Flow Rate Measurement in Pneumatic Conveying Pipelines with an Audio Frequency Acoustic Meter Stephen Tallon , Clive E Davies, Peter Wypych + , and David Hastie + Industrial Research Limited, PO Box 31-310, Gracefield, Lower Hutt, New Zealand + University of Wollongong, Wollongong NSW 2522, Australia 1 SUMMARY This paper describes measurements of acoustic waves propagating axially through pneumatic conveying lines, operated in both dilute and dense phase conveying modes, and the application of these measurements to analysis of the flow and in-line measurement of the solids mass flow rate. In dilute phase flows, earlier work has demonstrated that measurements of the velocity and attenuation of sound pulses introduced into the pipeline can be used reliably to estimate the solids velocity and concentration, and hence mass flow rate. For the larger particles used in this current work however, knowledge of the slip velocity between the particles and the gas becomes important. For dense phase flows, measurements of a continuous introduced sound signal are used to calculate slug periods, slug velocities, and slug densities. The product of the measured slug density and velocity is shown to correlate approximately with the solids mass flow rate, and to have some potential as the basis of an in-line flow measurement instrument for dense phase flows. 2 INTRODUCTION The operation of pneumatic conveying lines is generally described as either dilute or dense phase flow, depending on the respective flow rates of the gas and solids phases [1]. Flows at high velocities where the particles are well suspended, and do not remain in contact for long with neighbouring particles, are generally described as dilute phase. The high velocities required can result in high rates of abrasion of the pipeline and attrition of the particles, and the energy requirements are generally higher. However, this mode of conveying still has many useful applications, for example where the conveying line is part of a process that involves processing a dilute solids phase, such as in pulverized fuel combustion. Dilute phase conveying systems are also generally simpler to operate. If the conveying velocity is reduced, the behaviour of the flow changes and the particles begin to concentrate near the bottom of the pipe (for horizontal conveying) and may flow as a dense bed of material or as periodic waves or slugs of particles. These flows are generally described as dense phase flows. The lower velocities employed can result in lower rates of damage to the particles and pipeline. Dense phase systems, however, generally require more careful control over the design and operation of the pipeline to ensure a stable flow. An increasingly common practice, for example, to improve the stability is to use proactive methods at the solids inlet and along the pipeline to promote uniform and regular plugs of material [2]. In both dilute and dense phase conveying there are processing systems where it would be beneficial to have an in-line measurement of the solids mass flow. For example, where flow splitters are used in dilute phase conveying lines, the flow in the downstream branches is often indeterminate. Also, in many conveying systems that form intermediate steps in a process it is often not practicable to measure solids flows as they are entering of exiting the conveying line. An in-line measurement of the flow in dense phase conveying systems may also give better, and more immediate, information on the quality and stability of the flow in the pipeline. Previous work by the authors [3-5] has demonstrated the use of an acoustic based instrument for solids flow measurement in dilute phase conveying. The acoustic signals lend themselves well to the analysis of such systems; the signals are strongly dependent on a number of key process variables including the flow velocity of the gas and solids phases, the solids concentration, and a number of the particle physical properties such as particle size. Acoustic signals will pass through some systems that are opaque to light, and can be generated and recorded without needing transducers that intrude into the flow stream. The transducers required are also relatively inexpensive, and simple and reliable to operate. In earlier work [3-5], an in-line measurement of dilute solids flows was effected by introducing short pulses of sound into the pipeline and taking measurements of these pulses travelling axially upstream and downstream through the pipe. The solids mass flow rate was calculated from independent measurements of the solids velocity