PROFILE BAR SCREEN FOR DIGESTER VESSELS

Abstract

The invention relates to an improved profile bar screen for draining treatment liquor from a suspension of comminuted cellulose material and treatment liquor in an essentially cylindrical digester vessel. According to the invention is a profile bar screen designed with horizontal support arches 11 with integrated support shoulders 12 only at the outer ends of the support arch which support shoulders rest against the inside of the vessel wall 1. The invention combines the techniques from self-supporting screens with support members of weaker screen designs, avoiding need to make any additional welds in the classified pressure vessel wall of the digester. Installation of new screens in compartments previously equipped with blind plates in checkered screen rows may be done quickly and at less costs during shorter down time of digester.

Claims

1. A profile bar screen for draining treatment liquor from a suspension of comminuted cellulose material and treatment liquor in an essentially cylindrical digester vessel having an outer wall, said profile bar screen being arranged inside the outer wall of the cylindrical digester vessel thereby forming a withdrawal chamber between the outer wall of the digester vessel and the profile bar screen, the profile bar screen comprising: vertical profile bar elements facing the suspension of comminuted cellulose material, said vertical profile bar elements arranged in parallel to each other and forming a withdrawal slot for treatment liquor between neighboring ones of said vertical profile bar elements; and a plurality of horizontal support arches having vertical mounting slots for mounting said vertical profile bar elements in said mounting slots, the plurality of horizontal support arches having integrated support shoulders resting against the inside of the outer wall of the cylindrical digester vessel, each of said plurality of horizontal support arches having only two of said integrated support shoulders located at each respective end of each of said plurality of support arches whereby the plurality of horizontal support arches between said integrated support shoulders is located at a distance from the outer wall of the cylindrical digester vessel, enabling a free unobstructed flow of withdrawn cooking liquor between said outer wall of said cylindrical digester vessel and said plurality of horizontal support arches and between said two integrated support shoulders in each of said plurality of horizontal support arches.

2. The profile bar screen as claimed in claim 1, wherein each of said plurality of horizontal support arches with including the two integrated support shoulders comprises a single piece made from a metal plate with a thickness in the range of from 12- to 18 mm.

3. The profile bar screen as claimed in claim 2, including a support bar for said outer wall of the cylindrical digester vessel and an L-shaped frame with a first leg and a second leg arranged orthogonally towards each other attached to the plurality of horizontal support arches and having an inwardly facing side and an outwardly facing side, with the first leg attached to the plurality of horizontal support arches, and oriented in the radial direction of the cylindrical digester vessel and the second leg oriented in the circumferential direction of the cylindrical digester vessel, and with the inwardly facing side of the second leg being flush with the surface of the vertical profile bar elements facing the suspension of comminuted cellulose material kept inside the digester and the outwardly facing side of the second leg abutting the support bar finding final load support in the outer wall of the cylindrical digester vessel.

4. The profile bar screen as claimed in claim 3, wherein said plurality of horizontal support arches include expansion slots having an open end facing towards the interior of the cylindrical digester vessel and a closed end in the area of one of said integral support shoulders, thereby allowing for a flexibility of the plurality of horizontal support arches such that said integrated support shoulders may rest against the outer wall of the cylindrical digester vessel despite any local deviations from a perfect cylindrical shape of the outer wall of the cylindrical digester vessel.

5. The profile bar screen as claimed in claim 2, wherein the length of the plurality of horizontal support arches in the circumferential direction of the cylindrical digester vessel of the screen is in the range of from 200- to 600 mm.

6. The profile bar screen as claimed in claim 5, wherein the length of the plurality of support shoulders in the circumferential direction of the cylindrical digester vessel is in the range from 20- to 60 mm, and is less than 10% in relation to the length of the plurality of horizontal support arches, thereby providing a large unhindered withdrawal flow channel between said integrated support shoulders providing less exposure for sedimentation to build up.

7. The profile bar screen as claimed in claim 5, including a plurality of support members welded to a part of said outer wall of said cylindrical digester vessel by means of fillet welds, and wherein the integrated support shoulders includes an outer edge closest to outer wall of the cylindrical digester vessel and a closest one of said support members includes a large radius that provides clearance to any of said fillet welds, said large radius exceeding 10 mm.

8. The profile bar screen as claimed in claim 7, including a plurality of support members welded to a part of said outer wall of said cylindrical digester vessel, wherein the integrated support shoulders include an inner edge closest to the large unencumbered withdrawal flow channel and furthest away from a closest one of said plurality of support members includes a smaller radius exceeding 3 mm, no larger than said large radius.

9. The profile bar screen as claimed in claim 2, wherein the plurality of horizontal support arches have a thickness of about 15 mm.

10. The profile bar screen as claimed in claim 2, wherein said plurality of horizontal support arches are cut from said single piece by means of water jet or a laser.

11. The profile bar screen as claimed in claim 6, wherein the length of the plurality of horizontal support arches is about 40 mm.

12. The profile bar screen as claimed in claim 6, wherein the plurality of horizontal support arches are produced from a simple piece of sheet metal.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0028] In the following a preferred embodiment of the invention will be described with reference to the attached drawing, in which

[0029] FIG. 1 shown a continuous digester with cut-away sections in 2 screen areas of the digester;

[0030] FIG. 2 shows a slotted screen plate according to prior art;

[0031] FIG. 3 shows a profile bar screen according to prior art;

[0032] FIG. 4 shows an alternative profile bar screen in a vertical section view according to prior art;

[0033] FIG. 5 shows the profile bar screen as seen in section A-A in FIG. 4 according to prior art;

[0034] FIG. 6a shows yet another profile bar screen as installed in a screen row with “checkered design” as shown in FIG. 1 with a blind plate in each second screen compartment;

[0035] FIG. 6B shows how a screen row design according to FIG. 6a may be modified according to prior art with screens also in compartments that previously have had blind plates;

[0036] FIG. 7 show a screen design according to the invention; and

[0037] FIG. 8a-d shows different profile bars usable in the claimed invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0038] In FIG. 1 is shown a tall and cylindrical continuous digester of a conventional design, with in feed of comminuted cellulose material in upper part and out feed of cooked pulp in bottom. The digester is shown with cut-away sections in 2 screen areas of the digester. In the upper cut-away section are three screen rows shown. Here is the screen area designed with the “checkered” screen area, i.e. with alternating blind plates and screens 61, 62, 63 in each screen row.

[0039] The same “checkered” screen area is shown in the lower cut-away section, also with 3 screen rows with alternating blind plates and screens 71, 72, 73 in each screen row.

[0040] Essentially the same withdrawal capacity could be obtained with such “checkered” screen if profile bar screens are used instead of slotted plates, as profile bar screens have larger open slot area. The main problem with withdrawal capacity lies in the flow resistance trough the chip (or pulp) bed inside the digester, which could be very high in digesters with a diameter of 10-14 meter. So, a profile bar screen with “checkered” screen area could have same withdrawal capacity as a screen row with slotted screen plates all around the circumference, as the screens are located only some 700-1000 mm apart, which distance is neglect able compared to chip bed distance to center of vessel.

[0041] In FIG. 2 is shown a conventional slotted screen plate 74 according to prior art. A metal plate 75 is equipped with withdrawal slots 76 which are made by either machine milling or water cutting jet technique. The entire screen plate is only supported by a multitude of support rods 78. In FIG. 3 is shown a conventional profile bar screen 64 according to prior art. Profile bars 66 are supported by horizontal support arches 65, which in turn is supported by a multitude of support rods 68. Here is also shown a clean-out plate 69, which could be opened in order to clean out the lower part of screen assembly. Both these type of screens are shown on page A537 in “Chemical Pulping”, book 6A (1999), ISBN 952-5216-06-3, as screens promoted by Ahlstrom Machinery (now Andritz).

[0042] In FIG. 4 is shown a profile bar screen according to WO2013/105888 as mounted in a continuous digester. The digester wall 1 is the actual pressure vessel wall and inside of the wall is the profile bar screen 2 located with vertical profile bar elements 10 facing the suspension of comminuted cellulose material as it descends down through the digester as a pulp column in the downwards direction PC. The vertical profile bar elements 10 are mounted on several horizontal support arches 11 located preferably at a set vertical distance between each support arch 11. Thus, the force upon the profile bar screen from the pulp column is first exposed to the profile bar elements 10 and via support arches 11 to the inside of the digester wall 1.

[0043] As indicated could a blind plate 32 be located above the screen bar elements 10, supported on a horizontal thrust beam 30. The upper blind plate 32 has its inwardly facing surface located flush with the inwardly facing surface of the screen bar elements, allowing the descending pulp column to descend without any obstructions to the bar screen area.

[0044] Below the profile bar elements 10 is a guide plate 31 mounted, which in its upper part has a retracted position allowing any chip fragments caught in slots between the profile bar elements from being pushed out from the slots as the pulp column descends.

[0045] The withdrawn treatment liquor is collected in a number of withdrawal chambers 20 between the profile bar elements 10 and the inside of the digester wall. The withdrawal chambers 20 are all in fluid communication with each other and withdrawn treatment liquor will finally be collected in a header chamber 21 located below chambers 20 before being withdrawn from the digester trough an outlet 22.

[0046] The features of the profile bar screen as shown in WO2013105888 are shown in FIG. 5. The support arches 11 has on the surface 14 facing the interior of the digester vertical mounting slots for holding a profile bar element 10 (only 3 profile bar elements numbered in figure) in said mounting slots. The profile bar elements have a T-shaped form. The bottom part of the T-shaped profile bar element is mounted in the vertical mounting slots of the horizontal support arches, with the upper flat face of the T-shaped vertical profile bar facing the inside of the treatment vessel and the suspension of comminuted cellulose material contained therein.

[0047] A withdrawal slot 15 is thus formed between profile bar elements 10. The screen slot size is varying from some 5-6 mm in upper part of digester and down to some 3 mm in lower part of digester, as the cellulose material is subjected to increased delignification and softening during cooking in the digester and slot size needs to be smaller in the final phases of the cook.

[0048] The horizontal support arches 11 further have integrated protruding support shoulders 12 resting against the inside of the outer wall 1 of the cylindrical digester vessel, and in FIG. 5 are 3 such support shoulders 12 shown, each located some distance apart creating an open flow channel 13 connecting the withdrawal chambers 20 to each other.

[0049] According to WO2013105888 are the horizontal support arches 11 equipped with additional expansion slots 16 having an open end 16a facing towards the interior of the digester vessel and a closed end 16b in the area of a support shoulder 12, allowing a flexibility of the horizontal support arches such that all support shoulder may rest against the wall of the digester vessel despite any local deviations from a perfect cylindrical shape of the digester vessel wall.

[0050] In the shown embodiment could some 12 profile bar elements be mounted between 2 neighboring expansion slots, but could be as few as 10 or as many as 20. The non flexing part of the support arch would thus hold the profile bar elements fixed in relation to each other and the withdrawal slot 15 thus kept constant. The withdrawal slot 15 coincident with the expansion slot 16 would instead allow some alteration of the slot width as the support arch will flex. However, the impact of this alteration is low, and if for example 10 profile bar elements are mounted between two expansion slots will only 10% of slot area be subjected to alteration due to flexing of the support arch. The relative alteration of individual slots will decrease with less profile bar elements between expansion slots as flexing due to uneven cylindrical form of vessel will be distributed over more expansion slots.

[0051] The expansion slots preferably has a depth X trough the horizontal support arches exceeding the depth Y of the horizontal support arches 11 in the area of the support arches wherein no support shoulder is located radially outside of the support arches.

[0052] The length X of the expansion slots preferably exceeds 50% of the total depth Z of the horizontal support arches 11, as counted from the inwardly facing surface 14 of the support arches 11 and to the outwardly facing surface of the support shoulder 12 of the support arches 11.

[0053] The depth of the support shoulder (Z-Y in FIG. 5) could vary depending upon the depth of the chambers 20 in each individual digester.

[0054] In order to avoid stress cracking due to local stress load as the support arch 11 will flex is the closed end 16b of the expansion slot preferably widened with an essentially circular slot part with a radius exceeding 4 millimeter as shown in principle in FIG. 5.

[0055] The expansion slot 16 could have a width being substantially constant before mounting the profile bar screen in said digester vessel or alternatively have a larger width at the open end 16a facing towards the interior of the digester vessel than the width at the closed end 16b before mounting the profile bar screen in said digester vessel. The alternative with increasing width towards open end 16a could preferably be used when the digester vessel is expected to have a form that deviates more from a perfect cylindrical form, thus needing more flexing in the support arch 11. Thus, with increasing width the slot could be some 1 mm at closed end 16b and some 2-4 mm at open end 16a.

[0056] In FIG. 6a is disclosed a screen design as used according to prior art in checkered screen rows as disclosed in FIG. 1. In this design is the screen comprising a self-supporting sturdy support arch 11, having slots for mounting the individual screen bars 10. A L-shaped frame 82 surrounds the screen area, attached to the support arch by fillet welds. The support arch 11 finds its load support in a number of support members 80 welded to the digester wall with fillet welds 90. There are typically a multitude of support members 80 located over the height of the screen, and in numbers corresponding to the number of support arches 11 shown in FIG. 4. As each such support member 80 is welded to a part of the pressure vessel, each weld 90 has to be inspected by a third party certification member for appropriate approval of the pressure vessel design. The fillet weld also prevents penetration of corrosive cooking liquors between the support members 80 and the pressure vessel wall 1, and needs to be continuous around the entire support member. Such inspection needs most often crack inspection of the weld 90 using either penetrants or magnaflux methods or similar, thus needing extensive time and review for all welds. There is also a vertical support bar 81 which provides support for the L-shaped frame 82, with a fillet weld 92 around the circumference of the L-shaped frame, and with a fillet weld 91 attached firmly to the support member 80. A screen section could thus be replaced if needed without any changes in the pressure vessel wall design, as only welds 92 need to be removed with a grinding disc or similar. The support members 80 are only extended towards the withdrawal compartment 20 such that necessary support area is obtained for the support arches 11. In the next compartment, covered by a blind plate 84, the blind plate has support pins 88 arranged in a similar way as disclosed in FIG. 2. Now, if the operating conditions of the digester need increased withdrawal capacity there is an option to replace the blind plates with screen sections. How this is conventionally implemented is shown in FIG. 6b, where the blind plate 84 is removed. What is needed here is an additional support member 80e extending the support area for new support arch 11 stretching over the compartment previously covered by a blind plate. The L-shaped frame 82 and the individual screen bars 10 are not shown in this drawing for the new screen, but is of course included in the final design. The additional support member 80e needs to be welded around the entire circumference with a fillet weld 90e. This conventional design has the drawbacks that the welding work is time consuming and the additional welds needs thorough inspection before approval as a change in the pressure vessel design is made, besides high expenses for welding material/electrodes. In a typical digester with 4 screen rows needing replacement of the blind plates, FIG. 1 show 6 screen rows with checkered screen area, is over 1000 support members installed, thus requiring installation of over 1000 additional support members 80e. A certified welder may apply one weld for one additional support member in 5-10 minutes, and hence the total welding work alone may require in excess of 100-200 man hours. Then the necessary time for inspection using penetrant or magnaflux methods may require additional time in the same order. After inspection work could the installation of the new screens start, which takes additional time to complete the rebuild.

[0057] This amount of time is normally not available in the short down time of a digester where maintenance or rebuild work needs to be finished.

[0058] The inventive screen design is shown in FIG. 7, and avoid any alterations in the digester shell when installing the new screen. As no changes in the pressure vessel is made, no welds need to be inspected by a third party certification member for appropriate approval of the changed pressure vessel design. The screen design is similar to that disclosed in WO2013105888 but differs in that only two support shoulders 12 are located at each end and integrated with the support arch 11. The support arch 11 is preferably made with somewhat larger dimension Y′ than the corresponding dimension Y in FIG. 5, and the thickness of the support arch 11 is preferably extended from some 8 mm to about 15 mm. As indicated previously has not the digester a perfect circular form so the distance between the inner facing side of the support bar 81 and the inside of the digester wall 1 may change, but this may be compensated by delivering the screen with somewhat oversized support shoulder 12 in the radial direction. If the outer L-shaped frame 82 is not abutting the support bar 81, could excess material easily be ground off from the support shoulder at site, and as there are only 2 support shoulders on each support arch 11 is less adjustment time necessary during installation. Besides a dramatic reduction in necessary installation time will the screen design according to the invention provide with less hindrance for the downward flow of the withdrawn spent cooking liquors in the withdrawal compartment 20, and less amount of surfaces where the spent cooking liquors could form depositions precipitated from the spent cooking liquors.

[0059] The entire support arch 11 and integrated support shoulders 12 is preferably made in one single piece, preferably cut from about 15 mm thick metal plates using water jet or laser cutting techniques.

[0060] The invention is preferably applicable for screens with an arch length AL in the circumferential direction of the digester in the range of 200-600 mm. The screens are in a first installation made with an arch length of about 495 mm and expected to be put into operation late 2014.

[0061] The length SL in the circumferential direction of the digester of the support shoulders 12 when made from a single sheet metal piece are preferably in the range 20-60 mm, preferably 40 mm, and in relation to the total arch length AL less than 10%, leaving a large unhindered flow channel 13 between support shoulder 12 less exposed for sedimentations to build up.

[0062] In a most preferred embodiment is the outer edge of the support shoulder closest to digester wall and closest support member 80 designed with a large radius R.sub.1 that provides clearance to any fillet welds 90, and said radius exceeding 10 mm, preferably 15 mm. The inner edge of the support shoulder closest to flow channel 13 and furthest away from the support member 80 is preferably designed with a smaller radius R.sub.2, and said radius exceeding 3 mm, preferably 5 mm and at the most same as R.sub.1. The object of the smaller radius is to avoid a sharp edge from penetrating the pressure vessel wall 1. The clearance between outer edge of support shoulders 12 and support member 80 should preferably be kept at a minimum, and only made so large that it may accumulate any local dislocation of for example the support member in form of burrs or weld spots. No withdrawal flow is intended to be developed in this clearance and it will most likely be blocked by sedimentations after only a short time of operation, which will happen even if the clearance would be as large as 10-30 mm, at expense of reducing the major withdrawal flow channel 13.

[0063] But in an alternative solution could also the support shoulders be attached to the support arch as a separate piece, and preferably attached by welding. Alternatively could the support shoulders be designed as adjustable screws that after adjustment is locked by a weld, but both these alternatives result in a more expensive screen.

[0064] As is the case in the design according to WO2013105888 are the support arches 11 preferably equipped with additional expansion slots 16 having an open end 16a facing towards the interior of the digester vessel and a closed end 16b in the area of a support shoulder 12, allowing a flexibility of the horizontal support arches such that the 2 support shoulders may rest against the wall of the digester vessel despite any local deviations from a perfect cylindrical shape of the digester vessel wall.

[0065] It is to be understood that the above description and the related figures are only intended to illustrate the present solution. Thus, the solution is not restricted only to the embodiment described above and defined in the claims, but many different variations and modifications, which are possible within the scope of the idea defined in the attached claims, will be obvious to a person skilled in the art.

[0066] Thus the profile bar screen may preferably be used in other cylindrical pressure vessels such as digesters, either in continuous or batch digesters with a cylindrical form. In FIGS. 8a-8d are shown different types of profile bars that could be used.

[0067] In FIG. 8a is shown a profile bar with vertical profile bar elements having a closed Y-shaped form, with bottom part of the Y-shape mounted in the vertical mounting slots of the horizontal support arches for holding a profile bar element in said mounting slots, and with the upper flat face of the closed Y-shaped vertical profile bar facing the inside of the treatment vessel and the suspension of comminuted cellulose material contained therein.

[0068] In FIG. 8 b is shown a profile bar with vertical profile bar elements 10b having a circular form facing the interior of the digester, with an integrated flat bar portion at its bottom part mounted in the vertical mounting slots of the horizontal support arches for holding a profile bar element in said mounting slots, and with the upper faces of the circular bars facing the inside of the treatment vessel and the suspension of comminuted cellulose material contained therein.

[0069] In FIG. 8c is shown a profile bar with vertical profile bar elements 10c having a T-shaped form, with bottom part of the T-shaped bar mounted in the vertical mounting slots of the horizontal support arches for holding a profile bar element in said mounting slots, and with the upper flat face of the T-shaped vertical profile bar facing the inside of the treatment vessel and the suspension of comminuted cellulose material contained therein. But in this embodiment is each second bar element a flat bar element 10c″.

[0070] In FIG. 8d is shown an alternative profile bar arrangement according to FIG. 8c, but where each second bar element is recessed a distance B from the upper flat face of neighboring T-shaped bar elements.