Fighting fire with foam as an extinguishing agent has been around since the late 1800s. Despite its proven effectiveness, not all fire departments have adopted foam as a primary fire suppression agent for their fire ground operations. Here are a few basic points to get you started in your understanding on how foam suppresses fire, how it’s made and the basic types available today.
The Effects of Foam on Fire
The role of firefighting foam is to enhance the fire extinguishing properties of water. It has the following effects on extinguishing flammable or combustible liquid fires:
Decreases the fuel’s exposure to oxygen by separating the flames from the fuel surface. Creates a barrier between the fuel and fire to cut of the supply of oxygen to the flammable material
Cools the flammable material and surrounding surface when the water that is drained from the foam evaporates.
Eliminates vapor release from fuel surface by stopping any further gas evaporation from burning materials
Saturates the fuel source by flooding all the spaces
Insulates flammable material that is not yet caught fire or already extinguished
How Foam is Made
Foam is simply a mixture of air, water, and foam concentrate resulting in low density bubbles. Foam is generated in two steps.
Step 1: Mix foam concentrate and water to form a foam solution using a proportioner. Mixing can be done at the pump, in the hose line or at the foam nozzle.
Step 2: Introduce air to the mixture using various types of nozzles or application methods
Firefighting foams are classified as two primary categories: Class A or Class B. These categories correspond to the types of fuels that the foams are designed to be used on.
Class A Foam
Class A Foams are suitable for Class A ordinary combustible fuel fires, such as wood, cloth, paper, rubber and many plastics, and used as a firefighting agent or as a fire barrier. Water extinguishes ordinary combustible fuels, but when that water is mixed with a Class A foam concentrate it does it more effectively and faster. The finished foam reduces the water’s surface tension allowing it to wet and penetrate combustible fuel fires enabling extinguishment up to 3 times faster than plain water. Class A foam concentrates are generally biodegradable, environmentally safe, non-corrosive and non-flammable. It’s used at very low concentrates typically less than 1%.
Class B Foam
Class B Foams are most effective when the fuel source is a flammable or combustible liquid. Using water alone for suppression is largely ineffective. The water sinks below the fuel source and allows the fire to continue to burn. A foam proportioning system that is capable of delivering Class B foams permits the finished foam to form a blanket to cover the fuel source’s surface and starve it of oxygen.
Common types of Class B foam include:
- Film Forming FluoroProtein (FFFP)
- Aqueous Film Forming Foam (AFFF)
- Alcohol Resistant-Aqueous Film Forming Foam (AR-AFFF)
- High performance Fluorine Free Foams (FFHPF) which are more environmentally friendly
Proper Foam Expansion
In order to make effective use of your foam, proper expansion or air aspiration is necessary to deliver quality foam to extinguish your fire. Foam nozzles are designed to provide three types of expansion.
Low Expansion foam is ‘wet’ foam with fine bubbles of high weight and small volume. It has good flowability that allows itself to distribute over surfaces quickly and can create an air-tight seal in class B applications.
Low Expansion foams have an expansion ratio up to 20:1 and have been proven in actual use to be an effective means of controlling extinguishing, and securing most flammable liquid (Class B) fires.
Mid Expansion foams are ‘damp’ with fine bubbles of low weight and high volume. Mid-Expansion foams are primarily used to suppress the vaporization of hazardous chemicals. Mid Expansion refers to foams that have an expansion of between 20:1 and 200:1. Foams with expansions between 45:1 and 55:1 have been found to produce the optimal foam blanket for vapor mitigation of highly water reactive chemicals and low boiling organics. Due to its wide expansion range, medium expansion foam can be used for numerous applications.
High Expansion foam liquid concentrate is a synthetic, detergent type foaming agent with extremely high foam volume and low weight and appears ‘dry’ with big bubbles. It primarily used to fill basements, aircraft hangers or other enclosed structures. Application is at 2% from a High Expansion Generator. Expansion rate is from 200-1000:1