Paper making and fiber liberation – Apparatus – Automatic control
Reexamination Certificate
1999-06-10
2001-03-20
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Apparatus
Automatic control
C162S258000, C162S259000, C162S263000, C162SDIG006
Reexamination Certificate
active
06203667
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for regulating the basis weight of paper or board in a paper or board machine in which the machine stock comprises several component stocks and in which the basis weight of the dry paper or board is measured, preferably by means of on-line basis-weight measurement at the end of the machine.
BACKGROUND OF THE INVENTION
Regarding its principal features, the stock feed at a paper machine is generally as follows. The stock components are stored at the paper mill in separate storage towers. From the storage towers, the stocks are fed into stock chests, and from the stock chests further into a common blend chest, in which the stock components are mixed with each other. From the blend chest, the stock is fed into a machine chest, and from the machine chest there is an overflow back into the blend chest.
From the machine chest, the stock is fed into a dilution part of the wire pit, in which the stock is diluted with white water recovered from the wire section and serving as dilution water. From the wire pit, the stock is fed through centrifugal cleaners into a deaeration tank. From the deaeration tank, stock free from air is fed through a machine screen into the headbox, i.e., into the inlet header thereof, and through the slice opening of the headbox to the wire section. A bypass flow of the headbox is fed back into the deaeration tank, and the white water recovered from the wire section is fed into the wire pit.
The basis weight and the ash content of the paper are measured on-line right before reeling from a ready, dry paper, usually by means of measurement apparatuses based on beta radiation and x-radiation. Based on this measurement, the basis weight of the paper is regulated, for example, by means of a so-called basis weight valve by whose means the stock flow after the machine chest is controlled. A second possibility is regulation of the speed of rotation of the pump that feeds stock from the machine chest into the wire pit. The ash content is controlled by dosing of fillers. The basis weight profile of the paper in the cross direction is obtained when the measurement apparatus is installed to move back and forth across the web.
In prior art arrangements of regulation in a paper machine, the metering of component stocks usually takes place with the aid of the surface level in the blend chest, the consistency of component stock, and a pre-determined stock proportion reference value. The ash contents of component stocks are not used for controlling the metering of component stocks. The measurement values obtained by means of measurement of basis weight are used in the control of the basis weight valve situated after the machine chest, but these values are not used for controlling the metering of component stocks.
In prior art regulation of basis weight, exclusively the overall consistency and the overall flow rate of the stock are controlled. Regulation of basis weight taking place by means of the flow departing from the machine chest is disturbed, among other things, by disturbance in the consistency of the machine stock, which disturbance arises from incomplete mixing of the stock in the blend chest and in the machine chest. The volumes of the blend chest and of the machine chest are considerable, so that the regulation of their surface levels readily starts oscillating, which results in disturbance in the regulation of the basis weight. From the recovery of fibers, ash disturbance arises in the blend chest. The dynamics of the recovery of fibers produce different dynamics and dwell times for a part of the stock. Owing to the large volumes of the machine chest and the blend chest, a long stilling time is required before the basis weight can be set at the desired level. For this reason, a change in the paper grade being produced by the machine is slow.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a new and improved method for regulating the basis weight of a paper or board web in a paper or board machine.
It is another object of the present invention to provide a method for regulating the basis weight of a paper or board web which in a paper or board machine enables a quick change in the grade of paper or board being produced.
In order to achieve these objects and others, a method for regulating basis weight of paper or board in a paper or board machine formed from a machine stock comprised of a plurality of component stocks in accordance with the invention comprises the steps of determining a stock target of the machine stock which is the amount of fibers in the machine stock per unit time and relates to the basis weight of the paper or board, determining a metering target for each component stock based on the stock target, the metering target being the amount of the component stock per unit time flowing to form the machine stock and passing each component stock from a source thereof through a respective feed line. The consistency of each component stock is measured, preferably in a respective one of the feed lines, and a flow target for each component stock is determined based on the measured consistency of the component stock and the metering target determined for the component stock. The flow target is the volume of the component stock per unit time flowing to form the machine stock. The flow of each component stock through the respective feed line is then regulated based on the flow target determined for the component stock.
Further, desired proportions of the component stocks in the paper or board can be determined and the stock target determined based at least in part on the determined proportions of the component stocks. For example, the desired proportions of the component stocks can be determined by determining a fiber length target of the machine stock which is a desired fiber length of the machine stock, measuring data relating to the length of fibers in the component stocks, e.g., in the feed lines, and optimizing the proportions of the component stocks based at least in part on the measured fiber length data and the determined fiber length target of the machine stock. The proportions of the component stocks may be optimized additionally based on the cost of the component stocks and/or the availability of the component stocks. The proportions of the component stocks may be optimized by determining an average value of the fiber length of each component stock from the measured fiber length data, determining a weighted average value of the fiber length of each component stock from the measured fiber length data or forming a distribution of the fiber length of each component stock from the measured fiber length data at specified time intervals.
In additional embodiments, the ash content of each component stock is measured, e.g., in the feed line, and the flow target of each component stock is determined additionally based on the measured ash content of the component stock.
The method in accordance with the present invention for regulation of the basis weight by means of metering of component stocks is particularly well suited for process arrangements in which there is no blend chest/machine chest arrangement that equalizes the pumping and the consistencies. For precise regulation of basis weight, in the method in accordance with the invention the following properties have been discovered:
the dilution of component stocks to metering consistency takes place before the component-stock stock chest,
the regulation of the basis weight takes place from the component-stock stock chest by means of regulation of the flows of component stocks, and
the diluting to the headbox consistency takes place in two stages, of which in the first stage there is a substantially constant flow, and in the second stage the flow is regulated by means of a control signal received from the headbox pressure regulation.
The method in accordance with the invention for regulating the basis weight by means of metering of component stocks can also be used in conventional process arrangements in
Neles Paper Automation Oy
Nguyen Dean T.
Steinberg & Raskin, P.C.
LandOfFree
Method for regulating basis weight of paper or board in a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for regulating basis weight of paper or board in a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for regulating basis weight of paper or board in a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2462622