Foam: The Fire Service’s Voodoo Science - Part 4

Mike Wieder resumes his series with a discussion on apparatus mounted foam proportioning systems.


In our October 1997 installment we looked at in-line eductors for foam proportioning. For many years, in-line eductors have been the most common foam proportioning equipment used by the municipal fire service. However, in recent years apparatus-mounted foam proportioning systems have become...


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In our October 1997 installment we looked at in-line eductors for foam proportioning. For many years, in-line eductors have been the most common foam proportioning equipment used by the municipal fire service. However, in recent years apparatus-mounted foam proportioning systems have become increasingly popular in the municipal fire service. This rise in popularity can be traced to several reasons:

  • The technology of these systems have improved to the point where they can be expected to operate reliably, even on apparatus that make a lot of runs.
  • The advances in technology have resulted in these systems becoming more economically feasible for most fire departments.
  • The fire service's ever-increasing role in hazardous materials emergencies means that foam is needed on a more frequent basis.
  • The use of Class A foam concentrates for structural and wildland fire-fighting is impractical with portable foam proportioning equipment.

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Photo by Mike Wieder
This digital readout can provide much useful information for the driver/operator and the incident commander.

In this article we will examine the most common types of apparatus-mounted foam proportioning systems used in today's fire service. The systems described in this article are called low-energy foam systems. A low-energy foam system imparts pressure on the foam solution solely by the use of a fire pump. These systems introduce air into the solution when it either reaches the nozzle or is discharged from the nozzle. High-energy foam systems introduce compressed air into the foam solution before it is discharged into the hoseline. We'll explore high-energy foam systems (commonly referred to as "compressed air foam systems" or CAFS) in a future article.

Installed In-Line Eductor Systems

Installed in-line eductors use the same principles of operation as do the portable in-line eductors that were discussed in the previous article in this series. The only difference is that these eductors are permanently attached to the apparatus pumping system. The same precautions regarding hose lengths, matching nozzle and eductor flows, and inlet pressures listed for portable in-line eductors also apply to installed in-line eductors. Foam concentrate may be supplied to these devices from either pickup tubes using five-gallon (20 L) pails or from foam concentrate tanks installed on the apparatus.

Installed in-line eductors are most commonly used to proportion Class B foams. Take care to adhere to the operating rules for using these devices. Because Class A foam concentrates are normally used at very low concentrations (0.1% to 1.0%), installed in-line eductors are probably not the first choice for proportioning Class A foams. The very small orifice required for educting Class A foam can easily become clogged, and excess foam concentrate may be wasted by over-proportioning.

Around-The-Pump Proportioners

This type of proportioner is the most common type of built-in proportioner installed in mobile fire apparatus today. This proportioner is also installed on some fixed systems. It is especially useful when there is low water pressure or when a motor is not available for a separate foam concentrate pump.

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Diagram Courtesy of IFSTA/Fire Protection Publications
The basic design of a variable-flow, variable-rate direct injection foam proportioning system.

The around-the-pump proportioning system consists of a small return (bypass) water line connected from the discharge side of the pump back to the intake side of the pump (see diagram at left). An in-line eductor is positioned on this bypass line. A valve positioned on the bypass line, just off the pump discharge piping, controls the flow of water through the bypass line.

When the valve is open, a small amount of water (10 to 40 gpm [40 L/min to 160 L/min]) discharged from the pump is directed through the bypass piping. As this water passes through the eductor, the resulting Venturi effect draws foam concentrate out of the foam concentrate tank and into the bypass piping. The resulting foam solution is then supplied back to the intake side of the pump, where it is then pumped to the discharge and into the hoseline.

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