Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
2000-02-25
2001-03-27
Hastings, Karen M. (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C162S258000
Reexamination Certificate
active
06207018
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a headbox of a paper machine that includes a common feed line for a pulp slurry, a plurality of feed line sections into which the common feed line branches, and a device for influencing the composition of the pulp slurry flows in each of the feed line sections. The present invention also relates to a control device for a headbox of a paper machine wherein a cross-sectional profile sensor captures the weight basis of a paper web, and the control device controls the cross-sectional profile of the web and includes a device for influencing the composition of the pulp slurry in each of a plurality of feed line sections. The present invention also relates to a process for controlling a headbox of a paper machine which includes determining the weight basis cross-sectional profile of a paper web outside the headbox at a sensing rate, comparing this cross-sectional profile with a set point, and actuating a final control element in each section of the headbox in the event of variation between the actual value and the set point.
2. Discussion of Background Information
In the current state of the art, as shown by German Patent DE 43 16 054, a headbox of a paper machine is known which, at least in part, is formed of sections, enabling one to sectionally influence a feed pulp slurry. In headboxes of this type a weight basis cross-sectional control adjustment can occur, such that the pulp density is changed by section, while the fiber orientation cross-sectional profile is adjusted by sectionally altering the volumetric flows. The metered addition of diluting fluid generally is regulated by a control device. The actual cross-sectional profile of the pulp stream is thus determined outside the headbox, for example, in the screen section of the paper machine at certain time intervals &Dgr;T, and the actual cross-sectional profile is compared with set points stored in a control device. As a result of fluctuations between the actual and desired profiles, the pulp consistency is adjusted to the desired value or set point by actuating the control elements in the individual sections of the headbox. Headboxes, which operate with this type of cross-sectional profile control are shown, for example, in German Patent DE 42 39 845, the disclosure of which is hereby incorporated by reference.
However, the cross-sectional quality which can be achieved with such weight basis cross-sectional adjustment controllers is limited, in particular due to the longitudinal weight basis fluctuations occurring in certain processing systems. A primary cause for such longitudinal weight basis fluctuations is the occurrence of consistency fluctuations in the pulp stream. The main causes for consistency fluctuations in the pulp stream, in turn, are variations in thick matter and water in the screen, as well as uneven mixing of thick matter and water in the screen. De-mixing processes can also occur in the feed lines due to surface wall abrasion. It is also possible that interference in the control circuit, such as an actuator hysteresis, as well as measuring noise and regulator parameterization, can result in fluctuating metered quantities of thick material. Additionally, consistency fluctuations can occur in the headbox due to transient effects after a grade change, or after start-up of the processing system.
With previously known processes for controlling pulp stream consistency, the poor quality of measuring signals of the weight basis longitudinal profile were unable to suppress such consistency fluctuations. Moreover, the unavoidable weight basis longitudinal and residual profile variations result in a type of process noise, when measuring the actual cross-sectional profile. In order to accurately measure values for cross-sectional profile control, the process noise has to be suppressed by filtering the measurements. Due to this filtering, however, actuation of the cross-sectional control only occurs at large time intervals and/or with an inadequately low amplitude. As a result, the transient response of the control system slows down, and the cross section, which is adjustable by means of the control system, is limited by the random portion in the cross-sectional profile due to noisy profile data.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a headbox in which longitudinal profile interferences are reduced. It also is an object of the present invention to provide a control device and a process for controlling a headbox of a paper machine which allows improved and faster extraction of web cross-sectional profile from noisy profile data, and therefore allows more precise control of the headbox than the prior art.
According to the present invention, pulp consistency may be controlled in each individual section of the headbox. This allows for an improved and faster method of balancing the longitudinal web profile variations caused by consistency fluctuations in the feed pulp suspension. The web profiles recorded by the weight basis cross-sectional adjustment sensors located in an area behind the headbox of the paper machine encounter less noise, and therefore can be determined more quickly. This allows fast stabilization of potential cross-sectional variations. The determination of the pulp consistency via pulp consistency sensors is possible at various locations of the headbox. In a very cost-effective embodiment of the present invention, the pulp consistency sensor can be placed in the common pulp feed line, but it can also be placed in the return line.
In a particular embodiment of the present invention, each pulp feed line section is allocated its own pulp consistency sensor, enabling exact stabilization of the longitudinal profile variations in each section of the paper web. This contrasts with adjusting the consistency of the mean composition of the common feed of the distributing pipe, for example.
According to the present invention, pulp consistency sensors may use measuring principles to determine pulp consistency based on variations or fluctuations around a given mean value, rather than determining consistency based on an absolute value. These variations or fluctuations, however, have to be determined at very high resolution. Therefore, a particular embodiment of the invention provides a pulp consistency sensor that uses a light signal as a measuring device. On the basis of this measuring system, pulp consistency may be determined based on light scattering or light reflection, or via a particle counter.
According to another embodiment of the invention, the pulp consistency sensor includes a laser light source and a photo diode that detects the light signal. The number of particles passing through the section of the headbox in which the pulp consistency sensor is placed can be counted per unit time with this configuration. From this information, the variation with respect to a given mean value can be determined. However, it is not necessary to determine the exact number of particles since only variations of consistency have to be determined. Thus, in another embodiment of the invention that represents a modification of the above, it is acceptable to use an even more indirect measurement technique, for example, by determining the number of light-dark transitions per second on the photo diode or its “flickering intensity.” Instead of using a light as a measuring signal, it is also possible to measure pulp consistency based on other measuring techniques, such as with microwave signals, for example.
The present invention also provides a control device for a sectioned headbox which, in addition to the weight basis cross-sectional adjustment controller for the longitudinal cross-sectional profile, includes a pulp consistency controller which is located subjacent to the weight basis cross-sectional adjustment controller. Thus, the pulp consistency controller may be inserted in the feed line after the weight basis cross-sectional adjustment controller. The pulp consistency contro
Begemann Ulrich
Munch Rudolf
Greenblum & Bernstein P.L.C.
Hastings Karen M.
Voith Sulzer Papiermaschinen GmbH
LandOfFree
Process for controlling headbox with pulp consistency control does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for controlling headbox with pulp consistency control, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for controlling headbox with pulp consistency control will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2511234