Foam: Fire Service's Voodoo Science

In all of my years of fire service response and training experience, I do not believe that I have seen a topic that is any more vague to the fire service than the subject of foam firefighting (other than maybe what color fire trucks should be). There is so much information and misinformation about foam floating around the fire service (no pun intended) that it is often difficult for your average firefighter-type person (like me) to determine what is valid and what is not.

Photo by Mike Wieder
Aspirated foam is applied using a foam nozzle.

In the past year, I had the opportunity to learn more about foam than I ever thought possible. I looked forward to my assignment as the editor/writer of the new International Fire Service Training Association (IFSTA) Principles of Foam Fire Fighting manual with great enthusiasm. Despite the fact that I had been in the municipal fire business for over 17 years and had even done a short stint in petrochemical industrial fire protection, I knew little about how foam really worked. Sure, like other firefighters, I knew all the mechanical basics: how to assemble a foam stream, make sure your proportioner, concentrate and nozzle all match, and things like that. But I was really going through the motions without truly understanding what I was doing or why I was doing it.

The opportunity to work with the IFSTA foam committee would provide me the chance to work with some of the most knowledgeable people on foam firefighting in North America. Included on the committee were representatives of all of the major foam concentrate and equipment companies, fire service people and foam educators. However, in working with these outstanding people, it did not take long to figure out why foam science remains such a mystery to the fire service.

It was clear from the beginning that it would be difficult to gain quick consensus on almost every issue we wished to address in the manual. Each of the major foam manufacturers has its own ideas about how its products should be used and they are very protective of those ideas. In the foam business, there is little consensus about what flow rates should be used, methods of application, acceptable nozzles to use and general firefighting tactics using foam. This is certainly not meant to be a negative shot at the manufacturers. Each has gone to great lengths to develop and test its products. Unfortunately for the fire service, the manufacturers tend to come to different conclusions on how to use their products. They also tend to dispute the others' claims for their products. None of them particularly agree with the figures provided in NFPA 11, Low Expansion Foam Extinguishing Systems.

This leaves the fire service in a precarious position. For example, suppose you are the chief of a fire department that has never before had foam capabilities on any of its apparatus. Now you are buying a new pumper and wish to have an onboard foam system added to the rig. The apparatus manufacturer will likely give you four or five choices for types of foam systems that can be used to dispense the concentrate from the 30-gallon concentrate tank you have specified. In reviewing the various manufacturers' literature, and perhaps even talking to manufacturers' reps, you get a variety of opinions about which system is the best. You will probably also get a variety of answers on how much fire 30 gallons of concentrate will put out. The decision of which foam system to pick turns out to be not as easy as you thought.

Photo by Mike Wieder
Nonaspirated foam is applied using a fog nozzle.

And then it gets worse. Now you have to decide which foam concentrate to put in your tank. The manufacturers have different information on what the required flow rates should be, what proportioning percentage to use on a certain fuel, and what type of nozzle can be used in a given circumstance. Your head spinning, you decide to contact the instructor at your county or state fire academy to get an "unbiased" opinion. You soon learn that unbiased opinions are hard to find. Most of these instructors have been taught what they know by one of the foam manufacturers (usually the one that gives or loans them equipment for their school and sells them training concentrate at the lowest price). The instructors do not have an across-the-board, generic understanding of foam principles.

You could call one of the major foam consulting and response firms to get their advice on the issue. However, as with the foam instructors you have dealt with, you soon figure out that most of these people are aligned with one particular manufacturer and may not provide you with an accurate evaluation of all other manufacturers' products. Furthermore, these consultants do not want to teach you too much about foam firefighting, or else no one will hire them to come out and put the next gasoline storage tank fire out.

With seemingly no place left to turn, you do one of two things:

  • Say the heck with it, we haven't needed a foam system up until now, we'll just scrap the idea and call for mutual aid if we do.
  • Use the dartboard approach to picking your foam equipment (let it fly and see where it hits).

Obviously, neither of these choices is very desirable. There should be some place where you can get some good, generic, unbiased information on foam firefighting and equipment. To the credit of the members of the IFSTA foam committee, consensus was reached on many of the questions that the fire service has. To my knowledge, this is really the first time anyone has accomplished this feat to any degree of success. In this article, and future ones, I will examine some of the most-asked questions regarding foam firefighting and provide generally agreed upon information that answers them. Perhaps we will be able to lift the voodoo hex off of you!

Do We Need A Foam Nozzle Or Can We Use Our Fog Nozzle?

This is one of the most common questions I hear on the topic of foam firefighting. However, what you are really asking in this case is whether you need aspirated foam or nonaspirated foam. So let's take a look at all sides of this issue.

Photo by Mike Wieder
In high radiant heat situations, fog streams may be employed to shield firefighters.

Aspirated foam is foam that has air entrained into it prior to being discharged from the nozzle. This air may be entrained by air inlets on a foam nozzle or by the addition of compressed air within the foam proportioning system (known as compressed air foam systems, or CAFS).

CAFS streams are usually discharged from a solid stream nozzle, not a foam or fog nozzle. Aspirated low-expansion foam has an air-to-foam solution expansion ratio of about 10:1. This foam will have a thick, fluffy appearance.

Fog nozzles produce nonaspirated foam. Nonaspirated foam has no air entrained into it prior to discharge from the fog nozzle. A small amount of air may be entrained as it flies through the air but it is fairly insignificant when compared to the air entrained in foam discharged from a foam nozzle or CAFS system. At best, you can expect an expansion ration of 3:1 or 4:1 when using a fog nozzle. This foam will have a soupy or milky appearance.

It should be noted that the decision to use a foam nozzle or fog nozzle applies only when using aqueous film-forming foam (AFFF) or film-forming fluoroprotein foam (FFFP) concentrates on hydrocarbon fuels. Foam nozzles must be used when applying protein or fluoroprotein concentrates to any type fuel or alcohol-resistant AFFF or FFFP foam to polar solvent fires.

Firefighters familiar with foam operations have debated the merits of aspirated vs. nonaspirated foam as long as AFFF and FFFP concentrates have been available. Tests conducted in the United States and Great Britain have given conflicting results as to which method is most effective. Actually, each application method has advantages and limitations that are generally agreed upon by foam firefighting professionals. Each fire-fighting situation needs to be evaluated separately to determine which method is most suitable. The following sections give the advantages and limitations of aspirated foam and nonaspirated foam.

Advantages of aspirated foam. Aspirated foam is considered to be the highest quality, most stable foam. Because it has a substantial amount of air entrained into it (which means it has lots of bubbles), aspirated foam is said to have the longest drainage time.

In simple terms, this means that the foam blanket will last longer than one that was laid down from a fog nozzle. Thus, when covering an unignited spill, less-frequent reapplication will be required. This saves on the amount of concentrate that will be needed to do the job.

Limitations of aspirated foam. While technically speaking, aspirated foam is said to be the highest quality foam, it may not be the most suitable form of foam in some circumstances. Aspirated foam streams have a shorter reach than nonaspirated streams because of their high air content. Thus, they are more subject to adverse effects of windy conditions.

Most aspirating foam nozzles are not capable of having an adjustable stream pattern. Because of this, a fog pattern cannot be provided to protect firefighters when they make a close approach to a fire. Lastly, foam nozzles are bulky and can be used only for applying foam. This factor means additional equipment costs for those who choose to carry and use them.

Advantages of nonaspirated foam. Nonaspirated foam can be applied through any fixed gallonage, adjustable gallonage or automatic fog nozzle. A straight stream or fog pattern of 10 degrees or less is most desirable for foam application. This pattern allows for maximum aspiration of the foam.

Fog patterns may be employed if it becomes necessary to provide protective cover for firefighters. Nonaspirated foam streams have greater penetrating power and reach than do aspirated streams. Tests conducted by the U.S. Navy indicate that nonaspirated foam provides a faster knockdown of fire than does aspirated foam.

Limitations of nonaspirated foam. Although nonaspirated foam may knock down a hydrocarbon fire more quickly than aspirated foam, it will not form as thick a blanket. Thus, its vapor suppression capabilities are substantially less than with aspirated foam. Nonaspirated foam requires more frequent application than does aspirated foam to keep extinguished spills adequately covered.

Keep in mind that moisture is stripped from the foam bubble in a nonaspirated foam stream as it moves through the air. This reduces the foam's long-term cooling ability. The stripped water sinks quickly to the bottom of a hydrocarbon fuel pool where it provides no assistance with the situation.


So when you boil it all down, what do all these factors add up to? The following list is a quick summary of points on this issue:

  • Fog nozzles may be used to produce nonaspirated foam. This will provide a quick knockdown on hydrocarbon fires.
  • Foam nozzles must be used to provide an effective foam blanket for vapor suppression after a fire is extinguished, on unignited spills, or polar solvent fires.
  • You may choose to begin an attack on a fire with a fog nozzle. Once the fire is knocked down, switch to a foam nozzle to cover the fuel with aspirated foam for maximum vapor suppression.

It is hoped that this article has answered some questions for you. In our next installment, we will tackle the subject of what size incident you can expect to handle with the foam equipment on your truck or in your station.

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.