When firefighters arrive at the scene of a fire and begin applying water, they often expect the flames to diminish. However, many have witnessed a startling opposite effect—the fire spreads quickly into unburned spaces, causing sudden flare-ups. This phenomenon, referred to as “pushing fire,” has been the subject of debate in firefighting circles.

While the term “pushing fire” is used commonly in the United States, the exact causes and mechanics of this phenomenon have been less understood until recent years. Thanks to the collaborative research efforts of organizations like the Fire Safety Research Institute (FSRI) and the International Society of Fire Service Instructors (ISFSI), experiments have shed light on the conditions that create pushing fire and why it seems to occur in some situations but not others.

What is Pushing Fire?

Pushing fire happens when flammable gases from a fire in an oxygen-deprived space are forced by pressure into other unburned areas, where they mix with fresh oxygen and ignite. This can cause a sudden and intense spread of fire, posing serious risks to both firefighters and those trapped in burning buildings.

For a fire to push, several conditions must be met:

1. Oxygen deprivation in the original fire space: The fire burns with limited oxygen, producing smoke filled with flammable gases that have not fully combusted.
2. Ventilation or openings between spaces: These openings allow air to flow between the fire room and the next room, or even outside.
3. A pressure difference: This can be caused by natural airflow, exhaust fans, or even firefighting techniques like water application. When pressure in the fire room is higher than the adjoining space, the gases will flow from the fire room into the next space.

When these factors align, the result can be explosive. Water application, while intended to suppress flames, can cause this unwanted spread of fire by altering the airflow around the fire. But why does this happen, and how can it be controlled?

The Role of Water in Pushing Fire

Water is essential for firefighting, yet it can unintentionally cause or worsen a pushing fire. The water itself is not the problem; rather, it is the airflow and pressure changes caused by the water stream.

When water is sprayed onto a fire, air is entrained along both sides of the stream. As the water hits, it can push this air ahead of it, creating a pressure difference. This airflow can then drive the movement of smoke and flammable gases, pushing them into unburned areas. If these gases mix with oxygen in another room, the fire can reignite and intensify, rather than being suppressed.

This interaction was observed in FSRI’s experiment, where a water mist was applied to cover the window of a fire room. Instead of extinguishing the flames, the mist created a pressure that pushed the fire and smoke into another area, causing a flare-up in the adjacent room. The pressure from the water-driven airflow had exceeded the natural pressure in the fire room, preventing the smoke from escaping and pushing it into an unburned space.

Factors Contributing to Pushing Fire

Pushing fire is a complex phenomenon, and several factors can influence whether or not it occurs. These include:

• Distance of water application: The farther away the water stream is from the fire, the more airflow it will generate. This can increase the risk of pushing fire.
• Nozzle mode: Using a spray pattern drives more air than a straight stream. Spray patterns can, therefore, be more prone to pushing fire.
• Nozzle movement: Rapid or circular movements of the nozzle can create additional airflow, further exacerbating the pressure difference.

Interestingly, the type of nozzle used—whether a combination or smooth bore—does not significantly impact the likelihood of pushing fire. Similarly, hose diameter, water pressure, and nozzle brand are not determining factors, although higher water pressure will naturally result in more water being delivered to the fire.

Conditions to Watch Out For

For firefighters, understanding the specific conditions that lead to pushing fire is crucial in preventing it. The most critical conditions include:

• Ventilation-controlled fire: Fires that are oxygen-deprived are particularly prone to pushing fire.
• Presence of openings between spaces: These act as channels for flammable gases to move between rooms.
• Pressure difference: Firefighters should be aware of any actions that might create pressure differences, such as using exhaust fans or applying water from the outside.

When these conditions are present, firefighters must be especially cautious when applying water, as improper techniques can inadvertently cause the fire to spread.

Recommendations for Preventing Pushing Fire

Given the risks posed by pushing fire, what can be done to prevent it? Firefighters can take several measures to mitigate this phenomenon and ensure safer, more effective firefighting:

1. Use a steady, straight stream: Applying water in a straight stream, as close to the fire as possible, minimizes the amount of airflow generated. Avoid using spray patterns unless necessary, as they drive more air and increase the likelihood of pushing fire.
2. Avoid water application from outside: Applying water from an exterior window can force air into the building, increasing pressure and pushing smoke and fire into unburned areas. Whenever possible, apply water from within the structure or use transitional attack techniques.
3. Reassess after 10-30 seconds: If applying water for more than a few seconds, pause and reassess the fire’s behavior. This allows firefighters to evaluate whether their efforts are suppressing the fire or if it’s spreading unintentionally.

By following these guidelines, firefighters can reduce the risk of pushing fire and improve the overall safety of their firefighting operations.

Conclusion

Pushing fire is a serious phenomenon that can make firefighting more dangerous and complex. Understanding the conditions that lead to pushing fire and adopting proper water application techniques can make all the difference in controlling a fire and preventing its spread. As research continues, it’s vital for firefighters to stay informed and adapt their methods to minimize the risks associated with this unpredictable but preventable occurrence.

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