CN102666978A - Unit, structure and method for screening cellulose pulp - Google Patents

Abstract

The present invention relates to a screening unit (100), a screening arrangement (200) and a method for inspecting and/or adjusting a slot within a screening arrangement, wherein the screening unit (100) comprises an outer circumferential axial slot (122) formed on the outer perimeter of the screening unit (100) between a first opposing outer surface (123) of a rotating part (124) and a second opposing outer surface (133) of a stationary part (134), the first opposing outer surface (123) and the second opposing outer surface (133) forming the outer axial slot (122) both being located at the perimeter on the screening unit (100), whereby the outer axial slot (122) is accessible and adjustable before insertion of the screening unit (100) into the screening arrangement (200) and/or upon removal of the screening unit (100) from the screening arrangement (200).

Description

Unit, structure and method for screening cellulose pulp

technical field

The present invention relates to the treatment of cellulose pulp and in particular to a screening unit for a screening arrangement for cellulose pulp, a screening arrangement and a method related to screening cellulose pulp.

Background technique

A variety of "standard" operations are often performed when preparing cellulose pulp to be used for further processing, eg for papermaking and the like. For chemical pulp, the lignin of the chips is dissolved in a digester to separate the fibers within the chips from each other. The cooked pulp is then conveyed to a washing and screening structure. During the cooking process, not all wood chips are digested equally well, so the resulting pulp contains not only individually separated fibers, but also uncooked wood chips, knots and fiber bundles known as chips. Chips, knots and other impurities (eg sand, bark) that remain in the pulp can cause problems later in the pulp processing and therefore need to be removed. There are a number of well-known operations used alone or in combination to separate impurities from pulp, including settling, screening and vortex cleaning.

Screening generally refers to the operation in which the fibers of a pulp suspension are passed through a perforated plate with holes or slots while impurities are retained. Screening can also be done with some other kind of confinement area, such as a screen basket made of longitudinal bars with slots for passing a certain portion of the pulp suspension between the longitudinal bars. In a pulp mill there are often several screening operations at different points throughout the process.

There are several different types of screening equipment. One commonly used screening device is the so-called combination screen, which means that two or more screening stages are combined within the same screen housing. An example of such an apparatus is disclosed in EP 1 165 882, wherein a first cylindrical screening device is at least partially located within a second cylindrical screening device. In order to make the process less expensive, combined screening devices have been developed in which two different screening devices are combined into one device, thereby eliminating the need for eg a separate knot remover.

In order for a screening device of the type with cylindrical screens to function properly, it is necessary to have at least one rotating part generating pressure pulses in order to prevent the pulp suspension from merely covering the screening area without passing through it. In some embodiments, the screen itself rotates and cooperates with the stator, but a stationary screen and a separate rotor could also be used instead. The rotor or stator is usually provided with a pulsation device that generates pressure pulses to clean the screening openings for the fibrous suspension. For combined screening configurations, one of the screening elements is rotatable while the other is stationary. There are also two screening parts that are rotatable combined screening structures. Having one or several rotating parts creates several challenges, one of which is how to solve the problem of keeping the different flows within the device separated from each other while still being able to rotate. In combined screening structures, at least one such separation barrier is required, and typically one separation barrier per screening stage is included in the combined structure. For example a separation barrier may also be arranged to separate the flow from the receiving section of one stage from the flow to the injection section of the same stage. Barriers may also be provided to separate the injection flow to one stage from the injection flow to another stage. Basically, the rotating and stationary parts are arranged to seal against each other to ensure that the pulp suspension will flow through the screen drum and not between the stationary and rotating parts.

To enable rotation, a circumferential groove is formed between the rotating and stationary parts. The dimensions of the trough are designed to minimize the flow of pulp suspension through the trough during operation, thereby ensuring that there is no contact between the rotating surface and the stationary surface as the rotating surface turns. In some screening configurations, the slots are radial, which means that the width of the slots extends in a radial direction. The size of such radial slots is not adjustable due to the difference in diameter of the corresponding rotating and stationary cylindrical parts which determine the size of the slots. Another way to design slots is by forming axial slots, which means that the width of the slot extends in the axial direction. The size of the axial slots can be adjusted by adjusting the opposing surfaces forming the slots. In this context, the terms slot width and slot size are used interchangeably and mean radial extension when the slot is radial and axial extension when the slot is axial. The slot dimension of an axial slot may also be referred to as slot height.

During operation of the screening structure, the surfaces forming the grooves are subject to wear, which implies that the dimensions of the circumferential grooves change as the surfaces wear away. Temperature changes during operation can also affect tank dimensions. If the slot becomes too large, the function of separating the different streams within the screening structure will be worse, since the channels will be large enough to allow the separation volumes of the pulp suspension, eg the different receiving and injection parts, to mix. The risk of fibers entering the grooves and getting stuck will also increase, thereby further and more significantly increasing the wear on the surface forming the grooves, since the stuck fibers will cause an increased amount of wear.

In screening structures of the prior art, especially when more than one screening stage is involved, the inspection and adjustment of the axial slot dimensions is a rather complex operation due to the proximity contained in the screening housing for inspection and for subsequent adjustment The slots (if this is deemed necessary) are cumbersome.

In some screening configurations combining two different screening stages and having an inner and an outer screen, a removable screening unit comprising the inner screen and the rotor has been provided. This removable screening unit allows easy access to the inner screen. In one previously known construction, the screening unit is removed by first removing the top cover covering the screening housing and then lifting the removable screening unit upwards. In previously known screening structures, several disassembly steps were required before the screening unit could be removed.

Contents of the invention

A general object of the present invention is to provide an improved screening unit and structure for screening cellulose pulp. Another object is to provide a screening unit, a screening construct and a method for improving the performance of a screening construct comprising a screening unit. The specific object is to achieve a simpler, more efficient and more reliable method of checking and/or adjusting the slots between rotating and stationary parts in a screening structure comprising more than one screening stage. Another object is to provide screening units and screening structures which minimize the need to adjust such slots due to wear.

These objects are achieved according to the appended claims.

Briefly, the present invention proposes a screening unit, screening structure and method for inspecting and/or adjusting slots within the screening structure, wherein the outer circumferential axial slots are accessible and adjustable outside the screening structure. By having two surfaces on the removable screening unit forming an axial slot, the slot can be easily accessed, inspected and adjusted. Furthermore, the slot configuration does not change when inserted into the screening structure, and the slot dimensions are less sensitive to changes in temperature and/or pressure induced during operation of the screening structure containing the screening units. In previous constructions in which one of the surfaces forming the outer axial grooves was contained within the screening housing, pressure and temperature changes would affect the screening housing and the screening unit differently, resulting in eg differences in thermal expansion.

More specifically, according to the present invention, there is provided a screening unit, a screening structure and a method for inspecting and/or adjusting a tank, wherein the screening unit comprises a screen formed on the outer periphery of the screening unit, between the first opposite outer surface of the rotating part and The outer circumferential axial groove between the second opposing outer surfaces of the stationary part, the first opposing outer surface and the second opposing outer surface forming the outer axial groove are located at the upper periphery of the screening unit, thus, in the removable screening The outer axial slot is accessible and adjustable prior to insertion of the unit into the screening structure and/or upon removal of the removable screening unit from the screening structure.

According to one embodiment, the inner circumferential axial groove between the first opposing inner surface of the inner rotating part and the second opposing inner surface of the inner stationary part is also accessible prior to insertion into the screening structure and when removed from the screening structure and adjustable. Thus, the circumferential axial grooves can be accessed and adjusted in a simple manner and it is ensured that the grooves maintain their setting when inserted into the screening structure. Another advantage is that at least the entire circumference of the outer axial groove is inspectable and accessible, which allows changes/adjustments to be made only at specific points if required. The tank can also be easily inspected after adjustment and will not change due to pressure and temperature changes while running the screening structure. In embodiments with more than two screening stages and thus possibly more than two circumferential axial slots, all slot forming surfaces may be provided on the removable screening unit. Even in a configuration with only two screening stages, there may be more than two circumferential axial slots and all slot forming surfaces are advantageously arranged on the removable screening unit.

The inner rotating part may be the first screening means and the inner stationary part may be the part connected directly or indirectly to the support unit. However, other components may also constitute corresponding stationary and rotating components. For example, the first screening means may constitute an inner stationary part.

In one embodiment, the corresponding opposing surfaces forming the circumferential axial groove have extensions in the radial direction (R) and in the axial direction (A), and the corresponding opposing surfaces forming the groove are arranged as viewed from the central axis C about Corresponding slots are aligned in pairs over substantially the same radial distance. At least one of the corresponding opposing surfaces may comprise circumferentially arranged replaceable wear rings, which advantageously may consist of at least two circumferentially separable parts, so that the wear rings are easily removed for replacement. In one embodiment, the wear ring consists of four separable parts.

According to yet another embodiment, the wear rings have a clearance angle that passes through a portion of the thickness T of the wear rings, whereby the axial grooves formed between the wear rings diverge in a direction outwards seen from the center of the screening unit, the wear rings The thickness T corresponds to their extension in the radial direction R, while the grinding rings also have a height H corresponding to their extension in the axial direction A. In traditional designs, there is a risk of fibers getting caught between the rotating and stationary wear rings. By diverging the grooves in this way, the risk of fiber jamming and thus increased wear on the wear ring is reduced.

According to one embodiment, the wear rings have a corresponding clearance angle on the opposite side not facing the groove, so that they are rotatable. When adjusting the groove, an alternative to replacing the wear ring or part of the wear ring with a new wear ring may be to turn the wear ring or part/parts of the wear ring 180 degrees to create a new surface that maintains the predetermined groove dimensions.

The invention also relates to a method for inspecting and/or adjusting at least one outer circumferential axial groove using a screening unit and a screening structure as described above, wherein the screening unit is removed from the screening structure, the slot/slots are inspected and possibly adjustment, and the screening unit is then returned to its position within the screening structure.

Description of drawings

The present invention and its further objects and advantages are best understood by reference to the following description and accompanying drawings, in which:

Fig. 1 is a schematic cross-sectional view of a screening unit according to an exemplary embodiment of the present invention;

FIG. 2 shows a schematic longitudinal sectional view of a screening structure to which a screening unit is suitable, which shows the absence of a screening unit as shown in FIG. 1 within the screening structure.

Fig. 3 is a schematic cross-sectional view of a screening structure showing a removable screening unit in place within the screening structure according to an exemplary embodiment of the present invention.

Figure 4 shows removal/insertion of a screening unit according to the invention from/insertion of a screening unit according to the invention into a screening structure according to the invention.

5A-C show cross-sectional views of an outer circumferential axial groove according to an exemplary embodiment of the present invention, wherein FIG. 5A is an enlarged view of the enclosed portion in FIG. Exemplary embodiment of .

Fig. 6 is a schematic flow diagram of a method for inspecting and/or adjusting slots in a screening structure according to an exemplary embodiment of the present invention.

Detailed ways

In the drawings, similar or corresponding elements will be indicated by the same reference numerals.

It should be noted that although the following description primarily refers to combined screening structures in which the fine screen is located outside the coarse screen as viewed from the center of the screening structure, the invention is also applicable to "inside-out" configurations, i.e., the fine screen is located outside the coarse screen. inside. Therefore, it is within the scope of the present invention for the coarse screen to be located outside the fine screen for an inside-out configuration. It is also noted that these teachings are applicable to multi-stage screens in which several screening stages are performed in the same structure. Accordingly, these teachings are applicable whether or not the multi-stage screening apparatus includes coarse screens. The screening means in different stages may for example be the same type of opening with the same size. The screening devices of different stages can of course also have other variations of the opening size, instead of having to define a coarse screen or a fine screen separately.

The screening unit 100 in FIG. 1 includes a stator 110 installed on a support unit 111 . The rotor 121 is provided to be rotatable about a rotor shaft 125 included in the support unit 111 . The first screening device 101 is arranged on the rotor 121 , and the screening device is arranged to rotate together with the rotor 121 . The first screening device 101 has a cylindrical shape and is located outside the support unit 111 and the stator 110 coaxially with respect to the central axis C therewith. The stator may be provided with a plurality of impulse means 115 arranged to generate pressure pulses for cleaning the first screening means 101 as the rotary first screening means 101 rotates.

The outer stationary part 134 is arranged to cooperate with the rotor 121 to separate the flow of pulp suspension intended to be screened by the first screening device 101 from the flow of pulp suspension intended to be screened by the second screening device, so that when in the screening structure Operates separately from, but in conjunction with, screening unit 100 . The first portion 134a of the outer stationary part 134 is a cylindrical tubular device coaxially arranged with the first screening device 101, which has a diameter greater than the first screening device 101, thus, between the screening device 101 and the first external stationary part 134a A space is formed between them, and this space constitutes the first screening chamber 105. The first outer stationary part 134a is arranged to extend in an upward direction, whereby a substantial part of the first screening means 101 is surrounded by the stationary tubular means. In this figure, the first outer stationary part 134a is connected to a plurality of spaced rods 135 which in turn are connected to the base 131 . The bottom is connected to the support unit 111 and extends radially between the support unit 111 and the spaced rods 135 connected to the first outer stationary part 134a. The openings/slots between the bars allow access to the inner axial slots 102, as will be described further on. This access can also be achieved by having the first outer stationary part 134a extending down to the bottom 131 and providing its lower part with an opening giving sufficient access to said inner axial groove 102 .

The first outer stationary part 134a also has the function of forcing the pulp flow into the first screening chamber 105 mainly near the upper part of the first screening device 101 in operation. The stationary part 134 also includes a second outer stationary part 134b. The second outer stationary part 134b is arranged so that its outermost part is aligned on the same radial distance as the outer rotating part 124 as viewed from the central axis C. When using a screening unit in a screening structure, the second outer stationary part 134b and Together the outer rotating parts 124 act as a barrier to flow in the radial direction. According to one embodiment, the second outer stationary part 134b is a retaining means protruding from the first outer stationary part 134a.

An outer circumferential axial groove 122 is formed between the first opposing outer surface 123 and the second opposing outer surface 133 . The first opposing outer surface 123 is located on the outer rotating member 124 and the second opposing outer surface is located on the second outer stationary member 134b. The circumferential axial groove 122 has an extension in the radial direction R and an extension in the axial direction A, the axial extension being adjustable by adjusting the corresponding first and second surfaces forming the groove. The axial extension defines the dimensions of the groove. According to a preferred embodiment, the first opposite surface 123 and the second opposite surface 133 of the outer axial groove 122 are circumferential rings having a thickness T corresponding to their extension in the radial direction R and a thickness T corresponding to their extension in the axial direction A The height H of the extension. Circumferential rings may also represent wear rings, since they are suitable for interchange or change due to wear potentially occurring in connection with the groove. For example, the grooves can be adjusted by adjusting the ring into position, turning the ring to expose a "new", unworn surface, or by replacing a worn ring with a new ring. The extent of the groove in radial direction R is set by the thickness T of the wear ring. Thickness T is chosen such that the wear rings are thick enough to retain their original shape, regardless of the forces to which the rings are exposed during operation of the screening unit. The circumferential ring may advantageously consist of several parts and thus be easily removable. This design also enables adjustment, interchange or rotation of only a specific part or some specific parts of the circumferential ring.

An inner circumferential axial groove 102 is formed between a first inner opposing surface 103 and a second inner opposing surface 113, which are connected to an inner rotating part 104 and an inner stationary part 114, respectively. The inner rotating part 104 together with the inner stationary part 114 serves to form a radial barrier which prevents the flow of pulp suspension from the first screening chamber 105 from passing through the first screening means 101 when the screening unit is operating within the screening arrangement. And flow to the first receiving chamber 106 .

In Fig. 2 the screening structure according to the invention is shown when the screening unit described with respect to Fig. 1 is not in place within the screening structure. The screening structure comprises a screening housing 202 and a second screening device 201 intended to cooperate with the screening unit 100 described with respect to FIG. 1 . The term screening structure should be understood to describe the remaining components when the removable screening unit is removed and the overall structure when the removable screening unit is in place within the screening housing. The screening structure also includes a top cover 203 (as shown in FIG. 3 ) and a drive unit (not shown) connected to the rotor shaft 125 .

Figure 3 shows a screening structure comprising a removable screening unit. In operation, the pulp suspension is screened in two stages, first passing through the first screening device 101 . Receptacles from the first screening stage are then brought forward to the second screening stage; screening by the second screening means 201 . Outer and inner axial slots 122 and 102 are formed between the rotating and stationary parts of the screening structure.

In operation, a stream of pulp to be screened enters the first screening chamber 105 as it enters the screening structure via the main pulp inlet 211 , and the pulp is then fed towards the first screening device 101 . As before, the pulp is forced to flow upwards by the stationary parts 134 so that the pulp is introduced into the first screening chamber 105 mainly near the upper part of the first screening device 101 . Fibers smaller than the size of the openings in the perforated screen pass through the screening device 101 and into the first receiving chamber 106 . The receiving chamber is bounded by the screening device 101 and the stator 110 inside the screening device 101 . In the example shown here, the screening is carried out from the outside to the inside, which is especially performed in the first screening stage due to the centrifugal force which prevents the close contact of large and heavy particles with the screening device. section) is preferred. This screening stage is commonly known as a deknotter or deknotter.

The waste fraction, ie the particles that do not pass through the screen, is taken out via the first waste outlet 212 . The receiving portion is further fed to the second screening chamber 205 to form an infusion to the second screening device 201 . The pulp passing through the second screening device 201 is taken out via the receiving outlet 213, while the waste part is taken out via at least one waste outlet (not shown in the figures). The second screening device 201 may, for example, perform the task of mainly separating debris from fibers.

In one embodiment, the screening structure further comprises a secondary pulp inlet 215, which is arranged such that the pulp flowing through the secondary pulp inlet 215 is combined with the receiving portion from the first screening device 101 before entering the second screening device 201 mix. The pulp entering the secondary inlet 215 consists of an intake fraction from a subsequent screening arrangement (not shown) where the reject fraction from the second screening device 201 is screened again. The receptacle of the subsequent screening structure together with the receptacle from the first screening structure 101 forms an infusion to the second screening device 201 . In this way, the "good fibers" contained in the reject fraction from the second screening device 201 are brought back in the process to the pulp flow along which it travels, thus fiber losses are minimized. Such a secondary pulp inlet may be of the type described in Swedish patent application 0900351-8.

In one embodiment, the first screening device 101 is a coarse screen and the second screening device 201 is a fine screen. The term coarse screen refers to screens designed primarily to separate larger impurities such as nodules. Typically, the diameter of the opening may be 6-10 mm, usually 8-10 mm. The term fine screen refers to screening elements designed primarily to separate debris from fibres. For fine screens with grooves, the grooves may be in the range of 0.15-0.60 mm, typically 0.15-0.40 mm. Slots or holes can be used depending on the process parameters to be optimized.

FIG. 4 shows removal of the removable screening unit from the screening structure 200 . The eyebolt 126 is arranged on the rotor shaft 125 and viewed in the axial direction A at the top end of the rotor shaft. When the lifting device is connected to the eyebolts 126 and the screening unit 100, the top cover 203 shown in FIG.

5A-C illustrate an embodiment of a surface forming an axial groove, with FIG. 5A being an enlarged view of the area enclosed in FIG. 4 . Two embodiments of groove forming surfaces are shown in Figures 5B and 5C. In FIG. 5B an alternative embodiment is shown in which the surface forming the groove is provided with a clearance angle. This embodiment can be advantageously used in combination with the outer axial groove 122 . Figure 5C shows a more conventional slot design.

Figure 5B shows a first outer facing surface 123 and a second outer facing surface 133 which form the axial groove 122 therebetween. The opposite surface has an extension H in the axial direction A and an extension T in the radial direction R.

The clearance angle through a portion of the thickness T is chosen to be large enough to achieve the effect of preventing or at least reducing the amount of jammed fibers, but not yet large enough to affect the strength of the wear ring. If the clearance angle is set through the entire thickness, a sharp front end will be formed at the groove forming surface, making it difficult to define the groove size to be adjusted. Furthermore, there may be slight diameter variations around the entire perimeter of the circumferential groove, which need to be compensated for by choosing the clearance angle not to pass through the entire thickness T of the wear ring. That is, if the diameters of the opposing surfaces are different at certain points of the common perimeter, in the case of a sharp front the groove will be very large at that point. However, the larger the planar surface, the greater the degree of grinding and shearing compared to a "clean cut" of stuck fibers.

In embodiments where the first screening device 101 performs the function of the first screening stage and the first screening device 101 is a coarse screen, the outer axial slots 122 are generally subject to more wear than the corresponding inner axial slots 102 . This is due to the fact that the setting of the outer axial groove 122 is more stringent than the setting of the inner axial groove 102 and therefore requires a tighter setting. Since the opening of the first screening device 101 is relatively large in this case, the ratio of the inner axial groove 102 can be in the same range as the opening. When this concerns the outer axial grooves 122, it is more important to maintain the groove dimensions at a predetermined minimum distance to ensure that undesired mixing of the parts occurs. Smaller groove sizes mean a greater risk of contact between surfaces when fibers get stuck, leading to potential wear problems. Therefore, the alternative embodiment shown in FIG. 5B is particularly beneficial in combination with the outer axial groove 122 .

Fig. 6 shows schematically the inspection and possible adjustment of at least one circumferential groove in the screening unit 100 of the screening structure 200 by representing the steps of the method within boxes. In a first step, the screening unit is removed from the screening structure. The second step consists in inspecting at least one of the circumferential holes around its periphery. Inspection may be performed, for example, by visual inspection or measurement or a combination thereof. Upon noticing a deviation from the intended slot size, decide if any adjustments are required. If yes, take measures according to the third step. In cases where no difference is noticed, or if the difference is deemed to be within acceptable tolerances, the decision may be that no adjustment is necessary. In this case, the fourth step will be carried out directly. In the third step, there are a number of options on how to carry out the required adjustments, which are shown in dashed boxes in the figure. An alternative could be to adjust the position of one or several of the opposing surfaces to bring the slot size back to its predetermined size. In case the opposite surface comprises replaceable wear rings, a second option may be to replace one or several wear rings or parts of wear rings. Especially in the case where the opposite surface comprises a wear ring with a clearance angle provided on both sides of the wear ring, a third alternative is to turn the wear ring or part of the wear ring by 180 degrees, and when the adjustment step is completed, the screening unit Back inside the filter structure.

By allowing the possibility to adjust the slot dimensions of all circumferential slots outside the screening structure, the slot dimensions will not change due to pressure and temperature changes when operating the screening structure. In addition, the entire circumference, especially of the outer axial groove, can be easily inspected.

Although the invention has been described with reference to particular illustrated embodiments, it is emphasized that it also covers equivalents of the disclosed features as well as modifications and variations obvious to those skilled in the art. Accordingly, the protection scope of the present invention is limited only by the appended claims.

Claims (15)

1. A screening unit (100) used in a screening structure (200), the screening unit (100) comprising a rotor (121), a first screening device ( 101), a stator (110) arranged inside the first screening device and a support unit (111) arranged inside the stator, and the rotor (121) is rotatably mounted on the support unit (111) , the screening unit is adapted to be inserted into the screening structure (200) and removed from the screening structure (200), characterized in that, The screening unit comprises an outer circumferential axial groove (122) formed on the outer periphery of the screening unit between a first opposite outer surface (123) of an outer rotating part (124) and an outer stationary between the second opposing outer surfaces (133) of the component (134), Both said first opposite outer surface (123) and said second opposite outer surface (133) forming said outer axial groove (122) are located on the perimeter of said screening unit (100), thus, can be The outer shaft is accessible and adjustable prior to insertion of a removable screening unit (100) into said screening structure (200) and/or upon removal of said removable screening unit (100) from said screening structure (200) to the slot (122). 2. The screening unit of claim 1, wherein The screening unit also includes an inner circumferential axial groove (102) formed on a first opposing inner surface (103) of the inner rotating part (104) and a second opposing inner surface (114) of the inner stationary part (114) Between the inner surfaces (113), the removable screening unit (100) is removed before insertion into said screening structure (200) and/or from said screening structure (200). ) to access and adjust the inner axial groove (102). 3. The screening unit of claim 2, wherein: The inner rotating part (104) is the first screening device (101), and the inner stationary part (114) is the part connected with the supporting unit (111). 4. The screening unit according to any one of claims 1 to 3, wherein The first opposite outer surface (123) is located on the outer rotating part (124) of the rotor (121), and the second opposite outer surface (133) is located on the outer part connected to the support unit (111). on the stationary part (134). 5. The screening unit according to any one of claims 1 to 4, wherein Corresponding opposing surfaces (103, 113, 123, 133) forming said circumferential axial grooves (102, 122) have radial (R) and axial (A) extensions and form corresponding The opposing surfaces ( 103 , 113 , 123 , 133 ) are arranged to be aligned in pairs at substantially the same radial distance with respect to the corresponding groove ( 102 , 122 ), as seen from the center axis C. 6. The screening unit according to any one of claims 1 to 5, wherein At least one of said corresponding opposing surfaces (103, 113, 123, 133) comprises a circumferentially arranged replaceable wear ring. 7. The screening unit of claim 6, wherein Said corresponding opposing surfaces (123, 133) forming said outer axial groove (122) comprise circumferentially arranged replaceable wear rings. 8. The screening unit of claim 6 or 7, wherein At least one of said wear rings (103, 113, 123, 133) has a clearance angle through at least a portion of thickness T, whereby said wear rings (103, 113, 123, 133) formed between said At least one of the axial grooves (102, 122) is bifurcated in a direction looking outward from the center of the screening unit (100), and the thickness T corresponds to the radial direction R of the wear ring extension, and said wear rings (103, 113, 123, 133) also have a height H corresponding to their extension along said axial direction A. 9. The screening unit of claim 8, wherein The at least one wear ring (103, 113, 123, 133) has a corresponding clearance angle on the opposite side not facing the groove to enable rotation of the wear ring. 10. The screening unit according to any one of claims 6 to 9, wherein The wear ring (103, 113, 123, 133) comprises at least two parts which are separable in the circumferential direction to easily remove the wear ring for replacement. 11. A structure (200) for screening cellulose pulp in two or more stages, said structure comprising at least two screening devices (101, 201) surrounded by a casing (202), said screening The device is arranged coaxially with the central axis C, and the first screening device (101) is arranged to rotate together with the rotor (121), and the first screening device (101) and the rotor (121) are arranged as further comprising The removable screening unit (100) of the central support unit (111) is removed from the housing, the structure also includes a first opposing surface (123) formed on the outer rotating part (124) and a corresponding outer stationary part (134 ), said first and second opposing surfaces (123, 133) are arranged to act together to pass said second screening means (201) Separating the screened pulp suspension from the pulp suspension intended to be screened by said first screening device (101), characterized in that said screening structure comprises a filter unit. 12. The screening structure of claim 11, wherein The first screening device (101) is arranged partially inside the second screening device (201). 13. A method for inspecting and/or adjusting at least one axial groove (122, 102) in a screening structure (200) according to any one of the preceding claims, characterized in that removing said screening unit (100) from said screening structure (200), said screening structure comprising a screening housing (202) and a second screening device (201), inspecting and/or adjusting at least one of said first opposing surface (103, 123) and second opposing surface (113, 133) outside said screening housing (202), whereby said axial groove (122 , 102) to obtain the predetermined slot size; and The screening unit (100) is turned into position within the screening housing (202) of the screening structure (200). 14. The method of claim 13, wherein, At least one of the opposing surfaces (103, 113, 123, 133) includes a wear ring, and the step of adjusting includes turning the wear ring or part of the wear ring 180 degrees. 15. The method of claim 14, wherein, At least one of said opposing surfaces (103, 113, 123, 133) comprises a wear ring, and said step of adjusting comprises altering said wear ring or part of said wear ring.

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