In comparison to other fire types, the occurrence of flammable metal fires would be significantly lower than Class A and B fires. Statistics would show that of all structural fires attended by fire fighters, most will be controlled within an accepted time period due to the repetitive, rigorous training routines and modern equipment that allow for a fast response.
When a fire fighting crew arrive on the scene of an Industrial scale flammable metal fire, they do not carry any equipment or extinguishing agents specifically designed to ensure the safe control of a flammable metal fire. The resultant success rates in terms of controlling the fire and limiting the damage to property is minimal. It is not always possible for the fire service to have advance knowledge of the type of fire they are about to face and indeed these metal fires are often characterised by the increase in their intensity when conventional foam and water extinguishing agents are applied.
The need to introduce more lightweight components into automobiles and aviation make magnesium a desirable material to use and processing plants, die casters, machining plants and manufacturing plants are becoming busier due to these modern day requirements. The result is the increased use of metals such as magnesium in every day industrial applications and the frequency and scale of these fires is likely to increase significantly.
In the event of an industrial scale fire the application of a small number of 9kg fire extinguishers which could be carried on the vehicle, will not realistically prevent the fire from spreading. An incident in Staffordshire in 2014 saw a flammable metal fire at a local metal waste recycling plant where there was a large store of Magnesium swarf. This incident required the resources of numerous fire engines and more than 40 fire fighters were present. They were largely ineffectual due to the fact that they could only contain the fire to a single building whilst it burned itself out.
The only control measure which was available to the fire service was to spray foam into the building to assist in the cooling of the magnesium waste as it burned. In doing this, they were adding water to the fire, which in this category of fire only serves to offer further energy and to increase the intensity of the fire.
So new thinking and a new approach is required to ensure that fire fighters in attendance at flammable metal fires have the right materials available to tackle this type of fire. Fortunately there is now a new product designed specifically to combat this fire type. The product is Aqueous Vermiculite Dispersion, which has been extensively tested on magnesium powder, magnesium chips and magnesium swarf alloys (AZ 91 and AM60) during its developmental phases with successful results.
So what is Aqueous Vermiculite Dispersion(AVD)?
Unlike traditional flammable metal fire extinguishing agents, AVD is a water based agent whose success lies in the platelet shaped vermiculite particles (aluminium-iron-magnesium-silicates) that are contained within the self suspending dispersion. This is achieved through a carefully monitored and controlled manufacturing process at Dupré Minerals’ manufacturing plant in Newcastle-under-Lyme. AVD is a product that compliments the more traditional thermally expanded vermiculite products that are also produced on site. The chemical process removes the inter-laminar trapped water and yields microscopic, individual platelets that are freely suspended in water.
AVD is fine tuned from this vermiculite suspension to allow it to be deployed from traditional liquid fire extinguishing equipment and is typically 20% vermiculite concentration/80% water concentration.
How does AVD work?
A water based extinguishing agent, would not normally be recommended for any Class D fires due to the reaction between the flammable metal and the water. However, AVD is unique in this regard if applied as a fine mist or a foam.
As a mist
When it is applied as a mist with a droplet size ranging between 100 microns and 200 microns, the water content of the fine droplet evaporates due to the high heat of the fire, which can be burning at up to 3000°C. This means that instead of depositing a wet mist onto the fire, it is actually a dry mist that initially comes into contact with the burning metal. This initial layer of vermiculite platelets, being dry in nature, do not react with the fire, instead form a layer over the top of the fire and in a short period of time will form an oxygen barrier between the fuel and the oxygen present in the atmosphere. There are no bonding agents, adhesion promoters or other organic additives in AVD. The vermiculite platelets in AVD naturally overlap and bond together whilst drying to form a tight oxygen seal. Subsequent layering of the AVD on the fire will tighten the oxygen barrier and in doing so starve the fire completely of oxygen and will extinguish the fire completely.
As a foam
AVD foam works on much the same principal as AVD mist. To be successful as a foam, it needs to almost be a dry foam, thus ensuring that as AVD is applied, the thin wall structure of the foam bubbles dries instantly on the fuel source creating almost an instant barrier between the fuel and subsequent layers of foam. Foam application ensures that you can apply a thicker, less dense AVD barrier quicker than an AVD mist barrier whilst still offering the same oxygen barrier properties that ensure the fire is actually extinguished underneath. The AVD barrier is ultimately wetter, simply due to the quantity of AVD being applied in comparison to a mist deployment. However this just serves to offer better cooling properties to the fire underneath.
There are no real limitations in terms of how you can deploy AVD. It is not restricted as Class D powders currently are and as a liquid extinguishing agent, it benefits from utilising all the standard extinguisher bottles, backpack systems, mobile trolley systems as well as installed fixed systems that can deploy liquids. Importantly, where dry powders need to be deployed in very close proximity to the fire, AVD can be deployed from a distance of up to 10 metres with portable equipment, thereby reducing safety concerns. Additionally, AVD has been proven to be able to utilise standard fire fighting equipment that is carried on the fire engine such as the LPP’s, standard hoses and branches that are carried around as standard. By combining these standard Fire Fighting pieces of equipment with a 1000 litre IBC of AVD you have a system that is capable of tackling an industrial fire from a distance of 15 metres. Taking the unfortunate incident in Staffordshire last year, if this extinguishing agent were available on the day, then the fire would certainly have been controlled much quicker, reducing business disruption.
Current Class D Powders
Current Class D powders such as those based on graphite and sodium chloride offer good control to small scale Class D fires, such as a small swarf pile or spillage of powder but can they be used in large scale industrial fires? The answer is probably yes, however the proportion of Class D powder that you would need to store versus the size of the potential fire would be enormous, making it an almost impossible consideration. Controlled tests have also shown that graphite and sodium chloride powders offer good smothering properties to the fire, but do not fully extinguish the fire and instead simply cover the fire whilst it burns itself out.
Problems with Accreditation and Certification
Classifying and certifying Class D fire extinguishing agents has always been problematic and difficult. Read a copy of most versions of EN 3-7 and there is a glaring absence of test criteria for class D extinguishing agents, and almost no reference at all to them. In comparison, there are clear and defined performance criteria for Class A and B extinguishers. (DIN EN 3-7 is an example of where there are considerations for Class D extinguishing agents, but this is not shared between other EN3-7 versions). UL711 also provides specific test criteria for Class D extinguishing agents and in both instances, UL711 and DIN EN 3-7, there is a requirement to test the agent on magnesium (powder, swarf, chips) as well as sodium and this poses a problem where new developments of Class D agents are concerned.
Class D fires involve extremely high temperatures and highly reactive fuels, for example, burning magnesium metal breaks water down to hydrogen gas and can causes explosions in some cases. Burning magnesium can also break halon down to toxic phosgene and fluorophosgene and may cause a rapid phase transition explosion. It will continue to burn even when completely smothered by nitrogen gas or carbon dioxide, and with carbon dioxide it can yield toxic carbon monoxide. Consequently there is no one type of extinguishing agent that is approved for all class D fires, rather there are several common types and a few rarer ones. Each must be compatibility approved for the particular hazard being guarded.
This issue has been taken up with both the BS EN Committee and with NFPA to review how Class D agents are classified. Rather than have just a generic Class D, lobbying has taken place to sub categorize Class D to make extinguishing agents more specific and into the following 2 categories;
Flammable metals that burn as a solid (Magnesium, Titanium, Aluminium, Zirconium)
Flammable metals that burn as a Liquid (Sodium, Potassium, Lithium)
The proposed sub categories would ensure that all existing and new fire extinguishing agents are certified to either solid metal fires, liquid metal fires or even both. Standards testing could then be tailored to ensure optimisation of extinguishing agents to the hazard present and will ensure that when a first responder tackles a fire, they at least are in possession of the right kind of extinguishing agent for the hazard, thus making the potential to extinguish the fire better and reduce the overall hazard. Also, if the first responder has been able to tackle the fire with the most effective extinguishing agent to begin with, the fire fighters arriving on the scene may be performing a role of damage limitation and securing the fire area rather than trying to contain a fire that quickly spreads in the early stages due to the wrong extinguishing agent being used in the initial stages of the fire.
For further information, go to www.avdfire.com