WO2000024964A1

WO2000024964A1 - Method and device for attenuation of screen pulsations in the short circulation in a paper/board machine

Info

Publication number: WO2000024964A1
Application number: PCT/FI1999/000880
Authority: WO
Inventors: Harri Mustonen
Original assignee: Metso Paper, Inc.
Priority date: 1998-10-27
Filing date: 1999-10-22
Publication date: 2000-05-04
Also published as: FI982330A0; FI982330A; AU1048200A; FI103997B1; FI103997B

Abstract

The invention concerns a method for elimination of pressure pulsations occurring in the short circulation in a paper/board machine. The invention is applied in a paper/board machine which is provided with two or more machine screens (S1, S2, S3). The machine screens (S1, S2, S3) are provided with phase metering detectors (22, 23, 24), and the information on the phase metered by said detectors is passed to a measurement unit (28). Based on the measurement results, the relative speeds of rotation and/or phase differences of the machine screens (S1, S2, S3) are regulated so that the pressure variations arising from the machine screens (S1, S2, S3) neutralize each other. As a result of this, the pressure pulse produced in the sum flow (Fin) is minimized. The invention also concerns a device for elimination of pressure pulsations occurring in the short circulation in a paper/board machine.

Description

Method and device for attenuation of screen pulsations in the short circulation in a paper/board machine

The invention concerns a method and a device for elimination of pressure pulsations occurring in the short circulation in a paper/board machine, which paper/board machine is provided with two or more machine screens.

The functions of the short circulation in a paper machine include recycling of the raw-materials and the water that have passed through the wire back into the headbox, diluting of the stock to be fed into the headbox, cleaning of the stock to be fed into the headbox, and mixing of additives, media and chemicals into the stock. In the short circulation, air is removed from the liquid to be circulated, and attempts are made to accomplish the circulation so that any disturbance in the flow rate, consistency and pressure is equalized.

Constantly increasing running speeds of paper machines also impose ever increasing requirements on the process of the short circulation in a paper machine both in respect of the stability of the process of manufacture, the freedom from disturbance of said process, and in respect of control of the quality of the paper. Also, printing machines with constantly higher running speeds impose ever stricter requirements on the quality of the paper.

After a deaeration tank had become a part of the process of the short circulation in high-speed paper machines that produce printing papers, it was, at the same time, necessary to move from the traditional, so-called single-pump process to a process that operates with two principal flow pumps. In each process arrangement, the fresh stock is mixed with white water in the area of the wire pit. However, in the twin- pump system, a factor that disturbs the quality of the paper and the runnability is variation of the basis weight of the paper in the machine direction, which arises from pressure pulsations in the pumps and in the screens, on one hand, and from pressure and mixing disturbance resulting from fluctuations of the fluid levels in tanks and pipes, on the other hand. Similar disturbance also arises from other causes.

With respect to the prior art, reference is made to the FI Patent No. 88,415, in which a process arrangement is described for production of headbox stock for a paper machine in the short circulation. In said arrangement, a first compartment in a combination wire pit has been arranged as a feed tank for deaeration, and a second compartment has been arranged as a dilution tank for headbox stock, into which tank the fresh stock has been arranged to be fed.

The process of the short circulation in a paper machine has, as a rule, been arranged so that all new solid materials, i.e. fibres and fillers, are diluted with white water in the area of the wire pit. It is also possible to feed fillers in later process stages. As a rule, all fibrous material that has been fed into the headbox of a paper machine has been treated in deaeration and in preceding or following vortex cleaning. Thus, the consistency dynamism of the short circulation has been quite slow, which comes from the large quantities of material that move in the short circulation, which quantities are of an order of 200 cubic metres. The above also results, among other things, in the drawback that, in a situation of change of paper grade, a large amount of paper ends up in broke, because, after a change of grade or a similar change, the process of short circulation requires quite a long time to be stabilized.

At high machine speeds, when traditional connecting of a bypass flow either to the deaerator or to the wire pit is employed, losses of power arise. The loss of power can be in the bypass flow valve, because of the high pressure loss and the high flow rate, even as high as 300 kW.

In processes of the short circulation in a paper machine in which there are more than one machine screen, the machine screens are tuned to revolve at speeds of rotation slightly different from one another. This has the consequence that the phase difference between the pressure pulsations caused by different machine screens changes all the time. When the pressure pulsations are at the same phase, they amplify each other, and when they are at opposite phases, they attenuate each other efficiently. In the process, the variation in the pressure pulsations of the machine screens is seen as a regularly attenuated and amplified disturbance.

The object of the present invention is further development of the prior-art process arrangements of the short circulation in a paper machine and providing of novel, more advantageous solutions for said arrangements.

It is a particular object of the present invention to provide a novel process arrangement by whose means losses of power are reduced.

It is a further particular object of the present invention to provide a method and a device by whose means the pulsations of machine screens are attenuated.

In view of achieving the objectives stated above and those that will come out later, the method in accordance with the invention is mainly characterized in that the machine screens are provided with phase metering detectors, the phase of the rotation of each machine screen is determined, and, based on the determination of phase, the relative speeds of rotation and/or phase differences of the machine screens are regulated so that the pressure variations arising from the machine screens neutralize each other and the pressure pulse produced in the sum flow is at the minimum.

On the other hand, the device in accordance with the invention is characterized in that the device comprises metering detectors installed correspondingly on the machine screens for determination of the speeds of rotation and/or of the phases of rotation and corresponding signal lines which pass the information on metering of the speed and/or phase of rotation to a measurement unit.

In the arrangement in accordance with the invention, the machine screens of the short circulation are provided with detectors which meter the speed and/or phase of rotation of the machine screens. When two machine screens are in operation, the speeds of the machine screens are adjusted equally high, and the phases of rotation are synchronized at phases opposite to one another. In such a case, the pressure pulsations produced by the machine screens neutralize each other, and the disturb- ance pulse produced in the overall stock flow is attenuated as efficiently as possible. In the case of three machine screens, the pressure pulsations are attenuated most efficiently when the relative phase difference between the machine screens is 120°. When the number of the machine screens in operation is N, the pressure pulsations are attenuated most efficiently when the relative phase difference between the machine screens is 360°/N.

By means of the method in accordance with the invention, elimination of the pressure pulsations produced by the machine screens is achieved efficiently. The advantage obtained from this comes out in the form of improved quality of the final product and in the form of reduced losses.

In the following, the invention will be described in detail with reference to the exemplifying embodiments illustrated in the figures in the accompanying drawing, the invention being not supposed to be confined to said embodiments alone.

Figure 1 illustrates the connections of the machine screens in the short circulation of a paper machine.

Figure 2 illustrates the mode of carrying out the method and the device in accord- ance with the invention in metering of the pressure pulsations of the machine screens.

Figure 3 shows an example of a result of metering of the pressure pulsations caused by machine screens.

Fig. 1 shows a prior-art process of the short circulation in a paper machine, to which process the method in accordance with the present invention can be applied. As is shown in Fig. 1, the headbox 10 feeds a stock suspension jet J through its slice opening 11 onto the forming wire 13 running over the breast roll 14. The water collecting devices 15 placed inside the loop of the wire 13 pass the water drained through the wire 13 as a flow F₁ into the wire pit 16. The white water of the flow F, is used in the wire pit 16 for dilution of the fresh stock to the headbox consistency. A return flow F coming from vortex cleaners 18 is also passed into the wire pit 16.

A flow pump 17a feeds the stock flow F₅, which has been diluted to the headbox consistency, through vortex cleaners 18 as the flow F₆ into the deaeration tank 20. In the deaeration tank 20, there is an air space 20a subjected to a vacuum above the free level S₀ of the stock. The stock level is determined by an overflow 21 of the tank, over which a stock flow Fg flows, from which the air has been removed. This flow F₈ is passed into the wire pit 16.

From the bottom part of the deaeration tank 20, a stock flow F₇ is passed to the suction side of the flow pump 17b. The pressure side of the flow pump 17b communicates with the parallel machine screens S_j ,S₂,S₃. The accept from the machine screens Si ,S₂,S₃ is passed as the intake stock flow F_in into the inlet header 12 of the headbox 10. The bypass flow F_out of the inlet header 12 of the headbox 10 is returned to the bottom part of the deaeration tank 20.

Fig. 2 illustrates an exemplifying embodiment of the method and the device in accordance with the present invention. In the exemplifying embodiment shown in Fig. 2, three machine screens Sι ,S₂,S₃ have been connected in parallel. Into the parallel machine screens 82,82,83, a stock flow F₇ is fed, and from their accept side a stock flow F_in is obtained, which is passed to the headbox 10. The machine screens 8^82,83 are correspondingly provided with metering detectors 22,23,24, from which phase and speed signals are passed correspondingly through conductors 25,26,27 to the measurement unit 28. The metering detectors 22,23,24 are, for example, optical detectors or pulse detectors. The measurement unit 28 determines the phases and speeds of rotation of the machine screens Si ,S₂ and S₃ and transmits the data to the control unit 29 through the conductor 30. The control unit 29 regulates the relative speeds and/or phases of rotation of the machine screens S^,S₂ and S₃ so that the pressure pulsation that is formed is eliminated as efficiently as possible. The regulation of the speed and phase of rotation can be accomplished, for example, by braking down the speed of a machine screen S, , S₂ and S₃ or by connecting the machine screens 8₂,8₂,8₃ with a rpm-controlled drive. The machine screens Si , S₂ and S₃ can also be connected after one drive, or their speeds can be regulated by means of a frequency converter.

In the exemplifying embodiment described above, a wiring was examined which comprises three machine screens. The method in accordance with the invention can also be applied to wiring arrangements which comprise, for example, two parallel machine screens or a higher number of screens. The relative setting of the phases of the machine screens is carried out so that the phase measurement of one machine screen operates as a reference, and the phases of the other machine screens are regulated in relation to the reference. Application of the method in accordance with the invention does not require that the parallel machine screens are identical. For example, the numbers of blades in the machine screens can be different. If the screens differ from one another, their speeds of rotation are regulated so that the interference frequency caused by the screens is equal in respect of all machine screens. In such a case it is possible to accomplish regulation of the phase difference.

Fig. 3 illustrates results of metering of the pressure variations of machine screens, which results have been metered while three parallel machine screens have been in operation. Each curve represents a relative pressure variation metered from the intake side of the inlet header within a time period of 200 ms. For metering of the pressure variations, a piezoelectric pressure detector has been used. The speeds of rotation of the machine screens have been determined by means of an optical photocell. In each case, the metering has been carried out from a period of time slightly longer than one revolution. Each measured curve has been produced as an average from 300 samples. The metering cycles have been timed so that they start precisely at the same phase of the rotation of the device, in which case random variations present in the signal have been produced as an average equal to zero, and the variation produced by the device has remained. The frequencies of rotation of different machine screens always differ from one another, so that the results, computed by means of a timed time level analysis, represent the pulsation caused by each device.

By means of the method in accordance with the present invention, the pressure variation curves shown in Fig. 3 can be synchronized so that the variation of their sum pressure is minimized. In the case of three screens, the frequencies of rotation are adjusted equal, and the phase difference is adjusted to be 360°/3, i.e. 120° . An active regulation of the phase difference can be accomplished, for example, so that, when necessary, one or several screens is/are braked down on the basis of the information obtained from the phase metering.

In the following, the patent claims will be given, and the details of the invention can show variation within the scope of the inventive idea defined in said claims and differ even to a considerable extent from what has been stated above by way of example only.

Claims

1. A method for elimination of pressure pulsations occurring in the short circulation in a paper/board machine, which paper/board machine is provided with two or more machine screens (82,82,83), characterized in that the machine screens (81 ,82,83) are provided with phase metering detectors (22,23,24), the phase of the rotation of each machine screen (82 ,82,83) is determined, and, based on the determination of phase, the relative speeds of rotation and/or phase differences of the machine screens (82,82,83) are regulated so that the pressure variations arising from the machine screens (8 ,82,83) neutralize each other and the pressure pulse produced in the sum flow (F_in) is at the minimum.

2. A method as claimed in claim 1 , characterized in that the phase of rotation of one machine screen (8 ,82,83) is chosen as a reference, and the speeds and/or phases of rotation of the other machine screens (S2,S₂,S₃) are regulated in relation to the reference phase.

3. A method as claimed in claim 1 or 2, characterized in that the metering signals from the phase metering detectors (22,23,24) are passed to the measurement unit (28) through signal lines (25,26,27).

4. A method as claimed in any of the claims 1 to 3, characterized in that the speeds and/or phases of rotation of the machine screens (S2,S₂,S₃) are controlled by means of a control unit (29).

5. A method as claimed in any of the claims 1 to 4, characterized in that the speeds and/or phases of rotation of the machine screens (S2,S₂,S₃) are regulated by braking down a machine screen (S2,S₂,S₃).

6. A method as claimed in any of the claims 1 to 5, characterized in that the speeds and/or phases of rotation of the machine screens (85,82,83) are regulated by means of a rpm-controlled drive.

7. A device for elimination of pressure pulsations occurring in the short circulation in a paper /board machine, which paper/board machine is provided with two or more machine screens (S_j^.S_β), characterized in that the device comprises metering detectors (22,23,24) installed correspondingly on the machine screens (82,82,83) for determination of the speeds of rotation and/or of the phases of rotation and corresponding signal lines (25,26,27) which pass the information on metering of the speed and/or phase of rotation to a measurement unit (28) .

8. A device as claimed in claim 7, characterized in that the device comprises a signal line (30) for transfer of the information on the speed and/or phase of rotation from the measurement unit (28) to the machine screen (82 ,82,83) control unit (29).

9. A device as claimed in any of the claims 6 to 8, characterized in that the metering detectors (22,23,24) are optical detectors.

10. A device as claimed in any of the claims 6 to 9, characterized in that the metering detectors (22,23,24) are pulse detectors.