GB2035824A

GB2035824A - Fibre-suspension screen apparatus

Info

Publication number: GB2035824A
Application number: GB7938719A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-11-17
Filing date: 1979-11-08
Publication date: 1980-06-25
Also published as: BR7907433A; ES486024A1; DE2946356A1; SE7901076L; FR2441406A1; AU5275579A; IT1124991B; FI793509A; ES486029A1; IT7927356A0

Abstract

The rotary agitator 20 of a fibre-suspension screening apparatus is driven at a speed varied according to at least one physical parameter of the fibre suspension measured on one or both sides of the screen 1, 2 to prevent clogging thereof. The parameter may be the pressure, flow rate or concentration of the suspension, and any variations from a predetermined level produce an automatic change in agitator speed to restore the status quo. As a result of this increase in speed of said relative movement, fibres and the like clogging the perforations of the screen plate or plates are completely or partially torn away, thereby increasing the screening capacity to a normal value. <IMAGE>

Description

SPECIFICATION A method of unclogging the screening plate of a screen means for screening fibre suspensions, and apparatus for carrying out the method A known screen means for fibre suspensions comprises at least one screen compartment having at least one screen plate whose surface facing the screen compartment is arranged to co-operate with an agitating or stirring means in a manner to prevent fibres, slivers etc.

which do not pass through the perforations in the plate from fastening thereto. The agitating means and/or the screen drum is or are rotated in a manner such as to agitate continuously the fibre suspension nearest the plate to prevent clogging of the perforations therein. Despite this agitation of the suspension in an area adjacent the screen plate, fibre agglomerates, fibre skins, slivers etc are still likely to adhere to the plate and block the perforation therein especially when the suspension concentration is excessively high, it being necessary to remove these agglomerates etc in order to maintain the desired efficiency of the screen.As will be understood, such clogging of the plate is difficult to establish visually when the screen is in operation and hence it is not until the efficiency of the screen has fallen to a markedly low level that said clogging becomes apparent and the plate then manually freed of the substances adhering thereto.

The same difficulties are encountered in those screening apparatus which are designated de-waterers. Consequently, it is a prime object of the invention to provide a method of automatically cleansing screening apparatus immediately there is a tendency to clog, and apparatus for carrying out the method.

The invention is based on the observation that even when a small part of the total screening or de-watering area of the screen plate becomes clogged, there is a resultant change in the physical parameters of the fibre suspension, for example an increase in pressure on the input side of the plate and a decrease in pressure on the output side thereof.

In accordance with the present invention the aforementioned prime object is achieved by a method of removing from a screen plate in a screen means for fibre suspensions material adhering to said plate, said screen means comprising at least one screen compartment having at least one screen plate whose surface facing the screen compartment is arranged to co-operate with agitating or stirring means, a drive means being arranged to cause relative movement between the screen plate and the agitating means, said method being characterized by measuring the magnitude of at least one physical parameter of the fibre suspension on at least one side of the screen plate; and controlling the drive means in dependence upon the deviation of the measured value from a set value of said parameter within a given range, thereby to increase the speed of said relative rotation.

A screen means according to the invention for carrying out the said method comprises at least one screen compartment having fibresuspension inlet means; at least one screen plate with first outlet means for collected screened material; agitating or stirring means arranged to co-operate with the side of the screen plate facing the screen compartment to prevent clogging of the perforations in said plate; second outlet means for discharging liquid including screened material of a given size exiting through the screen plate; and drive means for moving the agitating means relative to the screen plate at a given speed, said screen means being characterized by measuring means arranged to measure the magnitude of a physical fibre-suspension parameter which is dependent upon the extent to which the suspension is able to pass through said plate, said measurement being taken at least on one side of the plate, and to produce a control signal when the value of the parameter measured deviates from a given range and to send said signal to a control means for increasing the speed of relative rotation between said plate and said agitating means.

Because the drive means increases the speed of said relative rotation in response to the receipt of said control signal, fibre agglomerates will be rapidly removed from the screen plate, whereby as soon as the parameter value measured by the measuring means return to the set value, as a result of unclogging the plate, the drive means will be re-set to its normal working speed.

Conveniently, two measuring means are used, of which one is arranged to sense the selected parameter in the input fibre suspension and the other is arranged to sense the parameter in the output fibre suspension, said output suspension having been freed of coarse particles.

Other characterizing features of the invention are disclosed in the claims. Exemplary embodiments of the invention will now be described with reference to the accompanying schematic drawings in which Figure 1 is a first embodiment of a pressure-sensing measuring means, Figure 2 is a cross sectional view taken on the line Il-Il in Fig. 1, Figure 3 illustrates a second embodiment of a a pressure-sensing measuring means, Figure 4 illustrates a flow meter connected to an outlet for screened fibre suspension, and Figure 5 illustrates two flow meters connected to a respective outlet for screened fibre suspension and an outlet for fibre suspension which includes excessively coarse fibre particles.

The screen means shown in Figs. 1 and 2 comprises a stationary screening device hav ing two pairs of ring-shaped screen plates 1, 2 which form therebetween screen compart ments 39 and 40 respectively and which are provided with perforations in the form of holes or slits to enable particles exceeding a given size to be screened from the fibre suspension.

The two screen plates 1, 2 are of curved configuration and arranged axially side by side and form the outer walls of a truncated cone.

The outer edge surfaces of the screen plates 1, 2 are fastened to the inside of a cylindrical casing or housing 3a, 3b surrounding said plates with the concave sides of the pairs of plates facing towards each other. In this way, each pair of screen plates forms an annular groove 38 in which the material to be screened is introduced at the inner edge from a central inlet 6 arranged adjacent one end wall 4, 5 of the casing 3a, 3b.That part of the material to be screened which passes through the screen plates, which material in an extreme case may comprise mainly liquid without any appreciable quantity of fibres, is collected in receiving chambers 7, 8, 9 located between the casing 3a, 3b and an internal partition wall concentrical with the casing, said partition wall comprising three cylindrical wall parts 10, 1 1, 12, of which the centre wall part 11 joins together the mutually adjacent inner edges of the two central screen plates, while the two other partition parts 10, 12 join the inner edges of the outermost screen plates to a respective end wall 4, 5 of the casing. Each of the chambers 7, 8, 9 is separated from its adjacent chamber by means of the screen plates, and is provided with an outlet 13, 14, 15 respectively.That part of the material to be screened which does not pass through the screen plates, because it contains particles, e.g. wood chips, which exceed a given size, departs to an outlet 1 6a, 1 7a through openings 16, 17 arranged peripherally of the grooves 38. A shaft 19 is rotatably journal led in a bearing housing 18 arranged in the end wall 5 and provided with requisite seals. The shaft 19 is driven by means of a motor 41, for example an electric or hydraulic motor, the rotary speed of which is controlled in a manner hereinafter described.The shaft 19 carries an agitating or stirring device comprising two stirrup-like arms 20 which extend in mutually opposite directions and which are formed in a manner such as to conform to the inner surfaces of the screen plates and to terminate at a given distance therefrom and, during rotation of the shaft 19, to sweep over the plates in order to prevent fibre-suspension particles retained by the plates from fastening thereto and blocking the perforation thereof. To facilitate mounting of the arms 20, said arms are arranged to be attached in mutually different positions on the outside of a sleeve 22 by means of nuts 21, which sleeve is non-rotatably connected to the shaft 19 by means of a screw means 23, said screw means permitting axial adjustment of the sleeve 22 along the shaft 19.As will be seen from Fig. 2, the casing 3a, 3b comprises two halves of which the upper half 3a is hingedly connected to the lower half 3b at 24, enabling one half of the casing to be lifted away from the other. The screen plates 1, 2 and the cylindrical parts 10, 11, 12 are also divided into two halves, of which the upper half accompanies the casing part 3a when said part is lifted up.

The motor 41 is arranged to drive the shaft 19 in normal events at a constant speed, e.g.

150 rpm, although, in accordance with the invention, the speed at which the shaft 19 is rotated can be automatically increased when a screen plate becomes clogged, for example the screen plate 2. When the fibre suspension constantly fed in at a constant pressure, this clogging of a screen plate is manifested as an increase in pressure in the screen compartment 39 having a clogged screen plate, i.e.

also as an increase in pressure in the inlet 6 or as a decrease in pressure externally of the clogged plate, i.e. also as a decrease in pressure in the associated outlet 13, 14 or 15.

In Fig. 1 there is illustrated a pressuresensing means 42 which may be of any suitable type and which is arranged to sense the pressure in the chamber 7 externally of the screen plate 2 and, when the sensed pressure falls beneath a given, minimum value, sends a control signal, via a line 43, to a control means 44 for controlling the speed of the motor 41, and more specifically to increase the motor speed upon receipt of the control signal.The motor 41 may, for example, comprise a hydraulic motor, in which case the control means 44 comprises a valve means which is actuated in response to the control signal from the pressure-sensing means 42, to increase the flow to the motor 41 upon receipt of said control signal, thereby to increase the speed of the motor 41, and thus also the speed of the shaft 19 and the agitator 20, for example to twice the normal speed, thereby to impart to the fibre agglomerates on the screen plate or plates such tearing forces as to clean the plate or plates of said agglomeraters. As beforementioned, the arms 20 terminate at a distance from the inner surfaces of the screen plates, although in certain cases it may be suitable to cause the arms to slide against said inner surfaces.

This latter alternative is particularly applicable when the screen means operates as a dewatering means. Immediately the screen plate or plates has or have been cleansed, the pressure in the chamber 7 will return to its normal value and the motor 41 will be re-set to its nominal speed. The illustrated embodi ment has three outlet chambers 7, 8, 9 with associated inlets 13, 14, 15 which do not communicate directly with each other, and hence a respective pressure-sensing means is provided for each chamber or outlet.

For the purpose of sensing the pressure in the central outlet chamber 8, and therewith for detecting the possible clogging of the two screen plates 1, 2 defining said chamber, there is arranged in the outlet 14a pressure sensing means 42' which has the same function as the pressure sensing means 42 and which is connected in parallel thereto. A pressure-sensing means is also provided in the inlet chamber 9, said pressure-sensing means also being connected in parallel with the pressure-sensing means 42.

As beforementioned, clogging of one or more screen plates is manifested by an increase in pressure within respective screen chambers 39, 40 and in the inlet 6. Fig. 1 illustrates a pressure-sensing means 42"' which is arranged to sense the pressure in the inlet 6 and, when the sensed pressure exceeds a nominal value, to generate a control signal which is used to increase the speed of the motor 41 in the aforedescribed manner.

The screen means illustrated in Fig. 3 has two pairs of curved ring-shaped screen plates 24, 25 of the aforedescribed kind. The concave sides of the two central screen plates 24, 24 face one another to form therebetween a screen compartment 45, while the two outer screen plates 25 form together with walls 26 and 27 a respective screen compartment 46 and 47. The material to be screened is introduced from a central inlet 28 arranged adjacent an end wall 27 of the casing. The screen plates form together with the cylindrical surface 29 of the housing two outlets 30, 31 from collecting chambers 32, 33 for screened material passing through the plates. The material which does not pass through the screen plates is removed through outlet openings 34 and an outlet 35. As with the embodiment of Figs. 1 and 2, a rotatable shaft 36 carries pairs of agitating devices 37.The cone angle of the screen plates lies within 90 to 1 75', and preferably from 120-150 . The shaft 36 is driven by an electric motor 48 via a gearing 49. The gearing 49 has two gearchanges, namely a first gear change in which the motor 48 drives the shaft 36 at a given, normal speed, and a second gear-change in which the shaft 36 is driven at twice said normal speed, in order to unclog the plates.

The gearing 48 is operated electromechanically and is normally in its first gearchange position, but is electromechanically re-set upon receipt of a control signal indicating that the perforations of one or more screen plates, for example the opening 50 in Fig. 3, have become clogged. In this embodiment there is provided a first pressure sensor 51 arranged in the screen compartment 46, and a second pressure sensor 52 arranged in the co-operating outlet chamber 32. The pressure transducer or the pressure-sensing means 51 continuously measures the pressure prevailing in the screen compartment 41 and sends a signal proportional thereto to a signal comparison circuit 53. The pressure transducer or the pressure-sensing means 52 continuously measures the pressure externally of the screen plate 25 and sends a signal proportional to said pressure to the comparator 53.The two signals obtained from the pressure transducers 51 and 52 are compared in the comparator 53 and should the signals received deviate from one another by a given value, this will indicate that the left screen plate 25 has become clogged. The comparator 53 will then send a control signal to an amplifier 54. The signal is amplified in said amplifier and is then sent to the electromagnetic switching means in the gearing 49, which switches the gearing to its second gear-change position causing the shaft 46, and therewith the arms 37, to be rotated more rapidly.

As will be understood, additional pressuresensing means must be provided for detecting the condition of the remaining screen plates, as described with reference to Fig. 1.

As before indicated, it is also possible to measure a further parameter of the flow suspension within and/or externally of a chamber. For the purpose of measuring the output flow of screened material there is arranged adjacent the outlet 13, 14 and 15 in Fig. 1 a flow meter of any suitable kind. In Fig. 4 there is illustrated a flow meter 56 which is coupled to the flange 15a of the outlet 15 and through which the screened fibre suspension flows. The flow meter may, for example, comprise a magnetic flow meter, such as a flow meter of the kind sold by Fischer a Porter of the Federal Republic ofl Germany under the designation 10D1423A and which transmits on a signal line 55 a signal which is proportional to the rate of flow of said suspension.With a given infeed of fibre suspension to the inlet 6 in Fig. 1, there will be a given flow of screened fibre suspension, the accept, will depart through the outlet 15 provided that the screen plates in the compartment 40 are not clogged. If one of the screen plates becomes clogged, however, the flow will decrease and the signal on the line 55 will deviate from the set value and cause the control means 45 to be actuated in a manner such as to increase the relative speed between the agitators and the screen plates.

It is also possible to measure, at the same time, the flow of fibre suspension departing through the reject outlet 17 of the compartment by means of a flow meter 57 and to feed the signals from the two flow meters to the signal comparator 45, which in turn controls the control means 44 in Fig. 1 in the manner described with reference to Fig. 3.

Another parameter value which changes with a change in permeability of a screen plate or the screen plates is the concentration of the fibre suspension. Thus, if a screen plate becomes completely or partially clogged, the concentration of the fibre suspension within the corresponding screen compartment will increase, while the suspension concentration will decrease externally of said compartment.

For the purpose of measuring such a change in concentration, a concentration-measuring device is arranged in the accept outlet and/or in the reject outlet.

The illustrated embodiments of the invention and the actual screen means have merely been given by way of example and various modifications can be made within the scope of the claims. Thus, the control system for the drive motor can be modified and the screen means may include a single screen compartment, such as the screen compartment 46 of the Fig. 3 embodiment.

In the aforegoing it has been assumed that the agitators are moved at a constant or substantially constant speed relative to the screen plates, although in normal operation this speed may also be periodic. The essential feature is that the relative speed is increased when clogging is indicated and, for example, this increase in speed shall be effected so rapidly as to enable a pressure wave to form, said pressure wave facilitating the release of agglomerates or layers of material adhering to the plates.

It will also be obvious that the invention is not limited to its application with the specific type of screen means illustrated. Thus, the agitating means may be stationary and the screen compartment rotated, and optionally both the screen chamber and agitating means may be driven, but at mutually different speeds, or in mutually opposite directions so as to obtain the desired relative movement.

Further, in the majority of cases the measuring means are not arranged to generate signals at an exact given value, but are arranged to generate said signals only within a given range which is characteristic of one or more clogged screen plates.

It is also possible to combine different kinds of measuring means and, for example, to measure both pressure and concentration and permit these two parameters to control the drive means.

The measuring means may also be placed at other locations than those described above and thus, for example, a flow meter or concentration meter may be mounted in the inlet 6.

Although it has been assumed above that the change in relative speed between the screen plate or screen plates and the agitators takes place step-wise and rapidly, it is preferred to produce continuous changes in speed which are completely dependent upon the magnitude of the control signal sent to the control means. Thus, if the control signal indicates slight clogging of a screen plate, the increase in speed is directly proportional to the magnitude of the signal. If clogging does not cease as a result of this increase in speed, but perhaps increases instead, this is indicated by an increase in the value of the control signal, whereupon a corresponding further increase in the speed is obtained. When the control signal gradually decreases in value, the speed will also decrease to a corresponding extent, and finally the control signal will indicate a normal value, i.e. a value indicating that the screen plate has been completely unclogged, whereupon the control means returns to operation at normal speed.

Claims

1. A method of unclogging a screen plate in a screen means for fibre suspensions, said screen means comprising at least one screen compartment (e.g. 39; 46) having at least one screen plate (1, 2; 24, 25) whose surface facing the screen compartment co-acts with agitating means (20, 37); a drive means (41; 48, 49) being arranged to cause relative rotation between the screen plate and the agitating means, characterized by measuring the magnitude of at least one physical parameter of the fibre suspension on at least one side of the screen plate and controlling the drive means in dependence upon a deviation of the measured value from a parameter value within a a given range, thereby to increase the speed of said relative movement.

2. A method according to claim 1, characterized by measuring the magnitude of the parameter within the screen compartment (40).

3. A method according to claim 1, characterized by measuring the magnitude of the parameter externally of the screen compartment.

4. A method according to claim 1, characterized by measuring the magnitude of the parameter on both sides of the screen plate.

5. A method according to claim 4, characterized by comparing the two measured pa rameters with one another and to increase the speed of said relative movement through said drive means upon a given deviation between said compared parameters.

6. A method according to anyone of claims 1-5, characterized in that the measured parameter or parameters comprises the pressure of the fibre suspension.

7. A method according to anyone of claims 1-5, characterized in that the measured parameter or parameters comprises the concentration of the fibre suspension.

8. A method according to anyone of claims 1-5, characterized in that the measured parameter or parameters comprises the flow of fibre suspension.

9. A fibre suspension screen means for carrying out the method according to claim 1, said screen means comprising at least one screen compartment (e.g. 39; 46) having inlet means (6; 28) for fibre suspension and having at least one screen plate (1; 24, 25) having first outlet means (16a, 1 7a; 35) for collected screened goods, the side of said screen plate facing the screen compartment co-acting with agitating means (20; 37) for preventing clogging of the perforation of said plate; second outlet means (13-15; 30, 31) for discharging liquid and screened material of a given site passing through the screen plate; and a drive means (41; 48, 49) for moving the screen plate and the agitating means relative to one another at a given speed, characterized by measuring means (42; 51, 52; 56) arranged to measure the magnitude of a physical fibresuspension parameter which is dependent upon the permeability of the screen plate, said measurement being taken at least on one side of the screen plate, said measuring means being arranged, when the measured value deviates from a parameter value within a given range, to send a control signal to a control means (44; 49) for increasing said relative movement.

10. A screen means according to claim 9, characterized in that the measuring means (42) is arranged to measure the pressure in the fibre suspension.

11. A screen means according to claim 9, characterized in that the measuring means (46) is arranged to measure the flow rate of the fibre suspension.

12. A screen means according to claim 9, characterized in that the measuring means is arranged to measure the concentration of the fibre suspension.

13. A screen means according to anyone of claims 9-12, characterized in that the measuring means (42"') is arranged to measure the parameter in the fibre suspension fed to the screen compartment.

14. A screen means according to any one of claims 9-12, characterized in that the measuring means (42; 46) is arranged to measure said parameter in the fibre suspension externally of the surface of the screen plate (22) remote from the screen compartment (39; 40).

15. A screen means according to any one of claims 9-12, characterized in that the measuring means (51, 52; 56, 57) is arranged to measure said parameter in the fibre suspension on both sides of the screen plate.