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PROCESSING SOLUTIONS Enhanced Technology for Blocked Chute Detection By Mark Kulp Acoustic Switches Arrive The microwave blocked-chute switch shown here is a non-contact system. In the current "high volume equals higher profits" mining world, is there any excuse for settling for less than the best technology for the detection of blockage in chutes? Installing point detection devices in transfer chutes for blockage detection is necessary as it is an inexpensive way of pre-empting a chute blockage. These transfer chutes are everywhere at mining sites, and one plugged chute can stop production. Screens, both wet and sizing, are also key to the smooth operation of a facili- ty, and they are just as susceptible (if not more) to blockages based on the wet materials they handle. Add in clean-up costs for any production stop- page and the entire event becomes extremely expensive. There are many options for the detection of blockages in chutes. Some of them are invasive, others just intru- sive, and some are even completely non-contacting. Today's demands are for high reliability, a preference for a 158 E&MJ; • SEPTEMBER 2012 non-invasive technology, and an avoid- ance of nuclear-based detectors when possible. Plant personnel are always looking for ways to increase throughput, reduce downtime and improve process efficiencies. Companies employing cut- ting edge technology are designing process instrumentation that offers many different types of techniques for providing reliable point level detection solutions for tough applications. To be successful in this instrumen- tation market, a company must offer solutions that add value for customers and offer user-friendly configurations with high reliability. With today's tech- nology, upgrading of instrumentation at a plant location from older measure- ment techniques to newer designs will definitely lower maintenance costs, improve process efficiency, and provide higher reliability devices, which will provide many benefits. With safety as the goal, any blocked-chute detector must be reliable, robust and accurate. Consider the technologies that have been in use—vibrating devices (tuning fork type), capacitance (or admittance as some prefer to call it), mechanical devices (such as the tilt switch—either mercury filled or its newer non-mercury version). In addition to the contacting type devices there are also the nuclear- or microwave-based detection systems, which have no intrusion into the chute. But now there is a new member of the detection family that falls somewhere in between. Acoustic switches require an opening into the chute, but don't protrude into the flow stream. Vibrating technology uses the princi- ple of exciting a piezo-crystal to induce vibration onto a set of tines. When product comes into contact with the tines, the vibration frequency is damp- ened, and an alarm relay is triggered. It is fine for measurement in lighter, small particle applications, but some- what out of place in the size fraction typically encountered in a coal prep or mineral processing plant. These sys- tems are potentially subject to issues of false indication due to build-up of fines that need to be washed off. Capacitance (or admittance) tech- nology uses the principle of applying a small radio frequency voltage to an ele- ment, and measuring the capacitance in picofarads of the element by an antenna installed into the chute. An electrical A self-cleaning acoustic switch may represent a better option for blocked chute detection. www.e-mj.com