Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
1999-03-05
2001-07-10
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C162S190000, C162SDIG004, C700S129000, C700S128000
Reexamination Certificate
active
06258213
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for controlling the variation of the basis weight of a paper web in the machine direction in which the basis weight of the dried web is measured and the flow of thick stock into a wire pit containing white water from the paper machine is controlled based on the measured basis weight of the web whereby a mixed flow of the thick stock and white water is passed into a headbox of the paper machine.
The present invention also relates to a regulation method in a paper machine for controlling the variation of the basis weight of a paper web in the machine direction, in which the basis weight profile of the paper web is measured by means of a measurement device. A basis weight regulation system receives the signal of the measured basis weight from the measurement device and generates a regulation signal by whose means the flow of thick stock passed into the short circulation in the paper machine is regulated by means of an actuator, such as a basis weight valve and/or a regulation pump.
BACKGROUND OF THE INVENTION
As is known in the prior art, the systems of regulation of the machine-direction basis weight of the paper produced by means of paper machines operate as follows. The flow of the thick stock entering into the wire pit is regulated by means of a basis weight valve based on the measurement of the basis weight in the dry end of the paper machine. The basis weight of the paper web is measured by means of measurement detectors traversing in its cross direction, and the result of measurement of the cross-direction basis weight profile is produced as an average value and passed into the system of regulation as a feedback signal. From the basis weight valve, the flow of thick stock is passed, in a manner known in the prior art, into the wire pit and into which wire pit, the wire waters are also passed from the wire part of the paper machine. In the wire pit, the thick stock flow and the wire waters are mixed together, and the diluted stock flow thus obtained is passed, in a manner in itself known, through stock cleaning and deaeration devices into the inlet header in the headbox and from there further through the distributor manifold, possible stilling chamber and the turbulence generator of the headbox into a slice duct of the headbox. Out of the slice duct, the stock suspension jet is discharged onto a forming wire or into a forming gap defined between a pair of forming wires.
In the prior art, the cross-direction basis weight profile of the paper produced by means of paper machines is often regulated by means of profiling of the height of the slice opening based on the measurement of basis weight taking place in the dry end of the paper machine and described above. In recent years, what is called dilution regulations have also become more common, in which a dilution medium, typically wire water or clear filtrate or, in exceptional cases, a stock more dilute than the stock in the headbox, is fed into discrete feed points separate in the cross direction in connection with the headbox. By means of this dilution water feed system, the cross-direction basis weight profile of the slice jet is profiled together with, or without, regulation of the top slice bar. It is a particular advantage of the dilution regulation that the headbox can be run with a slice opening at a uniform height, so that the cross-direction flows in and after the slice jet, arising from the profiling of the height of the slice opening, and the distortions in the fiber orientation profile in the paper, as a result of such cross-direction flows, can be substantially avoided.
It is the most pronounced drawback of the prior art systems of regulation that monitor the machine-direction basis weight profile of paper that the response time of the system of regulation is very long. The delay in these systems of regulation from the actuator to the basis weight measurement point consists of the delay caused by the stock flow from the wire pit to the headbox and of the time of the passage of the paper web from the headbox to the measurement frame in the dry end of the machine. The magnitude of the delay depends on the stock flow velocity in the pipe system of the short circulation and on the machine speed.
The dead time in this type of system of regulation is typically of an order of one minute. Further, the regulation is made slower by the fact that, after a change in the position of the basis weight valve, reaching of a stable situation requires equalization of the consistencies in the entire short circulation. The time constant arising from this in the process is typically several dozens of seconds.
The long dead time and the time constant restrict the operation of this system of regulation as follows:
In a stable running situation, the regulation is capable of eliminating variations of very long cycles only. As a rough upper limit are considered disturbances of a cycle of 100 seconds, but in practice even this estimate is in most cases clearly excessively optimistic.
In situations of change in the basis weight, such as change of paper grade, reaching a new stable situation takes a long time.
Optimal tuning of the regulation is difficult, because the dead time varies in compliance with the process conditions, and it cannot be predicted precisely by means of computing in different situations.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method of regulation in a paper machine by whose means expressly the machine-direction basis weight profile of the paper produced by means of the paper machine can be regulated more favorably than in the prior art so that the drawbacks discussed above can be substantially avoided.
It is a specific object of the present invention, in a novel and inventive way, to combine the prior art method of regulation of the cross-direction basis weight profile of paper based on dilution of the stock flow in the headbox and a method of regulation of the machine-direction basis weight profile.
It is another object of the present invention to provide a new and improved method for controlling the basis weight profile of a fibrous and/or material web.
In view of achieving the objects stated above and others, in the invention, the system of regulation used in the regulation method includes control means for controlling the variation of the basis weight in the machine direction in the paper machine, besides by means of a slow circuit of regulation-the use of a basis weight actuator to control the flow of thick stock into the wire pit, also by means of a substantially quicker second circuit of regulation which comprises an actuator or actuators and is associated with the dilution profiling system of the headbox. By means of these actuators, the level or flow rate of the dilution water flow in the dilution profiling system of the headbox and/or the consistency of the dilution water flow is/are controlled across the entire width of the paper web.
According to the invention, the regulation of the machine-direction basis weight profile is not accomplished exclusively by means of the system of regulation of the headbox, but in combination with regulation of a prior art basis weight valve or equivalent. In the invention, by means of the headbox, controlled changes are made in the overall dilution, i.e., in the level of dilution extending across the entire width of the paper web, by means of which controlled changes some of the slowness related to the regulation of the basis weight valve is compensated for.
Owing to the system of regulation in accordance with the invention, the dead time of the process can be reduced typically to about 20 seconds. Moreover, the detrimental effect of the time constant arising from the stabilization of the consistencies can be reduced substantially by connecting the regulation with measurement of the consistency of the stock entering into the headbox. This measurement can be used as a feedforward control for the dilution regulation, in which case, in a situation of
Fagerlund John
Mustonen Harri
Vahatalo Harri
Fortuna Jose
Steinberg & Raskin, P.C.
Valmet Corporation
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