Textiles: fluid treating apparatus – Machines – Liquid flowing
Patent
1996-12-27
1998-07-14
Vanatta, Amy B.
Textiles: fluid treating apparatus
Machines
Liquid flowing
162 60, D21D 502
Patent
active
057787042
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an arrangement for a continuous diffuser for washing pulp, comprising a number of hydraulic cylinders distributed in a ring, arranged so as to repeatedly raise and then lower a screen assembly which is included in the diffuser.
PRIOR ART
The washing of paper pulp for the purpose of removing undissolved residues of wood and chemicals, for example after cooking and bleaching, can take place in a so-called continuous diffuser. The washing is in this case effected by means of the pulp being pumped into the bottom of a tank and moving upwards in the latter. The liquid with dissolved wood residues and chemicals which surrounds the pulp runs into, and is pressed out in, a screen assembly which is situated in the diffuser and which moves upwards at approximately the same speed as the pulp, while at the same time washing liquid is conveyed to the pulp in connection with the screen assembly. When the screen assembly has reached its uppermost position, it is drawn quickly down to its lowest position, after which the procedure is repeated.
The screen assembly consists of screen elements made of metal, carried by radial support arms, arranged in concentric rings with diameters of, at present, up to about 9 m. On account of the size, weight and structure of the screen assembly, it is sensitive to uneven loads which can lead to damage and/or operational shutdown. The movement of the screen assembly is effected with the aid of a plurality of double-acting hydraulic cylinders which are distributed in a ring and whose upper piston rods are connected to the radial supporting arms of the screen assembly.
In order to obtain a simultaneous parallel movement of the hydraulic cylinders, these are coupled in series, in one or more circuits. The alternative involving a purely parallel coupling of the hydraulic cylinders means that a drop in pressure in the conduits of the hydraulic system has a greater effect, necessitates simultaneous valve movements in the hydraulic system, and therefore places greater demands on monitoring equipment. Depending on the size of the screen assembly, the number of hydraulic cylinders used at present is either three, four or six. If there are three hydraulic cylinders in the diffuser, these are coupled in series; if there are four, they are coupled 2 by 2, i.e. two circuits of two series-coupled hydraulic cylinders. If there are six, they are coupled 2 by 3, i.e. two circuits of three series-coupled hydraulic cylinders.
In a series-coupled circuit of hydraulic cylinders, the compressibility of the hydraulic medium leads to the hydraulic cylinders situated in the middle, if there are more that two of them, working with less power than the first and last, on account of the compressibility of the hydraulic medium. In addition, leakage in a hydraulic cylinder can result in its not participating in the work at all, or only to a very small degree. This leads to overloading of the radial supporting arms which lie nearest the region to which the leaking hydraulic cylinder is connected. The compressibility of the hydraulic medium and possible leakage in the hydraulic cylinders thus leads to stresses in the screen assembly and its radial supporting arms during the upward and downward movements of the said screen assembly, and can additionally lead to damage or to the movement of the screen assembly being arrested, with operational shutdown as a consequence.
The downward movement of the screen assembly is rapid, in order among other reasons to clear the screens of pulp by means of so-called back-flushing. In order to slow down this movement before the hydraulic pistons have reached their lowest position, the movement of the hydraulic pistons is slowed down by means of the lower part of the hydraulic pistons and the hydraulic cylinders being designed as brake chambers. The brake chambers have very small mechanical tolerances, and it is for this reason, and on account of temperature variations and the compressibility of the hydraulic medium, that the slowing down can di
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patent: 4827741 (1989-05-01), Luthi
patent: 5027620 (1991-07-01), Richter
patent: 5560229 (1996-10-01), Jonsson et al.
Kvaerner Pulping AB
Vanatta Amy B.
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