Classifying – separating – and assorting solids – Plural – diverse separating operations – Aqueous suspension and sifting
Reexamination Certificate
1999-09-17
2001-02-13
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Plural, diverse separating operations
Aqueous suspension and sifting
C209S155000, C209S156000, C209S268000, C209S273000, C209S246000
Reexamination Certificate
active
06186333
ABSTRACT:
The invention relates to a method for controlling a multi-step screening apparatus, said method comprising measuring flows of accept and reject channels of the screening apparatus comprising at least two steps, and adjusting their flow by means of regulating valves mounted in the channels.
The invention further relates to an apparatus for controlling a multi-step screening apparatus, said apparatus comprising a control unit arranged to measure flows of accept and reject channels of the screening apparatus by means of measuring devices provided in the channels, and to regulate accept and reject flows of the screening apparatus by means of regulating valves mounted in the channels.
Screening apparatuses are used for screening pulp in order to obtain the most suitable pulp for the manufacture of paper or board. The screening removes various impurities, such as splinters, any fibre bundles, and other impurities disadvantageous in view of the manufacture and quality of the finished web. The screening is performed by screening apparatuses comprising one or more screen cylinders or corresponding members provided with apertures. The area and apertures or slots of these cylinders or corresponding members are dimensioned such that the accepted pulp fraction, i.e. accept, is allowed to pass through the apertures of the screen as easily as possible, whereas impurities and fibres that are too big are not allowed to pass therethrough. In modern papermaking, it is very important to achieve as pure accept and as high screening capacity as possible. As a result of this, part of the acceptable material ends up in the rejected pulp fraction, i.e. reject, wherefore it is economical to screen the reject again for recovering such fibre material. Multi-step screening is thus used to ensure that the desired quality is obtained.
There are various prior art multi-step screens, so-called combined screens, in which the screening of pulp and of reject is effected in two or three successive steps. The apparatuses thus comprise a plurality of operationally successive screening steps. The pulp to be screened is usually supplied to the first step of the screening apparatus, i.e. to the coarse screen. The coarse screen separates the coarsest material, such as splinters and fibre bundles, from the pulp. This material is removed from the screening apparatus and supplied through channels to a reject container. The pulp that has passed through the coarse screening flows to the actual screening steps, in which the pulp that has passed through the screen is supplied through channels to an accept container or the like. The reject is diluted with water, if necessary, and supplied to the following screening step, in which the pulp fraction that has passed through the screen drum is recirculated to the accept channel. Usually a screening apparatus thus comprises a plurality of successive screens which screen the pulp such that the last reject is finally discharged from the screening apparatus through a channel to the reject container.
In prior art solutions, the screening apparatus is controlled by a complicated automation system with a high calculation capacity. The aim of the control system is to estimate the internal flows of the screening apparatus by calculating. However, such a control system has proved to be rather complicated and to set high requirements for the capacity of the automation system.
The object of the present invention is to provide a simpler solution for controlling a multi-step screening apparatus.
The method of the invention is characterized in that the flows of the reject channels and the other accept channels of the screening apparatus are controlled in relation to the flow of the accept channel of the first step of the screening apparatus.
The apparatus of the invention is characterized in that the control unit is arranged to control the flows of other accept and reject channels in relation to the flow of the accept channel of the first step of the screening apparatus.
A basic idea of the invention is that a multi-step screening apparatus is controlled by controlling other accept and reject flows of the screening apparatus in relation to the accept flow of the first screening step, which renders the complicated estimation of internal flows by calculating unnecessary. The idea of another embodiment of the invention is that in the steps following the first screening step, dilution water is supplied to the pulp in relation to the accept flow of the first step. The idea of yet another embodiment of the invention is that the accept flow of the first step of the screening apparatus is adjusted according to the need of either the feeding or the receiving accept container.
An advantage of the invention is that the control system can be simplified to a great extent, and that the solution can be successfully applied to even simple existing automation systems. The solution of the invention does not set as high requirements for the calculation capacity of the automation system as the known solutions. A simpler control system is naturally also less expensive. A further advantage is that a system in which the invention is applied rapidly adapts to variations in production, is easily controllable and stable, and allows an accept of equal quality to be produced.
REFERENCES:
patent: 4207141 (1980-06-01), Seymour
patent: 4504016 (1985-03-01), Wikdahl
patent: 4994145 (1991-02-01), Seymour
patent: 5534139 (1996-07-01), Cadek et al.
Hautala Jouko
Petajisto Jorma
Valli Juhani
Vuorinen Timo
Ladas & Parry
Martin Brett C.
Valment Corporation
Walsh Donald P.
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