The addition of foaming agents to fire-fighting water as a fire suppression agent has been recognized as early as the 1870s, when the first such use was reported to have been patented in England. Since then, through the years, further advances have been made. These advances have included better understanding of the function of the foaming agent, the type of delivery system needed to dispense and handle the foaming agent and, most significantly, the foam-to-water ratio for particular fire applications.
For many years, Class A type fire-fighting foams of the A-FFF type have never been used to combat fuel fires such as JP4 jet fuel, gasoline and diesel fuels. More recently, however, Class A foams, such as the agents sold under the trademark Sylvex of the Ansul Company, have been found to increase the fire suppression efficiency of water from three to up to nine times when used on wild land, Class A structure fires, and on hydrocarbon fires. This recognition has greatly expanded the utility and usage of foam as an efficient fire-fighting agent.
In the past, a fairly standard foam-to-water ratio of 6:94, or 6 parts foam to 94 parts water, was used and an apparatus capable of maintaining such a ratio at various water volumetric flow rates was disclosed in a number of patents, such as U.S. Pat. No. 4,064,891 to Eberhart and U.S. Pat. No. 4,448,256 to Eberhart, et al. The '891 patent to Eberhart discloses a balanced pressure valve. The '256 patent to Eberhart, et al., employs a positive displacement gear pump to maintain the standard ratio of foam to water despite changes in water volumetric flow rates.
Most recently, it has been recognized that the foam-to-water ratio of 3:97 over water volumetric flow rate variations from 20 to 1,000 gallons per minute (GPM) were needed, along with a more complex proportioning valve to ensure a stable foam-to-water ratio over the wide volumetric flow rate. U.S. Pat. No. 4,633,895, also to Eberhart, discloses a proportioning valve comprising a shuttle-type system for accommodating such foam-to-water ratio over the above-identified volumetric flow rate. However, with further development of foam suppression fire-fighting, it became apparent that foam-to-water ratios as low as 0.2:99.8 were needed and precise proportioning at 0.2% increments up to a foam-to-water ratio of 1:99 could be important in various applications.
U.S. Pat. No. 5,165,442 to Fiala discloses a differential valve for accommodating the mixing of foam and water at ratios as low as 0.2% and as high as 1%, in 0.2% increments. Generally speaking, the differential valve is constructed having an internal mechanism that produces a pressure drop or reduction in pressure through the valve, between the water inlet and water outlet, which increases in proportion to an increase of water through the valve. The pressure differential that is produced by the valve is used to pressurize foam, contained in an external foam dispensing tank and the like, for delivery at a preset ratio into the water stream passing through the valve. However, it has been discovered that at increasing water flow rates above about 150 GPM the delivery ratio of foam to water does not remain constant but, rather, varies from the desired constant ratio and is unpredictable. The inability of the differential valve to provide a desired consistent delivery ratio of foam to water at volumetric flow rates greater than about 150 GPM adversely affects the fire-fighting efficiency of the foam-water system.
With further development of foam suppression fire-fighting, it has become apparent that foam-to-water ratios as low as 0.1:99.9 may be needed in certain applications, and precise proportioning of foam to water up to a 1:99 ratio could be important in various applications under various water volumetric flow rates that include volumetric flow rates greater than 150 GPM, i.e., at volumetric flow rates where other known differential valves are usable to provide an accurate proportion of foam to water.
It is, therefore, desirable that a differential valve for delivering foam into a liquid stream be constructed in a manner that produces a predictable and controlled pressure differential, between the inlet and outlet of the valve in liquid passing through the valve, under a wide range of flow rate conditions to facilitate accurate foam delivery from the valve into the liquid passing therethrough. It is desirable that a differential valve be constructed so that it is compatible with existing foam-water delivery systems. It is also desirable that a differential valve be constructed from relatively conventional materials using conventional manufacturing techniques, comprise a minimum number of moving parts, and be simple to operate.