Mass flow monitoring using electrical resistance tomography

Abstract

An ERT system is disclosed for measuring mass flow rate of an electrically conductive product along a pipeline. The system comprises two sensing modules 5, 5’ to be placed around an interior surface o

Description

Field of the invention The invention relates to monitoring of mass flow using electrical resistance tomography. Background of the invention Electrical resistance (or impedance) tomography, herein termed ERT, is a low-energy, high-speed technique that can be used to derive in-line, real-time process information regarding the nature and distribution of electrically conductive components within an industrial process space. An ERT system comprises three modules, namely a sensor module, a data acquisition module, and an image reconstruction module. The sensor module comprises an array of electrodes placed around the internal periphery of a process space, such as a pipeline. The electrodes can serve both as transmitting electrodes, to cause current to flow and thereby create an electric field within the process space, and as receiving electrodes, to sense the resultant field at different locations around the periphery of process space. Electrically conductive regions distributed within the process space will modify the electric field and therefore affect the output signals of the electrodes acting as sensing electrodes. The data acquisition module causes different electrodes about the periphery of the process space to operate sequentially as transmitting electrodes. While an a.c. current is applied via different transmitting electrodes, the data acquisition module records the resultant voltages at the receiving electrodes The image reconstruction module, by analysing the measured voltages created when a.c. current is applied using different electrodes around the periphery of the process space, calculates an image of the cross section of the process space, showing the distribution of the electrically conducting regions within the process space. In this way. an ERT system can monitor a flow of an electrically conductive product being transported within a process fluid, to determine the density of the product. ERT systems are known and have been used previously in a variety of applications. It is not therefore believed necessary, within the present context, to provide a more detailed explanation of the data acquisition module nor the image construction module. Instead, reference is made, for example, to WO2023 / 026045 which describes in more detail an ERT system used to monitor a chemical process in a column apparatus, and to EP 2992364 in which an ERT system forms part of an apparatus for monitoring the flow of mixtures of a fluid in a pipeline. In order to measure mass flow rate of a product, it is not sufficient to measure the density of the product in the fluid but it is also necessary to measure the velocity of the product. As explained in EP 2992364, measurement of velocity is a multiphase flow is complicated because the different phases may not flow at the same velocity. The latter reference provides a solution requiring multiple monitoring modules to measure, electrical permittivity, electrical conductivity and the velocity of at least one of the phases. In WO2020 / 104761, the present Applicant disclosed a mass flow monitoring system comprising one or more processors, a magnetic source operable to selectively provide a magnetic field through a section of flow conduit to be monitored and an electric source comprising a plurality of electrodes arranged around the circumference of the section of said flow conduit to provide conductive paths therein. The one or more processor is operable to select an electrode-pair, from the plurality of electrodes. In a flow density mode, the processor is operable to apply an electrical signal across the electrode-pair and measure the responsive electrical signal across one or more other electrode-pairs. In a flow rate mode, the processor is operable to cause the magnetic field and the plurality of conductor paths to be angularly displaceable relative to each other, and to measure the responsive electrical signal of a selected electrode-pair. The measured flow density mode responsive electric signals and the measured flow rate mode responsive electric signals are computed to determine the mass flow through said section of conduit. Summary of the invention The present invention provides an ERT system for measuring mass flow of an electrically conductive product along a pipeline, comprising two sensing modules to be placed an interior surface of the pipeline, one downstream of the other, each sensing module comprising an array of electrodes that intersect a common cross sectional plane and are circumferentially spaced from one another, and each sensing module being associated with a data acquisition module and with an image construction module that serve to determine the distribution of the product within the cross sectional plane of the respective module, the system further comprising a processor for correlating data derived at different times from the two sensor modules to determine the flow velocity of the product and thereby enable calculation of the mass flow rate of the product. The ERT system of the invention generates for each sensing module a series of tomograms showing the distribution of the product over the cross section of the pipeline as the flow passes the respective sensing module. At each sensing module, the distribution will change from one tomogram to the next, each being representative of a different “slice” of the flow through the pipeline, but the sequences of tomograms derived from the two sensing modules will be generally similar to one another. By comparison of the two sequences, it is possible to determine the length of time taken by a particular cross sectional slice of the conductive product in the fluid flow to travel between the two sensing modules, thereby allowing the velocity and the mass flow of the product to be determined. While the ERT system may be regarded as each sensing module having separate respective data acquisition and image construction modules, it will be appreciated that the modules may be implemented as functions performed by a processor. Furthermore, while each sensing module may be associated with its own processor, a single processor with sufficient power may serve as the data acquisition and the image construction modules of both sensing modules, as well as being the processor serving to correlate the data from the two sensing modules. The correlation of the data derived from the two sensing modules may be effected by correlating either one, or both, of the tomogram pixels calculated by the image reconstruction module and the raw data derived from the receiving electrodes. Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically the sensor module of an ERT system for monitoring mass flow of a product along a pipeline, Figure 2 depicts method steps employed by the ERT system of Figure 1, Figure 3 shows the current flow7 paths and the lines of equipotential within a cross section of the pipeline when current is applied to two adjacent electrodes, Figure 4 is a schematic diagram of the hardware of the ERT system of Figure 1, and Figure 5 shows calculated density images through the cross section of a pipeline taken at different times. Detailed description of the drawings. The measurement system in Figure 1 comprises an electrical resistance tomography sensor which comprises tw7o sensing modules, formed by arrays of electrodes 5 and 5’. The electrode arrays 5 and 5' are arranged around the interior of a pipeline 3 at locations that are axially spaced from one another. The electrodes 5 and 5’ within each array are centred at a plane of the pipeline 3 which is perpendicular to the flow axis. Whereas a plane has zero depth, each sensing location does have depth in the flow direction, because the elongate nature of the electrodes 5 and 5’. The electrical stimulation applied to the electrodes permeates in all directions into the material adjacent the electrodes 5. As such, each sensing location may be considered as a volume which is defined by (e.g. centred about) the plane. Each of the electrodes 5 and 5’ is formed from a conductive material, allowing electrical contact to be made between the electrodes 5 and the material within the pipeline 3. The electrodes 5 may be made from an erosion resistant material, such as, for example, stainless steel. The electrodes 5 may be inlaid into the pipeline 3 such that the inner wall of the pipeline 3 which is not covered by an electrode is formed from the same material as the pipeline 3. Alternatively, the electrodes 5 may be mounted on a collar which is fitted to the inner wall of the pipeline 3. The internal surface of the collar (e.g. the surface which w7ould be in contact with the material in the pipeline 3) which is not covered by the electrodes may be formed from an erosion resistant material. For example, the internal surface of the collar may be formed from a polyether ether ketone (PEEK) or another polyurethane material that has a sufficient chemical resistivity Each electrode array is shown in Figure 1 as being associated with a respective current source 6 or 6’ operable to apply an electrical stimulation to a pair of (or multiple pairs of) electrodes 5 or 5’, a voltage monitor 7 or 7’ operable to receive an electrical signal from a pair of (or multiple pairs of) electrodes 5 or 5’ following the permeation of the electrical signal through the material 2, and a controller 8 or 8’. While this diagrammatic representation helps in explaining that the apparatus comprises two ERT systems located one downstream of the other, it will be appreciated that a single current source, a single voltage monitor may be connected to both sensing module and the apparatus requires but a single controller. The or each controller 8 may be a programmable logic controller (PLC), such as, for example, a PLC manufactured by Bachmann electronic GmbH, Feldkirch, Austria. The controller 8 controls the current source 6 and the voltage monitor 7. The controller 8 also performs processing as described further below in more detail. Each ERT system is intended to a produce a series of tomograms, such as shown in Figure 5, of the density distribution of an electrically conductive product within the pipeline at the sensing location, the tomogram in a series varying with the time when it was generated. Figure 3, which is a schematic cross section of one of the sensor modules, shows the current line I and the lines of equipotential V which occur when a.c. current is applied through the electrodes labelled 5a and 5b. These lines I and V will however be modified if the electrical resistance of the medium through which the current flows is not homogeneous. The voltages measured using the electrodes labelled 5c to 5p will therefore depend not only on the current flowing through electrodes 5a and 5b, but also on the distribution of the conductive paths defined by the medium. By stimulating a current flow sequentially from different directions using different electrode pairs and measuring the voltages at the other electrodes, one can obtain data which when fed into an inverse algorithm will reconstruct tomograms, such as shown in Figure 5, that represent the density distribution of the conductive regions within the sensing location. Thus, as represented schematically in Figure 2, the steps carried by the ERT system at each sensing module are: SI Apply electrical stimulation, i.e. connected an a.c. current source to a pair of electrodes of the array, S2 Receive electrical signal, i.e. measure the voltages at the other electrodes of the same array, and after repeating steps SI and S2 using different pairs of electrodes in step SI S3 Determine the characteris'd c(s), i.e. calculate using an inverse algorithm an image of the conductivity distribution within the sensing location. The physical apparatus that may be used to perform the stimulation, voltage measurement and image calculation at each sensing module is shown diagrammatically in Figure 4. The controller 8 in this embodiment is essentially a micro-computer. The computer has a monitor screen 8e, with a keyboard 8f and a mouse as input devices. The computer also includes a central processing unit (CPU) 8a, random access memory (RAM) 8b, nonvolatile memory? in the form of a hard disk drive 8c, all communicating with one another via a data bus 8i. The computer also includes a network interface 8h. The computer is programmed to control two peripheral units, namely the voltage source(s) 6 and the voltage nionitor(s) 7 by way of an Input / Output (I / O) interface 8d. The program to bee run on the CPU instructs the current source 6 to energise different pairs of electrodes in the two sensor modules and to gather data from the other electrodes using the voltage monitor 7. This data is store in RAM for processing to arrive at tomograms at the different sensing modules. As so far described, the steps performed at each sensing location separately are conventional and have been used to produce tomograms as shown in Figure 5 indicative of density / distribution of conductive material within a pipeline. Hitherto, to measure mass flow, as described previously, separate equipment was required to obtain a measurement of the flow velocity of the conductive material. In the present invention, such a measurement is achieved by providing two ERT systems and comparing the data from the two systems. As material flows through the pipeline, the tomograms generated at each sensing module will change with time. Thus, the two tomograms shown in Figure 5 may be generated at the first sensor module two seconds apart. Two similar tomograms will be generated at the second sensor module when the same slice of the flow of the product arrives at the second sensor module. If, by cross correlating the data from the two sensor modules one determines that a maximum correlation is achieved after, say, six seconds, then one can deduce that this is the time taken for the conductive material in the flow medium to cover the distance between the two sensor modules. The cross-correlation may either be performed on the values of the computed pixels in the tomograms or on the raw data obtained from the voltage monitor when like positioned electrodes are used to apply the excitation current. The precision of the velocity measurement will in practice depend on the time between generated tomograms at each sensing module and the distance between the sensing modules. Increasing the distance between the sensing modules, will increase the precision of velocity measurement but as the distribution of the product with the measurement cross section may change as a slice travels along the pipeline the correlation factor will be decreased. Increasing the rate of generation of tomograms would of course increase the measurement precision but require higher processing power. In practice, the optimum separation of the sensing modules is determined in dependence on the available processing power. The invention enables production measurement to be achieved using one single technology (ERT) and permits ERT to be used in assessment of multi-phase flow, such as hydro-transportation of slurry in dredging and mining industries.

Claims

1. An ERT system for measuring mass flow of an electrically conductive product along a pipeline, comprising two sensing modules to be placed an interior surface of the pipeline, one downstream of the other, each sensing module comprising an array of electrodes that intersect a common cross sectional plane and are circumferentially spaced from one another, and each sensing module being associated with a data acquisition module and with an image construction module that serve to determine the distribution of the product within the cross sectional plane of the respective module, the system further comprising a processor for correlating data derived at different times from the two sensor modules to determine the flow velocity of the product and thereby enable calculation of the mass flow rate of the product.

2. An ERT system as claimed in claim 1, wherein each sensing module is associated with a respective data acquisition module and a respective the image construction module.

3. An ERT system as claimed in claim 1, wherein a single data acquisition module and a single image construction module is associated with the two sensing modules.

4. An ERT system as claimed in claim 3, wherein the data acquisition and image construction modules are implemented by means of a single processor.

5. An ERT system as claimed in claim 4, wherein the processor serves additionally to perform the correlation of the data derived from the two sensing modules.

6. An ERT system as claimed in any preceding claim, wherein the correlation of the data derived from the two sensing modules is effected by correlating the tomogram pixels calculated by the image reconstruction module.

7. An ERT system as claimed in any preceding claim, wherein the correlation of the data derived from the two sensing modules is effected by correlating raw data derived from receiving electrodes of the sensing modules.

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