Automatic Nozzle Doubles As Foam-Making Nozzle
The combustible materials involved in today’s fires are different than those of even 10 or 15 years ago. New equipment, new techniques, and, in some cases, new firefighting agents [in particular, AFFF (aqueous film forming foam)] are being employed. Therefore, it is important that the equipment used in today’s progressive fire departments be compatible with a variety of tactical applications.
When equipment is upgraded to automatic nozzles and 1 3/4-inch hose, fire departments often ask, “Will this combination work with our existing eductors and AFFF foam?” The answer is YES, providing certain guidelines are followed.
Foam making is simply the addition of a proper amount of foam concentrate to water. This solution is then mixed with air (aeration) at the nozzle to produce a finished foam product, which is then applied to the flammable liquid.
By-pass or in-line eductors are preengineered systems that require specific inlet pressures for operation. A large amount of the inlet pressure is lost in creating the vacuum necessary to pull the foam concentrate from the container into the eductor, and then into the water. Part of the remaining pressure is used to overcome the friction loss in moving the water/foam concentrate solution through the hose to the nozzle. The pressure that is left must be sufficient to match the flow requirement and operating pressure of the nozzle. The most common nozzle pressure specified is 100 psi, and is considered a standard for fire nozzle operations.
Let’s look at a typical 95-gpm foam application using a 95-gpm eductor, conventional 95-gpm nozzle, and 1 1/2inch hose. Most eductor manufacturers recommend an eductor inlet pressure of 200 psi and no more than 150 feet of hose to the matching nozzle. This inlet or pump pressure is required to accomplish the following:
If an automatic nozzle is substituted for the conventional 95-gpm nozzle, the results will be identical because the inlet pressure to the eductor is sufficient to overcome the pressure loss in the eductor and hose. An automatic nozzle is designed to provide the optimum pressure of 100 psi regardless of the flow supplied. Therefore, the automatic nozzle “adjusts” itself to become the required 95-gpm nozzle operating at the correct 100 psi that remains.
Because l 3/4-inch hose has considerably less friction loss than l 1/2-inch hose when moving the same amount of water the same distance, adjustments must be made to the pump pressure or to the length of hose, or both. With 95 gpm flowing (a typical eductor capacity), the friction loss in 100 feet of 1 1/2inch hose is about 30 psi. For an equal length of 1 3/4-inch hose, the loss would be only 12 psi. Whatever the hose friction loss may be, we must add the required nozzle pressure (100 psi) and the loss through the eductor (about 55 psi). Using these figures we can then determine the approximate pump pressures necessary for satisfactory operation with various lengths of 1 ¾inch hose and a 95-gpm eductor.
When the approximate engine pressure is determined, compare the time required to empty a five-gallon container of water to the eductor table (on page 12) for the particular eductor setup that is being tested. This comparison is necessary to develop a more accurate pump pressure and to make changes for differences between different hose brands and different eductor types. During the comparison testing, water instead of the AFFF concentrate should be used for cost-saving considerations. This method can also be used to verify the efficiency of eductors used with conventional nozzles. The actual foam concentrate consumption rate is also listed.
As a comparison example: A 95-gpm eductor is used with 200 feet of 1 3/4-inch hose with a mixture setting of 3%. An inlet pressure of 180 psi is supplied to the eductor. If the flow through the eductor is correct (95 gpm) and the mixture setting (3%) is proper, it should take 1 minute and 45 seconds to fully empty a five-gallon container of water. The foam must be used up at a rate of 2.8 gpm to correctly mix 95 gpm of water at a 3% ratio. The engine pressure can then be adjusted up or down to accomplish the correct times for your particular equipment.
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Now that we have established the proper engine pressure for our particular application, let’s focus on proper nozzle operation for optimum foam making capability.
To gain the maximum reach and at the same time produce the maximum expansion ratio of the foam /water solution, the nozzle should be adjusted to a very narrow 10°-15° fog pattern. Wider fog patterns will result in a thinner foam with a low expansion ratio and reduced reach. The expansion ratio is the amount of finished foam produced compared to the volume of solution used to generate the foam.
Foam manufacturers currently recommend an 8 to 1 expansion ratio of finished foam to solution volume, up to an 11 to 1 expansion ratio for AFFF. For example, an 8 to 1 expansion ratio means 800 gallons of finished foam is produced from 100 gallons of the foam concentrate/water solution. The difference in volume is basically the air that is trapped with the foam bubble.
Automatic nozzles can produce expansion ratios between 7 and 10 to 1 at a 10°-15° fog pattern. Conventional fog nozzles will normally produce expansion ratios of 3 or 4 to 1. This difference is a result of the consistent high velocity of the stream and the typically greater area of water exposed.
By maintaining a constant nozzle pressure, automatic nozzles keep the velocity of the stream high. Large amounts of air are pulled into the stream and mix with the foam/water solution at the end of the stream. This mixture of air and foam solution expands and “snowflakes” down gently on the burning liquid. Additional expansion can be created by deflecting the foam stream on a horizontal or vertical surface, further entraining air and adding to the expansion ratio. It must be remembered that when using any nozzle with an eductor, the nozzle must be fully open to allow proper flow across the eductor. This produces the vacuum necessary to pick up the foam concentrate and mix it with the water.
Although originally designed for use with 1 1/2-inch hose and a nozzle of matching gpm, most eductors will function correctly with an automatic nozzle and 1 3/4-inch hose. The important factors to remember are that the inlet pressure of the eductor must be sufficient to overcome the total of:
- The pressure loss through the eductor.
- The friction loss for the hose lay between the eductor and the nozzle.
- The required 100 psi nozzle pressure.
By experimenting with various engine pressures on the drill ground, correct operation can be assured for actual fire conditions.
Automatic nozzles have an excellent performance record when used as structural firefighting nozzles. If these guidelines are followed, the automatic nozzles will perform equally well as foam making nozzles.
