This invention relates to an airborne foam delivery apparatus for applying fire suppressant foam to wildland fires.

Background Art

Wildland fires are a threat in many areas of the world. Effectively controlling and containing these fires is essential to the preservation of national parks, forests, and private property. Significant efforts are therefore directed to developing and improving wildland firefighting techniques.

Wildlands are characterized by difficult and often inaccessible terrain. Fighting fires in such rugged terrain is difficult. In many situations, the only realistic access to burning areas is by air. Therefore, wildland firefighting efforts quite often include the application of water from fixed wing airplanes and helicopters. Water, as it is converted into steam, has tremendous capacity to absorb and carry away heat. It has the further advantage of being environmentally safe. In addition, adequate and convenient reservoirs of water can often be found near burning fires. Helicopters equipped with "slung buckets" can quickly load from such reservoirs without landing. The close proximity of a reservoir also minimizes round trip flying time.

Despite the advantages of water in fighting wildland fires, water alone can be inefficient—especially when applied from the air. Much of the water evaporates before reaching the ground. Water which does reach the ground beads up and rolls off fuels before absorbing its full heat capacity. It has been estimated that water is only 5% to 10% efficient.

Many fire retarding or suppressing chemicals are available which are more efficient than water. However, they are often difficult to use against wildland fires. One problem with such chemicals is that they are environmentally toxic. They are also difficult to transport to remote locations in large quantities. In many cases, fire retardant chemicals require specialized and bulky application equipment. Both the chemicals and the application equipment are expensive.

Foam is one type of fire suppressant which is particularly efficient in terms of both effectiveness and cost. Foam fire suppressant can be created by different methods. However, most methods start with adding a foam concentrate to water in concentrations ranging from 0.1% to 1.0%. "Phos-Chek" fire

suppressant foam concentrate is one popular foam concentrate, available from Monsanto Chemical Company of St. Louis, Missouri. One reason for its popularity in fighting wildland fires is its relative biodegradability and its approval by necessary government agencies. Foam varies from being wet and runny to being as stiff as lather. The stiffness depends on bubble size and expansion ratio. Expansion ratio is the increase in volume as the water becomes foam. Applying foamable liquid under pressure through a standard nozzle results in a relatively low expansion ratio, and a somewhat runny foam. Special air-aspirated nozzles can result in much higher expansion ratios, producing much drier foam.

Foam fire suppressant can be applied from standard fire engines and other vehicles. Foam suppresses fires by surrounding fuel with a thick layer of water, which does not bead up and roll off. This allows the water to absorb its full capacity of heat, and also allows more water to be absorbed into the fuel. Furthermore, a foam blanket creates a vapor barrier between fuel and oncoming fire. It reflects oncoming radiant heat, insulates fuel, continuously releases water from the foam's bubble structure, and helps smother the fuel. A further advantage is that firefighters can easily see where foam has been applied.

Until recently, experts were skeptical of the utility of applying foam to wildland fires from the air. However, this skepticism has now been overcome, and foam concentrate is sometimes added to water applied from aircraft. However, because of the large volume of water which must be quickly dumped to effectively fight wildland fires, applying the water under pressure through nozzles has been impractical. Rather, the water is dumped from conventional slung buckets or fixed tanks. Foam is generated by air turbulence produced by free fall of the water.

The foam created in this manner is relatively runny, having a low expansion ratio. However, the surfactant qualities of the foam concentrate, even without thick foam, are enough of an advantage over water alone to justify the addition of foam concentrate. Nevertheless, it would be desirable to provide thicker foam from airborne applicators.

The most significant obstacle to creating high foam expansion ratios from airborne applicators is the extremely high volume of water which must be applied, at a very high flow rate. The need for a high flow rate is brought about by the speed of the aircraft. In a fixed wing airplane, minimum ground speeds are determined by the very nature of the aircraft. When a helicopter

is used to apply water, downdraft or "rotor wash" from the helicopter rotors mandates minimum ground speeds. Helicopter rotor wash is a serious problem for helicopters in wildland fire fighting. It can quickly defeat the effects of applied fire suppressants. The ground turbulence from rotor wash causes the fire to expand, and can endanger nearby fire crews. To minimize rotor wash problems, water applicators are usually suspended a considerable length beneath the helicopter, such as by a distance of 15 meters or more. In addition, minimum ground speeds are maintained to reduce the rotor wash impact on any particular ground area. The minimum ground speeds required of aircraft when fighting wildland fires mandates a very high application rate of fire suppressant. The flow from a standard firehose nozzle is simply not adequate. However, equipment to produce adequately high application rates of high-expansion foam has not been available for airborne operation. Accordingly, most firefighting efforts continue to rely on standard slung buckets, which depend on gravity to quickly jettison their entire water loads. Foam concentrate is added to the water to act as a surfactant and to provide a minimal amount of foaming through turbulence with the air.

Airborne firefighting effectiveness could be significantly improved if it were possible to apply high-expansion foam from aircraft at application rates approaching those achieved with standard slung buckets. To this date, however, there exists no practical high-expansion foam applicator which will achieve these rates. The invention described below fills the need for such an airborne foam applicator.

Brief Description of the Drawings

A preferred embodiment of the invention is described below with reference to the accompanying drawings:

Fig. 1 is a side view of an aerial foam delivery apparatus in accordance with a preferred embodiment of the invention;

Fig. 2 is a top view of the aerial foam delivery apparatus of Fig. 1; Fig. 3 is a bottom view of the aerial foam delivery apparatus of Fig. 1; Fig. 4 is a sectional view of the aerial foam delivery apparatus of Fig. 1, taken along the line 4-4 of Fig. 3; and Fig. 5 is a schematic diagram of pneumatic components associated with the aerial foam delivery apparatus of Fig. 1.

Best Modes for Carrying Out the Invention and Disclosure of Invention

Figs. 1-4 show an airborne foam applicator or foam delivery apparatus in accordance with a preferred embodiment of the invention, generally designated by the reference numeral 10. Foam applicator 10 is designed for operation beneath a helicopter to deliver a high volume of high-expansion fire suppressant foam.

Foam applicator 10 comprises a sealed or sealable liquid tank 12 for holding a foamable liquid such as a water and foam concentrate solution. Liquid tank 12 is generally cylindrical in shape, having a diameter of approximately 183 centimeters and an interior capacity of approximately 3785 liters. It is constructed of stainless steel to reduce interaction between internal tank surfaces and contained foamable liquid. Liquid tank 12 is designed to be capable of withstanding positive internal pressures of at least 689.5 kPa.

Liquid tank 12 includes mounting features which allow it to be connected to a helicopter sling, in a fashion similar to a helicopter slung bucket, for supporting the liquid tank from beneath the helicopter. Specifically, liquid tank 12 includes a support frame 14 which extends downward from liquid tank 12 to support it above the ground. Support frame 14 includes a circular upper mounting ring 16 which encircles the upper portion of liquid tank 12. Support frame 14 also includes a circular lower support ring 18. Both rings 16 and 18 have a diameter slightly larger than the outer diameter of liquid tank 12. Four support frame legs 20 are spaced about foam applicator 10, and extend vertically between upper ring 16 and lower ring 18.

Support frame 14 serves several functions. For instance, upper ring 16 is connectable to a helicopter sling, allowing foam applicator 10 to be conveniently connected for operation beneath a helicopter. Support frame 14 also serves as a pedestal or stand for supporting liquid tank 12 above the ground when it is not in use or when it is being refilled. In addition, the support frame acts as a bumper or protective guard to reduce damage from collision of foam applicator 10 with other objects. A plurality of mounting brackets 22 extend between legs 20 and liquid tank 12 to position liquid tank 12 within support frame 14.

A protective shroud or apron 30 extends downward from the circular periphery of liquid tank 12 to enclose a central nozzle area 31 directly beneath liquid tank 12. Apron 30 has an upper screened section 32 and a lower solid section 34. Upper screened section 32 is formed of a screen mesh to allow

substantial quantities of air to pass through protective apron 30 and into the central nozzle area. Lower solid section 34 is fabricated from a material which blocks air. Protective apron 30 serves both as a physical guard to the enclosed nozzles, and as an air block or wind deflector as will be more fully described below.

Three aerating nozzle assemblies 40 are positioned within central nozzle area 31. Nozzle assemblies 40 are best observed in Fig. 4, being directed generally downwardly from beneath liquid tank 12. The three nozzle assemblies are equally spaced from each other about an imaginary circle beneath liquid tank 12. Each of nozzle assemblies 40 includes a turret pipe 42 which is in fluid communication with the interior of liquid tank 12 and which is directed generally downwardly from the bottom of liquid tank 12. The turret pipes are also directed slightly outward and away from each other, an approximately 12 degree angle from vertical. Each turret pipe 42 has an approximately 6.35 centimeter internal diameter.

Each turret pipe 42 extends from liquid tank 12 to a liquid valve 44, specifically an air-actuated butterfly valve. In the preferred embodiment, butterfly valve 44 is a 2Vz inch "type 75" valve manufactured by Asahi/America of Maiden, Massachusetts. It is equipped with a pneumatic actuator and an electric solenoid valve 96 for remote actuation. This valve is capable of being fully opened in half a second. While other types of valves could be used, such as diaphragm and gate valves, they would not provide the quick response of the butterfly valve described above. The solenoid valve 96 accepts a 24-volt electrical signal from a helicopter through a control line 97 to release foamable liquid from liquid tank 12.

A nozzle 46 extends beyond each valve 44. Each nozzle 46 has a 5 centimeter orifice with a 90 degree full cone projection, allowing a flow of about 60 liters per second at 689.5 kPa water pressure. At this flow rate, water or foamable liquid emerges from nozzles 46 at a velocity of approximately 29.4 meters per second. A liquid velocity of 18.3 meters per minute is a preferable minimum for obtaining optimum foam expansion ratios. Nozzles such as used in the preferred embodiment are available from Bete Fog Nozzle of Greenfield, Massachusetts.

One liquid valve 44 thus corresponds to each nozzle, with the liquid valve being operably interposed between the liquid tank and the corresponding nozzle. Liquid valve 44 is remotely operable between a closed position and an open

position to allow passage of the foamable liquid through the corresponding nozzle.

An aerating screen 48 is positioned beneath each nozzle 46 to aerate and foam the liquid emerging from the nozzle. The distance from screen 48 to nozzle 46 is important in achieving high foam expansion, and must be determined experimentally for different nozzle types and sizes. In the preferred embodiment, maximum expansion is obtained at a screen-to-nozzle spacing of about 36 centimeters. Each aerating screen 48 is concave or spherically shaped, having a radius of curvature approximately equal to the distance from the nozzle to the aerating screen. Individual aerating screens are formed in quadrants of stainless steel mesh having a wire spacing of from 12 to 24 wires per centimeter. The wire spacing in the preferred embodiment is 16 wires per centimeter. Each screen extends over an arc of at least ninety degrees to fully encompass the cone projection of the corresponding nozzle. The screens are mounted to the nozzle assembly by sets of mounting struts.

Liquid tank 12 has a top access hatch 50 which is about 46 centimeters in width to allow entry of a person into the tank for inspection. Access hatch 50 incorporates a pressure safety head 52 with a rupturable disk to provide secondary protection against excessive internal pressures within tank 12. Access hatch 50 is sealable against a rim 54 with appropriate fastening mechanisms (not shown).

Liquid tank 12 also has a fill port or tank coupling 55 positioned near the bottom of tank 12 to allow filling or refilling the tank with foamable liquid. Locating the fill port on or near the bottom of the liquid tank reduces agitation and aeration within the tank during filling.

Three high-pressure gas containers or canisters 60 are securely mounted within liquid tank 12 for containing compressed gas, and preferably for containing compressed air. Gas canisters 60 have high pressure outputs which are manifolded together through a high-pressure supply line 62. High-pressure supply line 62 extends through the wall of liquid tank 12 to a pneumatic control system 64. A low-pressure supply line 66 extends back into liquid tank 12 from pneumatic control system 64 and upward to the top of liquid tank 12. A high- pressure gas coupling 68 is in fluid communication with pneumatic control system 64 to allow refilling or recharging gas canisters 60 with compressed air from an external source or compressor.

Pneumatic control system 64 is shown schematically in Fig. 5, along with associated pneumatic and hydraulic components. As mentioned above, gas canisters 60 are manifolded together through high-pressure supply line 62. Supply line 62 is connected to a plurality of parallel pressure regulators 82, so that the pressure regulators receive compressed air from gas canisters 60. Supply line 62 is also connected through a check valve 84 to high-pressure gas coupling 68. A pressure relief valve 86 is connected to supply line 62.

At least three and possibly four pressure regulators are required to meet the high air flow requirements of the system. Pressure regulators 82 have pressure outputs which are manifolded together and connected for fluid communication with the interior of liquid tank 12. Regulators 82 are operable to controllably discharge compressed air from canisters 60 into liquid tank 12 and to maintain a positive regulated pressure within liquid tank 12. The regulators are set to produce a regulated output of about 689.5 kPa. In the preferred embodiment, they must be capable, together, of supplying about 85 cubic meters of air per minute at 689.5 kPa.

The pressure-regulated air is connected through a check valve 88 to a two position valve 90. In a first position, valve 90 allows fluid communication between the regulated outputs of regulators 82 and low-pressure supply line 66. In a second position (not shown), valve 90 allows fluid communication between low-pressure supply line 66 and the ambient atmosphere, or between the interior and exterior of tank 12.

A low-pressure relief valve 92 is also connected to the outputs of pressure regulators 82, and the pressure-regulated air is supplied through a check valve 94 to drive butterfly valves 44, which include close-coupled solenoid actuators 96. A 24-volt valve control line 97 is connected to actuate solenoids 96.

The various control components are contained within apron 30 to be protected from collision with external objects. They are accessed by doors (not shown), with appropriate interlocks (not shown) to protect operators. While still providing a protective function, upper screened section 32 allows entry of air into central nozzle area 31 to permit aeration of the foamable liquid. Nozzles 46 are preferably positioned just below upper screened section 32, within lower solid section 34. Lower solid section 34 protects the nozzles from wind to prevent the wind from interfering with the interaction between the nozzles and aerating screens 48 as applicator 10 is carried by a helicopter.

In operation, foam applicator 10 is prepared for operation by filling liquid tank 12 through fill port 55 with foamable liquid, and by charging gas canisters

60 with compressed air. During filling, two position valve 90 is set in its second position to isolate the pneumatic control system from the liquid tank

J interior and to allow air to escape from liquid tank 12.

Filling liquid tank 12 with foamable liquid requires connecting a water supply line (not shown) to fill port 55 and pumping water into the tank. Foam concentrate is added to the water either prior to its entry into tank 12 or by dumping it into tank 12 from access hatch 50. 0 , Concurrently with filling tank 12 with foamable water, an external compressor is connected to high-pressure gas coupling 68 to charge canisters 60 with compressed air. Minimum fill times are desired, resulting in a need for one or more high-horsepower compressors.

The embodiment described above is designed for a working liquid capacity 5 of about 3400 liters. The gas canisters are sized to displace all 3400 liters of liquid under a pressure of 689.5 kPa. To meet this requirement, each canister has a capacity of 8.5 cubic meters at 12,411 kPa. A minimum desirable fill time is five minutes. A more preferable fill time is three minutes. Charging each of these cylinders to approximately 12,411 kPa within three minutes requires an 0 external compressor or several smaller compressors capable of supplying approximately 142 liters per second.

Locating high-pressure canisters 60 within liquid tank 12 results in at least two significant advantages. First, the canisters are protected from collisions with external objects. Second, the heat generated within the canisters by rapidly charging them with compressed air is dissipated by the water within liquid tank 12. This warms the water, resulting in more efficient formation and expansion of foam.

Once the applicator has been filled and charged, two position valve 90 is returned to its first position. With the valve in this position, the pressure regulators are connected to maintain a positive regulated pressure within the liquid tank of approximately 689.5 kPa.

Foam applicator 10 is then borne by a helicopter to a target location for application of foamed fire suppressant over a wildland fire. Applicator 10 is preferably suspended by cables at least 15 meters below the helicopter so that the foam applicator can be operated close to the ground while minimizing downdraft from rotor wash. When over the target, a helicopter pilot actuates

a remote release actuator which is connected to supply a 24-volt signal through control line 97 to solenoids 96. This opens butterfly valves 44. The positive pressure within the liquid tank, regulated to 689.5 kPa, expels the foamable liquid through nozzles 46 when butterfly valves 44 are operated. The pressure regulators maintain the positive regulated pressure within the liquid tank during expulsion of the foamable liquid, to maintain a constant liquid velocity to aerating screens 48. The foamable liquid is expelled under pressure through nozzles 46 and through aerating screens 48.

The desired maximum time for expulsion of all liquid from the tank is about 30 seconds, corresponding to an expulsion rate of about 114 liters per second. In practice, the particular selection and arrangement of components and regulated pressures as described above are responsible for achieving an expulsion time of less than 19 seconds, corresponding to an expulsion rate of greater than 177 liters per second and a velocity of about 29 meters per second through the nozzle. This rate has not previously been achieved in a device which is practical for airborne operation.

The use of plural nozzles is one feature of the invention which allows such a high expulsion rate. While it would be possible to use a single nozzle, with a larger orifice, this would present practical difficulties. For instance, a larger nozzle would be disproportionately heavier than the combination of three smaller nozzles. The corresponding aerating screen would not work as efficiently with a large nozzle to achieve high expansion rates. Air actuated valves would also be a problem— a 5 inch air-actuated butterfly valve requires four seconds to open, rather than the 0.5 seconds of the specified 2Vι inch valve. Avoiding the use of liquid pumps is another feature which makes the invention practical. An approximately 500 horsepower pump would be required to achieve flow rates of 177 liters per second through the specified nozzles. Such a pump would be as heavy as the entire apparatus described above.

The use of regulated air pressure within a pressurized liquid tank is a further feature which contributes to the practicality of the invention. This feature is in contrast to much smaller devices (such as hand-held fire extinguishers) which charge the entire internal volume of a liquid vessel to a pressure which is high enough to expel all liquid from the vessel. This type of charging would require that liquid tank 12 be capable of withstanding extremely high internal pressures-an impractical requirement in a 3785 liter tank. Furthermore, it would result in very high initial discharge rates, tapering off to

very low discharge rates as the tank was emptied. In contrast, the embodiment of the invention described above achieves a constant discharge rate as the tank is emptied, and requires that the tank be designed for maximum internal pressures of only about 689.5 kPa. The particular nozzle and aerating screen arrangement described above results in very high expansion of foam-much better than is achieved by simply opening dump gates and relying on turbulence generated by the free fall of water through the atmosphere. The degree of foam expansion is also much higher than would be achieved by use of nozzles alone. Using foam concentrate in conventional slung buckets results in expansion ratios of 12:1. In contrast, tests have indicated that the nozzle and aerating screen described above, when operating at a water pressure of about 689.5 kPa, are capable of producing foam expansion ratios of 40:1. These results are achieved when using Phos-Chek WD 881 foam concentrate in a 1% solution.