EP2516734A1

EP2516734A1 - Unit, arrangement and method for screening of cellulose pulp

Info

Publication number: EP2516734A1
Application number: EP10839885A
Authority: EP
Inventors: Anders Bergdahl; David Lindkvist
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2009-12-21
Filing date: 2010-12-13
Publication date: 2012-10-31
Anticipated expiration: 2030-12-13
Also published as: WO2011078764A1; BR112012015644A2; CA2782867C; CN102666978B; SE534497C2; EP2516734A4; CA2782867A1; EP2516734B1; SE0901591A1; BR112012015644B1; CN102666978A

Abstract

The invention relates to a screening unit (100), a screening arrangement (200) and a method for inspecting and/or adjusting slots in a screening arrangement where the screening unit 100 comprises a circumferential outer axial slot(122)formed on the outer periphery of the screening unit (100) between a first opposite outer surface (123) of a rotary part (124) and a second opposite outer surface (133) of a stationary part(134), the first opposite outer surface (123) and the second opposite outer surface(133) forming the outer axial slot (122) both being located on the screening unit(100) at its periphery so that the outer axial slot (122) is accessible and adjustable before insertion into and/or upon removal of the removable screening unit (100) from a screening arrangement (200).

Description

Unit , arrangement and method for screening of cellulose pulp

TECHNICAL FIELD

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

BACKGROUND

When preparing a cellulose pulp to be used for further processing, such as for example papermaking or the like, a number of "standard" operations are often performed. For chemical pulp, lignin of wood chips is dissolved in a digester in order to separate the fibers within the wood chip from each other. The cooked pulp is then transported to washing and screening arrangements. During cooking, not all wood chips are equally well digested and the resulting pulp thus contains not only individually separated fibers, but also pieces of uncooked wood chips, knots and fiber bundles known as shives. The shives, knots and other impurities (e.g. sand, bark) still remaining in the pulp may cause problems in later stages of the pulp processing and thus needs to be removed. There are a number of well-known operations used, separately or in combination, to separate impurities from the pulp, including sedimentation, screening and vortex cleaning.

Screening commonly refers to an operation in which fibers of a pulp suspension pass through a perforated plate with holes or slots, while the impurities are retained. Screening can also be effectuated through some other kind of restricted area, e.g. a screening basket made up of longitudinal bars with slots for passage of a certain fraction of pulp suspension in between the bars. In a pulp mill, there are often several screening operations at different locations throughout the process.

There are several different kinds of screening apparatuses. One commonly used screening apparatus is a so-called combi-screen, meaning that two or several screening stages are combined within the same screen housing. An example of such an apparatus is disclosed in EP 1 165 882, where a first cylindrical screening means is located at least partly within a second cylindrical screening means. Such combined screening apparatuses have been developed in order to bring about a cheaper process where two different screening apparatuses are combined into one apparatus, eliminating the need for e.g. a separate knotter.

In order for a screening apparatus of the kind with cylindrical screens to function properly there needs to be at least one rotating part creating pressure pulses preventing the pulp suspension from simply covering the screening area and not passing through. In some embodiments the screen itself rotates and cooperates with a stator but it can also be the other way around with a stationary screen and a separate rotor. The rotor or stator is commonly provided with pulse means creating pressure pulses clearing the screen openings of fiber suspension. For combined screening arrangements, one of the screens may rotate while the other is stationary. There are also combined screening arrangements in which both screens are rotatable. Having one or several rotary parts give rise to several challenges, among which one is how to solve the problem of keeping different flows in the apparatus separated from each other while still enabling rotation. In a combined screening arrangement, at least one such separation barrier is needed, and most commonly one separation barrier per screening stage included in the combined arrangement is present. A separation barrier may for example be provided to separate the flow of an accept fraction from one stage from the flow of inject fraction to the same stage. A barrier may further be arranged to separate the flow of inject to a one stage from the flow of inject to another stage. Basically, the rotary and stationary parts are arranged to seal against each other so as to ensure that the pulp suspension will flow through the screening cylinders and not between the stationary and rotary parts.

In order to enable rotation, circumferential slots are created between the rotary and stationary parts. The slots are dimensioned to minimize the flow of pulp suspension therethrough when in operation, yet ensuring no contact between the rotary surface and the stationary surface upon rotation of the rotary surface. In some screening arrangements the slots are radial, meaning that the slot width extends in the radial direction. Such radial slots are not adjustable in size since it is the differences in diameter of the corresponding rotating and stationary cylindrical parts that determine the slot size. Another way of designing the slots is by creating axial slots, meaning that the slot width extends in the axial direction. The size of the axial slots can be adjusted by adjusting the opposite surfaces forming the slot. In this context, the terms slot width and slot size are used interchangeably and refers to the radial extension of the slot when the slot is radial and the axial extension of the slot when the slot is axial. The slot size of an axial slot may also be referred to as the slot height.

During operation of a screening arrangement the surfaces forming the slots may be subjected to wear, implying that the circumferential slot size may change as the surfaces wear down. Temperature changes during operation may also affect the size of the slot. If the slot gets too large, the function of separating different flows within the screening arrangement may be deteriorated, since the passage will be large enough to allow the separated volumes of pulp suspension, e.g. different accept and inject fractions to mix. The risk of fibers entering the slot and getting stuck will also increase, increasing the wear on the surfaces creating the slot even further and more dramatic, since a fiber getting stuck will cause an increased amount of friction.

In prior art screening arrangements, especially when comprising more than one screening stage, the inspection and adjustment of axial slot size is a fairly complicated operation, since it is tricky both to access the slot contained within the screening housing for an inspection as well as for the subsequent adjustment, should that be considered necessary.

In some screening arrangements combining two different screening stages and having an inner and an outer screen, a removable screening unit containing the inner screen and the rotor, has been provided. Such a removable screening unit enables easy access to the inner screen. In one previously known arrangement, the screening unit is removed by first removing a top lid covering a screening housing and then lifting the removable screening unit upwards. In the previously known screening arrangement, a number of demounting steps are needed before the screening unit can be removed.

SUMMARY

A general object of the invention is to provide an improved screening unit and arrangement for screening of cellulose pulp. A further object is to provide a screening unit, a screening arrangement and a method for improving the performance of a screening arrangement containing the screening unit. A specific object is to achieve a simpler, more efficient and more reliable method of inspecting and/or adjusting slots between rotary and stationary parts in a screening arrangement comprising more than one screening stage, and a screening unit for that purpose. A further object is to provide a screening unit and a screening arrangement which minimizes the need for adjustment of such slots due to wear.

These objects are achieved in accordance with the attached claims.

Briefly, the present invention proposes a screening unit, a screening arrangement and a method for inspecting and/or adjusting slots in a screening arrangement in which a circumferential outer axial slot can be accessed and adjusted outside of the screening arrangement. By having both of the surfaces forming the axial slot situated on a removable screening unit, the slot can easily be accessed, inspected and adjusted. Furthermore, the settings of the slot will not be altered upon insertion into the screening arrangement and the slot size is also less sensible for temperature and/or pressure induced alterations during operation of the screening arrangement containing the screening unit. In previous arrangements where one of the surfaces creating the outer axial slot is contained in the screening housing, pressure and temperature variations may affect the screening housing and the screening unit differently, leading to discrepancies in e.g. thermal expansion.

More specifically, in accordance with the invention is provided a screening unit, a screening arrangement and a method for inspecting and/or adjusting slots in which the screening unit comprises a circumferential outer axial slot formed on the outer periphery of the screening unit between a first opposite outer surface of a rotary part and a second opposite outer surface of a stationary part, the first opposite outer surface and the second opposite outer surface forming the outer axial slot both being located on the screening unit at its periphery so that the outer axial slot is accessible and adjustable before insertion into and/or upon removal of the removable screening unit from a screening arrangement.

According to one embodiment, a circumferential inner axial slot between a first opposite inner surface of an inner rotary part and a second opposite inner surface of an inner stationary part is also accessible and adjustable before insertion into and upon removal from the screening arrangement. Both the circumferential axial slots can thus be accessed and adjusted in an easy manner and it is ensured that the slots maintain their setting upon insertion into the screening arrangement. A further advantage is that the entire circumference of at least the outer axial slot is inspectable and accessible, making it possible to make alterations/adjustments only at specific points if needed. The slots can also easily be inspected after adjustment and will not change due to pressure and temperature variations upon bringing the screening arrangement into operation. In embodiments with more than two screening stages and thus possibly more than two circumferential axial slots, all slot-creating surfaces may be arranged on the removable screening unit. Even in arrangements with only two screening stages, there might be more than two circumferential axial slots and all of the slot-creating surfaces are then advantageously arranged on the removable screening unit.

The inner rotary part may be the first screening means and the inner stationary part may be a part directly or indirectly connected to the bearing unit. However, also other parts may constitute the respective stationary and rotary parts. As an example, the first screening means may constitute the inner stationary part.

In one embodiment, the respective opposite surfaces forming the circumferential axial slots have an extension both in a radial direction (R) and in an axial direction (A) and the respective opposite surfaces forming a slot are arranged to be aligned on substantially the same radial distance pairwise, in relation to the respective slot, seen from a center axis C. At least one of the respective opposite surfaces may comprise circumferentially arranged replaceable wear rings, which advantageously may consist of at least two in the circumferential direction separable parts for simple removal of the wear rings upon replacement. In one embodiment, the wear rings consist of four separable parts.

According to yet another embodiment, the wear rings have a clearance angle through a part of a thickness T of a wear ring so that the axial slot formed between the wear rings diverges in an outward direction seen from the centre of the screening unit, the thickness T of the wear rings corresponding to their extension in the radial direction R and the wear rings further having a height H corresponding to their extension in the axial direction A. In a conventional design, there may be a risk that fibers get stuck in between the rotary and stationary wear ring. By having such a divergence of the slot, the risk of fibers getting stuck and thus increasing the wear on the wear rings, is diminished.

According to one embodiment, the wear rings have a corresponding clearance angle on the opposite side not facing the slot so as to make them turnable. When adjusting the slot, an alternative to exchanging the wear rings or parts of the wear ring to new ones may be to turn the wear rings or part/parts of the wear ring 180 degrees in order to enable a fresh surface which maintains the predetermined slot size.

The invention also relates to a method for inspecting and/or adjusting at least one circumferential outer axial slot, using the screening unit and screening arrangement described above, in which the screening unit is removed from the screening arrangement, the slot/slots is /are inspected and possibly adjusted and the screening unit is subsequently returned into position in the screening arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description and appended drawings, in which:

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

Fig. 2 shows a schematic longitudinal cross-section of a screening arrangement for which the screening unit is adapted, illustrating the situation where a screening unit as illustrated in figure 1 is not present within the screening arrangement.

Fig. 3 is a schematic cross-sectional view of a screening arrangement according to an exemplifying embodiment of the invention, illustrating the situation where the removable screening unit is in position within the screening arrangement

Fig. 4 illustrates the removal/insertion of a screening unit according to the invention from a screening arrangement according to the invention

Figs. 5A-C illustrate cross-sectional views of an outer circumferential axial slot according to exemplifying embodiments of the invention, where figure 5 A is an enlargement of the encircled section in figure 4 and figures 5B-C illustrate exemplifying embodiments relating to the slot-creating surfaces.

Fig 6 is a schematic flow chart of a method for inspecting and/or adjusting slots in a screening arrangement in accordance with an exemplifying embodiment of the invention

DETAILED DESCRIPTION

In the drawings, similar or corresponding elements will be denoted by the same reference numbers.

It should be noted that, although the following description primarily refers to a combined screening arrangement in which a fine screen is located outwardly of a coarse screen, seen from the centre of the screening arrangement, the teachings are applicable also on "inside-out" arrangements, i.e. where the fine screen is located on the inside of the coarse screen. Inside-out arrangements with the coarse screen located on the outside of the fine screen hence lie within the scope of the present invention. It is also to be noted that the teachings are applicable to other multi-stage screens where several screening stages are performed in the same arrangement. The teachings are thus applicable irrespective of whether the multi-stage screening apparatus contains a coarse screen or not. The screening means in the different stages may for example be of the same kind with openings of the same size. The screening means in the different stages may of course also have other variations in opening sizes, not being bound to a definition of a coarse or a fine screen, respectively.

The screening unit 100 in Figure 1 comprises a stator 110 mounted on a bearing unit 111. A rotor 121 is arranged to be rotatable about a rotor shaft 125 contained in the bearing unit 111. A first screening means 101 is arranged on the rotor 121 and the screening means is arranged to rotate with the rotor 121. The first screening means 101 has a cylindrical shape and is located outside of and coaxially with the bearing unit 111 and the stator 110, seen with regard to a center axis C. The stator may be provided with a number of pulse means 115, arranged upon rotation of the rotary first screening means 101 to create pressure pulses for clearing the first screening means 101. An outer stationary part 134 is arranged to act together with the rotor 121 in separating the flow of pulp suspension intended to be screened through the first screening means 101 from the flow of pulp suspension intended to be screened through a second screening means, separate from but acting in cooperation with the screening unit 100 when in operation in a screening arrangement. A first part 134a of the outer stationary part 134 is a cylindrical tubular means arranged coaxially with the first screening means 101, having a diameter larger than the first screening means 101 so that a space is formed between the screening means 101 and the first outer stationary part 134a, the space constituting a first screening chamber 105. The first outer stationary part 134a is arranged to extend in an upward direction so that 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 number of spaced apart rods 135 which are in turn connected to a bottom part 131. The bottom part is connected to the bearing unit 111 and extends in a radial direction between the bearing unit 111 and the spaced apart rods 135 connected to the first outer stationary part 134a. The openings/slots between the bars enable access to an inner axial slot 102, which will be described further. Such access may also be enabled by extending the first outer stationary part 134a down to the bottom part 131 and providing the lower part thereof with openings enabling adequate access to said inner axial slot 102.

The first outer stationary part 134a also has the function of, when in operation, forcing the pulp flow to be introduced into the first screening chamber 105 mainly in the vicinity of the upper part of the first screening means 101. The stationary part 134 further comprises a second outer stationary part 134b. The second outer stationary part 134b is arranged to be aligned with its outermost part on the same radial distance seen from the center axis C as an outer rotary part 124, the second outer stationary part 134b and the outer rotary part 124 together acting as a barrier for flow in a radial direction when using the screening unit in a screening arrangement. According to one embodiment, the second outer stationary part 134b is a holder means protruding from the first outer stationary part 134a.

A circumferential outer axial slot 122 is formed between a first opposite outer surface 123 and a second opposite outer surface 133. The first opposite outer surface 123 is situated on the outer rotary part 124 and the second opposite outer surface is situated on the second other stationary part 134b. The circumferential axial slot 122 has an extension in a radial direction R and an extension in the axial direction A, the extension in axial direction being adjustable by adjusting the respective first and second surfaces forming the slot. The extension in the axial direction is defining the slot size. According to a preferred embodiment, the first opposite surface 123 and the second opposite surface 133 of the outer axial slot 122 are circumferential rings with a thickness T corresponding to their extension in the radial direction R and a height H, corresponding to their extension in the axial direction A. The circumferential rings may also be denoted wear rings, since they are adapted to be interchanged or altered due to wear potentially occurring in connection with the slot. The adjustment of the slot can for example be made by adjusting the rings into proper position, turning the rings in order to reveal a "fresh", unworn surface or by replacing worn rings with new ones. The extension of the slot in a radial direction R is set by the thickness T of the wear rings. The thickness T is chosen such that the wear rings are thick enough to retain their original shape, irrespective of the forces they are subjected to during operation of the screening unit. The circumferential rings may advantageously comprise several parts, so that an easy removal is possible. This design also enables only a specific part or some specific parts of the circumferential rings to be adjusted, interchanged or turned.

An inner circumferential axial slot 102 is formed between a first inner opposite surface 103 and a second inner opposite surface 113, the opposite surfaces being connected to an inner rotary part 104 and an inner stationary part 114, respectively. When the screening unit is in operation within a screening arrangement, the inner rotary part 104 acts together with the inner stationary part 114 in creating a radial barrier preventing flow of pulp suspension from a first screening chamber 105 to a first accept chamber 106 without passing the first screening means 101.

In figure 2, a screening arrangement according to the invention is illustrated when the screening unit described in relation to figure 1 is not in position within the screening arrangement. The screening arrangement comprises a screen housing 202 and a second screening means 201 intended to cooperate with a screening unit 100 as described in relation to figure 1. The term screening arrangement is to be understood both as describing the parts remaining when the removable screening unit is removed as well as the entire arrangement when the removable screening unit is in position within the screening housing. The screening arrangement further comprises a top cover 203 (as illustrated in figure 3) as well as a driving unit (not illustrated) connected to the rotor shaft 125. Figure 3 illustrates the screening arrangement containing the removable screening unit. In operation, pulp suspension is screened in two stages, first through the first screening means 101. The accept portion from the first screening stage is then brought forward to a second screening stage; screening taking place through the second screening means 201. Outer and inner axial slots 122 and 102 are formed between the rotary and stationary parts of the screening arrangement.

In operation, a pulp flow to be screened enters the first screening chamber 105, upon entry into the screening arrangement through a main pulp inlet 211, and the pulp is then fed towards the first screening means 101. As previously described, the pulp is forced to flow upwardly due to the stationary part 134 in order for the pulp to be introduced into the first screening chamber 105 mainly in the vicinity of the upper part of the first screening means 101. Fibers of a size smaller than the size of the openings in the perforated screen plate pass through the screening means 101 and enter a first accept chamber 106. The accept chamber is limited by the screening means 101 and by the stator 110 located inside the screening means 101. The screening is in this illustrated embodiment performed from the outside- in, which is preferable especially when the first screening stage performs the task of separating mainly larger impurities (e.g. knots), due to the centrifugal force preventing large and heavy particles being in close contact with the screening means. Such a screening stage is commonly known as a knotter or deknotter.

The reject portion, i.e. the particles not passing through the screen, is taken out through a first reject outlet 212. The accept fraction is further fed to a second screening chamber 205 to form an inject to the second screening means 201. The pulp passing the second screening means 201 is taken out through an accept outlet 213 while the reject portion is taken out through at least one second reject outlet (not illustrated in the figure). The second screening means 201 may for example perform the task of separating mainly shives from fibers.

In one embodiment, the screening arrangement may further comprise a secondary pulp inlet 215 being arranged so that pulp flowing through the secondary pulp inlet 215 is mixed with the accept portion from the first screening means 101 before entering the second screening means 201. The pulp entering the secondary inlet 215 may consist of an accept fraction from a subsequent screening arrangement (not illustrated) in which the reject fraction from the second screening means 201 has been screened again. The accept portion of the subsequent screening arrangement, together with the accept portion from the first screening means 101 forms the inject to the second screening means 201. In this way, the "good fibers" contained in the reject portion from the second screening means 201 is brought back to the pulp flow moving along in the process and fiber losses are minimized. Such a secondary pulp inlet may be of the kind described in Swedish patent application 0900351-8.

In one embodiment, the first screening means 101 is a coarse screen and the second screening means 201 is a fine screen. By the term coarse screen is meant a screen designed primarily to separate larger impurities such as knots. Typically the openings may be 6-10 mm, normally 8- 10 mm in diameter. By the term fine screen is meant a screen designed to primarily separate shives from fibers. For a slotted fine screen, the slots may be in the range of 0.15-0.60 mm, typically 0.15-0.40 mm. Slots or holes may be used dependent on which process parameters are to be optimized.

Figure 4 illustrates the removal of the removable screening unit from the screening arrangement 200. A lifting eye bolt 126 is provided on the rotor shaft 125 at its top end seen in the axial direction A. The top cover 203 illustrated in Figure 3 is removed whereupon a lifting device is connected to the lifting eye bolt 126 and the screening unit 100, after demounting from the driving unit, is lifted upwards to be removed from the screening arrangement 200.

Figures 5A-C illustrate embodiments of the surfaces creating the axial slots, where figure 5A is an enlarged view of the encircled area in figure 4. In figures 5B and 5C, two embodiments of the slot-creating surfaces are shown. In fig 5B, an alternative embodiment, in which the surfaces creating a slot are provided with a clearance angle, is illustrated. This embodiment may advantageously be used in connection with the outer axial slot 122. Figure 5C illustrates a more conventional slot design.

Figure 5 B shows a first outer opposite surface 123 and a second outer opposite surface 133 forming in between themselves an axial slot 122. The opposite surfaces have an extension H in the axial direction A and an extension T in the radial direction R. The clearance angle through a part of the thickness T is chosen such that it is large enough to get the effect of preventing or at least reducing the amount of fibers getting stuck, but yet not large enough to impair the strength of the wear ring. If the clearance angle should be provided through the entire thickness, sharp tips would be formed at the slot creating surfaces, making it hard to define the slot size to be adjusted. In addition, there might be slight diametrical variations around the entire periphery of the circumferential slot which needs to be compensated for by choosing the clearance angle not to go through the entire thickness T of the wear ring. I.e., if the diameter of the opposing surfaces at some point of the common periphery is different, the slot will at that point be very large in case of sharp tips. However, the larger the flat surface the larger the degree of grinding and shearing as opposed to a "clean cut" of a fiber about to get stuck.

In an embodiment where the first screening means 101 performs the function of the first screening stage and the first screening means 101 is a rather coarse screen, the outer axial slot 122 is commonly subjected to more wear than the corresponding inner axial slot 102. This is due to the fact that the settings of the outer axial slot 122 is more critical than the settings of the inner axial slot 102 and thus needs to be tighter set. Since the openings of the first screening means 101, in such a case, are comparably large, the proportions of the inner axial slot 102 may be in the same range as the openings. When it comes to the outer axial slot 122, it is more important to maintain the slot size at the predetermined minimum distance in order to make sure that undesired mixing of fractions occur. A smaller slot size implies an increased risk of contact between surfaces as well as fibers getting stuck, leading to potential wear problems. The alternative embodiment illustrated in 5B is therefore especially beneficial in connection with the outer axial slot 122.

Figure 6 schematically illustrates the method of inspecting and possibly adjusting at least one circumferential slot in a screening unit 100 of a screening arrangement 200, by indicating the steps of the method in boxes. In the first step the screening unit is removed from the screening arrangement. The second step comprises inspecting at least one of the circumferential slots around its circumference. The inspection may be carried out for example by visual inspection or measurements or combinations thereof. In case discrepancies from the predetermined slot size are noticed, it is decided whether any adjustments are needed or not. If yes, measures according to the third step are taken. In case no discrepancies are noted, or if they are considered to be within an acceptable tolerance, the decision might be that no adjustments are necessary. In such a case, the fourth step will be carried out directly. In the third step there are a number of options regarding how to carry out the required adjustment, in the figure illustrated in dashed boxes. One alternative may be to adjust the positions of one or several of the opposing surfaces in order to bring the slot size back to its predetermined size. A second option, in case the opposing surfaces comprise replaceable wear rings, may be to replace one or several of the wear rings or parts of wear rings. A third alternative, especially in case the opposing surfaces comprises wear rings with a clearance angle provided on both sides of the wear ring, is to turn the wear ring or parts of the wear ring 180 degrees, When the adjustment step is completed, the screening unit is returned into the screening arrangement.

By allowing the possibility of adjusting the slot size of all circumferential slots outside of the screening arrangement, the slot size will not change due to pressure and temperature variations upon bringing the screening arrangement into operation. Additionally, it enables an easy inspection of the entire circumference especially of an outer axial slot.

Although the invention has been described with reference to specific illustrated embodiments, it is emphasized that it also covers equivalents to the disclosed features, as well as changes and variants obvious to a person skilled in the art. Thus, the scope of the invention is only limited by the appended claims.

Claims

1. A screening unit (100) for use in a screening arrangement (200), the screening unit (100) comprising a rotor (121), a first screening means (101) arranged to be rotatable with the rotor (121), a stator (1 10) arranged inwardly of the first screening means and a bearing unit (1 1 1) arranged inwardly of the stator (110) on which bearing unit (11 1) the rotor (121) is rotatably mounted, the screening unit being adapted for insertion into and removal out of the screening arrangement (200), characterized in that

the screening unit comprises a circumferential outer axial slot (122) formed on the outer periphery of the screening unit between a first opposite outer surface (123) of an outer rotary part (124) and a second opposite outer surface (133) of an outer stationary part (134), the first opposite outer surface (123) and the second opposite outer surface (133) forming the outer axial slot (122) both being located on the screening unit (100) at its periphery so that the outer axial slot (122) is accessible and adjustable before insertion into and/or upon removal of the removable screening unit (100) from a screening arrangement (200).

2. The screening unit of claim 1, characterized in that

the screening unit further comprises a circumferential inner axial slot (102) formed between a first opposite inner surface (103) of an inner rotary part (104) and a second opposite inner surface (113) of an inner stationary part (114), the inner axial slot (102) also being accessible and adjustable before insertion into and/or upon removal of the removable screening unit (100) from the screening arrangement (200).

3. The screening unit of claim 2, characterized in

the inner rotary part (104) being the first screening means (101) and the inner stationary part (114) being a part in connection with the bearing unit (111).

4. The screening unit of any of claims 1-3, characterized in that

the first opposite outer surface (123) is situated on an outer rotary part (124) being the rotor (121) and the second opposite outer surface (133) is situated on a outer stationary part (134) connected to the bearing unit (111).

5. The screening unit of any of claims 1-4, characterized in that

the respective opposite surfaces (103, 113, 123, 133,) forming the circumferential axial slots (102, 122) have an extension both in a radial direction (R) and in an axial direction (A) and that the respective opposite surfaces (103, 113, 123,133) forming a slot are arranged to be aligned on substantially the same radial distance pairwise in relation to the respective slot (102, 122) seen from a center axis C.

6. The screening unit of any of claims 1-5, characterized in that

at least one of the respective opposite surfaces (103, 1 13 , 123, 133) comprises circumferentially arranged replaceable wear rings.

7. The screening unit of claim 6, characterized in that

the respective opposite surfaces (123, 133) forming the outer axial slot (122) comprises circumferentially arranged replaceable wear rings.

8. The screening unit of claim 6 or 7, characterized in that

at least one of the wear rings (103, 113, 123, 133) have a clearance angle through at least a part of a thickness T so that at least one of the axial slots (102, 122) formed between the wear rings (103, 113, 123, 133) diverges in an outward direction seen from the centre of the screening unit (100), the thickness T corresponding to their extension in the radial direction R and the wear rings (103, 113, 123, 133) further having a height H corresponding to their extension in the axial direction A.

9. The screening unit of claim 8, characterized in that

the at least one wear ring (103, 113, 123, 133) have a corresponding clearance angle on the opposite side not facing the slot so as to make it turnable.

10. The screening unit of any of claims 6-9, characterized in that

the wear rings (103, 113, 123, 133) comprise at least two in the circumferential direction separable parts for simple removal of the wear rings upon replacement.

11. An arrangement (200) for screening of cellulose pulp in two or more stages comprising at least two screening means (101, 201) enclosed by a housing (202), the screening means being arranged co-axially with a center axis C and the first screening means (101) being arranged to rotate with a rotor (121), the first screening means (101) and the rotor (121) being arranged to be removable from the housing as a removable screening unit (100) also comprising a central bearing unit (111), the arrangement further comprising an circumferential outer axial slot (122) formed between a first opposite surface (123) of an outer rotary part (124) and a second opposite surface (133) of a corresponding outer stationary part (134), the first and second opposite surfaces (123, 133) being arranged to act together to separate pulp suspension intended to be screened through the second screening means (201) from pulp suspension intended to be screened through the first screening means (101) characterized in that the screening arrangement comprises a screening unit according to any of claims 1-10.

12. A screening arrangement according to claim 1 1 characterized in that

the first screening means (101) is arranged to be located partly within the second screening means (201).

13. A method for inspecting and/or adjusting at least one axial slot (122, 102) in a screening arrangement (200) according to any of the previous claims, characterized by

removing the screening unit (100) from the screening arrangement (200) comprising the screening housing (202) and also the second screening means (201),

inspecting and /or adjusting at least one of the first opposite surfaces (103,123) and the second opposite surfaces (113,133) outside of the screening housing (202) so that the axial slot (122, 102) obtains a predetermined slot size; and

returning the screening unit (100) into position within the screening housing (202) of the screening arrangement (200).

14. The method of claim 13, characterized in that

at least one of the opposite surfaces (103, 113, 123, 133) comprises wear rings and that the adjusting step comprises turning the wear rings or parts of them 180 degrees.

15. The method of claim 14, characterized in that

at least one the opposite surfaces (103, 113, 123, 133) comprises wear rings and that the adjusting step comprises changing the wear rings or parts of them.