Monday 24th Jan, 2022

Troubleshooting belt conveyors with DEM analysis

Shaun Reid, Consulting Engineer with TUNRA Bulk Solids, shares how TUNRA has applied DEM models to troubleshoot belt conveying operations.

Shaun Reid, Consulting Engineer with TUNRA Bulk Solids, shares how TUNRA has applied DEM models to troubleshoot belt conveying operations.

Belt conveyors offer an efficient solution for high throughput, continuous transport of various goods in a wide range of operating conditions. However, their operation involves a range of challenges that must be addressed to minimise expense on maintenance and downtime. An important tool for troubleshooting belt conveyor systems is discrete element method (DEM) modelling. The application of DEM generally focuses on the interactions occurring between bulk material stream and the conveyor belt at a transfer or feeding point. The following issues lend themselves to investigation and remediation using DEM, provided that the model is properly calibrated.

Belt wear

Wear of the conveyor belt top cover occurs primarily at the locations of belt loading which may involve either high-speed, free flowing discharge from a transfer chute; or low-speed, high pressure loading from a hopper onto a belt feeder. DEM may be applied to inform the way in which interactions with bulk materials contribute to wear of the conveyor belt, along with other forms of conveyor damage such as belt punctures and tears.

Wear occurs during interactions between two bodies at a non-zero relative velocity. Wear arising from interactions with bulk materials may be classified as either:

Impact – as a function of the relative velocity in the direction normal to the worn surface; or

Abrasion – as a function of tangential relative velocity, normal pressure and friction and is most often a combination of both mechanisms.

By predicting these dynamics, DEM simulations can inform the design and operation of conveying systems to minimise belt wear. This process foremostly considers the design of the transfer chute or hopper that is tasked with feeding bulk material onto the belt, with the aim of reducing the relative velocity between the bulk material stream and the conveyor belt; or optimising the pressure acting above a feeder belt. The geometry of the transfer chute or feeder interface influences the dynamics of the bulk material at the point of impact with the belt and thereby governs the propensity for wear. The development of a DEM model to predict these characteristics enables an efficient study of the complex interplay between these factors, so that a conveying system can be designed to reduce wear on the belt.

Many commercial DEM packages include analysis features for quantifying the wear conditions arising from the aforementioned dynamics. These typically involve basic models to determine the energy imparted upon the worn surface in both abrasive and impact interactions. This approach enables the comparison of the wear conditions that arise in different operating conditions, but only in terms of the change in dynamics that influence wear and not by the resultant wear performance of the surface. This approach is useful for relative comparisons between transfer chute or hopper designs and/or operating conditions so that performance may be optimised with respect to belt wear.

Extension of the above comparisons to the prediction of physical resultant wear requires a model that relates the energy imparted by the particles upon the surface to the surface removal caused by this energy. Several DEM packages have implemented such models but their application to prediction of wear in general instances requires caution. The multifaceted characteristics that influence belt cover wear, including bulk material, belt, environmental and operating properties, are not adequately captured by current wear prediction methods. The development of more rigorous models and subsequent methods for their calibration is an ongoing area of research.

While the above commentary has focused on wear, DEM simulations may also be applied to other forms of damage caused by interactions between the bulk material and the conveyor, such as belt puncture, belt tears or gouging, and impact damage to the idler assemblies. This form of damage is most prevalent in conveying instances that involve the transport of large and irregularly shaped particles, for which DEM approaches are particularly advantageous in predicting flow characteristics. In considering the impact conditions upon loading, DEM simulations also lend themselves to coupling with structural simulation methods such as Finite Element Analysis (FEA).

Belt tracking

Mis-tracking of a conveyor belt occurs when the belt drifts (laterally) from the centreline of the conveyor. This is a common phenomenon and one that causes significant delays in operation and jeopardises the condition of the belt. A range of factors contribute towards belt mis-tracking, with one of the primary causes being non-central distribution of the bulk material across the belt. In this instance the centre of gravity of the conveyed material is offset from the centreline of the belt, causing an uneven lateral reaction from the idler set, resulting in the belt drifting towards the underloaded side.

Non-central presentation of material onto the belt is a function of the transfer chute design and operating conditions. It is most prevalent in transfer chutes that involve a change in conveying direction or a misalignment of the incoming and receiving conveyors. The flow characteristics of such transfers are often inadequately predicted by fundamental analysis methods such as a continuum based approach, thereby benefiting from simulation with DEM techniques. The advantages of DEM based analyses also extend to the simulation of bulk materials that include coarse and irregularly shaped particles. In these instances, DEM may be applied to predict segregation of the coarse and fine fractions, which may impact downstream processing. 

The centrality of discharge from a transfer chute is often dependent on the operating conditions. While central loading may be observed for a certain bulk material at a certain throughput or moisture content, it may not be achieved in alternate conditions. Once a DEM model has been established, the sensitivity of discharge conditions to variations in operating parameters may be readily assessed, often more efficiently than could occur via physical testing methods using scaled conveying geometry. The lateral distribution of material on the receiving conveyor may be assessed quantitatively in DEM by analysis of the mass proportion on each side of the belt and/or by determination of the offset of the conveyed materials centre of gravity to the belt centreline.

Spillage and dust

DEM analysis also lends itself to the prevention of spillage from belt conveying systems. Spillage may occur due to a combination of volumetric and interfacing characteristics, both of which may be informed by DEM analysis. While the volumetric requirements of a bulk material atop a conveyor may be readily determined via analytical methods in steady-state operation, the transient conditions that occur during belt loading often benefit from simulation. Further to the conditions described for belt wear in the loading zone, if bulk material is discharged with relative velocity to the conveyor belt, a transition region must exist in which the material is not fully accelerated to the belt speed. To satisfy continuum principles, this results in a greater volumetric requirement in this region than is required in the steady state conveying region. Simulation of these characteristics, along with particle trajectories in the discharge zone, enables an interface and skirting design that prevents spillage of bulk material. This analysis may also be extended to the prevention of bulk material ‘rollback’ upon presentation to inclined conveying systems, particularly for coarse particles or dry materials.

DEM and Computational Fluid Dynamics (CFD) methods may be applied in both discrete and/or coupled approaches to analyse dust emissions, with particular application to belt loading locations. The application of these simulation techniques enables the optimisation of bulk material interactions through the transfer chute and upon presentation to the receiving conveyor, such that the propensity for dust emission is reduced. These techniques can also be applied to the design of dust control equipment, such as loading hoods and extraction systems, that are required for applications in which dust is unable to be eliminated through handling design or material conditioning.

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