Cement Americas

WIN 2018

Cement Americas provides comprehensive coverage of the North and South American cement markets from raw material extraction to delivery and tranportation to end user.

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24 CEMENT AMERICAS • Winter 2018 • www.cementamericas.com FEATURE Dust particles from filling a vessel pose another measure- ment challenge, causing vapor and coating inside the ves- sel. In some cases, especially with grains and corn, these dust particles cause an explosive environment and can self-ignite. Another element challenging solid level measurement is the dielectric of the product. Liquids, especially water- based ones, are simple from a dielectric standpoint; an electromagnetic pulse will create a very large reflection. By contrast, solids generally have a low dielectric. There is a lot of space between the particles, or the particles them- selves have a low dielectric. With grains and other solids, sound energy is absorbed and there is a weak echo. These non-reflective solids with a low dielectric have a negative effect on solid level measurements using some methods, including time domain reflectometry (TDR) and radio fre- quency (RF) capacitance. These general solid level measurement issues are made even more challenging by hybrid applications that may begin with a solid measurement, but then transition to liq- uid measurement. One example is corn milling, where corn is placed in a vessel, after which water is added and the mixture is heated to break down the corn into a corn mash. Another challenging hybrid example is thermomechanical and chemithermomechanical processes used in the manu- facturing of wood pulp. Commonly Used Solids Level Measurement Methods There are a variety of solid level measurement technolo- gies, which vary in accuracy and ease of use. The most com- monly used are manual gauging (bin-bobs), switches, time domain reflectometry (TDR), radio frequency (RF) capaci- tance, nuclear gauges, load cells, and ultrasonic, laser, or radar level meters. Table 1 provides an overview of the advantages and disadvantages of the available methods. Manual Gauging Using Mechanical Devices Manual gauging is very popular, but unfortunately quite inefficient and prone to failures. One frequently used man- ual gauging method is use of bin-bobs, a mechanical device consisting of a weight and a small motor, with a sensor at the top. Once a day, the motor releases the bob (a ball), which descends into the vessel on a cable until the weight lessens, indicating it has reached the top of the product surface. At that point, operators take a measurement and the motor then pulls up the cable. Switches are widely used for minimizing overfill and elimi- nating the chance of overdrawing or underdrawing product out of a vessel. Switches come in a variety of construction types, including vibrating fork, paddle, radio frequency capacitance or electromagnetic. Another popular manual method is use of a load cell, a transducer that is used to create an electrical signal whose magnitude is directly proportional to the weight force being measured. The load cell is mounted under the base of a tank vessel and takes a measurement of how much force is exerted by the weight of the product. Load cells are quite reliable. However, anything that causes the vessel to vibrate will translate into a vibration or fluc- tuation in the output signal. In addition, maintaining load cells can be a challenge; should anything go wrong, the entire weight of the vessel is on top of the load cell. Contact Methods Two methods that are quite accurate for solids level mea- surement are time domain reflectometry (TDR) and radio frequency (RF) capacitance. Both rely on a probe that is in contact with the product. Table 1 – Solid level measurement technologies Type Accuracy Cost Advantages Disadvantages Manual/mechanical Bin-bob Inexpensive Easy to use Inefficient Prone to failure Switch Good for minimizing overfill Load cell Very reliable Vibration causes fluctuation in output signal Contact TDR Very accurate Not affected by coating or dust buildup Must account for drag force RF capacitance Very accurate Affected by coating or dust buildup Must account for drag force Non-contact Nucleonic No disruption to existing process Good where there are logistical challenges to opening up the vessel Requires permitting and regular inspection Ultrasonic Radar Very accurate Less affected by dust buildup Longer range Handles extraneous noise

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