Aqueous Film Forming Foam (AFFF) and Alcohol Resistant-Aqueous Film Forming Foam (AR-AFFF) concentrates are used to extinguish flammable and combustible liquids either in surface or spill type fires. Flammable liquids emit volatile vapors in normal storage. Combustible liquids do not emit volatile vapors in normal storage; they emit flammable vapors when heated.
In surface- and spill-type fires, the liquid does not burn, the vapor burns. When you see a large pan fire of heptane burning, the fire starts out calmly. The base of the fire will actually be above the liquid. As the pan heats, the vapors are emitted at a higher rate. The longer the liquid burns, the higher the rate of vapor being expelled and the higher the base of the fire above the liquid.
AFFF and AR-AFFF separate the vapor from the liquid. Eliminating the vapor eliminates the burning property.
Foam concentratesThere are three groups of foam expansion: low, medium and high. Low expansion foam concentrates are typically used for extinguishing flammable and combustible liquids. Flammable and combustible liquids are identified as hydrocarbons or polar solvents.
They are identified this way to indicate what type of foam concentrate is used to extinguish the fire.
AR-AFFF concentrates are known by a variety of nicknames: ARC, ATC, alcohol type foam, 3x3, 3x6, to name a few. AR-AFFF concentrates are not limited to polar solvents; these concentrates can also be used on hydrocarbon fuels. AR-AFFFs are used when a mixture of hydrocarbon and alcohol-type fuel is to be stored or mixed together. If a polar solvent is added to a hydrocarbon fuel, the resulting mixture takes on the polar solvent characteristics and needs to be protected as such.
The same is true when storage has different types of liquids present. 3x3 and 3x6 refer to the concentration of foam into the water stream to protect the hazard. The first number indicates the concentration to protect hydrocarbon fuels; the second number indicates the concentration for alcohol-type liquids or polar solvents.
Foam system designThe most frequently asked question of a foam vendor or device manufacturer is, "How much foam will I need?"
The best way to answer this is to perform system calculations. This is the most accurate way to determine the required foam supply, although there is not usually enough time during a bid situation.
If the calculations have been run, the system will have a system-flow requirement measured in gallons per minute. The flow given is multiplied by the percentage of foam required or desired to protect the hazard. This is then multiplied by the duration (or the amount of time) the foam will have to run.
Note: Remember to make sure that the system demand or flow is going to be a final calculation.
If this information is not available, figure the amount of foam concentrate required using the following information:
- What is the required density?Over what area is this density required?
- What percentage of foam is specified?
- What is the duration for foam discharge?
The density multiplied by the area gives the minimum required gpms or system flow. When the system hydraulic calculations are performed, area multiplied by density is not required, nor is the safety factor. The safety factor is used to allow for system gallon adjustments that are apparent in all systems when calculations are run.
It is important to remember that duration times change as you move through the NFPA standards or when an insurance authority is involved. Following is information from current standards.
Foam proportioning devicesViking provides three methods of proportioning or induction: vacuum induction, metered pressure enduction, pressure induction and vacuum induction.
Vacuum induction uses a proportioning device that draws or pulls the foam concentrate into the water stream through the use of an eductor. An eductor uses a strong Venturi method of induction where a negative- or low-pressure area inside the device produces a vacuum. This area is equipped with a check valve and tube, which terminates at the foam concentrate. Essentially we have a straw in a bucket. When water passes by the eductor, our "straw" begins lifting concentrate from the bucket.
Because of the energy required to pull the concentrate from the storage container, eductors have a high-pressure loss through the device. The nominal loss through the eductor is 35 percent, which means if 100 psi is provided at the inlet of the device, 65 psi will be available at the discharge of the device for friction loss, starting head pressure and elevation.
An important element of designing with eductors is matching the system demand pressure to the available pressure at the discharge side of the outlet. If the required pressure is less than the available discharge pressure, the proportioning will be rich. Rich proportioning would mean rather than 3 percent concentration, you might actually get 5 percent, 8 percent or even 10 percent proportion of concentrate into the water stream. Richer proportioning will use up the supply faster than designed.
There is a minimum working pressure for eductors based on the foam concentrate being proportioned. This minimum pressure should go into the preliminary design to determine if an eductor is correct for the installation. Eductors are to be used in deluge applications. It is necessary to have equal lengths or matching equivalent lengths of pipe from the eductor to each individual discharge outlet.
When lengths of piping from an eductor are not equal between discharge devices, the proportioning will be incorrect. At times it is easier to add eductors for multiple sprinkler applications.
Metered pressure inductionMetered pressure induction utilizes a foam bladder tank, concentrate controller (proportioner), water pressure around the bladder tank to propel the foam and an orifice inside the concentrate controller to proportion the foam to the correct percentage.
The concentrate controller requires a minimum flow through the device before proportioned foam/water concentrate will be correct.
The minimum flow changes for the same concentrate controller depending on what type of concentrate is being proportioned. AFFF concentrates have a consistency of water, and AR-AFFF is more viscous (thicker); therefore, AFFF concentrates require a lower minimum flow through a proportioner than an AR-AFFF.
The flow through the proportioner has to be equal or greater than the friction loss through the water piping feed to the tank and the concentrate discharge piping to the proportioner. Water flows less than this will result in foam/water solution at a lower foam concentration (i.e. instead of 3 percent foam you may end up with 2 percent, 1 percent or even less).
Metered pressure induction proportioning can be used with many applications. The key to this design is that the minimum rated flow of the concentrate controller is met.
Pressure inductionPressure induction proportioning is a method of injecting foam concentrate into a proportioner at a higher pressure than the water pressure flowing through the proportioner. This is accomplished either with a foam pump or through water pressure manipulation of the discharge pressure to a bladder tank and concentrate controller (Low Flow Foam System).
Foam pumps have been used on foam systems to supply multiple risers on a single project. The foam pump supply pressure is set 20 psi higher than the water supply. An In-Line Balanced Proportioner (ILBP) is used to balance the foam pressure to the water supply pressure. A foam pump is like a fire pump in that a controller, a pump driver and an activation switch are required. An atmospheric tank that houses foam concentrate supplies the foam pump. The foam tank is made of fiberglass or plastic and is equipped with vacuum vents, discharge outlet, return line outlet, inspection hatch and low-level liquid indicator. The low-level liquid indicator will shut down the pump when the tank is almost out of concentrate.
Foam pumps are ideal for high capacity foam systems, low flow applications and multiple foam/water risers. The deciding factor for using a foam pump is a material cost element. Multiple risers will require a foam supply piping either underground or overhead that is compatible with the foam concentrate, meaning stainless steel or brass piping.
The Low Flow Foam System manipulates the water pressure to accomplish what a foam pump does, without the electrical considerations. A bladder tank is used rather than an atmospheric tank, and a pilot-operated pressure control valve is used to create a higher bladder tank pressure than the water pressure. An ILBP is used at the point of proportioning to balance the higher foam concentrate pressure to the lower water pressure. The pilot-operated pressure control valve reduces the water pressure to the bladder tank feed. This system is rated for a minimum differential of 15 psi, although 20 psi is the recommended planned pressure loss at the pilot-operated pressure control valve. This 5 psi "safety factor" is recommended because installation conditions may be less than ideal, and more regulation may be required.
A Low Flow Foam System, when used as a wet system, uses an A-1 pilot-operated pressure control valve. The A-1 valve is a flow control valve with pressure regulating trim that is shipped as a trimmed unit from Viking. This is a UL requirement. The trim is equipped with a Model C-1 Pilot Pressure Regulating Valve that is furnished with three springs. The medium pressure spring is installed at the factory. If lower or higher pressures are present at the job site, the springs are easily replaced. Pressure regulation is accomplished by tightening or loosening the screw bolt on the top of the C-1 valve. Tightening the bolt increases the discharge pressure; loosening the bolt decreases the discharge pressure. A minimum discharge flow of 25 gpms is required to correctly set the discharge pressure. If the system is fed by a fire pump, the fire pump must be running when setting the discharge pressure.
An A-1 Speed Control assembly is located on the priming chamber port of the flow control valve. The purpose of the speed control assembly is to provide smooth regulation by setting the speed at which the flow control valve clapper opens and closes. The speed control assembly is a loop with needle valves on the supply and discharge side of the priming chamber inlet. To adjust the speed control assembly, close the needle valves completely and then open them about a quarter turn. The needle valves have a small hole in them that ensures the flow control valve will not become primed closed.
When Low Flow Foam systems are installed as preaction systems, pressure regulation trim is available to be installed on the flow control valve. A special drain loop module is required on the conventional deluge valve trim, and the addition of the speed control assembly and the pilot-operated pressure regulating trim is necessary.
Low Flow Foam systems are ideal as single risers where closed heads are installed or as a main supply to multiple risers where many different systems are expected.
Foam discharge devicesFoam chambers and low-level foam makers are devices that dump foam from fixed piping onto a hazard. They utilize orifices to proportion the foam solution coming from a supply.
Foam chambers are typically used on the sides of large bulk storage tanks such as those found in refineries. The foam chambers are placed near the top of the tank with their outlets discharging through the tank onto the flammable liquid.
Low-level foam makers are used for dike spill protection. The foam makers are placed around a tank farm dike with piping connected to the discharge outlet of the foam maker. This piping rises up the outside of the dike wall, crosses over and drops to the floor of the diked area terminating in a 45-degree elbow.
Foam monitors are used extensively in aircraft hangars, but are also used as hazard protection for dikes around tank farms, bulk storage tanks or any hazard where a massive discharge flow is needed. Foam monitors provide cooling initially, which is important because if a tank wall is hot, it will burn the foam away. Once the hazard is cool, the remaining foam blankets the hazard to smother the fire.
Fire sprinklers are very good at spill protection and extinguishment. Sprinklers make good foam and can be placed in buildings to protect the entire hazard and conform to NFPA standards. Only sprinklers listed for ordinary hazard can be listed as foam sprinklers.
Sprinklers do not perform the same with different foam concentrates. The sprinkler manufacturer or foam concentrate manufacturer should be contacted prior to designing a foam sprinkler system.