Eductor Basics

Dominic Colletti Jr. details the use of eductors as an easy and less-expensive way to become involved in the application of nozzle-aspirated Class A foam.


You have had success using Class A foam during fire training exercises to combat automobile interior and other ordinary combustible fires, and you’ve now decided to implement nozzle-aspirated Class A foam. You already own a 95-gpm eductor that can be used during fire responses. The device has...


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When using an eductor, nozzle selection is important. Choose a nozzle that is rated at the same flow as the eductor. For example, with a 95-gpm eductor, you must have a 95-gpm fixed-flow fog nozzle, a variable-gallon adjustable fog nozzle set at 95 gpm or an automatic fog nozzle capable of flowing at least 95 gpm.

After optimizing hose diameter, hose length and nozzle selections, choosing a brand of Class A foam concentrate required to get the job done is another consideration. Make sure a high-quality and environmentally friendly foam concentrate is purchased. There are wide selections of Class A foam makes and brands available. Make sure you get what you pay for. Low-bid Class A foams may not be the best choice for effective fire suppression or even, in field use, be as economical as higher-priced Class A foam concentrates.

Situation Sensitivity

You’ve made all the right choices, equipped the apparatus and told your crew to get to work with Class A foam. However, even after making all the right decisions to help ensure the team’s success, there are situations out in the field that may cause an inline eductor to fail to pick up concentrate. Here are a few items to look at in keeping eductors operational.

Inlet pressure. If the pump operator fails to provide 200 psi at the inlet side of the eductor, it may not operate effectively or at all. If your apparatus has high friction loss in the discharge plumbing, or if the eductor is located away from the apparatus (such as at the end of 300 feet of 2½-inch hoseline), you will need a higher pressure than 200 psi on the master pump discharge gauge to make the device work. In this situation, friction loss in the piping and/or hose is robbing the eductor of the inlet pressure needed to make it operate properly. Calculate the friction loss in the piping and/or hose and make sure you add it to the 200 psi required at the eductor inlet to arrive at the needed discharge pressure on the master pump pressure gauge.

Flushing. If eductor metering and check valves are not flushed after use, foam concentrate deposits may plug them. Dried solids stuck on the inside of the metering and check valves cause a blockage and a “no foam concentrate†flow condition at the eductor’s throat. Always follow the manufacturer’s maintenance instructions for your particular eductor after each use.

Nozzles. Is the hose team opening up the nozzle completely during fire attack? Is there an internal blockage at the nozzle, like rocks plugging the inlet screen, causing the nozzle to flow less than its rating? Using a nozzle rated less than the gpm rating of the eductor, opening a properly rated nozzle less than all the way or having a properly rated nozzle with an obstruction will cause less gpm flow to occur, potentially leading to no concentrate pickup. Also, using a fixed- or variable-gallonage nozzle rated significantly above the flow rating of the eductor may result in poor foam stream reach, even though foam concentrate pickup is occurring.

Hoselines. Is the hose team exceeding the maximum recommended hoselay length by the eductor manufacturer? Is the hoseline kinked? Is the hoseline operating at a high elevation? These conditions lead to excessive back pressure. If your maximum hoseline length is 200 feet, but the situation requires 400 feet of hose, you can run 200 feet of large-diameter hose to the inlet of the eductor and then run 200 feet of “attack†hose after that. As long as the operator calculates the friction loss in the supply hose prior to the eductor, and ensures that 200 psi is provided at its inlet, foam concentrate pickup will occur.

Delivery Rate

After making sure that your eductor system is operational, and ensuring that pump operators, nozzle teams and fire officers alike understand the capabilities and limitations of your foam system, keep in mind when using an inline eductor you have set your fire suppression capacity, your gpm flow rate, to whatever the inline eductor flow rate is (such as 95- or 125-gpm of Class A foam solution, for example).

Even if you are using an automatic fog nozzle, your hose team still can flow only whatever the rated gpm capacity of the in-line eductor is, whether using plain water or Class A foam solution. The gpm rating of the eductor becomes the limiting factor to liquid delivery rate out of the nozzle.