By Jonathan M. Hinson
Firefighting foam has been around for many years, from the powder foam to protein foam to the synthetic foam in use today.
Originally designed to extinguish simple hydrocarbon-based fuel fires, today’s foams can be used on Class A fires and alcohol-based fuel fires and, of course, foam is still used for hydrocarbon fuel fires. Technology has advanced to make foam operations more effective, simplistic, and cost-effective. Flammable and combustible liquids are everywhere and being transported through all modes of transportation. Hopefully, all fire departments have enough foam resources readily available to effect a rescue from a flammable liquid incident. Some communities may have a higher risk or threat, so more foam resources are needed. Whether a department has an eductor and three buckets of foam or thousands of gallons of foam with master stream devices, there are still some basic principles and tips that can apply to both situations that will facilitate a successful foam operation.
Foam has been a source of frustration for many firefighters over the years, as making an effective finished foam blanket can be a challenge. Most foam equipment is very simple in operation, but there are complex systems on apparatus that require in-depth knowledge for operation. Regardless of the system being used, making finished foam without failure requires knowledge of basic principles and the equipment and supplies being used. The foam must also be properly applied to ensure complete extinguishment and prevent reignition.
As taught in most basic firefighting classes, four components are required to make finished Class B foam. Foam concentrate is the first component needed, which is commercially manufactured by both domestic and international companies. The foam concentrate is then introduced into water to form a foam solution. Next, the foam solution is mixed with air either by expelling it into the air through a standard nozzle or while passing through an air-aspirating foam nozzle. The foam must also be agitated, which can occur in a foam nozzle or on contact with a surface after leaving the nozzle. When these four components come together, the result is finished foam that can be used to extinguish flammable/combustible liquid fires or prevent ignition of flammable vapors. If any of these four components are missing, the Class B foam operation will not be effective and possibly fail completely.
There are two common types of foam, Class A and Class B, that equate to the types of fires they are effective on. Class B foams create a film or membrane over the surface of a fuel to prevent vapor production. The higher the expansion rate of the foam application, the longer the blanket will last, and also less foam concentrate will be used during reapplications during longer term incidents. Class A foams break down the surface tension of a fuel, allowing the water to penetrate quicker and deeper. Making a thick foam blanket is not required with Class A foam because the bubbles have nothing to do with the effectiveness of the foam.
Using the wrong type of foam can cause an ineffective firefight, leading to someone getting hurt. Class A foam cannot be used on a Class B fire. Class A foams do not make the needed film to smother the fire and prevent the production of vapors needed for combustion. Class A foam could slow the fire down and even extinguish a small flammable liquid fire somewhat, but even the small fire is going to quickly degrade the foam and allow the fire to reignite. The sudden and often unexpected reignition can injure the firefighters on scene who have possibly let their guard down after seeing extinguishment.
Applying Class B foam to a Class A fire is not as dangerous but can be ineffective. Some brands of Class B advertise and boast that their products can be used on Class A fires as well as Class B. Just remember that Class B foams are not penetrating but rather smothering the Class A material like a blanket. If enough water has not reached the fire to cool it, the fire will still be able to burn after the foam breaks down. For example, a fire department is attempting complete extinguishment of a garage fire that burned to the ground. Crews on scene covered the smoldering ashes with class B foam. The high winds quickly blew the finished foam off the ashes, causing the fire to continue to smolder. Class A foam was applied, and the fire was quickly extinguished in the heavy timbers that were smoldering.
Selecting the wrong style of Class B can also cause a foam operation to be ineffective. Class B foams come as standard aqueous film forming foam (AFFF), which forms a film over the flammable liquid, separating the foam from the flammable liquid and suppressing the flammable vapors. Standard AFFF is only effective against hydrocarbon (oil)-based flammable liquids because hydrocarbons do not want to mix with water. Alcohol-based fuels, on the other hand, are very miscible with water, which could create havoc with the foam blanket of a standard AFFF by pulling the water out of the blanket. Water and alcohol have an opposite electrical polarity, meaning they are readily attracted to each other, and they form a solution rather than a mixture when they come together that will eventually separate like oil and water.
Should foam be needed for a fire or spill of an alcohol-based flammable liquid such as ethanol, an alcohol-resistive AFFF (AR-AFFF) is required because of alcohol’s extreme desire to mix with water. AR-AFFF creates a plastic-like membrane that prevents water in the foam from mixing with the burning alcohol. If standard AFFF is used on an alcohol-based fire, the water will immediately be pulled out of the foam, which will immediately break down the foam blanket and allow the fire to continually burn despite the foam application. Most, if not all, gasoline consumed in this country now contains the alcohol-based additive ethanol, making AR-AFFF the best style of foam to combat any gasoline fire. With the use, manufacture, and transport of ethanol growing, fire departments should carry AR-AFFF foams rather than AFFF to ensure they are always best prepared for any type of flammable liquid fire. It is important to note that AR-AFFF foams can be used on hydrocarbon fires as well as polar solvent fires and not lose any effectiveness or create any safety issues.
|1 Mixing Class A and Class B foam in storage will create a problem that will be fatal to any foam operation. When these two types of foam are mixed, the chemical process that creates the plastic-like membrane in AR-AFFF is started. This process will cause plastic-like globules to form. (Photos by author.)|
Issues from Mixing Foams
Mixing different types of foam is another way your foam operation can fail. When applying foams, the best recommendation is to use the same brand and type of foam throughout the incident. However, experience has shown firefighters can still be effective using different brands so long as the foams are the same type and designed to be proportioned at the same amount. The more differences between the foams the less effective and compatible they will be. Class A and Class B foam should never be applied simultaneously or on top of one another.
While mixing foams during foam application on scenes can render an operation ineffective, mixing foams in storage can cause the foam to degrade and become ineffective when used. Manufacturers recommend that foam concentrates be stored in their original containers (pails, barrels, totes). These containers must remain sealed and airtight, out of direct sunlight, and in a climate-controlled environment. Introducing air, sunlight, and temperature extremes can all cause foam to degrade, causing it to not create an effective foam blanket when used on an incident scene. Mixing different brands of foams is also not recommended, as that can also affect the foam’s ability to work properly when applied. Underwriters Laboratories (UL) tests foam concentrates to ensure they meet set standards and will perform as expected. Mixing different foams together, not storing foams properly, and mixing other materials with foam voids that UL test, creating uncertainty and no guarantee the foam will work as prescribed.
Firefighters need to make sure that mechanics or anyone who works on their onboard foam systems understands proper foam storage and that foreign substances should not be added to apparatus foam storage tanks or systems. For mechanics to find leaks, many times they add dye to different liquids to help locate the leaks. This should not be done under any circumstances to find a leak within an onboard foam system. Many of these dyes contain oils that could be very detrimental to foam concentrate and void the UL test. Mechanics may understand and be certified to work on fire apparatus and foam systems, but they may not understand how foam works and the problems any additives to foam concentrate can create.
Mixing Class A and Class B foam in storage will create a problem that will be fatal to any foam operation. When these two types of foam are mixed, the chemical process that creates the plastic-like membrane in AR-AFFF is started. This process will cause plastic-like globules to form. Photo 1 shows the plastic-like globules that form when mixing Class A and Class B foam in the same container. The globules will become lodged in any system they are pumped or pulled through. Eductors will clog, and onboard foam systems could be completely ruined. The onboard foam systems are not cheap to repair and even more expensive to replace, which will probably be needed if the plastic-like globules pass through it.
Improperly using or storing foam concentrate is just one area that foam operations can fail; problems with the equipment used in the foam operation can also cause failures when not used correctly. Using inline foam eductors is the most common method of proportioning foam concentrate into a water stream. These devices are very simple, have no moving parts, work off the venturi principle, and can cause a lot of problems and heartache for firefighters.
The venturi principle creates a vacuum because of the pressure differences of the water entering the eductor at a high pressure and rapidly moving into an area of low pressure. Most modern eductors have a metering valve to adjust the percentage at which the foam concentrate is proportioned. Some eductors are designed for only one percentage. Eductors come in different sizes and flow rates. The most common eductors are designed for a flow of 100 gallons per minute (gpm) and used with 1½- or 1¾-inch handlines. Eductors designed for 250 gpm are made for 2½-inch handlines, and there are eductors capable of flowing 500 gpm for master streams.
One of the biggest reasons an eductor fails to make foam is because of too much pressure on the discharge or low-pressure side of the eductor, which prevents the venturi from occurring. The low-pressure side can have no more than 65 percent of the intake pressure. Most eductors require 200 pounds per square inch (psi) on the intake side, so any pressure greater than 130 psi on the discharge side will cause the eductor to not function. Most nozzles require 100 psi and then you have approximately 20 to 25 psi in friction loss, leaving only 5 to 10 psi for other factors such as elevation or a hose that has a higher amount of friction loss.
There are some simple solutions to overcome these pressure problems. Simply moving the eductor closer to the nozzle will eliminate many of the problems, but that option is not always feasible. Increasing the size of the hose will reduce friction loss as well. Simply using two reducers (one at the eductor and one at the nozzle) and 2½-inch hose will increase the available distance because of less friction loss in the bigger hose. Using a different nozzle, such as a low-pressure 75-psi nozzle, will also increase the amount of pressure available to be used for friction or elevation loss.
|2 An inline foam eductor with a gauge on the low-pressure side of the eductor that is used to show the amount of back pressure.|
Back pressure on the foam eductor can also be created from other factors. Two of the factors are kinks in the hose and nozzles being partially opened. Some firefighters partially close the nozzle to make handling the hose a little easier. But, this is not acceptable in foam operations. Incident commanders must instead make sure there are enough firefighters on a hoseline to control it without having to partially close the nozzle. Trash in hoselines and nozzles can also create additional back pressure, causing foam operations to fail. This can happen many times in rural situations when water is being drafted from static water sources. Leaves, sticks, rocks, etc. that are pulled into pumps unknowingly ultimately end up in nozzles and bring foam operations to a halt.
The easiest way to determine if back pressure is a problem is to place a gauge on the discharge side of the eductor to measure actual back pressure. These gauges can quickly identify if there is a problem with back pressure, allowing operators to narrow down their investigation into a problem. Gauges can be permanently installed on all eductors or mounted inline between the eductor and first section of hose. Photo 2 shows an inline foam eductor with a gauge on the low-pressure side of the eductor that is used to show the amount of back pressure. This gauge is color-coded to help the operator quickly notice a problem. If the needle is in the green, all is good. But if the needle is in the red, foam is probably not being produced and adjustments are necessary.
In addition to back pressure, there are some other reasons eductor-based foam operations can fail. One such reason is that the pickup hose is too long and the eductor is too far above the foam source. Just like pumpers cannot draft water at greater than 30 feet below the pump, an eductor cannot draft foam at more than six feet below the eductor. Foam system operators must make sure that the bottoms of foam containers are no more than six feet from the eductor, so all foam can be used, and the operation will not stop halfway through. A simple solution to this problem is to ensure that eductor pickup tubes are no longer than six feet.
Failing to flush all eductors and other foam equipment after use can lead to future foam operations failing. Even though a foam eductor is very simple with no moving parts, dried foam concentrate can block the small internal passages and cause the metering valve to lock up. Flushing eductors, hoses, and nozzles with clean water after every use can prevent this. Clean water should be pulled through the eductor the same way foam concentrate is while the metering valve is operated to ensure all internal areas are flushed. Some newly designed foam eductors have a button that will allow clean water being pumped through the eductor to be pushed out the foam pickup tube, simplifying the flushing process.
Onboard Foam Systems
Foam eductors are probably the backbone of foam operations in the fire service, but apparatus-mounted foam systems are quickly becoming widespread. Most modern systems have separate foam pumps that pump foam into the discharge streams. The metering and induction of the foam concentrate are done through different methods depending on the type of system installed. These onboard systems generally do not have all the back pressure problems, making them a better option in some cases, but these systems can still cause foam operations to fail mainly because of improper use. There are many different types, brands, and versions of onboard foam systems. All discussions here are based on generalities of onboard foam systems and may not apply to all systems.
Following manufacturers’ instructions and recommendations is key to ensuring onboard foam systems function correctly and efficiently when operated on incident scenes. Operators should have a complete understanding of their foam systems, which will allow them to maximize the use of and troubleshoot problems with the system. Cleaning filters and operating valves regularly are easy and simple ways to prevent problems. In some departments, onboard foam systems may be rarely used on incident scenes, so regular testing and operation (monthly or bimonthly) are good ideas to make sure the systems stay in working order and to keep operators familiar with their operation.
Onboard foam system operators need to ensure that the right foam is used in the system, as not all systems are designed for all types of foam. Using the wrong foam can be system-fatal, making the entire system inoperable and in many cases requiring complete system replacement. Many older systems cannot handle AR-AFFF foams because of their higher viscosity. Some onboard systems may be dual-agent systems (Class A and Class B), so operators must ensure that the right foam is put in the right tank, as this may cause the system and ensuing foam operation to fail. The same rules discussed earlier about mixing and storing foam apply when storing in apparatus tanks of onboard foam systems. Improper storage and any type of mixing can cause failures. Foam stored in apparatus tanks in firehouse bays with heaters close to the foam fill can cause foam to heat to levels beyond recommended storage temperatures. Foam apparatus tanks should remain full at all times to prevent sloshing and foaming within the tank while traveling.
Just like foam eductors, flush onboard systems and other equipment after use. This generally involves just shutting off the foam system and allowing clean water to follow through the piping, hoses, appliances, and nozzles until there are no signs of foam in the water being discharged. Just like eductor-based systems, foam left in the equipment can dry and lock up smaller openings, causing foam operations to come to a halt.
Another important fact operators must understand about flushing their onboard foam system is if the system should be stored “wet” or “dry.” Some systems are designed to have foam in the system at all times (wet), while others must be stored empty or dry at all times. Wet systems can just be shut down and all downstream equipment flushed with clean water. Dry systems must be shut down and completely flushed with water using valves designed for such use. Operators with both Class A and Class B foam systems must also ensure they leave the correct type of foam in the pump. If the other type of foam is required, the system must be flushed before switching over. This could be a manual process, or some of the computer-operated systems may do it automatically when the switch is requested by the operator.
Just like eductors, onboard foam systems have a limit or capacity. Operators must understand how much foam solution their systems can make and not exceed their capabilities. Fire engines are rated for their pump capacity, such as 1,000 gpm, and foam systems have this same rating except it applies to how much foam concentrate it can move. Most engineers who design these systems and fire trucks will limit the number of discharges that are foam-capable to help keep operators from overextending a foam operation. However, operators must understand what their flows are and how much foam is being used. For example, if the system is flowing 200 gpm at 3 percent cencentration, the foam system is moving 6 gpm of foam concentrate. Trying to overpump the foam system can create system damage and cause the entire foam operation to become inefficient or completely fail.
JONATHAN M. HINSON is a captain and a 15-year veteran of the Chesapeake (Virginia) Fire Department, serving on the department’s Foam Team. He is a 20-year fire service veteran. He also serves as chief of the Newsoms (VA) Volunteer Fire Department.