Foam: The Fire Service's Voodoo Science - Part 3

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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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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Photo by Mike Wieder
Figure 5. A common self-educting master stream foam nozzle.

As we continue down the path to greater foam consciousness, it is important for us to understand the equipment we use to make foam. In order for us to be successful in developing a meaningful foam stream, we must understand how our equipment works and what limitations it has.

Foam Terminology

There are two basic manners in which foam can be generated: chemically and mechanically. The first foams that were introduced to the fire service more than 70 years ago were chemical foams. Chemical foams are those produced as a result of a reaction between two chemicals. Chemical foams are considered obsolete and are rarely, if ever, found in use today.

Foams in use today are of the mechanical type. Mechanical foams must be proportioned (mixed with water) and aerated (mixed with air) before they can be used. To produce quality firefighting foam, foam concentrate, water, air and mechanical aeration are needed. These elements must be present and blended in the correct ratios.

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Diagram Courtesy of IFSTA/Fire Protection Publications
Figure 1. The foam tetrahedron.

The four elements need to create foam can thought of as a tetrahedron in a manner similar to the elements that are need for flaming combustion (Figure 1). Removing any element results either in no foam production or a poor quality foam, just as removing one of the combustion elements results in extinguishing a fire. Before discussing the foam-making process, it is important to understand the following terms:

Foam concentrate The raw foam liquid as it rests in its storage container or the apparatus tank before the introduction of water and air.

Foam proportioner The device that introduces foam concentrate into the water stream to make the foam solution.

Foam solution The mixture of foam concentrate and water before the introduction of air. In basic terms, this is the stuff in the hose between the proportioner and the nozzle unless you are dealing with a compressed air foam system (CAFS).

Foam The completed product after air is introduced into the foam solution (also known as finished foam). In basic terms, this is the agent after it is discharged from the nozzle.

Foam Proportioning

The term "foam proportioning" is used to describe the mixing of water with foam concentrate to form a foam solution. Most firefighting foam concentrates are intended to be mixed with 94% to 99.9% water. For example, when utilizing 3% foam concentrate, 97 parts water mixed with three parts foam concentrate equals 100 parts foam solution. For 6% foam concentrate, 94 parts water mixed with six parts foam concentrate equals 100 percent foam solution.

The four basic methods by which foam may be proportioned are induction, injection, batch mixing and premixing.

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Photo by Mike Wieder
Figure 3. A typical in-line foam eductor.


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Photo by Mike Wieder
Figure 6. A jet ratio controller that is connected into a foam supply operation.

Induction. The induction, sometimes referred to as eduction, method of proportioning foam uses the pressure energy in the stream of water to induct (draft) foam concentrate into the fire stream. This is achieved by passing the stream of water through a device called an eductor that has a restricted diameter (Figure 2).

Within the restricted area is a separate orifice that is attached via a hose (called a pick-up tube) to the foam concentrate container. The pressure differential created by the water going through the restricted area and over the orifice creates a suction that draws the foam concentrate into the fire stream.

In-line eductors and foam nozzle eductors are examples of foam proportioners that work by this method. These devices are covered in more detail later in this article.

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Diagram Courtesy of IFSTA/Fire Protection Publications
Figure 2. This diagram shows the operating principle of an induction-type foam proportioner.

Injection. The injection method of proportioning foam uses an external pump or head pressure to force foam concentrate into the fire stream at the correct ratio in comparison to the flow. These systems are commonly employed in apparatus-mounted or fixed fire protection system applications. These proportioners will be covered in detail in the next article in this series.

Batch mixing. Batch mixing is the most simple method of mixing foam concentrate and water. It is commonly used to mix foam within a fire apparatus water tank or a portable water tank when no other foam proportioning equipment is available. It also allows for accurate proportioning of foam. Batch mixing is commonly practiced with Class A foams but should only be used as a last resort with Class B foams.

Batch mixing may not be effective on large incidents, because when the tank becomes empty, the foam attack lines must be shut down until the tank is completely filled with water and more foam concentrate is added. Another drawback of batch mixing is that Class B concentrates and tank water must be circulated for a while to ensure thorough mixing before being discharged. The time required for mixing depends on the viscosity and solubility of the foam concentrate.

Premixing. Premixing is one of the more commonly used methods of proportioning. With this method, premeasured portions of water and foam concentrate are mixed in a container. Typically, the premix method is used with portable extinguishers, wheeled extinguishers, skid-mounted twin-agent units, and vehicle-mounted tank systems.

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Diagram Courtesy of IFSTA/Fire Protection Publications
Figure 4. The in-line foam eductor should be no more than six feet (two meters) above the level of the foam concentrate.

In most cases, premixed solutions are discharged from a pressure-rated tank using either a compressed inert gas or air. An alternative method of discharge uses a pump and a non-pressure-rated atmospheric storage tank. The pump discharges the foam solution through piping or hose to the discharge devices. Premix systems are limited to a one-time application. When used, they must be completely emptied and then refilled before they can be used again.

In-Line Foam Eductors

There is a wide variety of different types of foam proportioners available to the fire service. The remainder of this article will focus on in-line foam eductors. The in-line eductor is the most common type of foam proportioner used in the fire service (Figure 3). This eductor is designed to be either directly attached to the pump panel discharge or connected at some point in the hose lay. When using an in-line eductor, it is very important to follow the manufacturer's instructions about inlet pressure and the maximum hose lay between the eductor and the appropriate nozzle.

In-line eductors use the Venturi Principle to draft foam concentrate into the water stream. As water at high pressure passes over a reduced opening, it creates a low-pressure area near the outlet side of the eductor. This low-pressure area creates a suction effect, called the Venturi Principle. The eductor pickup tube is connected to the eductor at this low-pressure point. A pickup tube submerged in the foam concentrate draws concentrate into the water stream, creating a foam solution.

Several very important operating rules must be observed when using eductors. Failure to follow these rules lessens the performance of the eductor:

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Diagram Courtesy of IFSTA/Fire Protection Publications
Figure 7. A typical hose layout for a foam operation using a jet ratio controller.

Rule 1: The eductor must control the flow through the system. In other words, the flow through the eductor should not exceed the rated capacity of the eductor. Exceeding this capacity results in either poor quality foam or no foam at all.

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.

A jet ratio controller (JRC) may be used to supply foam concentrate to a self-educting master stream nozzle (Figure 6). The JRC is a type of in-line eductor that allows the foam concentrate supply to be as far away as 3,000 feet (900 m) from the self-educting master stream foam nozzle (Figure 7). This allows firefighters, who are involved in operating fire pumps and maintaining the foam concentrate supply, to be a safe distance from the fire. The JRC also allows an elevation change of up to 50 feet (15 m).

The JRC is supplied by a hoseline from the same pump that is supplying other hoses to the nozzle. The flow of water to the JRC represents about 2.5% of the total flow in the system. As with a standard in-line eductor, as water flows through the JRC a Venturi is created that draws concentrate through the pickup tube and into the hoseline. The difference is that the JRC proportions the concentrate at a 66.5% solution. This rich solution is then pumped to a self-educting master stream foam nozzle where it is further proportioned with the water supplied by the fire pump down to a discharge proportion of 3%. To achieve a proper proportion, it is important that the JRC and nozzle match.

Conclusion

In this article we have acquainted you with some important foam terminology and the most basic type of foam proportioning equipment: in-line eductors. In our next installment we will examine the various types of apparatus-mounted proportioning systems that are available to the fire service.


Mike Wieder is a senior editor at IFSTA/Fire Protection Publications at Oklahoma State University. He holds several degrees in fire protection and adult education. He is a former member of the Pennsburg, PA, and Stillwater, OK, fire departments. Part 1 of this series was published in the April 1997 issue; Part 2 appeared in May 1997.

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