Fire Science
This section covers Fire Science: combustion, heat transfer, fire development and the properties that make a fuel hazardous. The exam tests the fire tetrahedron, flashpoint, flammable range, vapour density and the distinction between flashover and backdraught.
Combustion and fuel properties
- The fire tetrahedron is fuel, oxygen, heat and chemical chain reaction; removing one factor extinguishes.
- Flashpoint is the lowest temperature at which a liquid gives off enough vapour to ignite; autoignition needs no pilot.
- The flammable range lies between the lower and upper flammable limits; below the LFL the mixture is too lean, above the UFL too rich.
- Vapour density relative to air controls where vapour collects; petrol vapour is heavier than air and gathers at low level.
Heat transfer and development
- Conduction, convection and radiation spread heat; a hot smoke layer radiates back to contents and accelerates growth.
- Fire development: incipient, growth, fully developed, decay.
- Flashover is the transition to simultaneous ignition of combustible surfaces in a compartment.
- Backdraught is a rapid deflagration when air enters an oxygen-deficient, hot compartment.
Worked example
A petrol spill in a basement: petrol's flammable range is roughly 1.4–7.6% by volume and its vapour is heavier than air, so it accumulates at low level and can find an ignition source remote from the spill. The tactical lesson is to isolate ignition, control ventilation and use gas monitoring rather than smell.
Traps
Treating flashover and backdraught as the same; assuming a high-flashpoint liquid is safe; ignoring vapour density; assuming water suits every fuel.