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

Mike Wieder continues his foam firefighting series by describing foam-making equipment.


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...


To access the remainder of this piece of premium content, you must be registered with Firehouse. Already have an account? Login

Register in seconds by connecting with your preferred Social Network.

OR

Complete the registration form.

Required
Required
Required
Required
Required
Required
Required
Required
Required
Required

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.

10_97_foam5.jpg
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.

10_97_foam6.jpg
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:

10_97_foam7.jpg
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.