Truck tyres catching fire can pose a serious hazard, the most common causes are flat or poorly inflated tyres or tyres rubbing against hard surfaces such as the chassis, guards or adjoining tyres until they are hot enough to ignite. Once alight, tyres are extremely difficult to extinguish. Using dry chemical powder (DCP) fire extinguishers will put out the fire but because they do not cool the heat source, there is a potential for the fire to reignite.
The greatest hazard associated with tyres is the risk of explosion.
Tyre explosions are much more violent than a simple blow out. Tyres fitted to trucks, cranes or other heavy vehicles may catch fire in certain circumstances and with fire comes the potential for the tyres to explode. An exploding tyre releases huge amounts of energy which in turn can lead to significant equipment damage, serious injuries or fatalities.
Pyrolysis - a less understood danger.
When rubber is heated sufficiently it will release combustible gases inside the tyre, and this may continue until the lower explosive limit is reached. When the temperature exceeds the flash point of the gases generated, the tyre will rupture or explode.
Common causes for tyre pyrolysis are using an oxy- acetylene torch to loosen wheel nuts, overheated brakes, wheel motor fires and welding on wheel rims. Additionally, tyre pyrolysis has been known to happen when vehicles are involved in high voltage electrical incidents which cause electrical arcing and current flow, for example when cranes or tip trucks, come into contact with overhead supply lines or a similar source.
Pyrolysis related explosions are unpredictable. They can occur spontaneously or up to 24 hours after the incident. In most cases there is no visible sign of fire. The danger can zone associated with a pyrolysis related fire is up to 300 metres from the tyre.
Tyres do not easily catch fire, but once they do, they burn with increasing intensity, and due to their composition, can pose significant Health and Safety risks. Firefighters have reported tyres and rims being projected dangerously sideways after a tyre has exploded with sufficient energy to sheer off wheel studs at the hub.
In Australia, mine sites have well documented safety procedures to follow if you suspect there is danger of a tyre explosion, for example after a rubber tyred vehicle has contacted overhead powerlines.
These procedures include:
- parking the vehicle and establishing an isolation zone with a minimum 300 metre radius
- evacuating all personnel from the area
- securing the area to ensure no one is allowed to re-enter the isolation zone for 24 hours
- alerting fire-fighting and emergency services
Any tyre fires involving heavy machinery must be approached from the front or back, away from the sidewall of the tyre. The fire should be fought from a shielded position, and once extinguished, the vehicle should remain isolated for at least eight hours.