METHOD OF PREVENTING CLOGGING A SCREENING MEANS

Abstract: In screening a fibre suspension, an accept and a reject are tapped off, tapping the reject being controlled by a valve (6). According to the invention, the pressure drop across the screening means is measured, and when this drop exceeds a predetermined first value the valve (6) is opened from a first to a second position and when the pressure drop falls below a second predetermined value or when a given predetermined time has passed the valve is returned to its position prior to opening. In this second position the reject valve (6) is preferably completely open. The accept flow may possibly also be constricted when the reject valve is in the second position.

Description: Method of preventing clogging a screening means The present invention relates to a method of preventi clogging a screening means in a screen, and/or blocking up a reject valve located in a reject conduit in screening a f bre suspension, a flow of accept being withdrawn through an accept conduit, and the pressure drop across the screening means being sensed for controlling tapping off a reject by means of the valve in the reject conduit. Screens of different implementation are used in pulp paper mills for cleaning suspensions of cellulose fibres. I the pressure screens most often used today the screening su faces are either stationary or rotating. To prevent slammin up or clogging of the screening means the screens are equipp with foils, which are given a relative speed in relation to the screen surface and during screening have the task of de- structively affecting the fibre network formed on the screen surface, thereby preventing complete clogging of the holes o slits in the screening means. A tendency to slamming-up can often, but not always, be prevented by increasing the relati rate of revolutions between the screening means surface and foil- A substantial increase of this revolutional rate can lead to the desired cleaning, although to the price of in¬ creased costs for the required power consumption. Slamming-up or clogging can also occur in the valve us to control tapping off the reject. This clogging occurs more often when the reject rate is low. SE-B 304 677 discloses a screening machine comprising valve in the screen reject conduit. Said conduit has a regul ting means designed to be positioned in a predetermined best possible regulating position, an actuating means for opening the valve to a position essentially more open than said best regulating position without disturbing the setting of said regulating means, and a sensor responding to a predetermined smallest flow rate in said reject conduit to open the valve said actuating means from said best regulating position, sai sensor being designed not to open said valve from said best regulating position in response to variations in the flow ra through said reject conduit which variations are insufficien for reducing the flow rate to said smallest value, and said sensor is acting to return said valve to said best regulatin position when the flow rate has increased to a value essen- tially above said smallest value. One object of the present invention is to prevent hole or slits in a screening means from clogging. Another object of the present invention is to prevent blockage in the valve in the screen reject conduit. These objects are achieved in accordance with the inve tion by sensing the pressure drop across the screening means and controlling the tapping off of reject by means of a reje valve, opening this valve from a first position to a second position when the sensed pressure drop exceeds a first pre- determined value, and after a while returning the valve to i first position, i.e. to its position prior to opening. The r ject valve is preferably opened entirely, i.e. the second po sition is the one in which the valve is entirely open. The valve in the reject conduit is returned to its first positio i.e. that which the valve had before the change, either afte a given predetermined time, or when the sensed pressure drop across the screen falls below a second given value. In order to further amplify the effect obtained in accordance with the invention, the flow through the accept conduit can be constricted by a valve therein. This constric tion may take place when the reject valve is in its second position and preferably simultaneously with opening the reje valve. It is particularly preferred that opening the reject valve is done rapidly, i.e. it is instantaneous. The invention will now be described in detail with the aid of the drawing, on which Figure 1 schematically illustrates a screen controlled in accordance with the invention, and Figure 2 schematically illustrates the degree of separ tion for two different impurities as a function of the rejec rate. The screen S in Figure 1 may be a screen with stationa screening means and rotating foils, e.g. as described in the Swedish Patent Specification No. 343 621. The fibre suspension which is to be screened is taken through a conduit 1 to a screen S. A portion of this suspen¬ sion, i.e. the main portion in nearly all applications, pass through the holes or slits in a screening means 4 and leaves the screen S through a conduit 2 as accept. The remaining po tion of the incoming suspension, which does not pass through the openings of the screening means 4, is tapped off through a conduit 3 as reject. The flow rate through the conduits 2 and 3 is regulated by means of valves 5 and 6, respectively. The pressure drop across the screen S is sensed by a pressure sensing means PG. A signal proportional to the pressure drop is fed to a control device MP. The control device MP controls the setting of the valves 5 and 6 in response to the signal from the pressure sensing device PG and the programming of a microprocessor in the control device MP by means of two valve actuating means 7 and 8, respectively. Also, as shown in the preferred embodyment of Figure 1, a flow meter can be provided in the reject conduit 3. A sig- nal proportional to the sensed reject flow rate is led to a control device FC being designed to keep the reject flow rate constant. When the sensed reject flow rate deviates from a predetermined value, a signal proportional to the deviation i fed to the actuating means 8 changing the valve opening towar the predetermined flow rate value. This flow control device F is only able to open the valve 6 to a predetermined greatest - opening, far from being completely open. The control device FC is only able to change the valve opening in a narrow range It is desired to achieve purification of the fibre sus- pension by means of screening. Accordingly, the impurities ar concentrated in the lesser of the two flows leaving the screen, i.e. in the reject flow, while the other larger flow is impoverished of impurities. Apart from a larger content of impurities the reject thus also contains prime fibres. It is quite natural to strive to keep the content of fibres in the reject as low as possible. This purification or separation is a function of the reject rate, i.e. the ratio between the re¬ ject flow rate and the feed flow rate, apart from parameters depending on the screen and screening means used, i.e. its hole diameter and total orifice area. Figure 2 schematically illustrated the relative separation of impurities of two dif¬ ferent kinds as a function of the reject rate. As will be see from Figure 2, the reject rate must exceed a given minimum value in order to achieve an acceptable purification of the suspension without fibre loss being unacceptedly great. The reject rate is usually 10 — 25 per cent by volume, preferably 10 - 15 per cent by volume. When there is a certain thickenin of the reject during screening, which may be of the order of magnitude of 2, the fibre content is approximately twice as great as in the feed or inject. This means that for a reject rate of about 10 - 15 per cent by volume there is a fibre los of about 20 - 30 per cent. Tapping the reject usually takes place continuously, an is then preferably a constant proportion of the inject. In certain applications periodical tapping of the reject is possible. Usually the reject is withdrawn continuously. For small reject rates the valve 6, controlling the re- ject rate is heavily constricted, i.e. the low-through in the valve has small cross-sectional area. Valves are often de signed so that for a small degree of opening they have a larg ratio between the cross-sectional circumference and area in the low-through portion of the valve. This results in that the risk of clogging .increases for heavy constriction of the valve. In turn, this means that when the flow- hrough area in the reject valve is kept constant, there is occurring a cer¬ tain amount of clogging in it. This is sensed by the pressure sensing means as a pressure increase across the screening means. If this clogging is not cleared, it will finally lead to complete stoppage. In screening, there is also a slow clogging of the screening means. Prevention of this clogging is usually at¬ tempted by increasing the rotational speed of the foils, with resulting increased power consumption. In certain cases this is not sufficient, and the result will be that the screen be¬ comes completely blocked. In accordance with the invention, clogging of the screening means openings and/or blocking up of the screen re ject valve is monitored by measuring the pressure drop acros the screen. A signal varying in response to this pressure dr is applied to the controlling means MP including a micro- processor. When this signal exceeds a predetermined value, the controlling means, as one of its functions, actuates the valve 6 via an actuating means 8, the valve then being broug into a second, pr'eferably entirely open position. The pressu drop across the screening means 4 thus falls, the reject flo will be greater and entrain with it a possible collection of fibres in the valve opening, and the load on the screening means will be less. The moving foils can now more easily re¬ move possible clogging. When the pressure drop across the screening means has fallen below a second predetermined value there is an actuation of the valve 6 such that it returns to its previous position. The clearing effect in question can, if necessary, be amplified by the valve 5 also being actuated when the valve 6 is in or is being brought into its open position. This actua- tion provides a suitable constriction of the accept flow. The valve 5 is preferably caused to return to its earlier positio simultaneously as the valve 6 assumes its first position. In the illustrated example, the return of the valve 6 and possible the valve 5 to their first positions is con- trolled by the pressure drop across the screening means. This return, i.e. the duration of the clearing proce"ss, can also b controlled so that it takes place after a given predetermined time. In this case the microprocessor of the controlling mean can be programmed so that, independent of the sensed pressure drop, it gives a signal after a given number of seconds, e.g. 5 seconds, said signal actuating the valves 5 and 6 via actua ting means 7 and 8, respectively, so that these return to their first positions. A further possibility is for the microprocessor also to be programmed such that the stated clearing is carried out periodically and independent of the pressure drop across the screen S. The valves 5 and 6 should be implemented such that the ratio between the cross-sectional area of the flow-through and its periphery is as large as possible. Examples of pre¬ ferred valves are ball and ball sector valves. The valves 5 and 6, particularly the valve 6, should have a structure such that rapid regulation can take place thereby increasing the clearing effect.

Claims: CLAIMS 1. A method of preventing clogging a screening means in screen and/or blocking up of a reject valve provided in a r ject conduit in screening a fibre suspension, a flow of acc being removed through an accept conduit, and the pressure d across the screening means being sensed for controlling tap ping off a reject by means of the valve in the reject condu c h a r a c t e r i z e d in that the valve is opened from first to a second position when the pressure drop exceeds a first predetermined value and after a time the valve is re- turned to its first position. 2. Method as claimed in claim 1, c h a r a c t e r i z e in that the valve is completely open in the second position. 3. Method as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the valve is returned to its previous or first position when the pressure drop falls below a second predetermined value. 4. Method as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the valve is returned to its previous or first position after a predetermined length of time. 5. Method as claimed in any of claims 1 - 4, c h a r a c t e r i z e d in that the tapping off of reject takes place continuously. 6. Method as claimed in claim 5, c h a r a c t e r i z e in that the rate of reject tapping is constant. 7. Method as claimed in any of claims 1 - 6, c h a r a c t e r i z e d in that in addition the accept flow is con¬ stricted by means of a second valve in the accept conduit wh the reject valve is in its second position. 8. Method as claimed in claim 7, c h a r a c t e r i z e in that constriction of the accept flow and opening of the r ject valve takq place simultaneously. 9. Method as claimed in any of claims 1 - 8, c h a r a c t e r i z e d in that the screen has one or more stationary screening means. 10. Method as claimed in claim 9, c h a r a c t e r i z e in that the screen has rotating foils.

Inventors: WIKDAHL NILS ANDERS LENNART, WIKDAHL, NILS, ANDERS, LENNART
Applicants: WIKDAHL NILS ANDERS LENNART, WIKDAHL, NILS, ANDERS, LENNART
Application year: 1982
Main classification: D21D5
IPC codes: B01D37, D21D5, D21G9