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March 2017 www.coalage.com 41 operating ideas continued of the chute. During normal "free flowing" conditions, the rate of absorption of the emitted radiation is low. It rises significant- ly when a blockage occurs and is used to trigger the alarm. Proper positioning and alignment of the components is required since the signals used are relatively small. This is the most expensive method to use and maintain since it is subject to yearly NRC licensing, regular inspections, and mandates the employment of a nuclear safety officer for the site. These devices are also the most difficult to remove and dis- pose of after their life cycle due to the nu- clear technology. These devices can have false trips by material building up on the wall of the chute, which necessitates either cleaning the chute or making adjustments to the sensitivity settings. Without proper care and maintenance, the ability to see a blockage can be compromised. Microwave Switch — Microwave tech- nology uses high frequency electromagnet- ic waves of radar that are pulsed between a sender and receiver. The units are mounted opposite each other outside of the vessel behind a wear-resistant window. Once ma- terial blocks the path between the sender and receiver, the receiver will no longer de- tect the complete transmission chain and sends a signal indicating blockage. This in turn closes or opens a switch (depending upon the electrical configuration), which then activates or deactivates an external circuit giving indication of blockage. Previously, the technology used lin- ear polarization that transmitted between the two units, which required the units to be perfectly aligned. The technology has evolved to circular polarization so the units do not have to be in perfect alignment, making installation easier. Since this technology does not make any intrusion into the chute wall but through a high-grade transparent window, the material cannot wear down the sensor face. For temperatures above 160°F, the sensors can be remotely mounted with a wave guide extension to direct the signal to a remote amplifier. This technology is best suited for dry granular material, lumps and fines that can absorb or reflect micro- wave energy. If the material cannot absorb or reflect the energy, a blockage will not be detected. They also should be in a dry envi- ronment. Moisture in the process causes a potential for coating the instrument with a dust film. Acoustic Wave Technology — Acoustic wave technology relies on a very low fre- quency (15 kHZ), high-powered transduc- er pair. This technology requires a pair of transducers to be located apart but aligned with each other. They are installed on either side of the chute or silo through a cutout in the vessel and do not require contact with the material. The transducers both pulse and receive signals from each other, and as soon as the signal is blocked, the attenuated acoustic signal is amplified and sent to the plant monitoring system. The low frequency and high power ap- plied to the sensors generates a pressure wave on the sensor face of each transduc- er, creating a self-cleaning feature. This pulsing pressure wave keeps material from adhering to the face and provides for main- tenance-free operation in critical applica- tions since they are immune to dust, parti- cles in suspension and water sprays. With all the available technologies, se- lecting the best system can be overwhelm- ing. As mentioned, all these technologies have a place in blockage detection applica- tions. This is a quick overview of the criteria for selection and the technologies available. To pick the best system, one needs to thor- oughly understand the process conditions. Explore all the available technologies and their track record in similar conditions to the environment and process. It is also ben- eficial to discuss the application and the point-level detection goals with the manu- facturer or manufacturer's representative to fine tune one's needs. They can guide one through the technologies available and give a fresh perspective on what technologies have been used in similar environments and processes and their track records, sav- ing time related to research. Steve Stone is vice president of sales and marketing for Hawk, a leader in innovative level measurement, positioning and flow solutions. He can be reached at (978) 304- 3000 or at firstname.lastname@example.org. Microwaves can also be used to monitor material flow. The sender units can be seen in the inset.