COMPRESSED AIR FOAM SYSTEMS AND FIRE HOSE
BY DOMINIC J. COLLETTI
Editor`s note: This article was chosen for this “Fighting Fire with Water” editorial package for the following reason: Foam agents simply enhance the properties of water, which is useful for cooling the fuel and combustion by-products below their ignition points, thus darkening the fire. Even when you use a foam agent, you essentially still are “fighting fire with water.”
In North America today, a growing number of urban and suburban fire departments are using compressed air foam systems (CAFS) for the full range of ordinary combustible fire responses. Their collective experience using Class A foam generated by CAFS during working fire alarms continues to expand the body of knowledge available on CAFS foam application strategy and tactics.
The question of fire hose compatibility with CAFS recently has surfaced. This issue affects firefighter health and safety and must be closely examined to ensure that the highest level of protection is afforded for fire personnel engaged in training and manual fire combat with CAFS pumping apparatus.
(Note: The following discussion has no bearing or implications on the use, practice, care, and so on of large-diameter supply hose (LDH) and LDH intake relief valves or other supply hose appliances. Conditions pertaining to trapped air in LDH supply hose and LDH appliances are potentially dangerous and have no relationship to CAFS finished foam delivery in fire attack hose.)
Fire Hose/CAFS Compatibility
The design of CAFS is directly related to fire hose/CAFS compatibility. CAFS used for structure fire attack and wildland interface fire attack, for example, must be of an advanced (vs. basic) design in that the flow rate capacities must be adequate for safe structure firefighting, the systems must be compatible with the fire hose used, and the systems should have built-in safety features. The pumping mechanism of an advance-design compressed air foam system generally includes at the minimum the following features:
An automatic pressure-balancing control that continuously adjusts the air compressor discharge pound per square inch (psi) to match the fire pump psi without pump operator intervention. A manual control should be provided for backup should the automatic control fail.
An automatic foam liquid concentrate proportioning system capable of accurately working between the psi and gallons per minute (gpm) water flow rates the compressed air foam system is expected to discharge during typical fire operations without pump operator intervention.
An automatic compressed air system that proportions the required ratios of compressed air and foam solution into apparatus discharges without pump operator intervention.
An automatic safety control that monitors fire pump pressure, foam solution flow rate, and the status of the foam concentrate-proportioning system to ensure compressed air is never injected into hoselines when the fire pump is dry or the foam-proportioning system is shut down or has run out of foam liquid concentrate.
A mechanical secondary safety compressed air relief valve on the discharge side of the air compressor to prevent a runaway air pressure condition from occurring, and an ASME-certified oil separator/compressed-air receiver tank.
Motionless mixing chambers installed at all CAFS discharges to ensure that a homogenized mixture of bubbles enters fire hoses and prepiped master-stream devices.
A method of heat rejection to cool the air compressor oil that does not use the apparatus booster tank water supply for cooling purposes.
HOSE PRESSURE AND WATER HAMMER
A commonly misunderstood CAFS/fire hose issue is the pressure operating range of CAFS–minimum and maximum obtainable hoseline operating pressures–and the exact pressure effect an abruptly shut compressed air foam system nozzle has on a flowing compressed air foam system hoseline. Some firefighters–relating to past fireground experiences involving the delivery of plain water–believe that a high pressure surge develops when a flowing compressed air foam system hoseline is suddenly closed. Normally, when a hoseline flowing water is slammed shut by the nozzleman, the hose is subjected to a high hydraulic kinetic energy that causes a “water-hammer” effect. The misconception that the same thing happens with a compressed air foam system hoseline is widely held. Even the vice president of a major hose manufacturing company telephoned me to voice his concern that he feared an abruptly closed CAF system hoseline could develop a pressure of 1,000 to 2,000 psi because of a compressible “slug of air.” In an advance-design CAF system, this simply is not true.
Preset pressure controls are one example of an advance-design feature. The preset controls for the air compressor and fire pump automatically maintain a typical 100-psi pressure at static conditions–this means that when the nozzle is shut, the maximum pressure in the hoseline is 100 psi. One fully adjustable compressed air foam system on the market makes it possible to control hoseline working pressure at any setting between 75 and 175 psi. A 150-foot-long 134-inch attack hoseline used for interior attack typically is operated at 100 psi (static) compressed air foam system pump pressure and usually has a 138-inch inside diameter (I.D.) ball valve and smooth-bore nozzle as the discharge device. When the nozzle is fully open, the flow is in the 80-gpm foam solution and 80-standard cubic feet per minute (scfm) air ranges, with residual pressure at the compressed air foam device discharge at about 85 psi. Slamming the device`s nozzle shut will not raise the pressure above the static setting because the finished foam within the hoseline (which is about 50 percent compressed air by volume) acts as a “cushion,” preventing a pressure “spike” from a water-hammer condition. In other words, any developing kinetic energy in the velocity head of water within the hydraulic side of the compressed air foam device`s pumping system will be absorbed and diffused by the pneumatic component of compressed air contained within the hoseline. How does this happen? Remember, the residual pressure of the flowing compressed air foam device`s hoseline is about 85 psi. After the nozzle on the CAF system`s flowing hoseline is shut, foam solution and air continue to fill the hoseline for a few seconds (because the air contained within the hose is a compressible medium) until the hoseline reaches static pressure–in this case, 100 psi–the same as the static pressures of the CAF device`s air compressor and fire pump.
This dynamic condition–foam solution and compressed air continuing to enter the hoseline after the nozzle has been shut–is referred to as packing. The physical dynamic of packing is an important CAF system hydro-pneumatic foam flow property that prevents a high-pressure “spike” from hydraulic kinetic energy.
Another concern is the effect of the CAF system`s finished-foam “pressure” on fire hose material and construction. A question often asked is, What effect does 100 psi of CAF system finished foam vs. 100 psi of plain water have on fire hose? This is akin to asking what weighs more and exerts more force, a cubic foot of feathers weighing 100 pounds or a cubic foot of bricks weighing 100 pounds? Pound-per-square-inch pressure is exactly that–a measurement of force per unit area, regardless of whether it is air or water pressure, so this clearly is not an issue.
Does CAF system finished foam have a higher derogatory effect on fire hose material and construction than plain water? Does it reduce the service life of the hose and cause premature failure? Here, we should mention that the term “fluid” as used in the definition of many studies of physical properties, such as hydraulics, may mean either liquid or gas. As such, the flowing properties of CAF system finished foam are much the same as water because its makeup within the pressurized hose is about a 1:1 mix of foam solution and compressed air by volume. The movement of the finished foam within the hose has a center turbulent layer and an outer boundary layer.
BASIC CAFS APPARATUS: SLUG-FLOW AND CHATTER
Unfortunately, the hardware designs of some basic-design CAFS may present problems for fire hose, including accelerated degradation and shortening of the hose`s life. Two somewhat similar operating conditions–slug-flow and chatter–can accelerate hose wear through chafing of the fabric/coupling, separating of the interior hose liner, and failure of the coupling hose fastener.
Slug-flow refers to a condition caused by the absence of foam liquid concentrate within the CAF system discharge hose. It occurs when the foam liquid concentrate proportioning system is accidentally shut off, malfunctions, or runs out of liquid concentrate. The result is that only plain water and air fill the hose. Since plain water and air do not mix, they “slug” and separate as they move through the hoseline toward the nozzle, causing a rapid forward and aft pulsation, constituting a dangerous hose-handling situation, a totally useless fire stream, chafing of the hose`s exterior, and increasing stress on hose couplings.
Chatter. Some CAF systems are not equipped with mixing chambers. The absence of a mixing chamber on the CAF system discharge will cause the first (and possibly the second) section of hoseline to vibrate as all three finished-foam components (air, water, and foam liquid concentrate) scrub together, making bubbles within the hoseline itself. Mixing chambers must be specified for all CAFS. These chambers prevent this disjointed, separate mixture of air, water, and concentrate from being injected in the hoseline. Mixing chambers create finished foam within the device, discharging a homogenized mixture of foam bubbles into the hose, meaning that the hoseline does not have to do the scrubbing work, thereby preventing chatter and exterior hose chafing.
MANUFACTURERS` SAFETY WARNING
In Spring 1994, there evidently was enough question in the minds of hose manufacturers regarding the compatibility of fire attack hose with CAFS to cause them to take action as a group through their trade association, the Fire Equipment Manufacturers Association (FEMA). The following is an excerpt of a FEMA-issued Safety Alert Bulletin (June 15, 1994):
…FEMA strongly recommends that no hose be used on a CAF system unless such use is recommended by the manufacturer of the system and the hose manufacturer. Use of non-approved hose can be dangerous and may cause a hose or coupling failure, producing property damage, bodily injury, or death. Always follow the manufacturer`s recommendations and instructions on proper use and maintenance.
Although all attack hose (whether filled with CAF system foam or water) is prone to a potential failure from a variety of fireground circumstances, departments using CAFS should always use attack hose approved by the manufacturer for the particular make and model CAF system device to minimize the chance of hose failure. In addition to complying with NFPA 1961, Standard on Fire Hose–1992, and NFPA 1962, Standard for Care, use, and Service Testing of Fire Hose Including Connections and Nozzles–1993, specifying optimum acceptance test, burst, service, and working pressures is recommended, as is networking with other fire departments to learn of their experience with CAFS and fire hose.
Since any compromised hoseline (whether containing water or foam) puts interior (or exterior) attack crews in danger, the best policy is to use the highest quality hose and perform testing to meet or exceed hose manufacturer and NFPA recommendations. It is important to have a working understanding of the type of service for which the hose is designed, the physical properties of the agents transported within, and the effects your particular CAF system will have on the hose.
According to NFPA 1961, fire hose should have a 3:1 factor of safety based on the relationship of maximum working pressure vs. burst pressure. Most industrial hoses (nonfire service) generally carry a 4:1 safety factor regardless of the fluid they carry (water or air). Most fire service hose manufacturers offer hose with pressure ratings within the margin of safety for a CAF system. At least one manufacturer offers the following pressure ratings:
Proof Burst Annual Working
1,000 psi 1,500 psi 500 psi 425 psi
When using any hose with a CAF system, keep in mind that if a partial or catastrophic separation of a hoseline were to occur, the CAF hoseline may release more energy than a line filled with plain water. Depending on the plain water pressure and flow it is contrasted against, a CAF system hose may have a higher or lower energy content. Fire personnel must be aware of this fact.
* * *
Proper selection of CAF system features, such as the design of the pumping system, and equipment, such as the attack hose and valve/nozzle, is essential for safe, effective use. The importance of being adequately trained in pump operation, hose handling, and foam application and of properly maintaining the equipment cannot be overstated. n
I have logged more than 200 live fire evolutions in acquired structures to develop CAFS application tactics through live fire training. Fire scenarios ran the gamut from room and contents fires to fully involved structures of various sizes.
This past April, I was the instructor-in-charge at a CAFS live fire training exercise at the U.S. Army`s Fort Indiantown Gap, in Annville, Pennsylvania. The exercise involved large wood-frame, single-story World War II vintage barracks. Fire evolutions ranged in size from 12-foot by 12-foot rooms with contents to fully involved 150-foot by 25-foot single-story Army barracks. Ninety-five firefighters, representing 12 fire departments (all currently using CAFS in daily fire operations)–some from as far away as Boulder, Colorado, and Peshtigo, Wisconsin–participated. An additional 200 firefighters, representing 63 fire departments from the northeastern United States, also attended as observers only.
Fire industry professionals gave a live fire demonstration the last day of training. The objective was to prove to the students that CAFS have the capacity to extinguish a large structure fire using only marginal personnel and water-supply resources. The demonstration was conducted in a 150-foot by 25-foot by 12-foot (3,750-square-foot area) single-story barrack loaded with straw and wood pallets from end to end and then ignited by the ignition team. Within seven minutes after ignition, total building flashover occurred–with the help of a stiff 15- to 20-mph prevailing wind.
Using the Iowa Rate-of-Flow formula, it was determined that a 450-gallons-per-minute (gpm) water delivery rate would be needed. This was low, however, when contrasted with the National Fire Academy formula, which would have required a 1,041-gpm delivery rate for this size structure. We could draw on little prior experience with regard to the use of CAFS for a fully involved building of this large size and construction. We estimated that a 212-inch compressed air foam system line flowing 180 gpm of foam solution (Class A foam liquid concentrate proportioning at 0.4 percent) and 180 scfm of compressed air probably was the minimum flow required for fire control. It was also evident that the 15- to 20-mph prevailing wind would accelerate the building`s rate of combustion as more oxygen was dumped inside and that, therefore, the fire load would demand an even greater water delivery rate than would normally be needed under calm atmospheric conditions.
Three firefighters, under the direction of one officer, attacked the fire with a single 212-inch handline. The hose discharge device consisted of a 212-inch play pipe, a 212-inch ball valve, and a two-inch smooth-bore nozzle. Fire attack was from the exterior only. Ninety-eight percent fire control was achieved within six minutes with a total volume of 1,080 gallons of Class A foam solution. At that point, it was agreed that a 134-inch compressed air foam system line could be advanced into the structure for overhaul and total extinguishment. The building used for this exercise was not in full compliance with NFPA (National Fire Protection Association) 1403, Standard for Live Fire Training Evolutions in Structures–1992, so no attempt was made to enter the building. n
–DOMINIC J. COLLETTI
(Top to bottom) Students, attending a CAFS live fire training symposium, move a CAF system hoseline from right to left in a well-involved structure attack at the U.S. Army`s fort Indiantown Gap, in Annville, Pennsylvania. Two 212-inch handlines, each flowing 180 gpm of foam solution and 180 scfm of compressed air, were used. The fire was under control in 14 minutes.
n DOMINIC J. COLLETTI, a fire protection systems engineer, is the national OEM accounts manager and foam systems product manager for Hale Products Inc., in Conshohocken, Pennsylvania. He is a volunteer firefighter with the Humane Fire Company in Royersford, Pennsylvania, and has been involved with engineering mechanical fire protection systems for 13 years.