PLIES

DESCRIPTION

TECHNICAL FIELD [0001] The present invention relates to components of machinery for cellulose ply manufacturing facilities, in particular for manufacturing paper. Embodiments described herein concern in particular drying cylinders for machinery for manufacturing cellulose plies or webs, such as paper or tissue paper. In particular, there are described hollow cylinders adapted to be pressurized with steam or another heat- transfer fluid, and intended to dry cellulose plies or webs guided around the drying cylinders. In the present description and in the appended claims, the term drying cylinder also comprises in particular Yankee cylinders.

BACKGROUND ART

[0002] To manufacture plies, webs or sheets based on cellulose fibers, such as paper plies, wet methods are normally used. These methods provide for the production of a suspension of cellulose fibers in water. The suspension is distributed on forming fabrics by means of headboxes, to form a layer of cellulose slurry. With subsequent passages through draining systems, the water content in the slurry is reduced, until forming a layer of cellulose fibers with a content of solid material sufficient to transfer the layer onto a drying cylinder or several drying cylinders, which remove further moisture from the cellulose layer through evaporation caused by the heat transferred from the inside of the cylinder through its cylindrical wall to the cellulose layer. The heat is usually provided by a heat-transfer fluid that circulates in the drying cylinder. For this purpose, the drying cylinder is hollow. [0003] Usually the heat-transfer fluid used is high pressure saturated steam. The saturated steam has the advantage, through change of status, of producing a high thermal energy transfer per surface unit toward the body of the cylinder. The pressure values used are high to allow operating temperatures to be obtained that are sufficiently high to effectively remove the moisture from the cellulose layer until obtaining the desired degree of residual moisture before removing the cellulose ply thus obtained from the drying cylinder.

[0004] Therefore, the drying cylinders must be designed to withstand high pressures in dynamic load conditions and at high operating temperatures.

[0005] The operating conditions are particularly heavy in the case of Yankee cylinders, commonly used for manufacturing paper, in particular tissue paper, and also in dryer section cylinders. These machine members are very large and critical from the viewpoint of safety, due to the operating pressures and temperatures, and resulting thermal and mechanical stresses. The high speed rotation of the Yankee cylinders, and of other drying cylinders used in papermaking machinery, generate fatigue stresses, particularly critical due to the risk of breakages that can be triggered at cracks or defects in the metal material of which the cylinders are produced.

[0006] Originally, Yankee cylinders and dryer section cylinders were mainly made of cast iron. In more recent times various systems have been studied for the production of drying cylinders and Yankee cylinders made of steel. This material is particularly suitable for producing dryer section cylinders and Yankee cylinders, as it allows better heat transfer toward the cellulose ply driven around the outer surface of the cylinder. Steel cylinders are composed of parts welded together, typically a cylindrical shell with two axial ends, to which respective heads, which carry the pins that rotatingly support the cylinder, are welded or screwed.

[0007] Examples of welded Yankee cylinders are disclosed in US, 438, 752, EP 1838922, WO2020/194358, DE202020106010, WO2019/209164, EP3556936, US2016/0228969.

[0008] In the case of joining of the shell-heads by welding, the welds that join the heads to the cylindrical shell in Yankee cylinders and in other drying cylinders for papermaking machinery must meet very high quality and safety standards, as in practice these cylinders form pressure tanks. The welds must withstand stresses generated by pressure, by the weight of the cylinder itself and by thermal expansions. These stresses vary with the temperature and can be cyclical due to rotation of the cylinders and to the variation in the operating conditions, causing fatigue stresses. Therefore, the welds must not have defects that can trigger dangerous breakages. Consequently, welding must be carried out by highly specialized personnel and must undergo various types of non-destructive testing (X-rays, ultrasonic, acoustic emissions), both initial and at regular intervals over the course of time. This has a high impact on the manufacturing cost of the cylinder and on the production times, as well as on the operating cost during the whole of the useful life of the cylinder.

[0009] Moreover, the welds in any case still remain possible causes of breakages, even catastrophic, of the cylinder, with risks of accidents which may even be fatal due to the operating temperatures and pressures of these machine members.

[0010] Coupling of the shell-heads by means of screws or bolts, which is a mandatory condition for cylinders made of cast iron, can be used for steel cylinders to avoid the criticalities linked to welding, but has a high impact on the manufacturing cost of the cylinders, as it requires numerous drilling and threading operations, as well as lengthy assembly times.

[0011] DE102013213197 discloses a method for thermal coupling between a shell and two heads of a Yankee cylinder. To this end, a difference in temperature is generated between each head and the respective axial end of the cylindrical shell such that, as a result of thermally induced deformations, the inner diameter of an inner projection of the shell becomes larger than the maximum outer diameter of the first head, allowing the head to be inserted into the shell. By then returning the shell and head to the same temperature, the inner diameter of the inner projection of the shell becomes smaller than the outer diameter of the head, blocking the latter.

[0012] It has been found that this coupling system is not safe, as during use the shell of the cylinder is subjected to a greater expansion at the ends than at the center, due to a difference in temperature between central area and ends of the shell. This difference in temperature can be of several tens of degrees and is due to the fact that in the central area the shell is constantly cooled by the cellulose sheet that is driven around the cylinder. This leads to a tendency for the ends of the cylindrical shell to become detached from the heads of the cylinder, with the risk of serious accidents or, at the very least, of penetration of residues and moisture between edge of the head and shell, with the consequent risk of corrosion.

[0013] The thermal coupling system of the prior art is therefore unreliable. [0014] It would thus be useful to provide Yankee cylinders and other cylinders for use in papermaking machinery that fully or partly overcome the drawbacks of the aforesaid cylinders.

SUMMARY [0015] According to the present disclosure, a drying cylinder for papermaking machinery, for example a Yankee cylinder, is provided which comprises a cylindrical shell defining an axis of the cylinder, a first head fixed to a first axial end of the cylindrical shell and a second head fixed to a second axial end of the cylindrical shell. The first head, the second head and the cylindrical shell form a pressurizable hollow inner volume of the cylinder, for example by means of steam. At least one, and preferably both, of the axial ends of the cylindrical shell comprise an inner annular projection extending radially toward the axis of the cylinder and having a minimum inner diameter smaller than a maximum diameter of the respective head. Moreover, the respective head is mounted so that at least a portion of the head having the maximum diameter is arranged in an axially more internal position with respect to a portion of the inner annular projection having the minimum inner diameter. Characteristically, the, or each, head has an outer perimeter circular portion deformable radially, formed by an annular recess on the surface of the head facing the inside of the cylinder, said outer perimeter circular portion having the maximum diameter.

[0016] As the outer perimeter circular portion of the head is deformable in radial direction, it is possible to maintain mutual contact between the perimeter edge of the head and the cylindrical shell, avoiding the risk of detachment due, for example, to a difference in temperature of the central zone of the cylindrical shell caused by the paper driven around the drying cylinder.

[0017] Further advantageous features and embodiments of the cylinder and of the method of manufacturing it are described below and defined in the appended claims, which form an integral part of the present description.

BRIEF DESCRIPTION OF THE DRAWINGS [0018] The invention will be better understood by following the description and the accompanying drawings, which illustrate non-limiting exemplary embodiments of the invention. More in particular, in the drawing:

Fig.l shows a schematic section of a Yankee cylinder in an embodiment; Figs. 2A a 2F show an assembly sequence of the Yankee cylinder of Fig.l; Figs. 3 and 4 show enlargements of the coupling area between a head and an axial end of the shell of the cylinder, in various steps of the assembly process; and

Fig.5 shows an enlargement of the coupling area between a head and an axial end of the shell of the cylinder.

DETAILED DESCRIPTION [0019] Fig. 1 shows, in a section according to a plane containing the axis of rotation, a simplified embodiment of a drying cylinder according to the invention. The cylinder is indicated as a whole with 1 and its axis of rotation is indicated with A-A.

[0020] The cylinder 1 can be a Yankee cylinder for a papermaking machine using a wet process. The cylinder 1 can also be a drying cylinder of another type, such as a dryer section cylinder, for a multi-cylinder papermaking machine.

[0021] The cylinder 1 comprises a cylindrical shell 3 that has a first axial end and a second axial end, both indicated with 3 A. Each axial end is closed by a respective head 5. A hollow volume 7 is formed inside the heads 5 and the cylindrical shell 3, into which a heat-transfer fluid can be fed and/or can circulate. If the cylinder l is a drying cylinder or a Yankee cylinder for papermaking machinery, the hollow volume 7 can be used for the circulation of saturated steam, which transfers heat to the paper driven around the cylindrical surface of the cylindrical shell 3. The heads 5 are integral with respective pins 9 for rotatably supporting the cylinder 1.

[0022] In the case of a Yankee cylinder for tissue or mono-glazed paper, due to the large diameter of the cylinder, a tie-rod 10 which extends between the two heads 5 is arranged coaxially to the shell 3. Each axial end 10A of the tie-rod 10 is anchored to the inside of the respective head 5.

[0023] During use, the inside 7 of the cylinder 1 is pressurized by the heat-transfer fluid, for example saturated steam. A part of the axial stress, generated by this pressure, on the heads 5 is transmitted to the inner tie-rod 10. The shell 3 and the tie-rod 10 are sized so that, during operation of the Yankee cylinder 1, the tie-rod 10 is subjected to tensile stress, and supports a part of the axial force resulting from the internal pressure generated by the heat-transfer fluid on the two heads 5. The residual part of the axial force is transferred to the shell 3. During operation, the tie-rod 10 is typically at a higher temperature than the temperature of the shell 3, and therefore the axial dimensions of the tie-rod 10 and of the shell 3 must take account of the fact that the latter is subjected to higher thermal elongation deformation than the shell 3. In practice, the tie-rod 10 is designed with an axial dimension smaller than the distance between the inner faces of the two heads 5 so that during assembly the spacer is pre- tensioned and the shell 3 pre-compressed. This solution ensures that the tie-rod is always tensioned performing its function in any operating condition of the Yankee cylinder 1.

[0024] Conversely, the shell 3 is subject to a reversal of stress, passing from a condition of compression, when cold and at ambient pressure, to the operating condition of tension, when hot and pressurized. In fact, when cold, the heads 5 will be pulled toward each other and toward the inside of the shell 3 by pre-tensioning of the tie-rod, while, when hot, the heads 5 will be pushed outward by the pressure of the steam or other heat-transfer fluid circulating in the empty volume 7 of the cylinder 1, for the portion of thrust not transferred to the tie-rod 10. This residual thrust is transferred to the inner annular projection 10 of each axial end 3 A of the shell 3.

[0025] Therefore, in axial direction the constraint between the heads 5 and the shell 3 must be sufficient to support relative thrusts in the two directions and have no axial clearances, so that reversal of the condition of stress does not cause relative movements between heads 5 and shell 3. [0026] The techniques used to date for this purpose use screw couplings or structural welds between each head 5 and the respective axial end 3 A of the shell 3. Screw couplings, typically used in cast iron cylinders, have problems of seal and of corrosion at the interfaces between shell and heads. Structural welds, typically used for steel cylinders, have problems of geometric distortions of the cylinder caused by localized overheating of the material, or problems of development of cracks caused by incomplete relaxation of the welded material. [0027] As mentioned in the introduction of the present description, systems for thermal coupling through interference fit, obtained by expanding the end of the cylindrical shell by heating and inserting the head inside it, have also been investigated. However, these systems have not given positive results due to the poor quality of the coupling obtained.

[0028] These problems are overcome with the cylinder 1 disclosed herein. For this purpose, at least one of the axial ends 3 A of the cylindrical shell 3 comprises an inner annular projection. In the example illustrated, the two axial ends each comprise a respective inner annular projection 11 which extends radially toward the axis A-A of the cylinder 1. Figs.4 and 5 illustrate an enlargement of a detail of one of the two inner annular projections 11 in an embodiment. The enlarged portion corresponds to the area indicated with IV in Fig.1. The annular projections 11 are defined “inner” in the sense that they extend from the inner surface of the shell 3 toward the axis A-A of the cylinder 1. [0029] In Fig. 4, D1 indicates the minimum inner diameter of the inner annular projection 11 and D2 indicates the maximum diameter of the respective head 5. The minimum inner diameter D1 of the inner projection 11 is smaller than the maximum diameter D2 of the head 5. In this way, each inner annular projection 11 forms a support for the head 5, to which at least part of the force generated on the respective head 5 by the pressure of the fluid that fills the volume 7 of the cylinder and facing the outside of the cylinder 1 is transferred.

[0030] In advantageous embodiments, adjacent to each inner annular projection 11 the shell 3 comprises an annular groove 13 that has a diameter D2. In the region axially more internal with respect to the annular grooves 13, grooves 15 are provided in a known manner on the inner surface of the shell 3, for collecting the condensate that forms due to the transfer of heat from the steam fed into the cylinder 1 to the paper that is driven around the outer cylindrical surface of the shell 3.

[0031] In the illustrated embodiment, as can be seen in particular in Figs. 4 and 5, on the inner surface facing the inside of the cylinder 1 each head 5 has an annular recess 5X, which defines an outer perimeter circular portion 5Y of limited thickness along the edge of the head 5. The circular portion 5Y is in substance a cylindrical wall that can have a variable radial thickness from the face facing the inside of the cylinder 1 toward the bottom of the annular recess 5X. More in particular, the thickness of the circular portion 5Y increases from the inner surface of the head 5 toward the bottom of the annular recess 5X. [0032] In some embodiments, in practice, the perimeter circular portion 5Y of the head 5 forms a lip that is inserted into the respective annular groove 13 of the shell 3. This circular lip portion 5Y forms an area with high radial deformability that can expand radially outward when a high pressure is applied inside the cylinder 1, and/or when the axial end 3 A of the cylindrical shell 3 expands as a result of heating. [0033] High pressures inside the cylinder 1 can occur during use and even more so during hydraulic seal tests. The circular lip portion 5Y with high radial deformability ensures that no accidental detachment between the head 5 and the end 3 A of the shell 3 occurs even when the pressure inside the cylinder 1 is very high and/or when the respective axial end 3 A of the cylindrical shell 3 expands radially as a result of heating. [0034] In practical embodiments, the circular portion 5Y is defined by a circular outer cylindrical surface 52, coaxial to the cylinder 1 and the outer diameter of which is equal to the maximum diameter D2 of the head, greater than the minimum diameter D1 of the inner projection 11 of the axial end 3 A of the cylindrical shell 3. The circular portion 5Y can also be defined internally by a conical surface 54, coaxial to the axis A-A of the cylinder 1 with a vertex outside the cylinder 1. In other words, as can be seen in the drawing, in a section of the radial plane containing the axis A-A of the cylinder 1, the inner surface 54 is inclined and moves toward the axis A-A of the cylinder 1 from the inside toward the outside of the cylinder 1.

[0035] To allow each head 5 to be mounted inside the respective inner annular projection 11, inserting the circular portion 5Yinto the annular groove 13, a thermal process can be used, exploiting differential thermal expansions between head 5 and shell 3. Advantageously, in conditions at rest and during normal use of the cylinder 1, the perimeter circular portion 5Y of each head 5 has a dimension in axial direction (according to the axis A-A of the cylinder 1) equal to the axial dimension of the groove 13, so as to obtain a two-way constraint in axial direction of the head 5 with respect to the shell 3. More precisely, the dimension in axial direction, i.e., the thickness, of the head 5 in the radially outermost area in which the circular portion 5Y extends is equal to the dimension in axial direction of the annular groove 13.

[0036] In practice, each axial end 3A of the shell 3 and the respective head 5 are taken to different temperatures, to cause an expansion of the axial end 3 A of the shell and/or a radial contraction of the head 5. The thermal deformation differential is greater than the difference between the inner diameter D1 of the inner annular projection and the maximum diameter D2 of the head 5.

[0037] Heating can be obtained with a flame, for example using gas nozzles, or with an electric heating system, possibly induction heating, or in any other suitable way. [0038] Figs. 3 and 4 conceptually illustrate this way of assembling each head 5 on the shell 3. Fig.3 shows the same enlarged detail of Fig.4 in a condition of thermal differential between the shell 3 and the head 5. As can be observed in Fig.3, as a result of the difference in temperature between shell 3 (or at least its axial end 3 A) and head 5, the maximum diameter D2’ of the latter becomes smaller than the minimum diameter D 1 ’ of the inner annular projection 11. ER indicates the radial thermal expansion between the inner annular projection 11 and the head 5 and EA indicates the axial thermal expansion.

[0039] Due to this thermal expansion, it is possible to insert the head 5 inside the axial end 3 A of the shell 5. By then returning the axial end 3 A of the shell 3 and the head 5 to approximately the same temperature, reaching the condition of Fig.4, mechanical blocking of the head 5 with respect to the shell 3 is obtained due to the fact that the circular portion 5Yof the head remains interlocked in the annular groove 13 and the radially outermost portion of the outer surface 5A of the head 5 abuts against the inner annular projection 11. In this way, the head 5 is radially and axially blocked (in both directions) with respect to the cylindrical shell 3.

[0040] To ensure tightness of the coupling thus formed between head 5 and shell 3 it is possible to provide an external seal and/or an internal seal through respective weld beads. An outer weld bead is indicated with 51 and an inner weld bead is indicated with 53. It would also be possible to provide the seals in the form of beads made of synthetic sealants. Synthetic sealants can be removed and replaced by new beads, if necessary. [0041] The welds 51, 53- if present - are non-structural welds as in the case of prior art cylinders, as they do not require to withstand the axial thrusts on the heads 5. Therefore, the welds 51, 53 are much lighter, require a smaller amount of welding material and cause fewer thermal stresses. Any cracks in the weld beads do not cause risks of breakage of the cylinder 1, as the welds are not subjected to mechanical stresses.

[0042] The inner weld bead 53 (with filler material in metal or, alternatively, with synthetic sealing material) prevents steam from penetrating from inside the cylinder toward the interface between the cylindrical surface 52 of the head 5 and the bottom of the annular groove 13 of the end 3 A of the cylindrical shell 3. Likewise, the outer weld bead 51 (with filler material in metal or, alternatively, with synthetic sealing material) prevents moisture or other pollutants from outside the cylinder from penetrating toward the interface between the cylindrical surface 52 of the head 5 and the bottom of the annular groove 13. In this way, the risk of corrosion of the surfaces of the head 5 and of the cylindrical shell 3 in the mutual interlocking area is avoided.

[0043] The sequence of Figs. 2A-2F shows a way of implementing the method described above of assembling the heads 5 on the shell 3. Although in this embodiment both heads 5 are assembled with the same technique, it must be understood that some advantages of the present invention can also be obtained in the case in which only one of the heads 5 is assembled according to the method described herein.

[0044] Moreover, the method schematized in the sequence of Figs.2A-2F also provides for assembling the tie-rod 10. However, some of the advantages of the method illustrated herein can also be obtained in the production of cylinders, for example dryer section cylinders, without internal tie-rod. [0045] A first step of the method is illustrated schematically in Fig.2A. In this first step, the shell 3 of the cylinder 1 is preferably arranged in vertical position. At least the area of the axial end 3 A facing upward is heated by means of the heat supply Q so that a first head 5 can be inserted, lowering it from above (arrow FI) once the maximum diameter D2 of the head is smaller than the minimum diameter DL of the inner annular projection 11 obtained through thermal expansion.

[0046] In this step, instead of heating the shell 3, or in addition to heating the shell 3, the head 5 can be cooled.

[0047] Once the head 5 has been inserted into the upper axial end 3A of the shell 3, as shown schematically in Fig.2B, the shell 3 is cooled (and/or the head 5 is heated) until the head 5 is blocked in the end 3 A of the shell 3 (Fig.4). [0048] In the subsequent step, shown in Fig.2C, the shell 3 with the first head 5 mounted, is rotated to take the head 5 downward and the axial end 3A still without head upward.

[0049] In this condition, the tie-rod 10 is lowered inside the shell 3, taking to rest with its lower axial end on the head 5. The tie-rod 10 is secured, for example with screws 31, to the lower head 5 (Fig.2D).

[0050] With a process similar to the one described with reference to Figs. 2A and 2B, the second head 5 (Fig.2E) is applied by lowering it from above and inserting it into the shell 3 inside the inner annular projection 11 through thermal deformation of the head 5 and/or of the shell 3 (or of its axial end 3 A). Fig.2F shows the final step of blocking the tie-rod 10 to the upper head 5 through screws 31.