In previous editions of Firehouse® Magazine I began to unshroud some of the mystery that surrounds firefighting foams for many of us. In those articles we looked at nozzle selection and required flow rates for incidents involving Class B fuels (flammable and combustible liquids...
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Rule 2: The pressure at the outlet of the eductor (also called back pressure) must not exceed 65 to 70% of the eductor inlet pressure. Eductor back pressure is determined by the sum of the nozzle pressure, friction loss in the hose between the eductor and the nozzle, and the elevation pressure. If back pressure is excessive, no foam concentrate is inducted into the water.
Rule 3: Foam solution concentration is correct only at the rated inlet pressure of the eductor, usually 150 to 200 psi (1,050 kPa to 1,400 kPa). Using eductor inlet pressures lower than the rated pressure for the eductor results in rich foam concentrations (too much concentrate for the given amount of water). Conversely, using inlet pressures greater than the rated inlet pressure produces lean foam concentrations (not enough foam concentrate for the given amount of water). A too rich or too lean concentration might not extinguish the fire or cover the spill properly.
Rule 4: Eductors must be properly maintained and flushed after each use. Flush the eductor by submerging the foam pickup tube in a pail of clear water and inducting water through it for at least one minute or until clear water is being discharged from the nozzle. At the station, thoroughly clean and check the eductor, including the strainer, after each use.
Rule 5: Metering valves must be set to match the foam concentrate percentage and the burning fuel. Failure to do so results in poor quality foam.
Rule 6: The foam concentrate inlet to the eductor should not be more than six feet (two meters) above the liquid surface of the foam concentrate (Figure 4). If the inlet is too high, the foam concentration will be very lean, or foam may not be inducted at all.
In order for the nozzle and eductor to operate properly, both must have the same rating in gpm (L/min). Remember that the eductor, not the nozzle, must control the flow. If the nozzle has a lower flow rating than the eductor, the eductor will not flow enough water to pick up concentrate. An example of this situation is a 60-gpm (240 L/min) nozzle with a 95-gpm (380 L/min) eductor.
Using a nozzle with a higher rating than the eductor also gives poor results. A 125-gpm (500 L/min) nozzle used with a 95-gpm (380 L/min) eductor will not result in proper eduction of the foam concentrate. Low nozzle inlet pressure, however, results in poor quality foam.
One type of foam proportioning equipment that is related to the in-line eductor is the foam nozzle eductor. The foam nozzle eductor operates on the same basic principle as the in-line eductor. However, this eductor is built into the nozzle rather than into the hoseline. As a result, its use requires the foam concentrate to be available where the nozzle is operated.
If the foam nozzle is moved, the foam concentrate will also need to be moved. The logistical problems of relocation are magnified by the amount of concentrate required. Use of a foam nozzle eductor compromises firefighter safety: firefighters cannot move quickly, and they must leave their concentrate behind if they are required to back out for any reason.
The self-educting foam nozzle has several advantages:
- It is easy to use.
- It is inexpensive.
- It works with lower pressures than required by in-line eductors.
A wide variety of flow rate versions are available, including large- scale master-stream versions.
The self-educting master stream foam nozzle is used where flows in excess of 350 gpm (1,400 L/min) are required (Figure 5). These nozzles are available with flow capabilities of up to 12,000 gpm (48,000 L/min). The self-educting master stream nozzle uses a modified Venturi design to draw foam concentrate into the water stream. The Venturi pickup tube is located in the center bore of the nozzle. This results in a "rich" (overproportioned) solution that is diluted at the deflector plates in the nozzle as the solution is discharged. The advantage of this style of foam nozzle eductor is a much lower pressure drop (10% or less) than typically associated with standard foam nozzle eductors. This allows for increased stream reach capabilities.