Understanding Extinguishing Agents

Choosing a fire suppression system for mobile equipment may seem a daunting task, especially considering the number of options available. To assist, this blog series provides a general overview of the main components and factors that influence the design of an effective system. 

Here, we start off with the essential component of any fire suppression system, the extinguishing agent.

Part 1 of 8: Extinguishing Agents

Fire suppression agents work in a number of ways - either by separating the fuel and air,  absorbing the heat from the combustion, or reducing oxygen concentration in the atmosphere surrounding a fire.

The agents currently favoured for protecting off-road plant are:

• Multi-purpose dry chemical (monoammonium phosphate-based powder)
• 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. Here, we share their main characteristics.

Foam system fire protection

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 for foam system fire protection systems in vehicles is Aqueous Film Forming Foam (AFFF). This 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 ‘runoff’ agent may also suppress secondary fires that may occur under the vehicle, should fuel or debris have collected and subsequently ignited.

• Pros of this foam system fire protection: 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 needs to be at least one minute to allow sufficient cooling.
• Cons of this foam system fire protection: Since 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 AFFF but provides a strong measure of freeze protection (as low as -50°C).

• Pros of wet chemical: Freeze protection to a possible -50°C, as well as the foaming properties of other foams. Discharge times up to two minutes to give a good cooling effect.
• Cons of wet chemical: 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

Dry chemical or powder are currently the most widely used agents 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 composed 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. These are available for almost all types of fires and almost instantaneously quench flames by chemical inhibition.

• Pros of dry chemical: Proven effectiveness on all classes of fires. Rapid fire knockdown. Compact and light.
• Cons of dry chemical: Poor post-fire security and 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, meanwhile (and at the same time), the AFFF/wet chemical agent will discharge to give effective cooling, preventing the fire from reigniting.

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

Want to learn more about how fire suppression systems work? Read our next blog in the series: Fire Suppression Basics Series: Detection Systems