Technical field

The present invention relates to the technical field of paper making and especially tissue paper making and the field of the paper machine itself.

Background art

If tissue paper is to be made out of pulp, the process essentially comprises

a forming that includes the headbox and the forming wire section,

b the drying section (TAD (through air drying) ) or conventional drying on a yankee cylinder) that also usually includes the crepe process essential for tissues,

c typically the monitoring and winding section.

Paper can be formed by placing the fibers, in an oriented or random manner, on one or between two continuously revolving wires of a paper making machine while simultaneously removing the main quantity of water of dilution until dry-solids contents of usually between 12 and 35 % are obtained.

Drying the formed primary fibrous web occurs in one or more steps by mechanical and thermal means until a final dry-solids content of usually about 93 to 97 %. In the case of tissue making, this stage is followed by the crepe process which crucially influences the properties of the finished tissue product in conventional processes. The conventional

dry crepe process involves creping on a usually 4.5 to 6 m diameter drying cylinder, the so-called yankee cylinder, by means of a crepe doctor with the aforementioned final dry- solids content of the raw tissue paper (wet creping can be used if lower demands are made of the tissue quality) . The creped, finally dry raw tissue paper (raw tissue) is then available for further processing into the paper product or tissue paper product according to the invention.

Instead of the conventional tissue making process described above, the use of a modified technique is possible in which an improvement in specific volume is achieved by a special kind of drying within process section b and in this way an improvement in the bulk softness of the thus made tissue paper is achieved. This process, which exists in a variety of subtypes, is termed the TAD (through air drying) technique. It is characterized by the fact that the "primary" fibrous web (like a nonwoven) that leaves the forming and sheet making stage is pre-dried to a dry-solids content of about 80% before final contact drying on the yankee cylinder by blowing hot air through the fibrous web. The fibrous web is supported by an air-permeable wire or belt (or TAD-fabric) and during its transport is guided over the surface of an air-permeable rotating cylinder drum (TAD-cylinder) . Structuring the supporting wire or belt makes it possible to produce any pattern of compressed zones broken up by deformation in the moist state, also named moulding, resulting in increased mean specific volumes and consequently leading to an increase in bulk softness without decisively decreasing the strength of the fibrous web. Such a pattern is fixed in the area of the TAD-cylinder.

A tissue paper is defined as a soft absorbent paper having a low basis weight. One generally selects a basis weight of 8 to 30 g/m ² , especially 10 to 25 g/m ² per ply. The total basis weight of multiple-ply tissue products is preferably equal to a maximum of 65 g/m ² , more preferably to a maximum of 50

g/m ² . Its density is typically below 0.6 g/cm ³ , preferably below 0.30 g/cm ³ and more preferably between 0.08 and 0.20 g/cm ³ .

The production of tissue is distinguished from paper production by the its extremely low basis weight and its much higher tensile energy absorption index (see DIN EN 12625-4 and DIN EN 12625-5) . Paper and tissue paper also differ in general with regard to the modulus of elasticity that characterizes the stress-strain properties of these planar products as a material parameter.

A tissue's high tensile energy absorption index results from the outer or inner creping. The former is produced by compression of the paper web adhering to a dry cylinder as a result of the action of a crepe doctor or in the latter instance as a result of a difference in speed between two wires ("fabrics"). This causes the still moist, plastically deformable paper web to be internally broken up by compression and shearing, thereby rendering it more stretchable under load than an uncreped paper.

Moist tissue paper webs are usually dried by the so-called Yankee drying or the through air drying (TAD) .

The fibers contained in the tissue paper are mainly cellulosic fibres, such as pulp fibers from chemical pulp (e.g. Kraft sulfite and sulfate pulps), mechanical pulp (e.g. ground wood) , thermo mechanical pulp, chemo-mechanical pulp and/or chemo-thermo mechanical pulp (CTMP) . Pulps derived from both deciduous (hardwood) and coniferous (softwood) can be used. The fibers may also be or include recycled fibers, which may contain any or all of the above categories. The fibers can be treated with additives - e.g. softeners, such as quaternary ammonium compounds, and binders, such as conventional dry-strength agents or wet-strength agents used to facilitate the original paper making or to adjust the properties thereof. The tissue paper may also contain other

types of fibers, e.g. regenerated cellulosic fibres or synthetic fibers enhancing, for instance, strength, absorption, smoothness or softness of the paper.

Tissue paper may be converted to the final tissue product in many ways, for example, by embossing or laminating it into a multi-ply product, rolled or folded. Such products may be paper towels, paper napkins, paper handkerchiefs, facials etc .

Disclosure of invention

The inventor recognized that by increase of environment temperature in special areas of the paper machine especially behind the most downstream drying section, e.g. yankee cylinder, sparks from fibers or fiber conglomerations may be caused and by the high speed web-run these sparks may be entrained and transported to the winding section and are trapped between the winding layers of the paper web and cause glowing fire nests within the paper roll (mother roll) . If this mother roll then is stored in a mother-roll storage room and the glow within the mother roll develops to a fire there is a high risk that fire destroys the whole content of the storage .

It is the problem (object) of the present invention to avoid this and to take care that such a disaster can be avoided.

This problem is solved by a paper making machine according to claim 1 in which behind the most downstream drying section, or at least upstream the winding-up section a spark detector is provided in the vicinity of the paper web running to the winding-up section.

If then the presence of sparks as described above is detected by the detector there exists not only a possibility to remove formation of the fiber sparks but to store the corresponding

mother roll separate from the mother roll storage in a safe area at least for a safety while.

Preferably in a tissue paper machine the spark detector is provided downstream the yankee cylinder where most probably sparks may appear.

In order to avoid malfunction of the spark detector if soiled by paper dust and other environment debris, cleaning air jet means is provided, wherein the air jet thereof is directed to the spark detector.

According to another possibility the spark detector is combined with a web run-guiding means stabilising the web run. There the spark detector is positioned close to the running paper web so that sparks safely can be detected.

In order to avoid extra cleaning air jet means the web run- guiding means comprises at least one guiding surface behind which a space is provided surrounded at least partly by a casing, in which an air-inlet-opening is provided such that air sucked in by an under pressure created by the web run is directed to the spark detector for cleaning purposes . Nevertheless, this system may be combined with a cleaning air jet means.

A preferred location for the spark detector is downstream a web measuring means, where basis weight and water content of the web is measured. This is a place where paper web- stabilisation can be kept as short as possible and no other means has to be installed there.

By the invention the spark detector remains clean and it is not necessary to take care for special cleaning for which the machine run would have to be interrupted. Anyway, in spite of the presence of spark detector for instance between the yankee cylinder and the winding-up section a good paper web stability is maintained.

Brief description of the drawings

Fig.l shows schematically an embodiment of a tissue paper machine, and

Fig.2 shows enlarged in comparison to Fig.l a paper web guiding foil containing a spark detector.

Embodiments of invention

In Fig.l is schematically illustrated a tissue paper machine comprising a headbox 1, a forming section 2 a drying section 3 and a winding-up section 4. Reference number 5 denotes a forming fabric and the reference number 6 downstream thereof a felt guiding the paper web W from the forming section 2 to the drying section 3. There is a pick-up roll 7 transferring the paper web from the forming section 2 to the felt 6. Reference number 8 denotes a forming roll. In the drying section 3 is provided a yankee-cylinder 9 partly surrounded by a drying hood 10. A creping blade 11 removes the yankee- cylinder coated with a special chemical agent from the circumferential surface of the yankee-cylinder. From there the paper web W is guided to a measuring device 12 where for instance the moisture content and the basis weight of the paper web is measured. Behind, e.g. downstream the measuring device a spark detector 13 is provided in the vicinity of the paper web, from where the paper web is guided to the winding- up section where the paper web is wound up a tissue tambour roll 14 forming from the paper web a paper roll as a so- called mother-roll 15. Especially such sparks may be present and created when fiber material possibly combined with dust and paper lumps probably combined with the chemical agent by which the yankee-cylinder is coated fill the space between the dryer hood 10 and the yankee-cylinder 9. This result in friction, which could cause higher temperatures leading to

the fiber sparks. The paper web W runs in Fig.l according to arrow 16 from right to left.

The spark detector 13 is illustrated in more detail as one embodiment in Fig.2. It is not necessary to use a spark detector in this way. The single spark detector may be placed anywhere downstream the yankee-cylinder and upstream the tambour 14. It is necessary to ensure that the detector is kept clean, which can be done by a positive air jet directed to the detecting part 17 of the spark detector 13.

In the special embodiment illustrated in Fig.2 the spark detector 13 is combined with a paper web run-guiding device (foil) 18 which comprises a closed casing 19 being provided with guiding surfaces 20 for stabilising the high speed paper web run. Such guiding means are used anyway in the paper making machine and it is possible to form the guiding means in the manner illustrated in Fig.2 and combine the guiding device 18 with the spark detector 13. By the run of the paper web W along the guiding surfaces 20 a sub pressure is created in the space 21 of the casing and via an opening 22 in the vicinity of the spark detector 13 air is sucked into the space 21 along the dotted lines in Fig.2. Thus, the sensing part 17 of the spark detector 13 is kept clean during the operation of the paper machine so that the operation of the paper machine has not to be interrupted for cleaning the spark detector 13. In this manner the spark detector 13 is disposed in the vicinity of the paper web W and sparks of glowing fibers of fiber conglomerations can be detected when being transported on top of the paper web W and wound into the mother-roll 15.

Separately or in addition a positive air jet device 23 may be provided in the wall 24 of the casing 19. The air jet is directed to the sensing part 17 of the spark detector 13, which is illustrated by the dotted line. The means for supply

of air to this device 23 is not illustrated and may be any- known air supply means .

In such a way it is not necessary to provide the spark detector immediately upstream the tambour 14, where normally less space is present because the distance between the yankee-cylinder 9 and the tambour shall be kept as short as possible for stabilising the paper web.

When afore mentioned sparks are detected it is possible on the one hand to stop the machine and to take care for cancelling the creation of such sparks. On the other hand one knows that such sparks were possibly trapped by the paper web windings in a specific mother-roll and it is possible to separate this mother-roll from the normal mother-roll storage in order to avoid a big damage by fire within the mother-roll storage .

The casing 19 may be a simple steel box as illustrated schematically in Fig.2.