It’s important that the metering valve be set at the correct percentage setting otherwise the ratio of concentrate to water may be too lean. If the department plans on using a 3% mix (3 gallons of concentrate to 97 gallons of water), then the metering valve should be set appropriately. Failure to match this setting with what a particular spill calls for could result in a weak foam blanket and a potentially unsafe situation. Therefore, always make sure that the eductor is set on the proper ratio for the situation.
Don’t confuse the setting of the metering valve with the amount of foam solution it flows. For example, a foam eductor may be rated at 250 gallons per minute (GPM), with an inlet pressure of 200 pounds per square inch (PSI). The metering valve is then set for any of several settings such as 1%, 3% or 6%, for example. In other words, using this eductor can have a flow range of 2.5 gallons of concentrate and 247.5 gallons of water per minute with the metering valve set at 1%. With the control set at 3%, the foam solution ratio is 7.5 gallons of concentrate and 242.5 gallons of water per minute. Finally, with the valve set at 6%, we get 15 gallons of concentrate mixed with 235 gallons of water per minute.
If an eductor receives less than the correct inlet pressure, let’s say 150 PSI instead of 200 PSI, the foam solution will be “richer” in concentrate. If the eductor had an inlet pressure of say, 225 PSI instead of 200 PSI, then the foam stream would be too lean. Foam that is too lean doesn’t create the proper application it’s designed to and this can create a weak foam blanket.
One important note about picking up foam concentrate and adding into the stream; the pick-up tube should be no more than six feet above the surface of the concentrate. Beyond this height, no foam concentrate will be picked up, or at the very least, certainly not the proper ratio.
Also, both the alcohol resistant Aqueous Film Forming Foam (AFFF) and alcohol resistant Film Forming Fluoroprotein (FFFP) are a thicker concentrate than non-alcohol resistant concentrates. This greater viscosity requires more effort to pickup. Colder temperatures can also affect viscosity. All UL-approved foam concentrates are proportion tested to as low as 35 degrees. Some manufacturers offer a pickup with a slightly larger diameter and some sources recommend that the small strainer at the end of the pickup tube be removed to aid in concentrate pickup and delivery.
The inlet pressure of the eductor varies with the manufacturer, but most operate at an inlet pressure of 200 PSI. The next bit of information will determine if your foam streams are successful or not. Backpressure becomes a serious issue with foam operations and the portable inline eductor. With a 200 PSI inlet pressure, the engine company cannot exceed 65% of this number for nozzle pressure, friction loss, and elevation loss on the discharge side of the eductor.
As an example, suppose we have a small fire and are tasked with going into action with our foam eductor. The eductor has an inlet pressure of 200 PSI. We know that 65% of this, or 130 PSI, can be allotted for hose, appliances, and elevation. If the nozzle has an operating pressure of 100 PSI, we have only 30 PSI remaining for use in friction loss and/or elevation loss.
Let’s say that the nozzle in this scenario is level with the eductor and therefore elevation is not an issue in this stretch. That gives us 30 PSI remaining for overcoming friction loss. If the eductor is designed to flow 125 GPM and we are using 1¾” hose, then the friction loss in this hose at 125 GPM is about 20 PSI per hundred feet. Therefore, we can stretch a line as long as 150 feet of 1¾” hose to apply the foam stream.
Let’s take a look at that again, here are the highlights:
- We are using a 125 GPM eductor with a rated inlet pressure of 200 PSI
- We can allot no more than 130 PSI for elevation, friction loss, and nozzle pressure (65% of the rated eductor inlet pressure)
- Friction loss in 150 feet of 1¾” hose at 125 GPM is about 30 PSI (maybe more or less depending on the brand of hose, age, and wear on it)
- Nozzle pressure is 100 PSI in this scenario
- Elevation is zero in this scenario
- Therefore we can stretch no more than 150 feet of 1¾” hose with a 100 PSI nozzle from the eductor to the scene