WO2005024394A1

WO2005024394A1 - Method for measuring porosity of moving web-like paper

Info

Publication number: WO2005024394A1
Application number: PCT/FI2004/000520
Authority: WO
Inventors: Pekka Komulainen
Original assignee: Aca Systems Oy
Priority date: 2003-09-08
Filing date: 2004-09-08
Publication date: 2005-03-17
Also published as: FI20031273A0; FI20031273A

Abstract

The object of the invention is a method for measuring the porosity of moving web-like paper, in which method on one side of a paper web a measuring head (1) vacuumed with a suction device (18, 19) has been placed, with wich measuring head the flow rate of air due to vacuum through a paper area of certain size is measured and the porosity of paper is defined from the flow rate, and in which method the access of dust coming loose from paper to the measuring device is prevented with a dust filter (30, 31) placed between the suction device and the measuring head. Characteristic to the method in accordance with the invention is that the state of blockage of the dust filters (30, 31) is determined by measuring the pressure before the dust filter and after it, and that the pressure is measured with at least one pressure sensor (32, 34) placed between the measuring head (1) and the dust filter (30, 31), and with at least one pressure sensor (33, 35) between the dust filter (30, 31) and the suction device (18, 19).

Description

METHOD FOR MEASURING POROSITY OF MOVING WEB-LIKE PAPER

The present invention relates to a method for measuring the porosity of moving web-like paper, in which method on one side of a paper web a measuring head vacuumed with a suction device has been placed and with which measuring head the flow rate of air due to vacuum through a paper area of certain size is measured and the porosity of paper is defined from the flow rate, and in which method the access of dust from paper to the measuring device is prevented with a dust filter placed between the suction device and the measuring head.

While measuring the porosity of a moving paper web many materials to be measured make dust or fibres or other particles on the paper surface come loose. Therefore, measuring devices based on air penetration are often equipped with dust filters, which prevent the access of dust or some other particles to regulating unit (the suction device) of the device. Unless filters are changed often enough they block up, which causes errors in measuring results. The amount of dust coming loose from the surface of paper varies depending on paper type, among other things. Therefore dust filters sometimes block up remarkably faster than expected, what causes false measuring results. To avoid this, dust filters must be changed (to be on the safe side) too often what increases measuring costs.

The object of the invention is to provide a method, with which the earlier mentioned problems related to measuring of porosity of paper are eliminated. Especially, the object of the invention is to provide a method, with which it is possible to measure the porosity of paper from a moving paper web such that it will be known when the dust filter, which is a part of the measuring device, must be changed for avoiding false measuring results due to a blocked-up dust filter. In addition, the object of the invention is to provide a method, with which it is possible to define the roughness of paper in addition to the porosity.

The object of the invention is accomplished by a method, the characteristics of which are presented in the claims.

Characteristic to the method in accordance with the invention is that the degree of blockage in the dust filter is defined by measuring the pressure before the dust filter and after it, and that the pressure is measured with at least one pressure sensor placed between the measuring head and the dust filter, and with at least one pressure sensor between the dust filter and the suction device. When a dust filter blocks up, the pressure drops on the suction device side and increases on the measuring device side. In this way, by means of pressure sensors placed on both sides of the dust filter, the difference between pressures (the pressure difference) is discovered, which difference is compared with the earlier defined limit value describing the excessive blockage of a dust filter and the state of blockage may easily be defined during the measuring. In this way, the possible blockage of the dust filter is noticed in time, in which case measuring errors due to a blocked up filter may be prevented. Due to this, dust filters may always be used their maximum operating time, thus preventing too early changes of dust filters and extra costs due to them. In addition, by means of measuring results of the pressure sensor placed between the measuring head and the dust filter, and the pressure sensor placed between the dust filter and the suction device the vacuum affecting in the measuring head, among others, may be regulated such that the vacuum is kept constant during the whole time of measuring, that is, it is regulated nonstop. Further, due to this kind of pressure measuring, the volume of vacuum affecting in the sealing groove of the measuring head may be optimized by means of the regulating system suitable for the material to be measured such that the leaking air between the measuring head and the paper web stays minimal but the vacuum does not cause any marks on the surface of the material to be measured.

In the second advantageous application of the invention the pressure measured before the dust filter and the pressure measured after the dust filter are compared with each other, and based on the difference of pressures the state of blockage of the dust filter is defined. In this way, the state of the dust filter may easily be monitored automatically during the measuring. In addition, while in this kind of method pressures affecting before the dust filter and after it are measured, and not only the pressure difference itself, a slight blockage may be compensated by directing the power of suctions devices based on the pressure difference such that the flow in the measuring head side of the blocked-up dust filter is kept the same the whole time as when the dust filter is completely clean.

In the third advantageous application of the invention, the difference between pressures is defined by subtracting measured pressure values from each other and comparing the acquired difference with the earlier defined limit value of the pressure difference, which corresponds to the pressure difference caused by a blocked-up dust filter. Due to the limit value defined in this way, which limit value is clear and easy to be automatically controlled, the carrying out of measuring by accident when the dust filter is too blocked-up may be reliably prevented. From the measuring results those, which have been gathered when the pressure difference is smaller than the limit value, may be afterwards accepted. In this way, all measuring results need not be rejected although the limit value was exceeded during measuring.

In the fourth advantageous application of the invention, the airflow rate is measured with a flange detector of pressure difference. While measuring porosity by means of airflow, it is possible to gather more reliable results than by measuring merely vacuum. The airflow rate may be measured with a flange detector of pressure difference exactly and reliably.

In the fifth advantageous application of the invention, the measuring head is moved with respect to the paper web to be measured. It is often important that measuring information is gathered from various points of the paper web. By means of a movable measuring head, this is possible simply and reliably with one relatively small measuring head without the otherwise needed several measuring heads mounted next to each other, each of them needing suction channels and a vacuum regulating system.

In the sixth advantageous application of the invention there is a counter part mounted in the measuring head on the opposite side of the paper web, which prevents airflow through the paper web. By measuring the leaking air rate between the counter part, paper web and the measuring head it is possible to determine the roughness of paper. In this way, with simple and advantageous additions/changes needed in the device in accordance with the invention, it is possible to define the roughness of paper, which apart from porosity is another important characteristic describing paper quality, and for which, in other situation, a separate measuring device would be needed.

In the seventh advantageous application of the invention excess pressure is created to the measuring head while measuring the roughness of a paper web. While using excess pressure, the flow rate between the counter part and the paper web is made strong due to which the accuracy of measuring improves.

Next, the invention will be explained in more detail with reference to the accompanying drawings, in which,

Figure 1 illustrates a principle drawing of a device in accordance with the invention, and figure 2 illustrates a measuring head included in the device in accordance with figure 1 viewed from side. The device in accordance with figure 1 includes a measuring head 1 and a regulating system 2 connected to it, and which regulating system comprises a pressure regulating valves 16 and 17 of the measuring opening 10, the suction devices 18 and 19, vacuum meters 20 and 21, pressure sensors 32-35 of the dust filters 30 and 31, a flow rate meter 22 placed in the vacuum channel of the measuring opening as well as the pressure balancing valves 23 and 24. In addition, the regulating system includes a controlling unit 25, with which it is possible to control the pressure regulating valves and collect and save measuring data from pressure sensors, vacuum meters and flow rate meters. The device comprises also two vacuum channels between the regulating system 2 and the measuring head 1, the first being the vacuum channel 5 of the sealing groove 12 of the measuring head 1 and the second being the vacuum channel 6 of the measuring opening 10. There are dust filters 30 and 31 at the measuring head end of the vacuum channels, with which dust filters the access of dust and other particles coming loose from the surface of paper to the regulating system 2 and the suction devices 18 and 19 is prevented.

Pressure sensors 32, 33, 34, and 35 have been mounted to the vacuum channels 5 and 6 near the dust filters 30 and 31 between the dust filters and the measuring head 1 and between the dust filters and the regulating system 2, with which sensors the pressure is measured before the dust filters and after them. These pressure sensors have been connected to the controlling unit 25, which calculates the pressure difference appearing before the dust filters and after them and on that basis informs the user of the measuring device about a blockage in the filters.

The construction of the measuring head 1 is presented in more detail in figure 2. In this case, the measuring head is, as viewed from above, an oval-shaped object, which comprises a frame part 3, and to that detachably attached measuring part 4. The frame part 3 includes a suction connection 15 of the measuring opening and the suction connection 14 of the sealing groove. In the middle of the frame part 3 there is, as viewed from above, an oval-shaped recess 7, which is somewhat bigger than the measuring part, at the bottom of which there is an oval-shaped flange 8 reaching from the bottom of the recess upwards for locking and sealing the measuring part to the recess as presented in figure 2. The frame part comprises also mounting screws 9 for mounting the measuring part.

The measuring part 4 is a separate part detachably attached to the frame part 3 with mounting screws 9. It comprises a measuring opening 10 to be placed against the surface of the material 13 to be measured, the size and shape of which opening has been defined in accordance with the test standard of the web-like material to be measured (For example, in the Bendtsen-method the area of the measuring opening must equal 10 cm ). The measuring opening 10 reaches through the measuring part to the recess 11 situated on the lower surface of the measuring part. On the outer edge of the upper surface of the measuring part, there is an open groove on its outer edges square by its cross section, which groove forms the sealing groove 12 of the measuring head, when the measuring part is attached to the frame part. The measuring part is attached to the frame part as presented in figure 2 such that there will remain air spaces 26 and 27 below the sealing groove and between the recess 11 in the measuring part and the recess in the frame part. The edges of the measuring part 4 have been bevelled in order to make the airflow rate increase inside the sealing groove for obtaining as even vacuum as possible. The flange 8 at the bottom of the recess 7 of the frame part seals the air space 26 situated under the measuring opening 10 and the air space 27 situated under the sealing groove 12 from each other. Due to detachability of the measuring part in the same measuring head, measuring parts with measuring openings of different sizes and shapes may be used for measuring, for example various materials and measuring in accordance with the orders of test standards connected with those.

The vacuum channel 6 of the measuring opening has been connected to the air space 26 between the measuring part 4 of the measuring head, and the frame part 3, in which case vacuum created by the vacuum channel to the air space creates the desired vacuum in the measuring opening 10 and a flow through the material to be measured. Respectively the vacuum channel 5 of the sealing groove has been connected to the air space 27 at the sealing groove 12. In this application, there are dust filters 30 and 31 in the measuring head end of both vacuum channels, and there are pressure sensors 32-35 on the measuring head end of both dust filters and on the suction device side, which pressure sensors have been connected to the controlling system 25, in which case, by means of the measuring data from them, the blockage of the dust filters may be monitored. Desired vacuum to the vacuum channels 5 and 6 may be regulated by pressure regulating valves 16 and 17. Vacuum and flow are created by means of suction devices 18 and 19, which, in this case, are pneumatic vacuum ejectors, suction connections of which have been connected to vacuum channels on the measuring head side, and blowing connections to the side of pressure regulating valves 16 and 17. While measuring the roughness of surface the blowing side of the suction device 19 is connected to the vacuum channel 6 on the measuring head side, in which case vacuum is created in the measuring head. The suction device 18 of the sealing groove is at this point switched off. Pressure regulating valves 16 and 17 are magnetic valves, which can be controlled by means of the controlling unit 25 programmatically. In the vacuum channel 6 of the measuring opening in the suction side of the ejector, there is a flow meter 22 and a vacuum meter 21 for measuring the flow and the vacuum. In the vacuum channel of the sealing groove there is mere a vacuum meter 20 at the respective point for regulating the vacuum in the sealing groove 12 to be similar with the vacuum in the measuring opening 10. All previously mentioned meters are by constructions recognized electrically functioning meters. They are connected to the controlling unit 25 of the regulating system, which unit utilizes the measuring data from the meters for measuring porosity and for regulating the vacuum in the measuring head. Pressure valves 23 and 24 are by construction and functioning principles recognized three way valves, with which the vacuum is maintained as even as possible by feeding suitably compressed air to vacuum channels 5 and 6 as well as limiting, if needed, air flow in the vacuum channels.

While using the device in accordance with figures 1 and 2, the measuring head is placed above or below the web-like material as presented in figure 1. As the paper moves forward, the measuring head touches the surface of the material. Desired vacuum (for example in accordance with the standard of the material) is adjusted to the measuring opening and the sealing groove. Usually, the vacuum in the sealing groove is adjusted manually during each mounting to be suitable to each device. Adjusting pressure is carried out in the controlling unit, which comprises a suitable user interface (a keyboard and a screen), with which the setting value of the vacuum may be fed to the memory of the controlling unit before starting the measuring. During the measuring, the controlling unit gathers data from the flow meter and the vacuum meter and defines the porosity out of those automatically. The measuring results of vacuum from the vacuum meters are utilized for controlling the vacuum of the measuring head such that it stays as close as possible to the earlier defined setting value. The pressure valves adjust the pressure to reach the desired value while the pressure sensors measure the vacuum of the measuring groove and the sealing groove nonstop. In this application, the measuring time is about 1 ms. The values of porosity in the preset vacuum value are saved to the memory of the controlling unit. During the measuring, the regulating system continuously controls the state of the dust filters by comparing the results from the pressure sensors measuring the state of blockage of the dust filter. In case the pressure difference caused by the dust filters exceeds a certain limit value representing a value of blocked up filters the controlling unit gives a warning text on the screen about a blockage of the filters. After measuring, the acquired results stay in the memory of the controlling unit, from where they may be transmitted, for example, to the process data system or somewhere else for saving/printing for later use. In case the dust filters have been blocked up during the measuring, a notice has been added in the saved results to those values which were gathered during the blockage of the dust filters.

The method in accordance with the invention and the device in accordance with it may be realized in many ways differing from the application in accordance with the figures 1 and 2. In one application, the measuring head has been organized to move in reciprocating motion in cross wise direction of the material during the measuring. In this way, so-called traversed measuring is created, by means of which with one measuring head the measuring data about the porosity of a paper web is gathered in the direction of the breadth in different points of the paper.

The regulating system 2 may as well be realized in many ways differing from earlier presented example application. The device may include channels controlled with magnetic valves for measuring the roughness of the surface of the paper, with which channels it is possible to change the direction of the flow of the air in the vacuum channels (an excess pressure is created to the measuring head). Many various meters may be used to measure the flow or the measuring may be carried out with a suitable flow sensor, the measuring data from which is converted into flow rate in the controlling unit, which is a part of the regulating system. For example, in one application the flow is measured with a flange sensor of pressure difference. Also, pressure meters/sensors functioning in various principles may be used for measuring the vacuum. There is not necessarily any need for a separate pressure sensor after a dust filter but often it is possible to use the measuring data from flow and pressure meters for defining porosity values while comparing the pressure before a dust filter to the pressure after it for defining the stage of blockage of dust filters.

The invention is not limited to the presented advantageous application but it can vary within the frames of the idea of the invention formed in the claims.

Claims

1. Method for measuring the porosity of moving web-like paper, in which method on one side of a paper web a measuring head (1) vacuumed with a suction device (18, 19) has been placed, and with which measuring head the flow rate of air due to vacuum through a paper area of certain size is measured, and the porosity of paper is defined from the flow rate, and in which method the access of dust coming loose from paper to the measuring device is prevented with a dust filter (30, 31) placed between the suction device and the measuring head, ch ara cteriz e d in that the degree of blockage of the dust filters (30, 31) is defined by measuring the pressure before the dust filter and after it, and that the pressure is measured with at least one pressure sensor (32, 34) placed between the measuring head (1) and the dust filter (30, 31) and with at least one pressure sensor (33, 35) placed between the dust filter (30, 31) and the suction device (18, 19).

2. Method in accordance with claim 1, c hara c teri z e d in that the pressure measured before the dust filter (30, 31) and the pressure measured after the dust filter (30, 31) are compared with each other, and based on the difference between the pressures, the blockage degree of the dust filter is defined.

3. Method in accordance with claim 2, chara c t eri z e d in that the difference between pressures is defined by subtracting measured pressure values from each other, and that the difference is compared with the earlier defined limit value of the pressure difference, which corresponds to the pressure difference caused by a blocked up dust filter (30, 31).

4. Method in accordance with any of claims 1-3, characterized in that the flow rate of air is measured with a flange detector of pressure difference.

5. Method in accordance with any of claims 1-5, characteriz ed in that the measuring head (1) is moved with respect to a paper web.

6. Method in accordance with any of claims 1-6, charact eriz ed in that there is a counter part mounted at the point of the measuring head (1), on the opposite side of the paper web, which prevents the air flow through the paper web, and that by measuring the flow of leaking air among the counter part, paper web and the measuring head (1) the roughness of paper is defined.

7. Method in accordance with claim 7, characterized in that an excess pressure is created in the measuring head (1) while measuring the roughness of the surface of a paper web.