EMtek recently completed a review and work on several downhill conveyor systems, identifying some common deficiencies in sizing of safety critical systems that may leave a typical installation at risk of a runaway situation.
Long and high-capacity conveying systems play a critical role in materials handling industries. These systems offer means of efficient material transportation, and they are becoming particularly significant lately due to persistent efforts to decarbonise mining operations.
Combining downhill topography with high-capacity conveying presents a special engineering challenge which must balance critical safety requirements with operational and project investment efficiency.
The challenge
Downhill conveyors are subject to unique engineering challenges where the drive train must not only be sufficiently sized to support production at the nameplate capacity, but it must also be able to maintain a safe conveyor speed over a wide range of operating loads. The system must maintain a reasonable level of braking redundancy even if elements of the drive-train or control system fail. On the other hand, excessive oversizing of drives and braking components could lead to excessive shock loadings during emergency stopping scenarios, whilst negatively impact the project economics.
Undersizing the brake system may be a result of misidentifying probabilities of certain types of failures, reliance on standards that may not be up to date with the latest industry trends, or insufficiently conservative selection of the hypothetical friction coefficient. Also, the use of secondary resistances which are typically used to conservatively estimate additional friction, may be reduced or non-existent in reality.
Friction coefficient
Selection of representative friction coefficient is always a challenge during conveyor design. Large deviations between design value and measured value could impact project economics, and in case of a downhill conveyor, negatively impact capacity of brakes and safety critical systems.
Whilst some conveyor design references offer detailed methodologies for friction coefficient estimates (i.e. CEMA 7) others offer only high-level guidance. For instance, ISO 5048 simply suggests calculating the downhill conveyor braking requirements using a value which is 40 per cent lower than the one used for calculations of driven conveyors (and it suggests using a value of 0.012). Whilst this is a good approach, it is important to ensure that the starting point adequately takes into account the latest industry trends and efficiency improvements offered by modern belting compounds, high quality idlers, and modern construction and alignment techniques. In short, 0.012 is unlikely to be adequately conservative value given the current industry trends.
During a recent third-party review of a downhill conveyor system design, EMtek identified that the friction factor used in design scenarios was insufficiently conservative. The utilised friction factor failed to account for the most probable range of frictions that are likely to be observed in practice.
Figure 1 shows calibrated friction factors for a number of equivalent conveyors that EMtek reviewed over a number of years. Whilst the assumed friction factor was generally in line with typical frictions used for overland conveyors, it was higher than the average calibrated value of equivalent systems, and it did not take into account a possible lower end of the range.
During the review, EMtek was able to demonstrate that the conveyor is likely to operate at frictions lower than the values used in the design, and should that be the case, the regenerative load on variable speed drives (VSD) will exceed the nominally selected capacity of 150%. This scenario would lead to progressive tripping of VSD controllers and loss of regenerative speed control and capacity. This exact scenario was responsible for loss of downhill conveyor control at one of the North American mines where EMtek was engaged to identify the root cause of the problem and develop adequate retrofit upgrade solution for the braking and drive-train system aimed at eliminating repeat occurrences of a dangerous event which led to equipment damage in the past.
A 40 per cent reduction is still a good rule of thumb, but with the latest industry trends, the most likely starting point is lower than the value suggested by standards such as ISO 5048.
Importance of scenario-based design
The same review also highlighted that conveyor system designers may overlook very realistic brake failure scenarios. The system under review had four drives, each with twin brake callipers acting on a low-speed disk brake which was mounted on the low-speed drive coupling. The designers assumed a single brake calliper failure and ensured that the remaining brakes have at least 1.5 times the minimum required braking capacity. A possibility of twin calliper failure was identified as remotely low risk and excluded from the scenario-based design and consideration. It was highlighted that the design was not subject to the requirements of AS4024, and the 150 per cent capacity was deemed adequate under European standards.
Subsequently EMtek contacted several maintenance representatives on this customers sites and identified several instances where drive pulley shaft failures, or reducer shaft failures have occurred due to manufacturing defects or improper installation and alignment. Such failures would either lead to immediate loss of braking capacity from both callipers (i.e. drive pulley shaft failure) or have the potential to damage the disk and multiple callipers (i.e. reducer shaft failure).
Therefore, while the design was not subject to redundancy requirements of AS4024, the design was nevertheless subject to requirements of EN620, which requires safety critical braking systems to be sized for PLr of c (Performance Level requirement, c according to ISO 13849-1), which considers a range of failure probabilities between 0.0001 per cent and 0.0003 per cent.
Downhill conveyors present a unique challenge where the need for adequate redundancy needs to be balanced with a possibility of unwanted dynamic effects from excessive redundancy. However, the review identified that should a failure of this type occur, the remaining braking efficiency would be inadequate to safely stop the conveyor, so design modifications were required.
Engineering solutions for improved safety
Addressing the complex problem of downhill conveyor braking system selection and design requires a systematic approach which should take following steps into account:
Representative and accurate conveyor operating calculations. Conveyor calculations, which are as accurate and as representative as possible are foundation of effective braking and drive system selection and design. Design should be based on factors which have taken into account the latest industry trends and achievements (i.e. Low rolling resistance belting compounds, GPS levelled conveyor formations, accurate conveyor alignment techniques, etc.). Ideally, the design should be based on calibrated data which is obtained from equivalent design and installation, and which had an appropriate factor of safety applied to it.
Redundancy in design. A safe and well-designed downhill conveyor braking and drive train system must have adequate levels of redundancy incorporated into the design. A good starting point is AS4024 which offers specific guidance that appears to be superior to all other industry standards from around the world. However, it is important to consider this early in the design. Late review and inclusion can result in system-wide changes.
Scenario based testing. The proposed design should be tested against a common range of failure scenarios, many of which can be verified against operational experiences. The system should maintain a reasonable level of redundancy with all possible failure scenarios. Although, reduced levels of redundancy may need to be accepted for most adverse (and least likely) cases.
Advanced monitoring and control systems. Modern braking systems can benefit from integration of monitoring and control technologies. These systems should be considered during design and equipment selection phase to improved overall reliability of the conveyor system.
Conclusion
Design and selection of drive-train and braking systems is of critical importance for safe, reliable, and efficient operation of downhill conveyor systems. EMtek’s recent review highlighted design assumptions that may lead to undersizing of brakes and drives. These observations have coincidentally been confirmed with operational experiences where EMtek has been involved with root cause analysis and problem solving of existing downhill conveyor installations.
Considering the plausible failure scenarios in design, application of verified design factors and assumption, and application of adequate redundancy factors is a standard design approach at EMtek which comes from many years of design and engineering experience, completed with a diverse workforce which has a range of operational, design, engineering, maintenance, and construction experience. EMtek with its offices in Melbourne, Perth, and Brisbane is well placed to help clients with downhill conveyor engineering enquires.