Radio-frequency slurry-density measurement for dredging pipelines

Hydraulic dredgers make use of a density meter to measure the instantaneous density in the slurry transport pipeline, primarily for process control and production calculation. the current ‘golden’ standard for slurry density measurement is the radioactive density meter. It is based on a slurry density-dependent absorption of gamma radiation. As such, it contains a radioactive source. the use of a radioactive source is legally justified [72], as long as there are no suitable effective non-radioactive alternatives. Moreover, transport and handling of devices containing radioactive material is often difficult and costly due to governmental regulations, certified personnel is required to operate radioactive devices, and the use of radioactive sources is a sensitive issue in society. The motivation for this research is to find a non-radioactive alternative for this slurry density meter. In this project, we have developed a prototype of an alternative density meter, based on transmission of electromagnetic waves through the slurry: the Radio Frequency (RF) density meter. The electromagnetic slurry density measurement relies on the differences in (electrical) permittivity and conductivity of water and soil, which result in a change in propagation time and amplitude of an electromagnetic wave, as it travels through the slurry. Though the permittivity of the soil varies slightly for different soil-types, this variation will only have a limited influence on the density measurement, due to the large difference in permittivity between water and soil. The operating window of the radioactive density meter covers the entire practical density range ¿ = [1.0, 1.8] ton m3. It is insensitive to the electrical conductivity of the slurry, and is commercially available for pipe diameters up to 1.3 m. In contrast, we restrict the operating window of the prototype RF density meter to densities in the range ¿ = [1.0, 1.8] ton m3 and an electrical conductivity in the range s = [0.05, 1.0] S m. Though the prototype has a /.2 m diameter, the pipe diameter is primarily limited by the availability of high powered RF amplifiers. In the course of the project, we built and tested two prototypes. Both prototypes are ‘hybrid’ density meters: they also contain a standard radioactive density meter, which enables us to make a direct comparison between the two methods. To measure the change in propagation time as a function of varying density (and conductivity), we developed an amplitude-modulated continuous-wave phase-shift measurement circuit. Conceptually, an electromagnetic density measurement seems an easy task: electromagnetic waves are transmitted through the slurry. The permittivity of the slurry determines the propagation velocity of electromagnetic waves and its conductivity attenuates the waves. By measuring these two parameters, we can calculate the sand volume-fraction and thus the density of the mixture. However, complications arise from the interactions of the electromagnetic waves with the metal pipe, the near-field behavior of the electromagnetic waves, and the gradual transition from propagation to diffusion. Moreover, the conductivity of the slurry does not only attenuate the signal, it also induces a significant phase-shift. The first prototype contained an abrasion resistant polyurethane coating, which yielded yet another unanticipated side effect: a whispering-gallery mode. therefore, the second prototype has a bare metal pipe wall. Real-life tests of the second prototype at a cutter dredger uncovered some serious issues with the RF density meter. It was only after we constructed a new test setup, which enabled us to vary both the density and the conductivity of the slurry, that we discovered the source of these problems: the third-harmonic of the RF signal was interfering with the phase-shift measurement. The construction of this new test setup was an important step in the development of the RF density meter: not only can it be used to test the density meter under controlled conditions with a wide variety of densities, soil types, and conductivities; it can also be used as a calibration tool for the first production series of RF density meters. An accurate measurement of the conductivity of the slurry turns out to be vital for a proper RF density measurement. For this purpose, we have developed a conductivity sensor, a flange probe which can be mounted flush with the pipe wall. Because the measured conductivity exhibits hysteresis with respect to the phase-shift that it induces, we have tried to use the built-in conductivity measurement of a magnetic flow meter. Though the measured conductivity of the flow meter exhibits no visible hysteresis, its standard deviation is much larger than from the flange probe. Eventually, we have switched to a model which uses a measurement of the attenuation of the electromagnetic wave instead of a conductivity measurement. Using the new laboratory setup, we have tested the second prototype with the improved phase-shift measurement electronics, varying the density and the conductivity within the operating window. Using a curve- fitting procedure, we obtain an RF density measurement with a standard deviation of 0.043 ton/m3 as compared to the radioactive density meter, for ¿ = [1.0, 1.8] ton/m3 and s = [0.05, 1.0] S/m. Within the reduced operating window ¿ = [1.0, 1.6] ton/m3 and s = [0.05, 0.3] S/m, which corresponds to the circumstances observed during real-life tests at the Haringvliet, this standard deviation improves to 0.014 ton/m3. In order to transform the second prototype to an operational device with the required robustness, that can be sold to customers of IHC Systems as a reliable tool for river-water operated dredgers, the complexity of the construction should be reduced, abandoning the concept of replaceable antennas. Also the electronics should be revised, making these more reliable.