There are many foam proportioners on the market today – ranging from inline eductors to direct injection systems – and selecting the proper one for your compressed air foam system (CAFS) involves a range of considerations.
Proportioners can be divided into two broad categories – manual and automatic.
Manual proportioners should not be used with compressed air foam (CAF) because a consistently accurate percentage foam solution must be maintained at any pump pressure or flow rate. If the proportioner does not add enough concentrate, the foam solution will be lean, which at best will make poor finished foam. At worst, a lean solution will create a condition called slug flow – when water and air do not mix because there is insufficient concentrate to create bubbles. Slug flow causes the hose line to jump and create severe nozzle reactions.
Manual proportioners require user adjustment and/or specific operating parameters in order to function accurately. The most common manual proportioner is the inline eductor. It requires a typical inlet pressure of 200 psi and usually specific flow rate, of 95 gpm. When operating in these parameters it provides an accurate foam solution percentage. Variations outside these parameters will cause inaccuracy or failure to operate.
Inline eductors work well for tactical operations where a specific constant flow is required. An example would be a large Class B flammable liquid fires using air aspirating nozzles.
The typical wildland or structural fire attack requires a constantly varying fire flow as nozzles are opened and closed and lines are added or shut down. Because of this varying flow, an automatic foam proportioner is required.
Automatic proportioners compensate for changes in both pressure and flow to maintain an accurate foam solution percentage. There are several categories of automatic proportioners – balanced pressure bladder, balanced pressure pump and direct injection systems.
Automatic proportioners used in CAFS are built by a number of manufacturers, including W.S. Darley, Foam Pro, Hale Products, Pierce Manufacturing, Robwen and Waterous.
Balanced pressure bladder and pump systems both use the water flow pressure to inject the foam concentrate through a metering valve, maintaining the balance between water flow and the amount of concentrate added to the discharge stream.
The foam concentrate then enters the water stream through a pressure differential valve. As the volume of water flowing through the valve increases, the pressure differential within the valve increases. This increased pressure differential allows more concentrate to enter the stream and treat the larger volume of water to maintain an accurate percentage.
Direct injection proportioners use a high-pressure pump – up to around 600 psi – which injects the concentrate into the foam manifold water stream. Parameters such as water flow are monitored and the information is fed to a computer, which calculates the amount of concentrate needed. The computer commands the foam injection pump to add the proper amount of foam concentrate, maintaining the correct percentage of foam solution.
CAF systems always use automatic-type proprotioners, and they typically are the direct injection type. They are sized from approximately 1.5 gpm to 12 gpm of concentrate pumping capacity. The size is determined by the volume of water to be treated and the percentage of foam solution required.
Wildland operations fall on the low flow end for water and concentrate, structural operations in the middle, and Class B incidents on the high end. As an example, a wildland CAF line flowing 20 gpm at 0.3 percent will require 0.06 gpm of concentrate; a structure attack line flowing 100 gpm at 0.3 percent will require 0.3 gpm; and a Class B line flowing 150 gpm at 6.0 percent will require 9 gpm.
Matching The Need
The proportioner’s size should match the need. If a small proportioner is used for high percentage Class B operations, it will drastically limit the volume of water that can be treated. However bigger is not always better. A large-volume proportioner is often inaccurate at low flows.
Because there is a potential for slug flow if the proportioner runs lean at low flows, a high-volume proportioner is not the best choice when selecting a system for wildland or even most structural operations.
Direct injection systems are powered by either electric or hydraulic motors. Electric motors are used on systems up to about 6 gpm. These motors range from 0.5 to 1 hp. They can draw large amounts of electrical current when running at full capacity. Larger systems use a pump driven by the truck power train to drive a hydraulic motor to power the proportioner.
All direct injection systems use flow meters to monitor water flow rate. For some systems this is the only control over the amount of concentrate injected. Other systems take advantage of high technology to sense multiple parameters and feed that information to a computer in addition to water flow.
More information allows a more accurate foam solution percentage to be maintained. Knowing water temperature is useful. It is easier to mix concentrate with warm water than with cold water. By sensing temperature, injection rate adjustments can be made as changes occur.
Another good bit of information is knowing the water’s conductivity. Water quality will affect foam quality as it is more difficult to make foam with hard water. Also foam concentrate types are not created equal. Some perform better than others.
When mixing only by volume, the use concentrates by different manufactueres may yield different results in foam consistency. By measuring water conductivity entering the proportioner and the foam solution exiting the proportioner, variations in the foam solution can be minimized.
Small direct injection systems usually have minimal controls and provide little information to the operator. Controls consist of an on/off switch and a dial to set the percentage. These systems are generally designed only for Class A foam and will proportion between 0.1 percent and 1.0 percent. These systems are also the least expensive.
Larger systems have a digital display panel. It shows current water flow, foam percentage, water volume and foam concentrate used. Controls include an on/off switch, a select switch to change the display and arrow buttons to make adjustments. Foam solution percentage rates of 0.1 percent to as high as 10 percent are available with either Class A or Class B foam concentrates. The larger these systems are, the more they cost.
Most direct injection systems can be set up “Auto On.” Some systems require the purchase of specific items such as control heads, and others simply require power from the pump switch. Auto On is a desirable feature as it defaults the fire pump to foam operation and takes a step out of the pump operator set up. If plain water is desired, the system can be just switched off.
Every foam capable apparatus should be equipped with a remote foam tank fill system. This is a convenience, but more a safety feature as it removes the need for firefighters to climb on top of the apparatus with buckets to refill the foam tank which can lead to falls as foam concentrate is very slippery.
Some systems use the proportioner pump as the transfer pump to fill the tank. Others use a separate independent pump. These transfer pumps are typically in the 5-gpm range.
Some systems are manual while others have an automatic feature. The auto systems have sensors to detect the foam tank level to automatically refill the foam tank.
But do not rely soley on the refill system to keep your high-volume operation supplied with concentrate. Regardless of the proprotioner’s capacity, if concentrate demand exceeds the refill pump’s capacity, the foam tank supply will be depleted.
Foam tanks on most apparatus range from five to 50-gallons. Tank size is determined the apparatus mission and oftem pumpers carry both Class A and B foams.
Dual tanks provide the ability to carry two different concentrates. Dissimilar concentrates – Class A and Class B for example – are not compatible and must not be mixed. If mixing occurs, the concentrate may turn to a gel-like substance and plug lines, filters, and the proportioner itself.
To prevent mixing, systems with dual tanks will have a tank selector valve with a flush feature. This allows the concentrate to be flushed out of the proportioner before switching to a different type concentrate. Both manual and electrically operated dual tank selectors are available. Some electric dual tank selectors are designed to automatically flush during certain operations. This can cause some issues in CAF when foam solution is lost due to the flush operation resulting in slug flow.
Product research, consideration of the mission of the apparatus, selection of a proportioner to fit that mission and selection of the appropriate options will ensure a user-friendly and properly functioning foam proportioner on a compressed air foam system.
Editor’s Note: Keith Klassen, who has more than 30 years in the fire service, is a career captain with the Summit Fire District in Flagstaff, Ariz. He is an Arizona State Fire Marshal Office certified instructor and instructor evaluator with over 20 years of foam experience. He is the CAFS instructional program manager for Waterous Company, overseeing domestic and international training with a staff of 12 Waterous-certified CAFS instructors. He is also a co-owner of inFOAMation associates LLC, which provides foam tactical training.