Hot work on wheels, lightning, or trucks striking overhead power lines, pose the risk of massive and potentially fatal tyre explosions, known as tyre pyrolysis.
The hidden danger lurks in highly explosive gas formed by partial decomposition of the interior of rubber tyres, for up to 24 hours after a super-heating event. Chemical decomposition of the rubber compounds into hydrocarbons such as methane, butane and propane, follows short exposure to extreme heat. The resulting chemical explosion often has fatal consequences.
No known factors could predict the time of the explosion. The longest recorded time lag was 24 hours after a lightning strike. The shortest could be two minutes.
After welding on the rim, the temperature in the tyre increases from ambient to over 1300 degrees Fahrenheit before the tyre explodes.
In one power line strike, the power lines were cut, some of the tyres exploded, and the wires remained live, lying on top of the truck bowl. The driver was not injured, but when he opened the cab door, he was electrocuted by touching the steel frame. The tyres had insulated the truck and the wires were still live.
Two fitters using a cutting torch on wheel nuts, were killed in Qatar when the rubber heated to pyrolysis level and the wheel exploded.
A fitter used an oxy-acetylene torch to heat and loosen 12 nuts to enable removal of a wheel. Two wheel nuts were removed without heat, and the tyre burst after heat was applied to the third and fourth nut.
The RLC Safety Department reports in alert T009-04 that hydrocarbon vapours that caused this type of explosion, could come from more than one source; from the pyrolysis of the rubber of the tyre, or from the vaporisation of tyre lubricating grease, or oil spilled onto the hub.
Deflated tyres behave similarly to inflated tyres. The pressure produced in the tyre by an explosion could be up to eight times the initial internal pressure. Tyre explosions are much more violent than simple blow-outs.
Pyrolysis transforms hazardous organic materials into gaseous components, small quantities of liquid, and a solid residue (coke) containing fixed carbon and ash. Pyrolysis of organic materials produces combustible gases, including carbon monoxide, hydrogen and methane, and other hydrocarbons.
Following similar incidents in Australia, it is now leading practice there for large rims to be fitted with pressure relief valves, similar to pressure vessels.
Managers and engineers should review their codes of practice to include a procedure in the event of a rubber-tyred vehicle being struck by lightning or power lines.
Evacuation is essential. Risk assessments should be conducted to determine responsibilities and actions in such events. Drivers and passengers should remain in their seats after striking power lines, until an electrical foreman confirms that the power is off.
A tyre fire site procedure requires three steps: Barricade the area as a no-go zone. Report incident to relevant authorities. Keep no-go zone for 24 hours and re-assess.
Review the risk assessment of the mine’s response when thunder storms are approaching, and consider these measures:
Distance and time of exclusion zone from a suspect heating tyre.
Parking procedures and locations around buildings.
Develop a lightning management plan.
Review emergency procedures.
Review lightning protection standards and measures.
Wheel work procedures
Relevant procedures should include these measures:
• Hot work is not permitted on a wheel rim attached to a tyre. Deflated wheels behave just like inflated wheels under extreme heat.
• Do not use a hammer or object to force rim components in place.
• Do not attempt to take rim components apart on inflated tyres.
• Use a clip-on chuck that permits the inflater to stand clear of the potential path of rim components when inflating tyres.
• Deflate tyres before removing a wheel from an axle, and before removing a tyre from a rim, by removing the valve core.
• Do not rework or reuse damaged rim components.
• A rubber-type lubricant must be applied to the tyre bead and the contact surfaces of the rim during assembly of wheel and inflation.
• Tyres should be inside a cage during inflation.
• Do not rest or lean equipment against a cage during tyre inflation.
• Do not inflate a tyre above 350 kPa while it is outside a cage, unless the area can be isolated.
• Inspect the tyre after inflation, while still in a cage, to ensure all components are correctly fitted.
The above article was published in Miner’s Choice in 2009 Aug-Sep.
See a video of a wheel rim welding robotic test at http://www.youtube.com/watch?v=HiLeji8bLOK