TECHNICAL FIELD

The invention described here is a screen intended to be used when withdrawing or extracting liquid from column of cellulosic material descending down in a vertical treatment vessel in continuous cooking systems using single or multiple treatment vessels. The preferable field of application is the withdrawal or extraction of impregnation or cooking liquors when producing chemical paper pulp in a continuous digester or impregnation vessel. STATE OF THE ART AND PROBLEMS

In state of the art digesters the problem with limited available service time during shut down requires digester screen designs that enable quick and easy access to withdrawal compartments behind screens. During operation these withdrawal compartment behind the screens tends to be blocked by different kind of precipitations, either lignin precipitation or harder scaling such as calcium carbonate scaling. Thus, the withdrawal compartment must be made ready available for access using high pressure jet cleaning or even acidic cleaning operations.

A cost efficient way of installing sufficient withdrawal capacity in screens rows (at least one row of screens and sometimes up to 7 rows) usually consist of screens in a row which are often arranged in a chessboard-like pattern at the desired withdrawal or extraction positions in the digester wall. In this design each second screen has withdrawal slots while each second has a blind plate. The reason for this less expensive screen row is that each slotted screen has capacity to withdraw liquor from column passing also the blind plates at each side of the slotted screen, as the major problem for withdrawal capacity is not the pressure drop over the slotted screens per se, instead it is the pressure drop radially into the column of chips, as the treatment vessel could have a diameter over 8 meter. The width of an individual screen (or blind plate) is conventionally between 500 mm to 1000 mm, so the withdrawal distance from a screen and sideways to middle of a neighboring blind plate are only fractions of the distance to center of pulp column.

Preferably each slotted screen consists of a multiplicity of vertically arranged staves in the conventional well proven stave design, but also slotted screen plates may be used. Between each stave there is a gap through which liquid can be withdrawn. The staves are fixed in parallel at horizontal backing members and the screen is mounted in a permanent mounting frame in the digester shell. In order to provide access to the withdrawal compartment behind screens has in principle two different designs been used in commercial digester, i.e. using screens with easily demountable inspection covers that either are installed in blinded screen sections or in a smaller lower part of the slotted screen section, or using screens that have a hinge on one vertical side of the screen.

The solution with inspection covers only enable a limited access to a part of the withdrawal compartment, and the entire withdrawal compartment becomes accessible only via special longer nozzles and without full visual view of the cleaning effect in distant parts of the withdrawal compartment. Thus, some precipitations may still be left in areas of the withdrawal compartment after cleaning.

The solution with vertical hinges on the screens enable full visual view the withdrawal compartment behind the screen during cleaning but are susceptible for mechanical wear on the hinges from the descending chip column, and the hinge designs may also create an obstacle for chip column movement and in the extreme may cause plugging by chip bridging over these parts of the hinge that protrudes into the chip column pathway.

It is thus of importance to find alternative designs for screens that both provides for a less expensive screen design that provides easy and quick access to the withdrawal compartment, and do not provide potential chip column blocking during operation. The essential function to be obtained is a free access to the lowermost part of the withdrawal compartment and an unobstructed flush out flow during cleaning that makes the cleaning operation both quick and effective.

SOLUTION AND ADVANTAGES

The inventive treatment vessel screen provides for an inexpensive screen design that enable a flush inside of the treatment vessel wall while still providing for a quick and easy access to the lower part of the entire withdrawal chamber. The inventive screen is based upon a simple slide in and slide down mounting procedure that during service access for cleaning the withdrawal chamber is reversed in order for exposing the withdrawal chamber.

Thus, the inventive treatment vessel screen is used for withdrawing treatment liquors in vertically oriented and generally cylindrical treatment vessels in continuous systems. Said treatment vessel comprising an outer cylindrical pressure vessel wall with a withdrawal compartment between the pressure wall and a screen with withdrawal slots facing the descending column of cellulosic material in the treatment vessel. A mounting frame for said screen is attached to the pressure wall. The treatment vessel screen is characterized in that the screen has an upper and lower flat vertical tongue, where first the upper flat tongue could slide into an upper recess in an upper edge of the frame while holding the screen slightly inclined versus the vertical and thereafter while first tilting the screen to a vertical position sliding the lower flat vertical tongue into a lower recess in a lower edge of the frame into a seating position where the screen in the seating position is form locked radially into position by both the upper and lower recess.

In the most basic version could no other complementary locking members be used, no filled welds or bolts, as the force exposed to the screen are in principal only the radial force pushing the screen outwards due to pressure drop over the screen, and the downward rubbing force from the descending pulp column. When the withdrawal space behind the screen is to be inspected and/or cleaned, then the screen may simply be lifted by a jack applying a lifting force on the screen to such an extent that the lower flat tongue comes free from the lower recess and the screen could be swung out of its vertical position around the upper end of the screen.

Further, in detail the screen and dimensions therefore could in a preferred embodiment have following features:

• the vertical dimension of the screen has a length exceeding the vertical dimension between an upper and a lower horizontal frame member of the mounting frame, and

• wherein an upper edge of the screen is formed like a vertically directed first flat tongue insert able in a vertically oriented upper recess located below and radially outside of an interior treatment vessel plate forming part of the mounting frame above the screen, and

· where a lower edge of the screen includes a second flat tongue insert able in a vertically oriented lower recess located in the mounting frame below the screen,

• whereas the vertical distance between the bottoms of the upper and lower recess exceed the vertical distance between the upper end of the upper tongue and the lower end of the lower tongue, and

· where the upper recess has a vertical depth equal to or exceeding the vertical depth of the lower recess, and

• allowing mounting of the screen to the mounting frame by sliding the upper tongue into the upper recess and pushing the lower part of the screen towards the mounting frame and finally dropping the screen to locking position with lower tongue located in the lower recess. In a preferred embodiment of the inventive treatment vessel screen is a locking member attached to a screen or frame preventing any vertical movement of the screen.

In another preferred embodiment of the inventive treatment vessel screen is the screen equipped with a hinge mechanism at upper end of screen allowing the lower end of the screen to swing away from frame while swiveling the upper end of the screen around the hinge mechanism.

In yet another preferred embodiment of the inventive treatment vessel screen is the hinge mechanism including a hook like member and a hinge rod, each mounted respectively on either the frame or the screen.

In yet an alternative embodiment of the inventive treatment vessel screen is the hinge mechanism including a radially directed rod mounted to the frame and passing through the upper recess and a vertically elongated eye on the upper tongue, with the radially directed rod passing through the elongated eye.

The treatment vessel screen according to a preferred alternative may be designed such that the screen and frame have a width less than 5 % of the circumference of the circular treatment vessel, such that at least 20 screens are needed to form a continuous screen row around the circumference of the circular treatment vessel, and where each screen and associated frame are oriented in one common plane and thus deviates from the circular interior wall of the treatment vessel. The flat design of the screen and associated frame and recesses, provides for a simple and inexpensive manufacturing of the slide in/slide out screen and associated frame.

The treatment vessel screen according to a preferred alternative may be designed such that the screen has a slotted face facing the column of cellulosic material as the column descends down the treatment vessel, and the slotted face is supported by backing members that take support from the cylindrical pressure vessel wall via support legs integrated with the backing members. The load on the screen will then in major part be distributed to the cylindrical pressure vessel wall, and the frame will only partly assist in load distribution and mainly function as a form locking member during mounting. BRIEF DESCRIPTION OF THE FIGURES

The invention will be described below using the attached figures: Fig. 1 shows a state of the art digester with a production capacity of 5000 ADMT/24h, and the large screen sections installed;

Fig. 2 shows a state of the art screen section with two screen rows, with screens arranged in the "chessboard"-like pattern design;

Fig. 3 shows a state of the art stave screen with a hinge at one side;

Fig. 4 shows the screen in Fig. 3 as seen in section Ill-Ill;

Figs. 5a-5c show a first embodiment of the inventive screen in 3 different positions, i.e. fully mounted in frame, swiveled out of position, and finally removed from frame;

Fig. 6 shows a detail view of the encircled upper part A in Fig. 5a;

Fig. 7 shows a detail view of the encircled lower part B in Fig. 5a;

Fig. 8 shows an alternative inventive screen design;

Fig. 9 shows a version of the screen seen in Fig. 8;

Fig. 10a shows an upper hinge mechanism for the screen;

Fig. 10b shows the screen in Fig.10a as seen in section X-X;

Fig. 11 shows an alternative upper hinge mechanism for the screen;

Fig. 12 shows the principle with flat screens arranged in a cylindrical vessel; and

Figs. 13a-13d show different kind of slotted screens that could be used, with an upper frontal view of the screen as viewed from the inside of the vessel and a lower sectional view of the screen.

DETAILED DESCRIPTION

Fig. 1 shows a state of the art digester 1 operated with Compact Cooking™ cooking profiling with a production capacity of 5000 ADMT/24h hardwood. Two large screen sections SSi and SS2 are needed in order to enable withdrawal of treatment liquors at middle of cook, SSi , and at end of cook, SS2, according to the cooking profiling used. Each screen section may include as many as 5-6 screen rows, 2a-2e and 3a-3f respectively, with as many as over 40 individual screens in each row. Hence, a total of more than 400 screens needs to be accessed in order to clean out precipitations that have formed in withdrawal compartments of the screens. Normally, the cleaning operations are typically planned to take place during annual service shut downs, but the numbers of screens installed may involve such an amount of work for dismantling, cleaning and inspection that a total clean out process is not able to be completely done during such single shut down. The available shut down time in pulp mills for regular service is normally restricted in time as each day involve a loss of income in order of 500.000 USD per day for a 5000 ADMT/24h digester.

In Fig. 2 is shown a screen section in a small digester with two screen rows, with screens arranged in a chessboard-like pattern at the desired withdrawal or extraction positions in the digester wall. In this design each second screen SC has withdrawal slots while each second screen has a blind plate BP. The reason for this less expensive screen row is that each slotted screen has capacity to withdraw liquor from column passing the blind plates BP, as the major problem for withdrawal capacity is not the pressure drop over the slotted screens per se, instead it is the pressure drop in the column of chips. The screen rows are conventionally arranged in a step out section of the digester pressure shell DS, forming a withdrawal chamber between the screen SC and the digester pressure shell. The spent treatment liquor withdrawn through the screens is first collected in the withdrawal chamber, which in turn drains down to a header HEi located below the withdrawal chamber via a draining hole (not shown here but seen in Fig. 7) in a wall separating the withdrawal chamber from the header HEi , and finally the spent treatment liquor BLEX is extracted from header through the pressure vessel shell via outlet studs.

In these designs the precipitation problems typically occurs in areas where the flow velocity is low and where precipitate accumulates in bottom due to gravity, i.e. in lower part of the withdrawal chamber. As indicated, access to the withdrawal chambers may be obtained by arranging inspection covers IC in the blind plates BP (as shown in Fig. 2), or alternatively arranged in lowermost part of the screen SC (not seen in this figure). An alternative solution enabling access to the withdrawal chamber for cleaning is shown in Fig. 3, using a state of the art stave screen SC with vertical slots with a hinge 31 at one side. The screen is mounted onto a load supporting mounting frame welded to digester shell having an upper horizontal frame member 20a and a lower horizontal frame member 20b, and, first and second vertical frame members 21a and 21 b respectively along each side of the screen. The individual staves 10 in the stave screen are mounted, typically by welding, to horizontally oriented backing members 22. Fig. 4 shows the screen in section view IV-IV, and the first and second vertical frame members 21a and 21 b are supporting the screen and are welded to the pressure vessel wall DS. As seen here are all individual T-shaped staves 10 in the stave screen mounted, typically by welding, into corresponding mounting slots of the backing members 22. In this embodiment are also radially directed support legs 22a of the backing members finding structural support in the pressure vessel wall DS. This hinge design is exposed to wear from the descending column of cellulose material, and material may be caught piling up at the upper surface of the hinge. If the digester is equipped with 20-40 screens in a screen row a corresponding number of plug column hang up positions are distributed over the circumference. Normally is the screen either held in position with filled welds along the top and sides, or as shown here with locking bolts 30 distributed over the screen surface frame 1 1. Now, welds introduce cumbersome work to remove them during service and bolts may be eroded to such an extent that small dismantling plates needs to be spot-welded to each head of the bolts for removal of bolts, even if the bolts may be recessed into the screen frame.

In Figs. 5a, 5b and 5c is shown a first embodiment of the inventive treatment vessel screen, with Fig. 5a showing the treatment vessel screen SC fully mounted, Fig. 5b with the treatment vessel screen SC swung out of position, and in Fig. 5c with the treatment vessel screen completely removed. The vertical dimension Vsc of the screen SC has a length exceeding the vertical dimension VFR between an upper 20a and a lower 20b horizontal frame member of the mounting frame. The upper edge SCUE of the screen is formed like a vertically directed first flat tongue insert able in a vertically oriented upper recess URE located below and radially outside of an interior treatment vessel plate DPL forming part of the mounting frame above the screen. The lower edge SCLE of the screen includes a second flat tongue insert able in a vertically oriented lower recess LRE located in the mounting frame below the screen. The vertical distance VB between the bottoms of the upper and lower recess exceed the vertical distance Vsc between the upper end of the upper tongue and the lower end of the lower tongue of the screen. Also, the upper recess URE has a vertical depth exceeding the vertical depth of the lower recess LRE. Hence, this design is allowing mounting of the screen to the mounting frame by sliding the upper tongue into the upper recess and pushing the lower part of the screen towards the mounting frame and finally dropping the screen to locking position with lower tongue located in the lower recess. As indicated in Fig. 6, after the screen has been dropped into the locking position, locked in aspects of radial movement, could also a locking member LMi , here in form of a screw, be mounted in the upper tongue, and directly below the interior treatment vessel plate DPL, such that the screen also becomes locked in place in aspects of vertical movement as well. However, this locking member will not assume any substantial stress load as the screen in operation is subjected to a vertically downwards rubbing action from the descending pulp column in the treatment vessel, and preferably could only a single locking member LMi be installed. In this embodiment is the screen SC equipped with horizontal backing members 22 with support legs 22a that comes into contact with the digester pressure shell DS.

In Fig. 8 is shown an alternative with a self-supported screen design, i.e. where the load on the screen is fully supported by the surrounding frame, and no support legs in backing members are used finding support in digester pressure shell. In such support structure both the upper recess URE and the lower recess LRE has a radial extension congruent to, or slightly exceeding, the radial extension or thickness of the upper and lower edge, SCUE and SCLE respectively of the screen. In Fig. 9 is shown an alternative but with a screen design with support legs 22a on the backing members finding support in digester pressure shell. In this kind of design may the entire load support for the screen be assumed by the backing members and no load support from the surrounding frame. The frame only carries the load from the weight of the screen. The major 5 load exposed to the screen is the load applied on the screen from the pressure differential over the screen, i.e. a load directed in the radial direction. In such support structure both the upper recess URE and the lower recess LRE has a radial extension well exceeding, the radial extension or thickness of the upper and lower edge, SCUE and SCLE respectively of the screen, i.e. the recesses are formed between digester pressure shell DS and a lip of the horizontal frame 10 members. What is also shown here is an alternative locking member LM2, here in form of a horizontal bar that is introduced into a gap between the upper horizontal frame member and the screen once the screen has been dropped into position. This locking bar may be spot welded in position or locked by a number of regular fastening members, for example bolts.

15 In Figs. 10a and 10b is shown yet an alternative for holding the screen, wherein Fig. 10b is seen in the section X-X in Fig. 10a. Here the screen is equipped with a hinge mechanism HMi , EE at upper end SCUE of screen allowing the lower end of the screen to swing away from frame while swiveling the upper end of the screen around the hinge mechanism. The hinge mechanism includes a radially directed rod HMi mounted to the frame and passing through

20 the upper recess URE and a vertically elongated eye EE on the upper tongue. By this design could the screen be lifted to such an extent that the lower end of the screen comes free from the lower recess and may be swung out of its lower position enabling access to the withdrawal compartment.

25 In Fig. 11 is shown an alternative to the hinge mechanism of Figs. 10a-10b. In this embodiment the hinge mechanism includes a hook like member integrated with the upper end SCUE of the screen and a hinge rod HM2 mounted in and along the upper recess URE.

Fig. 12 shows the principle with flat screens SC in a cylindrical vessel DS, and here with only 30 12 screens around the circumference in order to fully visualize the concept. As has been realized with the inventive design is that the flat screens are preferably used as the slide in and slide out design of the inventive screen in larger cylindrical vessel with diameters above 8-8.5 meter and using standard screens with a screen width SCw in the range 500 mm and up to 1000 mm at the most. In a cylindrical vessel with a diameter of 8 meter and screens with a 35 width of 500 mm are as many as 50 screens installed in a row to cover the inside, and at least 25 screens each having a screen width of 1000 mm. Thus, the inventive screens may be designed with a strict flat design, where each individual screen and frame have a width less than 5 % of the circumference of the circular treatment vessel, such that at least 20 screens are needed to form a continuous screen row around the circumference of the circular treatment vessel, and where each screen and associated frame are oriented in one common plane and thus deviates from the circular interior wall of the treatment vessel. The flat screen design is to be preferred in order to make a module design for each kind of digester with larger diameter.

Figs. 13a to 13d show different type of screens that could be used in the inventive screen design. In Fig. 13a is shown the oldest conventional slotted digester plate, which is the less expensive screen that could be manufactured. The slots are made in a regular metal plate and are either machine milled or using water jets or similar methods. However, this design has several drawbacks such as: a) the total open slot area is in the lowest order obtainable; b) the plate could not be made self-supporting and need full support in digester shell; and c) has lower edges of slots that tend to pile-up wood fragments against lower edge of slot during operation that finally may block the entire slot. These slots may have strict vertical direction or inclined towards the vertical as shown for example in Valmet's patent SE525611.

In Fig. 13b is shown the conventional stave screen design, using staves with T-bar shaped staves. This design increase stiffness of the screen surface considerably and increase the total open slot area with at least 40 % compared to the slotted plate shown in Fig. 13a.

In Fig. 13c is shown generation 2 of the conventional stave screen design, using staves with modified T-bar shaped staves with a slight conical transition. This design increase stiffness of the screen surface even more compared to the conventional stave screen design shown in Fig. 13b.

In Fig. 13d is shown the latest generation 3 of the conventional stave screen design, using staves with modified T-bar shaped staves with a slight conical transition, and with each second stave slightly recessed. This design, patented by Valmet in US8088253, increase effective open area of the screen exposed to the descending pulp column. Thus, the screen may be designed in a number of different ways, either slotted screen plates or the preferred stave screen design, which latter design may provide for self-cleansing action as lower end of slots may lack any pile up edges for wood fragments that may be caught partially in the slots.

The invention is not limited to the principle designs shown in this description. The upper hinges shown may also be complemented by some biased locking member that prevents unintentional detachment from the hinge. The screen may also be complemented with a holding rod that may be inserted between the frame and the screen when swung out of position to the cleaning position.