Modern level measurement technology for precise results unaffected by dust and buildup
In aggregate, quarry and mining applications, the conditions are often harsh, noisy and dusty. Level measurement devices need to be robust with total reliability as standard to ensure a continuous stress-free production process in building-material plants; whether for mixing systems with heavy vibrating conditions, volume measurements within dosing tanks, stone crushers or especially storing processes. For a concrete and cement manufacturer in Germany who has implemented guided wave radar sensors this level measurement technology offered multiple support for his storage process optimization. The TDRs with their high compatibility are used for consistently accurate continuous level detection under difficult varying process conditions, such as high pull-off forces, caking or condensate formation. The application is very dusty and sticky and the sensor has to be strong and fit into the existing process connection and monitoring system
Every year, over 40 million cubic meters of ready-mixed concrete is processed in Germany. Already 85% of all concrete and fresh mortar required on construction sites are mixed in ready-mixed concrete plants. The selection of the right measurement technologies, their correct configuration and smart positioning save substantial costs and add up to a significant improvement in the plants’ economic efficiency.
A German cement and concrete plant operator was looking for a supplier of level sensors in his narrow storage silos. The stored raw products and additives undergo different processing stages on the way to the final mortar and different kinds of concrete, ready-mixed concrete and special concretes. The solids in particular have different densities, particle sizes with different dust development and a differing moisture content. The manufacturer attaches more and more importance to the use of modern technologies combined with his traditional expertise knowledge. Since 1936, starting with a gravel quarry, the successful development of the company with several production locations and concrete plants proves him right. Therefore, a safe and high-quality measuring technology was indispensable for the equipment of the plant. It depends on functionality, ease of use and long life of the products, in accordance with the high requirements of the building materials industry. Within the steel storage silos of his production facilities in Southern Germany he implemented the guided wave radar sensor from UWT to replace defective level measurement sensors that could not fit the demanding process conditions in the long run. Cement and other different raw material solids such as sand and chalk need to be stored properly and transparently. The storage devices of the plant have a measuring range up to 14 meters. Within the single chamber silos there are temperatures around 80°C (176°F) and a process pressure of up to 0,8bar (11,6psi). We have no vibration and no electrostatic charge but a dusty atmosphere with some low moisture. Furthermore there was a heavy buildup of the materials to be detected.
NivoGuide® radar sensors are used for continuous level measurement in liquid and bulk solids applications. The measuring range of up to 75m (246ft) and its high sensitivity from a DK value of 1.5 make them versatile usable in silos, tanks and pipes. The guided wave radar does not contain any moving parts inside the process vessel. The microwaves are emitted at low frequencies, so that even with high dust intensity, no spurious reflection is generated. As a result, the energy content of the microwave can be used exclusively for determining the specific filling level within the storage processes. It is protected against thawing explosions and precisely measures ±2mm even under difficult process conditions. Two series have been implemented, the NivoGuide® NG 3000, which is specifically tailored to the needs of solids, and the NivoGuide® NG 8000 for applications in liquids, pastes, foam, slurries, oils as well as for interface measurement. The device versions are each equipped with arms that can be individually adapted to the application with rod and cable extensions. For the storage applications of the cement plant in Germany the guided wave radar of the NG 3100 series as rope version was the perfect solution. These TDRs are specially designed for solids measurement detection with an amplified coupling for high pull-off forces and a reliable design that is not affected by strong dust. Moreover the sensor is really flexible and needs no recalibration when the material changes. The measuring signal is guided on the rope and is reflected by the medium, received again at the sensor and converted into a level signal. This makes it possible to measure reliably even in narrow and tall containers, regardless of the formation of condensation or buildup.
Plug & Play though compatible solutions
During the storage processes there occurred some attachments to the boom, which can lead to faulty circuits. Thus the rope of the NG 3100 radar has been equipped with a special PA coating to prevent adhesion and also to improve the abrasion resistance of the ropes. Fine-grained bulk materials cannot settle in the mesh of the rope. Further, an optimized coupling to the bulk goods area was installed, which is characterized by an axial support of the probe inside the process connection and which differs constructively from conventional guided wave radar sensors. As a result, the tensile forces acting on the probe were optimally distributed, which allowed a load of up to 30kN. Different rope diameters of 4 and 6 millimeters are optimally designed for the prevailing tensile forces. The rope therefore does not travel under high load and supports the endurance of the sensor.
Generally, a thicker rope should be used for higher silos. In terms of their construction, the ropes of the TDR solid, unlike the guided radar for liquids, are equipped with twisted thicker strands. In addition, the weave of the ropes is such that in liquids a flexible rope and for solids a rather rigid rope that hangs tightly in the silo is used. The installed rope could be fixed to the bottom, so that it will not touch the silo walls during the filling and emptying process. The setup has been quite easy and it works reliable from the beginning. It was a plug and play installation and replacement. A consistent overfill protection and empty message is ensured. Filling properties such as moisture content, mixing ratio or particle size did not matter and made the planning easy. There are also accurate results when there are changes in the temperature conditions, pressure, density (usually around 1500 g/l) and DK values (differ between 2.0 – 3.0) as there wasn’t the need to adjust the respective medium. Further NivoGuide® NG 3000 series are designed according to the normative requirements of functional safety to ensure safety functions in a risk mitigation in accordance with SIL2.
The flexible and software-compatible technology fit seamlessly into the existing process connection and monitoring system. A menu-driven quick start wizard made the five-step commissioning easy, secure and cost saving for the operator. The attachment of only two wires were necessary (2-wire 4...20 mA/HART electronic) with a low voltage power supply (9,6...35 VDC). The integrated display allows check of measurement and echo curve on-site. The sensor can learn and will know which signals are changed and which are constantly in one place. Disturbing installations can be identified and service technicians' operations are avoided due to faulty level signals.