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

• 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.

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.

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.

Around-the-pump proportioning systems are rated for a specific flow and should be used at this rate, although they do have some flexibility. For example, a unit designed to flow 500 gpm (2,000 L/min) at a 6% concentration proportions 1,000 gpm (4,000 L/min) at a 3% rate.

A major disadvantage of the around-the-pump proportioner is that the pump cannot take advantage of incoming pressure. If the intake water supply is greater than 10 psi (70 kPa), the foam concentrate will not enter into the pump intake. The inability of concentrate to enter the pump intake generally means that the proportioner may be used only when operating off the apparatus water tank. Newer units capable of handling intake pressures of up to 40 psi (280 kPa) are available but they are not in widespread use at this time. Another disadvantage is that the pump must be dedicated solely to foam operation. An around-the-pump proportioner does not allow plain water and foam to be discharged from the pump at the same time.

Bypass-Type Balanced Pressure Proportioners

The bypass-type balanced pressure proportioner is one of the most accurate methods of foam proportioning. It is most commonly on large-scale mobile apparatus such as airport crash vehicles and refinery firefighting apparatus.

The primary advantages of the bypass-type balanced pressure proportioner are its ability to monitor the demand for foam concentrate and to adjust the amount of concentrate supplied. Another major advantage of a bypass-type balanced pressure proportioner is its ability to discharge foam from some outlets and plain water from others at the same time. Thus, a single apparatus can supply both foam attack lines and protective cooling water lines simultaneously. Supplying different types of lines is not possible with around-the-pump proportioners.

Systems equipped with a bypass-type balanced pressure proportioner have a foam concentrate line connected to each fire pump discharge outlet. This line is supplied by a foam concentrate pump separate from the main fire pump. The foam concentrate pump draws the concentrate from a fixed tank. This pump is designed to supply foam concentrate to the outlet at the same pressure at which the fire pump is supplying water to that discharge. The pump discharge and the foam concentrate pressure from the foam concentrate pump are jointly monitored by a hydraulic pressure control valve that ensures the concentrate pressure and water pressure are balanced.

The orifice of the foam concentrate line is adjustable at the point where it connects to the discharge line. If 3% foam is used, the foam concentrate discharge orifice should be set to 3 percent of the total size of the water discharge outlet. If 6% foam is used, the foam concentrate discharge orifice is set to 6 percent of the total size of the water discharge outlet and so on. Because the foam and water are supplied at the same pressure and the sizes of the discharges are proportional, the foam is proportioned correctly. A limitation of the bypass-type balanced pressure proportioner is its need for a foam pump with PTO or other power source. Also, bypass of concentrate in this system can cause heating, turbulence and foam aeration.

Variable-Flow Variable-Rate Direct Injection Systems

This type of proportioner operates off power supplied from the apparatus electrical system. Large volume systems may use a combination of electric and hydraulic power. The foam concentrate injection is controlled by monitoring the water flow and controlling the speed of a positive displacement foam concentrate pump, thus injecting concentrate at the desired ratio. Because the water flow governs the foam concentrate injection, water pressure is not a factor. Full flow through the fire pump discharges is possible because there are no flow-restricting passages in the proportioning system.

Variable-flow variable-rate direct injection systems provide foam concentrate rates from 0.1% to 3%, although greater concentrate flows may be possible in some applications. The control unit has a digital display that shows the current water or foam solution flow rate, the total amount of water or solution flowed to this time, the current foam concentrate flow rate, and the amount of foam concentrate used to this time (see photo on page 120). These systems can be used with all Class A foam concentrates and many Class B concentrates but may not be used with alcohol-resistant foam concentrates due to the high viscosity of the concentrate. These systems are supplied from atmospheric pressure foam tanks on the apparatus.

There are several advantages to variable-flow variable-rate direct injections systems. One advantage is their ability to proportion at any flow rate or pressure within the design limits of the system. Another advantage is that the system automatically adjusts to changes in water flow when nozzles are either opened or closed. Also, nozzles may be either above or below the pump, without affecting the foam proportioning. And finally, this system may be used with high-energy foam systems (a topic to be discussed later in this series).

The disadvantage of these systems is that the foam injection point must be within the piping before any manifolds or distribution to multiple fire pump discharges.

Variable-Flow Demand-Type Balanced Pressure Proportioners

The variable-flow demand-type balanced pressure proportioning system, also called a pumped/ demand system, is a versatile system used in both fixed systems and mobile apparatus applications. In this system, a variable-speed mechanism, which is either hydraulically or electrically controlled, drives a foam concentrate pump. The foam concentrate pump supplies foam concentrate to a Venturi-type proportioning device built into the water line (see diagram on page 121). When activated, the foam concentrate pump output is automatically monitored so that the flow of foam concentrate is commensurate with the flow of water to produce an effective foam solution.

There are several advantages in variable-flow demand-type balanced pressure proportioning systems. First, the foam concentrate flow and pressure match system demand. Second, there is no recirculation back to the foam concentrate tank. The system is maintained in a ready-to-pump condition and requires no flushing after use. Last, water and/or foam solution can be discharged simultaneously from any combination of outlets up to rated capacity.

A limitation of these systems is that the fire pump discharges have ratio controllers (which reduce the discharge area), thus pressure drops across the discharge are higher than those seen on standard pumpers.

Conclusion

The type of foam proportioning system you choose to equip your apparatus with will be dependent on several factors:

• The frequency with which you expect to use the system.
• Whether you will use Class A concentrate, Class B concentrate, or both.
• The maximum amount of foam solution you wish to be able to flow.
• Whether or not you need to be able to flow both foam and plain water from the same pump, at the same time.
• Budgetary limitations.

Hopefully, this article has given you some insight into the basics of apparatus-mounted foam proportioning systems. Your local fire apparatus dealer should be able to provide more information on options that are available to you.