WO2000032868A1

WO2000032868A1 - System of loading and controlling a press nip in a paper or board machine

Info

Publication number: WO2000032868A1
Application number: PCT/FI1999/000991
Authority: WO
Inventors: Antti Ilmarinen; Juha Lahtinen; Sanna JÄRVENPÄÄ; Juha Kivimaa
Original assignee: Metso Paper, Inc.
Priority date: 1998-12-02
Filing date: 1999-11-29
Publication date: 2000-06-08
Also published as: FI982601A0; FI104755B

Abstract

The invention relates to a system of loading and controlling a press nip in a paper or board machine. The press nip (N) is formed as an extended nip press which comprises a loading shoe (14) which is arranged to be loaded by means of two loading cylinders (26, 27). The loading shoe (14) is pressed against a backing roll (2) which includes a backing shoe (28). The system of loading and controlling the press nip (N) comprises at least two valves (22, 23) and loading medium lines (30, 24, 25) passing from the valves (22,23) to the loading shoe (14) and the backing shoe (28). One valve (22) is arranged to be employed for loading the first loading cylinder (26) of the loading shoe (14) of an extended nip roll (1), as viewed in the running direction (S) of the web, and for loading a loading cylinder (29) of the backing shoe (28) of the backing roll (2). The other valve (23) is arranged to be employed for loading the second loading cylinder (27) of the loading shoe (14) of the extended nip roll (1), as viewed in the running direction (S) of the web. Check valves (31, 32, 33) are fitted in lines (34, 35) between the loading medium lines of the valves (22) and (23), which check valves allow pressure to pass from the side of higher pressure to the side of lower pressure when the loading pressure exceeds a given limit value.

Description

System of loading and controlling a press nip in a paper or board machine

The invention relates to a system of loading and controlling a press nip in a paper or board machine, which press nip is formed as an extended nip press which comprises a loading shoe which is arranged to be loaded by means of two loading cylinders, which loading shoe is pressed against a backing roll which includes a backing shoe.

With the development of the papermaking art, the drying of a paper web in the manufacturing process has turned out to be very important. One important stage in the drying of the paper web is a press section, in which the drying can be enhanced, as known, by using extended nip presses. The extended nip presses employ a press shoe which is loaded by means of two loading cylinders, pressing the shoe against a backing roll so that the paper web is under compression in the nip for a longer period of time than in conventional compression with two rolls pressed against each other, and in which the loading of the shoe can be regulated to achieve desired compression. In the extended nip presses, as known in prior art, the web is passed between press felts through a nip between a rotating backing roll and a press shoe of an extended nip roll and a glide belt gliding along its face.

With regard to prior art, reference is made to international patent application publication WO-97/28309, from which an extended nip press is known which comprises a shoe which is driven by two pistons to engage a blanket-supported web against a backing roll. In this known arrangement, the balance of forces is taken care of by means of a valve and the compression relationship in the running direction of the web (tilt) is taken care of by means of another valve. Both valves are special valves which have different surface areas. In this known system, the rolls have separate pumps, so that a lot of attention must be paid to the maintenance of a force balance between the rolls. In this known system, for an equalizer valve is used a special valve which is designed for this application, which equalizer compares the pressures in different lines and directs a higher pressure into a reservoir. Even a small disturbance between the lines causes the system to be activated.

An object of the invention is thus to provide an arrangement in which no special valves are needed for balancing the system and which is simple in structure and can be easily accomplished.

A further object of the invention is to create an arrangement in which the system is activated at a given desired pressure difference and not yet necessarily because of a small disturbance.

When in the arrangements known in the prior art, five profiling zones have been used in a transverse direction, fifteen valves have been required for control of profiling. The hydraulics, then, have become rather complicated because of the large number of valves.

Thus, an object of the invention is to propose an arrangement whose valve system and the hydraulics associated with it are simple and in which it is possible to regulate the compression relationship in the running direction of the web (tilt), but in which it has also been taken into account that a possible malfunction in some valve will not cause breaking of rolls and the system of loading. In the arrangements known from the prior art where a large number of valves (for example, 15 valves) have been used, a malfunction of one valve in itself has not caused a dangerous situation, because there have been many valves, and a malfunction of one valve does not very much affect the operation and the load equilibrium of the entire nip (for example, in a system of 10 valves, the effect of each valve is only about 10 % of the total load).

With a view to attaining the objectives stated above and those that will come out later, the invention is mainly characterized in that the system of loading and controlling a press nip comprises at least two valves and loading medium lines passing from the valves to a loading shoe and a backing shoe, of which valves, one valve is arranged to be employed for loading a first loading cylinder of the loading shoe of an extended nip roll, as viewed in the running direction of the web, and for loading a loading cylinder of the backing shoe of a backing roll, and the other valve is arranged to be employed for loading a second loading cylinder of the loading shoe of the extended nip roll, as viewed in the running direction of the web, and that check valves are fitted in lines between the loading medium lines of the valves, which check valves allow pressure to pass from the side of higher pressure to the side of lower pressure when the loading pressure exceeds a given limit value.

In accordance with the invention, the control of the loading and the compression relationship in the running direction of the web in an extended nip press includes a safety system and the loading of the entire press nip is controlled by means of two valves. One of the valves provides pressure for one of the loading cylinders of the extended nip roll and for the loading cylinder of the backing roll and the other of them provides pressure in a corresponding way for the other loading cylinder of the extended nip roll. Between the lines there are arranged check valves to balance the pressures in case of a possible disturbance. The surface areas of pistons of the loading cylinders are so arranged that, if an equal pressure is supplied to both sides of the nip, the forces are in balance. The shoe of the extended nip press is placed with respect to the loading cylinders such that a given predetermined tilt is provided even at equal pressures, in other words, when the ratio of the force produced by the first loading cylinder in the running direction of the web to the force produced by the second loading cylinder is one, and when the loading ratio is changed, the tilt value can be changed. In practice, the tilt value is changed by changing the pressure of the cylinder row on the outlet side.

In accordance with the invention, an uncomplicated loading system is provided which allows the compression ratio to be regulated and which is provided with a safety system against possible damage. The forces between the rolls are in equilibrium with a given predetermined value of the compression ratio and with other values a small imbalance is created. In the invention, loading pressure is supplied to both sides of the nip by using the same hydraulics. In accordance with the invention, the loading pressure is taken from the same pump to both rolls, and if the pump breaks down, both rolls will lose the loading at the same time. The balancing between the rolls is accomplished in the invention by using control check valves, which are conventional valves in themselves known in structure. The balancing of forces by means of the check valves is accomplished such that a loading medium is allowed to flow from the side of higher pressure to the side of lower pressure when a set pressure limit between the lines is exceeded. In the system, a given pressure difference is thus required before the system is activated.

In accordance with the invention, one zone is preferably used in a transverse direction, but, when needed, the invention may also be applied when the possibility of profiling is desired.

In the following, the invention will be described in more detail with reference to the figures of the accompanying drawing, to the details of which the invention is not by any means intended to be narrowly confined.

Figure 1 is a schematic view of a press nip.

Figure 2 is a schematic view of one embodiment of the invention.

Figure 3 schematically shows displacement of a press shoe.

Fig. 1 schematically shows a cross-sectional structure of an extended nip roll. The extended nip roll 1 and its backing roll 3 form a nip N. The extended nip roll 1 comprises a belt mantle 11, which is a flexible deformable mantle, which conforms to the shape determined by a loading shoe 14 of the extended nip roll 1 and by the backing roll 2. The central axle 12 of the extended nip roll 1 comprises loading means 13 by means of which the loading shoe 14 can be pressed against the backing roll 2. A web W and a felt H are passed through the nip N.

As shown in Fig. 2, the shoe 14 situated inside the extended nip roll 1 is loaded by means of loading cylinders 26 and 27. The backing shoes of the backing roll 2 are designated by the reference numeral 28. These backing shoes 28 are formed of glide elements and of loading pistons fixedly coupled to them. The loading pistons are mounted in cylinders 29 bored into the axle 2A of the roll 2. In accordance with the invention, a loading medium is passed from a hydraulic unit 21 through a valve 22 to the cylinder 29 on the side of the backing roll and to the cylinder 26 on the side of the extended nip roll 1. Medium lines are designated by the reference numerals 30 and 24, respectively. A medium flow is passed from the hydraulic unit 21 through a valve 23 along a line 25 to the loading cylinder 27 on the side of the extended nip roll 1. Between the lines 24,30; 25, check valves 31,32,33 are arranged to guarantee that, when the valve 22 or 23 is broken, the imbalance of loading pressures does not damage the press nip N, but a given pressure difference, which does not damage the nip, is produced between the lines. The check valves 31,32,33 are placed in two lines between the lines 24, 30; 25 of the valves 22,23 such that the valve 33 is placed in a line 34 between the lines 30 and 25 and the valves 31 and 32 are placed in a line 35 between the lines 24 and 25. A valve frame is designated by the reference numeral 37. In Fig. 2, the running direction of the web is denoted with the arrow S and the inlet side of the extended nip roll 1 with the reference sign IT and the outlet side with the reference sign 1J.

When the valve 22 is damaged such that it attempts to allow in the line 30 a higher pressure than that of the line 25, the check valve 33 opens and limits the pressure in the line 30 to a value which is 5 bar higher than the pressure acting in the line 25. If the valve 22 is damaged such that the pressure in the line 30 or 24 decreases to a value that is 10 bar lower than the pressure acting in the line 25, the pressure difference between the lines is stabilized to a value of 10 bar by means of the check valves 31 and 32 (2 check valves of 5 bar in series) . This series coupling of two check valves simultaneously limits the higher tilt value (the pressure difference between the cylinders 27 and 26 to a value of 10 bar). The system operates in a similar fashion in connection with the valve 23 being damaged.

In accordance with the invention, the loading and the compression relationship in the running direction of the web in the extended nip press are controlled in the nip N by means of two valves, preferably proportional valves 22 and 23, of which valves, one valve 22 provides pressure for the loading cylinder 26 on the side of the extended nip roll 1 and for the loading cylinder 29 on the side of the backing roll 2 and the valve 23 provides pressure for the loading cylinder 27 of the extended nip roll. The check valves 31,32,33 are arranged between the lines 24, 30 and 25 of the valves such that in case of a possible disturbance, the pressures are equalized and pressure flows from a higher side to a lower one. The surface areas of the pistons of the loading cylinders 26,27 are so arranged that if an equal pressure is supplied to the sides of the extended nip roll 1 and the backing roll 2, the forces are in equilibrium. The shoe 14 is displaced from the symmetrical position with respect to the loading cylinders 26,27 such that at equal pressures the tilt (loading cylinder 27 : loading cylinder 26) is higher than one, i.e. the resultant force R is on the outlet side 1J of the symmetry axis X of the shoe (Fig. 3). In accordance with the invention, a desired tilt of equal pressure is achieved by displacing the shoe 14 with respect to the loading cylinders 26,27, as shown in Fig. 3, in which the displacement of the shoe 14 is illustrated with the arrow Z.

In accordance with the invention, the loading pressure is taken from the same hydraulic unit 21 for both rolls 1 and 2, and therefore if the unit should be broken, both rolls will lose their loading at the same time with the result that there is no risk of damage. The balancing of forces is accomplished such that oil is allowed to flow from the side of higher pressure to the side of lower pressure when a given pressure limit between the lines has been exceeded, which pressure limit may be determined as desired. Conventional valves known in themselves in their structure can be used as the valves 22,23,31,32,33. Above, the invention has been described only with reference to some of its advantageous embodiment examples, to the details of which the invention is, however, not intended by any means to be narrowly confined. The invention can be applied, for example, in several types of press nips of a press section and in their applications.

Claims

1. A system of loading and controlling a press nip in a paper or board machine, which press nip (N) is formed as an extended nip press which comprises a loading shoe (14) which is arranged to be loaded by means of two loading cylinders (26,27), which loading shoe (14) is pressed against a backing roll (2) which includes a backing shoe (28), characterized in that the system of loading and controlling the press nip (N) comprises at least two valves (22,23) and loading medium lines (30,24,25) passing from the valves (22,23) to the loading shoe (14) and the backing shoe (28), of which valves, one valve (22) is arranged to be employed for loading the first loading cylinder (26) of the loading shoe (14) of an extended nip roll (1), as viewed in the running direction (S) of the web, and for loading a loading cylinder (29) of the backing shoe (28) of the backing roll (2), and the other valve (23) is arranged to be employed for loading the second loading cylinder (27) of the loading shoe (14) of the extended nip roll (1), as viewed in the running direction (S) of the web, and that check valves (31,32,33) are fitted in lines (34,35) between the loading medium lines of the valves (22) and (23), which check valves allow pressure to pass from the side of higher pressure to the side of lower pressure when the loading pressure exceeds a given limit value.

2. A loading and controlling system as claimed in claim 1, characterized in that a given desired predetermined tilt value is achieved by means of a compression ratio between the loading cylinders (26,27) of the loading shoe (14) by displacing the shoe (14) with respect to the loading cylinders (26,27) in the running direction (S) of the web.

3. A loading and controlling system as claimed in claim 1 or 2, characterized in that the check valves (31,32,33) are preferably placed in two lines which are arranged to pass between the lines (30,25) of the valves (22,23) in order to limit the pressure difference between the lines (30,25) for limiting the tilt value to a desired maximum value and for securing a force balance in the nip.

4. A loading and controlling system as claimed in any one of the preceding claims, characterized in that the surface areas of pistons of the loading cylinders (26,27) are arranged such that when an equal pressure is supplied to both sides of the nip (N), the loading forces are in balance.

5. A loading and controlling system as claimed in any one of the preceding claims, characterized in that the system includes a hydraulic unit (21) for producing loading pressures.

6. A loading and controlling system as claimed in any one of the preceding claims, characterized in that the loading shoe (14) is placed with respect to the loading cylinders such that the compression ratio in the running direction of the web is > 1.

AMENDED CLAIMS

[received by the International Bureau on 25 April 2000 (25.04.00); original claim 1 amended; remaining claims unchanged; (2 pages)]

1. A system of loading and controlling a press nip in a paper or board machine, which press nip (N) is formed as an extended nip press which comprises a loading shoe (14) which is arranged to be loaded by means of a first loading cylinder (26) as viewed in the running direction of the web and by means of a second loading cylinder (27) as viewed in the πinning direction of the web, which loading shoe (14) is pressed against a backing roll (2) which includes a backing shoe (28), characterized in that the system of loading and controlling the press nip (N) comprises at least two valves (22,23) and loading medium lines (30,24,25) passing from the valves (22,23) to the loading shoe (14) and the backing shoe (28), of which valves, one valve (22) is arranged to be employed for loading the first loading cylinder (26) of the loading shoe (14) of an extended nip roll (1), as viewed in the running direction (S) of the web, and for loading a loading cylinder (29) of the backing shoe (28) of the backing roll (2), and the other valve (23) is arranged to be employed for loading the second loading cylinder (27) of the loading shoe (14) of the extended nip roll (1), as viewed in the running direction (S) of the web, and that check valves (31,32,33) are fitted in lines (34,35) between the loading medium lines of the valves (22) and (23), which check valves allow pressure to pass from the side of higher pressure to the side of lower pressure when the loading pressure exceeds a given limit value.