Acoustic Measurement and Monitoring in Dry Particulate Systems: Inspiration, Education, Application, Appreciation

Techniques for the use of audio-frequency sound waves in the measurement of flow rate and particle size and the estimation of the volume of material contained in partially closed vessels are described and discussed. INTRODUCTION Non-invasive interrogation of systems and processes involving dry particulate materials is particularly challenging. X-rays and γ-rays, for example, offer the prospect of truly non-invasive monitoring of material inside metal pipe work or metal containment as typically found in industrial practice, but require extensive calibration and are usually subject to licensing regulations. Exploitation of inductance and capacitance is limited by constraints on the properties of the pipe work or process vessel holding the material which must be modified to be partly, and usually significantly, non-metallic. Interrogation by acoustic waves is not a universal panacea to these difficulties, but while especially attractive and effective in dilute flow systems, can also be used in some situations with bulk solids. In this paper we revisit earlier work on measurement and monitoring of flow rate in both dilute and dense flow systems, particle size measurement, the effect of frequency on acoustic propagation velocity in dense systems, and also report on work in progress on the use of Helmholtz resonance to determine the volume fraction of bulk solids in a partially closed vessel. DILUTE PNEUMATIC CONVEYING Natural pressure waves are generated in pneumatic conveying systems, particularly by reciprocating air movers. These waves propagate as sound waves along the pipe and in a dilute system such as pneumatic conveying lines can travel for some distance. Changes in the velocity and magnitude of these waves along the pipe contain information of the flow velocity and concentration of the gas and solids phases and can be used for flow measurement 1-3 . Approximately logarithmic decay in pressure wave intensity was observed with increasing particle concentration, and the conveying gas velocity correlated well with changes in the propagation velocity of the waves, as in figures 1,2. Measurements can be made using a single pressure sensor located downstream of the solids feed point, although it is necessary to minimise the dead volume between the pipeline and the sensing surface of the pressure transducer, and depends on the existence of a constant and reliable source of sound. To remove this latter limitation it was demonstrated 4 that an introduced wave of a constant frequency could also be used. The use of an introduced sound wave also allows a frequency to be picked that minimises interference effects from other sources of sound waves and from sound wave reflections related to the pipeline geometry. The preferred sound frequency is one which is low enough for the waves to propagate as a one dimensional wave. The frequency limit for this to occur decreases with increasing pipe diameter 4 , and is in the mid to low audible frequency range for typical pneumatic conveying pipe sizes. Tallon and Davies 4 report that the measurement method can be applied to either vertical or horizontal conveying. Measurement of the pressure waves at two consecutive points along the pipeline is also reported, so that a relative attenuation between the two points can be measured. This allows a localised measurement to be made, preferably in a region of fully developed flow. To further improve the measurement reliability it is possible to measure sound waves propagating in both an upstream and downstream direction from a centrally located sound source (figure 3), or between two axially spaced sound sources (figure 4) 5 . The velocity of the sound waves in the downstream direction is faster than the upstream velocity because of the effect of the convective motion of the gas phase that the waves are passing through. The convective velocity is calculated simply from the difference between the upstream and downstream velocities, i.e. the sum of the measured upstream velocity and downstream velocities divided by two. For turbulent and largely plug flow conveying conditions with small particles, the convective velocity is essentially equal to the mean gas velocity. To calculate a solids mass flow rate from the solids concentration, it is assumed that the solids velocity is related to the gas velocity by a small but constant difference, or slip velocity. The estimated solids velocity is then multiplied by the measured solids concentration to give a mass flow rate. The slip velocity varies under different flow conditions, and at different points in the pipeline where fully developed flow may not have developed 6 , but it has been shown that the assumption of a constant slip velocity has little effect on the accuracy of solids flow measurement if the system is calibrated against known solids flow rates 4 ; see figure 5. This calibration can be achieved, for 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 Independent measure of air flow rate (kg s) A ir m a s s f lo w r a te b y c ro s s c o rr e la ti o n ( k g s -1 ) 0.000 kg s 0.028 kg s 0.054 kg s 0.135 kg s 0.228 kg s 0.342 kg s 0.508 kg s Equal value line Solids mass flow rate Figure 2. Air mass flow rate calculated from the increase in the sound wave propagation speed due to the convective flow of the gas phase. Sound velocity measured between two points 3m apart. The measurement is influenced by the solids flow rate. 0 1 2 3 4 5 6 0 0.05 0.1 0.15 0.2 0.25 0.3 Volumetric solids concentration (%) N a tu ra l L o g a ri th m o f P re s s u re V a ri a n c e ( P a 2) Figure 1. Change in intensity of natural sound waves in a pneumatic conveying line with changing solids concentration. Measured at a single point downstream of the solids feeder. example, by injecting a known mass of solids into the system and integrating the response over time 5 . Solids flow rate measurement has been demonstrated in dilute conveying systems up to 8” in diameter 7 (see figure 6), for conveying velocities up to 35m s 1 , and solids concentration up to 0.2% by volume. Better results are achieved if the measurement is made a sufficient distance downstream from the solids feed point or from pipe bends so that the flow is more fully developed. Theory for the propagation of sound waves through a dilute suspension is described by Tallon, 8 based on theory by Gregor and Rumpf. 9 Accurate calibration of the relationship between attenuation and solids concentration is difficult to achieve theoretically, but good correlation between measured and calculated acoustic velocities was observed. Changes in the propagation velocity due to changes in the particle size could also be predicted and it is possible to use this to give an in-line measurement of particle size, and to compensate for particle size effects on the mass flow rate measurement 10 . DENSE PHASE FLOW SYSTEMS Sound waves will also propagate through dense particle beds and can be used for measurement of a number of flow and state properties in dense flow systems such as downcomers, fluidised beds, dense phase pneumatic conveying and granulation systems.