For final extinguishment, the application of hoselines is required, but the value of the high expansion foam can be seen. It is a special category that is an area of specialization unto itself. Again, consult the major manufacturers and some of today's exceptional fire service authors for specifics on working with high expansion foam. For our purposes, the reason for explaining high expansion foam is to present the reader with a sense of expansion ratios at the upper end of the spectrum. This is certainly not a typical tactic for today's fire service.
Medium expansion foam (up to 200:1) can be delivered via a specialized nozzle. It is not as dependent on wind conditions as high expansion foam, but the foam stream range is limited. Again, most of the stream is air and subjected to various limitations. Medium expansion foam is not a common application for the typical municipal fire department.
Low expansion foam has an air to foam solution ratio of up to 20:1. This offer a longer reaching stream and greater penetration of a thermal column. A foam blanket can be created successfully, but not with the same degree of thickness as would be experienced with medium expansion foam. Therefore the advantages and limitations of each application must be weighed with the situation at hand.
Nozzles And Air Aspiration
Most municipal foam operations are centered on low expansion foam applications. Nozzles can be divided into two primary categories, air aspirating and non-air aspirating.
Non air-aspirating nozzles are the typical fog nozzles used on our engine companies today. These nozzles can effectively apply an Aqueous Film Forming Foam (AFFF) or a Film Forming Fluoroprotein (FFFP) Foam solution at an expansion ratio of really no more than 6 or 7:1 depending on the manufacturer, and as low as 2:1. Set on a straight stream, the nozzle will deliver the greatest range of finished foam. This increase in range enhances the safety of our members as well. In addition, the straight stream setting offers the greatest expansion ratio compared to a wider fog pattern because the stream is initially impinging on itself as it leaves the nozzle. This agitation is a critical component of foam application and as such, the typical fire department fog nozzle offers great value and flexibility.
The non-air aspirating nozzle can be of great value for the typical hydrocarbon spill because of the aqueous film solution that is part of the finished foam. This film moves out quickly ahead of the main foam blanket and begins to cover the spill. The use of the non-air aspirating nozzle is of limited value on polar solvents. For these incidents, an air-aspirating nozzle allows the polymeric membrane to form and create the necessary barrier.
Air-aspirating nozzles are a more specialized nozzle that is designed to entrain air into the stream at a higher ratio than a typical fog nozzle. The types of air aspirating nozzles vary and are designed to flow various quantities of finished foam depending on the model. Air aspirating nozzles come in basically two varieties. The first is an attachment for the typical fog nozzle. This is an easy way to acquire a thicker finished foam blanket. The expansion ratio varies but a 10:1 expansion ratio is a good number to start at. Consult the various manufacturers for the specific details of these air aspirating nozzle attachments. They offer a shorter range but provide a better expansion ratio than the typical fog nozzle without such an attachment.
The second variety of air aspirating nozzle is specifically designed for foam application. These tube like nozzles induct air into the stream at about a 10:1 ratio or better. The range of the stream is fair and the expansion ratio is also better than any fog nozzle alone.
The application of foam via an air aspirating or non air-aspirating nozzle depends on the type of spill. For example, the use of an air-aspirating nozzle can be successfully used to apply finished foam to both polar solvents as well as hydrocarbons. But, the application of foam via a non air-aspirating nozzle can primarily be successful on hydrocarbon spills. These nozzles allow for the aqueous film to spread rapidly onto the spill but the membrane that would be needed for use on a polar solvent requires the air aspirating type of nozzle.