Tire explosions result from the ignition of an explosive gas mixture inside the air chamber of a tire. The gases can generate internal pressures ten times the regular tire inflation pressure. Under overheated conditions, the explosive gas mixture can auto-ignite or be triggered by an external high energy source. Two types of tire explosions are:
- Heat initiated explosions (most common)
When excessive heat is transmitted to the inner tire liner, and the temperature reaches 250°C, the rubber begins to decompose (a process called pyrolysis) and release combustible gases. When the internal temperature climbs to 400°C, the breakdown products of pyrolysis can auto-ignite and explode.
- Diffusion initiated explosion (extremely rare)
If there is wood, for example, inadvertently trapped inside a tire chamber, methanol diffuses out of the wood at 65°C. As well, the friction of the wood with the inner tire generates combustible dust particles which react with the methanol/air mixture to create explosive conditions.
Tire explosions can also be initiated by:
- Electrification of the vehicle by a lightning strike, or from contact with power lines.
- Application of heat from welding on the rim.
- A vehicle or tire fire.
Misconceptions about tire explosion hazards:
False: Deflating the tire will prevent an explosion.
FACT: Pressure is not required in the pyrolysis process.
False: Removing energy/ignition source will eliminate the risk of explosion.
FACT: Rubber is a very poor conductor of heat, and unseen ‘hot spots’ will remain long after the removal of an external heat source. Explosions can occur immediately, or even minutes or hours after exposure to a potential ignition source.
False: There is no risk of an explosion if a tire is inflated with nitrogen.
FACT: There have been incidents where nitrogen inflated tires have exploded. This is because only a small amount of oxygen is required for combustion under explosive conditions. Air can enter the chamber when the beads are seated, or from ‘topping-up’ inflation pressure in the field.
Reducing the risk of tire explosions:
- Low ambient and operating temperatures minimize the potential for ignition and combustion inside tire chambers.
- Correct tire selection helps control tire chamber temperatures.
- Constraining the operational tonne kilometers per hour (TKPH) below its design value reduces vulnerability to explosion.
- Removing tire-damaging conditions reduces the likelihood of tire explosions during operations.
- Sound preventative maintenance of equipment minimizes susceptibility to tire explosions.
- Removal, re-routing or guarding of electrical high voltage sources decreases the likelihood of electrical contact and subsequent tire explosions.
- The addition of a liquid tire additive assists to prevent escalation of internal tire fires.
- Nitrogen inflation may reduce the likelihood of pyrolysis, but should not be considered a foolproof means of eradicating heat initiated explosions.
Preventing tire explosions:
- Never heat or weld rims or wheels on a mounted tire, whether it is inflated or not.
- Use non-flammable tire lubricants and sealants.
- Remove all foreign objects, such as wood, from tires.
- Avoid working near power lines.
- Minimize the risk of fire by following proper braking procedures and by isolating and changing smoking tires.
- Install a tire pressure monitoring system (TPMS) to monitor tire temperatures and pressures.