The Fire Protection Blog.

Vehicle Fire Suppression Basics: Extinguishing Agents

3rd October 2017

Posted by: Ardent

Ardent

Choosing a fire suppression system for your mobile equipment may look like a daunting task, especially when considering the number of options available. To assist you in your choice of system, this Blog Series will give you a general overview of its main components and the factors that dictate the design of an effective system.

We invite you to read though the blog posts in this series to familiarise yourself with the key principles, technology and design decisions behind the current generation of vehicle fire suppression systems. We start off with the essential component of any fire suppression system, the extinguishing agent.

Part 1 of 9: Extinguishing Agents
Fire suppression agents work in a number of ways, for example, by separating the fuel and air, reducing the concentration of oxygen in the atmosphere around the fire below the amount needed to sustain combustion, or by absorbing the heat from the combustion.

The agents currently favoured for protecting off-road plant are multi-purpose dry chemical (monoammonium phosphate based powder) and wet chemical (either Aqueous Film Forming Foam – AFFF – or other liquid based agents). These extinguishing agents have proven to be the most effective on the types of fires commonly experienced in vehicle and mobile plant. Let’s have a look at their main characteristics.

Firefighting Foams
Firefighting foams are a watery suspension of gas – usually nitrogen or air – in the form of bubbles separated by films of solution. Foam is produced mechanically by mixing a liquid concentrate with water and forcing the gas into this solution, producing bubbles.

The most common foam used in vehicle fire protection systems is Aqueous Film Forming Foam (AFFF). AFFF is water based and therefore has the advantage that it can cool the area down, reducing the chance of the fire reigniting. An incidental benefit of these systems is that the ‘run off’ agent may suppress secondary fires which may occur under the vehicle should the fuel, debris, etc. have collected and subsequently ignited.

  • Pros: AFFF creates a thin film on the fuel surface stopping fuel from evaporating and reducing its oxygen supply. AFFF has controlled test fires in 36-60% of the time needed by other foams and can be used in concentrations as low as 1%. Discharge time at least 1 minute to give good cooling.
  • Cons: Because the premix solution is usually 94% water (can be 99%), the solution has a high freezing point of 0° This can be reduced to as low as -30°C and as high as 99°C by introducing additives, but this degrades the performance and life of the AFFF. The AFFF containers are also relatively large, which can sometimes pose difficulties. Nozzle placement/location is critical.

Wet Chemical
The wet chemical agent is a blend of organic and inorganic salts coupled with surface active agents. It has similar properties to the AFFF but it provides a strong measure of freeze protection (as low as -50°C).

  • Pros: Freeze protection to a possible -50°C and the foaming properties of other foams. Discharge times up to 2 minutes to give good cooling effect.
  • Cons: Similar to AFFF, wet chemical systems are relatively large compared to dry chemical systems. This may make installation of the systems difficult.

Dry Chemical / Powder
This agent is currently the most widely used agent in the vehicle suppression market. Dry chemical systems are relatively compact in size, requiring little if any modification to the vehicle during installation. On a weight basis, they are probably the most effective agent in extinguishing fires and provide rapid fire knockdown capabilities. These systems also have a broad operating temperature range of -54°C to 98.9°C without any form of modification.

Dry chemical extinguishing agents are comprised of a finely-divided powdered material that has been specifically treated to be water repellent and capable of being fluidised and free-flowing when under expellant gas pressure. They are available for almost all types of fires and almost instantaneously quench flames by chemical inhibition.

  • Pros: Proven effectiveness on all classes of fires. Rapid fire knockdown. Compact and light.
  • Cons: Poor post-fire security and a possibility of the fire reigniting once the powder has settled. Improperly sealed containers can also be contaminated with moisture, leading to caking of the powder and reduced suppression qualities.

Our Recommendation: Twin Agent
Twin agent systems use a combination of dry chemical and AFFF/wet chemical agents to give fire suppression and surface cooling in the hazard area. The multi-purpose dry chemical discharges to rapidly knockdown the fire. At the same time, the AFFF/wet chemical agent will discharge to give effective cooling, preventing the fire from reigniting.

  • Pros: Arguably twin agent systems are the best and most efficient solution on the market.
  • Cons: Twin agent systems tend to be large in terms of volume.

Next blog post in the Vehicle Fire Suppression Basics Series: Detection Systems

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