The challenge of our innovative success in mine plant equipment is reliability and maintainability, as the more complex the equipment, the more chance of downtime, writesCorrina Trimarchi .
The increased competitiveness of the global minerals market has underpinned the drive to achieve optimum economies of scale in mining production and transportation processes, leading to ever increasing modernisation and automation of mining operations. This, in turn, has led to a marked increase in capital investment in mining equipment.
Coupled with this increasing investment in ever more complex equipment, comes the greater likelihood of downtime issues and cost of maintenance.
With mining equipment maintenance costs conservatively estimated to range from 20% to 35% of total mine operating costs, effectively managing equipment maintenance presents as a significant consideration for mine operators.
Given that the total annual cost of engineering equipment maintenance in the Australian underground coal mining industry alone is approximately $450 million per annum, with approximately 10% of production time lost to unplanned maintenance, the development of a maintenance strategy that efficiently streamlines maintenance protocols to achieve maximum productivity at a minimum cost is a contemporary challenge facing mine production teams.
Traditional strategies enlisted to control these costs include: optimizing scheduled maintenance; deferring non-essential maintenance; more rigid control of spare parts inventories and reducing maintenance manpower, including the outsourcing of maintenance to contractors.
Recently the concept of a reliability-centred maintenance program has been touted as a more efficient strategy to preserve optimum operating levels whilst still observing the necessary equipment maintenance protocols. It has been suggested that the introduction of such a maintenance strategy has the potential to reduce lost productivity time, whether due to preventative or corrective maintenance, by between 40% and 70%. In dollar value this represents a significant reduction in operating costs.
Reliability-centred maintenance, therefore, is advocated as the most effective strategy to address mining equipment maintenance, both in terms of lost production time as well as cost minimisation. Where the time honoured strategy has been to implement a scheduled maintenance process, where equipment and vehicles were periodically assessed for evidence of wear and tear, and recognised parts automatically replaced as a general protocol to extend the working life of equipment, the implementation of condition-monitoring techniques, as part of a reliability-centred maintenance regime, is becoming a more popular strategy.
It’s been argued that the philosophy that underpins scheduled maintenance and the automatic replacement of parts and / or machinery overhaul which is based upon the notion that particular equipment parts wear out over operating time is a misconception. The implementation of a time-based service, parts replacement or equipment overhaul routine is based upon prescribed industry timeframes for the deterioration of parts rather than the actual condition of the parts themselves and often leads to unnecessary maintenance, resulting in lost production time.
This is where the concept of condition monitoring as a maintenance strategy is argued to be more efficient. Condition monitoring, an integral process of reliability-centred maintenance, relies upon the knowledge and technical expertise of staff to identify early warning signs to potential equipment failure. It suggests that all equipment will provide an early indication of impending inefficiency or failure and that recognising these indicators forms the crux of the condition monitoring process.
The benefit in adopting this style of maintenance strategy is that unnecessary downtime arising from time-based, scheduled maintenance, with the attendant loss of productivity, can be averted, with maintenance activities only performed when necessary. Condition monitoring, utilising precise evidence that can predict when a failure is imminent, is a far better means of avoiding failures and their consequences than scheduled inspections, equipment replacements and overhauls.
Condition monitoring is based upon the ability of production and maintenance staff to accurately identify indicators which signal the potential failure of equipment. Then, employing an understanding of the repair history for the specific type of equipment or machinery, the time between the indication of potential failure and the actual functional failure of components can be identified. This allows time to take corrective action.
During this period, the equipment is more carefully monitored and left in operation so that it can continue to perform its designated function, whilst corrective action is taken during the on-condition period to restore the equipment to its normal operation.
In the exploration of reliability-centred maintenance in the mining industry, it was identified an 8 step process to condition monitoring, utilising haulage vehicles as an example:
Step 1. Select the most critical equipment
For example, assume that the most critical equipment in a particular operation is the 172 t (190 st) haulage truck fleet.
Step 2. Identify the functions of the most critical equipment.
What exactly do these haulage trucks do in their operating context? The primary function of the 172 t (190 st) haulage truck fleet is to move ore or waste from a loading point to a crusher or waste dump.
Step 3. Establish performance standards
How well must the haulage truck perform in the conditions under which it operates? Each truck, for example, must be able to carry a 172-t (190-st) load up and down 12 percent grades at speeds of up to 48 km/h (30 mph) in all weather conditions for periods of 24 hours, stopping only for refueling, servicing, periodic operator checks and shift changes.
Step 4. Determine the types of failures.
Any equipment condition that does not permit a haulage truck to meet the performance standard would constitute a failure. A potential failure is an identifiable physical condition that indicates that the failure process has started. On haulage trucks, typical potential failures might be:
- Vibration signaling the onset of transmission failure.
- Cracks indicating the start of fatigue in the truck frame.
- Metal particles in engine oil, indicating possible bearing failure.
A functional failure is the inability to meet the specified performance standard. A haulage truck experiencing the following types of failures would not be able to meet its performance standards and would sustain a functional failure:
- Hydraulic pressure insufficient to raise truck bed.
- Low engine compression reducing engine power.
- Electrical shorts inactivating warning devices.
One must also be aware of hidden failures in which the failure is not apparent until the function is attempted by the operator and the truck fails to respond. Typically, these might include instances in which:
- The operator pushes the brakes pedals but the truck keeps going.
- The operator activates the lever to raise the bed but nothing happens.
Step 5. Enumerate the consequences of failures.
What will the result be if a specific failure occurs?
Consequences of failure can range from inconvenient to catastrophic. For example, a haulage truck with failed warning lights can be restored to its performance standard with little downtime as the offending fuse is found and replaced. However, a haulage truck without brakes can pick up speed, collide with another truck, damage both trucks, injure the drivers, block the road and require extensive repairs.
In the larger context, maintenance can affect all phases of the mining operation. Without reliable equipment, production targets cannot be met. Without dependable equipment, product quality and customer satisfaction goals are not met. Unreliable equipment can endanger personnel, create environmental hazards and even undermine energy efficiency. For all of these reasons, avoidance of the consequences of failure becomes a primary objective.
Step 6. Rank the consequences of failures.
Modern mining production equipment has increased in complexity, multiplying the number of ways it can fail. Therefore, failure consequences must be classified to guide preventive and corrective actions. For example:
- Safety failures endanger personnel as well as equipment.
- Operational failures result in product loss plus the cost of repair.
- Nonoperational failures result only in the cost of repair.
In industry, the most important aspects are the avoidance or reduction of the consequences of safety and operational failures. Therefore, the most competent types of condition-monitoring techniques are applied to the equipment most critical to the safety of individuals and the production process.
Step 7. Apply the most effective condition-monitoring techniques.
To detect potential failures early and accurately distinguish them from normal operating conditions, condition-monitoring techniques such as vibration monitoring are used. These techniques are capable of detecting deteriorating equipment conditions with much greater accuracy and reliability than can humans. These techniques also detect hidden failures that human beings would not be able to detect unless they tried a control mechanism and it did not respond (e.g., lifted lever and truck bed did not raise).
With the availability of more effective and reliable condition-monitoring techniques, equipment condition can be more accurately observed. This allows equipment to remain in service on condition that it continues to meet its performance standard rather than replacing the troublesome component at the first sign of potential failure. In turn, this approach yields significantly greater life from components and units.
Step 8. Establish an overall maintenance plan.
Based on the consequences of failure, a maintenance program featuring condition-monitoring techniques is applied to identify potential failures accurately and quickly to preclude their deterioration to functional failure levels. The most effective maintenance program is built on the preceding implementation steps. Then the condition monitoring techniques selected are fitted into existing, competent maintenance programs to protect the 172 t (190 st) truck fleet from functional failures and their consequences.
This same regime can be applied to all critical industrial equipment utilised within the mining sector. It requires the application, however, of a methodical maintenance strategy, informed by evidence, rather than the implementation of standardized assumptions of natural component wear and tear based upon age or period of use.
Contemporary mining processes place a great reliance on the veracity of the belt conveyor system given that, with the increasing popularity of long wall mining, potentially up to 90% of ore is extracted from the one face. Thus, with the economic viability of the operation contingent upon the reliability of such core equipment, enormous pressure is placed upon manufacturers of conveyor systems to fulfill equipment reliability expectations.
Long term conveyor reliability is contingent upon the consideration of a number of additional functional design parameters to enhance longevity, including: designing for simplicity; designing for unplanned events; designing for future requirements; designing for effective maintenance and designing for monitoring of equipment. These same parameters, however, can be applied right across the spectrum of industrial mining equipment.
In terms of maintenance of equipment, it is advocated that the design process has an obligation to consider the ongoing maintenance needs of the end-user and that maintainability itself is integral to the design process. He defines this through the identification of core considerations such as:
- Effective accessibility of all systems and components
- Predictability of all potential failures
- Proper initial start-up and commissioning
- Serviceability and repairability of equipment and components
Particular weaknesses in the design process can therefore inherently create greater headaches for mining maintenance personnel. According to Dhillon, these weaknesses can be identified as follows:
- Poor design and placement of components
- Poor component-machine interface design and component handling capability
- Unnecessary equipment complexity
- Inadequate design with respect to resources available
- Poor design for routine maintenance
- Design conveniences
- Poor fault isolation capability
- Accessibility limitations
Accessibility limitations present as the greatest impediment to maintenance, with a study indicating that simple design flaws such as: inadequate access opening size, poor layout of parts, the need for partial or total disassembly of components to locate simple items such as fasteners and interfaces and the inability of maintenance personnel to use specialised tools presenting as recurring maintenance issues.
The adoption of maintainability engineering protocols has been touted as the most effective method of overcoming these design issues. The primary advantages of maintainability engineering include standardisation, interchangeability, accessibility and safety, with each of these characteristics contributing to a reduction in equipment downtime and a simplification of the maintenance process. Standardisation refers to the restriction of the variety of differing parts and components, allowing for the maximisation of common or standard parts and components across a range of equipment types and models.
The main advantages of standardisation are in the improvement in equipment maintainability and a reduction in the use of incorrect parts, in addition to a reduction in component design and production costs. Interchangeability refers to ability to replace a particular component with one similar which operates as effectively. Accessibility speaks for itself, suggesting that the ease with which a component can be reached for repair or replacement has significant bearing to the downtime experienced by that equipment when under maintenance.
The safety characteristic, too, is a self-descriptive yet often under-rated consideration which can impact significantly on the maintenance regime prescribed for particular equipment.
Maintainability design engineers draw from a simple set of tenets when designing industrial mining equipment: design to minimise the need for tools and adjustments; design to minimise the need for high skill maintenance; provide straightforward troubleshooting guidelines; provide test points; use colour coding; allow for ease of visual inspection; label components; use standard, interchangeable parts; use plug-in rather than soldered components; and, above all, design for user and maintenance safety.
Despite these simple, seemingly obvious guidelines, recent reviews by statutory bodies have indicated an inconsistency in their application, noting that equipment design complexity is contributing to greater production downtime and inflated maintenance costs, whilst the risk to maintenance staff could still be significantly reduced through greater simplification of equipment design.
Given that 40% to 50% of equipment operating costs in the mining sector are incurred through maintenance and that approximately 25% of all injuries in underground coal mining, as an example, occur during maintenance processes, simplifying and streamlining maintenance protocols are a primary consideration, both for greater economic efficiency in addition to improved employee safety. As Dhillon has noted, however, the major factors contributing to mining maintenance costs are much broader than just design features alone.
He has identified the following factors, in addition to equipment design: equipment age, skill of management personnel, the maintenance culture of the entity, the maintenance environment itself, maintenance training and experience of personnel, regulatory compliance and maintenance errors. This diverse array of factors contribute to the cost of maintenance experienced by mining operations, with each presenting their own very particular challenges to management.
It is estimated that the cost of maintaining equipment in the field can incur between 2 and 20 times the original procurement cost of each piece of equipment. This estimate factors in variables such as lost opportunities in up-time, yield, rate and quality resulting from compromised functionality or non-operation of equipment. It also allows for the cost of meeting human safety, property and environmental reparation resulting from equipment malfunction.
In essence, with the need for greater economies of scale informing an increased reliance on the automation of the mining industry, in turn driving the development and implementation of more complex equipment, the focus is now on how to increase the functionality and the longevity of this equipment to justify the enormous capital investment it requires.
Given that the design element informs the complexity of maintenance procedures, the adoption of stringent maintainability engineering protocols offer the possibility of simplified maintenance processes for end-users, whilst the implementation of reliability-centred, condition-monitoring protocols allow for operators to exert greater control over, with the objective of minimising, their on-site maintenance costs. This is the challenge of mine plant maintenance in the 21st century.
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