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Gaining Traction: Tyre safety for earth-moving machinery on Western Australia mining operations

INTRODUCTION

Working with off-the-road tyres for earthmoving machinery is potentially dangerous because of their large size and mass, magnitude of air or gas pressures, and presence of combustible materials. The uncontrolled release of stored energy can have serious, even fatal, consequences.

This guideline describes the common hazards when working with tyres, rims, wheels and assemblies on mining equipment. It provides guidance on safe systems of work in a mining environment, while allowing for flexibility in both process and documentation.

Operators should adopt a risk management approach to develop a documented tyre management plan that is current and specific to site, with appropriate controls to manage the risks.

Four main elements should be considered:

  • competent people — training, knowledge, experience, assessment, fitness-for-work
  • safe systems of work — adequate procedures, information and instructions, record keeping
  • fit-for-purpose equipment — safety-in-design, adequate capacity, well maintained
  • a safe and controlled working environment — adequate workshop facilities and services.

TYRE MANAGEMENT PLAN AND RISK MANAGEMENT

Tyre assemblies are safety-critical components that should be selected, operated and maintained correctly to reduce the risk of workers’ exposure to associated hazards to as low as reasonably practicable.

A good tyre management plan clearly defines the selection, operation, maintenance and disposal (i.e. whole life cycle) of tyres, rims and wheel assemblies. The management of these items requires an integrated risk-based strategy from key departments on a mine, including management, production, maintenance, supply, occupational health and safety, and environment.

Taking a risk-based approach towards tyre management includes:

  • identifying hazards
  • examining information on tyre and rim failures (e.g. safety alerts)
  • understanding the influence of the operational environment on tyre life and safety assessing and selecting tyres, wheels or rims
  • assessing and preparing storage and work areas
  • selecting and implementing tyre-handling facilities, including plant, tools, equipment, and safe systems of work
  • monitoring and implementing component inspection, maintenance and repair (e.g. non-destructive testing of wheel and rims)
  • maintaining accurate record keeping
  • understanding mechanisms of tyre fires and explosions
  • providing appropriate emergency response capability
  • ensuring people are competent for the tasks they are assigned.

The tyre management plan should provide those involved with tyres with an understanding of the mining operation’s strategy for managing tyre assemblies and components, and their roles and responsibilities.

Site-specific safe work procedures (SWPs) or safe work instruction (SWIs) should be developed by competent persons to support the tyre management plan. Changes to work conditions (e.g. inclement weather, new equipment) need to be addressed by work teams using a task-based risk assessment, such as a job safety analysis (JSA). If necessary, information from this process should be used to modify the SWP or SWI using the site’s change management process.

Individual workers should also conduct appropriate personal risk assessments before commencing work.

UNDERSTANDING THE HAZARDS

Potential scenarios

The hazards associated with tyres include:

  • those related to handling and working with tyres, wheels and rims
  • tyre fires, bursts and explosions when tyres are in service
  • loss of control of vehicle due to tyre failure.

These hazards are dealt with separately in this guideline but care should be exercised to ensure that all scenarios are adequately risk assessed and managed within the operation’s safety management system.

Major sources of risk when working with or using tyre assemblies include:

  • sudden release of stored pressure energy — leading to projectiles (e.g. rim components, rocks) and percussive shock
  • compressed air or other gases (e.g. nitrogen)
  • noise
  • handling heavy objects
  • working with or operating heavy equipment
  • heat and fire
  • fuels and chemicals
  • pyrolysis or diffusion — leading to explosions.

Compressed air

Compressed air poses a significant injury risk to workers, particularly to their eyes. Appropriate personal protective equipment (PPE) should always be worn to protect workers from injury by high-velocity air jets, as well as particles of dust, metal, oil and other debris that can be mobilised by a high-velocity air stream.

Overalls and other PPE may protect the skin from light particles and debris. However, fabric does not offer protection against high-velocity air at close range. Particles can be blown through overalls and skin to penetrate the body. Compressed air jets at close range can inject air into the body, causing swelling and pain. Air bubbles injected into the bloodstream can be carried to small blood vessels of the brain, lungs or heart, potentially causing a serious or even fatal embolism (blockage).

The stored potential energy contained in an inflated tyre, air receiver or other pressure vessel depends on the air volume and pressure, and may be substantial. Tyre blasts release significant energy. Overpressures caused by incorrect inflation, overheating, fire or a pyrolysis reaction will increase the risk.

Note: The force of a burst tyre may have enough energy to lift a small car eight metres into the air.

One way to reduce the risks to workers associated with inflation and deflation work is to use remotely controlled inflation systems. The use of properly designed tyre inflation cages, where available, and other barriers and restraining devices may also control some of the risks. Workers should be trained and instructed in the need to stand out of the line of-fire during tyre inflation and during inspection of the tyre assembly during and following inflation.

Nitrogen
Nitrogen gas is virtually chemically inert, and at room temperature and atmospheric pressure has no taste, colour, odour or toxicity.

The correct use of nitrogen prevents pyrolysis. Inflating tyres with nitrogen reduces the potential for auto-ignition. When the oxygen concentration within a tyre is less than about 5.5 per cent by volume, it cannot auto-ignite because there is insufficient oxygen to support combustion. Some purging of tyres may be required to decrease the oxygen content to below this level.

The use of compressed nitrogen gas for inflating tyres may control the risks such as rim rust, aging of the inner tyre and poor sealing, leading to better inflation pressure stability and longer rim life.

Note: While nitrogen diffuses through rubber at only one-third the rate for air, most of the pressure loss in tyres can be attributed to leaky o-rings or valve hardware.

Using nitrogen gas can introduce additional hazards, including:

  • new source of stored energy — nitrogen gas in pressurised containers introduces additional handling and storage hazards
  • cryogenic hazard — liquid nitrogen is extremely cold and contact or breathing in associated cold vapours can lead to tissue damage

Note: Nitrogen is only slightly (about 3%) lighter than air at the same temperature and will therefore disperse slowly due to buoyancy alone. However, cold nitrogen gas can be denser than ambient air and will tend to settle at ground level and in low places, such as mechanics pits and floor sumps.

  • nitrogen acting as an asphyxiant — venting of nitrogen into an enclosed or poorly ventilated workshop space (e.g. inflation chamber, sea container) can lead to an oxygen deficient atmosphere and create an asphyxiation hazard.

Note: The minimum acceptable concentration of oxygen in a room’s atmosphere is 18 per cent by volume under normal pressure (Safe Work Australia, 2012, Guidance on the Interpretation of Workplace Exposure Standards for Airborne Contaminants), but ideally it should be maintained above 19.5 per cent. If the nitrogen containment fails, large volumes of the gas can be released suddenly. Nitrogen is colourless and odourless so leaks are difficult to detect.

To avoid potential oxygen depletion, nitrogen should only be used in well ventilated areas. For enclosed facilities, the number of fresh air volume changes required per hour to achieve sufficient ventilation will depend on factors such as:

  • the volume of ventilated space
  • the quantity of available nitrogen
  • whether oxygen levels are monitored
  • whether ventilation is forced or based on natural air movement.

Noise
Noise-induced hearing loss resulting from injury to the inner ear can follow excessive or prolonged exposure to noise.

In the workplace, activities that can raise noise levels include:

  • the rapid release of compressed gases
  • the operation of pneumatically powered air tools, such as impact wrenches
  • heavy wheel parts and tools dropping onto concrete floors
  • working close to diesel-powered mobile plant.

There is a regulatory requirement to manage noise exposure above specific action levels. Engineering controls are preferred over a reliance on PPE to reduce workplace noise levels.

Examples of engineering solutions include:

  • providing suitable noise-attenuating enclosures for mobile plant and stationary powered equipment (e.g. forklifts, compressors)
  • setting noise level criteria for the selection and purchasing of air tools and other equipment
  • using special floor surfaces or mats to reduce the impact noise from hard floors.

Where it is not practicable to further reduce noise exposure, appropriate PPE must be provided and workers trained in its correct use. Tyre fitters may be at risk of injury if appropriate hearing protection is not worn.

Note: For further information on noise management, see the Department of Mines and Petroleum’s guideline on the management of noise in Western Australian mining operations.

Handling of heavy objects
A tyre assembly fitted with a protector chain for a large front-end loader, for example, may weigh up to 15 tonnes and, as such, is too heavy to be manually handled. Purpose-designed tyre-handling machinery, assembly jigs, inspection stands and other specialised fit-for-purpose tools are required for moving the tyre casings, wheels, rims and finished tyre assemblies of heavy mining vehicles.

The large dimensions and weights involved mean that, if finished tyre assemblies or components fall or slip out of their restraints during transport, storage, handling, assembly or fitment, they present potentially fatal crushing hazards for tyre fitters and vehicle operators.

Some low speed vehicles (e.g. tyre dozers, tractors, front-end loaders) use ballast (e.g. dry ballast, water solution) or other fill materials (e.g. inserts, foram fill). While water-ballasted tyres may weigh 20 to 30 per cent more than an air-filled tyre, dryballasted tyres are even heavier and can further increase the hazard of handling a heavy object. It is therefore imperative that reliable information on the mass of tyre assemblies, and whether the tyres are gas filled or ballasted, is available to competent persons to select suitably rated tyre-handling machinery. The weight of wheel assemblies should be clearly marked on or close to the assembly.

There are also potentially fatal crushing hazards associated with the support of earth-moving machinery when their tyre assemblies have been removed. The vehicle should be positioned on near-level ground before any attempt is made to remove wheel assemblies. Suitably rated jacking pads, jacking equipment and stands are required. The supporting surface (e.g. concrete or soil surfaces) should be assessed by a competent person to ensure it can support the point loads.

Load-supporting plant (e.g. jacks, axle stands, extension dollies) must be fit-for-purpose and designed by competent persons to established design standards (i.e. rated), and used by competent persons in accordance with the instructions of the original equipment manufacturers (OEMs) of the load-supporting plant and the vehicle.

Note: The use of packing in conjunction with a jack or stand should be limited to material that is fit for purpose (e.g. sufficient load capacity, will not deform under load, will not slide sideways, inherently stable).

Working with heavy equipment
Tyres are handled at various times using cranes, forklifts or mobile plant fitted with two- or three-arm hydraulic tyrehandling attachments. The tyres are very heavy so attempting to move them without fit-for-purpose equipment can result in serious injury, as well as possible damage to the tyre.

The operating instructions provided by the OEM should be followed to ensure additional hazards are not introduced by the tyre-handling machinery. General principles for tyre handling are covered in Chapter 7.

Poor handling of an unmounted tyre can cause irreparable damage, particularly to the bead. If the damage is undetected and the tyre is put into service, its subsequent failure can increase risks associated with the vehicle, such as loss of control of the vehicle, or sudden depressurisation near workers (e.g. in a workshop).

The interaction of heavy mobile equipment with each other and with workers requires risk assessment by competent persons and the implementation of appropriate risk controls.

Tyre-handling attachments
Various tyre-handling attachments are available for use with vehicle-mounted cranes (e.g. Hiabs), multipurpose machines (e.g. integrated tool carriers or ITCs) and large fork lifts. Such devices usually consist of hydraulically actuated gripper arms, commonly two or three arms, which grip the outside of the tyre to lift, move and manipulate it. The devices rely on achieving and maintaining sufficient grip to securely control the load.

Significant hazards and risks may arise when:

  • workers operate close to the machine or between the gripper arms (e.g. when removing or fitting wheel fasteners) — appropriate procedural and engineering controls are required to ensure workers are not crushed by the gripper arms or a falling tyre
  • inflating the tyre while the gripper arms are gripping the tyre — this can lead to overloading of the hydraulic circuit, and hydraulic or structural failure, resulting in loss of grip of the gripper arms, with the potential for workers to be crushed, sprayed with hydraulic fluid or hit by debris.

Additional considerations fortyre-handling machines
For two-arm type manipulators, ensure that anti-fall back devices are fitted, in serviceable condition and locked in position before tyre-fitting workers enter the operating space or “footprint” of the manipulator arms.

To facilitate safer workplaces, never:

  • operate under or near a suspended or elevated tyre assembly
  • reduce the manipulator arms clamping force while a tyre assembly is suspended in the gripping pads
  • use a tyre handler as a bead breaker unless it is specifically designed for this purpose
  • inflate or deflate tyre assemblies while being gripped or supported by the clamp arms
  • handle a tyre or tyre assembly unless the handler is rated for the load.

Heat & fire
External fires or heating of any part of a tyre assembly, wheel end or hub can compromise the integrity of the tyre, in extreme cases resulting in dangerous, rapid deflation (i.e. bursting) or explosion (due to pyrolysis or diffusion). Such events often start in another area of the vehicle, such as the engine, brakes or wheel-motor, and spread to the tyre. A tyre burst or explosion can spread fire to other tyres and areas of the vehicle. Tyre fires are difficult to extinguish, and also produce large volumes of toxic fumes.

An overheated tyre should be treated like a “ticking bomb”. Trucks that have or are suspected to have overheated tyres should be parked-up in a safe place well away from other equipment and workers, and allowed to cool down slowly over at least 24 hours before any inspection or replacement of the tyre is attempted. The use of thermal imaging or remote tyre pressure monitoring (e.g. tyre pressure/temperature monitoring systems or TPMS) avoids the need for personnel to enter the danger zone of a suspected overheated tyre to conduct an initial assessment.

Fuels & chemicals
Rubber may react with some fuels, solvents and other hydrocarbon substances. Apart from potentially weakening the tyre, this can greatly increase the risk of the tyre catching fire if an ignition source is available. Other chemicals can introduce similar risks. For example, do not use a lubricant to assist with assembling tyres and rims unless it has been clearly identified by the manufacturer as being safe to use for its intended purpose and the flash point has been identified.

Pyrolysis
Pyrolysis is the decomposition of carbonaceous material inside the tyre. Heating of the rubber (inner liner) releases gaseous volatile organic compounds into the air chamber of the tyre. Under certain temperature, pressure and concentration conditions, this volatile mix of air and fuel can become an explosive mixture and achieve auto-ignition. Rapid spontaneous combustion typically results in large catastrophic failures with destructive outcomes — the pressure typically exceeds 6.9 MPa (1,000 psi). Such events can propel debris hundreds of metres, and are potentially lethal to any workers in the vicinity, including persons in vehicles.

Sources of heating that could result in a pyrolysis explosion include:

  • heating of stuck or “frozen” wheel fasteners
  • welding or grinding of wheel components
  • vehicle coming into contact with high voltage electrical conductors (e.g. overhead power lines)
  • vehicle struck by lightning
  • external fires (e.g. engine bay fires, hydraulic fires, electric fires, grass fires in parking area)
  • overheating brakes (e.g. due to brake overuse, misuse or dragging)
  • overheating of electric wheel motors
  • gross under-inflation of tyres
  • heat separation (i.e. separation of rubber layers in tyre leading to further heating from rubbing friction)
  • overloading or over-speeding of the vehicle (e.g. exceeding its tonne kilometre per hour or TKPH loadspeed rating).

The pyrolysis reaction cannot be detected and explosions can result spontaneously without warning or any obvious external visible signs that the tyre is progressing towards auto-ignition.

The risk of pyrolysis may be mitigated by:

  • ensuring the air in the tyre does not reach auto-ignition temperature
  • reducing the oxygen concentration in the tyre so there is insufficient oxygen to support combustion (e.g. use nitrogen for tyre inflation)
  • using a suitable liquid tyre additive
  • monitoring the vehicle’s speed and load using on-board data acquisition and recording systems to help manage driver behaviour to stay within the TKPH rating
  • using a TPMS to monitor tyre pressure and temperature in real-time to detect extreme air pressure or temperature anomalies.

Projectiles
Despite their size and mass, tyre, rim and wheel assembly components can be propelled long distances at great speed when tyres burst or explode. Not only do these projectiles represent a lethal hazard to workers, but they can also damage adjacent plant and structures, which could result in further hazards.

Multi-piece rims pose a greater risk than single-piece rims due to greater potential for a catastrophic disassembly and increase in the number and trajectories of potential projectiles.

Projectiles may be generated as a result of:

  • poor maintenance and housekeeping
  • incorrect assembly or fitment (e.g. mismatched, misfitted or missing components; incorrect seating of split rims or tyre beads)
  • incorrect work procedures (e.g. starting to dismount a multi-piece rim assembly before the tyre is fully deflated; when removing a tyre assembly, loosening any fasteners before the tyre is fully deflated)
  • the adjustment of an incorrectly or misfitted (e.g. installed backwards) lock ring without complete deflation
  • re-use of damaged parts (e.g. out-of-round, deformed lock rings)
  • use of non-original, or non-approved replacement parts
  • rims that are cracked or fatigued.

The use of fit-for-purpose tyre inflation cages or equivalent containment devices may provide suitable protection during initial inflation after assembly. Tyre inflation cages are typically used for light vehicle and highway-type truck tyres. However, they are generally not practicable for earth-moving machinery because of their tyre size.

Controls to reduce the risk of projectiles include staying out of the potential line-of-fire by using:

  • distance (e.g. long air hose with remote gauge and air valve, and large exclusion zone)
  • suitably designed barriers or guards (e.g. blast-proof wall, earthen bund).

Foreign objects inside tyre
Foreign objects left inside the tyre can physically damage the casing liner or prevent proper seating of the bead and, in the event of a tyre burst, they can become lethal projectiles.

Scraps of wood left inside the tyre, when subsequently heated can result in a wood distillation reaction that releases methanol (wood alcohol) vapour into the tyre’s air chamber. Under certain temperature, pressure and concentration conditions, this volatile mixture of air and fuel can become an explosive mixture and auto-ignite at a temperature much lower than that for a pyrolysis-related explosion.

Scraps of plastic and other materials can also release volatile organic compounds when heated.

Particles of foreign matter inside a tyre’s cavity can also block the valve stem and could give someone servicing the tyre the impression that it is fully deflated when it is not.


REFERENCE: Department of Mines and Petroleum, 2015, Tyre safety for earth-moving machinery on Western Australian mining operations — guideline: Resources Safety, Department of Mines and Petroleum, Western Australia, 63 pp.

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