Fire fighting foam is a collection of bubbles, formed by the mixing of water, foam concentrate (or foam compound) and air. The mixture of water and foam concentrate is called foam solution with water being 97% to 94% of the mixture, while 3% to 6% is the foam concentrate. The foam solution is mixed with air, the ratio of which can vary from 1:2 to over 1:1000 depending upon the type of foam equipment used and its application.
Fire fighting foam being lighter than water can float on flammable liquid (while water sinks below the liquid surface), hence is very effective in fighting flammable liquid (Class B) fires. It can also be used to cover flammable liquid spills and prevent fires. With higher expansion ratios (more than 1:500) it is also used to fight Class A fires, where it results in minimal water damage.
Fire fighting foam is classified into three types of foam, based on expansion ratio (and consequently, its application). Most fire manuals use this classification –
|Low expansion foam :||expansion ratio from 1:2 to 1:20 (most commonly used between 1:3 to 1:10, useful for flammable liquid fires)|
|Medium expansion foam :||expansion ratio from 1:21 to 1:200 (most commonly used between 1:30 to 1:60, useful for tackling spills/ spillfires)|
|High expansion foam :||expansion ratio from 1:201 to 1:2000 (most commonly used between 1:400 to 1:1500, useful in Class A and LNG applications|
Nowadays, low expansion foam is again sub-divided into two types :
|Primary aspirated foam :||mixing of foam solution with air occurs inside the equipment, resulting in expansion ratios from 1:7 to 1:12 (e.g. foam branches, foam barrels)|
|Secondary aspirated foam :||mixing of foam solution with air occurs after solution leaves the nozzle, resulting in comparatively lower expansion ratios i.e. 1:3 to 1:6 (e.g. Aqua foam nozzles, handline nozzles)|
It may be noted that for secondary aspirated foam, only certain types of foam concentrates (AFFF, AR-AFFF, FFFP) will give proper results.
Though both aspirated and unaspirated foam are low expansion foams, and used for the same application, unaspirated foam can be projected further due to higher density as compared to aspirated foams. Hence unaspirated foam is increasingly being preferred for fighting large flammable liquid fires where large capacity foam monitors are employed in the attack.
Though the flow capacity of Low Expansion (LX) Foam branches can vary from manufacturer to manufacturer, standard foam branches are normally designed for the following capacities – 200/225 lpm (FB 5X) 400/450 lpm (FB 10X), 800/900 lpm (FB20X) (Note – all flows at 7 kg/cm2 inlet pressure).
Flow capacities for AAAG LX Foam branches can be identified from the model – the ‘5’ & ‘10’ in the FB5X and FB10X refer to flow of 50 and 100 Gallons (British) per minute. This converts to 225 lpm (for FB5X) and 450 lpm (for FB10X) using conversion of 1 Gallon (British) = 4.5 Litres. The ‘X’ in the model number refers to expansion ratio of the branch i.e. roman numerical ‘X’ indicates approximately 10 times expansion.
Self inducting foam branches allow induction to be carried out at the branch, while for foam branches employing inline inductors, induction takes place at the inductor. Normally, operation of foam branches using Inline Inductors is preferred where the fire risk is higher, and it would not be safe to do the induction at the branch (as it involves supplying foam concentrate containers near the branch, and may hamper the movement of the foam branch operator). Hence foam branches with inline inductors are preferred where there is a risk of combustible and flammable liquid fires.
Technically, there is little difference in both types of operation i.e. operating range, expansion, etc at the same inlet pressure at branch. Foam quality also can be expected to be the same whether using self inducting foam branches or those employing inline inductors.
Like LX Foam branches, Medium expansion (MX) Foam branches are also available in matching capacities i.e. 200/225 lpm and 400/450 lpm. Higher capacities are available for MX pourers i.e. 600 lpm, 900 lpm and 1100 lpm.
Medium expansion foam has a higher expansion ratio (1:30 to 60) as compared to low expansion foam, hence can cover spills quicker. However, due to larger size bubbles, it cannot withstand the heat from flames, and breaks down much more quickly as compared to low expansion foam. Hence, it is not possible to use MX foam for LX foam applications (such as pool/ spill fires).
Different methods are employed for mixing the foam concentrate into the water stream. Most commonly used methods include – Inline foam inductor (portable or fixed), Jet Ratio Controller unit, Around the pump proportioner, Bladder tank proportioning, Balanced Pressure proportioning, etc.
Inline inductors work on the venturi principle. Hence for the induction to be accurate, it is necessary that the outlet pressure be at least 65% of the inlet pressure. This puts a limitation on the length of hose/ piping that can be connected between the inductor and the discharge device. It also puts a limitation on the elevation of the discharge device. Also, varying pressures can result in inaccurate induction.
Different type of Foam concentrates have different applications, and require different levels of ‘working’ (or aeration and agitation). Hence foam equipment is normally designed for a certain type (or types) of foam concentrate. Listed or approved Foam equipment is also approved/ listed with a specific foam concentrate. It is recommended that suitability of foam concentrate with the equipment be checked before use.
Jet Ratio Controllers (JRC) are high efficiency venturi devices that deliver a concentrated foam solution to the monitor. The advantage with JRCs is that it allows the foam concentrate mixing at some distance away from the monitor. Hence foam container deployment and handling can be at a safe distance away from the fire scene
A pressurized water supply using hose from a hydrant valve (or Pump) is connected to inlet of the JRC unit. A PVC suction tube with Dip tube is used to induct foam concentrate from the containers into the JRC unit. A hose (or hoses) connected from the JRC outlet to the Foam nozzle delivers a concentrated foam solution to the monitor.
When installing Foam monitors, ensure the following –
- Height of the monitor is not more than 3 m from grade level (for monitors where induction occurs at the monitor nozzle i.e. using pick up tube or hose).
- The mounting arrangement is suitable to the back thrust from the monitor.
- Ensure that the mating flange of the hydrant post is matching to the valve inlet flange.
- Use approved gasket between flanges, and good quality nuts/ bolts.
- Ensure that full rotation of the monitor is possible, and the height of the monitor handwheels is at proper height for operator use.
Quality of fire fighting foam produced by Foam equipment depends upon certain factors such as the quality of Foam concentrate, its proper induction in the water stream and mixing of foam solution with air (aeration). As foam equipment is designed for induction and aeration rates at a certain working pressure, it is important to ensure that the equipment is operated at or within the specified pressure range. Operation of the foam equipment at pressure that varies greatly from the working pressure can result in ineffective performance of the equipment (it may also be noted that working pressure normally refers to pressure at the inlet of the equipment, not the line or pump room pressure).
Foam equipment is also designed for use with a specific type of foam concentrate/s. Optimum performance of the equipment can be obtained only when used with recommended foam concentrates.
Many of the commonly used foam concentrates are corrosive to carbon steel, and it is important to ensure proper flushing with fresh water after each use. Proper drainage in foam equipment or fixed systems is therefore, recommended. Bronze or Stainless steel (SS304/ SS316) is therefore, normally preferred for foam equipment.
Parts requiring replacement are usually rubber washers/ rings. When replacing them, remember to use original washers/ rings. If metal parts/ components are required to be replaced, use original spares, as material properties are important for foam equipment. Equipment may be painted externally once in 2/3 years depending upon atmospheric conditions – do not paint parts which are not originally painted.