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March 2017 www.coalage.com 39 operating ideas Point-level Blockage Detection Technologies Reduce Downtime By Steve Stone Blockage detection for coal chutes is a key factor in the transfer of solid material in bulk handling applications. When moving coal continuously over long (or short) dis- tances, it's important to consider direction- al changes to maintain high productivity and keep production schedules on point. As coal transfers from conveyors to inlet/ outlet chutes to holding areas, such as silos and bins, to trucks or railcars for distribu- tion, the potential for jams and blockages is high. Production losses from a blocked trans- fer chute can cost thousands of dollars per hour. Once production stops to clear the blockage, downtime can stretch from a few minutes to a few hours depending on the severity of the blockage and the required cleanup. The unexpected disruption trans- lates to lost production, lost material, possi- ble equipment damage and safety hazards for employees. With the potential for hours of down- time, equipment damage and lost rev- enue, installing a detection system be- comes a necessary and inexpensive way of pre-empting blocked chute debacles. Point-level monitoring is the most com- monly used technology for avoiding block- ages. A point-level system indicates the blockage and sends a notification signal to a predetermined location, most commonly a control room. The point-level technologies range from invasive to non-evasive. The technol- ogy used depends on a variety of factors. Each technology has its place in the opera- tion of bulk solids handling and processing depending on the application. Selecting Chute Blockage Detection Systems Point-level technologies for blockage de- tection fall into two categories, contact (evasive) and non-contact (non-evasive). Contact technologies are vibrating devices (tuning fork type), capacitance (aka admit- tance), and mechanical devices (such as tilt switch, either mercury filled or non-mercu- ry filled) where contact with the material is required. Non-contact devices are micro- wave switches, acoustic switches and nu- cleonic (nuclear) switches where contact with the material is not required, but the material needs to be viewed by the sensors. Most non-contact technologies require a sender and receiver to communicate with each other mounted opposite of each other. With several options available for point-level blockage detection, what cri- teria should be followed? There are sever- al items to consider during the selection process. Points to consider are the material used (powder, granules, lumps, etc.), the vessel transferring or holding the material (conveyor, chute, etc.) and the environment (high temperatures, noisy, wet, dusty, etc.). Some questions to consider about the process: • What type of material? The material processed will determine the technology one uses. Microwave technology will pass through the material whereas acoustic technology and contact switches will not. Size of the material also matters. Is the ma- terial diameter large or small? The spacing gaps between the compacted large ma- terials will allow certain technologies to pass through, making it ineffective. Is the material abrasive? If it is abrasive, contact technology could be damaged from the material. • What are the environmental conditions? Is it a heavy dust area that will coat the sen- sor, therefore causing false trips? If buildup potential is high, coating and buildup on contact switches will cause false tripping. Are there spray washers or is it a high mois- ture area? If yes, it is best to look at a system that is not affected by moisture. Does the material cause static electricity? If the ves- sel containing the material is not ground- ed, the static eletricity will cause material buildup around all the process measure- ment devices. The downtime associated with coal chute blockages can cost thousands of dollars per hour in lost production.