WO1991007231A1 - A method for separating fibre suspensions - Google Patents

Abstract

In a multihydrocyclone assembly comprising a plurality of hydrocyclones (1, 2), a vibration detector (10, 11) is mounted adjacent to or externally of each individual hydrocyclone. A malfunctioning hydrocyclone, for instance a hydrocyclone blocked at its apex outlet, produces a changed vibration signal which can be analyzed to establish erroneous function.

Description

A Method for Separating Fibre Suspensions

The present invention relates to a method for separat¬ ing fibre suspensions in multihydrocyclone plants, in which plugging or blocking of the hydrocyclones is likely to occur.

When treating fibre suspensions in multihydrocyclone plants, it has long been known that individual hydro- cyclones in the plant assembly may become blocked or plugged by fibres and/or contaminants present in the suspension. Another problem is that certain hydrocyclo¬ nes can become worn in the region of their apex. Both of these occurrences cause disturbances in the separa- tion procedure, which necessitates stopping and dis¬ mantling of the plant as a whole, in order to clean or replace these individual hydrocyclones. This is highly expensive and creates a disturbance in continuous production processes.

Many attempts have been made to maintain control over the operational function of the individual hydro¬ cyclones of a multihydrocyclone plant. For instance, multihydrocyclone plants have been proposed in which the hydrocyclones are positioned with their apices directed outwardly and a window is provided in front of each apex, or pointed part of the hydrocyclone, in order to enable this part-to be viewed visually. In addition to the fact that such an arrangement obviates the possibility of densely packing the cyclones in practice, i.e. the dense packing that is obtained when the hydrocyclones are arranged in layers with the axes located on the radii of a circle and with the apices pointing inwardly, this multihydrocyclone plant con- struction has not been widely used for other reasons. In another method devised to solve the aforesaid prob¬ lem, flow detecting arms have been arranged adjacent the apices of the hydrocyclones, these arms springing back in the absence of flow and therewith operating switches, which are sensed. Such mechanical arrange¬ ments, however, readily malfunction, therewith render¬ ing them unsuitable for use in large multihydro- cyclones.

The present invention is based on the fact that in operation a hydrocyclone emits detectable sound which changes in intensity when wear or plugging occurs. It is assumed that the change in sound observed in the event of plugging is du*e to a change in the flow condi- tions, particularly in the flow conditions at the inlet and outlet of the separation chamber in the hydro¬ cyclone, causing the generation of vibrations through turbulence effects. This explanation, however, shall not be considered as restrictive of the scope of the invention.

Thus, in accordance with the invention, vibrations caused by liquid flow in individual hydrocyclones are detected and analyzed, and measures are taken to stop these vibrations when the ananlysis indicates plugging.

It is evident that an important advantage is afforded when only a small number of individual hydrocyclones need to be exchanged or cleaned instead of needing to totally dismantle the whole of the multihydrocyclone plant periodically or when a poor separation result is obtained.

It is necessary to direct this vibration detection on individual cyclones. Detection can be effected either by applying a detector manually in a detecting opera¬ tion, or by fixedly mounting detectors to the indi¬ vidual hydrocyclones concerned. In this latter case, it is possible to automatically monitor the individual hydrocyclones, which also enables the detection of transistory malfunctions caused by defects which are otherwise difficult to establish.

For instance, when practicing the first mentioned aspect, a detector may be applied manually to the individual hydrocyclones in carrying out periodic checks, or when considered suitable and, in such case, preferably when the multihydrocyclone plant is in operation.

When practicing the second aspect, in which a detector is allotted to each hydrocyclone, the detector may be mounted in or adjacent the hydrocyclone and the mal¬ function can be observed either directly or through a recording device.

A suitably mounted detector which produces good signal strength can, itself, collect not inconsiderable energy, which can be converted into a light signal, either directly or while triggering another energy source, e.g. from a light diode mounted on the cyclone. Thus, when it is noticed that certain cyclones in a cyclone plant signal a malfunction, these identified cyclones can be rectified or serviced during brief operational stops, even though the light sources of these cyclones are no longer ignited when the supply of electric current ceases.

In one embodiment of such recording devices, each hydrocyclone may be suitably provided with an "in- telligent" detector in the form of an integrated cir¬ cuit, and all circuits will be connected to the record¬ ing device through a common signal line, this recording device collecting information from the individual hydrocyclones in sequence through a so-called polling process. Since such circuits are well known, they will not be described in detail here.

It is not necessary, however, to convert vibrations/ sound to electric signals at the location where the vibrations/sound is detected, and the sound conductor may therefore be arranged in accordance with the prin¬ ciples of the speech tube or stethoscope, for instance. It is important that each hydrocyclone is observed separately with respect to sound emission, since it is necessary to eliminate "cross-talk" from other cyclones as far as possible.

The invention will be described in more detail with reference to non-limiting exemplifying embodiments thereof and with reference to the accompanying drawings.

Figures 1-4 are sound spectrograms relating to hydro- cyclones under normal operation and hydrocyclones where the apex has become plugged.

Figure 5 illustrates a multihydrocyclone plant with one of the hydrocyclones being. shown in side view.

Figure 6 illustrates an individual hydrocyclone of a multi-hydrocyclone plant.

Figures 7-9 are block schematics. Figures 1-4 illustrate sound spectrograms taken in a test apparatus where one single hydrocyclone was con¬ nected to and operated with liquid flows conventional for ultihydrocyclones. The hydrocyclone concerned was of a standard type having a largest diameter of 80 mm.

Figure 1 illustrates the sound spectrogram obtained with a standard hydrocyclone having an apex diameter of 18 mm and with the apex plugged or blocked, whereas Figure 2 illustrates a comparison measurement with the apex of the hydrocyclone open and the flow normal.

The scales along the y-axes are arbitrary and denote intensity. The scale in Figure 1 is expanded with a factor of approximately four in relation to the scale of Figure 2. it will readily be seen that the hydro¬ cyclone whose apex is plugged exhibits much higher maximum intensity values between about 800 Hz and about 2000 Hz than the non-plugged hydrocyclone.

Figures 3 and 4 illustrate corresponding results ob¬ tained with a hydrocyclone whose apex opening had become enlarged as a result of wear.

In these tests, the detector head of a commercially available accelerometer was connected mechanically in the close proximity of the base end of the cyclone and from the outside of the test apparatus. When applying the invention in multihydrocyclone plants, it is suit- able to try to come closer to the source of the vibra¬ tions or to obtain insulated measurements on individual cyclones in some other way, in order to reduce distur¬ bances or interference from the surroundings. Figure 5 illustrates schematically a multihydrocyclone plant 124 of a known kind. One of the hydrocyclones 1 is illustrated in side view in Figure 6. This hydro¬ cyclone is mounted in a casing 3 having concentrical cylinder walls 4, 5, 6 and 8. The walls 5, 6 define an inlet chamber, the walls 6, 8 define an outlet chamber for the apex fraction, and the walls 4, 5 define an outlet chamber for the base fraction. The various hydrocyclones are fitted by insertion from without and are held in place by a cover member 7, which is locked firmly to the wall 4 with the aid of bayonette fittings.

When pressurized suspension is delivered to the inlet chamber, the inlets of the hydrocyclones (one for each hydrocyclone illustrated at 9) will be supplied with suspension and a vortex is generated by means of which the suspension is separated into a base fraction and an apex fraction in a known manner.

In accordance with the inventive concept, it is desired to carry out a diagnosis on the hydrocyclones and to discern whether the cyclones operate correctly or not, by analyzing prevailing vibrations, which become dif- ferent when the hydrocyclone malfunctions. In accor¬ dance with preferred embodiments of the invention, vibration/sound detectors are mounted firmly in the hydrocyclones, for instance in the close proximity of the smallest flow area at the base outlets, such as at 10. According to another embodiment, the detector is placed in a region outside the apex of the hydro¬ cyclone, firmly mounted on the wall 8 and surrounded by a protective cover 12 which shields the apex flow of the hydrocyclone from other flow, as illustrated at 11. The base outlet of the hydrocyclones illustrated in Figure 5 is provided with a central body 14 and a detector 10 (see Figure 6) is mounted in front of the central body.

The detector may comprise a piezoelectric element in the form of a circular plate of a commercially avail¬ able kind used as a microphone element, which produces electric voltage signals when deformed.

Many types of known microphone elements or accelero- meters connected to analyzers for analyzing the vibra¬ tion spectrum are used in the art to determine inten¬ sity distribution at different frequencies.

According to one preferred embodiment illustrated in Figure 7, a light source, e.g. a light diode (LED) mounted in the base part of the cyclone is activated when the electric signal exceeds a given threshhold value, wherein there is optionally applied a filter circuit (L) which allows through those frequencies which are characteristic of a plugging state.

According to one particularly advantageous embodiment of this kind in which individual signalling is em¬ ployed, the detector is utilized as an energy source to drive the light source. When the continuous power from the detector is insufficient to drive the light source, energy can be stored in a capacitor or a charged bat- tery (Figure 8), and the light source can be arranged to flash on and off, which saves energy and also faci¬ litates detection. Because a fault indication will cease when the plant is brought to a halt, it is con¬ venient to mark those hydrocyclones with which a fault is signaled during operation. These hydrocyclones can be serviced during short stoppages in operation, with¬ out needing to dismantle and inspect all of the hydro¬ cyclones more or less periodically and often, as re¬ quired with earlier known techniques.

In accordance with another preferred embodiment, illustrated schematically in Figure 8, the detectors fitted in the hydrocyclones can be connected to a central recording arrangement in which the signals are processed in accordance with a predetermined program and a signal is obtained when a cyclone becomes plugged or when there is a risk that plugging will occur. There is suitably provided either a display panel or a data printer from which identification is obtained. In order to reduce the wiring or cable layout, it is suitable to work with a multiplexor system in which the central unit calls-up the different detector units in sequence on one and the same bus line, which can function, for instance, with a single wire pair similar to a tele- phone system. Transmission can take place in the form of analog signals, although it may be advantageous to work solely digitally and provide each detector unit with an analog-digital-converter. Because the important frequencies are relatively low, sampling can be effected with lower sampling frequencies than is normal in telephony and standard type mass produced telephony circuits can be used. The circuits of the various detectors can then be called-up, for instance, 4000 times per second during the sampling period and a digitalized PAM-signal produced in response to each call, in accordance with standardized logarithmic scales. When the frequency range of 500-2000 Hz is to be monitored, each sampling period will require a duration of about 1 ms, therewith enabling up to about 1000 hydrocyclones to be monitored in a few seconds, including the polling time, with the aid of one single twin-wire conductor which, similar to telephone sys¬ tems, may also form the supply source for the current supply to the detector circuits.

Claims

Claims

1. A method for separating fibre suspensions in multihydrocyclone plants in which the risk of plugging exists, c h a r a c t e r i z e d by detecting and analyzing vibrations caused by liquid flow in indivi¬ dual hydrocyclones; and by taking de-plugging measures when the analysis indicates that plugging has occurred.

2. A method according to Claim 1, c h a r a c ¬ te r i z e d by detecting vibrations through a vibra¬ tion detector in the base region of a hydrocyclone.

3. A method according to Claim 1, c h a r a c - t e r i z e d by detecting vibrations through a vibra¬ tion detector in the apex region of a hydrocyclone.

4. A method according to Claims 1-3, c h a r a c ¬ t e r i z e d by detecting vibrations through a vibra- tion detector fixedly mounted in a hydrocyclone.

5. A method according to Claims 1-3, c h a r a c ¬ t e r i z e d by detecting vibrations through a vibra¬ tion detector which is fixedly mounted outside a hydro- cyclone.

6. A method according to Claims 1-5, c h a r a c ¬ t e r i z e d in that the vibration detector comprises an accelerometer which records the intensity of oscil- lations on one axis and frequency on another axis, wherein a pronounced change in the intensity at a given frequency in relation to the normal value indicates a plugging tendency. 11

7. A method according to Claims 1-5, c h a r a c ¬ t e r i z e d in that the vibration detector comprises a piezoelectric crystal which produces an electric signal whose strength deviates from the normal value in the event of a plugging tendency.

8. A method according to Claim 7, c h a r a c ¬ t e r i z e d in that a light source disposed in the base part of the hydrocyclone is caused to produce a light signal when the electric signal exceeds a given threshhold value.

9. A method according to Claim 7, c h a r a c ¬ t e r i z e d in that the electric signal is connected to a recording arrangement to which other signals from other hydrocyclones in the same multihydrocyclone plant are also connected, said recording arrangement pro¬ cessing the signal in accordance with a predetermined program and producing a signal when there is a risk that a hydrocyclone will become plugged or blocked.

10. A method according to Claim 1, c h a r a c ¬ t e r i z e d by conducting the vibrations to a vibra¬ tion detector located externally of the multihydro- cyclone plant.

11. A method according to Claim 10, c h a r a c ¬ t e r i z e d by employing the detection methods defined in Claims 6-9.

12. A multihydrocyclone plant for carrying out the method according to any one of Claims 1-11, c h a r ¬ a c t e r i z e d in that the individual hydrocyclones are provided with vibration detectors which function to detect and analyze vibrations caused by the flow of liquid in the individual hydrocyclones.

13. A plant according to Claim 12, c h a r a c ¬ t e r i z e d in that the vibration detectors are fixedly mounted in a base region of the hydrocyclones.

14. A plant accortding to Claim 12, c h a r a c ¬ t e r i z e d in that the vibration detectors are fixedly mounted in an apex region of the hydrocyclones.

15. A plant according to Claim 14, c h a r a c ¬ t e r i z e d in that the vibration detectors are fixedly mounted at a wall of the outlet chamber of the plant and disposed in a region outside the apex of the hydrocyclones.

16. A plant according to Claims 12-15, c h a r a c ¬ t e r i z e d in that the vibration detector is an accelerometer which records the intensity of oscilla- tions on one axis and frequency on another axis, and produces an alarm signal when there is a pronounced change in the intensity at a given frequency in rela¬ tion to the normal value.

17. A plant according to Claims 12-15, c h a r a c ¬ t e r i z e d in that the vibration detector is a piezoelectric crystal which produces an electric signal whose strength deviates from the normal value upon the occurrence of a plugging tendency.

18. A plant according to Claim 17, c h a r a c ¬ t e r i z e d in that light sources arranged at the base part of the hydrocyclones are caused to produce light signals when the electric signal exceeds a given threshhold value.

19. A plant according to Claim 17, c h a r a c ¬ t e r i z e d in that the electric signal from each hydrocyclone is connected to a recording arrangement.

20. A hydrocyclone intended for a multihydrocyclone plant according to Claims 12-19 and provided with a vibration detector fixedly mounted at the base or apex of said cyclone.