GB2211938A

GB2211938A - Acoustic monitoring of plant operation

Info

Publication number: GB2211938A
Application number: GB8725918A
Authority: GB
Inventors: R Brian Charles George Haywood; R Roger David Watkins; R Christopher Brian Scruby
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1987-11-05
Filing date: 1987-11-05
Publication date: 1989-07-12
Also published as: GB8725918D0

Abstract

Apparatus is described for monitoring acoustic emission (AE) from a plant in which an operation, such as mixing components together, is being performed. The apparatus includes a signal processing and digitising circuit (20a, b or c) and a computer (22) to enable real-time digital signal processing to be performed. The monitor (10) may be used to monitor drying of wet particles, or the coating of solid particles with a liquid. The acoustic emission varies rapidly with liquid percentages less than about 5%. Application to solid/solid and to solid/liquid mixing is described. <IMAGE>

Description

Plant Monitor This invention relates to a method and to an apparatus, for monitoring operation of a plant.

It is well known that when solid particles are mixed in a container, or are mixed with a liquid, acoustic emission occurs during the mixing process, for example due to impacts between particles or between particles and the container. The acoustic emission varies in accordance with the nature of the material being mixed, for example that from coarse sand is different from that from powdered chalk.

According to the present invention there is provided a method of monitoring operation of a plant in which particles of a solid material are mixed with a liquid, and the proportion of liquid to solid is no more than sufficient to just coat the particles with liquid, wherein acoustic emission from the mixing process is detected and the amplitude of the acoustic emission is monitored.

Although for solid/liquid mixtures with between about 20% and 100% liquid the acoustic emission from a thoroughly-mixed mixture varies only slowly with the proportion of liquid, it has surprisingly been found that for much lower amounts of liquid (between zero and when the particles are just coated) the acoustic emission varies rapidly with the liquid quantity.

The plant whose operation is being monitored may be one in which the particles are being mixed with the addition of liquid in order to coat the particles, in which case the acoustic emission will decrease as the operation proceeds. Alternatively it may be a plant in which wetted particles are being dried, in which case the acoustic emission will increase as the operation proceeds.

The present invention also provides an apparatus for monitoring operation of a plant. In the simplest case the acoustic emission may be detected by coupling an ultrasonic or acoustic transducer to the container in which the operation is proceeding. Preferably an ultrasonic transducer is coupled to the container by a rod wave-guide which transmits extensional waves. Preferably too the plant is monitored in real-time by amplifying signals from the transducer, passing the amplified signals through one, two, or several band-pass filters in parallel, converting the signals from the band-pass filters to digital form, and supplying the digital signals to an analysing computer. In the preferred embodiment the amplified signals are supplied to thirteen band-pass filters whose outputs are sampled, digitised, and multiplexed for transmission to the computer.

Such a plant monitoring apparatus is particularly useful for monitoring the progress of a mixing process, as the nature of the acoustic emission will vary as the different components are mixed into each other but will reach a steady state when the components are thoroughly mixed. When this steady state is achieved the mixing process can be terminated. That is the case both for solid/solid mixtures and for solid/liquid mixtures. As discussed above such an apparatus can also be used for monitoring a process in which solid particles are dried while being mixed, or in which a liquid is gradually added to solid particles being mixed in order to coat the particles.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of a plant monitor, three alternative electronic circuits being shown; and Figure 2 shows graphically the variation of acoustic emission from thoroughly-mixed solid/liquid mixtures with the proportion of liquid present.

Referring to Figure 1 there is shown a monitor 10 for monitoring operations within a mixing vessel 12. A thin steel rod 14 of diameter 1.0 mm and of length 2 metres is welded at one end to the outside of the vessel 12, and acts as a waveguide for ultrasonic extensional waves generated within the vessel 12. To the other end of the rod 14 is attached a broadband ultrasonic transducer 16 connected to an amplifier 18. Amplified signals provided by the amplifier 18 are supplied, via one of three alternative signal processing and digitising circuits 20a, 20b or 20c (described below), to a computer 22 with a display. The computer 22 is arranged to perform further processing of the digital signals and to display the results. The monitor 10 can thus perform real-time digital signal processing of acoustic emission occurring within the vessel 12.In the circuitry shown, two three-position switches 24 are provided to enable the signals to be passed through a selected one of the circuits 20a, 20b and 20c; it should be understood that a practical plant monitor might incorporate only one such circuit 20a, 20b or 20c, in which case no such switches 24 would be necessary.

The first signal processing and digitising circuit 20a comprises a band-pass filter unit 30, which provides an output signal equal to the root mean square (r.m.s.) value of the signals received from the amplifier 18 within a single preset frequency range. This output signal is supplied to an analogue-to-digital converter (ADC) 32 which supplies corresponding digital signals to the computer 22.

The next signal processing and digitising circuit 20b comprises two band-pass filter units 34 and 36 arranged in parallel, which provide output signals equal to the r.m.s.

values of the signals received from the amplifier 18 within, respectively, a low frequency band and a high frequency band. These two output signals are supplied to a unit 38 which determines their ratio and provides a digital output signal representing the ratio. This digital signal is supplied to the computer 22.

The other signal processing and digitising circuit 20c comprises thirteen different band-pass filter units 40 (only four are shown) arranged in parallel, whose outputs represent the r.m.s. values of the received signals in thirteen different frequency bands covering the range 10 kHz to 1 MHz. These outputs are supplied to a sample-and-hold unit 42 connected to an analogue-to-digital converter 44, the corresponding digital signals from which are supplied through a multiplexer 46 to the computer 22.

Thus the monitor 10 using any one of the circuits 20 a-c in conjunction with the computer 22 enables real-time digital signal processing of any acoustic emission created by the operations performed within the vessel, and hence enables a range of different such operations to be monitored. The information available to the computer 22 depends upon which circuit 20a, b or c is used, different circuits consequently being suitable for monitoring different operations. The first circuit 20a only enables the r.m.s. acoustic emission to be monitored, and hence is suitable for monitoring those operations in the course of which the r.m.s. acoustic emission varies, for example (as discussed further below) the drying of a particulate solid material. The next circuit 20b only enables the ratio of high frequency to low frequency acoustic emission to be monitored.Finally the other circuit 20c enables changes in the entire acoustic emission spectrum to be monitored.

It will be appreciated that a monitor might differ from that described above. For example, the rod 14 used as a waveguide, although advantageous where the vessel 12 is hot, or where access to the vessel wall is otherwise restricted for example by cladding, may be dispensed with and the transducer 16 connected directly to the vessel 12.

Again, in the circuit 20c, the number of band-pass filter units 40 may differ from that specified above; the number preferably is at least ten.

A wide range of different processes can thus be monitored. These fall into two broad categories: firstly where the components present in the vessel 12 are gradually changing during the process; and secondly those where the components present in the vessel 12 do not change but changes occur in either the degree of mixing or in the particle size during the process. One example of the second category would be the mixing of sand with cement, where as mixing proceeds the acoustic emission will tend to an asymptotic value corresponding to a thoroughly mixed mixture. Another example of the second category would be the mixing of small damp particles of for example a fertilizer leading to agglomeration: as the mean particle size increases the spectrum of the acoustic emission changes with the lower frequency signals increasing.

Referring now to Figure 2 there is shown graphically the r.m.s. acoustic emission (AE) from a thoroughly mixed sand/water mixture for different proportions of water. The value of the graph at any particular water percentage (by volume) thus indicates the asymptotic value achieved after thorough mixing as discussed above. It will be observed that for water percentages between about 10% and 100% the AE is a slowly varying function of water percentage.

However for water percentages less than about 5% the AE varies rapidly with the water percentage. The AE is a maximum when the sand is totally dry, and decreases to a minimum when there is sufficient liquid to coat all the particles (the corresponding percentage depending on the size of the particles). Hence by monitoring the AE during a drying process the achievement of dry particles can readily be detected. Equally, in a process for coating grit with resin for making grinding wheels, by monitoring the AE the achievement of a coating on all the grit particles can readily be detected. Each of these processes falls into the first category mentioned in the previous paragraph.

Claims

1. A method of monitoring operation of a plant in which particles of a solid material are mixed with a liquid, and the proportion of liquid to solid is no more than sufficient to just coat the particles with liquid, wherein acoustic emission from the mixing process is detected and the amplitude of the acoustic emission is monitored.

2. A method as claimed in Claim 1 wherein real-time digital signal processing of acoustic emission signals is performed.