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Weighing in on belt conveyors STEVE DAVIS In his regular BULKtalk column, Steve Davis of Rio Tinto considers the basics of bulk handling that sites often struggle with. He shares his insights gained from more than 30 years in bulk materials handling. Steve Davis is the principal advisor – bulk materials process at Rio Tinto, based in Perth. Steve has worked in bulk handling for 30 years, for both resource companies and professional engineering firms, in Australia, South Africa, the Middle East and Canada. His experience encompasses such commodities as iron ore, coal, potash, phosphates, petcoke, sulphur, sands and grain.
RIGHT Precision 5-roll belt scale with 4 stage stored in place calibration weights.
Rio Tinto’s Steve Davis explains the ins and outs of belt weighing systems and how they should be selected, installed and maintained to get the best accuracy possible. When transporting bulk commodities on belt conveyors it can be important to know the tonnes moved. Reasons for this could be to inform process monitoring, storage management, custody transfer and feed metering. To gain this insight, belt weighing systems (belt scales) are used. However, these devices should be selected, installed and maintained to meet specific aspects that define whether the system will achieve accuracy goals.
General aspects Belt scales do not directly weigh bulk on the belt. They are electromechanical devices that convert an instrument output to a weight indication. There are four generic types: 1. Electromechanical belt scales, using load cells and a deflecting load support frame, are the most common by far, and generally the most accurate. Deflection of the frame under load varies strain on load cells. Belt speed is measured, and an integrator converts the outputs to a weight. 2. Optical systems measure cross section of the load on the belt. The belt shape and speed, and the load bulk density must be consistent as these define the weight in the integrator. These are best with finer materials and consistent feed rates. 3. Nuclear systems measure absorption of radiation in the load and converts to a weight. Variations in the belt and the material coefficient of absorption can affect accuracy. 4. Impact weighers are also available for discharge chute installation. Deflection of the impact plate correlates to the flow of material in the discharge. These are suited for dry fine product. There is a common misconception that belt scales are 100 per cent accurate, which can lead to concern when two weigh systems do not match exactly. Conversely, there might be something wrong when two systems match consistently closer
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than realistically possible, for example, within 100 tonnes on a 200,000-tonne batch with a combined error of 0.05 per cent. Belt scales are sold based on error bands, such as being accurate to ±0.5 per cent. This accuracy cannot be attained just by installation on a conveyor. The accuracy of all belt scales depends on calibration, and repeatability through good installation and maintenance. External influences must be considered and mitigated. When all aspects of the belt scale and installation are in good condition, calibrated correctly, and external influences are minimal, the integrated weight readout will vary between 99.5 per cent and 100.5 per cent of the true weight on a ±0.5 per cent accurate scale. For a detailed explanation, I recommend reading Control Systems Technology Knowledge Bases. Good accuracy is possible, but it doesn’t just happen. Belt scales have a span, which is the range in which they deliver repeatable outputs. Go beyond the span and accuracy will drop. The best accuracy is generally in middle to top of the span range. Zero checking is not calibration. It is a regular reset