Fire presents many challenges, not least because of the variables that decide which suppression medium is appropriate for a given fire scenario. Our objective on this two-part article is to supply steering based mostly on a hazard-specific approach and help you make an knowledgeable decision on appropriate fire-suppression measures, especially with regard to the processing and storage of flammable liquids.
Fire hazards are classified as follows:
Class A – carbonaceous or flamable supplies
Class B – flammable liquids
Class C – flammable gases
Class D – metals similar to magnesium
Class F – fires generally related to cooking corresponding to vegetable oils
Class E – electrical hazards.
As a producer of foam proportioners and firefighting displays, we specialise primarily in the Class B class: flammable liquids.
Foam is taken into account to be the simplest fire-extinguishing medium for each hydrocarbons and polar solvents. In addition, sure kinds of foam (known as wetting agents) are specifically for deep-seated Class A fires the place water only may not penetrate the hearth load effectively.
How will we make foam?
Finished foam answer includes a easy recipe of froth focus, water and air. Foam focus is usually blended in the ratio of 1% or 3% to either 99% or 97% water.
Once the solution (premix) is created, it is pumped to the chosen discharge system – we will cowl proportioning later in this article and discharge methods in Part 2. At this level air is entrained by utilizing devices designed to naturally aspirate the solution. The amount of aspiration and the kind of focus determines the growth ratio. This is usually split between low, medium and high growth. Low expansion is up to 20:1, medium is 200:1 and excessive enlargement is greater than 200:1. Only high-expansion foam may have the addition of a water-driven generator fan to expand the foam adequately, though naturally aspirated turbines are additionally used. The determination to choose a specific expansion ratio is determined by the hazard under evaluation.
Put merely, water and oil do not mix. Applying only water to Class B merchandise is not going to suppress the burning vapours. This is an important level to make as it is the vapours, not the liquid itself, that burns. We need a medium that allows for the formation of a vapour-suppressing and oxygen-depleting blanket and/or a barrier film to be successful. Whatever method is chosen to produce that blanket or film, the target is the same.
Steps of froth generation Images provided by Author / Contributor
A little of the history of froth development
We can date the development of recent foam concentrates from the Sixties onwards. Up to that time foam concentrates were comprised of animal-based protein (typically floor hoof and horn is the bottom component). The major disadvantage was as a result of limited gas tolerance, which meant it was vulnerable to gasoline pick-up (mixing). A breakthrough got here with aqueous film-forming foams or AFFF’s. Instead of utilizing protein as the base product, synthetic detergents gave foam a greater gasoline tolerance as nicely as forming a vapour-sealing film throughout the surface of the fuel. Performance is additional enhanced with the formation of a foam blanket when air aspirating gadgets are fitted.
Nevertheless, AFFF’s improvement didn’t completely exchange using protein-based foam. Fluoroprotein, or FP, meant that protein foam could both combat gasoline pick-up (mixing) and offer a good resistance to warmth, defined by the industry as ‘burn-back resistance’.
Further growth in the Nineteen Eighties took FP to the following degree with film-forming fluoroprotein or FFFP. We now had one of the best of each worlds: a film-forming foam that also had good burn-back resistance.
The 1980s additional added to our record of acronyms with FFFP-AR – film-forming fluoroprotein, alcohol resistant. Alcohols or polar solvents had at all times offered a further problem as they were water soluble and foam harmful. The FFFP-AR chemistry included a polymeric barrier, which protected the froth blanket from early destruction. The identical know-how additionally turned obtainable with the model new generation of AFFF-ARs.
In recent years fluorine, one of many cornerstone components to all foams, has turn into an environmental issue, as a end result of persistence in groundwater. The trade has been offered with a serious challenge to remove the ‘fluoro’ component of all of the totally different foam concentrate types. We have witnessed an era of manufacturers’ claims and counter claims concerning the efficacy of a complete range of newly formulated fluorine-free foam. The term ‘SFFF’ (synthetic fluorine free foam) or ‘F3’ is used to outline these foams, which have turn out to be the model new normal and first choice for operators changing existing inventory or reviewing new tasks. As a foam proportioner producer we must look at the physical properties of foam carefully (especially viscosity) as they influence choices on the overall proportioning resolution. More of this later.
Multi-purpose foams such as the F3 alcohol-resistant sorts are increasingly essential within the fuel-storage enviornment. The introduction of E10 petroleum, with its 10% ethanol-based biofuel content material, implies that the alcohol-resistant high quality of SFFF/F3 is necessary to fight the small diploma of water miscibility of the gasoline.
All foam focus manufacturers will naturally promote their product as being extremely efficient. The best means of evaluating their claims is to familiarise yourself with the empirically based commonplace EN 1568 or UL162. The checks cowl extinction time, foam stability (via drainage time tests) and post-fire security (burn-back test). Hydrocarbons and the extra demanding polar solvents are both included in the check protocols. This additionally covers fresh and seawater as nicely as mild and forceful software of the foam. Each foam is given a rating with IA being the most effective, IIID the worst. This means that you could challenge foam suppliers to be able to make an knowledgeable decision on one of the best foam for your wants.
Whilst EN 1568 is a superb benchmark normal, awareness of the results on foam performance from factors outdoors the standard checks must be noted. For example, significantly aggressive solvents can problem the firefighting effectiveness of certain foams. This can be exacerbated by completely different ambient air temperatures, the appliance method, gas depth and so forth. Our recommendation would be to understand the small print of the checks and try to match them to the foam’s appropriateness in your particular person hazards. We would all the time advocate consulting particular person foam manufacturers as they will usually present particular in-house fire-performance data in opposition to some of the extra uncommon fuels.
However, despite the foam manufacturers’ typically conflicting claims on performance, the earlier fundamentals of how foam is proportioned nonetheless stay: 1% & 3% concentrates are the norm for producing a finished foam.
Foams according to the growth ratio Images equipped by Author / Contributor
Critical elements for achievement: the time and utility fee matrix
Successful extinction of flammable liquid fires is decided by two converging and complementary elements: time and the speed at which finished foam is utilized. Both are determined by empirically primarily based requirements printed by our bodies such because the National Fire Protection Association (NFPA) within the United States and in Europe by the EN standards.
The ‘time’ issue implies that for a given dimension of fireplace area it is needed to use the froth for long enough to achieve extinction and to forestall re-ignition. This is especially critical in manual intervention as firefighter’s might be in danger throughout post-fire operations. When it involves fixed protection for hydrocarbon and polar solvent tanks, the applying time may be as long as 60 minutes for the largest tank diameters.
The application rate refers again to the quantity of finished foam applied per square metre. This varies in accordance with the gas and the foam sort however is currently no much less than four.0 litres per minute per sq. metre. One of the continuing developments in foam efficiency is the probability that this might be lowered, however standards usually take time to recognise product development. Anything less than the minimal software rate signifies that fireplace management is unlikely to be achieved. This signifies that firefighting sources, notably in a manual fireplace attack, have to be assembled earlier than firefighting commences and this takes time. The benefit in becoming fastened methods is that the sources are already designed and constructed into the system for quick application.
Principle of foam extinguishment Images equipped by Author / Contributor
Foam proportioning strategies: mixing water with foam concentrate
In order to allow readers to make an informed determination on how best to sort out their explicit hearth challenges, the following is an overview of the various foam proportioning strategies.
Typically associated with fire service deployment and limited fixed systems purposes. This makes use of the venturi principle to create a strain drop in a small aluminium or seawater materials appropriate system with built-in focus pick-up tube. The pressure drop created by the venturi attracts the froth focus from a container where it mixes with water to create foam resolution. Both the pick-up tube and body of the inductor are compact enough to be easily carried on a hearth appliance or fitted into a pipework system.
However, while it’s thought-about to be the best method, it has extreme drawbacks: (i) models are manufacturing unit calibrated so any modifications to the hydraulics (caused by length of discharge line after the inductor, the elevation of the discharge system or a changing quantity of discharge devices) will doubtless require a system re-design; (ii) system design and sign-off are critical as deviations attributable to pipe diameter adjustments will adversely have an effect on proportioning accuracy and doubtlessly stop any proportioning; (iii) changes in focus viscosity will undermine the flexibility of the venturi to draw focus into the downstream traces; (iv) stress loss is excessive, as much as 35%, so duty pump pressures need to be excessive sufficient to allow for it, probably adding additional cost into the system; (v) foam supply units must be sized in order not to prohibit design flows; (vi) testing cannot be achieved with out creating premix and discharging completed foam. This will add to whole lifetime prices on account of foam concentrate usage, which needs to be changed, and the disposal of premix or foam.
Types of foam agentsImages provided by Author / Contributor
Balanced strain proportioners:
These use an electrical or a mixture of electrical and diesel pumps to provide foam concentrate right into a proportioner at a better stress than the incoming water-line stress. The proportioner is put in into the water line and regulates the mixing.
They are now related to older methods, the place disadvantages embrace: (i) further prices in hardware and design time; (ii) the foam pump is normally restricted to a a lot tighter operating range, though the proportioner itself can accommodate broad ranges in flows; (iii) the foam pump wants an additional power supply, aside from water, so is not as reliably secure as a system with a potential single point of failure, such as an entirely water-driven system; (iv) sometimes, these methods are very advanced and cause high efforts at the customer with set-up, commissioning and testing; (v) the recirculation of unused foam concentrate creates unnecessary agitation that has the potential to wreck the froth concentrate and entrain air, which in flip can undermine accurate foam proportioning and foam extinguishing effectiveness.
Comprised of a steel stress vessel containing a versatile bladder (typically a butyl material) linked to a foam focus proportioner similar to the ones fitted to balanced strain proportioners. The incoming pressurised water compresses the foam crammed bladder in order that foam concentrate may be proportioned with water using the identical supply. The venturi precept is once more introduced into play as it creates a strain drop on the level of injection for the froth focus. Testing can’t be achieved without creating premix and discharging finished foam. This will add to complete lifetime costs as a result of foam concentrate utilization, which has to be replaced, and the disposal of premix or foam.
However, the bladder itself is seen as a weak point because great care is needed to keep away from a damaged bladder when commissioning a new installation. It is rarely easy to work within the steel strain vessel ought to issues occur, particularly if and when a alternative bladder could also be required: it all adds to the issue and cost.
Compressed air foam (CAFS):
This is not a proportioning methodology in the typical sense as the foam is already proportioned utilizing one of many earlier methods. However, pressurised air is then added forcefully somewhat than naturally entrained or aspirated. As the name suggests, CAFS injects compressed air into the foam resolution on the level of discharge. The consensus view is that CAFS enhances the finished foam’s capability to cling to vertical surfaces permitting better penetration and cooling. This can be a profit in wildfire conditions by coating vulnerable buildings to mitigate fireplace development. The limited water supplies related to remote wildfire areas means that the useful resource may be absolutely optimised. However, as with balanced pressure proportioner foam pumps, an extra point of failure is feasible due to the additional CAFS apparatus.
Water pushed or turbine foam proportioners:
FireDos have turn out to be synonymous with their distinctive water-driven foam proportioner. Now in its third technology, the product is purely mechanical and highly reliable. A water-driven rotor supplies the driving force to drive a immediately coupled positive-displacement plunger or piston pump. This is supplied with foam concentrate via an atmospheric foam tank adjoining to the FireDos unit. The proportioning pump is manufacturing unit set to the required proportioning price and can proportion the froth concentrate precisely throughout a large operating vary. The purely mechanical system provides cost-efficient and eco-friendly proportioning fee testing, fully with out utilizing any foam concentrate or creating premix or firefighting foam.
FireDos GEN III foam proportioned. Images provided by Author / Contributor
The advantages of FireDos Gen III FM permitted foam proportioners include:
Reduced prices: proportioning fee is examined with out utilizing foam or creating premix or foam – value effective and eco-friendly.
Improved pressure gauge 10 bar : fixed proportioning rate across a wide working vary.
Optimised supply of high-viscosity foam concentrates with improved suction capacity.
25% discount in strain loss, compared to older generations, via hydraulic optimisation.
Improved cylinder cut-off for straightforward and quick adjustment to a different proportioning rate.
FM approved FireDos Gen III proportioners are thus far the only FM permitted water motor coupled variable proportioners utilizing FM approved circulate meters. Using the take a look at return line this provides the chance to examine the proportioning fee during commissioning as nicely as for annual tests, without creating any premix, which is a very huge benefit for the consumer and relieves the setting.
In the second a part of this characteristic we will explore the results of viscosity on foam proportioning and typical foam utility areas and really helpful discharge units.
For extra info, go to www.firedos.com