary type) pumping and pressure proportioners. Positive displacement pumping is common to all fixed proportioning installations and includes balance pressure and indirect orificing systems.

Accurate proportioning with line proportioners or venturi-type induction devices may require modification of the equipment. Complete details relating to equipment and system design may be obtained from the “Engineering Manual” (National Foam System, Inc., 150 Gordon Drive, Lionville, Pa. 19353).

In emergency situations where proportioning systems are unavailable, the concentrate can be premixed at the site by bulk mixing since it has no transit time limitations. Long-term premix storage, however, is to be avoided.

Approved and listed

Universal Foam has been approved and listed by Underwriters’ Laboratories and Factory Mutual Laboratories for use on both petroleum and polar flammable liquids. It fits neatly into approproate portions of the 1974 edition of NFPA 11, and exceeds in performance all provisions of NFPA 11B, “Synthetic Foam and Combined Agent Systems.”

Basic to the application of any fire fighting foam is the avoidance of direct plunging or immersion of the foam into the fuel. Immersion into the fuel will produce one or more effects. The fuel picked up may enter into the foam and weaken the overall heat-resistant properties; oleophobic foams will carry fuel and vapors which, while burning off, will prolong extinguishment; or water-miscible flammable liquids (ethanol, isopropanol, acetone) will cause almost instant foam breakdown. Considerations are double important in cases involving lower amines and reactive chemicals (e.g. sulfur chloride).

In fixed systems, the proper application is designed into the system. Portable or manual applicators rely on the expertise and training of the fire fighter.

Fire fighting techniques

The fire fighter is instructed to:

  1. Use the full range and width of the foam pattern, letting the foam do the work.
  2. Lay, or skim, the foam pattern deliberately across the fuel surface with a low trajectory for rapid extinguishment of spills and shallow fires.
  3. Utilize tanks, dike walls, and other obstructions to deliver the foam indirectly to the fuel surface, minimizing immersion.
  4. Use a flaking pattern, especially on hydrocarbons.

Comparison of systems

To show the difference in effectiveness between a fixed system and portable application, we picked a highly volatile fuel, propylene oxide, which could not be extinguished by any previously available foam-type agent. Identical 100-square-foot propylene oxide fire tests with a fixed device and a manually held foam maker resulted in fire extinguishment times of 1 minute 30 seconds and 3 minutes, respectively. The tests dramatically demonstrated the importance of proper application.

The fire fighter, wearing standard turnout gear, was repelled by the intensity of the fire and required 2 minutes 5 seconds to bring the fire under control. The fixed device, activated remotely and discharging at dike level, attained control in 45 seconds with no unnecessary exposure to fire personnel.

Universal Foam has been tested and found effective as a fire fighting agent for subsurface injection in hydrocarbon fuel at a 3 percent concentration. Subsurface application cannot be used for flammable polar liquids. Conventional high back pressure or forcing foam makers can be used with no modification required.

Studies have been conducted on chlorosilanes, sulfur chloride, sodium methylate, lithium amide and some lower amines. All of these agents, except the amines, are highly reactive with water. This would normally preclude the use of aqueous foam. However, in all tests conducted, the reactivity was either arrested or greatly attenuated by Universal Foam.

It has also been demonstrated that effective fire protection can be designed for highly flammable and explosive fire hazards, such as the lower ethers and propylene oxide. □ □

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