Open Cut » Health and Safety
Tyre fires and explosions by nature are a high potential hazard for any operator of rubber tyred vehicles with a number of serious incidents and accidents including injuries and fatalities reported throughout the mining industry over the project duration at least 2 earthmover tyre fires occurred in Australia, both of which resulted in the loss of the haulage truck, but fortunately without any injury to personnel.
A literature review and experimental test work involving the burning of 15 fully instrumented tyres was carried out to gain a better understanding of the effects of heat and fire on tyre assemblies, and the subsequent escalation of such an event.
Both the review and observations during the experimental work reinforced that tyre fires will create extremely hazardous conditions -through potentially extreme heat and smoke generation as well as through the tyres potential violent rupture, and or explosion from pyrolysis. Irrespective of failure mode, such events release sufficient energy to cause severe injury to bystanders, even at a distance.
Neither the case history review nor the experimental work were able to find specific , timely and reliable characteristic s to differentiate how a tyre assembly would fail if subjected to intense heat, but did record the following observations.
Localised external tyre fires were found to cause the violent rupture and rapid venting of the tyre in the immediate vicinity of the fire, usually at a pressure only slightly above inflation pressure. It is of interest that no observable difference could be established for external fires of normally inflated and N2 inflated tyres.
Where tyres were subjected to complete engulfment by heat and flames, eg by a pool fire, it was shown that extinguishment of a hot tyre fire is not only difficult , but very hazardous. It is suspected that the intense heat surrounding the tyre and impinging on the tyre and the rim surfaces may in fact cause sufficiently higher pressures, from a temperature/pressure relationship in the tyre cavity causing the tyre to rupture, and or cause internal pyrolysis of the tyre (a breakdown of the rubber into a series of volatile & flammable products) followed by ignition and subsequent violent chemical explosion. The pyrolysis mechanisms triggered by accidental heating of wheel and rim components, and subsequent ignition of pyrolysis products is reported to cause the most violent wheel assembly explosions. The experimental work was not able to reliably reproduce complete internal pyrolysis events including explosion, despite continuous heat application. However, as most experiments showed some evidence of this failure mechanism, and given the ferocity of reported cases, risk management plans ought to be considered that eliminate internal tyre assembly heating at all cost.
Both potential exposures call for a careful review of current tyre fire risk management plans that make the prevention of tyre fire hazardous conditions a key priority, and provide for realistic and safe actions should a tyre fire occur. Key observations and recommendations are provided below.
Observations & Recommendations
Risk: Risks arising from tyre fires are directly linked to the failure modes of tyres that become subjected to extreme heating conditions. The case history and the experimental work indicate that there are 2 main modes of failure:
Tyre failure through the tread area or sidewall area
As the heat impacts the tyre its integrity is weakened - rubber is consumed by the combustion process and rubber bonding becomes affected by the heat. This results in the tyre bursting through the affected sidewall or tread area.
Tyre failure through very rapid buildup of internal pressures, caused either by a temperature/pressure relationship or by pyrolysis, ignition of the pyrolysis products and resultant explosion.
Pyrolysis is the decomposition of a chemical (the tyre rubber) by extreme heat. This breakdown process is reported to start when rubber reaches a temperature of ~ 250 degC, with auto ignition and subsequent explosion of the breakdown products reported to take place above 400 degC. .
External wheel assembly fires appear to cause failure through the tyre by rapid venting through the heat damaged area.
When air inflated tyres are completely engulfed, there is a risk of violent explosion, however hot external fires engulfing deflated tyres do not appear to fail violently.
Case histories, and some of the experimental results also suggest that hot external fires engulfing inflated tyres can also lead to Pyrolysis.
Heat application causing internal pyrolysis, ignition and explosion of pyrolysis products causes the violent disintegration of wheel assemblies with a very high potential to cause damage and injury, even at considerable distances. However, as shown by the experimental work, even prolonged heat application does not automatically trigger a pyrolysis event including explosive disassembly of the wheel.
Either failure mechanism was observed to be violent and likely to cause considerable harm and injury to personnel, even at distance and priority must be given to personal protection. A suspected fire shall be treated as an actual fire until confirmed otherwise.
Operational Controls: Tyres are a considerable source of fuel - however low ambient and or operating temperatures appear to control ignition and rate of combustion. Controlling tyre operating temperature, through correct tyre selection and maintaining the operational TKPH below its design TKPH, and removing tyre damaging conditions will reduce the likelihood of operations induced tyre fires.
Sound preventative equipment maintenance, and the removal, rerouting or guarding of any electrical high voltage sources will reduce the likelihood of electrical contact with fixed sources and subsequent potential pyrolysis type fire events.
The addition of a liquid tyre additive may also assist to prevent escalation of internal fires. Nitrogen inflation may reduce the likelihood of pyrolysis, but should not be considered as the only and major means of reducing an operations exposure to pyrolysis vents.
Detection of Fires using Thermal Tools: Measurement of external tyre temperatures using a thermal tool may be misleading and should only be used as a guide to the actual tyre temperature, as the remaining internal heat can cause the tire to reignite, even once extinguished on the outside.
Immediate Response: Because tyre fires are often hard to recognise and difficult to combat, priority must be given to personal protection in the first instance. A suspected fire shall be treated as an actual fire until confirmed otherwise.
Rapid and effective response to pool fires to minimise or avoid tyres being engulfed by heat and flames will reduce the likelihood of a tyre reaching critical temperatures where pyrolysis is possible.
The size of the tyre fire event and the amount of heat contained in a tyre appears to govern if a tyre fire can be brought under control, safely and effectively. Where it is deemed safe to fight a tyre fire, water appears the best extinguishing agent and must be applied liberally and over a considerable time to remove heat
Pyrolysis: Potential injury and damage consequence, uncertainty over the event together with the variable time lag between heat application and rapid pressure rise (believed to be the result of pyrolysis product ignition) in air inflated tyres supports immediate park-up of the equipment, evacuation of the driver, securing the vehicle/ area and a lengthy quarantine period as an appropriate emergency response measure.
Storage of tyres should aim to sufficiently segregate tyre stacks, and allow for ready access of fire fighting equipment.