Cement Americas

WIN 2018

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www.cementamericas.com • Winter 2018 • CEMENT AMERICAS 25 FEATURE Time domain reflectometry (TDR) in the megahertz (Mhz) range is among the most accurate methods used for solid lev- el measurement. A TDR device transmits low-intensity elec- tromagnetic pulses of approximately one nanosecond width along a rigid or flexible conductor. These pulses move at the speed of light. When the pulses reach the surface of the product to be mea- sured, the pulses are reflected with an intensity that depends on the dielectric constant. The TDR device uses a cable or rod that is inserted into the vessel. TDR devices are not affected by coating or buildup of dust on the probe. Radio frequency (RF) capacitance is also quite accurate for solids level measurement. RF capacitance devices operate in the low MHz radio frequency range, measuring admittance of an alternating current circuit that varies with level. RF devices tend to be more affected by any kind of coating or buildup on the probe. The fact that these are contact measurement methods can become a problem, because one must account for the drag force of anything that comes in contact with the product in a solids environment. If one uses a probe in cement, for example, there may be too much drag force from the mate- rial pulling on the cable, which may exceed the roof's rated capacity, possibly causing the roof to fail. This can be dealt with, but requires additional engineering considerations. Non-Contact Methods One non-contact option is a nucleonic device mounted on the side of a vessel, which sends a beam of nuclear energy through the silo, vessel or tank. On the other side a detector determines how much of the radioactive signal is blocked by the product. These devices are not usually considered desir- able for any food or beverage measurement application. They also require permitting and periodic inspection by gov- ernment agencies to ensure they are properly maintained. Nucleonic devices are the only way to provide reliable mea- surement for high-pressure vessels where it is not desirable to have a device reaching into the tank. They are an excel- lent option for retrofitting an existing vessel where there are logistical challenges to opening up the vessel or where one does not want to disrupt the existing process. Non-contact methods that use ultrasonic or radar operations are the newest technology deployed for solids level measure- ment. Ultrasonic devices operate in the kilohertz frequency range. Some recently developed solids measurement devic- es calculate the volume inside a tank by using multiple ultra- sonic signals to conduct 3D modeling. While quite accurate, such devices require significant startup time, as there can be no factory calibration. Radar devices using the gigahertz frequency range are much more accurate than ultrasonic devices – on par with, or slightly better than TDR and RF capacitance devices. In addi- tion, radar is less affected by any kind of dust particles in the open air above the product. Where dust is a problem, preventive measures can be taken with radar antennas, including developing antenna designs that are less susceptible to build up. Radar devices also have a longer range in the vessel, which can be a significant bene- fit for very tall silos. One example of new non-contact radar technology is Kro- hne's OPTIWAVE 6300 C non-contact radar, a frequen- cy-modulated continuous-wave (FMCW) level meter for measuring level, volume and mass of powders, granulates and other solids. It gives a much more stable measurement than pulse radar solids levels in dusty environments. The radar product was co-developed by KROHNE with the assistance of a research program at Ruhr-University Bochum, which was dedicated to creating the optimal antenna shape and signal processing software to handle solids. One important advantage is that it is a loop-powered device, which does not require running AC power or 24 volt DC as a separate set of wires up to the instrument. No purging is required, so operators do not have to bring in a nitrogen or air purge to the top of a vessel – a significant benefit, since these vessels can be 150 ft. tall. The antenna portion of the instrument is a permanent part of the vessel, so if any maintenance or pipe work has to be done on the electronics, the electronics can be quickly decoupled from the drop antenna portion with no loss of process seal. It also uses a unique drop antenna design for very dusty atmospheres. The software in the radar has been specifically designed to deal with the materials' low dielectric. It can also work accu- rately even in the face of very rapid level changes if there is a dramatic collapse of product. The device also handles extra- neous noise better because it is tuned to lower dielectric products with less reflection and less signal available. Now, whether you are measuring fine particles like carbon black, talcum, or fly ash, or much coarser products like aggre- gates or rocks, there is a radar technology that can easily adapt to provide the best possible results with high reliability. Choose the Right Technology for Solids Level Measurement There are many available solids level measurement tech- nologies available. Users should weigh the advantages and disadvantages of each and consider the importance of accu- racy for their particular application. Those requiring a high level of accuracy should consider new radar technology that can operate in the dusty conditions often found in large silos and other vessels. Tailored to the low dielectric of these materials, and with the ability to track very rapid changes that may lead to dramatic collapse, radar technology is an excellent new option. Joe Incontri is director of marketing, Americas at Krohne Inc.

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