Foam is a highly effective tool to include in a fire protection arsenal. It has many applications from fire extinguishment to exposure protection to hazardous material response. In order to make foam effective it is important to both understand how foam systems work and select the correct hardware.
Fire safety professionals are familiar with the fire tetrahedron, which graphically demonstrates the elements needed to create a fire. There is also a foam tetrahedron, which describes the components need to create foam. In order to make foam we need water, foam concentrate, air, and agitation. Water and foam concentrate are first combined in the exact percentages required to create foam solution. Foam solution and air are then agitated together to create finished foam bubbles.
Adding foam concentrate to the water is done by a foam proportioner. Adding air and creating agitation can be done by use of an aspirating nozzle. This is called a naturally aspirating foam system (NAFS). It can also be done by use of an air compressor that is used to inject air into the foam solution under pressure. The agitation then takes place in the hose or in a mixing chamber. This is called a compressed air foam system (CAFS).
In this article we will examine each of the foam hardware components, proportioner, nozzles, and compressed air foam systems individually.
There are many foam proportioners on the market today. They range from inline eductors to direct injection systems. Proportioners can be broken into the two broad categories of manual and automatic. Manual proportioners are systems that require user adjustment and/or specific operating parameters in order to function accurately. The most common manual proportioner is the inline eductor. It requires a typical inlet pressure of 14 bar and a specific flow rate, for example 430 litres-a-minute. When operating in these parameters it provides an accurate foam solution percentage. Variations outside these parameters will cause inaccuracy or no foam. These units work well for tactical operations where a specific constant flow is required. An example would be a large Class B incident.
The typical fire attack in either the wildland or structural arenas requires a constantly varying fire flow as nozzles are opened and closed and lines are added or deleted. Because of this varying flow an automatic foam proportioner is required. Automatic proportioners will allow for changes in both pressure and flow while automatically adjusting and maintaining an accurate foam solution percentage. There are several categories of automatic proportioners on the market today. They include balanced pressure bladder, balanced pressure pump, and direct injection systems. Balanced pressure bladder and pump systems both use some method of balancing the foam concentrate and water pressure. The foam concentrate then enters the water stream through a pressure differential valve. As the volume of water flowing through the differential valve increases the pressure differential within the valve increases. This increased pressure differential allows more concentrate to enter the stream and treat the larger volume of water maintaining an accurate percentage.
Direct injection proportioners use a high pressure pump, up to around 28 bar that injects the concentrate into the foam manifold water stream. Parameters such as water flow are monitored and the information is feed to a computer. The computer then calculates the amount of concentrate needed and commands the foam injection pump to add the proper amount of foam concentrate maintaining the correct percentage of foam solution.
CAFS systems will always use automatic type proportioners and they will typically be the direct injection type. They are sized from approximately 7 litres-a-minute to 55 litres-a-minute of concentrate pumping capacity. Direct injection systems are powered one of two ways; by electric motor or hydraulic motor depending on the size of the system.
Small direct injection systems usually have minimal controls and provide little information to the operator. Controls consist of an on/off switch and a dial to set the percentage. These systems are usually designed only for Class A foam and will proportion between 0.1 percent and 1.0 percent. These systems are also the least expensive. Larger systems will have a digital display on the control unit to provide information to the operator. This information includes current water flow, foam percentage, and volume of water and foam concentrate used. Controls include an on/off switch, a select switch to change the display, and arrow buttons to make adjustments. Percentage rates of 0.1 percent to as high as 10.0 percent are available with both Class A and Class B capability. The larger these systems are the higher the cost.
Every foam capable apparatus should be equipped with a foam tank fill system. This is a convenience, but more a safety feature as it removes the need to climb on top of the apparatus with buckets to refill the foam tank. This reduces exposure to slip and fall hazards. Some of these systems utilise the proportioner pump as the transfer pump to fill the tank. Others utilise a separate independent pump. These transfer pumps are typically in the 23 litres-a-minute range.
Aspirating Foam Nozzles
A method to add air and agitation to the foam solution and create bubbles is to use and aspirating foam nozzle. Aspirating nozzles are a great foam application tool to keep in the firefighting tool box. Air is drawn into the nozzle through a venturi effect. As the foam solution passes through a restriction in the nozzle a low pressure is created that allows the air to enter the nozzle. This process consumes energy; the more air that is drawn in the more energy is consumed.
There are a wide variety of aspirating nozzles on the market. Some are fixed tubes with no adjustment. Others are adjustable usually by changing the stream pattern. And finally, each of the nozzle manufacturers make clip-on aspirating nozzles that attach to the bumper of their fog nozzle to be added when needed.
A term commonly used when referring to aspirating nozzles is expansion ratio. Expansion ratio is the ratio between the volume of foam solution pumped into the nozzle and the volume of finished foam bubbles exiting the nozzle. For example if one litre of foam solution enters the nozzle and 50 litres of bubbles exit the nozzle the expansion ratio is 50 to 1. Expansion ratios are broken into three categories. They are low, medium, and high expansion. Low expansion is from 1 to 1 up to 20 to 1. Medium expansion starts at 20 to 1 and goes up to 200 to 1. High expansion begins at 200 to 1 and can go as high as 1000 to 1.
Low expansion nozzles are typically fixed tubes with no adjustment. They produce a wet foam. These nozzles typically operate at 5.5 bar to 7 bar nozzle pressure and a class A foam percentage of 0.5 percent.
Medium expansion nozzles are typically adjustable allowing variations in the volume of flow and the consistency of the foam. Operating pressure for these nozzles is typically 4 bar. The lower pressure is necessary because as the bubble size increases the bubbles become more fragile. Too high a pressure will simply break them reducing the effective production of the nozzle. Larger bubbles also require more structure which comes from an increase in foam percentage, usually 0.5 percent to 0.7 percent.
High expansion nozzles produce a large volume of dry foam. The dry consistence is due to the large volume of air and low water content. As the bubbles become even bigger the same principals discussed for medium expansion nozzles apply. Nozzles pressures drop to around 2.7 bar and the foam percentage must be increased to the range of 0.7 percent to 1.0 percent, which is the maximum percentage for class A foams.
Every foam capable apparatus including CAFS equipped rigs should have an adjustable medium expansion nozzle due to its versatility.
Compressed Air Foam Systems
As mentioned earlier, there must be agitation to force the mixing and form the bubbles. The most efficient way to create agitation is a compressed air foam system. In the system, air under pressure is injected into the foam solution as it leaves the discharge of the apparatus. The agitation takes place in a mixing chamber or the fire hose. As the mixture moves through the hose it tumbles and scrubs on the inside liner of the hose creating bubbles. CAFS is capable of producing very fine equally sized bubbles. These bubbles provide the maximum amount of surface area for a given volume of water and therefore the maximum heat absorbing ability.
The simplest way to think of a compressed air foam system is as three separate pumps tied together. They are a water pump, standard to any pumping fire apparatus; a foam pump or proportioner discussed earlier in the article; and an air pump commonly referred to as the air compressor. In order for these pumps to work properly together there must be check valves to keep the water, concentrate, and air in the proper place. There must also be an auto balance system. Its job is to keep the air and water pressures balanced. This is important as both air and foam solution are being added to the same hose line. If the pressures are not balanced the product with the higher pressure will override and the mixture in the hose will be incorrect. In most systems the air is the last product injected and is added to each individual discharge separately. This is done to provide full control over the foam being produced and to allow individual discharges to operate in different modes at the same time.
Most systems can operate in four modes. They are water only, foam solution, air only, or compressed air foam. Water can be flowed at any time through discharges not connected to the foam manifold or through foam manifold discharges when the foam proportioner is turned off.
Foam solution can be provided for a standard firefighting or an aspirating nozzle through discharges plumbed off the foam manifold by opening the discharge water valve with the proportioner turned on. Air only can be provided. This is done by closing the discharge water valve and opening the air valve. Compressed air foam is made by opening both the water and air discharge valves. The consistency of the foam can be adjusted simply by controlling the amount the water discharge is opened. The farther the valve is opened the more foam solution will enter the line displacing a portion of the air which is also entering the line.
Recent technical developments in compressed air foam systems have concentrated on making the systems more accurate and user friendly. The use of an auto tank fill device manages the water tank level automatically when the apparatus is hooked to a water supply. They make it easier for the operator of the CAFS to maintain consistent operations.
Nozzles for use with CAFS
In a CAFS, when the foam reaches the nozzle the bubbles are formed and ready to fight fire. The most appropriate nozzle is one that has minimal disruption of the bubble structure. Disruption of the bubble structure returns a portion of the finished product back to foam solution decreasing surface area and fire fighting effectiveness.
Smooth bore nozzles allow the bubbles to be discharged with little disruption and are therefore the weapon of choice for compressed air foam. Selection of a smooth bore nozzle typically includes a valve with a large diameter waterway with a tip which is roughly half of the line size. The smaller orifice size will break a portion of the bubbles removing some air and changing the foam consistency. This combination allows the user to change the foam consistence at the end of the line by simply adding or removing the tip.
With the many foam hardware choices on the market today it is easy to obtain a system that will fit the exact needs. To be certain that you are purchasing the correct system for your needs first determine your target hazards and the tactical applications in which you will use foam. Then research the available systems and determine which will best fit your needs.
For further information, go to www.waterousco.com