Understanding Fire and Water in Firefighting
Fire Classification and the Fire Tetrahedron
Fire Classifications in Chile (NCh 934)
Class A: Ordinary Combustibles
These fires involve materials like wood, paper, cloth, rubber, and resins. The symbol is the letter A on a green triangle.
Class B: Flammable Liquids and Gases
These fires involve flammable or combustible liquids, flammable gases, greases, and paints. The symbol is the letter B on a red square.
Class C: Electrical Fires
These fires involve energized electrical equipment. Safety requires using non-conductive extinguishing agents. Once de-energized, the fire may become Class A, B, or D depending on the fuel. The symbol is the letter C on a blue circle.
Class D: Combustible Metals
These fires involve combustible metals like magnesium, sodium, potassium, titanium, and zirconium, which burn at very high temperatures (over 2,500°C). They require specialized extinguishing agents. The symbol is the letter D on a yellow star.
The Fire Tetrahedron
Four elements are necessary for fire:
- Heat
- Oxygen
- Fuel
- Chain Reaction
Methods of Fire Extinction
- Cooling: Lowering the temperature to stop the release of flammable gases and vapors. Water is often used for this method.
- Suffocation: Removing oxygen to smother the fire. Blankets and foam can be used.
- Dispersion/Isolation of Fuel: Separating or removing the fuel source. Firewalls and closing fuel valves are examples.
- Inhibiting the Chain Reaction: Using chemicals to interrupt the combustion process. Chemical powder and halon extinguishers work this way.
Water as an Extinguishing Agent
Absorption of Heat
Water is effective due to its high specific heat and latent heat of vaporization. Applying water as a fog maximizes its cooling effect.
Increase in Volume
Water expands 1,700 times when turning into steam, displacing air and potentially removing oxygen.
Foams
Mixing water with chemicals and air creates foam, which can separate oxygen from fuel and cool the burning material.
Considerations When Using Water
Water can be damaging in certain situations, such as with reactive chemicals, delicate materials, or enclosed spaces where steam can cause burns.
Water Pressure and Flow
Understanding Pressure
- Static Pressure: Pressure exerted by water at rest.
- Dynamic Pressure: Pressure of water in motion.
- Residual Pressure: Pressure remaining after water is released.
- Atmospheric Pressure: Pressure exerted by the atmosphere.
Pumps and Pressure
Pumps increase pressure but not the total amount of water available. Overloading a pump can damage it and the water distribution system.
Pressure Losses
Pressure can be lost due to height, changes in diameter, friction, and changes in direction.
Measuring Water
Understanding metric and imperial units for pressure and flow is crucial for planning and effective firefighting.
Water Supply and Equipment
Sources of Water
- Public or private water systems
- Open water sources
- Tankers and other mobile water supplies
Limitations of Water
Water is not always suitable for firefighting, especially with reactive materials or sensitive environments.
Safety Considerations
Water as a Projectile
High-pressure water can cause injury and damage.
Water Hammer
Sudden changes in pressure can damage equipment and injure personnel.
Action and Reaction
The force of water exiting a nozzle can cause loss of balance or injury.
Proper Training and Discipline
Effective firefighting requires proper training, discipline, and understanding of water pressure, flow, and equipment limitations.