Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
Issue link: https://emj.epubxp.com/i/82657
PROCESSING SOLUTIONS bridge is set to measure an imbalance, caused by contact with the product, and triggers the alarm. While the develop- ment of various guard elements has improved their ability to ignore coatings, they are still subject to being fooled by a coating. Various probe styles have been developed, and some are actually flush with the chute wall, but they still are influenced by coatings. Mechanical or tilt switches use the principle of a floating element inside the chute. When the material rises to a preset level, the switch body is tilted by approximately 15°-25°, causing a con- ductive liquid (mercury in some cases) to produce an electrical connection across a pair of contacts, and activating the alarm. Simple and reliable in many installations, this device would seem less than optimum for use in abrasive and harsh environments. Nucleonic (nuclear) technology uses a radiation source and a detector, mounted on opposite sides of the chute. During normal free flowing con- ditions, the rate of absorption of the emitted radiation is low. It rises signifi- cantly when a blockage occurs, and is used to trigger the alarm. Proper posi- tioning and alignment of the compo- nents is required since the signals used are relatively weak. It does have the advantage of no contact at all with the process material; however, it is subject to licensing, regularly scheduled in- spections, and mandates the employ- ment of a nuclear safety officer for the site. These devices can have false trips caused by material building up on the wall of the chute, which necessitates either cleaning the chute, or making adjustments to the sensitivity setting, thereby leading to lack of an ability to see a real blockage Microwave technology uses the high- frequency electromagnetic waves of radar, with an emitter and a detector mounted on opposite sides of a chute. Alignment of the emitter and detector is important, as they must face each other rather precisely to minimize sig- nal losses. These devices are able to be tuned to accept free flowing material as the normal condition, and respond only to a level that attenuates the signal Acoustic switches employ relatively wide acoustic beam angles and thus are less prone to alignment problems than other technologies. www.e-mj.com SEPTEMBER 2012 • E&MJ; 159