Coal Age Magazine - For more than 100 years, Coal Age has been the magazine that readers can trust for guidance and insight on this important industry.
Issue link: http://coal.epubxp.com/i/805515
40 www.coalage.com March 2017 operating ideas continued • Where will the sensor be mounted? Mounting of the sensor will depend on the location of the equipment and where one wants the blockage detected. It brings into consideration what mounting accessories one will need and cable lengths required to communicate the data to the transmit- ter. The area size of the chute or silo being monitored also comes into consideration. Non-contact detection systems have a minimum and maximum range. If the area is too narrow, it may fall into the systems blanking range, making the system use- less. For contacting detection systems, the probe length would need to be thought out. • What type of output is needed? Once the potential blockage is detected, how will that information be processed and what will be done with that information? Do they want 4-20 mA output? Do they want it to trigger an alert or an automatic action? Where should the signal be sent? Do they want to have local indication or remote in- dication? Should the signal be viewable at the location of the equipment or have the information sent to a control room? Point-level Technologies for Chute Blockage Detection After factoring in the process conditions and the process needs, selecting the point level technology is a matter of reliability, ease-of-use and available budget. Mechanical Switch — Mechanical or "tilt" switches use the principle of a "hinged" element inside the chute. When the ma- terial rises to a preset level, the switch body is tilted by approximately 15° to 25°, causing a conductive liquid (mercury, in some of the older switches) to produce an electrical connection across a pair of con- tacts, activating an alarm. This method is a simple and reliable method for many installations when used with dry prod- ucts and in dry conditions. They are not recommended for harsh or abrasive ma- terials or wet, dusty environments, which are typically found in coal applications. These switches are mechanical in design so they can become damaged due to large, rough solids impacting them. Preferably, the switch should be mounted hanging inside a chute at a level that the material won't reach until a blockage occurs and under the feed conveyor in order to pre- vent material from beating on it. They are also susceptible to failure due to buildup when used in wet and dusty environments due to buildup potential. Capacitance Probe — Capacitance (Admittance) technology works on the principle of applying a small radio fre- quency voltage to an element and mea- suring the capacitance in Pico farads of the element as an antenna installed into the chute. An electrical "bridge" is set to measure an imbalance, caused by con- tact with the product and triggering the alarm. While the development of various "guard" elements has improved their abil- ity to ignore coatings, they are still subject to a false trip by a coating. Varying probe styles have been developed and some are actually flush with the chute wall but they are still subject to coatings. Capaci- tance switches also require calibration to the material being detected and, in some cases, require a large surface area for a plate probe for chute detection or probes for containment like silos. Installation re- quires inserting a probe through an open- ing in the vessel so that sensing element is positioned at the desired level. In the case of a flush-mounted sensor-plate, the unit is mounted to a cutout in the vessel at the desired level. Indication results when the material level reaches the probe. Like the tile switch, this technology is best for dry products and dry conditions since abra- sive or wet material can cause false trips. Vibration Probe — Vibrating tech- nology uses the principle of exciting a piezo-crystal to induce vibration on to a set of tines. When material touches the tines, the vibration frequency is dampened and an alarm relay is triggered. Because of the mechanical design, the vibration switch is susceptible to mechanical contact and high vibration. It works best for dry gran- ular material that is small in particle size. It is not recommended for high-impact applications. These systems are potential- ly subject to issues of false indication due to buildup on the tines. Washing the tines periodically is recommended. Like simi- lar invasive probes, the vibration switch is inserted into an opening in the vessel so that it is positioned at the desired high level. Indication results when the material reaches the tines. Nucleonic Switch — Nucleonic (nu- clear) technology uses a radiation source and a detector, mounted on opposite sides An acoustic switch can be mounted to a chute to monitor material levels.