An zyxwvu Intrinsically Safe Electrical Tomography System Trevor zyxwvutsr A. YORK', zyxwvut Member,IEEE, Quinton SMIT', John L. Davidson' and Bruce D. GRIEVE2 'Dept. Of EE&E, UMIST, Manchester, UK, e-mail zyxwv : tay@umist.ac.uk ' Syngenta, Huddersfield, UK, e-mail: bruce.grieve@syngenta.com zyxw Abstract- In recent years industrial tomography has emerged as a valuable tool for interrogating the in- ternals of a variety of process vessels. A popular ap- proach is to inject electrical current and then from measurements of the resulting boundary voltages to reconstruct a cross-sectional image representing the distribution of materials in the vessel. In many cases the process takes place in a hazardous environment and consequently there is a need for certified safe equipment. This paper reports on the design, con- struction and testing of the worlds first intrinsically safe electrical tomography system. To achieve certifi- cation zener barrier diodes and intrinsically safe relay modules provide electrical isolation from the hazard- ous environment. The instrument is certified for op- eration in a zone 0 environment and has been operat- ing successfully on a production pressure filter for 2 years. IiieX Term-Process Tomography, Pressure Filtration, Electrical Resistance, Intrinsic Safety I. INTRODUCTION The basic aim of tomography [l] is to determine the distribution of materials in some region of interest by ob- taining a set of measurements using sensors that are dis- tributed around the periphery. The measurements are non-intrusive. perhaps penetrating the "wall" of the ves- sel but not entering into the medium, and also, ideally, non-invasive such that the sensors are located on the out- side of the "wall". Each measurement is affected, to a greater or lesser degree, by the location of materials in the region of interest. Typically a source of energy is im- posed on the vessel from one orientation and a number of measurements are taken by distributed sensors to create a projection of data. The source is then moved to provide another projection and so on around the vessel until a frame of data is accumulated. For medical applications the source of energy is commonly X-rays which are at- tenuated as they pass through the vessel in a way that de- pends on the density distribution of materials. Usually the frame of data is translated, using software, into a cross-sectional image representing the distribution of ma- terials. Tomography has enjoyed considerable success in medical applications, for instance identification of tu- mours, particularly using X-rays as a source of energy and more recently using magnetic resonance, ultrasonics and electrical excitation. Tomography, therefore, is in- herently complex, involving energisation of a target re- 0-7803-7912-8/03/$17.00 zyxwvutsrq 0 2003 IEEE gion, multiple sensor electronics, data acquisition and data inversion. Fuelled by developments in personal computing and sensor design, research into applications of tomography to industrial processes began to gain popularity about 10 years ago. A summary of the state-of-the-art in this field can be found in, for instance, the "Proceedings zyx of the World Congress on lndzistrinl Process Tomogrn- phj,"[2,3]. "Process Tomography : Principles, Tech- niques and Applications"[4] or in special issues of "Mensirrenient Science and Technolog?> "[ 5 -6.71. Tech- niques have been influenced by successes in medicine. however. in many cases. the demands of industrial appli- cations are significantly different. It is not uncommon to require many cross-sectional images per second, at low cost, using "mobile" equipment. For these reasons nucle- onic techniques are often inappropriate and alternatives have emerged. For instance, the literature includes de- scriptions of instruments that are based on acoustic, opti- cal. infra-red and microwave sources of energy. A par- ticularly successful approach for industrial applications involves electrical tomography. Three "low" frequency electrical measurement modalities are used to determine distributions of conductivity (resistance), permittivity (capacitance) and permeability (inductance). The state- of-the-art in electrical tomography systems has been re- viewed by York [SI . Electrical tomography has motivated applications for process design and validation, on-line monitoring. and control. This can lead to improved product quality and process efficiency with improved profits through reduced time and waste. There are also important consequences for environmental issues. Typical fields of application in the early years of development included 2-phase flow, fluidised beds, mixing and environmental monitoring. Cross-sectional images can be interpreted to yield pa- rameters of interest, for instance void fraction and flow regime or mass flow of components if correlated images from pairs of sensors are used. Many industrial processes operate in hazardous envi- ronments. For instance, the use of solvents presents a po- tentially explosive atmosphere. In order to exploit the benefits of electrical tomography in such cases it is es- sential to provide certified safe equipment. This paper describes the design of the world's first, certified, intrin- sically safe (I.S.) electrical tomography system. This has been designed for a research project that is seeking to monitor progress during pressure filtration of agrochemi- cal products [9] but could be readily applied in other ap- plication areas which may or may not involve tomo- graphic processing. The following sections of the paper 946