In addition, a low expansion application results in a shorter drain time which means that due to limited agitation, the foam blanket is not as thick and will not last as long. The finished foam will be much wetter in a low expansion application, especially when applied through a traditional fog nozzle. As we said earlier, this does have certain advantages such as the ability to penetrate a heated thermal column. The finished foam is also less prone to be affected by wind, and the finished foam will spread out more rapidly on the spill surface.
A disadvantage of such low expansion foam is its rapid drain time. When the foam blanket begins to break down, it becomes vulnerable to ignitable vapors mixing with air. Air monitoring should be an ever-present concern around the entire area to include the monitoring within exposure buildings for any flammable gas buildup. Remain on the alert even after the foam blanket has been applied as any break in the seal could result in a rapidly ignited fire.
Quite the opposite can be said of medium to high expansion foams. A thick blanket of foam will have a longer drain time, but can be affected adversely by wind and even by the intensity of the fire.
Regardless of the type of finished foam used, if the fuel is very hot, there is the danger that the foam blanket could be degraded quickly. Since foam is primarily water, it does offer a cooling ability but in order to achieve this cooling, requires large applications of finished foam.
As with any foam operation, it is important that the nozzle match the eductor being used. Never should the nozzle be of a lower flow capability than the eductor. The nozzle should, as a minimum, be of the same flow or higher than the eductor. Lets take a look at an example that requires the application of foam via a typical adjustable gallonage fog nozzle.
A great advantage of knowing the math behind the foam process allows for even greater flexibility. Take the basics of what we have covered so far in the series. In this example let's use a 95-gpm eductor with a 200 psi rated inlet pressure. We know that our rule of thumb is that 65 percent of this number (130 psi) can be used for nozzle pressure, friction loss, and elevation. On the discharge end of the eductor we have 1 3/4-inch hose on level grade to a 100 psi adjustable gallonage nozzle that must be set on 95 gpm as a minimum.
The gallonage ring on the nozzle goes up to 125 gpm, then to 150 gpm and finally, the highest flow setting is 200 gpm. We must have a nozzle that flows the same amount as the eductor as a minimum. But what if we had the nozzle set on a higher flow than 95 gpm? The eductor will still flow the same amount of solution simply because the eductor is receiving the 200 psi inlet pressure. This allows us a lot more versatility and we can now conduct foam operations at a higher elevation or with a longer stretch of hose. Let me explain...
If we move the gallonage ring to 125 gpm, the nozzle will still flow 95 gpm of foam solution and offer finished foam at a low expansion ratio, but the nozzle pressure will only be 58 psi. This allows us to use the extra energy that was allotted to nozzle pressure to now be used for either friction loss or elevation.
In other words: " We have a total of 130 psi to use for friction loss, elevation, and nozzle pressure " Flowing 95 gpm through an orifice sized to flow 125 gpm requires a nozzle pressure of 58 psi " Taking the 130 psi (this is 65 percent of the 200 psi eductor inlet pressure that we cannot exceed) and subtracting the nozzle pressure of 58 psi, gives us 72 psi for friction loss and elevation " If the stretch was 200 feet of 1 3/4-inch hose, this has a friction loss of about 10 psi per hundred feet or a total loss of 20 psi " We now have a remaining 52-psi to overcome elevation, for say, a fire on the upper floor of a parking garage for example. At 5 psi per floor above the first, we can go over a hundred feet high!
Successful foam operations mean being able to interpret these numbers and come up with a solution to any specific emergency. For an eductor to work properly it must not experience too much backpressure. If it does, the foam will not be inducted into the pickup tube and we will not flow any foam!
Just remember, the key is no more than 65 percent of the eductor inlet pressure that can be allotted for nozzle pressure, elevation, and friction loss. Thus a 200-psi eductor can give you the flexibility of using up to 130 psi anyway you see fit.