|Nozzle Aspirating Foam Systems (NAFS) include medium expansion nozzles like the device shown here. They are excellent for filling voids found in burning tire carcasses with foam and for enveloping small structural voids found inside homes during overhaul operations.|
|A 1-3/4-inch attack hose with a 1-inch smoothbore nozzle. Notice the agent feathering as it exits the nozzle - this in an indication that the nozzle is too small a diameter for the flow rate being discharged.|
|The same hoseline and flow-rate as shown on the left, but this time discharged from a 1-3/8-inch inside diameter opening of a 1/4-turn ball valve. Notice that the stream does not "feather" after exiting the nozzle.|
|"Wet, Fluid and Dry" foam consistencies, as shown here, can be produced by compressed air foam systems. Each foam consistency provides a tool to meet various tactical firegound requirements. (Photos by Dominic Colletti)|
|A combination nozzle that allows for both fog and solid streams simultaneously.|
|This firefighter has the option of using either the 1/4-turn ball valve alone, or installing a 15/16-inch tip, dependant upon whether compressed air foam, foam solution or water will be discharged from the hoseline.|
One challenge when implementing Compressed Air Foam Systems (CAFS) into fire operations is choosing the most appropriate nozzle to enhance fireground foam applications.
Introducing firefighters to the considerations surrounding nozzle selection for the successful use of compressed air foam is an important part of any CAFS implementation program. Obtaining firefighter buy-in on a final nozzle choice is critical.
No matter what, nozzle selection is a highly personal choice for each fire department and is dependant upon a host of local factors. In this article, we will look at some of those factors and a few considerations in making the best choice.
To start, we will review foam production mechanics basics: To produce a Class A finished-foam blanket, four items are required: water, foam concentrate, air and mechanical agitation. When all four components are brought together, finished foam media is produced.
Used for many decades, Nozzle Aspirated Foam Systems (NAFS) use aspiration at the nozzle device (air is added at the nozzle) which also provides the mechanical agitation needed to produce a finished-foam blanket. With CAFS apparatus however, there is no need to use a nozzle to introduce air into foam solution to create the finished foam, since compressed air is added inside apparatus piping, prior to the point where hose lines and/or portable monitors connect to the fire apparatus.
NAFS are relatively common and understood fairly well by the fire service. NAFS require a method to introduce foam concentrate into plain water (a foam proportioner) as well as a method to introduce air into the foam solution stream.
Foam concentrate can be proportioned into plain water via several methods - batch mixing, eductor systems or by the most popular device today, an electronic discharge-side direct-injection system.
A suitable nozzle device at the point of foam application must be used to introduce an adequate volume of air to create finished foam. Fixed or automatic fog nozzles, air aspirating attachments and high-expansion foam generators introduce different volumes of air into foam solution to create various finished-foam blanket qualities.
Each device will produce a specific foam quality, such as finished-foam expansion ratio and foam blanket 25 percent drain time.
CAFS are relatively new to the structure fire service and are radically different from NAFS. CAFS inject compressed air into a foam solution stream inside apparatus piping on the fire apparatus.
As water, foam concentrate and compressed air move through a mixing chamber and hose line, the components become "scrubbed" together creating finished compressed air foam.
"Scrubbing" is the term used to define the turbulence that occurs inside piping and discharge hose lines that acts to form foam bubbles. Therefore, with CAFS, a nozzle is not required to entrain air or shape and finish a foam fire stream.
Some departments that are very successful using CAFS don't even use nozzles at all. Some simply use a 1/4-turn ball valve with 1 3/8-inch inside diameter bore. The 1/4-turn ball valve provides a means to turn the hose line on and off so the hose team can advance it.
When planning to conduct a nozzle evaluation for a new CAFS equipped apparatus, ensure that you have a wide selection of nozzle choices available. It is vital to understand their operating characteristics. If your department has traditionally used one particular type of nozzle, the time is now to consider potentially making a change into a design that will maximize the application of compressed air foam.
Using different nozzle systems with compressed air foam under live fire training conditions (in accordance with National Fire Protection Association (NFPA) 1403) can also be a big help and will strengthen your departments' knowledge base and possibly remove myths about nozzles and foam application techniques.
When evaluating various nozzles for use with compressed air foam for the structure fire attack, there are four vital criteria that affect firefighter safety and firefighting efficiency. They are: delivery rate; stream quality/finished foam quality; nozzle reaction force; and stream reach.
When selecting a nozzle device for use with CAFS, it is vital that your nozzle selection maximize finished foam delivery rates while maintaining the highest quality fire stream and also provide a manageable nozzle reaction force for hose teams.
The first consideration when evaluating any nozzle for use with CAFS is an evaluation of fire flow delivery rates. With CAFS hardware, delivery rates are expressed in gallons per minute (gpm) of foam solution and in cubic feet per minute (cfm) of compressed air (as compressed air foam).
Target Flow Rate
First, choose a target flow rate (gpm and cfm) for each different size diameter attack hose line on the apparatus. The target flow rates for foam solution gpm rates with CAFS should be close to, or at, those flow rates previously used with water for each given size diameter attack hose line.
For example, if a 1.75-inch attack hose line used to deliver water was typically delivering 120 gpm, then, when going with CAFS, try to match it with a 120 gpm foam solution and 60 cfm of compressed air delivery rate.
Evaluate what types and sizes of nozzles are needed to move the volume of compressed air foam out of fire attack hose lines. For example, if a 120 gpm and 60 cfm target delivery rate is set for an 1.75-inch hand line, a 1 3/8-inch inside diameter smoothbore or a 250 gpm at 50 psi fixed-gallonage fog nozzle may be required.
It is important to note that both of these nozzles would be too large for a water only application through a 1.75-inch hose. Since the movement and delivery of compressed air foam is very different from water, traditional hydraulic principles do not apply.
Simply, with CAFS, you will need much larger nozzle aperture openings than ever before.
Another consideration during a CAFS nozzle evaluation includes analyzing the quality - consistency - of the finished-foam agent produced. First, evaluate stream reach. Nozzles must be sized to provide good stream reach so firefighters are provided with adequate stand-off distance.
After providing firefighters with a good stream reach, examine the foam quality produced. Ensure the nozzle choice provides high-quality finished foam, from "wet" to "dry." Each nozzle tested will have trade-offs in finished foam quality with certain consistencies, but provide adequate quality with others.
To be able to produce different types of foam consistencies - from "wet" to "dry" - most CAFS use hardware that vary the air to foam solution discharge ratio.
During a nozzle evaluation, ensure that your final selection creates an acceptable quality of finished compressed air foam throughout the "wet" to "dry" foam consistency range. Dry compressed air foam has a high volume of air and a low volume of foam solution. Evaluate dry compressed air foam through both fog and smoothbore nozzles to see the foam quality differences various nozzle selections can make.
Using Stream Shapers
Examine the use of "stream shapers," especially on 2.5-inch hose lines. When using portable monitors at higher flows, stream shapers may be needed to reduce stream swirl and increase foam stream reach.
The third consideration for nozzle evaluation is choosing a nozzle system with a manageable nozzle reaction force for hose teams. Nozzle reaction is a function of flow (gpm) and size of the nozzle orifice (aperture).
When evaluating nozzles, watch for feathering, which is a function of the compressed air media exiting the nozzle and expanding to match atmospheric pressure while moving through a small orifice. When hose lines experience feathering, it is an indication that nozzle selection (orifice size) may be too small for the flow rate moving through it.
The fourth consideration in evaluating nozzles is creating a system that allows firefighters the greatest variety of choice when using compressed air foam. For example, when choosing to adopt large-diameter smoothbores (1 3/8-inch) for use on 1.75-inch hand lines, it is important to realize that if an officer decides to change media - from compressed air foam to foam solution or plain water for instance - a 1 3/8-inch nozzle at 50 psi requires 397 gpm to create an effective stream. That's much more than can be realistically moved through a 1.75-inch hand line.
Some departments choose a nozzle that will allow various fire stream choices. Those kinds can deliver compressed air foam, foam solution and plain water. Firefighters can prepare to deliver compressed air foam through a 1.75-inch line by removing the 15/16-inch stacked smoothbore tip from the 1/4-turn ball valve. Should the officer decide to change the media from compressed air foam back to plain water, or foam solution, the firefighter can simply replace the 15/16-inch tip to develop an effective fire stream.
The final consideration during an evaluation of a specific nozzle is how easily the system will be accepted by the end-users. This is a function of training and education. For example, firefighters may not feel as secure or be as effective when using a smoothbore nozzle if they have been traditionally using a narrow fog for fire attack or using a wide fog for personal protection.
Also, it is vital to look at current nozzles in use, current agent application techniques and a department's prior knowledge of nozzles and agent application. Some changes may be needed and these must be clearly outlined and addressed during training.
One option is a nozzle that allows a solid-stream through a smoothbore, a range of fog patterns, or both simultaneously.
Our discussion here was designed to give a broad overview of some of the issues and items for consideration when performing a CAFS nozzle evaluation. This was not designed or intended to be all inclusive and doesn't cover all topics regarding nozzles and nozzle evaluations.
The Compressed Air Foam Systems Handbook (available from Fire Protection Publications) has a full chapter including over 60 color photos covering hose, nozzles, and friction loss for compressed air foam.
The authority having jurisdiction in each fire district must decide which type of nozzle or nozzles work best for a particular purpose under local fire conditions.
One last note: an objective nozzle evaluation led by fire management, using the troops' input, will go a long way toward the successful and safe implementation of CAFS.
Don't miss or forget this section when planning your departmental compressed air foam implementation strategy.
Editor's Note: Dominic Colletti is the author of The Compressed Air Foam Systems Handbook and Class A Foam-Best Practice For Structure Firefighters. He is co-author of Foam Firefighting Operations I and The Rural Firefighting Handbook. Colletti is a former assistant fire chief and serves on the technical committee of NFPA 1500 Fire Department Occupation Safety and Health Program. He is the global foam systems product manager for Hale Products and can be reached at firstname.lastname@example.org.