Tire and wheel dangers at mine sites, particularly regarding electricity, is sometimes unseen, making tire safety crucial for all
By Donna Schmidt, Field Editor
Proper tire safety is not just a seasonal concern, though the unpredictability of summer weather often makes dangers greater. Knowledge of these, including the potential results from lightning strikes, and proper education on both the design and maintenance of the tire are paramount.
According to the National Lightning Safety Institute (NLSI), citing data from the National Oceanic and Atmospheric Administration (NOAA), there were 756 deaths from lightning strikes in the U.S. between 1990 and 2013 (the most recent available data). That doesn’t account for any individuals injured — civilians or miners — and considering the highest point of conductivity at some points during a thunderstorm is a large haul truck, those statistics suddenly mean much more.
One incident directly tied to mining that can put the hazards of lightning strikes into perspective is a July 2012 incident at a mine in Tumbler Ridge, British Columbia, Canada. As the evening crews began the 11th hour, the driver of a 777F haul truck sat parked at the waste dump waiting for an electrical storm to pass. The truck was struck, resulting in significant damage to its rear.
During the event, according to a report from the British Columbia Ministry of Energy, Mines and Natural Gas, the external left dual was propelled approximately 250 m (820 ft) from the truck, while the rim of the tire was propelled another 50 m (164 ft) beyond that. Additionally, all four rear tires were blown out and the cab windows were shattered. Thankfully, the driver escaped uninjured and was able to move to a safe location.
In another incident in Hunter Valley, New South Wales, Australia, in 2007, a large rear dump truck was struck by lightning while stationary and unattended; again, there were fortunately no injuries.
However, three tires were blown off the truck between two and five minutes after the strike, and two tires (positioned 1 and 3) exploded on the driver’s side of the truck — sending debris flying several hundred meters. The vehicle and other equipment were left with extensive damage.
Mining regulators in that case noted that one complete wheel, which weighed 1.6 metric tons (mt), was launched approximately 100 m (328 ft) and the flange of the wheel, which weighed 250 kg (551 lb) by itself, was propelled to the top of a nearby stockpile about 275 m (902 ft) away.
“The air blast and shockwave created by the lightning strike caused damage to the operator’s cab and to other structures and equipment located as far as 230 m (754 ft) from the truck,” Australian officials said, adding that the tires were also ejected and landed between 50 and 60 m (165 and 170 ft) from the truck.
Coal Age spoke with BKT Tires, the Tennessee-based Americas OTR mining and construction sales and service subsidiary of Indian group Balkrishna Industries Ltd., about lightning strike hazards, prevention and safety procedures for truck contacts with electric lines, and also today’s picture of tire safety.
What can one do if equipment is struck by lightning or comes in contact with an electric transmission line?
Quick and proper action could save serious injury or even loss of life. Since mine sites are often at high altitudes and have no trees, mining equipment is often the highest point and a target for lightning. It is also common for truck beds and crane booms to accidentally contact electric lines.
Lightning and high voltage transmission lines contain an enormous amount of electrical energy. When equipment is energized by a lightning strike or contact with a transmission line, that energy goes to the ground through the tires, the path of least resistance being the ply cords, which are typically steel. The very large surge of electricity through the ply cord material creates heat that can weaken the bond between the ply and the surrounding rubber.
Since an inflated tire and wheel assembly is a pressure vessel, any compromise of the integrity of the vessel could lead to a catastrophic event. Further, the heat created causes expansion of the gases and can turn any moisture inside the assembly into steam, which can very quickly increase the inflation pressure beyond the capacity of the wheel or the now-damaged tire, causing a rapid air loss event. This, however, is only half the story.
Pyrolysis: The Hidden Danger
According to Ron Tatlock, BKT’s manager of OTR training and engineering, heat can initiate the process of pyrolysis, which some consider as a “de-vulcanization” of sorts. In other words, the tire rubber’s chemical bonds in the compounds begin to break down in pyrolysis, which is exothermic, or generates heat. This heat, in turn, speeds the process of breaking down and weakening the rubber while also heating up gas and moisture in the tire and wheel assembly, causing an exponential rise in inflation pressure.
“Pyrolysis can take place very quickly or, it can take several hours to reach the point where the inflation pressure exceeds the strength of the rim or the now-weakened casing,” Tatlock said. “A tire that is undergoing pyrolysis gives no outward sign and that is the hidden danger after equipment is energized with electricity.”
What to Do in an Incident
First, according to BKT, get everyone away from the equipment immediately and create a safety barrier around it. Large tires, wheels and hubs have all been found more than 300 m (984 ft) away following events, and the most critical areas are those 45˚ outward from the end of the axle.
Second, keep all individuals away from the area for at least 24 hours. Historically, there have been reports of catastrophic loss of air from large tires more than 21 hours following electrical contact, and this has been tied to pyrolysis.
Finally, after 24 hours or more, make sure all tires on the equipment are scrapped. Even if they look okay, Tatlock noted, they may still have chemical bond damage that cannot be seen.
What is the safest way to get the operator off the equipment?
“Your first instinct may be to get off the truck as quickly as possible, but first there are important considerations to think about,” Tatlock said.
Is an electric current still running through the vehicle?
Is the vehicle still running?
Is the vehicle on fire?
Assume the process of pyrolysis started in one or all of the tires.
Has heat generated inside the tires, resulting in increased air pressure and potentially rapid air loss?
Personal safety procedures may vary depending upon the nature of electrical current passing through the vehicle. In the case of a vehicle coming into contact with electric transmission lines, he noted, the truck operator must determine if the vehicle is still in contact with the transmission lines.
So what should someone in this situation not do? First, do not assume everything is fine. In the case of electrical transmission line contact, do not open the cab of the vehicle until it has been verified that there are not live lines in contact with the unit.
Also, the vehicle should not be moved if there is danger of coming in contact with other vehicles or persons by dragging the transmission lines.
Instead, some procedures to follow include keeping safety as a prevailing directive, according to Richard Kithill Jr., founder and CEO of the NLSI. Call for assistance, but remain inside the cab; if there are set mine protocols for the situation, follow those emergency steps.
If the truck is not in contact with transmission lines, as quickly as possible pull to the side of the haul road, if this can be done safely, and turn the engine off,” Kithill said. “Operators should exit the cab and vacate the vehicle as quickly as possible. Remember, the hidden tire danger should be a primary concern. At this point, electrical current is no longer running through the truck, but the operator and nearby personnel should still exercise caution.”
The NLSI also recommends the use of alternate procedures to vacate haul trucks, as many are equipped with egress ladders that are well within the tire danger zone. Additionally, a similar size vehicle can be placed nose-to-nose (but not touching, as a safety precaution, should the vehicle still be or become energized) with the affected truck, after which the operator should carefully step from the vehicle to the unaffected vehicle and back away to a safe distance.
The magic number, the two said, is 330 — as in yards (300 m), or the distance from the truck all personnel should remain for at least 24 hours in case of pyrolysis.
What are the biggest aspects of overall tire safety?
It is important to remember that a tire and wheel/rim assembly is a pressure vessel, and that the air inside the tire actually supports the load.
“Equipment tires are specially designed to safely allow vehicles to move materials,” Tatlock said. “Therefore, the most important aspects of overall tire safety should include taking all measures necessary to preserve the original shape and design of the tire.”
There are some factors to consider, the first of which is maintaining air pressure to the recommended manufacturers’ specifications. Over- or under-inflated tires change the tire shape, and this can lead to safety issues; moreover, some under-inflation conditions can include ply, tread or bead separations; tire stress and/or fatigue; rapid or irregular wear; radial sidewall cracks; liner failures in tubeless tires; and poor handling.
Additionally, over-inflation creates excess stress on the tire and wheel assembly, which can create punctures, cuts and rock penetration, impact breaks and/or bruise damage, cut separations and growth of cuts, or even pin cuts.
“It is important to remember that the rim, though made of steel, has a load and inflation pressure limitation, which should be embossed on the rim,” he said. “As the rim is part of the pressure vessel, care must be taken not to exceed the rim capacities to avoid catastrophic air loss. Rims that are bent, cracked or otherwise damaged should be removed from service until inspected and repaired or certified for service by a rim expert.
“If you have any doubts about rim capacity or serviceability, consult your servicing dealer or rim manufacturer. Never weld on a tire/wheel assembly; this can be a fatal mistake,” he added.
More factors to consider include:
- Cuts and punctures to tire tread and sidewalls can affect design components of the tire;
- Combined speed and loads should be followed according to the tire manufacturer’s recommendations;
- Proper mounting and rim selection;
- Never make repairs on heavy equipment tires while tire is inflated and mounted;
- Tire selection is one of the most important factors of overall tire safety;
- Operating in a safe work environment, i.e., during severe weather where lightning is involved or operating perhaps under transmissions lines might be struck by the vehicle; and
- Compliance with Mine Safety and Health Administration regulations.
Not every operation has set procedures for emergency tire safety at the site, but all operations are required by law to conduct pre-shift inspections on vehicles. BKT stresses that this examination should always include tires.
For a complete tire inspection, one should do a visual review, looking for cuts, cracks and bulges, as well as damage from equipment interference.
“Look at tread, and both sidewalls of each tire,” Tatlock said. “Steel belts or casing cords that have been exposed to the air require immediate inspection by a qualified individual to determine the necessary next action. Steel cords may quickly begin to rust, and thus weaken the tire structure.”
Additionally, the company recommends checking for the presence of lubricants or fuels on the tires, as petrochemicals can severely damage and weaken tires. Hydraulic leaks should also be stopped.
While visually inspecting the tire, the rims should also be examined for cracks or parts that may be bent. If a liquid tire protectant (such as Tire Life) is found on the outside of the tire or rim, this is usually an indication that a problem is present and that further investigation is needed.
Finally, only a trained operator with a certified pressure gauge should check inflation; if not possible, inflation pressure checks should be done by the maintenance department or the contracted tire dealer per a site’s protocol no less often than once every two weeks; all valve caps and stems should also be in good condition.
BKT’s Role in Safer Design
Tatlock noted the extensive safety-related research and development that goes into BKT’s tires; the company uses the latest FEA design tools and techniques to ensure proper casing integrity.
“We also employ the latest in chemical and physical components and finished product testing during development and as ongoing quality assurance monitoring,” he said. “Our production process incorporates many in-process quality assurance checks to assure the integrity of compounds and components before they are assembled into BKT tires.”
BKT is also developing a mobile app for tire selection and maintenance, and is also planning on developing a best-in-class tire performance data collection and analysis system in the near future.
Currently, its most commonly used and popular products for mining are TRA tire types, including earthmover-haulage radial tires with TRA tread code “E” for articulated and rigid haul trucks, and it offers traction and rock designs for rougher surfaces. Loader tires with TRA tread code “L” are also used in most mines, he added.
“These tires are designed for high loads and slow speeds over short distances. Today’s mine sites use a wide range of loader tire sizes.”
Specialty tires are typically required for shuttle car vehicles operating in low-seam underground coal operations, Tatlock said, such as the BKT Mine Star 35×15-15. Low-seam operations, where there are restrictions regarding tire size, allow for bias tires as the ideal option. These are often urethane-filled to extend wear life, minimize downtime and to carry large, heavy loads in severe environments.