Modelling of Energy Accumulation in Belt Conveyor Systems Under Faulty Conditions

Belt conveyor systems have been extensively used since the first years of 900s in several kind of overland applications, such as in the mining sector and steel plants but also in totally different environments like indoor industries or warehouses. However, the main task required by such mechanisms are the same: belt systems are commonly employed to transport bulk materials or packages due to their economy, reliability, and wide range of capacity. Belt conveyors are an extremely versatile tool which can be easily embedded in a variety of plants typology and whose transportation distance can span from short to very long belts of several hundreds of metres. In order to avoid the detrimental complete slipping of the belt on the pulleys, usually a pretension mechanism is adopted to maintain a minimum amount of tension in the belt. In the design phase the system is usually considered as stationary, with the transported material moving at a constant speed guarantying a constant mass flow rate. As a consequence, the tensions in the various belt sections and the slipping angles of the belt on the pulleys are considered constant. Actually, during real operations, dynamics effect in the system can occurs due to multiple causes, such as not constant loading of the transporting side of the belt, eccentricity in the pulleys mounting or during the start and stop phases of the system. A dynamic analysis is therefore useful to understand the real tensioning state present in the various belt sections due to the variability of the operational conditions. Hitherto, several studies have been carried out to investigate the dynamic response of the belt conveyor system during stars and stops in nominal conditions. Different models have been created, representing the belt dynamics mainly by means of discrete elements or finite elements methods with consequent high computational time. However, faulty conditions of such a system have been rarely considered, although they represent very dangerous situations. In fact, specially for long belt systems, a system failure can lead to the accumulation of energy (elastic and gravitational) which can result in serious injuries of the operators if not appropriately trained. Indeed, it is not sufficient to cut off the energy supply to the electric components to put the plant in safe conditions. This work focuses on these types of scenario with the aim to extract indicators from the commonly observable signals available to the operators in order to report the insurgence of a faulty condition in such a system and to alert the workers on the possibility of residual accumulated energy. A dynamic lumped masses model has been developed to describe the tensions in the belt and the kinematics quantities of each component under healthy and faulty conditions. The slipping angles on each pulley are considered time-variant and the belt elongations are taken into account. The pretension of the system is realised by means of a gravity tensioner and the belt motion is imposed by two electric motors acting in parallel on the driving pulley. The results of a case study are presented and analysed, in which, due to dirt and spilled bulk material accumulation on the return pulley, the increase of the friction coefficient causes an anomalous stop of the system. Operators’ actions to try to restart the conveyor are simulated and the dangerous elastic energy accumulated in the belt as a consequence of the motors action is evaluated. It has been seen that the height of the counterweight can give an indication of the tensioning condition and can be used as a first indicators to monitor the health status of the system.